FLOOD07075

• County: Denver
• County: Jefferson
• Community: Denver, Edgewater, Lakewood, Wheatridge
• Stream Name: Sloans Lake Basin
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1977
• Prepared For: Denver / Lakewood
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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- HYDROLOGIC AND HYDRAULIC DETERMINATIONS -

Des i gn Ra i n fa 11

Basin Parameters

That portion (about 601) of the Sloans Lake Drainage Basin lying north
of Colfax Avenue and west of Sheridan Boulevard is located in Township 3
South, Range 69 West (Jefferson County). That portion (about 35%) of the
Basin lying north of Colfax and east of Sheridan is in Township 3 South,
Range 68 West (Denver County). A very small portion (about 5%) lies south
of Colfax Avenue and is in Township 4 South, Ranges 68 and 69 West. The
design rainfall presented in the Rainfall/Runoff Information Report pub-
lished under Project REUSE for these townships and ranges varies insig-
nificantly. The values for Jefferson County are generally sl ightly higher
and were the values used for runoff computations in the Sloans Lake Basin
hydrologic investigation. These data are presented in Tables B-1 and B-2
of Appendix B. The depths and durations were computed for the four design
rainfalls (5-, 10-,25- and 100-year) to include a breakdown for a five
and ten minute duration.

Each sub-basin was physically described in terms of tributary area to
each design point, sub-basin length, distances from sub-basin centroid
to design point, degree of perviousness throughout the basin and the
selection of representative coefficients typical of the basin. Numer-
ous field visits were made to become familiar with the physical charac-
teristics of each sub-basin, to aid in making necessary judgements con-
cerning adjustments to coefficients relating peak flows and time to
peak. All available records of existing storm sewers were gathered
from the municipalities in the Basin and were studied to determine the
extent, capacity and effects on runoff of the existing storm drainage
facilities.

Percent of imperviousness was determined by measuring impervious surface

areas of eight sample street blocks throughout the basin. The selected

blocks were judged to be typical of fully developed representative areas

for the various sub-basins. Field visits verified the development charac-

teristics of the sample blocks.

After evaluation of the soi 1 characteristics, infiltration rate and

pervious runoff, the following hydrological factors were selected and

used in the computation of storm hydrographs:

Sol I Infi ltration:

The coefficients used in the computation of the unit hydrographs, C
t
(coefficient reflecting time to peak) and C (coefficient related to peak
p
rate of runoff) were determined and adjusted in accordance with the pro-
cedure contained in the Urban Storm Drainage Criteria Manual.

First 1/2 Hour Remainder of Storm

5-year Return Period I inch/hour 0.5 inch/hour

10-year to 100-year Return

Period 0.5 inch/hour 0.5 inch/hour

Impervious Depression and Detention Storage: 0.1 inches

Pervious Depression and Detention Storage: 0.3 inches

Table A-l in Appendix A lists the information developed for the various
parameters associated with the Sloans Lake Basin hydrologic investigation,
based on a totally developed study area.

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