Water passing the dam for the purpose of maintaining a depth of water behind the dam would be drawn from the bottom of the dam. Water drawn from the base of the dam would carry silt and debris past the dam and thus prevent the loss of reservoir capacity due to silt buildup. Drawing from the base of the dam would be done by having open pipes on the upstream side of the dam extending from the base to the nominal spillway level. Water would have to pass through these pipes to go over the spillway. These pipes could be designed to be pulled up for maintenance and cleaning. Machinery for clearing the pipes of debris while in place would be useful. Water for power generation would continue to be inducted from near the surface where the water is cleaner.
-- hangingchad, Dec 30 2003

A spillway http://www.nae.usac...pillway release.jpg
Most often on the edge of the dam face as a release for water when levels become too high. [Klaatu, Oct 21 2004]

Dam gates http://www.pastfoun...ots/HugoLakeDam.jpg
[Klaatu, Oct 21 2004]

Longitudinal Plans of the Hoover Dam http://xroads.virgi...n/hoover/pre05.html
[hangingchad]'s link as a link. [phoenix, Oct 21 2004]

Three Gorges Dam Project http://www.criengli...3-6-6/21@17804.htm/
Chinese scheme to lessen silt buildup [hangingchad, Oct 21 2004]

If the spillway is at the bottom of the dam, then how would the reservoir ever fill? Are you talking about the dam "GATES" being at the bottom to pull excess silt from the reservoir?

Most of the heavier silt occurs at the top of the reservoir's lake where the heavier materials silt out. Generally at the dam face is the finer silts which are flushed by the turbine intakes which are located at the bottom of the lake.

If you have any seasonal reservoirs, you can see the effect as the lake drains in the dry season. The greatest amount of silt is where the turbid water enters the lake and the velocity slows.
-- Klaatu, Dec 30 2003


Here is the layout of Hoover Dam. http://xroads.virginia.edu/~MA98/haven/hoover/pre05.html If you will study this you see that the two spillways, the Arizona and the Nevada spillways, limit the maximum level of Lake Mead by allowing a place for surface water to overflow. You can also see that the water for the turbines comes from four towers a short distance from the face of the dam and that the water comes from the surface level.
-- hangingchad, Dec 31 2003


Guess I'm not very good at describing my concept. The pipes would not go through the dam or act as a siphon. They would merely ensure that the water going over the spillway came from the base of the dam. When the reservoir was at full capacity and the river was in flood more silt and trash would go over the spillway with this scheme. I'd suppose a surface spillway would still be of use to let the floating trash get past the dam.
-- hangingchad, Dec 31 2003


I think his idea is to have a pipe take the intake water (bottom) to the top, on the other side of the dam. This would stop working when the level of water in the resivoir reached the outlet level (well, it'd praticaly stop before that...). To phrase that differently, hole in side of wall conected to pipe leading to bottom. - sipon without down part.

And hangingchad, what's wrong with a closable hole in the bottem of the dam?
-- my-nep, Jan 01 2004


to my-nep - I think you have the idea of the pipe fed spillway. There's nothing inherently wrong with a closable hole in the bottom of a dam. If the hole is down 400 ft the pressure would be around 190 psi. Water at this pressure would blast through the dam carrying anything movable with it. That part is good because the pressure would be sufficient to move silt, sand ,gravel, stones at the base of the dam. However, blasting silt and sand and gravel through a valve of any sort is going to erode it. I think maintenance is going to be a problem. If the hole cannot be closed -- you have a hole in the dam. The chinese are smart people and will most likely make it work. The advantage of my pipe fed spillway scheme is that: (1) it could be easily retrofitted to an existing dam (2) when the river is in flood it would carry the most heavily silt laden water over the dam (3) when not in flood light silt settling at the base of the dam would be carried over.
-- hangingchad, Jan 02 2004


On a dam used to generate electricity drawing spillway water from the base of the dam instead of from the surface water would tend to leave more clean water (surface water) to go through the turbines possibly lessening wear. Another possibility with the bottom fed spillway is that a program of stirring up or agitating the existing silt in the impoundment would allow it to be mixed with the water and thus to be carried over the dam. Over a period of years a silted up dam could be restored or a new dam could limit it's silt buildup.
-- hangingchad, Nov 18 2004



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