Overview

Cascade Dam, near Cascade, Idaho, on the North Fork on the Payette River, is a zoned earthfill structure 785 feet across the crest. The initial total storage capacity was 703,200 acre-feet (active 653,200 acre-feet). The spillway is located on the right abutment of the dam. The invert is 45 feet wide at the crest under the radial gates and about 330 feet long excluding the open cut channel to the reservoir. The design capacity is 12,500 cubic feet per second with the water surface at elevation 4828.0 feet. Two 21-foot wide by 20-foot high radial gates are installed on the crest of the spillway to provide means for regulating the discharge of water over the spillway and to provide protection for the dam in the event of a sudden rise in reservoir water level. A sedimentation survey completed in 1995 at Lake Cascade estimated the total capacity at 693,200 acre-feet (active 646,500 acre-feet).

Geology

Cascade Dam is sited on igneous rock of granitic type, which generally is of fair quality, but there are numerous localized areas of poor quality. The rock `is light gray coarse-crystalline porphyritic granite and locally is gneissic. It is extremely sheared, conspicuously sheeted and jointed and zones of crushed rock are numerous. Much of the rock is hard, competent and relatively fresh. The dam embankment cutoff trench extends to bedrock and includes a concrete cutoff wall with footings in bedrock. A pressure grout curtain was placed to depth not exceeding 100 feet under the cutoff wall footings. An area of badly broken rock (by blasting) was consolidated with grout. Soils remaining in the embankment foundation upstream and downstream of the cutoff trench are mostly coarse-grained and bouldery, not types considered subject to liquefaction. The gated spillway structure and chute are in a rock cut on the right abutment. The outlet works intake structure and tunnel are on the right side. Foundation rock conditions for the spillway and outlet works are similar to rock described above. An inferred east-west trending fault has been mapped a short distance north on the left abutment. The dam is located within a notch in a bedrock ridge which is interpreted to be a tilted fault block nor an uplift block (horst) related to regional block faulting. The abutments upstream and downstream are natural bedrock slopes, which are stable. The excavated slope of rock adjacent to the spillway and outlet works is stable. None appear to be subject to landslides or rock falls. Basement igneous rocks enclose the basin, which makes the water holding capability of the reservoir satisfactory.

General

Dimensions

Hydraulics & Hydrology