FLOOD06564

• County: Boulder
• Community: Boulder
• Stream Name: Goose, Wonderland, Elmer's Twomile, 2-mile, 4-mile
• Basin: South Platte
• Title: Major Drainageway Planning Volume 1, Part B
• Date: 4/1/1969
• Prepared For: Boulder County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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tp = Ct (L Lca) .3

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where tp

= time to peak of hydrograph from midpoint of unit
rainfaJ 1 in hours

L

= length along stream from study point to upstream
I imits of the basin in miles

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Lca

= distance from study point along stream to the centroid
of the basin in miles

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Ct = a coefficient related to time to peak

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The second equation defines the unit peak of the unit hydrograph

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= Cp640
qp
tp

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where qp = peak rate of runoff in cfs per square mile

Cp = a coefficient related to the peak of the unit hydro-
graph.

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The Cp and Ct values are empirical and should be based on similar
basins that have been instrumented to obtain the actual rainfall-
runoff relationships. The Cp and Ct values used in this study are
based on values recommended in Table 4-1 of the Urban Storm Drainage
Criteria Manual. When selecting the Cp and Ct values for the various
basins, the area, shape, and slope were considered along with probable
future conditions such as type of development and percent of imper-
vious area. The urban Ct values ranged from .25 to .40 and the urban
Cp value ranged from ,40 to .60. The estimated percent of impervious
ranged from 20 percent in the foothills basin to 40 percent in basins
that have shopping centers. The shape of the unit hydrographs are
based mainly on the Harvard Gulch data collected by the City of Denver.

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The process of computing excess precipitation, and then the unit hydro-
graph, was followed by multiplying the two to obtain the design storm
hydrograph. This is lengthy and time consuming if done by hand. For
this reason, a computer was used. The data fed into the computer
for each basin consisted of: the area, the basin length parameters,
Cp, Ct, percent pervious area, and the precipitation infiltration
along with the design retentions for the pervious and impervious areas,
A sample of the computer print out is presented in Figure IV-3.

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All of the unit hydrograph and storm hydrographs that were obtained
from the computer were plotted and studied for reasonableness. Checks
for peak flow per unit area and the widths at 50 and 75 percent of the
peak flow were made on each unit hydrograph that was used.

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