CASE STUDY OF A DEWATERED RESERVOIR: 1994 FAILURE OF IVEX DAM, CHAGRIN RIVER, NE OHIO

Dam removals generally fall into two categories: removal of low-head dams and removals of larger dams. Low-head dams tend to have a high storage capacity loss due to sediment accumulation, and are often removed in one step, the so-called “blow and go” method. Larger dams usually require phased drawdown removals to manage both the existing storage capacity and the larger volumes of sediments.

The 13 August 1994 failure of IVEX Dam on the Chagrin River was an inadvertent dam removal by the “blow and go” method. The dam had lost 86% of its storage capacity over its 152-year history. The loss was caused by a seepage piping failure adjacent to the spillway which resulted in a 20 m wide breach and dewatered the reservoir in a matter of minutes. The breach was followed by four phases of reservoir modification: (1) incision up to 3 m into the reservoir sediments to re-establish the gradient of the Chagrin River across the reservoir, (2) channel widening by slumps, (3) reservoir sediment dewatering, and (4) slope stabilization. Observations suggest the first phase occurred within minutes and was accompanied by large, longitudinal scours. Channel widening by slumps probably initiated rapidly and was active over a several month period. Reservoir sediment dewatering was essentially complete within few weeks and added strength to the former reservoir sediments. Slope stabilization developed locally over the first year as slumps regraded slopes to more stable configurations. Incipient bank stability was provided in some cases when the Chagrin River re-exhumed bedrock channel banks or the root structures of pre-reservoir trees. Post-failure surveying and sediment coring revealed that up to 31,300 m3 of sediment (13% of the reservoir fill) was mobilized by the breach in the dam and removed. Over the past 11 years, natural and seeded vegetation has successfully stabilized the channel banks.