Debris Clogging Assessment of Dams
Debris such as large individual trees and mats of connected large wood and other items can dislodge during flood events and travel through the reservoir to the dam. These pieces of large debris can be floating or partially submerged which when approaching the dam can become racked against or lodged inside dam outflow structures such as radial gates, morning glory spillways, and submerged outlet works. When these outflow structures become partially or fully obstructed, the water surface elevation in the reservoir raises and there is greater risk of dam overtopping. Plugging of the reservoir outlet works can also lead to delay or inability to return the pool to normal operating elevation.
Concern regarding plugging of spillways and outlet works has been identified by many agencies as a research priority. Debris removal at spillway radial gates is currently on the top 25 list of Collaborative Research Needs from USACE, and is listed at number 17 for Reclamation's Research Priorities to Enhance Dam Infrastructure Sustainability (Merten, 2014). Debris management at intake structures is currently listed on the prize competition priority list for sediment related competitions, and is also listed in Reclamation's Research Priorities at number 24.
For the past 2 years, Reclamation's hydraulics laboratory has been researching impacts of large debris to reservoir water surface elevation (WSE) and discharge capacity for the Dam Safety Office. Results indicate that debris jams on radial gate ogee crested spillways can cause a raised WSE of up to 9 feet, and reduce discharge by up to 48% (Walker, 2018). Morning glory spillways resulted in raised WSE up to 22 feet above clear water and a 60% reduction in discharge (Walker, 2019). The impacts observed in the physical model illustrate the importance of researching the likelihood of debris clogging in Reclamation's portfolio. An existing Reclamation spillway inventory (Brom, 2014) lists spillway types and geometry, but did n
Need and Benefit
While debris impact to reservoirs is currently being studied in the hydraulics laboratory, how debris is being managed across the portfolio has not been studied and a ranking of projects with debris concerns has not been created. However, concerns of debris plugging at either spillways or intakes is listed as a top 25 priority of collaborative research needs for USACE and in the research priorities to enhance dam infrastructure sustainability for Reclamation.
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Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Debris Clogging Assessment at Dams (final, PDF, 3.4MB)
By Connie Svoboda, Jacob Carter-Gibb
Report completed on September 30, 2020