FLOOD09526

• Designation Number: 281
• County: Boulder
• Community: Boulder
• Stream Name: Boulder and Adjacent Boulder County Drainageways
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Boulder and Adjacent County Drainageways
• Date: 5/1/1987
• Designation Date: 10/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

of the basin (Lea), percent of impervious cover, coefficient reflecting time to
peak (Ct), and coefficient related to the peak rate of runoff (Cp).

A

=

basin area in square miles.

Basin areas, basin and channel slopes, and lengths of flow paths were measured on
the U.S. Geological Survey's 7.5 Minute Quadrangle mapping, and supplemental
information was taken as needed from the topographic mapping (Reference 5) as
provided by UD&FCD (Reference 6). Actual bas ins lopes were adjusted for use in
determining the coefficient reflecting time to peak (Ct) for use with the most
current data determined by the Urban Drainage and Flood Control District (Figure
3). Va lues of Ct and Cp were also determi ned from thi s 1 a test vers ion of CUHP
and were taken from Figures 4 and 5. The coefficients for Ct were adjusted for
use in the computer program. The equation used to convert Ct to *Ct is as
fo llows.

The impervious cover was determined by first identifying land uses throughout
each of the basins from planning and zoning maps of the City and County of
Boulder. Typical values for the percentage of impervious area for each land use
were then assigned and a weighted impervious area was computed for each land use.
Impervious percentages used in the CUHPB model are shown in Table 4.

TABLE 4

I_pervious Percentages

*C
t

= Ct

LL .18
ca
So.24

Land Use
Downtown Business Area,
Shopping Centers, etc.
Residential Dense
Residential Normal
Residential, Large
Scattered Mountain
Parks, Greenbelts,
Undeveloped Areas

Percent I_pervious

Lots

95 - 100
50 - 70
35 - 50
20 - 40

Based upon a new equation for time to peak:

LL .48
ca
So

In addition, the unit peak of the unit hydrograph is defi1\ed by:

Development,
Open Space,

tp = Ct

. . . . . . . . . . . .. . . . .

o - 10

In which qp
Cp

=

q = 640Cp
p tp
peak rate of runoff in cfs per square mile.
Coefficient related to peak rate of runoff.

Rainfall losses due to infiltration and depression storage are al so input
parameters to the CUHPB model. Infiltration rates were taken from the Urban
Storm Drainage Criteria Manual revised guidelines and are based on soils
classified by the Soil Conservation Service (Reference 7). The infiltration
rates used for the analysis ranged from 3.0 to 4.0 inches/hour initial and from
.015 to 0.6 inches/hour final, with a decay coefficient )f 0.0018. Depression
storage values ranged from 0.4 to 0.5 inches for pervious areas and were 0.1 inch
for impervious areas.

=

Once qp is determined, the peak of the unit hydrograph for the basin is computed
by:

Qp = qpA
In which Qp = peak of the unit hydrograph in cfs.
A = area of basin in square miles.

The drainage basin and channel input data parameters for the 11 drainageways are
summarized in Table 5.

Finally, the value for Cp was determined using the equation:
Cp = PCtAO.15

Where

P

=

peaking parameter from Figure 5.
Coefficient from Figure 4

Stream routing for each of the sub-basins was completed using the U.S. Army Corps
of Engineers HEC-1 Flood Hydrograph Computer Model. For this analysis, the storm
hydrographs computed by the CUHPB were input into the HEC-1 Model and routed

Ct =

8