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COrlditlonswereeomputedassUIllingrhatthec><tSt1ngSOklll1rrigatton dalDs and canals are rellloved but tlmt r"~d Htucturea efc 1n place ><ith culvena unob".ru~'n'. ." ,., ' . ~-~ ~~~ ".eVB. o"a are r"."fenced to mean Sea level from the National Ceodetic Vertical Datum of 1929. Table 3 "h~" the effect of future basin urbanization on the dis- charge and water aurface elevation of the tOO-year flood. Plates 23 through 28 dioplay water "urface elevations for the 100-year flood under ""laring conditions compared w1th 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 areaa. Projected urban- ization increaaea flooding r~lativ~ to e~iating conditione in the reach downstrea~ from County Road 25. The 100-year flood increases in elevation by nbout 0.1 to 0.7 feet and the discharge increaaea range up td'about 25 percent. Totalurbanizacionincrease:J.floodingthroug,ltthe entire atudy reach. Relative to existing conditions, the lOO~ye&r flood elevation increases by about 1.0 to 3.4 feet. The discharge incr~aseB by about 100 to 120 percent. InfoI'llllltionregarding the more frequent floods, such as the IO-year and 50-year floods ia uoeful for design of ~inor engineering works or for land use planning where a high failure riak is economically fessible and hazards to life and property are low. The 100-year flood ia often used for design when a lower risk-of f,,11ule is dedred. ItsOlost important use is as a atandard for flOOd plain design~tion snd flnod plsin regulat1on. The 500-year flood is useful tn remind tho publIc that floods large~ than the loo-year flood con and do OCcur. The 500- year flnod can also be uaed to regul. "e high ~isk developIIIents in the flood pla!n, such aa n"clear ,ower ,1_,~.~ ,<., . , ,".0 "o~. c ~t..r.a atorng... Table 2 lists the dhch"r5e~ ..nd wat-..r surface elevationa with exl"rIngconrlit1on.. for the 10- 50- '00 '500 , ,~~-, sn -y~n flood eventa at "elected reference pointl,l, 1'1ates 5 th"""llh \f> .how the arc" fl~~~ed by the IOa-yeur aoJ ~OO-)e"r flOods under existing conditIons. Plates 17 th~ough 22 show the Slre_bed ..1~"at1on and ",Her .urface profile. for the 10-, 50-, 100-, and 500-ye..r floods ond"r e~hting conditions. The flood bound"r1es"'ere located at eachcroos sectIonnnd the Intervening flood boundari~a were drawn bssed upon detailed topoKraphic \napping, engIneering judg_nt, and iield observ..".",. . . v . .~ i", h"..pv~[, possiblethatllloreor less flooding ahould be shown nnthe floodedar"a ""'pa. Par... specific situation, where 9\Ore detailed "ccuru~.y is rcqu Ired, ~he flood bOULld..r t"s can b~ lIlore .,ccurdt,'ly ~~t"bl i .hed by detenlli:\tn.g lhe ..ater surfae.. "tHation fro" tlw protile or re[..r"!l~_" t~ble aod th,>" loea~1ng that ..levat Ion by .urvey 00 th~ Ih'od plain. " "