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Item 12 - Drainage Improvements for Poway Library Site
TO: Honorable Mayor and Members ~f.~ City Council FROM: James L. Bowersox, City Man~ INITIATED BY: John D. Fitch, Assistant City Manager~/¥' ~,J Mark S. Weston, Director of ' .' ~Ser~i,ces.~v J. Bradley Kutzner, Senior Civil Engineer~/ DATE: May 23, 1995 SUBJECT: Drainage Improvements for the Poway Library Site ABSTRACT On February 16, 1995, the City Council selected a location for th Poway Ci y Library to be positi ned along Poway Road on the library site property (A tachment }. The entire site s w thin the 100-year floodplain and a portion is wi bin the 1 0 year floo way of att esnake Creek. To build on the site, the proper y has to e flood prod ed by e eva lng the building pads above the 1OD-year floodplain elevat on and cons ructing dra nage imp such that there is no additional flood impact to the adjo ning proper ies. Staff and the City's consultants have analyzed four alternative channel imp for Rattlesnake Creek to flood proof the library site for development on both the north and south portions of the property. Staff recommends Alternate D - Combination Concrete Channel and Naturalized Channel improvements. ENVIRONMENTAL REVIEW Additional tal review will be completed with site development plans. FISCAL IMPACT Flood proofing the library site will total $910,000 with Alternative D. ADDITIONAL PUBLIC NOTIFICATION AND CORRESPONDENCE Standard distribution. A copy of this report is being mailed to the Library Advisory C RECOMMENDATION t is recommended that th City Counci adop Alternative D as the preferred method to flood proof the library s te and direc star to complete the 1 review and site development process o implement his a creating a building site which would allow building deve opment on bo h nor h and south portions of the library site. ACTION 1 OF 7 ~AY 2 2 1995 ITEM 12 --'- AGENDA REPOR CITY OF POWAY TO: Honorable Mayor and Members of the City Council FROM: James L. Bowersox, City Manager INITIATED BY: John D. Fitch, Assistant City ManagerL~ Mark S. Weston, Director of Engineerin~Services~y~,' J. Bradley Kutzner, Senior Civil Engineerc~-- DATE: May 23, 1995 SUBJECT: Drainage Imp for the Poway Library Site BACKGROUND On February 16, 19 5, t'e City Council selected a location for the Poway City Library to be posi ione along Poway Road o the library site propert (Attachment I). T e en ire site is within he lO0-year floodplain an a portio is ithin he I 0 y ar floodway of attlesnake Creek. To bu ld on the si e, t e property as o be flood proo ed by elevating the build ng pads above he 1 O-year floodpla n elevation and constructing drainage improvements such t at t ere is no addit onal flood impact to the adjoining properties. The main channel of Rattlesnake Creek borders the northwest edge of the library site. The existing channel does not have the capacity to pass the flood waters that can be delivered to the property by the existing storm drainage facilities crossing Poway Road. taff has completed three studie relative to stream flood flows at th ibrary site ba ed on v rious si e developm nt plans. The characteris ics of he upstream 10 -year food cond tion impac s propo ed development on he ibrar site. he exis ing floo control cannel w ich originates at he south dge of M lleary ark and s located etween rikara and Wanesta has a capaci y f 290 fs. ,he lO0-year flood f ow t H lleary Park is app ox ma ely 3 0 cfs. Th difference, or 100 cfs, would flow south of Hil ear ark eas of anes a across vacant lan and 1 property north of owa oad in he v cini y of the Lively Cen er. The flows then travel sou h a ong Bowron Roa , an west alon Civic Center Drive to rejoin the main creek a Community Par . T is side-ch nnel flow which would occur during maximum flood events was analyzed by s aff and ur consultants to determine buildable areas on the site. The floo way of t is eastern flow en roaches on the southeast corner of the library si e. The ouncil has selecte the south portion of the library as a potential site. Staff has urther analyzed this development alternative as it relates to flood proof ng the site. ACTION': 2 OF 7 ~AY: 1995 Page 2 - Any site develo ment of the library site will require sig ificant drainage ' on the west edge of the property alon the ~xist ng alignment of a tle nake Creek. The existing channel will ot handle he flood flows o att esnake Creek delivered through the exis in storm rain system under oway oad. Previous de elopment plans have s ud ed loca lng both the library a the 1 s te on the north half of he property near Poway Road ma ntaining the exist ng fl odway of the overf ow channel along Bowron Road o the east and Civ c Cen er Drive to the south. By locating a 1 site on the sou h hal of the property, additional analysis w s ma e o assure that the proper" could be flood proofed. The analysis s udie four alternative channels vr the main channel of Rattlesnake Creek: Na ural zed channel, grass-lin d channel, concrete bo and channel, and a comb nation concrete channel wit a concrete box and n ural hannel. Our consul ants developed both cost es imates for the four a ernat ves and a qualit rive evaluation for channel con igu ations. The resu s of he analys s will be presented below. Develop ng the south port on of he library site w th a building is possible with no a ditional cost provided t e set backs rom Bowron Road and Civic Center Dr ve used in the analysis are observed. FINDINGS Staff and the Cit"s consultants have analyzed fou al channel improvements for attlesnake Creek to flood proof he library site for development on bo h the north and south portions o the property. Their report presents t e four channel configurations wh ch are described as follows: A1 A - N turalize Channel: The naturalize channel would include an open channel for attlesna e Creek approximately 50 eet wide t the bottom and be revegetate with naive species to create a r parian ha itat similar to Poway Creek behin Creeksi e Plaza. The channel wou d extend rom Poway Road to the connection in Com unity Park. The cost of the channel mprovements with property is about $ 65,000; however, should Tarascan be extended in the future, the cost of a br dge crossing would increase the cost of the naturalized channel to $ ,263,000. The bridge cost is approximately $700,000. A1 B - Grass-Lined Channel: This creek would result in a grass-lined channel similar to the channel t~ rough Community Park extendin from Poway Road to the connection at Commun ty Park. Channel improvement initially would cost $590,000. However, i Tarascan is to be extended, he crossing of the channel would cost approxima ely $700,000 with a total pro ect cost of $1,293,000. A1 C - Conc ere Box and Channel: This channel improvement would include the construc ion of an enclosed reinforced concrete box across the library site transit oning to an open conc ete tra ezoidal channel west of the proposed Taras an al gnment to the at C mmunity Park. The box culvert would e covered and the land oul be use over the concrete channel. The cost of th s alternative is $750,0 0 w th the ox serving as the bridge structure shou d Tarascan be extended o the south. No additional costs would - be assigned for the bridge crossing, n addition, more land is usable by the library with this al I~IAY 2 3 1995 ITEM 1,2 3 OF7 May 23~ 1995 Page 3 Alternative O - Combination Concrete Box and Natural Channel: T is lternative is a combin tion of constructing a reinforced concre e box from oway road to west of t e Tarascan extension. From Tarascan to he community ark the channel would e ~ as a naturalized channel s milar to 1 A. The cos of this al is $910,000 with no future cost or bridge at Tarascan road. The qualitative matrix of the al was also prepared rating usable area flood aesthetics, wetland impacts, future roadway extension, and &M costs, his anal sis suggests that the naturalized channel or the comb nation rein orced co.crete box and naturalized channel are equal qual tatively, ttachmen 2 presents both the quantitative and qualitative matr x developed in the L brary Site Flood Protection Alternatives Report. Staff recommends Alternate D - Combination Concrete Channel and Naturalized Channel imp {see Attachment 3). ENVIRONMENTAL REVIEW Additional 1 review will be completed with site development plans. FISCAL IMPACT Flood proofing the library site will total $910,000 with A1 D. ADDITIONAL PUBLIC NOTIFICATION AND Standard distribution. A copy of this report is being mailed to the Library Advisory C RECOMMENDATION It is recommended that the City Council adopt Alternative D as the preferred method to flood proof the library ite and direct staff to complete the tal review and site deve opment process to implement this al creating a building si e which would allow building development on both north and south portions of t e library site. JLB:JDF:MSW:JBK:mh Attachments: 1. Vicinity Map 2. Rattlesnake Creek Flood Protection Matrices 3. Alternative D 4 OF 7 ~AY 2 3 1995 ITEM 12 5 OF7 ATTACHMENT I ~IAY 23 1995 ITEM 12 RATTLESNAKE CREEK FLOOD PROTECTION MATRICES - LIBRARY SITE, P( QUANTITATIVE MATRIX I LANO TOTAL uSF'ABL ALUE OF TARASCAN LTERNATIVE COST CROSSING ~OST(OFFBiTE} COST ACRF.S 'CESS LA $516,00 -700,00 $ 51,000 ,265,00 4 5 O 8 (GRASS) $542.00 700.00 $ 51,000 ,293.00 4 5 0 C 709.00 $ 42.000 751.00 5 3 400, O0 ( 8~8.00 $ 51,000 909.00 -$ 8 50, OC QUALITATIVE MATRIX SEABL LOOP ETLANDSi ROADWAY & ~ ?OTA _$ (GRASS) : ! 2 ~ 2 ' t C (CONCRETE i 1 4 ~' I 1 12 D (COMBINAT !0N ~' ! ,3 2 i 2 DEF ! N I T T ONS USEABLE AREA - LAND AVAILABLE ON LIBRARY SITE FOR OVERFLOW PARK]NO FOR LIBRARY OR CONNUNiTY PARK. I=MOST 2=LEAST FLDOD PROTECTION - EXTENT TO ',~HICH FLOG[) mROTECTiON PROVIDED. ~E5THETICS - VISUAL !~PACT OF PROPOSED CHANNEL ','¢ETLAN[)S [NIPACTB - EXTENT TO ~[CH '~VEq. ANO ARE~ IN CREEK IS LIKELY :0 BE DECRA[)ED(i=LEAST, 4 = ~OSr) ROAONAY EXIENSION COSTS - COST IMPACTS & FEASIBILITY OF 'THE TARASCAN EXTENS[ON. O&Nl COSTS - RANKING OF COSTS OF EACH ALTERNATIVE (]=LEAST COSTLY, 4=,~OBT COSTLY) LAND V^LUE - $1 90/SOUARE FOOT fOR FLOOD',VAY LAND '~VITH VOST OF THE LANO BEING EXISTING C~E.K ~STE COST ,OF TAR,ASOAN R©AO -S WOULD BE E'3UAL FOR .ALL ALTERNATivES THiS L.O_,T IS NOT INQLL, OF.O i"q ANALYS',S 6 OF 7 ATTACHMENT 2 I~IAY 23 1995 Ii~..M 12 ATTACHMENT 3 ~1AY 231995 iTEM