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COrlditlonswereeomputedsssUIllingthatthec><tSt1ngSOklll1rrigatton dalDs and canals are rellloved hut tlmt r"~d Htucturea erc in place ><ith culverta unob".ru~.n'. ." ',' < . ~-~ ~~~ ".eva, 0"8 are teLefenced to mean Aea level from the Nationat Ceodetic Vertical Datum of 1929. Table 3 "h~" the effect of future basin urbanization on the dis- charge and water surface elevation of the laO-year flood. Plates 23 through 28 display water surface elevations for the IOO-year flood under ""isting conditions compared ,.,ith projected and total urbaniza- tion conditions. FINDINGS OF STUDY Under the projected urbanization condition, the downstream half of the baain 1a covered. with relatively impervious areas. Projected urban- ization increaaes flood1ng relative to ""tating conditions in the reach dOwnstream from County Road 25. The lOO-year flood lncrease~ in elevation by nbout 0.1 to 0.7 feet and tne diacharge tncr~a888 range up td'about 25 percent. Total urbanization increase:J.flooding througltthe entire study reach. Relative to existing condittons, the lOO~ye&r flood elevation increaaes by about 1.0 to 3.4 feet. The discharge increases by about 100 to 120 percent. InfoI'lllationregarding the more frequent floods, such as the IO-yes[ and 50-year floods Is unefal for design of ~lnor engineerlnR works Or for land use plannlng where a high failure risk is economically feasible and hazards to life and p<operty are low. The 100-year flood Is often used for design when a lower rhk.of faUule Is desired. ItsOlost important use Is as a standard for flOOd plain designation and flood plain regulation. The SOO-year flood is useful to remind the public that floods larger than tne lao-year flood can aod do OCcur. The 500- yearfloodcanalaobeusedtoregul,,,ehighrlskdeveloPIIIentsintbe flood plain, auch '"l nuclear ,ower ".,~.~ ,<., . , ".0 .ox. c ~ter.a storage. Table 2 liats the dhchar"ea ..nd>lat-er surface ele"atlonawith exl..rlngconrliUon.. terthe 10- SO- '00 '500 , ,~~-, ~11. -l'~n flood events at selected r"C"rence poInts. VIate.. ~ th"""llh \f> .h~w the n!"~" fl~~~~d by t~c lOa-yeur ~11.J ~OO-)""r flOods under existIng conditions. Plates 17 tnrough22 show the Slr.._bed..l..vaUonand",Hcr aurface profIles for the 10-, 50-, 100-, and 500-y""rfloodsund"rex1stin g conditions. The fIood bound"rteS"'ere located at e""h"roos section and the Intervening flood boundari..s were de""", based upon d"tailed topoKraphl" \napping, engIneering judg_nt, and Held observa"~",. . .v .[1..,hn.........., posaiblethatlllo..eor less fIooding should he sho>lr1 nnthe flOoded are" mapa. Par... ape"Hi" aitnation, where mnre detalled 'lccurac.y is reqolred, th"flolldbOULld,,<(<,s"nnhclIlore.,ccurdt,'iy..atabli.hedby determl:\in.g lhe >later sur fa".. <'lHation fro" ell<' p<otlle or re[erenc_e table "nd tl"'n lo"atlng that ..l"vsllon by ourv"y On tit.. Ih'o<l plaln. " "