CS8B - Concurrent Session 08B: Lake Ilo Revival: A Story of Dam Rehabilitation

Abstract Description: The Lake Ilo National Wildlife Refuge, located north of Dickinson, North Dakota is owned and operated by the U.S. Fish & Wildlife Service (USFWS). The refuge features Lake Ilo as its central habitat for waterfowl nesting and brood-rearing. The dam that impounds Lake Ilo is considered a high hazard potential dam.

Since 2006, intermittent seepage in the right abutment of Lake Ilo Dam has been observed during dam safety inspections. A comprehensive review and risk analysis of Lake Ilo Dam was performed in 2019 to help USFWS identify and focus on potential failure modes that were judged to be a significant risk. Contact erosion along coal seams at the right abutment embankment contact was judged as a significant risk and as a result USFWS began monitoring this area.

In September 2020, amber-colored seepage with an estimated flow rate of 5 gallons per minute (gpm) was exiting the right abutment, prompting the activation of the Emergency Action Plan at Event Level 1 and restriction of the reservoir level to Elevation 2188.3. Seepage flow rates fluctuated between 0 and 5 gpm from September 2020 to April 2022 while the reservoir level fluctuated between Elevations 2185.0 and 2188.3. In April 2023, the seepage appeared to increase; subsequent lowering of the reservoir halted the seepage when held below Elevation 2185.

A sand filter wall was designed to intersect the existing coal seams in the right abutment. The sand filter wall design consisted of a 275-foot-long, 8-foot wide trench that extended to depths up to 16 feet. To mitigate potential seepage exiting the right abutment upstream of the filter wall, a filter sand blanket was designed to extend along the dam embankment and abutment contact to an elevation near the normal high water line of the reservoir.

In addition to mitigating seepage at the right abutment, a sand filter diaphragm was constructed around the low level outlet conduit, and a 4-foot-wide stoplog slot was cut into the existing outlet works gate tower to lower the stoplog sill to near the bottom of the gate tower. The comprehensive review process proved to be a critical tool that helped USFWS understand the risk at Lake Ilo and focus on the necessary improvements to lower the dam safety risk.

Construction was completed between August and October of 2024. The Lake Ilo locals, both human and wild, anxiously await spring runoff and the filling of the reservoir.