Bonasa Breaks Ranch Dam Breach

Challenges

A 19-foot-high earthen dam at the Bonasa Breaks Ranch in Asotin County, Washington overtopped and breached following several days of heavy precipitation.  After the precipitation event, significant damage was observed along a 6-mile stretch of Rattlesnake Creek downstream of the dam including erosion, removal of trees, sediment deposition, and damage to roads and bridges.  Downstream property owners were pursuing legal action against the ranch owner for costs to repair the damage.

Solution

RJH was retained by Murray Dunham & Murray, a law firm representing the ranch owner.  RJH was tasked with developing a professional engineering opinion regarding the incremental damage resulting from the dam breach compared to the baseline flood damage that would have occurred without a breach.

RJH performed a site visit to observe the dam breach and downstream channel. Following the site visit, RJH performed hydrologic and hydraulic modeling to simulate conditions that occurred during the flood and breach. The hydrologic model was developed using NEXRAD precipitation data obtained from the National Oceanic and Atmospheric Administration. Outflow hydrographs from the hydrologic model were routed through a two-dimensional hydraulic model of Rattlesnake Creek.

RJH evaluated downstream flow depths, velocities, and shear stress for scenarios with and without the dam breach. The conclusions from this analysis were used to assist the owner in negotiating a settlement.