FLOOD09594

• Designation Number: 63
• County: Adams
• County: Arapahoe
• County: Douglas
• Community: Denver Metro Region
• Stream Name: SJCD Basins 6200 North and South, 6100.5
• Basin: South Platte
• Title: Major Drainageway Planning - Volume I
• Date: 7/1/1976
• Designation Date: 11/17/1976
• Floodplain - Doc Type: Floodplain Report/Masterplan

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do not contribute directly to the drainage facility. This
water may be held in small depressions in the ground or on
roof tops or may be absorbed or collected by plants. The
guidelines used for establisning depression and detention
losses were those given on Table 2-2 of the "Runoff" section
in the Urban storm Drainage Criteria Manual.

Infiltration. Infiltration is the amount of water that flows
through the soil surface and does not contribute directly to
surface runoff. Infiltration values established in the Urban
Storm Drainage Criteria Manual were used as the basis for
determining infiltration in the study area.

Unit Hydrograph

Under the CUHP, local hydrographs for specific areas are
developed by applying the effective rainfall data to a unit
hydrograph. A unit hydrograph was developed for each of the
study's subareas. It is a graph of direct runoff from 1 inch
of rainfall uniformly distributed over the drainage area for a
specific unit of storm duration. The hydrograph simulates the
basin's characteristics including tributary area, basin shape,
street patterns, channel capacities and slopes.

Formulas 4-1 and 4-2 in the "Runoff" section of the Urban
Storm Drainage Criteria Manual were used to determine the peak
of each subarea hydrograph. Two additional points were plotted
representing the width of the hydrograph at 75 percent and 50
percent of the peak discharge. These widths were derived from
Figure 4-5 in the Urban Storm Draina e Criteria Manual. A
trial plot of the un1t y rograph was s etc ed an t e area
under the curve was calculated. The shape of the unit hydrograph
was adjusted until the area under the curve matched, within 5
percent, the volume of a 1-inch runoff from the entire basin.

Local Flood Hydrograph

A local flood hydrograph is a graph of runoff versus time for
a storm event from a local drainage area. These hydrographs
were determined by distributing the effective rainfall times
the unit hydro graph ordinants during the unit storm time
periods. Pertinent local hydrograph data for each subarea of
Basin 6100 and the North Tributary to Basin 6200 are presented
on Tables 4-3 and 4-4.

Routed Hydrograph

At restricted waterway crossings where a ponding area is
formed, the local flood hydrograph has to be combined with the

4-5