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1'1-2 Avenue darn, drop structures at Santa Fe and Missi55ippi Avenue, the drop structure downstream of Florida Ayenue a'1d the dam uP5tream of Florida Avenue u5ed to div~rt water by the Denver Parks Department. The Puhl ic Serv i ce COIIIpany dam down5 treillll of 13th Street i 5 ~n i nfl atah 1 e d'llll wh i ch would be in its cnll apsed position in t irn~ of flnod. The call apsed elevd- tion of the dam Wd5 d55~ned tn be 1 foot above the concretp foundation. HISTORICAL FLOODS Flooddamage5 in the 5tudyredch hdve been cau5ed by high peak flo"5 thilt inunddte 1 and adj ~cent to the ri ver, and hy ~ustdi 'led hi qh flow~ that rn~y not inundate the floodplain, but cau~e dd'1lage by erosion dnd undercutting of5tructures. High peak f10w floods are most often dssocidtedwiththun- dentorm5 and su~tained rain>. Long duration flooding h mo~t often aS50ci dted wi th snO\\!11elt. Some signific~nt past flood5 are df>scribed in the <lCcornpanying FlOOd Hazard Are<l iJ.eline<ltion Study for the South Platte River. [ r r I r The 3rd Avenue di.l'11wJS given special attention becduse a boat chute was recently added which significantly increased the capacity of the darn to pa5s flood flows. Calcul~t!ons of the flow over the d~ were done al1d a rating curve established for use in theHEC-2model. The HEC-? 'IlOdel was rIIod ified in Reache5 3 through 7 ~ Oxford to Sand Creek, to reflect propo5oM improvements. Channpl imQrMf'ment, wpr",''1(jd..l"d usinq CI card5, and modifications to hridges were reflect..d on appropriate SB and X? card,. Water surfacp elevations for flow over dWls whiCh i'nuld have boat chutes added ufider the reCiYnmefided p1 an were c~l cul ated by hand. Only the laO-year profile was determined in Reach..s J through 7, since the chan- nel will bedesiqned to contain the lOa-year flood. The Mams County portion of th.. South Platt.. River, will to some extent, be fixed between protected banks with the tlottOlll elevatinn of the stred'll controlled via drops. I [ I I I I [ I Improved Conditions The study reaches are described in Table i-I. SEDIMENTATION AND EROSION Prior to develooment by man. the South Platte River regime. channel and floodplain characteristics were controlled by a1ternatiye periodS Of drought dnd wet cycles. Its width, depth, bends, bdnk height <lnd slope wereconstantlyreadjustingtonatur<ll phenOlTlena. Th!'currentregimebegan tu develop d~out the t Irne of the CIv il Wdr "I tll diversions of wdter for irrigation. In ReaCh 1 - Chatfield Reservoir to Columbine Da'11, no hydra'Jliccomputa- tions were performed sincenochafinel improvements affecting water surface e 1 evat ions are i ncl uded in thp Phase A p11J11. In Reach 2 _ Co 1 l.M~b ; n.. ~"'" to Oxford, the U. S. Army Corps of Engineers p~oject d~sign prOfi 1 es are con~ sidered adequate. Later, through Denver, the river was fhed between permanent banks dnd chdnn~l i ~ed. The U. S. Army Corps of Engineers dldnnel i lat ion project will fix the bed and banks of tile ri'ler from Columbine Valley to O~ford Avenue. !he river wi I 1, to an e~tent, he control led from Colurnhin~ '1alley upstre.J'1 to Ch~tfi~ld R~s~rvoir to CQntrol b~nk erosion through th~ littl~ton FloQd_ plain Park. In Reach A _ Sand Creek to Baseline Road, the Gingery HEC.2 model was modi- fiedtoreflecttherecorrmendedchdnnel a1i'lnment, profile, and width using th~ same cross sections as under e~istinq conditions. The lOO-Y~dr a~rl 10- year profi les were calcul ated sincl' the recorlJ1lend~d ch~nn!'l II/Ould contai n th~ la_year ~loo<1. Relocated divers ion ddms were model~~ by h,)nrl. The sediment load nf the South Platte River in the 40 mile study reach, prior to construcUon of the Tri.Lakes (Chatfi~ld. 8~M Cre"k, and Cherry Creek Reservoirs), is shown in Table IV-l. I I I I I