why can't they dredge dams with suction tubes?

I hear dams have a problem of filling up with silt.

Since they already got built in water pressure, why not include(or install) a through-fitting on the dam face (I"m thinking about 2ft wide) that would accept a big hose(on the water-side) which could then be used to dredge silt when the dam had excess water?

 

Now that's outside the box thinking.
So you would turn on the thru fitting, place the hose near the silt so the water flow would pick it up?
Sounds like you would have to have someone controlling the pickup point to keep it near but not plugged by the silt.

Hopefully that person would be on the surface some how.

I suppose you would not need a thru fitting, just run a siphon tube over the dam with the outlet at the base of the dam.

 

It would be far cheaper to use a powered system than to modify an existing dam.

Personally, for getting rid of silt near a dam I think a tethered submersible, power coming through the tether, would provide considerable saftey and comfort for what would be long, tedious shifts. Topside you could empty the hose, also connected via the tether, into waiting dump trucks from the surface tender(s). I imagine that the technology used to pump concrete could be adapted. These would be have to be transportable to be economical, moving from job to job (though jobs would probably not be short affairs). That way the cost to any operating dam would be less that a full system of its own over time.

The alternative to keep a whole reservoir from silting up is, of course, is to simply employ dredging barges which are well sorted out technologies. Wouldn't be surprised if some were already in use on some of our larger reservoirs.

 

Yes a dredging barge setup seems like it would work best.

You also have to keep in mind that in most cases the dredge spoils would have to be contained and not allowed to run down the river, although if it was a continuous process the amount running down river wouldn't be much different than if the river was not damned to begin with.

 

Yeah, always seems cheaper to use a 'powered' unit as Runudyne says. It just does what you want, when you want. But I'm not sure a barge and crane would be cheaper for a large amount of silt from a large dam. Just the wages for that operator pulling each tiny load up from a few hundred feet down seems problematic. I'm talking about moving "hydraulic mining" amounts of material.

Figure the fitting would be installed during a low water season, or whenever the dam starts becoming near useless and can be taken out of action for a couple seasons. IIRC most semi-modern dams already have some type of through fittings to siphon off 'city water'. Just use one to 'restore' the dams capacity over a few years.


Anyway, I was thinking a big hose would be attached to the fitting on water side of dam, and on the other end would be a big pipe about 2' diameter and maybe 20' long, with a bunch of 3" holes along its length. It would be held more or less vertical by a float, which in turn would be more or less 'anchored' in place by the pipe, but could be winched or towed to new location every so often along an arc from dam's water side face. But mostly it would dredge a 'lowest spot' and rely on silt finding its way to that spot.

The fitting would be about 40' below high water mark, and that would provide pressure even if the pipe was down several hundred feet.

It would try to mimic natural silt flows down river, since it would be most active at highest water, and mostly send 'fresh' silt down river from upstream sources in real time.

At low water the pipe could be winched up off the lake bed so it wouldn't get clogged sitting in silt without water flow.


A big issue would be "OK, now we are CHANGING some fundamentals of this dam's underpinnings by 'suddenly' removing a few million tons of supporting material from near the dam's interface with the earth, with a big city just down stream". I guess you wouldn't want to dredge too close to dam, even if that was where lots of silt lay. Put some 'swirlers' inside the semi-ridge hose to prevent silt from settling and clogging during the few hundred yard journey to outlet.


Siphon tubes only work for up to 32', and are weak at anything close to that. That would work OK for lots of smaller dams. Most dams have a Spillway that automatically comes into play to prevent overflowing. Just lay the hose over that and have 32' worth of water pressure even at the bottom of a deep lake. Should be enough for most de-silting. Have a shutoff at 50ft below the crest on the outlet side and a filler cap at the crest. Just close the outlet side, fill the crest-cap, close the crest cap and open the outlet to 'prime' the suction.

 

So, you take the silt from behind the dam and clog the river downstream. The problem only moved a few feet. The silt needs to be taken somewhere else and dumped.

 

Actually they are dredging a local lake right now.
A local "soil" supplier is taking (buying?) all the dredged material and selling it as garden soil. Heavy in lots of organics. All the good topsoil that has washed down from probably 100 sq miles.

 

you can not "suck" water from too deep or you vaporize the water due to vapor pressure, pump will not work. with silt and sludge, it takes that much more vacuum to get it move though the pipe. So it would only work in very shallow water.

 

You can "suck" water from any depth with a pump at the surface. The maximum height above the surface water can be sucked from is limited by cavitation.

 

No doubt studies are done on the feasibility of this in various situations, near to me the idea was dismissed as impractical, costing too much for what would be gained in extra storage capacity. Presumably in some cases, raising the dam wall would be more economical.

 

Have you looked at the volume of silt that is deposited by some of the major dams and the fact that the silt is deposited over the bottom of the whole upstream lake? Your hole is only in one place and that is near the dam. It would surely work but might not actually make a dent in the total amount of silt.

Consider the Colorado river or the Mississippi deltas where the silt was deposited over the past. These contain so much silt that it might not possible that any reasonable process could cope with the volume even if it could be gathered up. On our coast we have dredges working full time to even maintain a small navigable channel in active areas.

 

Sediment is (mostly) brought into a reservoir from the rivers and streams that feed it. Since the sediment is (mostly) heavier than water the only reason that it can be transported by the water is because water is moving rapidly and is turbulent. As soon as the water enters the lake/reservoir its velocity (usually) drops significantly, and so it drops its load of sediment at the mouth of the stream/river, not (necessarily) far down the lake where the dam is. Eventually, the sediments will reach the dam, but simply removing the sediment closest to the dam may not affect the bulk of the deposited sediment.

Sedimentation Problems with Dams | INTERNATIONAL RIVERS.org

 

It would create a hole in the deepest spot of sediment, and neighboring sediment would tend to fall towards that hole. Moving higher sediment to lower spots deep under a lake wouldn't be much of a problem. Just winch a big 'sled' or chain(chain with its ends on two winch cables, for a total of four land stations) from one side the other and back again.

I did mention the hose and end pipe would be held basically vertical by a buoy which in turn would be winched or towed to new locations every so often.

The sediments ejected just downstream of the dam wouldn't be moved 'just a few feet', they would more or less do what they did before the dam existed and travel all the way down stream(maybe stopping on the river banks here and there for a few seconds or few hundred years) to eventually form a river delta. This apparatus would be used during heavy river flows when there was excess water. The river wouldn't be 'clogged' downstream, it would only a be a certain(controllable) amount 'browner' than its normal rainy season fast running brownness, and more like its pre-dam natural brownness. This wouldn't be dumping 50yrs worth of silt in two months, it would be 50yrs worth over 10yrs of wet seasons.

Tell the eco-freaks it is restoring natural sediment/nutrient flows downstream, etc., which would actually be the truth. If no one wants to buy my invention for their own rational water management needs, I'll get some eco-freak lawyers to force them. JACKPOT! The NEW "American Dream"

I'm thinking a 'over the spillway' would be way to go, and be mostly gravity suction powered with possibility of "helper" pump with powered by 'grid' electricity during nighttime off peak. Like if the reservoir was a little low but you knew the rainy season was coming, and wanted to capture more of it.

If you wanted to get cute or 'elegant' or 'green'(spending lots of money and resources off-site so the dumb public doesn't see true costs) you could run the helper pump off neat little custom 'inline' generator units at the end of longer downstream hoses, depending on terrain.



LAST BUT NOT LEAST.....

Since the suction at the bottom of the pipe will be 32ft high of water weight (minus friction of water flow), regardless of how deep the reservoir was when it STARTED...I don't see why this couldn't be used to (gradually) dig a very big and deep reservoir behind a fairly small dam, as long as you dug far enough away from the dam/earth intersection, and do it all "for free" with only the cost of a big hose, a perforated end pipe, and a few winch cables.

 

It would be of great benefit if you were to look at these wonderful ideas with a realistic appraisal. Take the Colorado River for instance, By the time that water carrying your sediment to the ocean gets there, all the water has been transported elsewhere and the river does not exist anymore, so where does that sediment go? Any such fanciful scheme must be supported by economic benefit or it is DOA. This idea is no more promising than that submersible airplane you think is communist plot and probably offers less useful fallout.

Read Collossus, a good book by Michael Hiltzik describing the building of Hoover Dam as well as how it affected the flow of the Colorado and downstream areas like the Salton Sea and The Sea of Cortez. It will give some idea of the scale of things to consider when simple ideas are proposed to change nature..

 

today the 'movement' is towards doing intentional 'pulse' releases to restore some of the aspects of the natural river.

http://news.nationalgeographic.com/...elta-pulse-flow-morelos-dam-minute-319-water/

it is true that many if not most dams in California (or feeding CA like Hoover) haven't seen max capacity for years if not decades, just from ever increasing water use by exploding population.


I was thinking mostly of the modestly sized dams in my SF bay area, which have seen max capacity a few times in living memory. http://www.valleywater.org/Services/Reservoirs.aspx

I'm guessing most of these have pretty high silt problems because they are among steep hills and only feed sporadically during heavy rain storms.