FLOOD05980

• County: Gilpin
• Community: Black Hawk
• Stream Name: North Clear Creek, Gregory Gulch, Chase Gulch
• Basin: South Platte
• Title: Reconstitution of 1979 Flood Peaks, North Clear Creek, Gregory Gulch& Chase Gulch
• Date: 4/26/2001
• Prepared For: CWCB
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Flood Documentation Report

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Table 4. Comparison of 1979 Flood Peak Estimates to TR20 Model Estimates, 100-
Year Event.

time to

Cl
C2
Gl
G2

North Clear Creek
100- ear time to
Point 1979 est. 2001 Model
NCI 2694 2702
NC2 2528 2524
NC3 2280 2267
NC4 NtA 1924
NC5 NtA 1311
NC6 Nt A 929

Note: See Figure 4 for design points.

IMPACT OF RESERVOIR STORAGE ON FLOOD PEAKS

The impact of the Chase Gulch and Dorothy Lee Reservoirs on flood peak discharges
was identified using the TR20 hydrologic models calibrated to reconstitute 1979 100-year
flood peak: estimates. Storage/elevation curves (Figures 5 and 6) were developed from
design and as-constructed drawings of the two reservoirs (RMC, 1991, and RMC, 1994).
The discharge curve for Chase Gulch Reservoir uses the spillway discharge curve
developed for the reservoir shown in the Appendix. The discharge curve for Dorothy Lee
Reservoir was calculated using the FHW A HY -8 program in the Appendix.

The results of the analysis for the 10-, 50-, and 100-year event are summarized at selected
design points on Table 5. The results for the 100-year event are found in the Appendix.
The results indicate detention storage above the Chase Gulch water supply reservoir
spillway reduces flood peaks significantly at the outlet. At the outlet. design point C I.
100-year discharges are reduced from 1179 cfs to 551 cfs. Dorothy Lee Reservoir has
only a minor impact on flood peaks during the 100-year event at the outlet of Gregory
Gulch. At the outlet, design point G1, 100-year discharges are reduced from 1248 cfs to
ll59 cfs. This reduction is less than predicted by a recent HEC-1 analysis by I.F. Sato
(1996), which assumed a slightly lower curve number causing a lower peak discharge
than the current study. In addition, the current study defined the storage and discharge
relationship of Dorothy Lee Reservoir using design drawings and hydraulic calculations.
The 1996 Sato study used simplified hydraulic relationships and did not examine the as-
built infollDation when modeling the reservoir. The TR20 model indicates the reservoir
overtops during the 100-year event

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