Spiveyman said:
If you have a long valley Vs. a short valley that you plan to flood, but the height of the water at the dam is the same, can the dam be the same size? In reading this thread I recall somewhere in the middle a discussion about that and the force being applied to the dam being a result of the depth of water, not the width of the channel.
I have written some other stuff in this thread about dams but i'll do it again: It's easier for me to write it again than for you to wade through more than 120 pages of this thread
Just one thing: i'm from the Netherlands, and us people use the metric system
pressure can be measured in PSI, Bar, Atmosphere. Atmosphere is like Mach: 3 mach is the speed of sound (311 meter per second?) and 3 atmosphere is 3 times the pressure of the atmosphere, the average pressure of the weight of several kilometers of atmosphere around us. YES ! air has a weight !! it aint light either, because it puts a pressure on us of 1 kg per square centimeter, which equals 1 bar or 1 atmosphere.
We dont notice that pressure because the pressure is all around us: from above, from the inside (your lungs) and from beneath: unless your feet are sucked to the ground by an absolute vacuum, which is practically impossible.
anyways, enough about the weight of air.
There is just 1 more way of expressing pressure: by meters of water column.
If you have a pipe of 10 meters high, filled with water, with a cap on the end that's sitting on the ground, the pressure on the bottom cap of that pipe will be 1 kg per square centimeter, which equals 1 bar or 1 atmosphere.
There's the law of.... Bernouille ?? or did i forgot his name. But the law of this ancient scientist says that the pressure on a pressurised vessel (like an air compressor vessel) puts a force in ALL directions equally.
So the pressure from the meters of water column presses with the same force aside, as the weight of the water on top of it.
This means that if your dam is 1 meter high, the water pressure on a square meter of dam surface varies from 0 to 1 bar, which mediates 0,5 bar on the first meter of dam.
This means the water is pressing with a force of 500 kg against the dam body.
A standard gravity dam (held in place by the gravity on earth or rocks, not by concrete constructions) for this 1 meter deep pond will need to weigh at least 500 kg per meter to hold the water in place.
there are lots of variables not taken into account, like the cohesion of the sand particles in the dam, or the case where the dam starts to get soaked with water.
I will not advise a safety factor here, as some people on this board think it is dangerous if i do so, which will get me into endless discussions like in Koop's bridge thread...

anyhow my profession is steel, not dirt
Anyways, you need to increase dam weight only for the depth of the water, and not for the water behind it.
as someone stated earlier in this thread, if that was true, boats would be deeper in a small pond than in a large sea...
