Short Term
Long Term
Setting a Precedent


Long Term: Mississippi River
Written by Melinda Medlock

There are two main parts to the problem with the river:

  1. The wetlands are dying for lack of sediment
  2. The river is constantly depositing more sediment on its bed, building itself too high above the surrounding floodplain.  This is increasing the stress on the Old River control structure and increasing the elevation by which the river sits above New Orleans.

 The following proposal is structured around these two parts of the problem.  The number in parantheses next to each sub-heading indicates which of the two parts of the problem that section of the proposal is intended to solve. 

Distributaries (1)

We plan to construct two distributaries to build up floodplain suffering from lack of sediment.  Each will divert ¼ of the river’s discharge as measured at Baton Rouge USGS station during normal water levels, about 103,000 ft3 [National Parks Service, 2004].  A flood gate at the entry point of each distributary will control the amount of water that is allowed to pass into the distributary.  This gate can be opened wider during floods and partially or completely closed off during periods of low water.  The banks of each distributary will be armored to prevent widening of the channel and erosion of the surrounding wetlands.  Quarried limestone rock held in place with a wire mesh would be most effective and cost efficient, as limestone is mined at various locations along the river and could be cheaply rafted downstream. 

The Mississippi River Gulf Outlet (MRGO) was filled in with an average of about 15 ft of silt by hurricane Katrina [Brown, 2005], and will be accessible only by small, shallow draft vessels until dredged.  Rather than spending extra money to dredge this channel, it should be filled in from its intersection with the Intracoastal Waterway until the Violet Canal.  This will end the funnel effect that worsened the storm surge from hurricane Katrina.  Our eastern distributary will take advantage of the remaining channel below this point.  We plan to widen the banks of the existing Violet Canal to 300ft and deepen it to 30 ft to accommodate increased water levels.  This distributary will directly benefit the Breton Sound region.

There are currently two canals used to access the Lafayette oil and gas field: the Wilkinson Canal and the Barataria Waterway.  We recommend armoring the banks of the Barataria Waterway to prevent further erosion of the surrounding wetlands and establishing it as the main access canal to the field.  Our western distributary will take advantage of the existing Wilkinson Canal, establishing a uniform width of 300ft and depth of 30ft.  This distributary will directly benefit the Barataria Bay region, which includes many lakes and large pools of water that were formerly wetlands.  This means that the water here is shallower than water closer to the Gulf and will fill in with sediment faster than areas along the coast.  These shallow pools are also more protected than areas further down the coast.

Point of Entry (1)

Currently, the point of entry to the Mississippi river main shipping channel is at Head of Passes, where ships can enter from the either the east or the west.  In order to expedite rebuilding of the wetlands by the proposed sediment diversions, Buras should replace Head of Passes as the main entry point to the shipping channel.  Buras was chosen for the two existing canals that connect it to the Gulf from the east and west.  Both of these canals will be widened to 500ft and dredged to 50ft deep.  Their new banks will be armored to prevent unplanned channel expansion.  Current Gulf shipping channels that end at Head of Passes will need to be extended to Buras.  This mostly involves detailing the underwater topography of these new routes on navigation maps.  The depth of the Gulf between Buras and Head of Passes is not large enough to require large scale dredging to move the main shipping routes.  All levees south of Buras should be deconstructed.

A longshore current naturally carries sand along Gulf coast from East to West.  This current should help distribute sediment from both diversions over a wider area, including areas west of the actual channel.  This current, along with constant wave action, will also begin to erode the now unprotected bird-foot delta south of Buras, carrying the sediment westward into the Barataria Bay region. 

Crevasses (1)

Two crevasses should be cut in the earthen remains of levees between the southern tip of Plaquemines parish and Buras.  They should be spaced five miles apart and should speed up sediment transport to the floodplain.

Cutoff (1)

We plan a southern cutoff, below which no people will be allowed to live, unless on deep-water platforms like the oil companies currently use.  The main shipping channel will be maintained until Buras with dredging or the proposed wing dams described later.  However, nothing downriver from the Wilkinson Canal(on the main channel and western distributary), or downriver of the end of Highway 46(on the eastern distributary) will be protected from floods.  During any period of high water, the river will overflow its banks, depositing sediment on the surrounding wetlands.  Buras will be no exception; it is proposed as a checkpoint, not a port.  This cutoff will eliminate approximately 1/3 of Plaquemines parish, and the southern tip of St. Bernard parish.  In order to give people time to relocate, living outside the current parish boundaries could be disallowed immediately, and the boundary within the parish could be enforced 50 years from now.  This cutoff will allow the river to meander and change position, the natural process which the levees currently inhibit. 

Barrier Islands (1)

The Chandler barrier islands should be built up using dredged material.  This material could either come from Ship Shoal, the closer of two former delta lobes off the coast of Lousiana where sediment deposits are shallow and easily dredged, or the abandoned bird-foot delta south of Buras.  The eastern distributary will also supply an increasing amount of sediment to the islands as the wetlands are built out into the Gulf.  The strengthened barrier islands will provide more protection to wetlands developing as a result of the eastern distributary and also provide a line of defense against incoming storms.  Storms have always and will continue to regularly destroy these islands.  If the eastern distributary does not supply enough sediment to replenish them completely each time, dredged material may need to be supplied after major storms.  Whether and how much dredged material is needed can only be determined by continuous monitoring of the islands, especially after a major storm hits. 

Diversion etc.

Dredging (2)

The river is currently being dredged to maintain a channel navigable to ships with a draft of up to 45ft.  This is very costly and feasible only because of the economic importance of the river.  The main shipping channel should continue to be maintained by dredging while the following proposed solution is modeled in order to predict its effects and modify it accordingly. 

The rest of this proposal is only a suggestion for an experiment to model the behavior of the river when the following changes are made.  The aim of the experiment would be to prevent further sediment deposition along the riverbed and, at least partially, erode the current bed down closer to the level of the surrounding floodplain.  There are many regional and temporal factors that affect how the river will respond to the following changes.  Accordingly, we are not proposing that the following changes be implemented before their effects are tested on a model. 

Wing Dams (2)

A wing dam is a structure that spans only part of a river, while a dam, for example, spans the entire river.  Wing dams are used to control the depth of a river by deflecting its main channel [Mioduszewski and Maeno, 2003].  They can be used to decrease the width of the channel, thus increasing its hydraulic radius and efficiency at transporting sediment.  The current behind each dam will slow and its carrying capacity will decrease.  As sediment, mostly suspended clay particles, builds up behind the dam, it will become increasingly less porous, building the banks outward into the channel.  This means that the river channel will narrow and deepen.  The increased sediment carrying capacity and pressure on the bed will increase erosion, promoting a self-scouring process to bring the bed level closer to the current elevation of the surrounding floodplain.  Increased erosion rates means the existing banks, and potentially the river ends of the wing dams will need to be armored.  This armoring is particularly important because of the increase in velocity the wing dams will cause.

The wing dams should be constructed of porous rock rubble installed in layers.  Stone of quarry-run grade is available in mines upriver, in Illinois for example.  These rocks could be rafted down the river at a relatively low cost.  The crest of the wing dam should be built to the surface of the water at normal water levels, which is about 45ft from the bed as measured from the middle of the shipping channel.  The wing dams will be built on the banks, so their bottoms will be sloping to match the existing gradient.  Because they are only built to the water’s surface during normal water levels, they will be hard to see during normal and low water, and impossible to see during high water.  So, each dam needs to be buoyed to avoid becoming a constant boat hazar

    The angle that the upstream and downstream sides of the wing dam should make with the vertical depends on the natural repose of the type of stone used to build the dams.  This angle for dry materials generally increases with particle size.  (The figure below labels the angle of repose of the stone from one quarry in Illinois [Tisdale and Richetta, 1997]).  The top of the dam should be between 5 and 10 ft wide, depending on the cut of the stone laid in that particular spot.  Rather than require that all the stones be cut to exactly the same width, it is less expensive to require that all stones be cut to within a 5-10ft range. 


 The river end of each wing dam should make a 10-20º angle with the vertical.  The effect of a wing dam inclined at a slight angle and one that is normal to the current direction is the same.  The only reason to use slightly more material and incline the dam is to reduce the impact of the oncoming current and limit erosion of the structure on its river end.  The space allowed between successive dams should be between 1.5 and 2.5 times the length of the upstream dam, with a maximum of 3000-4000ft [US Army Corps of Engineers, 1997].  The actual lengths of the dams will vary with the existing geography of the region.  In order to allow two-way barge traffic, the channel should have a maintained width of 500ft from Buras up to the Wilkinson canal, 650ft from the Wilkinson canal through the Port of South Louisiana, and 500ft from the Port of South Louisiana through Baton Rouge.  (The figure below labels the proposed width of the main channel through New Orleans.)  The wing dams should be staggered on either side of the river to


 The shipping channel from Buras through Baton Rouge should be maintained at a 50ft depth because it needs to accommodate ships with a draft up to 45ft.  The sloping banks depend on the geography of the region, but the average depth of the bed from land to the edge of the main shipping channel should be about 35ft.  This depth will probably be altered by the addition of the wing dams, so the depth profile of the river should be constantly monitored. 


Modification Estimates (2)

The following estimates of how the addition of these wing dams would affect the flow and sediment transport rate of the river are based on several basic assumptions.  We assume that the water discharge of the river is held constant, that is, the average annual volume of water flowing past New Orleans, does not change and varies with the following equation:

            D = A x V [Nelson,2006]

where D is discharge in ft3/s, A is the average cross-sectional area of the channel in ft2, and V is the average velocity of the current in ft/s.  We also assume that the sediment carrying capacity varies with the sixth power of the velocity [Divener, 2006].  Our approximations for the dimensions of the current river channel at and below New Orleans are according to the US Army Corps of Engineers’ report on the Mississippi River Basin[US Army Corps of Engineers, 1997]. 

This restriction of the channel width could potentially increase the hydraulic radius of the river by 20%, making it more efficient. 

            (HRnew - HRold) / HRold ≈ .2

The average cross-sectional area of the river past New Orleans should decrease over time as the bed is eroded.  This will cause the velocity to increase.  Even a small increase in velocity translates to a large increase in the sediment capacity of the river, assuming the discharge of the river remains constant.  This increase in sediment carrying capacity means that the river could possibly carry about 150,000 tons more sediment each day.  If the banks are armored as previously mentioned, this erosion could take place along the riverbed up to a rate of approximately .73ft/yr.  The riverbed will gradually erode itself back towards sea level in New Orleans, where it is currently several feet above street level.  These numbers are only rough estimates; a model needs to be built to accurately measure these benefits.

River Profile and Old River (2)

The river bed is currently very high above sea level all the way out to Head of Passes, where the levees stop.  The southern cutoff (see map above) shortens the maintained channel considerably, allowing the river to reach base level, sea level, sooner.  This will alter the river’s profile up through the first barrier, namely the Old River control structure.  Because it will have a more direct route to the Gulf, our modifications to the channel should prevent any additional deposition and allow scouring of the existing bed.  Erosion of the bed down closer to sea level should decrease the height difference between the Atchafalaya and Mississippi river beds at the Old River control structure.  If this does start to happen, sediment that has built up behind the Old River control structure should be dredged to match the changing elevation of the bed of the lower Mississippi.  This will decrease the strain on Old River to maintain the current ratio of Mississippi flow directed down the Atchafalaya.  It will also increase the capacity of the structure for handling floods. 

Since the mean elevation of New Orleans is below sea level, the river will always be at least slightly above the city and the levees protecting the city from river floods will always be necessary.  However, if the river does erode its bottom, the levees would be able to handle higher and higher levels of flood waters and the city’s risk level for river floods would decrease accordingly.