FLOOD07758

• County: Denver
• County: Jefferson
• Community: Lakewood, Denver, Jefferson County
• Stream Name: Lakewood Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 12/1/1979
• Prepared For: Denver / Lakewood / Jefferson County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

8

1. Infiltration, 0.38 to 0.60 inches per hour, depending

on extents of grass cover and storm intensity.

2. Surface Retention, 0.20 to 0.40 inches average reten-

tion depth depending on ground slope and density of

may be reduced as they pass through long, narrow, gently

sloping watersheds because of the effect of natural channel

and flood plain storage capacity. A synthetic unit hydrograph

method was used to develop flood hydro graphs for the 24 hydro-

logic points in the Lakewood and l1cIntyre gulch basins. The

unit hydrograph method used is the Colorado Urban Hydrograph

Procedure as described in the Drainage Criteria Manual. In

this method, it is first necessary to determine the area, stream

length, location of the centroid, and the average slope of each

sub-basin drainage area. These physical characteristics of

each sub-basin plus two empirical coefficients were used to

develop the unit hydrographs. The empirical coefficients, Ct

and Cp' are coefficients of time of unit hydrograph peak and

peak rate of runoff, respectively. These coefficients are given

in the Drainage Criteria Manual for 60, 40 and 20 percent of

watershed imperviousness, and it was necessary to interpolate

to obtain coefficients for the estimated percent imperviousness

for present and future land use conditions of the Lakewood Gulch

and McIntyre Gulch basins. These drainage basin characteristics

are as shown on Table III-2.

ground cover.

The degree of urbanization in any sub-basin is important in

hydrograph analysis. A high degree of urbanization results in

extensive impervious areas and generally greater runoff than

would occur from unmodified watershed areas. Lakewood Gulch

and McIntyre Gulch watersheds have impervious surfaces presently

covering some 38 percent of their combined areas. Under future

urbanized conditions, it is expected that about 49 percent of

the area will be covered by impervious surfaces.

Hydrographs

Hydrograph shape depends on a number of watershed characteristics.

The configuration of the watershed and its slope are two of the

most important characteristics. A short, wide watershed can be

expected to have a high rate of peak discharge that develops

quickly, while a long, narrow watershed will have a relatively

lower peak discharge that develops more slowly. A steeply

sloped watershed will have a relatively higher peak rate of

discharge that develops more quickly than would a more gently

sloping watershed. Peak rates from upstream watershed areas

The foregoing hydrographic analysis was required to determine

the volume of flood runoff from excess rainfall depth. In

addition, it was necessary to determine the peak rate of