DAM FAILURE IN SOUTHEASTERN WASHINGTON

On 18 April 2000 an irrigation dam burst and released a flood which rushed downhill through a school and on to the Snake River. The earthen dam was filled to its capacity of 2.14x10⁶ ft³ for the first time. The leak probably started at the contact between the plastic liner and the pipe at the base of the 30-foot high dam. The geology of the area is loess overlying ancient fluvial sand and gravel overlying Miocene Columbia River basalts.

The volume of silt eroded from the dam was approximately 8x10⁴ ft³; this was augmented by erosion of loess and underlying alluvium as the flood deeply scoured a small canyon. The coarse sediment was deposited on an alluvial fan; thixotropic mud covered the school grounds.

The Manning equation is Q=A R^2/3 S^1/2 n^-1, where Q=discharge, A=cross-sectional area, R=hydraulic radius, S=slope, and n=Manning resistance factor. The paleodischarge was measured at six places along the flood path; depending in part on the estimated value of n, the estimations for peak flood discharge ranged from 3 to 10x10³ ft³/sec.

The dam was completely rebuilt within a year of the failure. Improvements included better compaction of the silt, a concrete upstream face, and a spillway.