FLOOD04566

• County: Boulder
• Community: Boulder
• Stream Name: Gregory, Bear Can, Bluebell Can Creeks, Viele Chan
• Basin: South Platte
• Title: Major Drainageway Planning Volume 1, Part C
• Date: 9/1/1970
• Prepared For: Boulder County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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slopes were considered along with probable future conditions such as type
of development and per cent of impervious area. The urban C values ranged
from .29 to .35 and the urban C value ranged from .45 to .62. The esti-
mated per cent of impervious raRged from 10 per cent in the foothills basin
to 40 per cent 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 Denve r.

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The process of computing excess precipitation, and then the unit hydrograph,
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 com-
puter was used. The data fed into the computer for each basin consisted of:
the area, the basin length parameters, C , C , per cent pervious area, pre-
cipitation, infiltration, and the designPret~ntions for the pervious and im-
pervious areas. A sample of the computer print out is presented in Table
IV-3, and graphically illustrated 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 by checking the peak
flow per unit area and the widths at 50 and 75 per cent of the peak flow.

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To obtain most of the design flow hydrographs, two or more of the "computer
hydrographs" were added graphically. This was necessary where the upper
portions of the larger basins have different hydrologic characteristics than
the lower portions of the basins, thus making it necessary to compute the dif-
ferent hydrologic regions separately. When adding hydrographs from different
basins together, such as where Skunk Creek enters Bear Creek, the absolute
time of the hydrograph had to be determined and the addition lagged appropriately
since the peak flow from each basin would not reach the confluence at the same
time. In the smaller channels the shape of the upper basin hydrograph as it
came downstream was assumed to retain its original shape. In the larger chan-
nels, some channel storage was accounted for which lowered the peak of the
hydrographs and spread out the hydrograph time. (This is an example of bene-
fits to be derived by the protection of channel storage areas from encroach-
ment).

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SPECIAL CONSIDERATIONS

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This study is concerned only with rather small tributaries to Boulder Creek;
however, when completed, these major channels will be significant factors in
the floods on Boulder Creek below Boulder. After urbanIzation of the esti-
mated combined 100-year flood flow peak from the project area of 12 square
miles will be approximately 5,000 cfs. This compares with the 11,000 cfs
estimate of the U.S. Corps of Engineers for Boulder Creek below the confluence
with South Boulder Creek under present conditions. This points out how sIg-
nificantly urbanization increases peak runoff flow. Luckily, the times of
concentration are varied so that runoff from the urban basins does not occur
at the same time as that on Boulder Creek from upstream.

There will be a perennial ,low flow in most of the proposed channels. This
low flow will prObably increase as urbanization expands. Since the inverts
of some of the proposed channels are below the present water table, the chan-
nels will act as a drain in some areas and lower the surrounding groundwater