WSP11325

• File Number: 8443.400
• Description: Narrows Unit - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 3/1/1978
• Author: Woodward-Clyde Consu
• Title: Review Of Data And Studies Developed By The Bureau Of Reclamation For The Feasibility Stage Design Of Narrows Dam Colorado
• Water Supply Pro - Doc Type: Report/Study

~

Woodward.Clyde Consuhants

1 "U.' ')
v .,;

~

4396. The profile drawing (Figure 2) indicates that the inter-

section of the active conservation and joint use reservoir

water levels with natural ground surface occurs around Stations

~

229 and 230. The profile plots available to us may not
accurately show the ground surface. It is likely that the

foundation zone downstream from the dam around Stations 230 and

~

232 will remain saturated to ground surface. We believe that

short term high water levels will create active steady-state

~

seepage flow conditions at the toe of the dam. This

distribution of hydraulic gradients along the seepage paths

will not be uniform (at the average value) since foundation

soil permeabilities vary. A large portion of the loss in total

t

head will be near the exit zone, causing above average

hydraulic exit gradients.

Average hydraulic gradients are shown in Figure 8 for two

.

cross sections, Stations 231 and 224. The maximum average

gradient at Station 231 is 0.16 under maximum water surface

conditions. Considerable additional safety exists at Station

t

224 since natural ground level is about 15 feet above the

expected water surface (saturation surface) within the soil and

also because the resulting partially saturated zone in the

~

foundation upstream and downstream of the embankment will

impede development of steady-state saturation flow conditions

during the brief periods that high water is against the

~

embankment. In addition, the higher downstream ground surface

- 24 -

t

.,