Subscribe to RSSCoastal sediment needs – let’s get real, folks!
Sheila Grissett wrote an eye-opening story for the August 1 Times-Picayune on the gigantic volume of clay and concrete ingredients for levee bolstering that will be moved into NOLA during the next several years and the impact on highway congestion, air quality, etc.
Grissett reported that 30 million cubic yards of sediments of various types will be required to upgrade the NOLA levee system to the 1% flood risk level by the target date of June 1, 2011. She compared 30 million cubic yards as equivalent to six times the volume of the New Orleans Superdome. A cubic yard of sediment is roughly equivalent to a ton.
In other words, late Louisiana Governor John McKeithen’s iconic legacy structure would theoretically hold five million tons of sand, silt or clay. I propose that future discussions of coastal sediment needs adopt the easy-to-remember 5 million ton Superdome equivalent (SDE) as an official volume unit, which is far more evocative and memorable than a plain number.
Imagine this space sans fans and brimming with 5 million tons of clay
In a post on June 29 The trouble with Harry I described the conspicuous absence in coastal planning of the world’s leading expert on the Mississippi delta, Dr. Harry Roberts. This Boyd professor and former director of the LSU Coastal Studies Institute is internationally known as an authority on the major deltas of the world but he has been relatively unrecognized by the lay community in Louisiana.
For years I have groused about this anomaly, hoping to arrange for an appropriate venue and audience for Harry. His perspective on the feasibility of rebuilding the Mississippi delta should be of the highest importance in coastal policy discussions but in my judgment the communication gap between science and policy remains a huge problem.
This disconnect was brought to a head recently when Harry and his colleague Mike Blum released a new technical report that clearly shows a serious deficit in riverborne sediment needed to restore the coast. Blum and Roberts’ projection of the skeletal remains of south Louisiana in the year 2100 is shown in the accompanying graphic. 
This stark report generated considerable media attention, which caught the local planning community by surprise.
Thus it was that on August 19 Harry Roberts took center seat at House Committee Room 6 at the State Capitol and presented his assessment to a non-scientist audience – the members of the Coastal Protection and Restoration Authority (CPRA).* Amy Wold described the occasion in the August 20 Advocate. I saw Harry’s presentation as an historic event and I’m curious to see the residual effect, if any, of his message, which contrasts markedly with the common mantra that all we need is money to save the coast.
Ironically, on the morning of Harry’s talk I had made an ill-timed effort to improve my auditory acuity with an ear syringe. This act left me temporarily deaf but I paid close attention to his slides from which I gleaned the following bottom line:
By the year 2100 (less than five generations from now) the Mississippi delta plain will have suffered a total net deficit of from 18-24 billion tons of sediment, or from 200 to 270 million tons of sediment each year. Furthermore, that figure includes only the first (and the least discussed) of the following three urgent sediment needs in south Louisiana: (1) infilling** to offset delta-wide subsidence, sea level rise, wetland deterioration and barrier shoreline erosion; (2) levee construction; and (3) marsh creation to mitigate damage caused by these levees.
From 200 to 270 million tons of sediment per year represents from 40 to 54 Superdome equivalents! That number dwarfs the six SDE’s that Sheila Grissett says will be needed during the next two years to upgrade NOLA levees. Now add in the sediment represented within the next few years for projects such as Morganza to the Gulf, Donaldsonville to the Gulf … plus an unknown volume needed to mitigate for damages done.
Pardon me but isn’t this the time for a reality check?
Len Bahr
*Harry told me that his invitation to speak came from King Milling (CPRA member; Chair, Governor’s Coastal Advisory Commission; and Director of the America’s Wetland Foundation).
**By a combination of river diversion projects and conveyance of dredged sediments via pipelines.
13 Comments
2009-09-15
21:05:14
I've always been troubled by one aspect of these calculations. I'm unclear on just how much of the delta is made from solids carried by the River. Plant mass comes primarily from the atmosphere and dissolved minerals in water. Shell material originates as dissolved material from the River or from saltwater. Certainly deltas form at the mouthes of rivers, but at least a part of this mass is originating as dissolved minerals and atmospheric carbon that is fixed into solid form by the high productivity at the estuarine interface.
This is analogous to the question "Where do trees come from?" The answer may surprise some until you think about it. I've read that most of the mass of a tree comes out of the air through fixation of carbon. If it did not, you would see big depressions next to big trees.
Thanks for the opportunity to comment and probably demonstrate my ignorance. --Mike
2009-11-02
03:32:16
All the more reason NOT to recycle paper, perhaps?
2009-09-07
09:52:47
[...] Coastal sediment needs – let’s get real, folks! [...]
2009-08-26
11:24:19
I just completed a (MS) research project tracking the response of marsh vegetation to hurricane sedimentation. I can tell you that Hurricane Ike pushed in more sediment onto a marsh in Cocodrie than Hurricane Gustav. Ike was a bigger storm and it moved west of the marsh.
Beyond knowing where the sediment was coming from when addressing storm sedimentation and sedimentation in general we need to remember that it is the volume of both the sediment and organic matter that sustains our marshes. I saw an increase in soil organic matter (root) volume post-sedimentation. It is a combination of sediment and organic matter that sustains our marshes. I've been convinced that it is the volume of dirt and plant material not the mass of the stuff that we need to look at.
2009-09-03
06:40:04
Thanks for your comment Jennifer. It's great to see input from folks with recent new data collected in the field!
2009-08-26
10:21:31
Len,
I think the jury is still out on this issue of storms. There is a body of data out there indicating substantial accretion from storms of various strengths. Gene Turner documented a lot of sediment accretion after Katrina and Rita. At the Pearl River, there was strong evidence of erosion, but also accretion, and until we measured soil elevation changes, which were positive by several centimeters, we were not sure if that ratio was negative or positive. From other storms (such as Ivan), people have documented postive elevation gains from sedimentation. Ivan is just one of numerous additional examples.
There are techniques to determine the source of the sediments, whether from the bays, the Gulf, or internally from the marshes them selves. We need to futher investigate.
And yes, in some cases, the storms do just move sediments around. I would assume that may be due to where in the storm a particular site may be. You'll get very different impacts if you are in the forward left quadrant of a storm, versus the much stronger right forward quadrant.
2009-08-24
14:31:57
Re sediment budget, has anyone calculated the amount of seds pushed inland by coastal storms? I remember seeing the buildup in St Bernard wetlands post Katrina at several inches. Not saying coastal storms are good, just that I've not seen gross estimates of this source of supply and its fate if the big linear levees go in. Even with levee gates, storm time would seem to be when the gates shut down.
2009-08-25
06:02:04
Oliver-
I had the same thought when I saw Harry's presentation to the CPRA. On the other hand, based on conversations with delta experts my sense is that there is a strong geological consensus that the storm accretion:erosion ratio is negative, not positive. Remember, the ultimate source of all deltaic sediment was/is the river. Storms just move it around and generally take away more than they put back where it's needed.
2009-08-24
11:42:31
Len:
Don't forget, most of the six SDEs for New Orleans 1 in 100 year protection will come from the delta, so in a sense you have to add that onto the deficit that Roberts projects. Thousands of acres of bottomland hardwood forest, a deltaic ecosystem that is actually on a steeper decline than delta marshes, will be destroyed, since the clay needed for levees underlays bottomland hardwoods for the most part. in other words, in order to save the delta, we have to destroy the delta. Morganza, Donaldsonville, the newly federalized Plaquemines levees, plus future lifts for enhanced LACPR protection will increase the deficit.
2009-08-25
05:49:06
David-
Your comment is well taken. I just saw a map of the corps' proposed borrow sites for the levee clay. As you said, the resulting holes are all within the very delta that we're trying to save by borrowing from Peter to pay Paul.
2009-08-23
16:14:28
I'm interested but I sure wish my dad could be with us to see what he would have said.
2009-08-24
10:13:10
For readers who aren't familiar with the name "Viosca," Charlie, who now lives in Dallas, is the son of Percy Viosca Jr.who will be featured in a series of posts for the first anniversary of LaCoastPost this October.
Percy was born in New Orleans in 1892 and passed away in 1961 (just as the devastating Mississippi River Gulf Outlet was being completed).
Joe Herring, former secretary of the Louisiana Department of Wildlife and Fisheries, says that in his opinion Percy Viosca ranks above John J. Audubon in terms of his knowledge of Louisiana natural history and his role in documenting its importance.
2009-08-23
13:28:39
Blum and Roberts have shown starkly what the coast will look like in 2100. But they used a conservative estimate of sea-level rise of 4 mm/yr by 2100. Recent studies indicate that sea level will rise by about a meter (or more) by 2100. I believe that only by vastly larger diversions can we hope to save even part of the coast. Increasing energy costs will likely make diversions the only affordable restoration tool that is sustainable.