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conditions ~re computed assuming that the existing .mall irrigation daros and canals sre removed but that road structures are in place with culverts unobstructed, All elevations ars referenced to mesn sss level 1 I , I I ! Table J shows the effect of future basin urbanization On the dis- ch"rgeaod..atersurbceelevatlonofth..lOO-yearflood. Plates 23 through 28 display water surface elevations for the 100-year flood under existing conditions compared ..ith projected and total urbaniza- tioncooditions. fro~ the Natlonal Gcodetic Vertical D~tum of 1929. FINDINGS OF STUOY ~nder the projected urbanization condition, the dO~9trea~ half of the basin is covered ..ith relatively impervious areas. Projected urban- ization increases flooding relative. to exiating conditions in the reach downstream from County Road 25. the laO-year flood increasea in elevatIon by about 0.1 to 0.7 feet and the discha~ge increaaes range Ul> to about ZS percent. Total urbanization increases flooding throug~ the entire atudy reach. Relative to e~i9ting conditions, the 100-year flood elevation increases by aboot 1.0 to 3.4 feet. The discharge increnes by about 100 to 120l'ercent. Information regarding the more frequent floods, soch as the la-year aod 50-year flooda i9 useful for deaign of ~inor engineering wurks Or fur land use planoing where a high failure risk is economically feasihle and hazards to life and property are low. The lOO-year flood is often used for design when a lower risk of failo." is desired. Its most i~l'ortant uae is aa a standard for flood plain deslgnation nnd flood plain regulation. The 500-year flood Is useful to remind the public that floods larger than the lOa-year flood can aod do OCcur. The 500- year flood cao also be used to regul.;e high risk development. in the flood platn, auch ss n\lclear .po"'er pl~ntB or toxic material storage. Table 2 li&ts the discharges and ",ater surface elevations ..ith edllUng conditIons for the 10-, 50-, 100-, and SOO-yest flood event.. at aelected referenc~ points. Plates 1 through 16 aho~ the area f~ooded "y l-h" 1oo-ye..r aod 500-ye"r floods under "xiatlng condittons, Plates 17 th~ough 22 show the str..a",bed ~lev...tton and vater surface profiles for the 10-, 50-, 100-, and 100-y~ar floods under exiating cOnditIona. The flood bound"'~ie" W~~e located at each cross scction and ti", intervening flOod bonndaries Weee drawn b....ed upon detailed topo~ranhl" m8pping,enginecringjudgment,andfi..ldob.etvatlona. It is, however, possIble that mor" or less floodIng should be shown on the flooded ...~ea IMPS, For" specIftc situation, where more detailed accuracy is raqutred, the Hood boundaries Can t>.. mon, a"cur"tely ","tahlhhed by determIning the Water surfaCe ~levation fr~ the profile or r~ference t",ble snd then loeatiI1e: that elevation l>y "urvey on the flood p13in, " "