FLOOD04833

• Designation Number: 421
• County: Arapahoe
• County: Adams
• Community: Aurora
• Stream Name: Upper Box Elder Creek
• Basin: South Platte
• Title: Hydrology Report - Upper Box Elder Creek Outfall Systems Planning Study
• Date: 3/1/1994
• Prepared For: UDFCD
• Prepared By: CH2MHILL
• Floodplain - Doc Type: Floodplain Report/Masterplan

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and corrected slopes were entered into Equation 4-5 of the Runoff chapter of the USDCM
(UDFCD, 1969) to determine the weighted subwatershed slope. Weighted subwatershed
slopes varied from 0.2 percent to 4.9 percent in the study area.

3.6.4 Time of Concentration

In accordance with the USDCM, the modified CUHP procedure was used for subwater-
sheds less than 90 acres (UDFCD, 1969). The modified CUHP procedure produces peak
flows similar to the rational method and requires a time of concentration value for each
subwatershed modelled (for example, subwatersheds less than 90 acres). For existing
development conditions, Equations 3-2 and 3-3 from the Runoff chapter of the USDCM
were used to determine the time of concentration (UDFCD, 1969). The velocity
parameter used to determine the travel time variable in Equation 3-2 was estimated from
Figure 3-1 of the Runoff chapter of the USDCM using the curve for short grass pasture
and lawns (UDFCD, 1969).

For future development conditions, Equation 3-4 from the Runoff chapter of the USDCM
was used to determine the time.9f concentration, since it yielded shorter concentration
times than Equation 3-2 and 3-3 and Figure 3-1 (UDFCD, 1969). Time of concentration
values generally ranged from 30 to 120 minutes for existing development conditions. For
future development conditions, the time of concentration values ranged from 17 to
48 minutes.

3.6.5 Retention Storage Losses

Retention storage losses were determined according to Table 2-1 of the Runoff chapter of
the USDCM (UDFCD, 1969). For existing conditions, the retention storage losses were
assumed to be 0.05 inch for impervious areas (representing sloped roof areas) and 0.40
inch for pervious areas (representing wooded areas and open fields). For future
development conditions, the retention storage losses were assumed to be 0.05 inch for
impervious areas (representing sloped roof areas) and 0.35 inch for pervious area
(representing lawn grass).

3.6.6 Inrdtration Losses

The infiltration parameters (initial infiltration, final infiltration, and decay coefficient)
were computed from Table 2-2 of the Runoff chapter of the USDCM based on a
weighted average of hydrologic soil type (UDFCD, 1969). The weighted average of
hydrologic soil type for each subwatershed was computed based on superimposing the
hydrologic soils maps (Appendix B) onto the subwatershed maps.

DENlOOISI86.WPS

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