the mississippi delta gary parker, university of illinois

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

The Mississippi DeltaGary Parker, University of Illinois

From NASA

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

Before and After Katrina

From the New York Times

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

and the rest of the delta subsides under compaction, with no replacement sediment, causing

the shoreline to advance.

From Scientific American

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

Fan-deltas evolve as channels aggrade and avulse.

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

exchanging with the floodplain.

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

So how do fans and fan-deltas work?•Aggradation and elongation•Avulsion

Kosi Fan, India

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

From GoogleEarth

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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.

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

•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.

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

•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.

What parts do we need for the science and engineering?

We have most of the parts, but assembly will be no easy job.

National Center for Earth-surface DynamicsNational Center for Earth-surface DynamicsENABLING LANDSCAPE SUSTAINABILITY

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