Appendix F Water QualityPoway Walmart Expansion Page 1 of 15 Job No. 106-054.1-13
Preliminary
STANDARD URBAN STORMWATER
MANAGEMENT PLAN (SUSMP)
For
Walmart #1700-05 Expansion
13425 Community Road
Poway, CA
Prepared for:
Wal-Mart Stores, Inc.
2001 SE 10th Street
Bentonville, AR 72716
Prepared by:
Nasland Engineering
4740 Ruffner Street
San Diego, CA 92111
858-292-7770
September 21, 2010
Cory Schrack R.C.E. 65976 Date
Poway Walmart Expansion Page 2 of 15 Job No. 106-054.1-13
Table of Contents
Section Description Page
Title Page…………………………………………………………………................................... 1
Table of Contents…………………………………………………………………………………. 2
1.0 Vicinity Map……………………………………………………………………………….. 4
2.0 Project Description……………………………………………………………………….. 5
3.0 Drainage and Stormdrain………………………………………………………………. 5
4.0 Pollutants and Conditions of Concern………………………………………………… 5
4.1 Watershed………………………………………………………………………… 5
4.2 Pollutants From the Project Area……………………………………………… 5
4.3 Pollutants of Concern in Impaired Downstream Bodies of Water…………. 6
4.4 Impact of Hydrologic Regime…………………………………………………… 6
5.0 Low Impact Development Site Design Best Management Practices………………. 6
5.1 Drain a Portion of Impervious Areas into Pervious Areas ………………….. 6
5.2 Properly Design Pervious Areas to Effectively Receive Runoff …………… 6
5.3 Construct a Portion of Low Traffic Areas With Pervious Surfaces ………… 7
5.4 Minimize Directly Connected Impervious Areas …………………………….. 7
5.5 Maintain Pre-Development Rainfall Runoff Conditions …………………….. 7
5.6 Conserve Natural Areas ………………………………………………………. 7
5.7 Construct Streets, Sidewalks and Parking Aisles to Minimum Widths ……. 7
5.8 Minimize Project’s Impervious Footprint ……………………………………… 7
5.9 Minimize Soil Compaction ……………………………………………………… 7
5.10 Maximize Canopy Interception by Preserving Existing Trees and Shrubs … 7
5.11 Preserve Natural Drainage Systems ………………………………………….. 7
6.0 Source Control Best Management Practices………………………………………….. 8
6.1 Provide Storm Water Conveyance System Stenciling and Signage 8
6.2 Design Outdoor Material Storage Area to Reduce Pollution Introduction… 8
6.3 Design Trash Storage Areas to Reduce Pollution Introduction……………… 8
6.4 Use Effective Irrigation Systems and Landscape Design…………………….. 8
7.0 Project Specific Best Management Practices…………………………………………. 8
7.1 Surface Parking Areas …………………………………………………………… 8
7.2 Dock Areas………………………………………………………………………… 8
8.0 Treatment Best Management Practices…………………………………… 9
8.1 Basis for Selection……………………………………………………………… 9
8.2 Targeted Pollutants of Concern……………………………………………… 9
8.3 Pollutants Not Present………………………………………………………… 9
8.4 Design Criteria………………………………………………………………….. 9
8.5 Volume Flow-Based Analysis…………………………………………… 11
8.6 Pollutant Removal Information………………………………………………… 13
8.7 Maintenance Mechanism……………………………………………………… 14
9.0 Conclusion………………………………………………………………………………… 15
10.0 Engineer of Work……………………………………………………………………….... 15
11.0 References………………………………………………………………………………... 15
Poway Walmart Expansion Page 3 of 15 Job No. 106-054.1-13
Attachments
Best Management Practices (BMP) Site Map
City of Poway SUSMP Checklist
2006 CWA Section 303(d) List of Water Quality Limited Segment List
Vegetated Swale Design Sheets from California Stormwater BMP Handbook
Vegetated Buffer Strip Sheets from the California Stormwater BMP Handbook
FloGard Roof Downspout Filter Manufacturer’s Information
FloGard Catch Basin Insert Manufacturer’s Information.
Contech Unit Manufacturer’s Information.
San Diego Hydrologic Basin Planning Area Map
Hydrology Report and Exhibits (Existing and Proposed Conditions) Attached Separately
Poway Walmart Expansion Page 4 of 15 Job No. 106-054.1-13
1.0 VICINITY MAP:
Poway Walmart Expansion Page 5 of 15 Job No. 106-054.1-13
2.0 PROJECT DESCRIPTION:
The project is located on approximately 16.5 acres near the intersection of Community Road and Hilleary Place.
The existing site is comprised of a single st ory existing Walmart building and associated paved parking lots and
truck loading areas.
The project proposes a rear expansion of the existing Walmart store. Work at the rear of the store includes
demolition of an existing driveway at Midland Road, demolition of portions of the existing structure including the
existing truck dock and Tire Lube Express (TLE), as well demolition of existing paving, parking and utilities within
the proposed expansion area. The adjacent Plowboy’s Market at the corner of Hilleary Place and Midland Road
will also be demolished in order to make room for the expansion and a new driveway at Hilleary Place. The rear
expansion will then be constructed which will include additional retail area, 2 truck docks, 2 trash compactors,
paving, utilities and landscaping. Small modifications are also proposed at the existing building front including
revisions to the entry vestibules and the front drive aisle. The approximate disturbed area for the entire site will be
approximately 9.5 acres.
The project proposes a significant increase in landscape area. The existing site is approximately 85% impervious.
The proposed site will be approximately 82% impervious.
As defined in the City of Poway SUSMP, the project qualifies for the following exception B to the numeric sizing
criteria:
“Where significant redevelopment results in an increase of less than 50 percent of the impervious
surfaces of a previously existing development, and the existing development was not subject to SUSMP
requirements, the numeric sizing criteria discussed for structural treatment control volume -based BMPs
apply only to the addition, and not to the entire development. (Ord. 569 § 2, 2002 ).”
Therefore proposed stormwater treatment facilities are required to be sized to treat runoff from the new
impervious surfaces only (see section 8.0). The project’s treatment BMP’s will be designed to exceed this
requirement where feasible.
3.0 DRAINAGE AND STORMDRAIN
The existing and proposed flows for this project are indicated in the W almart Expansion Preliminary Hydrology
Study dated September 21, 2010 prepared by Nasland Engineering. Please refer to the attached copy of the
Hydrology Study.
4.0 POLLUTANTS AND CONDITIONS OF CONCERN
4.1 WATERSHED
The project site is located in the San Diego Region, Penasquitos Hydraulic Unit, Poway HA identified as
906.20. The nearest receiving water is Poway Creek. Poway Creek is not listed on the 2006 303(d) list.
The nearest watershed listed in the 2006 303(d) list is the Los Penasquitos Creek 906.10 which lists
Phosphate and Total Dissolved Solids as pollutants for a 12 mile stretch. Potential sources are listed as
unknown.
4.2 POLLUTANTS FROM THE PROJECT AREA
The anticipated and potential pollutants from the Walmart Expansion project for Parking Lots and
Commercial Development are as follows (based on Table 1):
Project Category Anticipated Pollutants Potential Pollutants
Parking Lots
Commercial Development
- Heavy Metals
- Trash & Debris
- Oil & Grease
- Sediments
- Nutrients
- Organic Compounds
- Oxygen Demanding
Substances
- Pesticides
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4.3 POLLUTANTS OF CONCERN IN IMPAIRED DOWNSTREAM BODIES OF WATER
As indicated in section 4.1 the nearest receiving water is Poway Creek. Poway Creek is not listed on the
2006 303(d) list. The nearest watershed listed in the 2006 303(d) list is the Los Penasquitos Creek
906.10 which lists Phosphate and Total Dissolved Solids as pollutants for a 12 mile stretch. Common
sources of phosphates include agricultural runoff, untreated and partially treated sewage and some lawn
fertilizers. Common sources of Total Dissolved Solids include agricultural runoff, sediments, and urban
runoff (including heavy metals). Since the anticipated pollutants from the project site includes heavy
metals and heavy metals can occur in Total Dissolved Solids, heavy metals are a primary pollutant of
concern. Since Sediments are a potential pollutant from Parking Lots and Commercial Development, and
sediments can contribute to Total Dissolved Solids, sediments are a secondary pollutant of concern.
The project will propose permanent BMP’s to reduce the possibility of pollutants leaving the project site
(Section 8.0).
4.4 CONDITIONS OF CONCERN (IMPACT TO HYDROLOGIC REGIME)
The existing hydrologic regime will not be adversely impacted by the proposed pr oject and should not be
considered a condition of concern. The existing site is fully developed with a retail/commercial building
and associated paved parking and loading areas. There are no natural habitats, creeks or streams in the
project vicinity. The proposed project will not increase the runoff volume or velocity; the project will not
significantly reduce infiltration or increase runoff flow frequency. Therefore no downstream erosion will
occur. There are no habitats in the immediate vicinity of the project that would be affected. The existing
drainage patterns will be kept as close to existing as possible so as to not increase the run off to any one
drainage area. Refer to the Hydrology Study for a detailed basin analysis.
5.0 LOW IMPACT DEVELOPMENT SITE DESIGN BEST MANAGEMENT PRACTICES
The list of Low Impact Development Site Design Best Management Practices (City of Poway SUSMP Manual)
contains the following:
If project includes landscaped or pervious areas, drain a portion of impervious areas into pervious areas
prior to discharge to MS4 (See Section 5.1)
If project includes landscaped or pervious areas, properly design pervious areas to effectively receive and
infiltrate or treat runoff from impervious areas (See Section 5.2)
If project includes low-traffic areas (walkways, trails, patios, parking lots, alleys, etc.) and appropriate soil
conditions, construct a portion of low-traffic areas with permeable surfaces (See Section 5.3)
Minimize directly connected impervious areas (See Section 5.4)
Maintain pre-development rainfall runoff conditions (See Section 5.5)
Conserve natural areas (See Section 5.6)
Construct streets, sidewalks and parking lot aisles to minimum widths (See Section 5.7)
Minimize project's impervious footprint (See Section 5.8)
Minimize soil compaction (See Section 5.9)
Maximize canopy interception by preserving existing trees and shrubs (See Section 5.10)
Preserve natural drainage systems (See Section 5.11)
5.1 DRAIN A PORTION OF IMPERVIOUS AREAS INTO PERVIOUS AREAS
The new impervious pavement areas at the rear of the building will drain to vegetated swales and
vegetated strips on the eastern perimeter of the site.
5.2 PROPERLY DESIGN PERVIOUS AREAS TO EFFECTIVELY RECEIVE RUNOFF
The proposed vegetated swales and vegetated strips are designed per guidelines set forth in the
California Stormwater BMP Handbook. Calculations are shown in section 8.5 of this report.
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5.3 CONSTRUCT A PORTION OF LOW TRAFFIC AREAS WITH PERMEABLE SURFACES
Use of permeable pavements is not feasible for this project. The nature of a Walmart site is not conducive
to permeable surfaces other than landscape areas. All areas of the site experience high intensity vehicle
and pedestrian use. The intense use and heavy truck loads on the new paved areas at the rear of the
building would not permit the use of pervious concrete paving. All paved drive surfaces will be exposed to
significant vehicle traffic. The majority of the main parking area is to remain. The new concrete pavement
in front of the building is also subject to constant pedestrian and cart traffic.
5.4 MINIMIZE DIRECTLY CONNECTED IMPERVIOUS AREAS
The site design will incorporate vegetated swales and vegetated strips on the eastern perimeter of the
site. The vegetated swales will accept runoff from the new pavement areas behind the building. The
vegetated swales will filter the runoff before entering the underground storm drain system.
5.5 MAINTAIN PRE-DEVELOPMENT RAINFALL RUNOFF CONDITIONS
Per the Hydrology Study, the project will significantly reduce the overall peak stormwater runoff. This is
due to the increased landscape area for the site, which promotes natural infiltration.
5.6 CONSERVE NATURAL AREAS
Because this site is already developed with commercial buildings and surface parking lots, there are few
natural areas to conserve. Many existing mature and healthy trees will be retained throughout the site.
The landscape concept plan and tree removal plan as a part of the site plan package show specific
locations.
5.7 CONSTRUCT STREETS, SIDEWALKS AND PARKING AISLES TO MINIMUM WIDTHS
The proposed sidewalk along the southern portion of the site will be designed to the minimum width
required. The paved drive aisles will be constructed to the minimum width necessary for safe vehicle
movement and fire department access.
5.8 MINIMIZE PROJECT’S IMPERVIOUS FOOTPRINT
The project proposes a significant increase in landscape area. The existing site is approximately 85%
impervious. The proposed site will be approximately 82% impervious. The building expansion will occur
over existing impervious surface, and additional pervious landscaping will be added at the location of the
former plowboy’s market.
5.9 MINIMIZE SOIL COMPACTION
In landscape areas, soil compaction will be minimized to promote storm water infiltration.
5.10 MAXIMIZE CANOPY INTERCEPTION BY PRESERVIING EXISTING TREES AND SHRUBS
Moderate amounts of landscaping exist on the site currently. Existing mature trees that are healthy will
remain to help with canopy interception. Several dozen unhealthy trees will be removed as indicated on
the tree removal plan in the site development package. To replace the removed trees the project will
incorporate new landscaping to the City of Poway standards. To help conserve water, rain sensors & drip
irrigation shall be incorporated into the new landscape irrigation system. Proposed landscaping will be
drought tolerant and require minimal irrigation. Landscaping and irrigation will comply with Walmart’s
Xeriscape guidelines.
5.11 PRESERVE NATURAL DRAINAGE SYSTEMS
No natural creeks or channels exist in the vic inity of the site that could be used as natural drainage
systems. The site design proposes the use of vegetated swales along the eastern perimeter of the site.
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6.0 SOURCE CONTROL BEST MANAGEMENT PRACTICES
The list of Source Control Best Management Practices (City of Poway SUSMP Manual) contains the following:
Provide Storm Water Conveyance System Stenciling and Signage
Design Outdoor Material Storage Area to Reduce Pollution Introduction
Design Trash Storage Areas to Reduce Pollution Introduction
Use effective Irrigation Systems and Landscape Design
6.1 PROVIDE STORM WATER CONVEYANCE SYSTEM STENCILING AND SIGNAGE
Appropriate stenciling and signage will be incorporated into the site drainage str uctures. This will include
stenciling of inlet structures and catch basins in traveled ways and pedestrian pathways. Existing storm
drain facilities that do not have markings will be stenciled.
6.2 DESIGN OUTDOOR MATERIAL STORAGE AREA TO REDUCE POLLUTION INTRODUCTION
There will not be any materials stored that would be exposed to rainfall. There are two proposed pallet
and bale recycling areas that will hold cardboard boxes and wood pallets, but no materials that would be
create polluted runoff.
6.3 DESIGN TRASH STORAGE AREAS TO REDUCE POLLUTION INTRODUCTION
Two trash compactors are proposed adjacent to the truck docks. Trash in the compactors is not exposed
to rainfall. The compactor pads are graded so that no runoff from other areas enters the compact or pad
area. The concrete pad areas include a drain inlet that connects to the building sewer system for cleaning
purposes.
6.4 USE EFFECTIVE IRRIGATION SYSTEMS AND LANDSCAPE DESIGN
In order to prevent the introduction of nutrients and phosphates into the stormdrain system, the project’s
irrigation systems will be designed to effectively use irrigation water. Rain sensors are to be installed with
the main controller unit to prevent irrigation during and after precipitation. Pressure drop actuated shut off
valves are to be installed to control water loss in the event of broken lines or damaged spray heads.
Landscape maintenance is to be monitored to assure that excess use of fertilizers and pesticides does
not occur. Irrigation is to be adjusted to avoid runoff.
7.0 PROJECT SPECIFIC BEST MANAGEMENT PRACTICES
The list of Project Specific Best Management Practices (City of Poway SUSMP) lists two priority categories that
are applicable to this project: Parking Areas and Dock Areas.
7.1 PARKING AREAS
The project proposes some new paved drive aisles and vehicle maneuvering areas. The areas will flow to
vegetated swales and vegetated strips on the eastern side of the site then be filtered via FloGard catch
basin inserts before entering the underground stormdrain system. The new paved areas at the rear of the
building will be used by heavy trucks for maneuvering, so pervious paving is not recommended.
7.2 DOCK AREAS
The project proposes the removal of one existing lo ading dock and construction of two new dock areas.
The dock areas will be graded so no stormwater other than the immediate dock area will drain toward the
low point of the dock well. A Contech unit will be utilized to treat stormwater runoff for each proposed
truck dock.
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8.0 TREATMENT BEST MANAGEMENT PRACTICES
Structural Treatment Best Management Practices are slightly different th an previous categories of best
management practices because they are applied to each project site on a case by case basis. Structural
Treatment Best Management Practices are design considerations instead of a list of suggestions. The following
areas will be evaluated:
Basis for Selection
o Targeted Pollutants of Concern
o Pollutants Not Present
o Exclusions
Design Criteria
Pollutant Removal Information
Literature References
Maintenance Condition(s)
8.1 BASIS FOR SELECTION
The selected structural treatment systems will need to address the primary pollutant of concern as well as
the secondary pollutants concern to the maximum extent practicable (MEP) standard. Feasibi lity of
implementation is also important to consider when selecting a structural treatment BMP. The pollutants of
concern are included in Section 4.2 and discussed below.
Based on Table 3 of the City of Poway SUSMP and based on feasibility of implementation and known
removal efficiency, we recommend a combination of bio-filters (vegetated swales) and drainage inserts.
Using these treatment systems in series will create a “treatment train” and provide at least a medium level
of removal efficiency for anticipated and potential pollutants.
8.2 TARGETED POLLUTANTS OF CONCERN
As indicated in Section 4.2 Heavy Metals are the pollutant of concern and Sediment is a secondary
pollutant of concern. The project design will need to target this pollutant as well as the following
anticipated pollutants: Trash & Debris, Oil & Grease.
8.3 POLLUTANTS NOT PRESENT
Based on the uses defined in Section 4.2 and Table 1 of the City of Poway SUSMP, Bacteria and Viruses
are not anticipated or potential pollutants from the project site.
8.4 DESIGN CRITERIA
For the County of San Diego the 85th Percentile Storm is specified as the storm event to treat. Treatment
is to be provided either on a volume basis or a flow basis. For a volume based analysis the County has
provided an 85th Percentile Isopluvial Map for the 24-hour storm runoff. For the flow-based analysis the
County has accepted an intensity factor (I) of 0.2 inches per hour as the value to be used in the Rational
Method equation Q=CIA to determine flow rates in cubic feet per second.
As defined in the City of Poway SUSMP, the project qualifies for the following exception #2 to the numeric
sizing criteria:
“Where significant redevelopment results in an increase of less than 50 percent of the impervious
surfaces of a previously existing development, and the existing development was not subject to SUSMP
requirements, the numeric sizing criteria discussed for structural treatment control volume -based BMPs
apply only to the addition, and not to the entire development. (Ord. 569 § 2, 2002.”
Therefore proposed stormwater treatment facilities are required to be sized to treat runoff from the new
areas of impervious surfaces only. The project’s treatment BMP’s will be designed to exceed this
requirement where feasible.
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Wherever possible, stormwater runoff collected by the on-site drainage systems shall be routed through
vegetated swales before entering the drainage structures with filter inserts. Refer to the enclosed BMP
Site Map for locations of proposed treatment BMP’s. When these structural BMPs are used in series they
will provide an effective treatment technology that utilizes filtration and a biological process to remove
potential pollutants of concern from storm water before it reaches local water sources. The systems shall
be installed by the contractor and the owner will be required to regularly maintain the devices. Specific
information for the vegetated swales, vegetated strips, Contech units, FloGard roof downspout filters and
FloGard catch basin inserts is included at the end of this report. Treatment BMPs will be implemented as
depicted in the following table.
Proposed
Basins
Runoff
Coefficient
(C)
I
(in/hr)
Area
(Acres)
Treatment
Flow
(Qt)
Q100 Proposed BMP
P1 0.82 0.20 1.24 0.20 6.75 Not Applicable
P2 0.63 0.20 1.78 0.23 4.79 Vegetated Swale/FloGard Catch Basin Insert
P3 0.86 0.20 1.76 0.30 8.14 Not Applicable
P4 0.81 0.20 5.09 0.83 24.64 Not Applicable
P5 0.90 0.20 0.20 0.04 1.18 FloGard Roof Downspout Filter
P6 0.90 0.20 0.37 0.07 2.03 FloGard Roof Downspout Filter
P7 0.90 0.20 0.24 0.04 1.43 FloGard Roof Downspout Filter
P8 0.90 0.20 0.34 0.06 1.86 FloGard Roof Downspout Filter
P9 0.90 0.20 0.24 0.04 1.44 FloGard Roof Downspout Filter
P10 0.90 0.20 0.31 0.06 1.71 FloGard Roof Downspout Filter
P11 0.90 0.20 0.24 0.04 1.42 FloGard Roof Downspout Filter
P12 0.90 0.20 0.31 0.06 1.71 FloGard Roof Downspout Filter
P13 0.90 0.20 0.24 0.04 1.46 FloGard Roof Downspout Filter
P14 0.90 0.20 0.31 0.06 1.71 FloGard Roof Downspout Filter
P15 0.90 0.20 0.24 0.04 1.46 FloGard Roof Downspout Filter
P16 0.90 0.20 0.31 0.06 1.71 FloGard Roof Downspout Filter
P17 0.90 0.20 0.18 0.03 1.11 FloGard Roof Downspout Filter
P18 0.90 0.20 0.34 0.06 1.89 FloGard Roof Downspout Filter
P19 0.90 0.20 0.11 0.02 0.75 Contech Unit
P20 0.69 0.20 0.48 0.07 2.54 Vegetated Swale/FloGard Catch Basin Insert
P21 0.90 0.20 0.11 0.02 0.75 Contech Unit
P22 0.70 0.20 0.47 0.07 2.47 Vegetated Strip/FloGard Catch Basin Insert
P23 0.86 0.20 0.53 0.09 3.08 Not Applicable
P24 0.53 0.20 0.39 0.04 1.59 Not Applicable
P25 0.65 0.20 0.47 0.06 1.50 Vegetated Strip/FloGard Catch Basin Insert
P26 0.35 0.20 0.19 0.01 0.45 Not Applicable
Proposed Treatment Control BMPs
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8.5 VOLUME AND FLOW-BASED ANALYSIS
The City of Poway SUSMP states that the flow that must be treated is the 85% percentile flow. For the
site the 85th percentile volume will be calculated and the treatment BMP’s will be sized as follows:
The proposed vegetated swales are sized as follows to accommodate the required flow:
Q = CIA
Where C = Runoff Coefficient
I = Intensity = 0.20 in/hr
A = Basin Area (Acres)
Vegetated Swale:
The capacity of each proposed vegetated swale is calculated per guidelines set forth in the California
Stormwater BMP Handbook by using Manning’s Equation as follows:
Q = 1.49/n * A * R2/3 * S1/2
Where n = Manning’s Resistance Coefficient = 0.25
A = channel area
R = hydraulic radius
S = channel slope
Runoff Coefficient (C)0.63 -
Rainfall Intensity (I)0.20 in/hr
Basin Area (A)1.78 acres
Treatment Flow (Qt)0.23 cfs
Swale Bottom Width (b)3.00 ft
Swale Side Slope H:V (z)3.00 -
Swale Flow Slope (s)0.009 -
Depth of Flow (D)0.28 ft
Area (A)1.08 sf
Wetted Perimeter (P)4.77 ft
Hydraulic Radius (R)0.23 ft
Mannings Number (n)0.25 -
Design Flow Velocity (v)0.21 ft/s
Required Residence Time 10.00 Minutes
Design Length 126.89 ft
Treatment Capacity 0.23 cfs
Treatment Flow for Basin P2
Proposed Vegetated Swale for Basin P2
Vegetated Swale Calculation for Basin P2
Each proposed vegetated swale has been designed with a treatment capacity greater than the treatment
flow. For more information on vegetated strips refer to the attachments located at the end of the report.
Runoff Coefficient (C)0.69 -
Rainfall Intensity (I)0.20 in/hr
Basin Area (A)0.48 acres
Treatment Flow (Qt)0.07 cfs
Swale Bottom Width (b)1.00 ft
Swale Side Slope H:V (z)3.00 -
Swale Flow Slope (s)0.009 -
Depth of Flow (D)0.23 ft
Area (A)0.39 sf
Wetted Perimeter (P)2.45 ft
Hydraulic Radius (R)0.16 ft
Mannings Number (n)0.25 -
Design Flow Velocity (v)0.17 ft/s
Required Residence Time 10.00 Minutes
Design Length 100.52 ft
Treatment Capacity 0.07 cfs
Treatment Flow for Basin P20
Proposed Vegetated Swale for Basin P20
Vegetated Swale Calculation for Basin P20
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FloGard Roof Downspout Filters:
Proposed
Basins
Runoff
Coefficient
(C)
I
(in/hr)
Area
(Acres)
Treatment
Flow Qt
(CFS)
Q100
(CFS)
P5 0.90 0.20 0.20 0.04 1.18
P6 0.90 0.20 0.37 0.07 2.03
P7 0.90 0.20 0.24 0.04 1.43
P8 0.90 0.20 0.34 0.06 1.86
P9 0.90 0.20 0.24 0.04 1.44
P10 0.90 0.20 0.31 0.06 1.71
P11 0.90 0.20 0.24 0.04 1.42
P12 0.90 0.20 0.31 0.06 1.71
P13 0.90 0.20 0.24 0.04 1.46
P14 0.90 0.20 0.31 0.06 1.71
P15 0.90 0.20 0.24 0.04 1.46
P16 0.90 0.20 0.31 0.06 1.71
P17 0.90 0.20 0.18 0.03 1.11
P18 0.90 0.20 0.34 0.06 1.89
FloGard Roof Downspout Filter Calculations
10" FloGard roof downspout filters are capable of treating 0.72 cfs and bypassing 3.67 cfs. These filters
are more than adequate for treatment of roof stormwater runoff. For manufacturer’s information on
FloGard roof downspout filters refer to the attachments located at the end of the report.
Contech Unit:
Proposed
Basins
Runoff
Coefficient
(C)
I
(in/hr)
Area
(Acres)
Treatment
Flow Qt
(CFS)
Q100
(CFS)
P19 0.90 0.20 0.11 0.02 0.75
P21 0.90 0.20 0.11 0.02 0.75
Contech Unit Calculations
The Contech unit treats peak water quality design flows up to 0.13 cfs and has an internal weir overflow
capacity of 1.0 cfs. This is more than adequate for treatment of stormwater runoff within the truck dock
areas. For manufacturer’s information on the Contech unit refer to the attachments located at the end of
the report.
FloGard Catch Basin Insert:
Proposed
Basins
Runoff
Coefficient
(C)
I
(in/hr)
Area
(Acres)
Treatment
Flow Qt
(CFS)
Q100
(CFS)
P2 0.63 0.20 1.78 0.23 4.79
P20 0.69 0.20 0.48 0.07 2.54
P22 0.70 0.20 0.47 0.07 2.47
P25 0.65 0.20 0.47 0.06 1.50
FloGard Catch Basin Insert Calculations
An 18x18” FloGard catch basin insert inlet provides treatment for a filtered flow of 0.7 cfs and the
“ultimate” bypass feature is capable of bypassing the 100 year storm flow. For manufacturer’s information
on FloGard catch basin inserts refer to the attachments located at the end of the report.
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8.6 POLLUTANT REMOVAL INFORMATION
8.6.1 VEGETATED SWALE
A vegetated swale is an area of grass, shrubs, and/or close growing vegetation which impedes
the sheet flow of stormwater, encourages infiltration, and prevents direct runoff into adjacent
surface waters. Vegetated swales can achieve moderate to high levels of the majority of potential
pollutants. They can achieve moderate to high levels of removal of metals or nutrients that are
attached to suspended soil particles through the settling of solids by natural flocculation and
vegetation uptake. Flocculation is the process in which a solute comes out of a solution. After flow
through the vegetated swale metals and nutrients will not be transported to the receiving waters.
In the case of some pollutants, the microbiology of the soil can be used to filter dissolved
pollutants from runoff. The proposed swales will be adequately sized to meet the requirements of
minimum 10 minute residence time.
8.6.2 VEGETATED STRIPS
Vegetated strips are vegetated surfaces that are designed to treat sheet flow from adjacent
surfaces. Runoff velocities are slowed which allows sediment and other pollutants to settle.
Vegetated strips are generally effective in reducing the volume and mass of pollutants in runoff.
8.6.3 FLOGARD ROOF DOWNSPOUT FILTER
The FloGard roof downspout filter is made of a durable dual-wall geotextile fabric liner
encapsulating an adsorbent which is easily replaced and provides for flexibility, ease of
maintenance and economy. It is designed to collect particulates and debris, as well as metals and
petroleum hydrocarbons (oils and greases). The FloGard roof downspout filter performs as an
effective filtering device at low flows and, because of the built in high flow bypass, will not
impeded the system’s maximum design flow. FloGard roof downspout filters are typically installed
in commercial buildings for the removal or non-soluble pollutants normally found on building roofs
(sediment, gravel, hydrocarbons) from roof stormwater runoff.
8.6.4 CONTECH UNIT
The Contech unit consists of a multi-chamber steel, concrete or plastic catch basin unit that can
contain up to four StormFilter cartridges. The unit treats peak water quality design flows up to
0.13 cfs and has an internal weir overflow capacity of 1.0 cfs. The single cartridge Contech unit
consists of a sumped inlet chamber and a cartridge chamber. Runoff enters the sumped inlet
chamber either by sheet flow from a paved surface or from an inlet pipe dis charging directly to the
unit vault. The inlet chamber is equipped with an internal baffle, which traps debris and floating oil
and grease, and an overflow weir. While in the inlet chamber, heavier solids are allowed to settle
into the deep sump, while lighter solids and soluble pollutants are directed under the baffle and
into the cartridge chamber through a port between the baffle and the overflow weir. Once in the
cartridge chamber, polluted water ponds and percolates horizontally through the media in th e
filter cartridges. Treated water collects in the cartridge’s center tube from where it is directed by
an under-drain manifold to the outlet pipe on the downstream side of the overflow weir and
discharged. When flows into the unit exceed the water quality design value, excess water spills
over the overflow weir, bypassing the cartridge bay, and discharges to the outlet pipe.
8.6.5 FLOGARD CATCH BASIN INSERT
The FloGard multipurpose catch basin insert is designed to capture sediment, oil and grease,
trash and debris from low flows. A high flow bypass allows flows to bypass the device while
retaining sediment and larger floatables (debris and trash) and allows sustained maximum design
flows under extreme weather conditions. FloGard catch basin inserts are r ecommended for areas
subject to silt and debris as well as low to moderate levels of oils and grease.
Poway Walmart Expansion Page 14 of 15 Job No. 106-054.1-13
8.7 MAINTENANCE MECHANISMS
Maintenance is a major part of any successful best management practice. During Final Engineering an
Operation and Maintenance Plan will be prepared and provided separately. An effective maintenance
program should include the following key components:
Vegetated Swales
Inspection and repair to be scheduled annually before first seasonal rain
Trash removal and mowing (1-2 times/month)
Removal of sediment or buildup (when necessary)
Re-grading to eliminate standing pools of water (when necessary)
Vegetated Strips
Vegetated strips should be inspected at least twice annually for erosion or damage to vegetation,
preferably at the end of the wet season to schedule summer maintenance and before major fall run-off to
be sure the strip is ready for winter. However, additional inspection after periods of heavy run-off is most
desirable. The strip should be checked for debris and litter and areas of sediment accumulation. Mowing
may only be necessary once or twice a year for safety and aesthetics or to suppress weeds and woody
vegetation. The need for litter removal should be determined through periodic inspection but litter should
always be removed prior to mowing. Regularly inspect vegetated strips for pools of standing water.
FloGard Roof Downspout Filter
It is recommended that each installation be serviced a minimum of three times per year, with a change of
filter medium once per year. Service should be performed prior to, during and following the rainy season.
Service procedures generally consist of visual inspection, cleaning, filter replacement (if necessary) and
keeping maintenance records. For additional information on maintenan ce please see the manufactures
information attached at the end of this report.
Contech Unit
At least on scheduled inspection should take place per year with maintenance following as warranted.
First, an inspection should be done before the winter season. During the inspection the need for
maintenance should be determined and, if disposal during maintenance will be required, samples of the
accumulated sediments and media should be obtained. Second, if warranted, maintenance such as
replacement of the filter cartridges and removal of accumulated sediments should be performed during
periods of dry weather. In addition to these two activities, it is important to check the condition of the unit
after major storms for potential damage caused by high flows and for high sediment accumulation that
may be caused by localized erosion in the drainage area. It may be necessary to adjust the
inspection/maintenance schedule depending on the actual operating conditions encountered by the
system. In general, inspection activities can be conducted at any time, and maintenance should occur, if
warranted, in late summer to early fall when flows into the system are not likely to be present.
FloGard Catch Basin Insert
It is recommended that each installation be serviced a minimum of three times per year, with a change of
filter medium once per year. Service should be performed prior to, during and following the rainy season.
Service procedures generally consist of visual inspection, cleaning, filter and grate replacement (if
necessary) and keeping maintenance records. For additional information on maintenance please see the
manufactures information attached at the end of this report.
Source Control BMP’s such as stormwater inlet stenciling are indicated in the operation and maint enance
plan. Stormwater stencils should be maintained so they are legible at all times.
BMPs shall be inspected, cleaned and repaired when necessary, prior to and during each rainy season,
including conducting an annual inspection no later than September 30th each year. Should any of the
project’s surface or subsurface drainage/filtration structures or other BMPs fail or result in increased
erosion, the owner shall be responsible for any necessary repairs to the drainage/filtration system or
BMPs and restoration of the eroded area.
Poway Walmart Expansion Page 15 of 15 Job No. 106-054.1-13
9.0 CONCLUSION
The most effective and feasible means of treating the targeted pollutants to the MEP standard for the Walmart
Expansion project is to use a series of vegetated swales, vegetated strips, FloGard catch basin inserts, FloGard
roof downspout filters and Contech units to treat the primary pollutant of concern and other anticipated pollutants.
The project shall include vegetated swales as primary treatment that provides at a minimum a “medium” level of
removal efficiency for all pollutants. In addition drainage inserts will be utilized to capture any remaining trash,
debris or other pollutants. With the BMP’s in series, this system will create a “treatment train” and allow for higher
removal efficiency and provide effective treatment of storm water runoff. As indicated in the report, site control
and source control BMP’s shall be implemented in addition to the structural treatment BMP’s. The
recommendations in this report are based on current stormwater regulations, chan ges or modifications to
regulations or requirements will require revisions to this report.
10.0 ENGINEER OF WORK
Prepared under the supervision of
Cory Schrack
RCE 65976
Expires 06-30-12
11.0 REFERENCES
This report has been prepared in accordance with:
The City of Poway’s Standard Urban Stormwater Mitigation Plan (SUSMP)
Also Referenced:
Technical information pertaining to the proposed FloGard Catch Basin Inserts and FloGard Roof
Downspout Filters was obtained from the KriStar Enterprises, Inc. Technologies website and staff input.
KriStar Enterprises, Inc. can be found online at http://www.kristar.com/draininlet.html
Technical information pertaining to the proposed Contech Unit was o btained from the Contech
Stormwater Solutions website at http://www.contech-cpi.com/stormwater/products/filtration/stormfilter/15
2006 CWA Section 303(d) List of Water Quality Limited Segments Requireing TMDLs.
http://www.waterboards.ca.gov/water_issues/programs/tmdl/docs/303dlists2006/epa/r9_06_303d_r eqtmdl
s.pdf
California Stormwater BMP Handbook - New Development and Redevelopment, January 2003.
http://www.cabmphandbooks.com/Development.asp
BEST MANAGEMENT PRACTICES
(BMP) SITE MAP
CITY OF POWAY SUSMP CHECKLIST
1 of 6
CITY OF POWAY
Standard Urban Storm Water
Mitigation Plan (SUSMP) Checklist
Application No.: Public Private
Project Name: Walmart #1700-05 Expansion
Address: 13425 Community Road, Poway, CA
Project Description: Expansion of the existing Walmart building. Removal and replacement of
paved areas to the east of the building. Construction of new loading docks.
Does project discharge to an Environmentally Sensitive Area? Yes No
SUSMP Category (Check all SUSMP categories that apply to the project):
Residential development of 10 or more units
X Commercial or industrial development greater than 1 acre
Automotive repair shops
Restaurants
Steep hillside development >5,000 ft2
Project discharging to receiving waters within Environmentally Sensitive Areas (ESA)
that creates 2,500 ft2 or more of impervious surfaces or increases area of
imperviousness of project site to 10% or more of its naturally occurring condition and:
Project is within 200 ft of an ESA
Project is more than 200 ft from an ESA but discharges urban runoff to receiving
water within ESA without mixing with flows from adjacent land
X Parking lots >5,000 ft2 or with >15 parking spaces and potentially exposed to urban
runoff
Streets, roads, highways, and freeways which would create a new paved surface that is
≥5,000 ft2
Retail Gasoline Outlet ≥5,000 ft2 or with projected average daily traffic ≥100 vehicles/day
X Significant Redevelopment – creates or adds ≥5,000 ft2 of impervious surfaces on an
already developed site (and falls in one of the above categories)
No categories apply; project is not subject to SUSMP requirements (if so, there is
no need to complete the rest of the checklist)
Checklist prepared by: Nasland Engineering Date:4-14-09
2 of 6
Identify Pollutants from the Project Area
Check that all pollutants anticipated to be generated from the project area correspond
with the anticipated pollutants in Table 1 of the SUSMP Ordinance (PMC 16.103.030).
Identify Primary and Secondary Pollutants of Concern
Check that primary and secondary pollutants of concern for the project have been
correctly identified and compared with pollutants identified in Table 1.
Which receiving water(s) does the project discharge to? Poway Creek
What are the pollutants for which the receiving water is impaired? Poway Creek is not listed
on the 303(d) list. Nearest impaired water body is Los Penasquitos Creek (phosphate, total
dissolved solids)
Did project compare receiving water pollutants
with pollutants generated from project area? Yes No
What are the primary pollutants of concern? Heavy Metals
What are the secondary pollutants of concern? Sediments
Identify Conditions of Concern
Was a drainage study report prepared? Yes No
Was the report prepared by a registered civil engineer? Yes No
Name of engineer Cory Schrack, Nasland Engineering
Was a field reconnaissance conducted? Yes No
Did drainage study compute:
X Peak flow rate Retention volume
Flow velocity X 2-year frequency storm
Runoff volume X 10-year frequency storm
X Time of concentration
What duration storm was used?
X 6 hour 24 hour
Were conditions of concern adequately identified? Yes No
3 of 6
If so, has the project implemented site design, source control,
and/or treatment control BMPs to maintain pre-project hydrologic
conditions affecting downstream conditions of concern? Yes No
Hydromodification Management
Does the project disturb more than 50 acres? Yes No
If yes, has modeling of pre-project and post-project flows Yes No
been performed in accordance with the interim hydromodification
criteria described in PMC 16.103.050?
Are estimated post-project runoff durations and flows less than or Yes No
equal to pre-project values?
Comments:
Establish Storm Water BMPs
Low Impact Development Site Design BMPs
The following items must be implemented by all SUSMP priority projects:
X If project includes landscaped or pervious areas, drain a portion of impervious areas into
pervious areas prior to discharge to MS4
X If project includes landscaped or pervious areas, properly design pervious areas to
effectively receive and infiltrate or treat runoff from impervious areas
If project includes low-traffic areas (walkways, trails, patios, parking lots, alleys, etc.) and
appropriate soil conditions, construct a portion of low-traffic areas with permeable
surfaces (N/A, surfaces experience high volume/ high intensity uses)
The following items must be implemented where determined applicable and feasible (identify
reasons for infeasibility of any item)
X Minimize directly connected impervious areas
X Maintain pre-development rainfall runoff conditions
X Conserve natural areas
X Construct streets, sidewalks, and parking lot aisles to minimum widths
X Minimize project's impervious footprint
4 of 6
X Minimize soil compaction
X Maximize canopy interception by preserving existing trees and shrubs
Preserve natural drainage systems (N/A, no natural drainage systems exist on-
site)
Comments:
Source Control BMPs
Does the project:
X Provide storm drain system stenciling and signage
X Design outdoor material storage areas to reduce pollution introduction
X Design trash storage areas to reduce pollution introduction
X Use efficient irrigation systems & landscape design
X Incorporate requirements applicable to priority project categories (see SUSMP for
detailed requirements):
Private roads Equipment wash areas
Residential driveways & guest parking X Parking areas
X Dock areas Roadways
Maintenance bays Fueling areas
Vehicle wash areas Hillside landscaping
Outdoor processing areas
Treatment Control BMPs
Check the treatment control(s) selected:
X Biofilters (Vegetated swales) Wet pond
Detention basin Constructed wetland
Infiltration Filtration system
o Infiltration basin
o Infiltration trench
o Porous asphalt, concrete, or modular concrete block
X Drainage insert
o OiI/water separator
X Catch basin insert
o Other
Hydrodynamic separation system
5 of 6
Do treatment controls effectively address primary
pollutants of concern (High or Medium Effectiveness)? Yes No
If No, describe:
Verify the design of selected treatment controls:
Was the treatment control BMP designed for: Volume X Flow
Did the SUSMP present the BMPs design process
(e.g., the specific design criteria used)? Yes No
Did the SUSMP use the 85th percentile storm event for design? Yes No
What design storm was used to calculate numeric sizing criteria?
Was the BMP designed properly? Yes No
If not, describe: Detailed BMP sizing calculations will be included in the
SUSMP during final project design. The project is currently in discretionary
review.
Is BMP(s) located near pollutant sources to be treated? Yes No
Are there restrictions on use of infiltration BMPs? Yes No
Maintenance Requirements
Was an O&M plan attached? Yes No
Does plan require annual inspection and Yes No
as-needed maintenance of all structural BMPs?
Was an access easement/agreement included? Yes No
O&M plan will be prepared during final project design. The project is currently in
discretionary review.
Waiver of Structural Treatment BMP Requirements
Was a waiver of infeasibility granted? Yes No
If Yes, was RWQCB notified? Yes No
Other Information
6 of 6
Table 1. Anticipated and Potential Pollutants Generated by Land Use Type.
General Pollutant Categories
Priority
Project
Categories Sediments Nutrients
Heavy
Metals
Organic
Compounds
Trash
&
Debris
Oxygen
Demanding
Substances
Oil &
Grease
Bacteria
&
Viruses Pesticides
Detached
Residential
Development
X X X X X X X
Attached
Residential
Development
X X X P(1) P(2) P X
Commercial
Development
>One Acre
P(1) P(1) P(2) X P(5) X P(3) P(5)
Industrial
Development
>One Acre
X X X X X X
Automotive
Repair
Shops
X X(4)(5) X X
Restaurants X X X X
Hillside
Development
>5,000 ft2
X X X X X X
Parking Lots P(1) P(1) X X P(1) X P(1)
Retail
Gasoline
Outlets
X X X X X
Streets,
Highways &
Freeways
X P(1) X X(4) X P(5) X
X = anticipated
P = potential
(1) A potential pollutant if landscaping exists on-site.
(2) A potential pollutant if the project includes uncovered parking areas.
(3) A potential pollutant if land use involves food or animal waste products.
(4) Including petroleum hydrocarbons.
(5) Including solvents.
3/08
2006 CWA SECTION 303(d) LIST
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VEGETATED SWALE
Vegetated Swale TC-30
January 2003 California Stormwater BMP Handbook 1 of 13
New Development and Redevelopment
www.cabmphandbooks.com
Description
Vegetated swales are open, shallow channels with vegetation
covering the side slopes and bottom that collect and slowly
convey runoff flow to downstream discharge points. They are
designed to treat runoff through filtering by the vegetation in the
channel, filtering through a subsoil matrix, and/or infiltration
into the underlying soils. Swales can be natural or manmade.
They trap particulate pollutants (suspended solids and trace
metals), promote infiltration, and reduce the flow velocity of
stormwater runoff. Vegetated swales can serve as part of a
stormwater drainage system and can replace curbs, gutters and
storm sewer systems.
California Experience
Caltrans constructed and monitored six vegetated swales in
southern California. These swales were generally effective in
reducing the volume and mass of pollutants in runoff. Even in
the areas where the annual rainfall was only about 10 inches/yr,
the vegetation did not require additional irrigation. One factor
that strongly affected performance was the presence of large
numbers of gophers at most of the sites. The gophers created
earthen mounds, destroyed vegetation, and generally reduced the
effectiveness of the controls for TSS reduction.
Advantages
If properly designed, vegetated, and operated, swales can
serve as an aesthetic, potentially inexpensive urban
development or roadway drainage conveyance measure with
significant collateral water quality benefits.
Design Considerations
Tributary Area
Area Required
Slope
Water Availability
Targeted Constituents
Sediment ▲
Nutrients
Trash
Metals ▲
Bacteria
Oil and Grease ▲
Organics ▲
Legend (Removal Effectiveness)
Low High
▲ Medium
TC-30 Vegetated Swale
2 of 13 California Stormwater BMP Handbook January 2003
New Development and Redevelopment
www.cabmphandbooks.com
Roadside ditches should be regarded as significant potential swale/buffer strip sites and
should be utilized for this purpose whenever possible.
Limitations
Can be difficult to avoid channelization.
May not be appropriate for industrial sites or locations where spills may occur
Grassed swales cannot treat a very large drainage area. Large areas may be divided and
treated using multiple swales.
A thick vegetative cover is needed for these practices to function properly.
They are impractical in areas with steep topography.
They are not effective and may even erode when flow velocities are high, if the grass cover is
not properly maintained.
In some places, their use is restricted by law: many local municipalities require curb and
gutter systems in residential areas.
Swales are mores susceptible to failure if not properly maintained than other treatment
BMPs.
Design and Sizing Guidelines
Flow rate based design determined by local requirements or sized so that 85% of the annual
runoff volume is discharged at less than the design rainfall intensity.
Swale should be designed so that the water level does not exceed 2/3rds the height of the
grass or 4 inches, which ever is less, at the design treatment rate.
Longitudinal slopes should not exceed 2.5%
Trapezoidal channels are normally recommended but other configurations, such as
parabolic, can also provide substantial water quality improvement and may be easier to mow
than designs with sharp breaks in slope.
Swales constructed in cut are preferred, or in fill areas that are far enough from an adjacent
slope to minimize the potential for gopher damage. Do not use side slopes constructed of
fill, which are prone to structural damage by gophers and other burrowing animals.
A diverse selection of low growing, plants that thrive under the specific site, climatic, and
watering conditions should be specified. Vegetation whose growing season corresponds to
the wet season are preferred. Drought tolerant vegetation should be considered especially
for swales that are not part of a regularly irrigated landscaped area.
The width of the swale should be determined using Manning’s Equation using a value of
0.25 for Manning’s n.
Vegetated Swale TC-30
January 2003 California Stormwater BMP Handbook 3 of 13
New Development and Redevelopment
www.cabmphandbooks.com
Construction/Inspection Considerations
Include directions in the specifications for use of appropriate fertilizer and soil amendments
based on soil properties determined through testing and compared to the needs of the
vegetation requirements.
Install swales at the time of the year when there is a reasonable chance of successful
establishment without irrigation; however, it is recognized that rainfall in a given year may
not be sufficient and temporary irrigation may be used.
If sod tiles must be used, they should be placed so that there are no gaps between the tiles;
stagger the ends of the tiles to prevent the formation of channels along the swale or strip.
Use a roller on the sod to ensure that no air pockets form between the sod and the soil.
Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days
after the first rainfall of the season.
Performance
The literature suggests that vegetated swales represent a practical and potentially effective
technique for controlling urban runoff quality. While limited quantitative performance data
exists for vegetated swales, it is known that check dams, slight slopes, permeable soils, dense
grass cover, increased contact time, and small storm events all contribute to successful pollutant
removal by the swale system. Factors decreasing the effectiveness of swales include compacted
soils, short runoff contact time, large storm events, frozen ground, short grass heights, steep
slopes, and high runoff velocities and discharge rates.
Conventional vegetated swale designs have achieved mixed results in removing particulate
pollutants. A study performed by the Nationwide Urban Runoff Program (NURP) monitored
three grass swales in the Washington, D.C., area and found no significant improvement in urban
runoff quality for the pollutants analyzed. However, the weak performance of these swales was
attributed to the high flow velocities in the swales, soil compaction, steep slopes, and short grass
height.
Another project in Durham, NC, monitored the performance of a carefully designed artificial
swale that received runoff from a commercial parking lot. The project tracked 11 storms and
concluded that particulate concentrations of heavy metals (Cu, Pb, Zn, and Cd) were reduced by
approximately 50 percent. However, the swale proved largely ineffective for removing soluble
nutrients.
The effectiveness of vegetated swales can be enhanced by adding check dams at approximately
17 meter (50 foot) increments along their length (See Figure 1). These dams maximize the
retention time within the swale, decrease flow velocities, and promote particulate settling.
Finally, the incorporation of vegetated filter strips parallel to the top of the channel banks can
help to treat sheet flows entering the swale.
Only 9 studies have been conducted on all grassed channels designed for water quality (Table 1).
The data suggest relatively high removal rates for some pollutants, but negative removals for
some bacteria, and fair performance for phosphorus.
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Table 1 Grassed swale pollutant removal efficiency data
Removal Efficiencies (% Removal)
Study TSS TP TN NO3 Metals Bacteria Type
Caltrans 2002 77 8 67 66 83-90 -33 dry swales
Goldberg 1993 67.8 4.5 - 31.4 42–62 -100 grassed channel
Seattle Metro and Washington
Department of Ecology 1992 60 45 - -25 2–16 -25 grassed channel
Seattle Metro and Washington
Department of Ecology, 1992 83 29 - -25 46–73 -25 grassed channel
Wang et al., 1981 80 - - - 70–80 - dry swale
Dorman et al., 1989 98 18 - 45 37–81 - dry swale
Harper, 1988 87 83 84 80 88–90 - dry swale
Kercher et al., 1983 99 99 99 99 99 - dry swale
Harper, 1988. 81 17 40 52 37–69 - wet swale
Koon, 1995 67 39 - 9 -35 to 6 - wet swale
While it is difficult to distinguish between different designs based on the small amount of
available data, grassed channels generally have poorer removal rates than wet and dry swales,
although some swales appear to export soluble phosphorus (Harper, 1988; Koon, 1995). It is not
clear why swales export bacteria. One explanation is that bacteria thrive in the warm swale
soils.
Siting Criteria
The suitability of a swale at a site will depend on land use, size of the area serviced, soil type,
slope, imperviousness of the contributing watershed, and dimensions and slope of the swale
system (Schueler et al., 1992). In general, swales can be used to serve areas of less than 10 acres,
with slopes no greater than 5 %. Use of natural topographic lows is encouraged and natural
drainage courses should be regarded as significant local resources to be kept in use (Young et al.,
1996).
Selection Criteria (NCTCOG, 1993)
Comparable performance to wet basins
Limited to treating a few acres
Availability of water during dry periods to maintain vegetation
Sufficient available land area
Research in the Austin area indicates that vegetated controls are effective at removing pollutants
even when dormant. Therefore, irrigation is not required to maintain growth during dry
periods, but may be necessary only to prevent the vegetation from dying.
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The topography of the site should permit the design of a channel with appropriate slope and
cross-sectional area. Site topography may also dictate a need for additional structural controls.
Recommendations for longitudinal slopes range between 2 and 6 percent. Flatter slopes can be
used, if sufficient to provide adequate conveyance. Steep slopes increase flow velocity, decrease
detention time, and may require energy dissipating and grade check. Steep slopes also can be
managed using a series of check dams to terrace the swale and reduce the slope to within
acceptable limits. The use of check dams with swales also promotes infiltration.
Additional Design Guidelines
Most of the design guidelines adopted for swale design specify a minimum hydraulic residence
time of 9 minutes. This criterion is based on the results of a single study conducted in Seattle,
Washington (Seattle Metro and Washington Department of Ecology, 1992), and is not well
supported. Analysis of the data collected in that study indicates that pollutant removal at a
residence time of 5 minutes was not significantly different, although there is more variability in
that data. Therefore, additional research in the design criteria for swales is needed. Substantial
pollutant removal has also been observed for vegetated controls designed solely for conveyance
(Barrett et al, 1998); consequently, some flexibility in the design is warranted.
Many design guidelines recommend that grass be frequently mowed to maintain dense coverage
near the ground surface. Recent research (Colwell et al., 2000) has shown mowing frequency or
grass height has little or no effect on pollutant removal.
Summary of Design Recommendations
1) The swale should have a length that provides a minimum hydraulic residence time of
at least 10 minutes. The maximum bottom width should not exceed 10 feet unless a
dividing berm is provided. The depth of flow should not exceed 2/3rds the height of
the grass at the peak of the water quality design storm intensity. The channel slope
should not exceed 2.5%.
2) A design grass height of 6 inches is recommended.
3) Regardless of the recommended detention time, the swale should be not less than
100 feet in length.
4) The width of the swale should be determined using Manning’s Equation, at the peak
of the design storm, using a Manning’s n of 0.25.
5) The swale can be sized as both a treatment facility for the design storm and as a
conveyance system to pass the peak hydraulic flows of the 100-year storm if it is
located “on-line.” The side slopes should be no steeper than 3:1 (H:V).
6) Roadside ditches should be regarded as significant potential swale/buffer strip sites
and should be utilized for this purpose whenever possible. If flow is to be introduced
through curb cuts, place pavement slightly above the elevation of the vegetated areas.
Curb cuts should be at least 12 inches wide to prevent clogging.
7) Swales must be vegetated in order to provide adequate treatment of runoff. It is
important to maximize water contact with vegetation and the soil surface. For
general purposes, select fine, close-growing, water-resistant grasses. If possible,
divert runoff (other than necessary irrigation) during the period of vegetation
TC-30 Vegetated Swale
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establishment. Where runoff diversion is not possible, cover graded and seeded
areas with suitable erosion control materials.
Maintenance
The useful life of a vegetated swale system is directly proportional to its maintenance frequency.
If properly designed and regularly maintained, vegetated swales can last indefinitely. The
maintenance objectives for vegetated swale systems include keeping up the hydraulic and
removal efficiency of the channel and maintaining a dense, healthy grass cover.
Maintenance activities should include periodic mowing (with grass never cut shorter than the
design flow depth), weed control, watering during drought conditions, reseeding of bare areas,
and clearing of debris and blockages. Cuttings should be removed from the channel and
disposed in a local composting facility. Accumulated sediment should also be removed
manually to avoid concentrated flows in the swale. The application of fertilizers and pesticides
should be minimal.
Another aspect of a good maintenance plan is repairing damaged areas within a channel. For
example, if the channel develops ruts or holes, it should be repaired utilizing a suitable soil that
is properly tamped and seeded. The grass cover should be thick; if it is not, reseed as necessary.
Any standing water removed during the maintenance operation must be disposed to a sanitary
sewer at an approved discharge location. Residuals (e.g., silt, grass cuttings) must be disposed
in accordance with local or State requirements. Maintenance of grassed swales mostly involves
maintenance of the grass or wetland plant cover. Typical maintenance activities are
summarized below:
Inspect swales at least twice annually for erosion, damage to vegetation, and sediment and
debris accumulation preferably at the end of the wet season to schedule summer
maintenance and before major fall runoff to be sure the swale is ready for winter. However,
additional inspection after periods of heavy runoff is desirable. The swale should be checked
for debris and litter, and areas of sediment accumulation.
Grass height and mowing frequency may not have a large impact on pollutant removal.
Consequently, mowing may only be necessary once or twice a year for safety or aesthetics or
to suppress weeds and woody vegetation.
Trash tends to accumulate in swale areas, particularly along highways. The need for litter
removal is determined through periodic inspection, but litter should always be removed
prior to mowing.
Sediment accumulating near culverts and in channels should be removed when it builds up
to 75 mm (3 in.) at any spot, or covers vegetation.
Regularly inspect swales for pools of standing water. Swales can become a nuisance due to
mosquito breeding in standing water if obstructions develop (e.g. debris accumulation,
invasive vegetation) and/or if proper drainage slopes are not implemented and maintained.
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Cost
Construction Cost
Little data is available to estimate the difference in cost between various swale designs. One
study (SWRPC, 1991) estimated the construction cost of grassed channels at approximately
$0.25 per ft2. This price does not include design costs or contingencies. Brown and Schueler
(1997) estimate these costs at approximately 32 percent of construction costs for most
stormwater management practices. For swales, however, these costs would probably be
significantly higher since the construction costs are so low compared with other practices. A
more realistic estimate would be a total cost of approximately $0.50 per ft2, which compares
favorably with other stormwater management practices.
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TC-30 Vegetated Swale
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Maintenance Cost
Caltrans (2002) estimated the expected annual maintenance cost for a swale with a tributary
area of approximately 2 ha at approximately $2,700. Since almost all maintenance consists of
mowing, the cost is fundamentally a function of the mowing frequency. Unit costs developed by
SEWRPC are shown in Table 3. In many cases vegetated channels would be used to convey
runoff and would require periodic mowing as well, so there may be little additional cost for the
water quality component. Since essentially all the activities are related to vegetation
management, no special training is required for maintenance personnel.
References and Sources of Additional Information
Barrett, Michael E., Walsh, Patrick M., Malina, Joseph F., Jr., Charbeneau, Randall J, 1998,
“Performance of vegetative controls for treating highway runoff,” ASCE Journal of
Environmental Engineering, Vol. 124, No. 11, pp. 1121-1128.
Brown, W., and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic
Region. Prepared for the Chesapeake Research Consortium, Edgewater, MD, by the Center for
Watershed Protection, Ellicott City, MD.
Center for Watershed Protection (CWP). 1996. Design of Stormwater Filtering Systems.
Prepared for the Chesapeake Research Consortium, Solomons, MD, and USEPA Region V,
Chicago, IL, by the Center for Watershed Protection, Ellicott City, MD.
Colwell, Shanti R., Horner, Richard R., and Booth, Derek B., 2000. Characterization of
Performance Predictors and Evaluation of Mowing Practices in Biofiltration Swales. Report
to King County Land And Water Resources Division and others by Center for Urban Water
Resources Management, Department of Civil and Environmental Engineering, University of
Washington, Seattle, WA
Dorman, M.E., J. Hartigan, R.F. Steg, and T. Quasebarth. 1989. Retention, Detention and
Overland Flow for Pollutant Removal From Highway Stormwater Runoff. Vol. 1. FHWA/RD
89/202. Federal Highway Administration, Washington, DC.
Goldberg. 1993. Dayton Avenue Swale Biofiltration Study. Seattle Engineering Department,
Seattle, WA.
Harper, H. 1988. Effects of Stormwater Management Systems on Groundwater Quality.
Prepared for Florida Department of Environmental Regulation, Tallahassee, FL, by
Environmental Research and Design, Inc., Orlando, FL.
Kercher, W.C., J.C. Landon, and R. Massarelli. 1983. Grassy swales prove cost-effective for
water pollution control. Public Works, 16: 53–55.
Koon, J. 1995. Evaluation of Water Quality Ponds and Swales in the Issaquah/East Lake
Sammamish Basins. King County Surface Water Management, Seattle, WA, and Washington
Department of Ecology, Olympia, WA.
Metzger, M. E., D. F. Messer, C. L. Beitia, C. M. Myers, and V. L. Kramer. 2002. The Dark Side
Of Stormwater Runoff Management: Disease Vectors Associated With Structural BMPs.
Stormwater 3(2): 24-39.Oakland, P.H. 1983. An evaluation of stormwater pollutant removal
Vegetated Swale TC-30
January 2003 California Stormwater BMP Handbook 11 of 13
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through grassed swale treatment. In Proceedings of the International Symposium of Urban
Hydrology, Hydraulics and Sediment Control, Lexington, KY. pp. 173–182.
Occoquan Watershed Monitoring Laboratory. 1983. Final Report: Metropolitan Washington
Urban Runoff Project. Prepared for the Metropolitan Washington Council of Governments,
Washington, DC, by the Occoquan Watershed Monitoring Laboratory, Manassas, VA.
Pitt, R., and J. McLean. 1986. Toronto Area Watershed Management Strategy Study: Humber
River Pilot Watershed Project. Ontario Ministry of Environment, Toronto, ON.
Schueler, T. 1997. Comparative Pollutant Removal Capability of Urban BMPs: A reanalysis.
Watershed Protection Techniques 2(2):379–383.
Seattle Metro and Washington Department of Ecology. 1992. Biofiltration Swale Performance:
Recommendations and Design Considerations. Publication No. 657. Water Pollution Control
Department, Seattle, WA.
Southeastern Wisconsin Regional Planning Commission (SWRPC). 1991. Costs of Urban
Nonpoint Source Water Pollution Control Measures. Technical report no. 31. Southeastern
Wisconsin Regional Planning Commission, Waukesha, WI.
U.S. EPA, 1999, Stormwater Fact Sheet: Vegetated Swales, Report # 832-F-99-006
http://www.epa.gov/owm/mtb/vegswale.pdf, Office of Water, Washington DC.
Wang, T., D. Spyridakis, B. Mar, and R. Horner. 1981. Transport, Deposition and Control of
Heavy Metals in Highway Runoff. FHWA-WA-RD-39-10. University of Washington,
Department of Civil Engineering, Seattle, WA.
Washington State Department of Transportation, 1995, Highway Runoff Manual, Washington
State Department of Transportation, Olympia, Washington.
Welborn, C., and J. Veenhuis. 1987. Effects of Runoff Controls on the Quantity and Quality of
Urban Runoff in Two Locations in Austin, TX. USGS Water Resources Investigations Report
No. 87-4004. U.S. Geological Survey, Reston, VA.
Yousef, Y., M. Wanielista, H. Harper, D. Pearce, and R. Tolbert. 1985. Best Management
Practices: Removal of Highway Contaminants By Roadside Swales. University of Central
Florida and Florida Department of Transportation, Orlando, FL.
Yu, S., S. Barnes, and V. Gerde. 1993. Testing of Best Management Practices for Controlling
Highway Runoff. FHWA/VA-93-R16. Virginia Transportation Research Council,
Charlottesville, VA.
Information Resources
Maryland Department of the Environment (MDE). 2000. Maryland Stormwater Design
Manual. www.mde.state.md.us/environment/wma/stormwatermanual. Accessed May 22,
2001.
Reeves, E. 1994. Performance and Condition of Biofilters in the Pacific Northwest. Watershed
Protection Techniques 1(3):117–119.
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Seattle Metro and Washington Department of Ecology. 1992. Biofiltration Swale Performance.
Recommendations and Design Considerations. Publication No. 657. Seattle Metro and
Washington Department of Ecology, Olympia, WA.
USEPA 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in
Coastal Waters. EPA-840-B-92-002. U.S. Environmental Protection Agency, Office of Water.
Washington, DC.
Watershed Management Institute (WMI). 1997. Operation, Maintenance, and Management of
Stormwater Management Systems. Prepared for U.S. Environmental Protection Agency, Office
of Water. Washington, DC, by the Watershed Management Institute, Ingleside, MD.
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VEGETATED STRIP
Vegetated Buffer Strip TC-31
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Description
Grassed buffer strips (vegetated filter strips, filter strips, and
grassed filters) are vegetated surfaces that are designed to treat
sheet flow from adjacent surfaces. Filter strips function by
slowing runoff velocities and allowing sediment and other
pollutants to settle and by providing some infiltration into
underlying soils. Filter strips were originally used as an
agricultural treatment practice and have more recently evolved
into an urban practice. With proper design and maintenance,
filter strips can provide relatively high pollutant removal. In
addition, the public views them as landscaped amenities and not
as stormwater infrastructure. Consequently, there is little
resistance to their use.
California Experience
Caltrans constructed and monitored three vegetated buffer strips
in southern California and is currently evaluating their
performance at eight additional sites statewide. These strips were
generally effective in reducing the volume and mass of pollutants
in runoff. Even in the areas where the annual rainfall was only
about 10 inches/yr, the vegetation did not require additional
irrigation. One factor that strongly affected performance was the
presence of large numbers of gophers at most of the southern
California sites. The gophers created earthen mounds, destroyed
vegetation, and generally reduced the effectiveness of the
controls for TSS reduction.
Advantages
Buffers require minimal maintenance activity (generally just
erosion prevention and mowing).
If properly designed, vegetated, and operated, buffer strips can
provide reliable water quality benefits in conjunction with
high aesthetic appeal.
Design Considerations
Tributary Area
Slope
Water Availability
Aesthetics
Targeted Constituents
Sediment
Nutrients
Trash ▲
Metals
Bacteria
Oil and Grease
Organics ▲
Legend (Removal Effectiveness)
Low High
▲ Medium
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Flow characteristics and vegetation type and density can be closely controlled to maximize
BMP effectiveness.
Roadside shoulders act as effective buffer strips when slope and length meet criteria
described below.
Limitations
May not be appropriate for industrial sites or locations where spills may occur.
Buffer strips cannot treat a very large drainage area.
A thick vegetative cover is needed for these practices to function properly.
Buffer or vegetative filter length must be adequate and flow characteristics acceptable or
water quality performance can be severely limited.
Vegetative buffers may not provide treatment for dissolved constituents except to the extent
that flows across the vegetated surface are infiltrated into the soil profile.
This technology does not provide significant attenuation of the increased volume and flow
rate of runoff during intense rain events.
Design and Sizing Guidelines
Maximum length (in the direction of flow towards the buffer) of the tributary area should be
60 feet.
Slopes should not exceed 15%.
Minimum length (in direction of flow) is 15 feet.
Width should be the same as the tributary area.
Either grass or a diverse selection of other low growing, drought tolerant, native vegetation
should be specified. Vegetation whose growing season corresponds to the wet season is
preferred.
Construction/Inspection Considerations
Include directions in the specifications for use of appropriate fertilizer and soil amendments
based on soil properties determined through testing and compared to the needs of the
vegetation requirements.
Install strips at the time of the year when there is a reasonable chance of successful
establishment without irrigation; however, it is recognized that rainfall in a given year may
not be sufficient and temporary irrigation may be required.
If sod tiles must be used, they should be placed so that there are no gaps between the tiles;
stagger the ends of the tiles to prevent the formation of channels along the strip.
Use a roller on the sod to ensure that no air pockets form between the sod and the soil.
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Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days
after the first rainfall of the season.
Performance
Vegetated buffer strips tend to provide somewhat better treatment of stormwater runoff than
swales and have fewer tendencies for channelization or erosion. Table 1 documents the pollutant
removal observed in a recent study by Caltrans (2002) based on three sites in southern
California. The column labeled “Significance” is the probability that the mean influent and
effluent EMCs are not significantly different based on an analysis of variance.
The removal of sediment and dissolved metals was comparable to that observed in much more
complex controls. Reduction in nitrogen was not significant and all of the sites exported
phosphorus for the entire study period. This may have been the result of using salt grass, a warm
weather species that is dormant during the wet season, and which leaches phosphorus when
dormant.
Another Caltrans study (unpublished) of vegetated highway shoulders as buffer strips also found
substantial reductions often within a very short distance of the edge of pavement. Figure 1
presents a box and whisker plot of the concentrations of TSS in highway runoff after traveling
various distances (shown in meters) through a vegetated filter strip with a slope of about 10%.
One can see that the TSS median concentration reaches an irreducible minimum concentration
of about 20 mg/L within 5 meters of the pavement edge.
Table 1 Pollutant Reduction in a Vegetated Buffer Strip
Mean EMC
Constituent Influent
(mg/L)
Effluent
(mg/L)
Removal
%
Significance
P
TSS 119 31 74 <0.000
NO3-N 0.67 0.58 13 0.367
TKN-N 2.50 2.10 16 0.542
Total Na 3.17 2.68 15 -
Dissolved P 0.15 0.46 -206 0.047
Total P 0.42 0.62 -52 0.035
Total Cu 0.058 0.009 84 <0.000
Total Pb 0.046 0.006 88 <0.000
Total Zn 0.245 0.055 78 <0.000
Dissolved Cu 0.029 0.007 77 0.004
Dissolved Pb 0.004 0.002 66 0.006
Dissolved Zn 0.099 0.035 65 <0.000
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Filter strips also exhibit good removal of litter and other floatables because the water depth in
these systems is well below the vegetation height and consequently these materials are not easily
transported through them. Unfortunately little attenuation of peak runoff rates and volumes
(particularly for larger events) is normally observed, depending on the soil properties. Therefore
it may be prudent to follow the strips with another practice than can reduce flooding and
channel erosion downstream.
Siting Criteria
The use of buffer strips is limited to gently sloping areas where the vegetative cover is robust and
diffuse, and where shallow flow characteristics are possible. The practical water quality benefits
can be effectively eliminated with the occurrence of significant erosion or when flow
concentration occurs across the vegetated surface. Slopes should not exceed 15 percent or be less
than 1 percent. The vegetative surface should extend across the full width of the area being
drained. The upstream boundary of the filter should be located contiguous to the developed
area. Use of a level spreading device (vegetated berm, sawtooth concrete border, rock trench,
etc) to facilitate overland sheet flow is not normally recommended because of maintenance
considerations and the potential for standing water.
Filter strips are applicable in most regions, but are restricted in some situations because they
consume a large amount of space relative to other practices. Filter strips are best suited to
treating runoff from roads and highways, roof downspouts, small parking lots, and pervious
surfaces. They are also ideal components of the "outer zone" of a stream buffer or as
pretreatment to a structural practice. In arid areas, however, the cost of irrigating the grass on
the practice will most likely outweigh its water quality benefits, although aesthetic
considerations may be sufficient to overcome this constraint. Filter strips are generally
impractical in ultra-urban areas where little pervious surface exists.
Some cold water species, such as trout, are sensitive to changes in temperature. While some
treatment practices, such as wet ponds, can warm stormwater substantially, filter strips do not
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are not expected to increase stormwater temperatures. Thus, these practices are good for
protection of cold-water streams.
Filter strips should be separated from the ground water by between 2 and 4 ft to prevent
contamination and to ensure that the filter strip does not remain wet between storms.
Additional Design Guidelines
Filter strips appear to be a minimal design practice because they are basically no more than a
grassed slope. In general the slope of the strip should not exceed 15fc% and the strip should be
at least 15 feet long to provide water quality treatment. Both the top and toe of the slope should
be as flat as possible to encourage sheet flow and prevent erosion. The top of the strip should be
installed 2-5 inches below the adjacent pavement, so that vegetation and sediment accumulation
at the edge of the strip does not prevent runoff from entering.
A major question that remains unresolved is how large the drainage area to a strip can be.
Research has conclusively demonstrated that these are effective on roadside shoulders, where
the contributing area is about twice the buffer area. They have also been installed on the
perimeter of large parking lots where they performed fairly effectively; however much lower
slopes may be needed to provide adequate water quality treatment.
The filter area should be densely vegetated with a mix of erosion-resistant plant species that
effectively bind the soil. Native or adapted grasses, shrubs, and trees are preferred because they
generally require less fertilizer and are more drought resistant than exotic plants. Runoff flow
velocities should not exceed about 1 fps across the vegetated surface.
For engineered vegetative strips, the facility surface should be graded flat prior to placement of
vegetation. Initial establishment of vegetation requires attentive care including appropriate
watering, fertilization, and prevention of excessive flow across the facility until vegetation
completely covers the area and is well established. Use of a permanent irrigation system may
help provide maximal water quality performance.
In cold climates, filter strips provide a convenient area for snow storage and treatment. If used
for this purpose, vegetation in the filter strip should be salt-tolerant (e.g., creeping bentgrass),
and a maintenance schedule should include the removal of sand built up at the bottom of the
slope. In arid or semi-arid climates, designers should specify drought-tolerant grasses to
minimize irrigation requirements.
Maintenance
Filter strips require mainly vegetation management; therefore little special training is needed
for maintenance crews. Typical maintenance activities and frequencies include:
Inspect strips at least twice annually for erosion or damage to vegetation, preferably at the
end of the wet season to schedule summer maintenance and before major fall run-off to be
sure the strip is ready for winter. However, additional inspection after periods of heavy run-
off is most desirable. The strip should be checked for debris and litter and areas of sediment
accumulation.
Recent research on biofiltration swales, but likely applicable to strips (Colwell et al., 2000),
indicates that grass height and mowing frequency have little impact on pollutant removal;
TC-31 Vegetated Buffer Strip
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New Development and Redevelopment
www.cabmphandbooks.com
consequently, mowing may only be necessary once or twice a year for safety and aesthetics
or to suppress weeds and woody vegetation.
Trash tends to accumulate in strip areas, particularly along highways. The need for litter
removal should be determined through periodic inspection but litter should always be
removed prior to mowing.
Regularly inspect vegetated buffer strips for pools of standing water. Vegetated buffer strips
can become a nuisance due to mosquito breeding in level spreaders (unless designed to
dewater completely in 48-72 hours), in pools of standing water if obstructions develop (e.g.
debris accumulation, invasive vegetation), and/or if proper drainage slopes are not
implemented and maintained.
Cost
Construction Cost
Little data is available on the actual construction costs of filter strips. One rough estimate can be
the cost of seed or sod, which is approximately 30¢ per ft2 for seed or 70¢ per ft2 for sod. This
amounts to between $13,000 and $30,000 per acre of filter strip. This cost is relatively high
compared with other treatment practices. However, the grassed area used as a filter strip may
have been seeded or sodded even if it were not used for treatment. In these cases, the only
additional cost is the design. Typical maintenance costs are about $350/acre/year (adapted
from SWRPC, 1991). This cost is relatively inexpensive and, again, might overlap with regular
landscape maintenance costs.
The true cost of filter strips is the land they consume. In some situations this land is available as
wasted space beyond back yards or adjacent to roadsides, but this practice is cost-prohibitive
when land prices are high and land could be used for other purposes.
Maintenance Cost
Maintenance of vegetated buffer strips consists mainly of vegetation management (mowing,
irrigation if needed, weeding) and litter removal. Consequently the costs are quite variable
depending on the frequency of these activities and the local labor rate.
References and Sources of Additional Information
Caltrans, 2002, BMP Retrofit Pilot Program Proposed Final Report, Rpt. CTSW-RT-01-050,
California Dept. of Transportation, Sacramento, CA.
Center for Watershed Protection (CWP). 1996. Design of Stormwater Filtering Systems.
Prepared for Chesapeake Research Consortium, Solomons, MD, and EPA Region V, Chicago, IL.
Desbonette, A., P. Pogue, V. Lee, and N. Wolff. 1994. Vegetated Buffers in the Coastal Zone: A
Summary Review and Bibliography. Coastal Resources Center. University of Rhode Island,
Kingston, RI.
Magette, W., R. Brinsfield, R. Palmer and J. Wood. 1989. Nutrient and Sediment Removal by
Vegetated Filter Strips. Transactions of the American Society of Agricultural Engineers 32(2):
663–667.
Vegetated Buffer Strip TC-31
January 2003 California Stormwater BMP Handbook 7 of 8
New Development and Redevelopment
www.cabmphandbooks.com
Metzger, M. E., D. F. Messer, C. L. Beitia, C. M. Myers, and V. L. Kramer. 2002. The Dark Side
Of Stormwater Runoff Management: Disease Vectors Associated With Structural BMPs.
Stormwater 3(2): 24-39.
Southeastern Wisconsin Regional Planning Commission (SWRPC). 1991. Costs of Urban
Nonpoint Source Water Pollution Control Measures. Technical report no. 31. Southeastern
Wisconsin Regional Planning Commission, Waukesha, WI.
Yu, S., S. Barnes and V. Gerde. 1993. Testing of Best Management Practices for Controlling
Highway Runoff. FHWA/VA 93-R16. Virginia Transportation Research Council,
Charlottesville, VA.
Information Resources
Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold
Climates. Prepared for U.S. Environmental Protection Agency Office of Wetlands, Oceans and
Watersheds. Washington, DC.
Maryland Department of the Environment (MDE). 2000. Maryland Stormwater Design
Manual. http://www.mde.state.md.us/environment/wma/stormwatermanual. Accessed May
22, 2001.
TC-31 Vegetated Buffer Strip
8 of 8 California Stormwater BMP Handbook January 2003
New Development and Redevelopment
www.cabmphandbooks.com
FLOGARD ROOF DOWNSPOUT FILTER
FLOGARD CATCH BASIN INSERT
CONTECH UNIT
SAN DIEGO HYDROLOGIC BASIN PLANNING AREA
MAP
HYDROLOGY REPORT
(ATTACHED SEPARATELY)
Poway Walmart Expansion Page 10 of 10 Job No. 106-054.1-13
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WATER QUALITY MODELING REPORT
POWAY WALMART EXPANSION PROJECT
POWAY, CALIFORNIA
December 2010
WATER QUALITY MODELING REPORT
POWAY WALMART EXPANSION PROJECT
POWAY, CALIFORNIA
Prepared for:
City of Poway
13325 Civic Center Drive
Poway, California 92074-0789
Prepared by:
LSA Associates, Inc.
20 Executive Park, Suite 200
Irvine, California 92614-4731
(949) 553-0666
LSA Project No. PWY0901
December 2010
P:\PWY0901\Water Quality Modeling\wq modeling dec 2010.doc i
TABLE OF CONTENTS
WATER QUALITY MODELING REPORT.........................................................................................1
PROJECT LOCATION...............................................................................................................1
PROJECT CHARACTERISTICS...............................................................................................1
Lot Coverage Changes.......................................................................................................2
Construction Activities......................................................................................................3
Storm Drains and Water Quality Management.................................................................3
Source Control BMPs........................................................................................................4
Treatment Control BMPs...................................................................................................5
POLLUTANTS OF CONCERN..................................................................................................6
WATER QUALITY MODELING METHODOLOGY..............................................................8
WATER QUALITY MODELING RESULTS............................................................................9
LIMITATIONS..........................................................................................................................10
REFERENCES.....................................................................................................................................11
TABLES
Table A: Project Description of Walmart Store Square Footage Changes.............................................2
Table B: Anticipated and Potential Pollutants Generated by Land Use Type........................................7
Table C: Anticipated Pollutant Concentrations Pre- and Postconstruction............................................9
Table D: Anticipated Pollutant Loading Pre- and Postconstruction.....................................................10
APPENDIX
A: WATER QUALITY MODELING METHODOLOGY
LSA ASSOCIATES, INC. WATER QUALITY MODELING REPORT
DECEMBER 2010 POWAY WALMART EXPANSION PROJECT
CITY OF POWAY
P:\PWY0901\Water Quality Modeling\wq modeling dec 2010.doc 1
WATER QUALITY MODELING REPORT
The purpose of this report is to evaluate the impacts of the proposed project on storm water quality by
modeling existing and future water quality with implementation of the project. This report is intended
to support the project’s environmental document.
PROJECT LOCATION
The proposed project is located in the City of Poway (City), which itself is located in the central
eastern portion of San Diego County. The project site consists of two adjoining parcels located at
13425 Community Road and 13430 Midland Road. Both properties are located within an existing
developed shopping center. The cross streets of the project site are Community Road and Hilleary
Place.
The proposed project and the existing Walmart store are located on an approximately 16.49-acre (ac)
project site that is bound on three sides by roadways (Community Road to the west, Hilleary Place to
the north, and Midland Road to the east). Adjacent land uses include commercial land uses to the
south and the northwest corner of the developed shopping area. Multifamily residential land uses are
located to the north beyond Hilleary Place, to the east beyond Midland Road, and to the west beyond
Community Road. The proposed project site is developed with an existing Walmart store, an
associated parking facility, loading docks, landscaping, a building, and parking lot signage.
PROJECT CHARACTERISTICS
The proposed project consists of the expansion and remodeling of the existing Walmart retail store
located at 13425 Community Road in the City. The expansion consists of the addition of 36,996
square feet (sf) of commercial/retail uses to the existing 142,937 sf structure (including the garden
center and Tire and Lube Express), resulting in a 179,933 sf Walmart with a full-service grocery
department. The project would include demolition of the existing Tire and Lube Express and the
adjacent vacant 7,000 sf commercial structure. The existing 6,275 sf Tire and Lube Express would
not be replaced.
The proposed project would include extensive remodeling to both the exterior and interior of the
store. s shown in Table A, the remodeled and expanded store would include approximately 39,831 sf
of food sales area (4,331 sf is existing; 35,500 sf of food sales area would be added with the project),
11,814 sf of food sales support area (bakery, deli, etc.), 16,648 sf of stockroom receiving area,
11,194 sf of ancillary area, 89,963 sf of general merchandise sales area, and 8,346 sf of outdoor
garden center area. The project would include a new entrance for the grocery uses, new lighting,
parking lot improvements, and new landscaping.
LSA ASSOCIATES, INC. WATER QUALITY MODELING REPORT
DECEMBER 2010 POWAY WALMART EXPANSION PROJECT
CITY OF POWAY
P:\PWY0901\Water Quality Modeling\wq modeling dec 2010.doc 2
Table A: Project Description of Walmart Store Square Footage Changes
Existing Store
(sf)
Proposed Store
(sf)
Expansion
(sf)
General Merchandise Sales 94,748 89,963 (4,785)
Food Sales Area 4,331 39,831 35,500
Food Tenant Area 1,634 2,137 503
Stockroom/Receiving area 14,779 16,648 1,869
Ancillary Area 10,002 11,194 1,192
Food Sales Support Area 0 11,814 11,814
Tire and Lube Center 6,275 0 (6,275)
Total Walmart Building 131,769 171,587 39,818
Outdoor Garden Center 11,168 8,346 (2,822)
Total Walmart Store 142,937 179,933 36,996
sf = square feet
The existing loading dock will be replaced with two new loading docks, each with three doors and a
trash compactor. The existing truck entrance (i.e., driveway) on Midland Road would be closed, and a
new truck entrance would be constructed along Hilleary Place. The existing truck entrance would be
closed with the extension of a berm/wall combination barrier 8 ft in height measured from the street
level. A new sidewalk, curb, and gutter will be installed along the length of the new portion of the
berm/wall (i.e., in place of the previous driveway). The project would also construct a berm/wall
combination barrier 6 ft in height measured from the street level extending from the corner of
Midland Road and Hilleary Place and extending to the west at least 10 ft beyond the edge of the
expanded structure.
Although the existing Walmart is currently permitted to be open 24 hours a day year-round, the store
is currently open between 8:00 a.m. and 10:00 p.m., 7 days a week, with exception of the holiday
period in December, when the store is open 24 hours a day. The project proposes to extend business
hours to 24 hours a day year round.
Lot Coverage Changes
The maximum lot coverage allowed in the CG zone is 30 percent, which includes buildings and
excludes roads, parking areas, pathways, landscaping, and other impervious surfaces. As described
above, the project site is 16.49 ac in size and is currently developed with a 142,937 sf Walmart store
and a 7,000 sf vacant commercial structure, for a total of 149,937 sf of building lot coverage or
20.9 percent total lot coverage. The proposed project would remove the 7,000 sf vacant structure and
expand/remodel the Walmart store to be 36,996 sf larger. The project would result in 29,996 sf of
additional lot coverage, which is 4.2 percent greater than currently exists. Post-project, the 179,933 sf
store would cover 25 percent of the 16.49 ac (718,304 sf) project site. Table A shows the square
footage changes by store usage area.
LSA ASSOCIATES, INC. WATER QUALITY MODELING REPORT
DECEMBER 2010 POWAY WALMART EXPANSION PROJECT
CITY OF POWAY
P:\PWY0901\Water Quality Modeling\wq modeling dec 2010.doc 3
Construction Activities
Development of the proposed project will require excavation and grading of the site; delivery of
materials and personnel; demolition of the 7,000 sf vacant commercial structure and existing Walmart
Tire and Lube Express; removal of five above ground storage tanks (ASTs) associated with the Tire
and Lube Express; construction of the store expansion area and parking lot improvements; and
landscaping of the project site. Construction of the project is anticipated to commence in July 2011
and be completed in August 2012 (14 months). The store will remain open during construction.
Construction of the project will require removal of approximately 8,800 cubic yards (cy) of material.
This includes building demolition debris, site pavement demolition debris, and soil export. Trucks for
hauling away material will be staged at the southeastern portion of the site, behind the existing store
structure to avoid congestion on the residential streets adjacent to the site and to avoid parking
conflicts and other operational conflicts during construction.
Storm Drains and Water Quality Management
Careful consideration of site design is a critical first step in storm water pollution prevention for new
developments and redevelopments. In general, site design objectives include a combination of factors
that may include, but are not limited to: minimization of impervious surfaces, including roads and
parking lots; preservation of native vegetation and root systems; minimization of erosion and
sedimentation from susceptible areas such as slopes; incorporation of water quality wetlands,
biofiltration swales, etc., where such measures are likely to be effective and technically and
economically feasible; and minimization of impacts from storm water and urban runoff on the
biological integrity of natural drainage systems and water bodies.
The proposed project would incorporate several Site Design/Low Impact Development (LID) Best
Management Practices (BMPs) in accordance with the City’s Standard Urban Stormwater
Management Plan requirements in effect at the time of building permit issuance. The goal of using
Site Design/LID features is to mimic the site’s existing hydrology by using design measures that
capture, filter, store, evaporate, detain, and infiltrate runoff, rather than allowing runoff to flow
directly to piped or impervious systems. This includes directing runoff to vegetated areas, protecting
native vegetation, and reducing the amount of impervious surface area. The incorporation of Site
Design/LID BMPs may reduce the number and/or sizing of Treatment Control BMPs needed for the
site. These features should also be sized in accordance with the San Diego County LID Handbook.1
The Site Design/LID BMPs that are applicable to the proposed project and will be implemented are
discussed in more detail below.
• Drain a Portion of Impervious Areas into Pervious Areas and Design Pervious Areas to
Effectively Receive Runoff. The new impervious pavement areas at the rear of the Walmart
building will be designed to drain to vegetated swales and vegetated strips on the eastern
perimeter of the site.
1 County of San Diego, Department of Planning and Land Use. Low Impact Development Handbook
Stormwater Management Strategies. December 31, 2007.
LSA ASSOCIATES, INC. WATER QUALITY MODELING REPORT
DECEMBER 2010 POWAY WALMART EXPANSION PROJECT
CITY OF POWAY
P:\PWY0901\Water Quality Modeling\wq modeling dec 2010.doc 4
• Properly Design Pervious Areas to Effectively Receive Runoff. The proposed vegetated swales
and vegetated strips will be designed per guidelines set forth in the California Stormwater BMP
Handbook.
• Minimize Directly Connected Impervious Areas. The site design will incorporate vegetated
swales and vegetated strips on the eastern perimeter of the site. The vegetated swales and strips
will accept runoff from the new pavement areas behind the building. The vegetated swales and
strips will filter the runoff before it enters the underground storm drain system to minimize
directly connected impervious areas.
• Maintain Predevelopment Rainfall Runoff Conditions. The project will result in an increase in
landscaped area on site. The existing site is approximately 85 percent impervious; the proposed
project will result in approximately 82 percent of the site being impervious. As a result, the
project will reduce the overall peak storm water runoff due to the increased landscaped area on
site, which promotes natural infiltration.
• Conserve Natural Areas. Because this site is already developed with commercial buildings and
surface parking lots, there are few natural areas to conserve. Many existing mature and healthy
trees will be retained throughout the site.
• Construct Streets, Sidewalks, and Parking Aisles to Minimum Widths. The proposed sidewalk
along the southern portion of the site will be designed to the minimum width required. The paved
drive aisles will also be constructed to the minimum width necessary for safe vehicle movement
and fire department access.
• Minimize Project’s Impervious Footprint. The project proposes a 3 percent increase in
landscaped area on site. The existing site is approximately 85 percent impervious, and the
proposed site will be approximately 82 percent impervious. The building expansion will occur
over existing impervious surface, and additional pervious landscaping will be added at the
location of the existing vacant commercial structure. As a result, the project would not increase
impervious surfaces.
• Minimize Soil Compaction. In landscaped areas, soil compaction will be minimized to promote
storm water infiltration.
• Maximize Canopy Interception by Preserving Existing Trees and Shrubs. Moderate amounts of
landscaping currently exist on the site. Although several unhealthy trees will be removed, existing
mature trees that are healthy will remain and new trees will be planted to help with canopy
interception. To help conserve water, rain sensors and drip irrigation will be incorporated into the
new landscape irrigation system. Proposed landscaping will be drought tolerant and require
minimal irrigation. Landscaping and irrigation will comply with Walmart’s xeriscape guidelines.
The project landscaping will comply with the City’s landscape efficiency standards, Municipal
Code Chapter 17.41, adopted December 15, 2009.
Source Control BMPs
The second phase of water quality management includes Source Control BMPs. Source Control
BMPs effectively minimize the potential for typical urban pollutants to come into contact with runoff,
thereby limiting water quality impacts downstream. The Source Control BMPs that would be
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incorporated into the proposed project where applicable, based on the project design, are discussed in
more detail below.
• Provide Storm Drain System Stenciling and Signage. Storm drain inlets and catch basins in
traveled ways and pedestrian pathways would be stenciled with prohibitive language and/or
graphic icons to discourage illegal dumping. Existing storm drain facilities that do not have
markings will also be stenciled.
• Design Trash Storage Areas to Reduce Pollution Introduction. Two trash compactors are
proposed adjacent to the truck docks. Trash in the compactors would not be exposed to rainfall.
The compactor pad would be graded so that no runoff from other areas would enter the compactor
pad area. The concrete pad areas would include a drain inlet that connects to the building sewer
system for cleaning purposes.
• Use Efficient Irrigation Systems and Landscape Design. The irrigation system would be
designed to effectively use irrigation water. Rain sensors would be installed on the main
controller unit to prevent irrigation during and after precipitation. Pressure drop actuated shutoff
valves would be installed to control water loss in the event of broken lines or damaged spray
heads. Landscape maintenance would be monitored to ensure that excess use of fertilizers and
pesticides does not occur. Irrigation would be adjusted to avoid runoff.
• Parking Areas. Runoff from the new paved drive aisles and vehicle maneuvering areas would
flow to vegetated swales and vegetated strips on the eastern side of the site and then be filtered
through FloGard catch basin inserts before entering the underground storm drain system.
• Dock Areas. The dock areas would be graded so that no storm water other than the immediate
dock area would drain toward the low point of the dock well. Runoff from the proposed truck
docks would be treated through a Contech unit.
Treatment Control BMPs
The third component of water quality management is the Treatment Control BMPs designed to reduce
the impacts of urban development on downstream water bodies to the maximum extent practicable.
The purpose of Treatment Control BMPs is to remove the pollutants typically associated with each
type of urban land use prior to discharging into receiving waters. Treatment Control BMPs are
designed to infiltrate, filter, and/or treat runoff from the project footprint to one of the “Numeric
Sizing Treatment Standards.” The Treatment Control BMPs that will be incorporated into the
proposed project are described in more detail below. Figure 4.6-2 shows the proposed Treatment
Control BMPs for the proposed project.
• Vegetated Swale. A vegetated swale is an area of grass, shrubs, and/or close-growing vegetation
that impedes the sheet flow of storm water, encourages infiltration, and prevents direct runoff into
adjacent surface waters. Vegetated swales can achieve moderate to high levels of the majority of
potential pollutants. They can achieve moderate to high levels of removal of metals or nutrients
that are attached to suspended soil particles through the settling of solids by natural flocculation
and vegetation uptake. Flocculation is the process in which a solute comes out of a solution. After
flowing through the vegetated swale, metals and nutrients will be removed from the storm water
and therefore will not be transported to the receiving waters. In the case of some pollutants, the
microbiology of the soil can be used to filter dissolved pollutants from runoff. The proposed
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swales will be adequately sized to meet the requirements of the minimum 10-minute residence
time (i.e., the average amount of time storm water runoff would spend in the vegetated swale).
• Vegetated Strips. Vegetated strips are vegetated surfaces that are designed to treat sheet flow
from adjacent surfaces. Runoff velocities are slowed, which allows sediment and other pollutants
to settle. Vegetated strips are generally effective in reducing the volume and mass of pollutants in
runoff.
• Contech Unit. The Contech unit consists of a multichamber steel, concrete, or plastic catch basin
unit that can contain up to four StormFilter cartridges. The single-cartridge Contech unit consists
of a sumped inlet chamber and a cartridge chamber. Runoff enters the sumped inlet chamber
either by sheet flow from a paved surface or from an inlet pipe discharging directly to the unit
vault. The inlet chamber is equipped with an internal baffle, which traps debris and floating oil
and grease, and an overflow weir. While in the inlet chamber, heavier solids are allowed to settle
into the deep sump, while lighter solids and soluble pollutants are directed under the baffle and
into the cartridge chamber. Once in the cartridge chamber, polluted water ponds and percolates
horizontally through the media in the filter cartridges; treated water collects in the cartridge’s
center tube, from which it is directed to the outlet pipe and discharged. When flows into the unit
exceed the water quality design value, excess water spills over the overflow weir, bypassing the
cartridge bay, and discharges to the outlet pipe.
• FloGard Catch Basin Insert. The FloGard multipurpose catch basin insert is designed to capture
sediment, oil, grease, trash, and debris from low flows. A high-flow bypass allows flows to
bypass the device while retaining sediment and larger floatables (debris and trash), and allows
sustained maximum design flows under extreme weather conditions. FloGard catch basin inserts
are recommended for areas subject to silt and debris as well as low to moderate levels of oils and
grease.
• FloGard Roof Downspout Filters. The FloGard roof downspout filter is made of a durable dual-
wall geotextile fabric liner encapsulating an adsorbent that is easily replaced and provides for
flexibility, ease of maintenance, and economy. It is designed to collect particulates and debris, as
well as metals and petroleum hydrocarbons (oils and greases). The FloGard roof downspout filter
acts as an effective filtering device at low flows and, because of the built-in high-flow bypass,
will not impede the system’s maximum design flow. FloGard roof downspout filters are typically
installed in commercial buildings for the removal of nonsoluble pollutants normally found on
building roofs (sediment, gravel, hydrocarbons) from roof storm water runoff.
POLLUTANTS OF CONCERN
Several pollutants are commonly associated with urban runoff (dry weather and storm water runoff),
including sediment, nutrients, bacteria, oxygen-demanding substances, petroleum products, heavy
metals, toxic chemicals, and floatables. The anticipated and potential pollutants in urban runoff for
commercial land uses and parking lots are illustrated in Table B. Anticipated pollutants associated
with these uses and their impacts on water quality and aquatic habitat are described in more detail
below.
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Table B: Anticipated and Potential Pollutants Generated by Land Use Type
General Pollutant Categories
Priority
Project
Categories Sediments Nutrients
Heavy
Metals
Organic
Compounds
Trash
and
Debris
Oxygen-
Demanding
Substances
Oil
and
Grease
Bacteria
and
Viruses Pesticides
Commercial
Development
>1 Acre
P1 P1 – P2 A P3 A P4 P3
Parking Lots P1 P1 A – A P1 A – P1
Source: Standard Urban Stormwater Management Plan (SUSMP) for Wal-Mart #1700-05 Expansion, Nasland Engineering, September
21, 2010.
1 A potential pollutant if landscaping exists on site
2 A potential pollutant if the project includes uncovered parking areas
3 Includes solvents
4 A potential pollutant if land use involves food or animal waste products
A = Anticipated
P = Potential
Sediments. Natural sediment loads are important to downstream environments because they provide
habitat, substrate, and nutrition; however, increased sediment loads can result in several negative
effects to downstream environments. Excessive sediment can be detrimental to aquatic life by
interfering with photosynthesis, respiration, growth, and reproduction. In addition, pollutants that
adhere to sediment, such as nutrients, trace metals, and hydrocarbons, can have other harmful effects
on the aquatic environment when they occur in elevated levels.
Nutrients. Nutrients are typically composed of phosphorus and/or nitrogen. Fertilizers are a main
source of nitrogen and phosphorus in urban runoff. Other sources of phosphorus in runoff are lawn
clippings and tree leaves that accumulate on streets and in gutters. Elevated levels of nutrients in
surface waters cause algal blooms and excessive vegetative growth. As nutrients are absorbed, the
vegetative growth decomposes, using oxygen in the process and reducing dissolved oxygen levels.
Dissolved oxygen is critical for the support of aquatic life.
The ammonium form of nitrogen (found in wastewater discharges) converts to nitrite and nitrate in
the presence of oxygen, which further reduces the dissolved oxygen levels in water.
Kjeldahl-N is defined as the sum of organic nitrogen and ammonium nitrogen, and excludes nitrite
and nitrate.
Metals. Bioavailable forms of trace metals are toxic to aquatic life. The most common metals found
in urban runoff are lead, zinc, and copper. Sources of heavy metals in surface waters include
emissions and deposits from automobiles, industrial wastewater, and common household chemicals.
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Trash and Debris. Trash and debris can have a significant effect on the recreational value of a water
body and aquatic habitat. It also can interfere with aquatic life respiration and can be harmful or
hazardous to aquatic animals that mistakenly ingest floating debris.
Pesticides. A pesticide is a chemical agent designed to control pest organisms. Pesticides can persist
in the environment and can bioaccumulate (concentrate within the body) over several years, resulting
in health problems for the affected organism. Pesticides have been repeatedly detected in surface
waters and precipitation.
Organic Compounds. Organic compounds are carbon-based and are found in pesticides, solvents,
and hydrocarbons. Elevated levels can indirectly or directly constitute a hazard to life or health.
During cleaning activities, these compounds can be washed off into storm drains. Dirt, grease, and
grime may adsorb concentrations that are harmful or hazardous to aquatic life.
Oxygen-Demanding Substances. Oxygen-demanding substances include plant debris (such as leaves
and lawn clippings), animal waste, and other organic matter. Microorganisms utilize dissolved
oxygen during consumption of these substances, which reduces a water body’s capacity to support
aquatic life.
Bacteria. Bacteria levels in urban runoff can exceed public health standards for water contact
recreation. Bacteria levels in streams within natural watersheds also can exceed standards for water
contact recreation. A common source of bacteria is animal excrement.
Petroleum Hydrocarbons. Petroleum hydrocarbons include oil and grease, benzene, toluene, ethyl
benzene, xylene, and polyaromatic hydrocarbons. Sources of petroleum hydrocarbons include parking
lots and roadways, leaking storage tanks, auto emissions, and improper disposal of waste oil. Some of
these materials can be toxic to aquatic life at low concentrations.
WATER QUALITY MODELING METHODOLOGY
A volume-based pollutant loading model was used to assess storm water quality impacts associated
with redevelopment of the proposed project site. The empirical modeling approach was adapted from
the Simple Method.1 See Appendix A for more details regarding the modeling approach.
Modeling was performed on the following constituents:
• Total suspended solids
• Total phosphorus
1 Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban
BMPs.
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• Nitrate
• Copper
• Zinc
These constituents were selected based on the availability of storm water runoff concentrations for the
various constituents and land uses, as well as treatment efficiencies of the proposed Best Management
Practices (BMPs). Because pathogens are difficult to model unless the source is known, total coliform
was not modeled. In addition, pathogen data are often collected as grab samples; therefore, data are
highly variable, and reliable Event Mean Concentrations (EMCs) are not readily available. Oil and
grease, hydrocarbons, and trash and debris were also excluded from modeling. Instead of being
uniform, their release is often concentrated (for example, floating on the surface of the storm water
runoff). Because these constituents do not exhibit the traditional behavior associated with buildup and
runoff from impervious surfaces, they were excluded from the modeling.
WATER QUALITY MODELING RESULTS
Modeling results for the project site indicate that the project will result in a decrease in pollutant
concentrations and loading of total suspended solids, total phosphorus, nitrate, copper, and zinc with
implementation of treatment BMPs (Tables C and D).
Table C: Anticipated Pollutant Concentrations Pre- and Postconstruction
Existing
(mg/L)
Postconstruction,
No Treatment
(mg/L)
FloGard
Downspout
(mg/L)
Contech
Unit
(mg/L)
Swale/Strip
and
FloGard
Insert
(mg/L)
Average
Postconstruction
Concentration with
BMPs
Total Suspended
Solids 66.00 66.00 9.20
50.00 0.65 40.00
Total
Phosphorus 0.39 0.39 0.16 0.32 0.03 0.26
Nitrate 0.48 0.48 0.55 0.45 0.05 0.41
Copper 0.04 0.04 0.02 0.03 0.01 0.03
Zinc 0.24 0.24 0.03 0.19 0.01 0.14
Source: LSA Associates, Inc., December 2010.
BMP = Best Management Practice
mg/L = milligrams per liter
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Table D: Anticipated Pollutant Loading Pre- and Postconstruction
Existing
(lbs/year)
Postconstruction without BMPs
(lbs/year)
Postconstruction with BMPs
(lbs/year)
Total Suspended Solids 2,171 2,099 1,265
Total Phosphorus 12.8 12.4 8.4
Nitrate 15.8 15.3 13.1
Copper 1.28 1.24 0.89
Zinc 7.9 7.7 4.6
Source: LSA Associates, Inc., December 2010.
BMP = Best Management Practice
lbs/year = pounds per year
LIMITATIONS
This report includes water quality modeling results for a limited number of constituents based on a
proposed BMP and is not a Water Quality Assessment Report. The modeling results are conservative
since they do not evaluate the effectiveness of other City-required Site Design or Source Control
BMPs that may reduce concentrations of pollutants in runoff with implementation of the proposed
project.
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REFERENCES
Center for Watershed Protection. Simple and Complex Stormwater Pollutant Load Models Compared.
Technical Note from Watershed Protection Techniques. 2(2): 364–368.
Center for Watershed Protection. 2000. National Pollutant Removal Performance Database for
Stormwater Treatment Practices, 2nd Edition. March.
Los Angeles County. 2000. Los Angeles County 1994–2000 Integrated Receiving Water Impacts
Report.
Nasland Engineering. 2010. Standard Urban Stormwater Management Plan (SUSMP) for Wal-Mart
#1700-05 Expansion. September 21.
Pitt, R., A. Maestre, R. Morquecho, T. Brown, T. Schueler, K. Cappiella, P. Sturm, and C. Swann.
2004. Findings from the National Stormwater Quality Database. Research Progress Report.
Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing
Urban BMPs. Publication No. 87703, Metropolitan Washington Council of Governments,
Washington, D.C.
Stormwater Manager’s Resource Center. The Simple Method to Calculate Urban Stormwater Loads.
www.stormwatercenter.net. Site accessed December 15, 2009.
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APPENDIX A
WATER QUALITY MODELING METHODOLOGY
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APPENDIX A
WATER QUALITY MODELING METHODOLOGY
MODEL DESCRIPTION
A volume-based pollutant-loading model was used to assess storm water quality impacts associated
with the proposed project. The empirical modeling approach was modified from the Simple Method1
to use runoff monitoring data from specific land uses. The model was developed in a Microsoft Excel
spreadsheet.
MODELING PROCEDURE
The steps in the modeling process were as follows:
• Estimate the average annual rainfall.
• Determine the size and land use of the project area.
• Use the Rational Method to estimate runoff volumes.
• Estimate runoff quality based on data from similar land uses.
• Estimate the pre- and postconstruction pollutant loads and concentrations in storm water runoff.
• Estimate the percentage of runoff treated by the Best Management Practices (BMPs).
• Estimate the pollutant concentrations in the BMP effluent.
• Estimate the pollutant loads and concentrations for postconstruction with BMPs.
Average Annual Rainfall
An average annual rainfall of 12 inches was used.2
Project Site Characteristics
The project site was assumed to be 16.49 acres (ac) in size. The existing land use is Commercial
General, with 2.47 ac (15 percent) of pervious area. The planned land use is commercial, with 13.52
ac (82 percent) of impervious area.
1 Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban
BMPs.
2 World Climate. http://www.worldclimate.com/cgi-bin/data.pl?ref=N33W117+2200+047111C Site
accessed December 21, 2009.
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Constituents Modeled
Modeling was performed on the following constituents:
• Total suspended solids
• Total phosphorus
• Nitrate
• Copper
• Zinc
These constituents were selected based on the availability of storm water runoff concentrations for the
various constituents and land uses, as well as treatment efficiencies of the proposed BMPs. Because
pathogens are difficult to model unless the source is known, total coliform was not modeled. In
addition, pathogen data are often collected as grab samples; therefore, data are highly variable, and
reliable water quality data are not readily available. Oil and grease, hydrocarbons, and trash and
debris were also excluded from modeling. Instead of being uniform, their release is often
concentrated (for example, floating on the surface of the storm water runoff). Because these
constituents do not exhibit the traditional behavior associated with buildup and runoff from
impervious surfaces, they were excluded from the modeling.
Event Mean Concentrations (EMCs) for the modeled constituents were obtained from the Los Angeles
County 1994–2000 Integrated Receiving Water Impact Report (Los Angeles County 2000). These
data were used because EMCs were available for land uses similar to the existing and proposed
project land uses and the sampling stations are relatively close to the proposed project.
Table A.1: Event Mean Concentrations for Selected Constituents
Constituent Commercial EMCs (mg/L)
Total Suspended Solids 66.0
Total Phosphorus 0.39
Nitrate 0.48
Copper 0.039
Zinc 0.241
Source: Los Angeles County 1994–2000 Integrated Receiving Water Impacts Report, Los
Angeles County, 2000.
EMCs = Event Mean Concentrations
mg/L = milligrams per liter
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Runoff Volumes
Runoff volumes for each land use were calculated using the Rational Equation:1
Qlu = 0.9*Rv * I * A
Where:
Qlu = Average annual runoff volume (acre-feet, or af)
I = Rainfall depth (feet)
A = Drainage area (ac)
Rv = Runoff coefficient
0.9 = Fraction of precipitation that produces runoff
Where the runoff coefficient was assumed to be a linear function of the percent imperviousness (i):
Rv = 0.05 + 0.9*i
Pollutant Loads
Average annual pollutant loads for pre- and postconstruction were calculated by summing the
pollutant loads from all of the land uses in the project area. The pollutant load from one land use is
calculated by multiplying the average annual runoff volume generated from each land use by the land
use EMC listed in Table A.1.
Llu = 2.72 * Qlu * Clu
Where:
Llu = Pollutant load from each land use (pounds)
Qlu = Average annual runoff volume for each land use (af), calculated using the Rational Method
Clu = EMC from each land use (milligrams per liter)
Total runoff volume (Q) and pollutant loads (L) for the project were calculated by summing the
runoff volume and pollutant loads from the two land uses.
The average pollutant concentration (C) from the entire project area was then calculated using the
following equation:
C = L ÷ (2.72 * Q)
1 Nvotny and Olem. 1994. Water Quality: Prevention, Identification, and Management of Diffuse Pollution.
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Treatment BMPs
Runoff from 3.87 ac (23.5 percent) of the site would be treated with a FloGard roof downspout filter.
Runoff from 0.22 ac (1.3 percent) of the site would be treated with a Contech unit (catch basin storm
filter). Runoff from 3.2 ac (19.4 percent) of the site would receive treatment from a vegetated strip or
swale followed by a FloGard catch basin insert. Runoff from 9.2 ac (55.8 percent) of the site would
receive not treatment.
BMP Effluent Concentrations
A literature search was conducted to determine the removal efficiency of the Treatment BMPs.
Removal efficiencies summarized in Table A.2 were obtained for the BMPs using data from the
National Pollutant Removal Performance Database (Center for Watershed Protection 2000).
Table A.2: Treatment BMP Removal Efficiencies
Treatment BMP
Total Suspended
Solids
Total
Phosphorus Nitrate CopperZinc
FloGard Roof Downspout Filter 86 59 -14 49 88
Contech Catch Basin Storm Filter 25 19 6 30 21
Vegetated Swale/Strip 93 83 90 70 86
FloGard Catch Basin Insert 86 59 -14 49 88
Source: Center for Watershed Protection, National Pollutant Removal Performance Database, March 2000.
BMP = Best Management Practice
BMP effluent pollutant loads were calculated by multiplying the average pollutant concentration (C)
from the project area treated by a BMP by the removal efficiency for that BMP. Potential impacts of
the project were then assessed by comparing the pollutant loads and concentrations pre- and
postconstruction.