Wednesday, October 26, 2011

West Bay Diversion to close?

West Bay Diversion Project to End. Or at least, that was the headline in early 2010.

Backing up, West Bay was the first diversion project optimized for sediment diversion. It's also WAY larger than either Canarvon or Davis Pond (ea. ab. 10,000 CFS vs. 50k+ for W. Bay). Canarvon and Davis Pond are sometimes called failures, but that isn't really fair, since they were optimized to fight saltwater intrusion only and NOT to carry sediment.

West Bay is located near Head of Passes (scroll down on the link for the maps). Planning started in 1991 and it was the first diversion optimized for sediment diversion. It was opened and petered along for a while, but was considered a disappointment. The Corps, who hated the project from the start (they believed it would be a hazard to navigation or would just plug up), trumpeted the failure and pushed for its closure.

Here's a quote from Dr. Len Bahr about the Corps and West Bay:

Despite its recent construction in 2003, bathymetric surveys in 2009 showed that sediment accretion from the West Bay Sediment Diversion project had exceeded subsidence in parts of the project receiving area for the first time in almost a century. Nevertheless, the public was told that a lack of significant wetland accretion contributed to a decision to shut down this ‘youthful’ sediment diversion project.

They also forced CWPPRA to pay for the dredging of the Pilot Town anchorage, despite modeling that proved West Bay was responsible for 20% (or less) of the shoaling and that the anchorage was built in an area that was prone to shoaling anyway. Here's a presentation from a past Tulane Engineering Forum describing the sediment transport modeling [PDF]. Furthermore, the port people weren't really complaining because the anchorage is hardly ever used (pilots are almost 100% available within a few hours notice at Head of Passes; the anchorage is almost a vestige of the old sailing days).

Enter the 2008 and 2011 Mississippi River floods (both of which were BIG, especially 2011 which was at least a 100-year event). West Bay, which wasn't a total failure like the Corps was making out, but also wasn't lighting the world on fire, explodes in activity and they figure out the missing ingredient: sand and gravel. The normal river flow has almost no sands in it; it's all mud and fine sediment (good for nourishing plants, but not enough of a base for land-creation). In flood events, there's a totally different dynamic. During flood events, sediment load per unit volume of water is at least 50 times larger, with much coarser sediment mobilized. The Corps continues to call West Bay a failure.

There was also an interesting side story that happened about this time: CWPPRA was investigating barging sediment from one area to another and asked the Corps about shortcutting through the West Bay diversion instead of going all the way around to SW pass. The Corps said, 'bah, it'll be too shallow and you'll just get stuck on a sandbar and we know everything there is to know about river dynamics because we've been studying it for 100+ years.' Well, the barging project was shelved because of cost, but they did to a hydrographic survey and the depth of the West Bay channel: 84' (barges draw ~20' or less).

More and more, the science community wants to study West Bay (being wrong in science is a good thing; you learn something new). The Corps is doing what a bureaucratic organization does to something it sees as embarrassing them: kill it with fire. The Corps had finally blackmailed CWPPRA into closing West Bay in 2010 when 2 monkey wrenches were thrown into the mix: the 2011 floods and the shipping people 'stabbed the Corps in the back' (or so that's how the Corps felt, so I'm told). Note that that link is dated October 13th.

My theory behind the objection: the port people know SW Pass is hydraulically inefficient and a pain to keep dredged. They're looking into a new, deeper pass much closer to Venice and see West Bay as an interim solution to accommodate New Panamax-class ships (Panamax=39.5' draft, New=49.9' draft) in a cost effective manner (as opposed to Miami's expensive Deep Dredge Project that's caught up in Tea Party-type stalls). The Port of New Orleans has been doing extremely well with containerized cargo this year (highest ever for NOLA?) and want to get an even bigger slice of that pie going forward.

How will this all shake out? Will the Corps force the closure of West Bay? Will we ever learn how to save the coast? We shall see.

2 comments:

Ryan Waldron said...

The time scale for measuring the effects of sediment diversions is much larger than the 2003-2009 period over which measurements were taken. On the physical model I used in grad school, 30-50 years was the minimum time period we needed to see and measurable land building.

Two understand why it takes so long, one must first realize that a diversion is a man-made duplication of a natural event, a crevasse and following new distributary formation. hopefully that new distributary would result in the formation of its own delta.

The first thing that will occur though is not land building, rather it is scour. The new channel will likely be SCOURING FOR 20 YEARS. At the end of that initial time period hopefully levees and bifurcations in the channel begin to form. This is the point at which land can actually be successfully created.

Water that does make it over the levees or in the area between bifurcated flow will have a lower velocity than that in the newly scoured channel (which will have developed an equilibrium between its velocity and sediment carry capacity by this point). Thus slowing down the water at this point will not allow the flow to maintain the sediment which will then either fall out of suspension, stop saltating along the bottom, etc.

This is when land begins to truly form. 30-50 years out. Look at West Bay, look at Cubit's gap. Both are highly successful human caused deltas, and each follow this same time period. 7 years is much to soon to make a determination about Est bay. Period.

Clay said...

Take a look at Wax Lake:

http://en.wikipedia.org/wiki/Wax_Lake

Built in 1942. Took years to show results, but it now shows up as a "green" area on coastal land loss maps.

http://www.nwrc.usgs.gov/upload/landloss11X17.pdf

Wax Lake is one of the green spotches in central Louisiana.