Mud Mountain Dam Lower Cascade Creek Water Control Facility

Mud Mountain Dam is a zoned earth and rockfill structure with an 810- foot-long crest and a maximum height of 432 feet above bedrock. The structural height of 427 feet is measured from the bottom of a structural depression located approximately 170 feet upstream of the cutoff wall to the new top of dam at 1,257 feet NGVD29. The dam is 1,600 feet wide at the base, 25.5 feet wide at the crest, with a design crest elevation of 1,257 feet NGVD29. A concrete seepage cutoff wall extends from the crest into bedrock in the core of the dam. Mud Mountain Dam is constructed in a narrow canyon where rock walls on both sides of the gorge rise almost vertically to a height of nearly 275 feet above the White River channel. The upstream hillside slope at the left abutment is steep (approximately1:1). Lower Cascade Creek (LCC) drains into an existing Water Control System (WCS) consisting of debris screens, a concrete lined catch basin and pipes which convey flow along the upstream left groin to the pool. The upstream groin is armored with shotcrete to approximate EL 1167.8 feet (NAVD88). The catch basin and embankment abutment tie-in are separated by roughly 51 feet of sandy-gravel fill between EL 1,267.8 feet (NAVD88) (basin) and 1,264.3 feet (NAVD88) (crest). The current WCS was not designed to a handle a specific flow or event with a defined return interval, because there is very little available hydrologic data on flow-frequency for LCC. Rather, the existing inlet basin was constructed to address recurring erosion caused by flows from past events along the upstream groin. This lack of a specific design event or flow has led to failure to control flows at the inlet even without debris accumulation. The LCC watershed is approximately 0.4 square miles or 0.1 percent of the Mud Mountain Dam watershed

Dam/Tunnel, New Construction.

Division 02 -	Existing Conditions, Subsurface Investigation.
Division 31 -	Earthwork, Site Clearing, Grading, Excavation and Fill, Subgrade Preparation, Excavation, Shoring, Tunneling and Mining, Tunnel Excavation.
Division 35 -	Waterway and Marine Construction, Dam Construction and Equipment.