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<br />SECTION IV <br /> <br />HYDROLOGY <br /> <br />During Phase A of this study, It was necessary to perform extensIve <br />hydrological studies In order that reasonable volume and peak rates of <br />flow relatIonships could be derived for sizing of structures and for <br />routing of storm runoff through detention reservoirs. The text that <br />follows is basically Information that was presented In the Phase A Re- <br />port. During Phase B, this Information .,as reviewed to insure Its corn- <br />patibil Ity wIth the chosen alternate. <br /> <br />The size and character of major drainage works, as weil as other <br />urban storm drainage facil ities, are determIned by the magnitude of the <br />design runoff; that Is, how much water should they carry before being <br />overtopped. Furthermore, the desIgn of the works must be based upon ex- <br />pected sediment problems, Including both degradation and aggradation. <br />This fIeld is the domaIn of the hydrologist. <br /> <br />Any preliminary design in the Denver regien must be based on ade- <br />quate and thorough hydrological studies if the plan is to have signIfi- <br />cant value. <br /> <br />Hydrological studies must tlllke into consIderation volume of runoff, <br />as well as peak rates of flow. It is when the 'Ioiume characteristIcs <br />are known that one realizes the value of detention storage, both pond <br />type and channel storage, to reduce the peak rates of flow. <br /> <br />The magnitude of a flood cat] be judged by its maximum rate of flow, <br />even though the maximum rate may last for only fifteen mInutes. The <br />frequency of a flood is judged by its return period" The return period <br />is defined as the average Interval of time within which a given event <br />"Ill be equalled or exceeded once. Thus, a flood having a return period <br />of 100 years has a one per cen t p robllb i 1 i ty of be i nq equa 11 ed or ex- <br />ceeded In any given year. <br /> <br />In August of 1972, prelimInary flood flow estImates were <br />mined for fourteen combinations of basins in the study area. <br />time, floods with return periods of :;, 25, and 100 years were <br />both for existing and projected future development. <br /> <br />deter- <br />At that <br />studied, <br /> <br />During Phase A, the 100-year future return period WllS chosen for <br />design purposes because It resulted in lower annual cost. Experienced <br />planners and engIneers throughout the country have generally chosen <br />this frequency as a basis of design for urban areas. Furthermore, the <br /> <br />IV.l <br />