It’s the sediment, stupid
Coastal Plain All Out-Go, No In-Come
This is my soapbox column.
I know I am preaching to the choir – but from an environmental geoscience perspective I am presenting thoughts about some basic geologic process understandings that should be having more influence on a potentially multi-billion dollar reconstruction effort than they are having.
I begin with some geologic facts about the most important delta in the United States.
The coastal Louisiana landscape was built from sediments supplied by the Mississippi River and deposited on its delta or along adjacent shorelines, carried there by longshore currents.
Since the supply of sediments has been reduced in a substantial way by engineered structures that dam and confine the river, very little sediment from the river reaches these environments.
The wasting of the coastal plain is due in large part to the absence of major influxes of sediment, leaving the destructive mechanisms of subsidence and wetland attrition as the dominant forces there. Thus the root of the current dominance of subsidence in the delta and resulting wetland loss is the cutting off of floodwaters – and their sediment load – to most parts of the system.
Over the many years that restoration of the Mississippi delta plain in south Louisiana has been under consideration by the state and the U.S. Army Corps of Engineers, the size of proposed restoration projects has been generally small consisting of small diversions – mostly of water to refresh marshes, not sediment to build land.
This has been accompanied by hand-wringing over the continued loss of delta-plain wetlands.
The fact is, there is no sustainable approach to preserving and growing land in the Mississippi delta that does not include restoring large-scale sediment delivery to distal wetlands. Available data suggest that despite historical reductions in sediment load, the lower Mississippi River still transports sufficient sediment to meaningfully counteract current subsidence and sea level rise – if that sediment is effectively delivered to subsiding delta wetlands.
Accomplishing this will require creative, out-of-the-box approaches that go beyond minor diversions over and through existing and planned engineered structures.
It will require major river diversions that distribute sediment currently channeled off the edge of the continental shelf into distal coastal wetland environments.
This will need to be accompanied by other measures, such as getting water and sediment currently moving down the Atchafalaya River into wetlands and perhaps pipelines to deliver sediment into some areas.
While past arguments in favor of coastal-plain habitat restoration have focused on natural system values and the economically important productivity of these systems, post-Hurricane Katrina planning – centered around the protection of lives and property from hurricane surges, waves and flooding – includes the natural landscape as an important component of a protection system.
With this human-centered rationale for restoring the delta plain, the substantial challenges related to the infrastructure and societal issues associated with large-scale sediment diversions should receive the creative attention that has been lacking in the past.
Without approaches that deliver quantities of sediment more on a scale of those that built the delta, meaningful restoration and protection of distal parts of the delta plain is not possible, and plans should be laid for abandonment of these areas.