Posts Tagged sediment trap

Mississippi Flooding and the Atchafalaya spillway

News sources are quoting a figure of 1.5 million cubic feet per second for the current flow of the Mississippi River at the Morganza spillway. That is a lot of water!


To get a visceral idea of how much water that is, my brother Mike from Sandy, Utah sent me some calculations describing how the flow at Morganza (above Baton Rouge) compares to the flow of the Colorado River. 1.5 million cfs is equal to 34.43526 acre feet / second. By comparison, the total capacity of Lake Powell is 24,322,000 acre feet; currently 53.99% full, available capacity 11,205,145 acre feet. The Upper Colorado Basin is supposed to provide an average of 7.5 million acre feet each year past Lees Ferry, Arizona. On Wednesday (May 11, 2011) the flow into Lake Powell was 31,862 cfs, and outflow was 14,791 cfs. Little Cottonwood Creek flood stage is around 500 cfs (coming down into Sandy from Alta and Snowbird).


Lake Powell capacity and percent full numbers are taken from Lake Mead capacity and percent full numbers are taken from from


So at 1.5 million cfs, the Mississippi would deliver the annual flow past Lees Ferry in 2.52 days, would fill the currently available storage in Lake Powell in 3.77 days, and would then fill the available capacity in Lake Mead in another 4.96 days. That’s a lot of water! If I could somehow capture and store 1 second of the current Mississippi flow, I could easily water my yard for the rest of my life. And they would never miss it!


You can read more about this at “The Control of Nature: Atchafalaya”: (February 23, 1987):

That link is an old article by John McPhee about attempts to control nature, which is probably getting a lot of web traffic these days. If the Old River Control Structure were to fail, most of the flow of the Mississippi would flow down the Atchafalaya River, taking a much shorter route to the sea and leaving New Orleans and Baton Rouge along a backwater channel.


The interesting thing to Mike is that in the long term, the Mississippi Delta is eroding and subsiding while most of the sediment is funneled into deeper waters in the Gulf of Mexico. He further observes that to prevent the loss of the Delta and associated flooding from the ocean (especially during hurricanes), they should allow flooding from the Mississippi River to deposit sediment (mud) on the land, and probably need to let the Mississippi River take new channels to the Gulf every few hundred years.


And he’s right. That long lobe of the Mississippi delta past New Orleans is called a “bird’s-foot delta” because it looks like one. It’s unnatural for a delta to form like that, and it’s a consequence of 100 years of levees upriver and in the delta itself. He’s right – all the sediment is going to exactly the wrong place.


There is a way to fix this.


The Morganza diversion needs to be a sediment trap:

We need clean water to flow down the Mississippi to New Orleans (thereby eroding and deepening the built-up channel), and muddy water to pass down the Atchafalaya (thereby building up that lower-elevation lobe). How can we separate the muddy water from the clean water? Build a sediment trap!


The Army Corps of Engineers needs to create a small lake at the point of the diversion. When muddy water flows into a lake, the mud settles to the bottom as sediment. Clear water flows beyond that inflow delta, and eventually flows out of the lake. This is why Lake Powell is getting filled up with sediment in its upper reaches, and why the Grand Canyon is sediment-starved. The lake should have two major outflow pipes: one positioned at the lake bottom close to the inflow, and the second positioned near the lake surface at the downstream end of the lake. The “bottom near the inflow” pipe feeds sediment-laden water down the Atchafalaya river. The “surface downstream” pipe feeds clear water down the Mississippi. After decades, and some big floods like the current one, the Atchafalaya lobe will build up and the New Orleans bird’s-foot will erode away.


The Morganza spillway already has a stilling basin, but that’s designed to dissipate energy, not separate out the sediment.


Yes, it will cost a lot of money to build the sediment trap, but the alternatives are a lot more expensive. What is the cost of relocating New Orleans and Baton Rouge? We are hoping to accomplish here a controlled stream capture and relocation of the sediment load. By the way, the natural event when a river distributary carves a new channel is called a “crevasse event.” The river catastrophically breaks through the natural levee and forms a crevasse where the new channel pours out. A lot of people in the John McPhee article were terrified of a crevasse event happening on its own.


We need a sediment trap and decades of patience to let it work.


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Leaf Boats and Sediment Traps

Last week we had a big thunderstorm pass over the house during the evening, and my two boys (ages 10 and 6) put on their raincoats and went out into the front yard to see what was going on. They love to send leaf boats down the edge of the street when it rains. When I came out to check on them, they had piled up some landscaping gravel in the gutter to make a dam, causing a large puddle of murky rainwater to collect in the street near our driveway. The two of them were really excited about their civil engineering: “Look, Dad! We made a dam! But our lake keeps rising and flowing around the dam, so we need more gravel!” I grinned. This arms race continued until their dam started to leak through the gravel, kind of like the turbines at the bottom of Glen Canyon Dam. “What? You guys aren’t generating electricity?”

They noticed that the water coming into their little lake was murky with silt, while the water seeping out through the gravel was clear and clean. “Yep, you guys have built a sediment trap“, I explained. “The fast-flowing river water carries dirt with it, but in the calm lake water the dirt drops out to the bottom. That’s why it’s called a sediment trap, because the river sediment can’t get past the lake.” “Wow! We built a sediment trap!” they exulted. “Sediment trapping is a big problem for silty rivers like the Colorado and the Nile“, I elaborated. “The Glen Canyon Dam and the Aswan High Dam trap the river silt and prevent it from flowing downstream. That’s why the Grand Canyon and the Nile Delta are losing their sand.” [Yes, sometimes I talk in html.] The boys were thrilled to hear the world-wide implications of their little experiment.

After the rain we put the gravel back around the mailbox and washed our hands. And I thought to myself, What a great neighborhood / country / life I have! My kids can play safely in the yard, forget their video games, get dirty, have fun, and learn about fluid mechanics all at the same time!

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