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<br />S~CTION 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 c1etention 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 com- <br />patibll ity with the chosen alter~ate. <br /> <br />The size and character of m~jor drainage works, as well 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 de~;ign 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 ~tudies if the plan is to have signifi- <br />cant value. <br /> <br />Hydrological studies must t"ke into consideration volume of runoff, <br />as well as peak rates of flow. It is when the voiume characteristics <br />are known that one realizes the Vaiue 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 intervel of time within which a given event <br />will be equalled or exceeded once. Thus, a flood having a return period <br />of 100 years has a one per cen t I> robilb 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 S, 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 lOa-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 dreas. Furthermore, the <br /> <br />IV-I <br />