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.. ~ ~ ;; ~ ~ - ~ - '" ~ ~ = ~ = := " : o o '" - o '" ~ ... ~ = u ~ ~ - : ~ = - ." = o o - : ~ = ~ ~ o ~ ~ o ~ ~ ~ o o - ~ o z -< MO ~w ~~ ~o u ~ ~ ~ ~ w ~ U ~ ~ z ~ < ~ 0Ll gj - .... I- z = - .... "" - '" ,'". 8 @ ~ ~ ~ . ~ ,:l:Ll!! N z "!.~ ' ~ :~ "'J ", ,.; "'= "~'I&. ~'.' ..." ,~",@ @< 0<< ~ 'x~tl z gzz 'c:'~ NO:> ~... ... " ..~ ."0:;- <.,.::J._ '....1&. "..1,<"" L;iggg 888 0 0 .OM " ~81',"'oLigo.ri "':N<<J' 0; ...,'~'" ... M N " "78 ."::It::: .,., " ..;-.... ". , '1'0 . ~ . 1< 8 . . . '!'$:.. @ 0 z z 0, . , ,.- " ;if~ ~ ~ " . w_ ~ . N ~o' 0'. . > ""0 "~ii . ".0 ",."... ,i;;; 22 8 . . 0 - .. ',"CO - - . . ." S-''''t; "'...... N. N. . N . "~ N_ - N 0; . ..; ..w. ...... - - ;", o~1i .,,'ll!:,... z> -'-0 . ,,,. co - . ,,''''''. . . . 2 8 @ 0 ~o; z z Z N N 8 " 0 . 0 0 ~= - . , 'i8 . e;;C: . . , 0 . . 0 . 0 , 0 , . . .I~ ~ . " . . 0 , . '" 0 , . " . . , o . " , 'To . . . li jji~ . , N . . " , , IO 0 I I . < ~ - i K , I i e ' ,~ o. H 's E.>: . ,~ r ! ~ 5. H !~ << ~~ " , , " h .. .. 1~ .. I . ii-6 ij .~ n - . ~">f ., ! ~ . . g li . . '. H ! l i ! 1 ! ! ! ! , ,. oN -. , '" .' "~ .0 . - . .. >< 8Si -, .. 0- o ; ~ '" o I~ , ~ . ~ l . e o ~ . SECTlONTHREE HvdroloUlc/Hvdraullc Review 01 Flood MldgaUOIIAllemaUves alrcady consltUcled a bridge capable of passing the dt'sign flow of 8,200 cfs, Thernilroad opening has been partially blocked ,however, to limit conveyance to lhat oflhe e xisting Highway 6 opening. The proposed conditions model as prepared by the HMGP applicant included encroachments at the Highway 6 and UPRR crossings; however, the model did not include bridge lop of roadway information that is used to determine roadway overtopping. This model was modified as part of (his evahlalion 10 reflect the original bridge top ofroadway used at the Highway 6 and UPRR bridges in the NRCS HEC-2 model. An additioual modification to the original NRCS model was made to include the split flow option at Highway 6. "!11is revision was made to verify the discharge which could be conveyed east within the Pawnee Creek ov.:;.-fl::;w ehannelto the Riverside Ccmctcry. The estimated impacts associated with the proposed mitigation alternative arc prescnted in Tables 3-2 and 3-3. In general, the proposed mitigation alternative components will decrease floodingdeplhsupstrearnofHighway6by4t05feel. I'rovidingadditionalconveyanceatthe .::dsting Highway 6 and UPRR crossings willl;l-c"atly assist in reducing the potential flooding upstr.:.'lIll including events which exceed the design evcl;:. 3.2.2 Pawnee Creek Overflow The Pawnee Cre<:k overflow towards Slerling WllS hydraulically modeled in bo..': the USACE study and the NRCS study using HEC-2. However, the llSACE hydraulic modcl w"': not available ~t L'lC time of this study. Therefore, lh~ :'-/{CS hydraulic model was utili7..cd tor modeling ofprc- and post-project conditions. The downstream extent of the cxisting model docs' not inelude Highway 6 and the UPRR as the existing conditions model assumcs flow into Sterling only. In otdcr to estim3lethe impacts of the proposed levee and conveyance structure system, this ovcrtlow reach was revised using the HEC-2 model as part of this analysis. No bydtaulic modeling of the post.ptoject conditions was presented in the HMGP application report. Details of the proposed slructure and topography in thc overflow area and in th" vicini tyofthe planned conveyance structurc arc not currently available. Cross section data was extrapolated partly from additional ground surface points included on the available mapping, and partly from "S"S '__n~~1.:~ -, ~ _;~"'n nn~~___I~ _~__ "1~.._.;~__ ~...1.~ ,."_1."_)' ~ ,^_ _.._~~ ~~~ ~ ~ 'V>'V"'~I""V ,.~'"""v,v 't~'~''''V "~I'" ~,~..."v,~ v' "'" ."...'"~ v 'v>, v.,~ ~,y for re.grading within the reach between the highway and the new UPRR culverts were estimated based on available mapping. The actual extenl of overtopping of Highway 6 and the performance of the f100dwal] in tbe vicinily of the highway can only be roughly approximated with thc present infonnation. The planned conveyance structure at the UPRR railroad was prcliminarilymodeled with an opening of approximately 208 feetwideand4f eethighwith a~sumed slope and friction conditions, Pel'Sonnel from the CWeB communicated the anticipated dimcnsions of the bridge openings in a leJephone conversation. 1n both evaluations, the peak IOO.year discharges were similar (4,243 cis used by the NRCS c"'npared 10 5,oJ(i{i crs by lne uSACE). The mea~ured discimrge during tne 1997 event wa~ approximately 9,300 cfs. The proposed design discbarge for the overflow is approximately 5,ODOcfs. Flows III excess of the design discharge would be conveyed to the south and overtop both Highway 6 and possibly the railroad. However. flood flows into Sterling would be reduced by the ptoposed f100dwaH and levee upstrcam of the bridge crossings. URS G,...lno, Woodw,,,d Clydo 3-3 '~T~_"'__'''''V~