Laserfiche WebLink
<br />UNIT HYDROGRAPH DEVELOPMENT <br /> <br />1. General <br />The unit hydro graph for the area upstream of Cherry <br />Creek Dam was based on the reservoir inflow hydrograph <br />observed during the flood of June 1965. The method used to <br />develop the unit hydrograph is briefly described below. <br /> <br />2. June 1965 Flood and unit Hvdroqraph <br />The observed flood hydrograph into Cherry Creek <br />Reservoir began at 4:00 p.m. on 16 June and the peak of the <br />hydrograph occurred at 7:30 p.m. on the 16th which indicates a <br />3.5 hour period from beginning of runoff to time of peak. <br />This inflow hydrograph is shown in Figure 2. However, since <br />no recording rainfall data were available to indicate storm <br />distribution, the Tp value for the flood was not readily <br />apparent. Therefore, in order to derive a unit graph from <br />the flood, it was decided to apply Clark's Method, which is <br />based on the time from the end of the runoff producing rain to <br />the point of inflection on the recession side of the <br />hydrograph. The unit graph was adjusted to reproduce what was <br />believed to be a flood wave development that is characteristic <br />of major storms such as the June 1965 event. That <br />modification attempted to relate the point of inflection on <br />the flood hydrograph to the rainfall pattern that occurred in <br />the basin. The unit graph was based on an area of 146 miles <br />where rainfall was 2 inches or greater, as shown on the <br />isohyet of the storm in Figure 3. Once the unit graph was <br />derived for the area contributing to runoff during the 1965 <br />flood, Snyder's constants were calculated and applied to the <br />entire basin to develop the unit hydrograph to be used for the <br />Standard Project Flood and the Inflow Design Flood. The unit <br />hydro graph values derived from the June 1965 storm are shown <br />in Table 1 and displayed graphically in Figure 4. <br />Although the HEC-l model has the capability to derive <br />unit hydrograph parameters, it was not used in this example <br />since the rainfall distribution was unknown. Also, when using <br />HEC-1 to derive unit hydrograph parameters, more than one <br />flood event should be investigated. <br /> <br />3 <br />