Ord 712ORDINANCE NO. 712
AN ORDINANCE OF THE CITY OF POWAY, CALIFORNIA,
AMENDING SECTION 16.103.050 OF THE POWAY MUNICIPAL CODE
REGARDING HYDROMODIFICATION MANAGEMENT
WHEREAS, on February 21, 2001, the California Regional Water Quality Control
Board, San Diego Region (SDRWQCB) issued Order No. 2001 -01, National Pollutant
Discharge Elimination System (NPDES) Permit No. CAS0108758, naming the City of
Poway and other Copermittees responsible for the implementation of a Countywide
Stormwater Management Program; and
WHEREAS, on January 24, 2007, the SDRWQCB issued Order No. 2007 -0001,
which revised the requirements of the 2001 Permit; and
WHEREAS, Order No. 2007 -0001 requires all Copermittees, including the City of
Poway, to develop and implement a Hydromodification Management Plan; and
WHEREAS, the Hydromodification Management Plan is intended to manage
increases in runoff discharge rates and durations from all Priority Development Projects to
protect the environment;
WHEREAS, as Title 16, Division VI of the Poway Municipal Code has been updated
to comply with Permit Order No. 2007 -0001.
NOW, THEREFORE, THE CITY COUNCIL OF THE CITY OF POWAY DOES
ORDAIN AS FOLLOWS:
Section 1 : The City Council finds that this Ordinance is exempt from the requirements of
the California Environmental Quality Act (CEQA) pursuant to Section 15061(b)(3) of the
CEQA Guidelines, as it entails State - mandated updates to current City regulations, and
there is no possibility that this will have a significant effect on the environment.
Section 2 : Section 16.103.050, entitled "Identifying conditions of concern in receiving
waters" is amended to read as follows:
A. Common impacts to the hydrologic regime resulting from development typically
include increased runoff volume and velocity; reduced infiltration; increased flow
frequency, duration, and peaks; faster time to reach peak flow; and water quality
degradation. These changes have the potential to permanently impact downstream
channels and habitat integrity. A change to a priority project site's hydrologic regime
would be considered a condition of concern if the change would impact downstream
channels and habitat integrity. Because of these potential impacts, the following
steps shall be followed by each priority project:
1. Evaluate the project's conditions of concern in a drainage study report
prepared by a registered civil engineer in the State of California with
experience in fluvial geomorphology and water resources management.
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The report shall consider the project area's location (from the larger
watershed perspective), topography, soil and vegetation conditions, percent
impervious area, natural and infrastructure drainage features, and any other
relevant hydrologic and environmental factors to be protected specific to the
project area's watershed.
2. As part of the drainage study, a qualified, licensed professional shall provide
a report on proposed infiltration techniques (trenches, basins, dry wells,
permeable pavements with underground reservoir for infiltration) regarding
any potential adverse geotechnical concerns. Geotechnical conditions such
as slope stability, expansive soils, compressible soils, seepage, groundwater
depth, and loss of foundation or pavement subgrade strength should be
addressed, and mitigation measures provided.
3. As part of the drainage study, the civil engineer shall conduct a field
reconnaissance to observe and report on downstream conditions, including
undercutting erosion, slope stability, vegetative stress (due to flooding,
erosion, water quality degradation, or loss of water supplies) and the area's
susceptibility to erosion or habitat alteration as a result of an altered flow
regime.
4. The drainage study shall compute rainfall runoff characteristics from the
project area including, at a minimum, peak flow rate, flow velocity, runoff
volume, time of concentration, and retention volume. These characteristics
shall be developed for the two -year and 10 -year frequency, Type I storm, of
six -hour or 24 -hour duration (whichever is the closer approximation of the
site's time of concentration), during critical hydrologic conditions for soil and
vegetative cover. The drainage study shall report the project's conditions of
concern based on the hydrologic and downstream conditions discussed
above. Where downstream conditions of concern have been identified, the
drainage study shall establish that pre - project hydrologic conditions affecting
downstream conditions of concern would be maintained by the proposed
project, satisfactory to the Director of Development Services, by incorporating
the site design, source control, and treatment control requirements identified
in this division.
B. Priority development projects are subject to the City's Hydromodification
Management Plan (HMP) as incorporated in the Poway Local SUSMP. The HMP
shall demonstrate how priority projects will manage increases in runoff discharge
rates and durations where such increased rates and durations are likely to cause
increased erosion of channel beds and banks, sediment pollutant generation, or
other impacts to beneficial uses and stream habitat due to increased erosive force.
Projects will be required to utilize a continuous hydrological simulation and the
results of a hydromodification management analysis must adhere to the following
criteria:
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1. For flow rates between the pre - project lower flow threshold and the pre -
project 10 -year runoff event, the post - project discharge rates and durations
may not deviate above the pre - project discharge rates and durations by more
than 10 percent over more than 10 percent of the length of the flow duration
curve.
2. Lower flow thresholds may be determined using the tools provided in the local
SUSMP along with a critical flow calculator and channel screening tools
developed by the Southern California Coastal Water Research Project
( SCCWRP). These methods identify lower flow thresholds for a range of
channel conditions. The critical flow calculator recommends a lower flow
value of 0.1Q2, 0.302 or 0.5Q2 dependent on the receiving channel material
and dimensions, where Q2 is defined as the 2 -year runoff event. This value
will be compared to channel susceptibility rating (High, Medium or Low) as
determined from the most recent SCCWRP screening tools to determine the
final lower flow threshold.
3. The lower flow threshold may alternately be determined as 10 percent of the
pre - project 2 -year runoff event or 0.1Q2. This approach is available if the
project applicant chooses not to complete the channel screening analysis.
C. Priority Development Projects are required to implement hydromodification
mitigation measures so that post - project runoff flow rates and durations do not
exceed pre - project flow rates and durations. Hydromodification mitigation can be
provided as follows:
1. Demonstrate no post - project increase in impervious area and resultant peak
flow rates as compared to pre - project conditions.
2. Installation of LID BMPs such as bioretention facilities to control runoff flows
and durations from new impervious areas.
3. Mitigation of flow and durations through implementation of extended detention
flow duration control basins.
4. Preparation of continuous simulation hydrologic models and comparison of
the pre - project and mitigated post - project runoff peaks and durations (with
hydromodification flow controls) until compliance is achieved.
5. Implementation of in- stream rehabilitation controls to demonstrate that
projected increases in runoff peaks and /or durations would not accelerate
erosion to the rehabilitated receiving stream reach.
D. Priority development projects shall be exempt from the HMP criteria under the
following conditions.
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1. If the proposed project does not increase the impervious area or peak flows to
any discharge location.
2. If the proposed project discharges runoff directly to an exempt receiving water
such as the Pacific Ocean, an exempt river reach, an exempt reservoir, or a
tidally influenced area.
3. If the proposed project discharges to a stabilized conveyance system that
extends to the Pacific Ocean, a tidally influenced area, an exempt river reach
or reservoir.
4. If the contributing watershed area to which the project discharges has an
impervious area percentage greater than 70 percent.
5. If an urban infill project discharges to an existing hardened or rehabilitated
conveyance system that extends beyond the "domain of analysis ", the
potential for cumulative impacts in the watershed are low, and the ultimate
receiving channel has a low susceptibility to erosion as defined in the
SCCWRP channel assessment tool.
E. LID facilities must be designed to be practically built and maintained within the urban
environment. Projects are encouraged to use LID design approaches to provide
both treatment for the 85th percentile water quality event as well as flow control to
meet hydromodification criteria. To assure compliance with hydromodification flow
control requirements, design criteria, specifications, and long -term operations and
maintenance requirements have been provided in the local SUSMP for a variety of
LID -based flow control methods including bioretention basins, flow - through planter
boxes, and bioretention systems in combination with cisterns and vaults. Sizing
factor development shall include the use of continuous simulation of runoff from the
long -term rainfall record.
F. Proof of a long -term maintenance responsibility and mechanism will be required for
all post- construction BMP and flow control facilities. If not properly designed or
maintained, hydromodification flow control devices may create a habitat for vectors
such as mosquitoes or rodents. Maintenance activities for flow control and LID
devices shall be specified in the proposed Project Submittal.
G. The local SUSMP outlines the monitoring program to assess the effectiveness of
hydromodification management facilities. Monitoring activities will include inflow and
outflow monitoring from BMPs, baseline cross section monitoring, and flow -based
sediment monitoring.
Section 3 : This Ordinance shall be codified.
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EFFECTIVE DATE: This Ordinance shall take effect and be in force on January 14,
2011; and before the expiration of fifteen (15) days after its passage, it shall be published
once with the names of members voting for and against the same in the Poway News
Chieftain, a newspaper of general circulation published in the City of Poway.
Introduced and first read at a Regular Meeting of the City Council of the City of
Poway held the 16th day of November 2010, and thereafter PASSED AND ADOPTED at a
regular meeting of said City Council held the 7th day of December 2010.
Don Higginson, Mayor
ATTEST:
Ordinance No. 712
Page 6
STATE OF CALIFORNIA )
) ss.
COUNTY OF SAN DIEGO)
I, Linda A. Troyan, City Clerk of the City of Poway, do hereby certify that the
foregoing Ordinance No. 712, was duly adopted by the City Council at a meeting of said
City Council held on the 7th day of December 2010, and that it was so adopted by the
following vote:
AYES: BOYACK, GROSCH, MULLIN, CUNNINGHAM, HIGGINSON
NOES: NONE
ABSTAIN: NONE
DISQUALIFIED: NONE
�. Troyan, MMC, City
Poway