Rising Water, Flood Stage, Dams, Sluice Gates and Flood Control

Posted by PITHOCRATES - February 27th, 2013

Technology 101

We have spent much of our History trying to Tame the Awesome Power of Water

Water can be scary.  And very powerful.  Which helps with it being scary.  We saw what happened when that storm surge hit the East Coast.  It just swept everything in its path.  For water has mass.  Making it heavy.  Just try holding a couple of buckets of water with your arms outstretched.  You won’t be able to hold them up long.  Now think about the weight of a few billion buckets.  An amount no one could move.  But there are few things this amount of water can’t move.  Except maybe a levee.  A floodwall.  Or a dam.

Also making water scary is that you don’t know what is lurking beneath the surface.  During periods of heavy rains storm sewers quickly fill to capacity.  Water backs out onto streets.  Flooding intersections.  And basements.  Streams and rivers rise above their flood stage.  And overflow their banks.  Water saturating the soil may wash it away from underneath.  Creating large sink holes.  That from the surface may look like a puddle of water.  Water overflowing riverbanks can hide many dangers.  Submerged debris that can entangle you.  That have swift and dangerous currents flowing through them.

Rising water can get into areas where it doesn’t belong.  It got into the subway tunnels in New York.  Causing a lot of damage.  It got into the basement electrical rooms at the Fukushima Daiichi nuclear power plant in Japan.  Causing a lot of damage.  Including a partial meltdown of the reactor core.  A failed levee can flood a city.  Like New Orleans.  So untamed water can do a lot of damage.  And we have spent much of our history trying to tame the awesome power of water.

Each Spring the Snows melt and the Rains come Swelling Rivers beyond their Flood Stage

Early cities rose on rivers.  For rivers were our first highways.  The river’s current turned water wheels to power our mills and factories.  Provided irrigation for our farms.  Etc.  Rivers allowed cities to come to life.  Which is why a lot of our cities today have rivers flowing through them.  Yes, a river view is beautiful.  And the recreational opportunities are plentiful.  But they are not why we founded these cities on rivers.  It was all the benefits the river provided.  Things that allowed a civilization to grow.  But it wasn’t all good.

Each spring the snows melted.  And the rains came.  Swelling rivers beyond their flood stage.  Overflowing their banks.  Bringing great damage to life along the rivers.  Especially to the towns on its banks.  So we did something with these rivers that were prone to such damaging flooding.  And built a dam upstream.  To control that flooding.

They would choose an appropriate location upstream.  Some place where the river valley narrowed a bit.  So they could build a dam across the valley.  Once they did the water upstream of the dam rose into a lake or reservoir.  Providing a source of drinking water.  Irrigation water.  Recreation.  Or power generation with a hydroelectric dam.  Very beneficial things.  But all secondary to its main purpose.  To eliminate that recurring flooding.

A Dam’s Sluice Gates are the Key to Flood Control

If you’ve ever seen a dam on a river you probably noticed some things.  Turbulent water at the base of the dam on the downstream side.  Warning signs and some sort of a barricade (such as a chain stretched across the river held up with floats) a hundred feet or so in front of the dam on the upstream side.  Signs you would be wise to heed.  For great danger lurks beneath the surface of the water.  In that dam are underwater openings.  That have moving gates to make these openings bigger or smaller.  Sluice gates.  And you don’t want to be anywhere near these gates whenever they’re open.  For the weight of a few billion gallons of water creates a powerful force of water moving towards those gates and through the openings.  If you ever thought of diving off a small dam don’t.  You would be sucked quickly to these openings.  If they are not opened enough for your body to fit through the force of the water would hold you against the openings until you drowned.  If the opening is large enough the water will flush you through with great force and violence.  Discharging you into the turbulent water on the downstream side of the dam.

These gates are the key to flood control.  During the snowmelt runoff and heavy rains of spring we can close these gates to allow only a trickle of water flow.  Maintaining a safe river level downstream.  The excess snowmelt runoff and the rains will fill the lake or the reservoir upstream of the dam.  After the rains stop they can open the gates a little more to bring down the level of the lake or reservoir.  Without sending the river downstream above its flood stage.  If the level rises too high behind the dam the water will enter a spillway and flow over/around/through the dam.  Like an overflow in a sink.  Allowing the water to rise only to a maximum level behind the dam before spilling over/around/through the dam.  Joining that turbulent water on the downstream side.  Which you want to avoid as much as the dangers on the upstream side of the dam.

We haven’t always been successful in controlling the awesome power of water.  Some dams we’ve built have failed.  Like the Teton Dam in Idaho.  An earthen dam.  Just upstream of Wilford.  Built for flood control.  To protect the towns and farmlands downstream of the dam.  As it turned out, though, the Bureau of Reclamation did a poor job building the dam.  And the rains were heavy that year.  Raising the level behind the dam 3 feet a day instead of the designed 1 foot.  Water started leaking through the dam.  Saturating the soil making up the dam.  The water rose rapidly.  But before it could reach the spillway the dam gave way.  Sending some 80 billion gallons of water rushing downstream.  Wiping out Wilford.  And destroying most of Sugar City.  And Rexburg.  Causing damage as far away as 30 miles downstream in Idaho Falls.  Illustrating the awesome power of water.  And the price we pay when we don’t give it the proper respect.



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