A spur dike, or wing dam, is a structure that spans only part of a river, while a dam, for example, spans the entire river.  Spur dikes are used to control the depth of a river by deflecting its main channel (Mioduszewski).  They can be used to decrease the width of the channel, thus increasing its hydraulic radius and efficiency at transporting sediment.

Spur dikes should be constructed along the sides of the river, at an angle of 90 degrees to the flow velocity.  They should be constructed of porous material, such as willow branches strapped together and layered, again, with quarried rock or gravel.  Willow branches are also available at various locations along the river and could be rafted down at a low cost.  These dikes should be above the surface during normal water levels, and be submerged during floods, allowing excess water to flow over them.  Since the channel deepens as you move towards the middle, the dikes do not need to be as tall as the required 45 ft depth of the shipping channel.  The sloping banks are 35 ft deep during average water levels.  So, the spur dikes should be built to this height (35 ft).  The dikes should be constructed every 2 miles along the river from Kenner, just upriver from New Orleans, until Chalmette, just downriver from New Orleans.  Between Chalmette and Pointe a la Hache in the lower Plaquemines parish, the dikes should be constructed every 6 miles.  Below Pointe a la Hache, there should be at least one crevasse, that would allow water to flow out during periods of high water, every 4 miles.  This will allow flood water to deposit sediment on the wetlands surrounding the main channel without immediately displacing the people settled on the high ground near the river levees. 

The following estimates of how the addition of these spur dikes 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:

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 numbers for the dimensions of the river channel at and below New Orleans are according to the US Army Corps of Engineers’ report on the Mississippi River Basin (Mississippi River Basin).

Dikes should be constructed in pairs, one on each side of the river, each one extending about 100 ft into the channel.  This restriction of the channel will increase the hydraulic radius of the river by approximately 30%, making it more efficient.  The average cross-sectional area of the river past New Orleans should decrease by approximately 6%, causing the velocity to increase by approximately 6%.  This small increase in velocity translates to a 40% 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 will be able to carry about 150,000 tons more sediment each day.  If the banks are armored as previously mentioned, this erosion will take place along the riverbed at a rate of approximately .002 ft/day.  The riverbed will gradually erode itself back to sea level in New Orleans, where it is currently several feet above street level. These dams will initially trap more sediment, thus further decreasing the sediment load that the river is carrying to the wetlands.  However, as the dams are built up with trapped sediment, the channel will be narrowed.  The hydraulic radius of the channel will increase, putting more pressure on the sediment particles on the significantly elevated riverbed.  This increased pressure will decrease the flow rate needed to erode the bottom, increasing the sediment load of the river and causing it to down cut into its own bank.  The river will gradually return to the level of the city.

The river banks need to be armored to prevent further erosion.  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.  This armoring is particularly important in New Orleans proper because of the increase in channel velocity the spur dikes will cause.

The river profile upstream of Kenner will also be affected by the erosion of the river bed below the beginning of the dikes.  The point where the bed erosion will begin, originally at Kenner, will gradually move upstream, as the gradient of the bed downstream steepens.  This upstream movement will stop at the Old River control structure, where it will begin to erode the 15ft difference in elevation between the Atchafalaya and Mississippi riverbeds (Angert 2002). The riverbed will stop eroding when it reaches sea level.  Since the mean elevation of New Orleans is below sea level, the river will always be slightly above New Orleans and the levees protecting the city from river floods will always be necessary.  However, as the river erodes its bottom, the levees will be able to handle higher and higher levels of flood waters and the city’s risk level for river floods will decrease accordingly. 

There need to be two main distributaries leading to the east and west of the current channel.  The levees currently along the main channel should be removed below the southern boundary of the Plaquemines parish.  Below the boundaries of this parish, the river will be allowed to flood its banks periodically, both natural flood periods, to deposit sediment in the surrounding wetlands.  The eastern distributary channel will utilize the current Mississippi River Gulf Outlet (MR-GO).  This channel was filled in with an average of about 15 ft of silt by hurricane Katrina (Brown 2005), and is 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 southern tip of Lake Borne.  This will end the funnel effect that worsened the storm surge from hurricane Katrina. A distributary from the Mississippi river should be directed into the remaining lower section of the MR-GO.  This distributary will redirect a maximum ¼ of the river’s volume (General Information About the Mississippi River), or about 103,000 ft³ of water during normal water levels.  The intersection of the previous MR-GO channel and the new distributary will be marked by a flood gate that will control the amount of water allowed to enter the channel.  This way, during periods of low water, more of the river’s volume can be kept in the main channel to ensure a navigable channel for ships. 

The western distributary will take advantage of the Wilkinson Canal that starts just south of Bayou Dupont, from within the Plaquemines parish.  This is a canal dug for the oil and gas industry.  However, the major oil field in this area can also be reached by river traffic via the Barataria Bay waterway, which also provides a more direct route to the Lafayette oil and gas field.  This canal also runs through several 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. 

We also want to rebuild the Chandler barrier islands to the northeast of Head of Passes using dredged material from Ship Shoal, the closer of two areas off the coast of Lousiana where sediment deposits are shallow and easily dredged.  Both of these areas are sites of former delta lobes of the river.  This dredged material will provide more protection to wetlands developing as a result of the MR-GO eastern diversion.  This eastern diversion will also supply an increasing amount of sediment to the islands as the wetlands are built seaward. The longshore current, which flows from east to west along the Lousiana coastline, will carry some sediment westward from the mouth of the MR-GO channel, building up the wetlands and reinforcing the coastline between the MR-GO and the main river channel.

Image detailing our proposal: (click for larger version)

Estimated Costs:

• Refilling the Mississippi River Gulf Outlet (MR-GO) from its connection with the Intracoastal Waterway to intersection with the Violet Canal:
• 1.47 billion dollars (Dredged Material)

• Spur Dike Construction along the Mississippi River:
•  8,775,000 dollars (Matanuska)

• Building crevasses along the Mississippi River in the Plaquemines Parish
• 80,000 dollars (Boyer 1997)