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SECTION III HYDROLOGICAL SUMMARY GENERAL COMMENTS The extent of present flooding and the slzlng and character of major drainage works and other urban storm dral:n..ge facilities are deter- mined by the magn I tude of runoff. Fu rthe rmon~, the des I gn of the facilIties must be based upon expected sediment pt"Oblems, In<:ludl ng both degradation and aggradation, and water quality problems. Any master plan must be based on adequate and thorough hydrological studies If the plan Is to have a slgnlfl<:ant value and establIsh a firm factual basIs. HydrologIcal studIes must take Into considera- tion volume of runoff, as well as peak rates of flow. I t Is when the volume characterIstIcs are known that one realIzes the value of detention storage, both pond type and channel storage, t,) reduce the peak rates of flow. The magnItude of a flood can be Judged by Its maxImum rate of flow, even though the max Imum rate may I as t for on 1 y ten mInutes. The fre" quency of a flood Is Judged by Its n~turn pedod. The return period Is defined as the average Interval of time withIn whIch a given .,vent will be equalled or exceeded once. Thus, a f'lood havIng a r.,turn peri- od of 100 years has a one per oent probabl'll ty of beIng (~qualled or "x- ceeded In any given year. Runoff flows were determined at many 10cat10n5 for' return periods of 10, 25 and 100 years so that a determination could be made concern.. Ing the storm frequency for which drainage facilltl"s sh()uld be desIgned. The deta11ed Information ,)1' the computlltTonal methods are presented In appendIx form, wIth thi" section hign1ighting basic i nformat i on. The approach taken by the DRCOG U rbat, Stann D ra I nllge C rIte rl CI Manual In the determinatIon of runoff flows Is: 1. DetermInation of rainfall that \.ould most 11k.dy occur durlns an event with a given return perIod. ThIs Is done at present by US" of Isohyetal (line of equal dept~ rainfall) Depth-Duratlon- Frequency maps that were synthesized during the formation of the Manual. ThIs provides a basIc Input for the r'unoff computation. This Information was analyzed foIC the. Lellll Gulch 'watershed in sev- eral locatIons and found to be r<.latlvely constant. This Is some- what unusual as rainfall varies .::onslderably across the Denv,"r region, but remains more constan:: her'" because the wlltershed 1s In a flat saddle portion of the 1sohyetal map.