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CHAPT'ERIX HYDROLOGIC ANALYSIS is ~ontrotled by slope, flow length, depth of flow, and the channel surface rOl1ghness. reak rates of flow arc b..sed on the relationship of the above parameter. aa well as the drainage area. Flood Characteristics Procedures outlined .n the SCS National Engineering liandbook, Section 4, Hydrology were also used to derive hydrographs of runoff where necessary. Runoff hydrographa were developed to evaluate the effects of the on-stream det"ntionreservoir on peak flows. Raehhydrographwss routed 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 Elizabeth has been caused by cloudburst type thunderstorms which produce intense rainfall. The relatively steep terraln in the drainage basin produces short duration peak flowa which recede soon after the storm ceasea. The majority of the soils .n the drainage basin are of the Bresser aeries which is a sandy loam. These soils are characterized by relatively high infil- tration rates but are eroded easily. Soil erosion and deposition has caused continuing problem. with maintaining flood conveyance in channels and in cul- verts. Hydrolo~ic Procedure. Methods of Analysis The first .tep in the hydrologic analysis is to establish the amount and pa~tern of rsinfalL to be expected for the 5-, 10., 50-, and 100-year frequency storms. The National Weather Service has published rainfall data for the area which relates rainfall depth, duration, and frequency. Thi. data was analyzed to produce realistic rainstorms for each rainstorm frequency. The hydrologic Ilnalyaisw"s performeu to establish the 5-, 10-, 50-,,,nd tOO-year freque.ncy flOOd discharges at a number of pointa.o.long the study rc.o.ehea. Thia analysis is based on anticipated future development conditions en the town. Town zoning maps and evaluation of existing development densities were ll~cd to cst"blish the future rlln<>ff potential. The aoils in the watershed have been catalogued by the ses ~n a draft report for the weatern part of Elbert County according to theirhydrolo &icproperties. This information was used to estimate infiltration rate. for the various soils CO Elizabeth. Infiltration ratea in combination with vegetal couer and the percentage of impcrvcous area establishes a rainfall-runoff relationship for "ach drainage sub-basin. The variOus frequenty rainstorm. are then applied and tl",r"sulting runoff is computed. The Soil Conservation Service (SCS) procedure described in '~rban Hydrolo- gy for Small ,,"atersheds" Tcchnic"lRelc"seNo. 55wasusedtopr"dict flood ITSgnitudes. Thi~ method was developed specifically to ansly~e the effects of urbanization on rainfall-runoff relotionahip.. ~tOtM hydrogrsphs were computed for all de.ign points affected by the de ten. tion reservoir. The hydrographs provide a continuous reading of discharge at any time during a storm. Thus, the lagging of peak flows due to the effect of the reservoir i. accurately reflected in the peal< flow computation sdownstresm. Continued urbanization of open Rpace in the watersh~d increases the imper- viollsarNland thr>rcby increa.es peak rates of runoff. The volume of runoff is dete~ined by the amount of rainf"ll and by infiltration rstes related to soil type, type of vegetal cover, i~pcrvious surfaces, "nd surface retention. The travel time of the peak flow horn" given location to a dnwnstream point The detailed computations req'J.red in thehydr"logic a""lysis wcre.nbmittl'd totheCol.oradoWatcrConservationBoardlnthetechni.caladdendurn. Asummary ot the peak [lows is presented in Tabte-! Peak flows at other intermediate p"int. can be obtained frorn the tabulation in Tabh,-8 in the section on hydrau- lic "nalyais. -4&- .45-