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I I I I I I I I 'I I I I I I I I I I I III. FLOOD HISTORY IV. HYDROLOGY To the knowledge of the writers and the municipalities, no records of flood history exist for Moon Gulch, There are no available data regarding storm magnitudes or degrees of damage. Estimating the frequency and magnitude of future flooding is a major factor in drainageway planning. The physical makeup of a basin in addition to the existing and future land use are used in establishing the runoff volumes for the basin. These runoff volumes can be further estimated for different frequencies or time periods by varying the rainfall parameters. Time periods of floods are generally expressed as return periods indicating the percent probability of an event being equalled or exceeded in anyone year. For example, the 100- year flood represents a flood magnitude or discharge having a one percent chance or probability of being equalled or exceeded in anyone year. Time periods to be analyzed in this study are the 10, 50, 100, and 500-year design frequencies for the basin. Hydrologic analysis had not been completed for Moon Gulch on a basin-wide approach prior to this study. Past information had been established by residential development studies for their respective areas and provided discharges for each area and did not evaluate or take into account the effects on or of the entire Moon Gulch basin. The Colorado Urban Hydrograph Procedure (CuHP) was utilized in developing run-off hydro graphs for the 10, 50, and 100-year events. The 500-year frequency was developed using a log-normal probability distribution extrapolation. The study area 'and subsequent 31 sub-basins are shown in Figure 2. CUHP establishes runoff for each of tbese sub-basins -7- -8-