- Associated Press - Saturday, September 2, 2017

NEW ORLEANS (AP) - An oyster farmer by the name of Cubit had no grand designs when, in 1862, he dug a ditch into the east bank of the Mississippi River; he simply wanted a shortcut for his skiff. But his cut in the lower river delta at Pilottown, about 15 miles below Venice, unleashed the land-building power of the muddy Mississippi. It poured through, widening the ditch into a broad canal that allowed enough river sediment through to create more than 75 square miles of coastal wetland in less than a century.

On Aug. 22, marine scientist Alex Kolker tried stabbing a sediment borer into this young land, now a part of Delta National Wildlife Refuge, to see how it’s holding up. The blade landed with a thud rather than the squish he expected. “This land here is so strong,” he said.

With some added muscle from a colleague, the tool was pushed deep enough to extract a cylinder of soil. It was dense, dark and fertile - the foundation of a healthy marsh.

For Kolker, who works for Louisiana Universities Marine Consortium, the accident known as Cubit’s Gap hints at what two planned sediment diversion projects could do - on purpose, this time - to ease Louisiana’s land loss crisis. The first of their kind in the state and costing about $2 billion, the Mid-Barataria and Mid-Breton diversions, both planned upriver from Cubit’s Gap, would allow huge volumes of water and sediment to pour through the river’s extensive levee system and build land where much has been lost.

The diversions are part of an ambitious and multi-faceted initiative to restore and protect Louisiana’s coastline, which is receding at a rate of a football field every 100 minutes. Erosion, sea level rise, land subsidence and other factors are at play, but the diversions aim to alleviate the problem posed by the levees themselves, which cut off fragile coastal marshes from the sediment and nutrients that sustain them.

Cubit cut his ditch through the river’s natural bank, formed by the pileup of sediment at the edge of the water. The taller, stronger levees that line the river today were built decades later, to control flooding and enable the passage of large ships, but they also send much of the land-giving sediment past the marshes and straight into the Gulf of Mexico.

With Cubit’s Gap as a test case, Kolker and Tulane University environmental scientist Reda Amer recently completed research that indicates new sediment diversions will not only create land but make it out of sturdy stuff: mineral- and nutrient-rich matter that’s resistant to erosion and hurricane damage. Their findings are to be published next month in the journal Remote Sensing of Environment.

Kolker and Amer also found that sediment diversions build land surprisingly fast. For example, the spot where Kolker pulled his soil sample this week was, just 17 years ago, shallow water. By 2015, it had built up enough soil to support plants.

“There was no land here in 2000,” Amer said, looking over stalks of roseau cane and patches of arrowhead and sawgrass. “And now this land exists because of a sediment diversion.”

Cubit’s Gap allows flows of 50,000 and 100,000 cubic feet of water per second. That’s comparable to what’s envisioned for the Mid-Barataria and Mid-Breton diversions.

“It was great to study for both size and scale,” Kolker said. “It represents what the state wants to do with its master plan.”

Louisiana’s 50-year, $50 billion coastal master plan is the greatest allocation of resources for restoration and storm protection in the state’s history. The diversions are some of the more ambitious and expensive elements of the plan - and among the most controversial.

Oyster harvesters worry that the influx of sediment will smother the waterbottoms that they lease from the state. Shrimpers say the added freshwater might push their catch from bays to deeper waters, where it’s tougher and more expensive for them to work. Some coastal communities, such as Jean Lafitte, are concerned about flooding that could be triggered by the diversions.

But unlike Cubit’s Gap, the coastal plan’s sediment diversions would be carefully managed, advocates say, with flows regulated to balance land-building goals against the effects on fisheries and wildlife. Kolker and other members of the Sediment Diversion Operations Expert Working Group, which helps the state develop diversion plans, have recommended an operations strategy that closely tracks seasonal river flows and ecosystem conditions to determine how much sediment to release and when to do it.

While sediment diversions are new and largely untested, freshwater diversions have been operating in Louisiana for decades. Designed to freshen bays and improve oyster growing areas that have become too salty, the diversions have inadvertently moved significant quantities of sediment as well. Some scientists questioned the stability of wetlands created by these diversions, especially after Hurricane Katrina blasted out large sections of land near the Caernarvon diversion.

But Cubit’s Gap seems to indicate that freshwater diversions are simply too small to carry the high volumes of sediment needed for stable wetlands. “The problem with the Caernarvon marshes wasn’t so much that they were fresh; it was more that they were sediment-poor,” Kolker said.

In a small boat, Kolker zipped away from Cubit’s Gap to the outer reaches of the Mississippi Delta. Exposed to the waves and wind of the Gulf of Mexico and far from the flow of sediment, the marsh there is crumbling away. “Look at the soil here,” he said, grabbing a clump that resembled coffee grounds more than the sturdy stuff at Cubit’s Gap.

“It’s literally ground-up marsh, and it has no structural stability. It shows that sediment is such a precious resource. We need to be careful how we manage that resource, because we can use it to build stable and resilient land.”

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