CS37C - Concurrent Session 37C: How Low Can You Go - Whittier Narrows Erodibility Study

Whittier Narrows Dam (WND) is an earthen dam, approximately 3 miles long, located on the Rio Hondo and San Gabriel Rivers in Los Angeles County, California. The dam is one of the main elements of the flood control system for the Los Angeles County Drainage Area, while supporting water conservation. The probable maximum flood would overtop the dam by about 4 feet. Due to the high consequences of breach resulting from flow overtopping the dam, the U.S. Army Corps of Engineers is developing plans to armor the crest and downstream face of the dam to prevent failure during overtopping. The downstream face will include a stepped, non-erodible material designed to help dissipate energy of the overtopping flow.

Of prime interest was the erodibility of a proposed hardfill (soil-cement) mix used to form the steps of the downstream face. Hardfill mix design incorporates a low concentration of cement (usually ~10%), mixed with non-uniform gravel or site-specific material to a specified moisture content. Construction and placement techniques vary, however hardfill mix is typically placed in lifts to produce steps. Realizing that reducing cement content significantly benefited project costs, a prototype scale study was performed to quantify the erodibility of a hardfill mix with varying cement content. Site specific material and proposed mix components were identified and transported to the laboratory where multiple 15,000 pound specimens were constructed, cured and placed in an outdoor flume. Increasing flow rates, up to the prototype unit design discharge, were conveyed over each specimen and erosion quantified through post test LiDAR scans. Testing quickly identified flow stability concerns on the proposed step geometry and a 1:4 Froude scale sectional model was constructed and operated to confirm flow stability and tailwater requirements. This presentation will present the findings of this erodibility study as well as a lesson learned on the impact of chute slope on flow stability.