5 Jul 2012

Bleg: Dam(n) silt

Can any engineering types tell me why dams are not designed to flush silt from filling up the reservoir? I know that strength is an issue, but isn't it possible to make some kind of "silt-tunnel" in the same way that dams have turbine tunnels?


  1. Hello David,

    As I'm sure you know, moving sediment through a reservoir and dam is not an easy task. However, some dams are built with low level outlets specifically for this purpose. One of the main issues, particularly for large reservoirs, is that as flowing river water enters the reservoir it slows, dropping sediment at the upstream end of the reservoir. If this happens, one of the only ways to move that sediment through the dam is to significantly lower reservoir levels and then flush the sediment through the dam with a large volume of water(a costly task). To avoid this, some operators have tried (with limited success) to utilize turbidity currents to keep the silt in suspension throughout the reservoir, which requires a minimum flow velocity and duration.

    Some useful references:

    Morris GL and Fan J. 1998. Sedimentation handbook: design and management of dams, reservoirs, and watersheds for sustainable use.

    Palmieri et al. 2001. Economics of reservoir sedimentation. http://www.ncbi.nlm.nih.gov/pubmed/11381772

    Sloff, CJ. 1998. Modeling reservoir sedimentation processes for sediment management studies. Conference proceeding. “Hydropower into the next century”, Portoroz, Slovenia, 15-17 September 1997, p. 513-524, Aqua Media Int., UK.

  2. A really useful reference on sediment bypass and sluice gates (as well as other techniques for integrating e-flows into dams) is this world bank technical report: http://documents.worldbank.org/curated/en/2009/11/11510159/integrating-environmental-flows-hydropower-dam-planning-design-operations



  3. It isn’t my field, but there was a big dam in Puerto Rico that was built with penstocks to remove the sediment. They were never used and were eventually welded shut for fear that once opened they might not be able to close them. Apparently, there was a case of that happening somewhere in the world back in 1950’s. I’ll let you chase that down with the Bureau of Reclamation.

  4. I once saw a presentation on the Colorado river, where they release large qs of water at once to bring down more sediment and improve the ecology.


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