What Impact Does Sedimentation Have On Columbia River Basin Dams?

The following discussion of possible impacts of sedimentation on Columbia River Treaty dams is divided into three different sections.

1) Sedimentation and the Hugh L. Keenleyside Dam Structure

 The Hugh L. Keenleyside Dam is an earthfill dam with zones of building material with different degrees of porosity, permeability and density. The core of the dam is an upstream sloping glacial till with low permeability. This zone is connected to a till blanket that extends 670 metres into the Arrow Lakes Reservoir. Additional sediment on the upstream face of the till blanket would not cause dam safety concern.
Keenleyside dam’s concrete gravity structures are founded on bedrock. Significant sedimentation on the upstream face of the concrete structures would cause additional load and decrease stability. However, Arrow Lake Reservoir is a large reservoir, and to date, no sedimentation has been observed on the upstream faces of the concrete structures.

2) Sedimentation and Smaller Reservoirs

For smaller reservoirs, such as at Aberfeldie - where there is a large amount of sediment being carried along the river bed — sediment has essentially filled up the small reservoir despite the dredging work that has been done. To ensure the long-term safety of the dam, a decision was made to anchor the dam. The anchoring allows the dam to withstand additional sediment load for all load cases, including withstanding a seismic event.

3) Sedimentation and Operations of B.C.’s Large Reservoirs

Over time, some suspended sediment may settle in reservoirs. Depending on many variables (e.g. sediment transport rates into, and out of, the reservoir), there may be a slow build-up of sediment deposits in a reservoir. Occasional landslides may add additional sediment into the reservoir.

However, sedimentation is not a practical concern for the larger reservoirs in B.C., such as Arrow Lakes, Revelstoke, Kinbasket, and Duncan because the rate of sediment build up in these reservoirs is extremely small compared to the depth and volume of the reservoirs. If sedimentation were significant BC Hydro would notice changes to its reservoir operating data - specifically reservoir storage volume curves would become inaccurate, and this would result in obvious inaccuracies in the daily calculation of reservoir inflows.

Reservoir bathymetric surveys are used to determine contours of the reservoir bottom and the reservoir volume. The surveys are expensive and time consuming and BC Hydro typically only conducts new bathymetric surveys of its large reservoirs when the information provides benefits for some of their environmental projects.