Overview

Crawford Dam is an earthfill structure 162 feet high and 580 feet long, with a volume of 1,006,000 cubic yards. The uncontrolled overflow spillway is in the left abutment of the dam and has a capacity of 1,400 cubic feet per second. The outlet works in the right abutment of the dam carries water through a 34-inch-diameter steel pipe controlled by four 2.25-foot-square high-pressure gates. Maximum discharge capacity to Aspen Canal is 125 cubic feet per second. Crawford Reservoir has a total capacity of 14,395 acre-feet and an active capacity of 14,064 acre-feet. The reservoir has a surface area of 406.2 acres.

Geology

Crawford Dam is located in a narrow gouge. Dakota sandstone and Mancos shale are exposed on both abutments, which are steep. In the sandstone, a system of strike joints that trend N 15 degrees E is found along the gorge, and these joints have opened up most noticeably where the stream parallels the jointing. The weathered sandstone in some areas of the abutment required as much as 25 feet of excavation before acceptable rock was found. The valley section of Crawford Dam is founded on approximately 65 feet of predominately fine-grained alluvial deposits of silty and sandy clay with a few lenses of sand and gravel and sandstone boulders that accumulated as slope wash from the canyon sides. Logs of preconstruction drill holes in the alluvium (Qal) indicate the clay varies widely in thickness, ranging from less than 5 feet to over 50 feet; there also are a few zones of sand, sand and gravel, and gravel and cobbles, but they do not appear to be connected. Beneath the alluvial deposits are friable-to-well-cemented sandstone with thin interbeds of carbonaceous shale and siltstone of the Dakota Formation. The sandstone is often quite broken and has a conspicuous joint system. Beneath the Dakota Formation is the Morrison Formation which is composed of variegated siltstone and mudstone. Artesian flows of water were encountered in pre-construction drill holes coming from either the contact between the Dakota and Morrison Formations or the alluvial deposits and the underlying Morrison Formation. During construction grouting, drill holes extending to depths of 100 feet (near the top of the Morrison Formation) took as much as 2,200 sacks of cement per hole. Evidence suggests a highly permeable confined or semi-confined aquifer is present in the upper Morrison formation which is locally connected to pockets, lenses, and layers of coarse grained alluvium deposited on the bedrock surface and overlain by clay. The spillway excavation was predominately in the Dakota sandstone. The depth of overburden varied considerably as an old stream channel was found across the spillway alignment. The stilling basin was excavated in mudstone of the Morrison formation. The outlet works tunnel in the right abutment was entirely in the Dakota formation, consisting of friable- to well-cemented sandstone, as well as some thin beds of shale and siltstone, and fractured rock. Prior to construction, it was anticipated that the ground water was trapped water, and the water would drain out when the tunnel was driven. Contrary to anticipations, there was a persistent flow of ground water along the tunnel alignment.

General

Dimensions

Hydraulics & Hydrology