FLOOD05902 - Laserfiche WebLink

Home Browse Search

CHAPTER IX HYDROLOGIC A.~ALYSlS is controlled by slope, flow leagth, depth of flow, and the channel surfsee roughness. Peak rate. of flow are based on the relation.hipof the above parameters as well as the drainage area. Flood Characteristics Procedures outlined in the SCS National Engineering Handbook, Section 4, Hydrology were also used to derive hydrographs of runoff where necessary. Runoff hydrographs were developed to evaluate the effecta of the on-stream derention reservoir on peak flows. Each hydrograph was routcd through the reservoir and added to hydrographs developed at a number of downstream loca- tions to determine peak flows. Past flooding in the Town of Eli~abeth has been caused by cloudburst type thunderstorms which produce intense rainfall. n'e relatively .teep terrain in the drainage basin produces short duration peak flows which recede soon after the .torm ccaSeS. The majority of the soils in the drainage basin are of the Bre.ser series which is a sandy loam. The.e soils are charaeteri~ed by relatively high infil- tration rates but are eroded easily. Soil ~ro.io" and deposition has caused continuing problem. with maintaining flood conveyance in channels and in cul- verrs. Hydrolo~ic Procedures ~thod" of Analysis TI,e rirst step in the hydrologic analysis i. to esteblish the amount and pattern of rainfall to be expected for the 5-, 10-,50-, and lOO-year frequency storms. The National Weather Service haa published rainfall data for the area which relates rainfall depth, duration, and frequency. This data was analyzed to prod"ce rc".listic rainstotms for each rainstorm frequency. TIoe hyJ~"l~gl~ ""..ly.i. "a. p"-rf"roo,,d t" establi"h the 5., la~, 50-, .1r,d The soils in the watershed have been catalogued by the SCS in a draft report for the we.tern part of Elbert County according to their hydrologic properties. This information was used to estimate infiltration ratea for the various .oils in Hiz~hpth. Infiltr~tion rate. in combination with vegetal cover and the percentage of i~pervious area establishes a rainfall-runoff relationship for ..achdroinage.ub-basin. Thevariousfrequencyrsin.tocmsarethenapplieJand tl'e resulting runoff is computed. 100-y..ar frequency flood di~eh"rges at a number of points along the study reaches. This analysis is based on anticipated future development conditions ~n the tOlm. Town zoning map. and evaluation of existing develop_nt denaities were used to establish the future nmoff potential. The Soil Conscrvation Service (SCS) procedure dcscribed ~n "lJrban Hydrolo- ~y for Small Watersheds" Technical Release No. SS was used to predict flood magnitudes. This method was developed specifically to analy.e the effects of urbanization on rllinfall~runoff relationships. Storm hydrograph. were computed for all design points affected by the deten- tionrcservoir. Thehydrograph. provi<le a contimlOUs rca<ling of discharge at any tlme during a storm. Thus, the lagging of peak flows due to the effect of tl",r"servoir ill accurately reflected in the peak flow c=putationa down.tream. Continlledurbani:ationofop"n.paee in the watershed increase a th eimper- vioc. arc a :md thereby incr~ose$ peal< rates of runoff. The ""lume cd r,,,,,,if is de [ermined by the amount of rainfall and by infiltration rates related to .oil type, type of vegetal cover, imperviou~ surfaces, and surface retention. TIw tr"vel ciOle of the peak flow from a given location to II downatrea11l point fhedetailedcornputatlonsrcquiredinthehydrotogicanalysi."cresubmitted to the Colorado \lat,'r Cunll,"vation Board inche technical addendum. Asu_ary of the peak flows is pre'''nted in Table-I Peak flows at other intermediate points can be obtained frum the tabulation in Table-8 in the section on hydrau- licanalyais. -41,- -45-