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5 , Town of Haxtun Drainage Basin Description The area classified as "in-Town" covers approximately 472 acres. The area can be characteriz.ed as urban, and consists of commercial, industrial, open or pari: space, and single-family residential. Single-filnily homes are situated on large lots, and are all generally grassed and well maintained. The underlying soils are tile same as descr1bedunder"Solls". Figure 2 shows the"in-Town"drainagebasins,deslgnatedasbasinsAthroughE. The Town is drained by a street flow net syste<Jl, which ootfal1s into channels at the east terminus of Fletcher Street, and a channel running north- to-south along the redrlot lines of \lalker Street. No formal pipe system Is currently in~place. Bocause of the absence of a storm sewer system and low lyi ng SL.ll1p areas wi thin the Town 1 imi ts, runoff causes sand and grdvel deposl~ tion. which occurs on a routine basis rluring the Sll11mermonths (these areas aredescrlbed in a 1ater sectionl. In the winter. in these Sdme sump areas, ice btJild.up occurs. Sever..l of these 10101 SllfllpS or ponding areas act to storean<lattenuate frequently occurring events, however, those ponding are generally overgrown ..nd not maintained as detention areas. basins 1 through 3). the 2-, 6-, and 24-hour stom durations were used to '~odel the flows that enter fron the agricultural areas. Table 1 presents the 10- and 100~year ra; nfall depths used in the major basin hydrologic analysis. The di stributions appl ied were the SCS, Type II thunderstorm for the 6~ and 24-hour, and a 2.nour storm pattern developed by Urban Drainage and FlOOd Control District. Table 1. Summary of Rainfall Data - Major Basin Hydrology. 10-Year Duration Total Rainfall Depth {hour1 (inches) 2 2.6 6 2.7 " 3.2 2 3.' 6 4.0 " 4.8 Frequency 100-Year Hydrologic Analysis Hy<1rnlogi c ~nalysis w~, rerform"~ to <1<>t..r,"; ne tt,.;> p~ak flow. v(l1u"'e (If runoff, and flow rooting tendencies. The hydrologic analysis utilized both the U.S. Departrnent of Agriculture, Soil Conservation Service, TR-20 flyd rologl c Computer I~odel, and the Envi romenta 1 Protec ti on Age~cy Storm '~ater ~nagement Model (SWMM). as modified by the Urban Drainage and Flood Control District. The purpos(' of the hydrologic analysis is to predict the flOOd magnitude for the lO.year and lOO.yearevents. Tneexistingdevelopedboun- daries are at tne corporate limits of the Town andnomoresignifica ntgrowth isexpected,therefore. no future eonditionhydrologic analysis was performed, The hydr010gic analysis utilized rainfall data obtained frOO1 the "Precipi tatio~ - Frequern:;y Atl as of the Western Uni ted States. Volume 111 - Colorado", Nationd! Oceanic and Atmospheric Administrdtion, 1973. Peak 10- and 10n~year fl QWs for the exi sting condi tion were predlcte<J usi n9 a specific <1; s trilw ti or. of the 2- and 6-nour d~ r.}ticn s term. In the maj or bas i ns (1. l;'. , Thl;'rat;onal formo<1elling three different storm durations was toquan- tify peak flows and volumes for the ~arious types of storms Which can occur in Eastern Colorado. cor the ~ajo'" basins of siz~s under c~msidenticr. in this study, it 1s relati~ely unl ikely that a 24-nour thunderstorm will produce flood causing peak flows within Haxtun, due 1argely becauseofthesMall sur- face acreages l1ess than twn squaremiles1. The 24-hour stenn IS more typical in larger ri~er and creek systems (greater than 50 square miles). Wi thin the Town of Haxtun, a short~duration high-intensity thunderstorm pattern was chosen for use In preparing the hydrologic model, Thejustifica. tion for using a 2-hour duration storm is based largely on rainfall.runoff gauging data for urbdn areas, developed by the Urban Drainage and Flood Control District. The use of this type of storm pattern can also be substan. tlated through actual experiences, related to the prOJect teafll by Town staff and local SGS officials. Table 2 presents the 10- and lOO~year storm patterns and total depth for the 2-hour duration thunderstorm, applied;n basins A throughF.