Sediment management at small dam removal sites

Abstract

The removal of obsolete and unsafe dams for safety, environmental, or economic purposes frequently involves the exploration of sediments trapped within the impoundment and the subsequent assessment of sediment management needs and techniques. Sediment management planning requires a thorough understanding of the watershed’s surficial geology, topography, land cover, land use, and hydrology. The behavior of sediments is influenced by their age, consolidation, and stratigraphy. All watersheds have a history that helps forecast sediment loads, quality, gradation, and stratigraphy. Impounded sediment deposits may include coarse deltas and foreset slopes, fine or coarse bottom deposits, cohesive or organic matter, and wedge deposits immediately behind the dam. Some watersheds have anthropogenic pollutants from agricultural activities, mining, industries, or urban runoff. The volume and rate of sediment release during and after small dam removal can be limited by active management plans to reduce potential downstream impacts. Management strategies include natural erosion, phased breaches and drawdowns, natural revegetation of sediment surfaces, pre-excavation of an upstream channel, hazardous waste removal or containment, flow bypass plans, and sediment dredging.

Synthesis of Common Management Concerns Associated with Dam Removal

“…There are two management questions that are central to many dam removals: (1) How much of the sediment impounded within a reservoir will erode? and (2) How quickly will the eroded sediment move through the downstream river corridor (e.g., Downs et al ., ; Sawaske and Freyberg, ; MacBroom and Schiff, )? From a more practical perspective, will reservoir erosion be slow and incomplete, leaving behind exposed impoundment sediment that can be perceived as a “stinking mudflat” from which sediment bleeds out over time, or will erosion be rapid and complete, such that fluvial forms and processes are swiftly reestablished?…”

“…However, once the sediment from upstream deltas reached the dam sites, the multi‐year phased removal on the Elwha River ultimately released a sediment quantity many times greater than the average annual load and exceeded more than 50% of the reservoir‐deposit volume. Although the implications of reservoir erosion may be greatest for dams that store large sediment volumes, reservoir erosion is a concern across the spectrum of dam removals (e.g., MacBroom and Schiff, ), highlighting the need for ongoing evaluation and improvement of predictive tools to inform dam removal CMCs.…”

Ecological and Health Risk Assessments of Heavy Metals Contained in Sediments of Polish Dam Reservoirs

“…According to the report of the World Commission on Dams [34], a volume of 0.5 up to 1% is lost annually due to the sedimentation process. Therefore, periodical dredging works or other methods are applied, aimed at the restoration of the volume of sediments [35][36][37]. Deepening works and storage of sediments removed from the reservoir are very costly [38].…”

Rivers in the Critical Zone

“…The large amounts of sediment released by dam removal can endanger downstream aquatic organisms or result in aggradation and river planform change, although progressive notching of a dam, rather than instantaneous removal, can be used to control the rate of sediment release (MacBroom and Schiff, 2013). The large amounts of sediment released by dam removal can endanger downstream aquatic organisms or result in aggradation and river planform change, although progressive notching of a dam, rather than instantaneous removal, can be used to control the rate of sediment release (MacBroom and Schiff, 2013).…”