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The Mississippi Delta Gary Parker, University of Illinois

The Mississippi Delta Gary Parker, University of Illinois. From NASA. Before and After Katrina. From the New York Times. The river is diked along its entire length and is not allowed to avulse. Sediment is either stored in-channel or funneled out to sea.

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The Mississippi Delta Gary Parker, University of Illinois

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  1. The Mississippi Delta Gary Parker, University of Illinois From NASA

  2. Before and After Katrina From the New York Times

  3. The river is diked along its entire length and is not allowed to avulse. Sediment is either stored in-channel or funneled out to sea. From US Army Corps of Engineers website

  4. So the river bed gets higher and higher e.g. at the Old River Control Structure,

  5. The river mouth extends farther and farther into the Gulf of Mexico,

  6. and the rest of the delta subsides under compaction, with no replacement sediment, causing the shoreline to advance. From Scientific American

  7. Fan-deltas evolve as channels aggrade and avulse.

  8. Large, low-slope rivers co-evolve with their floodplains, with bed material load exchanging with the channel, and wash load exchanging with the floodplain.

  9. So how do fans and fan-deltas work? • Aggradation and elongation • Avulsion Kosi Fan, India

  10. What happens when humans interfere with this process by building dikes and preventing avulsions? From GoogleEarth

  11. What happens when humans interfere with this process by building dikes and preventing avulsions?

  12. The Kusatsu River forms a fan-delta on Lake Biwa, Japan which is densely populated. It has not been allowed to avulse since the 10th Century.

  13. Over geomorphic time the Mississippi Delta has been no different. The United States is so new, however, that there is no cultural memory of channel avulsions. Borrowed from C. Paola: original source not known

  14. The Mississippi Delta rapidly subsides by compaction under its own weight. • Under natural conditions this subsidence is balanced by overbank deposition of sediment abetted by channel avulsion. • The mud that would construct the floodplain is held behind levees and delivered out to sea. • Meanwhile the sand deposits on the channel between the levees as it elongates. • As a result, the levees and the prevention of avulsion is causing the shoreline to advance, not in geomorphic time, but in engineering time.

  15. “At this rate, New Orleans will be exposed to the open sea by 2090.”

  16. What parts do we need for the science and engineering? • Transport and sorting of mixed sizes of sand and mud. • Self-constructed, co-evolving channel and floodplain. • Subsidence under compaction. • Sea level variation. • Aggrading and avulsing channels. • Self-constructed deltaic region. • Option to place or remove levees at various times. • Option for partial diversions without complete avulsion. We have most of the parts, but assembly will be no easy job.

  17. For example, what effect would this controlled avulsion have on delta sedimentation and navigability of the river at New Orleans? Wax Lake Delta, Atchafalaya River

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