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Home My WebLink About0351.092r- . , - .July 23, 1992 THE HONORABLE CITY COUNCIL Pale Alto, California Duncan Pla,ce Bicycle/Pedes_tX.ij5,J\ Bridge --Infol:'"!!1ational. Report on Alternative Design Members of the Council! Report in Sri~ This is an ir.formational report on staff's evaluation of an alternative design for the Duncan P.~ce Bicycle/Pedestrian Bridge. No council action is required~ Background On July 13, 1992 1 Council authorized the City Manager 'Or his d~signee to execute an amendment to Agreeroent No. C0017013 with the santa Clara Valley Water District (District) in an amount up to $50,000 for design and construction of a modified bridge and related ilnprovements. The modified bridge is a S-toot-wide concrete structure constructed at the same elevation as the original bridge. The new design results in raised channel walls (up to a height of 5 feet above the backyard ground level) for four properties upstream of the bridge. During the public hearing, Mr. and MrS4 John Wall~ one o~ the affected parties, ex~ressed their concerns about the highet channel ~all behind their home and stated that they were opposed to a .... ·all height of more than 4211 above the ground. Upon approving the revised bridge design I council asked staff to reexalr.ine the possibility of a design .,.,hereby the bridge could be lO'Jcred .... hile limiting the channel wall at the Wa 11 property to a maxim:J.m of 42"4 specifically, staff 'Was asked to determine if the channel wall height could be reduced by lowering the channel bottom beneath the bridge. Discussion In response to council's request, staff asked the District to evaluate the effects of lowering the channel bottom beneath the bridge (see letter to District, Attachment A} 4 The District responded with a letter (see Attachment B) stating that the wall height upstream of the bridge could be reduced by lO\r,'ering the r.hannel bottom, but that this change would result in the need to raise the channel walls dO\olnstreajl\ of the bridge for a distance of 500 feet. The channel walls would have to be raised to a height ranqing from 4-1/2 to 5-1/2 feet above the ground at eleven properties 4 The fUndamental problem is that the slope of Adobe -1- CMR! 351.: 92 -1 - Creek i& very flat; and, if the ch~_'1.nel bottom were steeper.ed to provide a deeper cnannel beneath the bridge, the channel slope would then need to be correspondingly flattened do~nstream of the bridge. The resulting flat slope would cause what is kno~n as an -hydraulic jump"z where the water slows do~n and qjumps" up to a higher elevation. Since lo,,"erinq the cha:'lnel bottom beneath t.he bridge +ot/ould have greater neqative impact on more property owners, staff will proceed wi th the constructior. of the bridge and c!"lannel 'Wa lIs as disclJssp.d at the July 13 Council me~ting. Staff will continue to meet with the Walls in an effort to identify sujtable landscaping or other features to mitigate t..be impacts of the higher channel ... ·all ~ Respectfully submitted~ §~fAG§;~~ Assistant Director of Public Works OJ< VIO G. ADAMS Director of Public Works ~)iJ~.1Nv~ 1.ml-~I:· Assistant City Manager Attac!m.ents: A. Letter to District Related CMR: B. Response from District 251:92 332:92 cc: Mr. and Mrs. Wall Greenmeadow Community Association Walnut Grove Ho~eowneTs Association -2- CKR:351:92 " ~ - publ ic Works D8partrt,~nt Er.gi~eering Division (415) 329-2270 Ms. Nai Hsueh Santa Clara V~lley ~ater District 5750 Almaden EXF"'Y. San Jose, CA 95118 )l..ttachment A ,-- July 15, 1992 SUBJECT: Duncan Place 3icycl@/Pedestrian Bridge Dear Nai: Thank you for yowr at't"?ndarlce a.t Mor.day night's Cit}' Council meeting on the Duncan Place Bicycle/Pedestrian Bridge. Together ~e have develop2d a br idge repla.cement plan that addresses the concerns raised by the public at the community rneetin;. U~fortan~tely, the solution requires the c~~~nel wall to be raised to approximately 4~ feet above ,;!:"o'.l.'1d at tr.e "",'all property. Dut"in9 his r2marks, Mr, ~all indicated that a wall height of )~ f~et above ground would be sat~sfactory. Although th2 Ccuncil app~oved the revised bridge design, they requested that we explore once more a possible €:1gineering solution of lcl· ..... ering the bridge Nill .... ·al1s at the Wall property. In order to respond to the Cou~cil'5 co~ce~ns, 1 r€quest ~hat YDU re-exarnina the possibility of an alternative design with a 3~ fo~t high channel · ... ra]l at the Kall p'r'operty b.@ a r.e.....-br-idge .. ithi" £" of the elevaticn 0: the pre-ear-thquake bridg-e. Spe~ifical1y, please reospond to Counei l!':1<::mber Cobb's suggestion to lower tbe invect of the channel in the vicinity of the bridge. His. question had t",o parts: 1) Could the ch-annel .... "dl he.ight at the 10<.'311 proPerty be reduced b:l" lOI,,;ering th~ channel botto,l1 underneath t~c bridge?, and 2) In order to accomplish this, what would be the limits of the .. ·ork and """hat effect ..,ill this haVe on the channel 'Wall height upstream ~.nd downstrear.. of the bridge'? It is irr.por-tant that your a." s ..... 'e rs be concise and easil J' u~derstood by noT'l engineers ~ Please r.;o:.spond to these questi ens in ",.n i ting (including graphics, if needed) and m-::et .. ·ith Joe Te:-e~i to discuss your findings by Monday, July 20. Joe .. "ill contact you to arrange for a convenient meeting tim~ and place. l.-:·· .' '-:0 '. -'.~' Letter to Ms. Hsueh July 1 'So , 1992 Page 2 I ul"'.dersta"d that you .·i 11 brir,g t.he 2:!greement amendment to the District Bo~rd for their approval on July 28. Please expedite the start of construction sa that the bridge ..... ill be completed in time for the sta.:-t of school. If you have any questions or need further information, y'::'u may call ll',e at (41'Soj 329-2270 or Joe Teresi at (4~5) 329-2129. Thanks for your cooperation. Sincerely, ~g~ ;z.ssistant Director of Public ''';':>r}-.s GB:JT:cl cc: Hr-. &: ~:rs. h'all Greentt'teado ..... Comr.-:uni ty Assc.c i.3 tier. W~lnut Grave Horneo~ners Association Bill Zaner June F 1 ern ing Dave Adams Joe Teresi File 4;702.2()0 City Council li r , \ '" ~ ! 'j , , i -J n J1L 23 ',,2 0'3: =-.o.:tl 68-~, ','HLL[r i-Jj:.TE? U::::-:-. 40:=-':'S July 23" 1992 Mr. George BagdoD Asslstant Dile..."tOc of Publ1.; T~V()rh Cit)' of Palo Alto ;>.0" Box 10250 Palo Alto, CA 94)03 Dcar George: SUbject: Adobe Creek", Ph~e IV-D ... ncan Footbridg~ Hydraulics Attachment B As reqaested in your lell.tr of July 15, 1992, WI:: nave perfonned further hydraulic .analysis 00 Ad0be Creek to address Pilo AltO City Coundi coo.cerr.~ about the cbar.n=! wal:s behind the Wa.ll re.sidenc~. The rew~t:& were discussed at length ov~ the phone on July 17, 1992 betwe~D Mr. Jue Teres.i (1£ yt,ur staff and Mr. Roy Weese of our staff. Endosed is a..1oth~r copy of tr. e diagram FAXed to Mr. Joe TeresL Ott July 17, 1992 and a table summarizing the channel v • .'aI! heights llnd~r three j~j~n conditions: t.f.te ongtnar design,. the lowered bri.~ge design, and Coun~jlrnan Cobb's pr0Pl-)$.!:'.-'i d~ign. As we st.ated at the Cit)' GJu:ilcil meeting un July 13, 1992, the cna:mel wolll at the Wall property tar..ni1: be lowered to 3 feet~ inChes U1'Jess the footbrid:ge is rai.sM to the ~evation pro'p.o~ed frr our D~iEn Drawings. Thi.s. statemen'. was maae asliumlng that the!: original design \'~1Jes we usoo for ~·~r charmd bottom ~le ... atioru; .and chann~1 bottom s](lPes would r.ot be chll.ll eilgro. Per Councilffic:mber Cohb's requesl. we also examined the hycfraul;,,: cor.ditioru rc:s.uI ril'lg tT0:t'. lowering the channel oor.;'')rrl und~r 1,.'le footbridge. Lower:ng t1)e 'C~annd b:;Mm · ... ,'ill draw til,;. w"'"r down at U',e bridge locartoD and tb~ Wall p!"uPl!rty. but raise tile warer surfa.:t': dO\Hl.<;.[ream of the footbridge Th~ attacned tab!e-shows tile impa~ of !his de-.s[gn on ft;Sidents living do""rutr~ of the footbridge, Enclosures • I'l'I>JI<l'V Owuer Wost_ Joa...o. F""I SpbIr'Emcry PrcsIoo X1k footbrid,e M""""" City of PaIo Alto G~ TLIlchcr CTlJ122. - W.uL HEIGHT REQWREME!>'S OriI;lr.J Ile<iio Low..-.d lind:_ Lowrred Sri. A.d C Illld S. m 3' -8" l' -i' 5'S- 3' -5-3'-S'" 5'-4 .. 3'-S--3'-8-4''';- 3'-8-3' -S' 4·~· 3'-8' ]'-fj" 4' -6" _ ........... -_ ....................... ..................... 2',<)' 4'-8" 2' {J" 2'>()' ",,'-9" 2'-'.)' 4'-9~ 4'-9" 4"·9" 4'-9" 4.'·9" 4' .S;" NeweU S.ul Monis/Ol~ Bames&~l Foft<;r Foolhrid~ Fay WaM Hai~ Vm De RJ.:t , • " F. ~ _