7886

• UCREFRP Catalog Number: 7886
• Author: Hydrosphere Resource Consultants.
• Title: Yampa River Basin Recommended Alternative, Detailed Feasibility Study
• USFW Year: 1995.
• USFW - Doc Type: Final Report.
• Copyright Material: NO

I Reservoir Site Characterization
I Table 3-1
Comparison of PMF Estimates Elkhead Lake Dam
' Harza.1991 CDOW.1985
Probable Maximum Precipitation
Duration, hours
48.0
18.0
PMP, inches 16.2 9.2
Infiltration Losses
Rainfall excess, inches
10.5
4.8
Infiltration Loss, inches 5.7 4.7
Overall runoff coefficient 65.0 50.0
Unit Hydrograph
Method Dimen. Graph Tri-angular
Lag Curve
1 Time of concentration, hours -- 16.0
Lag time, hours 18.8
Time to peak, hours 12.0 10.5
Peak discharge, ft 3/sec 6,720 9,680
Baseflow, ft3/sec 1,000
Probable Maximum Flood
Peak, ft3/sec
40,600
37,400
3-day volume, ac-ft 110,000 55,800
Source: Harza, 1991.
SEDIMENT MOVEMENT

General
The placement of a dam on a river has an impact not only on streamflows but also on the
sediment moved by those flows. The reduced flow velocities and associated increase in settling
time available in the reservoir allows sediment to separate from the water and be retained in the
reservoir. Sediment starved water released downstream of the dam then picks up bed and bank
material to reestablish its sediment load until a stable slope or channel armoring occurs. Both
of these phenomena occur at the Elkhead Reservoir. Since a reservoir has existed at the site
for approximately twenty years, some sediment impact information is available from the
existing facility. In addition, investigations of "natural" physical conditions provides raw data
from which to make theoretical sediment and fluvial mechanics projections.
Sediment Inflow/Accumulation
Examination of the current elevation-area-volume relationship (Figure 3-3) in comparison
with the same relationship for pre-dam conditions reveals a reduction in reservoir surface areas
for elevations between 6,336 and 6,370 and a corresponding "hump" in the area curve. This
hump corresponds to 1,170 acre-feet of lost storage. This lost storage is in the range of
elevations where sand, larger sediment, and some fine sediment would be expected to
accumulate in the reservoir. The amount of fine sediment accumulation below elevation 6,336
1 3-15