FLOOD10493

• County: Adams
• County: Arapahoe
• Community: Aurora
• Stream Name: Upper Box Elder Creek
• Basin: South Platte
• Title: Upper Box Elder Creek Outfall Systems Planning Study - Alternative Evaluation Report
• Date: 12/1/1994
• Prepared For: Aurora, Arapahoe County, Adams County, UDFCD
• Prepared By: CH2M Hill
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.6.3 Slope

For area adjustment 1, 2-hour duration hyetographs were created by distributing the I-hour
rainfall depths according to Table 3-1 of the Rainfall chapter of the USDCM (UDFCD,
1969). No area adjustments were applied.

The slopes for each subwatershed were estimated according to the weighted slope method
documented in the USDCM. The length.of each sJlbwa.t~rshed was broken up into small
segments. The slope of each of the segments was computed and corrected according to
Figure 4-1 of the Runoff chapter of the USDCM (UDFCD, 1969). The segment lengths
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.

For area adjustment 2, 3-hour duration hyetographs were created by adding rainfall
exceeding the 2-hour storm depth uniformly over the extended duration. The 3-hour
hyetographs were then adjusted according to the 10 to 20 square mile area adjustments
shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969).

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).

Time of Concentration

3.6.4

For area adjustment 3, 6-hour duration hyetographs were created by adding rainfall
exceeding the 2-hour storm depth uniformly over the extended duration. The 6-hour
hyetographs were then adjusted according to the 20 to 30 square mile area adjustments
shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969).

For area adjustment 4, 6-hour duration hyetographs were created as described for area
adjustment 3 and adjusted according to the 50 to 75 square mile area adjustments shown
in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969).

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Hyetographs for the 500-year event were based on the same temporal distribution as the
l00-year event.

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

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).

3.6.5

CUHP Modelling

CUHP models for existing and future development conditions were developed for each
watershed (total of twelve base models). The 24 storms described in ta~e previous section
were modelled in each of the twelve base CUHP models. The subwatershed characteris-
tics required for the CUHP models included the identification number, area, length,
centroid length, slope, impervious percentage, retention storage losses, and infiltration
losses. The methods for estimating the subwatershed characteristics are described below
and a summary table of the characteristics is shown in Appendix H.

Subwatersheds were delineated on 1 :24,000 scale digitized USGS quadrangle maps. The
boundaries were determined based on the topography of the maps, existing roadways!
bridges, and existing culverts. The subwatershed areas, lengths, and centroid lengths were
estimated from the digitized 1 :24,000 scale USGS quadrangle maps.

3.6

Delineation of Subwatersheds

3.6.1

InfIltration Losses

The infIltration parameters (initial infIltration, fmal 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 A) onto the subwatershed maps.

3-3

3.6.6

Imperviousness

The imperviousness of each subwatershed was determined based on superimposing the
maps of existing and future development condition imperviousness (Appendices B and C)
onto the subwatershed maps. Based on the assumed impervious values for each land use
area, a weighted impervious percentage for each subwatershed was calculated. For exist-
ing conditions, imperviousness was typically 1 percent. For future conditions, the imper-
viousness ranged from 2 to 85 percent.

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3.6.2

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