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

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3. Estimate the effective rainfall by deducting watershed
retention and losses from total precipitation.
4. Determine hydrograph shape using watershed character-
istics.
A computer program was developed to calculate the large number
of hydrographs that were required.

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rainfall accumulation was divided into 10 or IS-minute incre-

ments for each storm duration, and a typical storm pattern was

determined from the study of Denver area rain storms. The total

rainfall amounts for each frequency rain were distributed accor-

ding to the typical rainfall pattern for the development of

runoff hydrographs.

Precipitation
Rainfall data were taken from Figures 7-1 through 7-8 of the
Drainage Criteria Manual. These figures show that depths of
given frequency rainfall events vary somewhat over the basin.
Accordingly, the basin was divided into rainfall regions and
rainfall depths were calculated for each region. Rainfall
depths for 2, S, 10, and 100-year frequency rainfall events in
the regions and the various hydrologic points to which they
apply are shown on Table III-I, which follows. Rainfall durations
shown in the Table III-l headings are the durations used for
hydrograph analysis as determined from the characteristics of
some 68 Denver area rain storms. The point rainfall amounts taken
from the source data were reduced by an area correction
factor for sub-basins larger than five square miles.

The time rate of accumulation of rainfall depth was determined

from the Drainage Criteria Manual. Depending on sub-basin area,

Runoff
Surface runoff from a basin equals total rainfall minus basin
retention and losses. Basin retention includes temporary
pondings, temporary interception of rainfall and the like that
delay surface runoff. Rainfall losses included phenomena such
as infiltration and evaporation that reduce the amount of water
that reaches the drainage channel. An important consideration
in estimating basin retention and losses is the appraisal of
antecedent moisture conditions on the watershed. Denver area
rainfall records indicate the major rainfall events are fre-
quently preceded by rains, so that the design storm losses
should be calculated in fairly high antecedent moisture condi-
tions. Basin retention and losses also depend on basin soil
characteristics and the extent of urbanization. The soils in
the Lakewood Gulch and McIntyre Gulch basins have low rates of
absorption. Typical rates used are as follows: