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I I I I I I 22 SECTION IV HYDROLOGY The size and character of major drainage works, as well as other urban storm drainage facilities, are determined by the magnitude of the design runoff, that is, how much water should they carry before being overtopped. Furthermore, the design of the works must be based upon expected sediment problems, including both degradation and aggradation. This field is the domain of the hydrologist. Any master plan in the Denver Region must be based on adequate and thorough hydrological studies if the plan is to have significant value. I I I I I Hydrological studies must take into consideration 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 to reduce the peak rates of flow. The magnitude of a flood can be judged by its maximum rate of flow even though the maximum rate may last for only 10 minutes, The frequency of a flood is judged by its return period. The return period Is defined as the average interval of time within which a given event will be equalled or exceeded once. Thus a flood having a return period of 100 years has a one per cent probability of being equalled or exceeded in any given year. I I I I I The 100-year return period was chosen for design purposes because it is reasonable to provide against major damage and loss of life in an urban area for a hazard which has a one per cent chance of happening in any given year. The 100-year flood is the present, commonly accepted flood nationally, It Is expected that most Federal regulations and flood insurance will be based on the 100-year flood. Furthermore, the Corps of Engineers studies for flood plain purposes define the Intermediate Regional Flood which generally is a 1 DO-year flood. The 25-year flood is generally inadequate for a prosperous and growing com- munity to use for planning. The 50-year flood is also generally considered to be inadequate In the Denver Region for major drainage design. The dif- ference in project cost between the 50-year and 100-year design is normally not sufficient to justify the lowering of flood protection standards. RA I NFALL I I I The 100-year design rainfall was determined from the nine isohyetal maps pre- sented In the Rainfall Part of the Denver Regional Council of Government's Storm Drainage CrIteria Manual. Figure IV-l is the Depth-Duration-Frequency curve for the South Boulder area for the 2. 5, 10, 25, 50, and 100-year storms. Table IV-l tabulates the lOO-year storm from Figure IV-I. The last column In the table represents the rearranged raInfall increments that were used for design purposes.