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I ." ~ " o ~ ~ . , ~ l . ~ ~ SECTIONTHREE Hvuroloule/HlIdraulle Review of Rood MlUgauon A1ternaUVes The impacts associated with the proposed project are sununarized in Table 34. In general, the proje<:t will increllSe flooding depths upstream of Highway 6 approximately 0.1 to 0.4 feet due to the levee, floodwaIl, and road closure system. However. the proposed system will limit flood flows from entering Sterling, therefore decrea.'ling damages greatly through Sterling. Conveyance at Highway 6 could limit the overal] flood depth iocrease. 3.2.3 Overflow at Atwood The overflows which occur as a result of split flows from t".': :;'...in stem of Pawnee Creek at Highway 3] have not been hydraulical]y mocid<:J in any of the previous analyses. The proposed mitigation pi.... ~.::."""er. includes.. ),000 cfs conveyance structure at Highway 6. Although the 5,000 cfs value appears to b<l ;e]ow the USACE JOO-ycar estimate and the estimated 1997 peak discharges, this structureshou]d reduce backwater and the duration of f100ding upstream. In order to estimate thc impacts of such a structure, the overflow was modeled using HEC.2 a.'l partofthisana]ysis.Details0ftheproposedstructure(suchasdesigoplaos,inverteIcvations, slope, and topography) in the oYelfl"w area z."d in the vicinity oft~e planoed conveyaocc structure are not currently available: Cross scctioodatll wasextrapo]ated partly fromaddiliorull ground surface points included on the available mapping, aod partiy from USGS topogtaphic 7.S-minute quadrangle maps. The planoed conveyance structures at Highway 6 and the UPRR ' were preliminarily modeled with an opening of approximately 208 feet wide and 4 feet high with assumed slope and friction conditions. Personnel from the CWCB communicated thc anticipated dimcnsions of the bridge openings ina telephone conversation, The impacts associated with the proposed project are summarized in Table 3-5. In general, the proposed mitigation alternative will reduce flooding depths upstream of Highway 6 by approxirrl\(t:;:y 1 to 6 feet. Px~'i:;!jng conveyallce struCl,",~~ will greatly a.~s;o;t::: ,cciucmg the potentia] for flooding upstream of Highway 6 at AlWOod, including events which exceed the design event. URS Gre/nor Woodward C/yd.. t~''''''''''''''_''____''''''''''''''' 3-S