quote:
The maps on the master plan show most areas of Plaquimine and St Bernard with brown dots, meaning they are scheduled to be improved with sediment diversion which is supposed to build marsh.

This is how it works. Historically the missisippi river was a huge bird foot delta. The distributaries meanreded, which means changed course over time. The bends in the river would grow away from the inside curve (point bar) until the river would eat through itself cutting off the bend resulting in an oxbow. Also natural annual floods would bring more depositions in the form of silt and clays to the adjacent lands on the other side of the natural levees.

We have done two things to disrupt this natural replenishment and growth.

First we have built levees to contain the floods. This stops the anual deposition of fertile silts and clays that runn off the greater portion of the continental US, on the adjacent lands. This sediment remains suspended in the water collum between the levees, until some is deposites in the venice area marshes. The majority o fthese sediments are carried out to sea.

Second we dredge the river to maintain shipping lanes. This reduces friction from the stream (river)bed causing more of the sediments to remain suspended due to the higher downstream velocities of the river current. This results in even more sediments being carried out to sea, and further.

Opening diversions will result in the deposition of sediment. Thre more sediment deposited by these diversions, the less effective the diversions will be. This means the job is partially done. With a lower velocity of fresh water moving in due to the deposition of these sediments, the saltier brackish waters, driven by tides will kill off fresh water species of vegetaton giving way to species more sutible to estuarian (brackish) enviroments.

The opening of these diversions will mimic a natural process no longer in process due to intervention of humans. This is a cyclical process and more diversions are needed and should be open and closed periodically to allow sediment loads to be distributed as needed. This is an expensive and long term project, but the models (wax, davis, carnarvon(sp.)) show this works.

2 ...

Report Post

Posted by Deege

Member since Dec 2007

845 posts

Posted on 3/27/13 at 11:03 am to hardhead

It is a little more complicated than that Hardhead

LINK

"Operating the diversion consistently in
response to saltwater intrusion events
may help to
maintain marsh species composition
along the estuarine gradient while
increasing species
survival and rates of production.
Whatever benefits result from increases
in the inorganic
sediment supply should be weighed
against the impact of adding nutrients
that may weaken the
soils."

and

LINK

Per RnR poster Steve: "This report is a study of how much the roadbed of highway 23 in Plaquemines Parish has sunk in the last 25 years.

Go to the bottom of the report, and click on the link that says HTML slide show, then click on slide 13.

What you will see is that highway 23 in Belle Chasse has sunk 15 centimeters (6 inches) in the last 25 years. 6 inches doesn't sound like a lot, but it is when you are near sea level.

Down by Myrtle Grove to Empire, highway 23 has sunk 26 centimeters, or just over 10 inches in the last 25 years! That means the entire land and roadbed between the 2 levees has sunk that much.

Want the reason? It sure ain't the oilfield canals, is the LEVEES. And thats just in 25 years, what about the 50 years before that? How much did it sink then?

Hate to say this, but the only solution is to either get a couple dozens dredges in the river and start filling the marsh, or wait until the entire marsh ecosystem is destroyed and open up the levees WIDE OPEN, and not these little piss ant diversions. Suggesting anything else is like talking about fishing conditions when the boat is sinking..."

This post was edited on 3/27/13 at 11:06 am

2 ...

Report Post

Posted by dat yat

Chef Pass

Member since Jun 2011

4345 posts

Posted on 3/27/13 at 11:23 am to hardhead

I understand the basic geology of our area, the effect of the levees over the last century and what diversions are supposed to do. And your explanation is excellent.

My question was how the sediment load of the proposed diversions will be different than the two major diversion already in place.

0 ...

Report Post

Outdoor Board - Replies to Post

Return To Board

Page 1

Return To Board