green_achers wrote:Tanada wrote:gg3 wrote:As for the silting problem, couldn't that be controlled by simply dredging the stuff out every so often?
More than that, if you are dredging the dam area anyway you can set up a pump dredge and recover all that lovely silt. A portion of the silt can be expelled downstream to keep the channels relatively healthy and the rest can be sold as soil amendment for all sorts of organic farming uses.
Well, then that brings up the design function of the dam. If it's primarily a hydroelectric project, or a "multi-purpose" structure in which a large part of the cost/benefit ratio depends on hydroelectric generation, then the energy cost of dredging has to be taken into account. Something tells me it takes a lot more energy to get silt out of a resevoir than it took to get in.
Silt goes in under solar power i.e. rain washes it into the river bed and then downstream. Getting it out mechanically involves physically lifting it from the half mile or so behind the dam up to the lake shore where it can be drained and then shipped out. A simple containment structure/drainage area see
CDF1 and
CDF2. Warning the first link has a lot of pictures of the many sites in use today in my area.
Obviously it does take energy to lift the dredging's the 1000 feet or so from the water channel bottom into the CDF dike area but if it takes more that 1% of the energy produced by the dam I would be shocked. Basically all you need is a high capacity pump that tolerates muddy water, once the muddy water is in the CDF passive filtering and drainage will put the water back into the lake. If you are slightly smart you build a two cell facility, when the firt cell is full you switch to the second and start hauling the dirt out of the first cell via train or truck for sale as topsoil amendments or fill dirt for construction/land reclimation projects. Worst comes to worst you just ship it down stream and use it to restore natural silt islands below the dam site.
So yes, the silt goes in through natural deposition and doesn't on the face of it cost humans anything so it does cost more to remove the silt than to put it in. In fact you can claim it costs infinitely more even if it costs $1.00 because it didn't cost anything to start with and dividing by zero gives you infinity....
It will cost money to build a CDF for each hydro dam and deal with the silt if the intention is to recover the soil. It would cost much much less to set up a mud pump with the sole purpose of sucking up the mud/silt and expelling it down stream. The dredge in that case simply pulls the mud up and dumps the muddy water into a half pipe feeding down ito the river channel on the downstream side of the dam. No fuss, no muss, you are restoring the natural flow of silt downstream without trying to recover the silt for other uses. You save the cost of all those CDF related operations. The simplest way to do this is to open the dam gates slightly during low demand periods and let natural pressure force the mud through the bypass system. To be effective you would need to do this for say an hour a day every day from midnite to 1 AM, but a lot of ranchers/farmers ect ect depend on the water in the lake and object to their congress persons about such a plan because it 'wastes the water' they want to buy.