Scientific Investigations Report 2006-5284
The Taum Sauk pump-storage hydroelectric power plant located in Reynolds County, Missouri, uses turbines that operate as pumps and hydraulic head generated by discharging water from an upper to a lower reservoir to produce electricity. A 55-acre upper reservoir with a 1.5- billion gallon capacity was built on top of Proffit Mountain, approximately 760 feet above the floodplain of the East Fork Black River. At approximately 5:16 am on December 14, 2005, a 680-foot wide section of the upper reservoir embankment failed suddenly, sending water rushing down the western side of Proffit Mountain and emptying into the floodplain of East Fork Black River. Flood waters from the upper reservoir flowed downstream through Johnson's Shut-Ins State Park and into the lower reservoir of the East Fork Black River. Floods such as this present unique challenges and opportunities to analyze and document peak-flow characteristics, flood profiles, inundation extents, and debris movement.
On December 16, 2005, Light Detection and Ranging (LiDAR) data were collected and used to support hydraulic analyses, forensic failure analyses, damage extent, and mitigation of future disasters. To evaluate the impact of sedimentation in the lower reservoir, a bathymetric survey conducted on December 22 and 23, 2005, was compared to a previous bathymetric survey conducted in April, 2005. Survey results indicated the maximum reservoir capacity difference of 147 acre-feet existed at a pool elevation of 730 feet.
Peak discharge estimates of 289,000 cubic feet per second along Proffit Mountain and 95,000 cubic feet per second along the East Fork Black River were determined through indirect measurement techniques. The magnitude of the embankment failure flood along the East Fork Black River was approximately 4 times greater than the 100-year flood frequency estimate of 21,900 cubic feet per second, and approximately 3 times greater than the 500-year flood frequency estimate of 30,500 cubic feet per second. Dynamic wave unsteady flow models Dam Break (DAMBRK) and Unsteady NETwork (UNET) were used to route the flood wave from the embankment failure breach of the upper reservoir to the spillway of the lower reservoir. Simulated velocities ranged from 20 to 51 feet per second along Proffit Mountain and 12 to 32 feet per second along the East Fork Black River. Simulated arrival time of the flood wave took approximately 5.5 to 6.0 minutes to enter into the floodplain of the East Fork Black River, and roughly 29 minutes to begin filling the lower reservoir. Simulated shear stress values reached as high as 232 pounds per square foot along the slope of Proffit Mountain and 144 pounds per square foot within the Shut-Ins. Flood depths from the embankment failure may have reached greater than 50 feet along Proffit Mountain and as much as 30 to 40 feet along the East Fork Black River.
A steady-state model was used to develop 2-, 5-, 10-, 25-, 50-, 100-, and 500-year flood frequency profiles along the East Fork Black River. A similar flood event, hypothetically resulting from a breach of the east embankment above Taum Sauk Creek, was simulated along with the 100- and 500-year flood profiles on Taum Sauk Creek. Estimated extents of flood inundation were developed for each profile.
Debris movement was extensive as a result of the flood wave moving down Proffit Mountain and through Johnson's Shut-Ins State Park. A quantitative assessment of debris movement was conducted to benefit rehabilitation efforts within the park. Approximately 180 acres of timber were affected as a result of the embankment failure flood.
Posted December 2006
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