FLOOD08354

• County: Boulder
• Community: Lafayette
• Stream Name: Coal Creek , Waneka Reservoir
• Basin: South Platte
• Title: Major Basinwide Planning
• Date: 12/1/1980
• Prepared For: Boulder County
• Prepared By: Camp Dresser & McKee
• Floodplain - Doc Type: Floodplain Report/Masterplan

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A survey of predominant soils indicates that the penneability of this soil

TABLE 4
5-MINUTE INCREMENT, 2-HOUR DURATION DESIGN STORMS

association ranges from .2 inches per hour to 6.0 inches per hour, with the

majority of this soil group having a range of penneabil ity between 2.0 inches

City of Lafayette, Colorado
For Use With The Colorado Urban Hydrograph Procedure

per hour to 6.0 inches per hour.

Time
(Minutes)

Incremental Rainfall Depth/Return Period
2 r. 5 r. 10 r. 100 r.

5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120

.01
.03
.07
.14
.24
.14
.07
,04
.04
.03
.03
.03
.02
.02
.02
.02
.02
,01
.01
.01
.01
.01
.01
.01

1.04

Total (2 hr)

.01
.04
,10
.20
.36
.18
.10
.06
.06
.04
.04
.04
.03
.03
.03
.03
.03
.01
.01
.01
.01
.01
.01
.01

1.27

.02
.05
.12
.24
.43
.19
.12
.07
.07
.05
.05
.05
.03
.03
.03
.03
.03
.03
.02
.02
.02
.02
.02
.02

1. 76

.03
.05
.11
.22
,41
.68
.38
.22
.14
,14
.11
.11
.11
.05
.05
.05
.05
.05
.03
.03
.03
.03
.03
.03

3.14

For the purpose of computing the runoff losses for each sub-basin, the
infiltration losses were calculated using Horton's equation with initial and
final infiltration rates of 3.0 and .5 inches per hour respectively, and a decay
rate of .0018. Detention and depression storage for impervious areas was

estimated at .1 inches and .4 inches for the pervious areas.

Existing and Future Condition Hydrology

The values listed in Table 3 give the basic drainage basin data and resulting

peak f1 ow rates for both ex i st i ng and future development condit ions. for each

sub-basin shown on Drawing B.

The hydrology for the selected Alternative 2A recognizes the interception of

existing major drainage patterns, The stonn hydrographs developed in the Phase

B study utilized existing basin development conditions, since a policy of

on-site detention is a basic assumption made in Alternative 2A. Hydrographs for

the 10- and 100-year recurrence intervals at various locations along channels

Reference: Urban Drainage and Flood Control District Preliminary
Design Storm Procedure, Hydrology Research Program,
August 1979.

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for each planned major drainageway are shown on Figure 4. Flood discharge

profiles for each major drainageway are contained on Profile Sheets 1 through 6

and are shown on Figure 3.

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