Floodplain Resilience Presentation Debut in Hammond, La.

On Thursday October 3rd, in Hammond, Healthy Gulf presented on Floodplain Resilience in the Lake Pontchartrain Basin to a group of interested community members from Tangipahoa, St. Tammany, and Ascension Parishes. The information presented was wetland fill permit data gathered and analyzed over 2014-2018 by Scott Eustis, Healthy Gulf’s Community Science Director who has done more wetland permit review since 2010 than anyone at Healthy Gulf. The data were mapped using publicly available Army Corps of Engineers wetland application information by Naomi Yoder, a geographer and marine scientist who shares wetland permit review duties with Scott.

I hosted the meeting at the Hammond Chamber of Commerce board room inside the historic train depot. The slide presentation focused on summary statistics and trends in quantity and location that Scott and Naomi see in wetland fill permit activity  in four Parishes, (St. Tammany, Tangipahoa, Livingston, and East Baton Rouge), along the Interstate 12 corridor. The presentation also included St. John and St. James Parishes at the lower end of the Lake Maurepas Basin. Those Parishes feel the effects of flooding from rivers flowing into Lake Maurepas, but also the effects of wind-forced high water events, and storm surges from Pontchartrain that push water into Lake Maurepas and the Manchac swamp.

Any land-use change for road-building, pipeline construction, housing, industry or any other activity that requires the filling of any acreage of wetlands must be submitted to the Army Corps of Engineers as a Clean Water Act (CWA) Section 404 permit application. When, or if the Corps grants the wetland fill permits based on the application, the project also needs a state certification that water quality will not be impaired by the wetland fill activity. For a decade, Scott Eustis has commented continuously on Army Corps CWA wetland fill permits, and this presentation sought to view their cumulative impact. We have not seen the Corps of Engineers or Parishes similarly analyze the mounting number of acres of wetland losses in the region, and so Healthy Gulf decided to present this data in order to encourage people, Parish governments and state agencies in Louisiana to pause and look at the big picture. The Foundation for Louisiana provided Healthy Gulf a grant for the data analysis and for sharing the information in meetings. Hammond was our first such presentation.

In all six Parishes, there have been 1828 acres or 2.8 square miles of wetlands at risk of fill just in the last 5 years. St Tammany (821 ac.) and Livingston Parish (587 ac.) have seen the most wetland fill applications. Western St. Tammany and the Livingston/East Baton Rouge boundary near the Amite River are the two hot spots of wetland loss. When wetlands are filled, the Army Corps of Engineers requires mitigation to be a part of permit requirements – but this mitigation is primarily for wildlife habitat, and does not consider how losing wetlands in one area and creating them in another is changing flood hazards every year across the landscape. A mitigation bank is a parcel of land that contains or has the potential for wetlands and that is protected and managed in perpetuity for restoration.  These banks (a land trust, state agency or specialized land holding corporation) sell wetland credits to fund the restoration of wetland trees and plants on a property, sometimes including the re-contouring of the land’s drainage patterns so water can be stored in wetland soils.

In the Hammond presentation, one of the most interesting pieces of information to the attendees was the fact that an acre of wetlands can store a million gallons of stormwater. So, if you fill 1828 acres, it means that 1.8 billion gallons of water now won’t be stored in wetland soils or evaporated by trees and plants, but will enter the surface drainage system. Engineered drainage systems in both new developments and established communities must attempt to handle this storm water.  Continued wetland filling creates a burden to communities. Many times this burden is borne by residents themselves in the form of higher insurance rates for flood prone areas. In the relatively flat landscape along the I-12 corridor, figuring out what to do with more and more water is an increasing challenge. This is particularly troubling when heavy precipitation fills local rivers and streams or tides or surges from southeast winds and tropical storm systems force coastal water into Lakes Pontchartrain and Maurepas.

Our presentation made the point that Parish and regional flood risk planning experts must track the wetland filling that goes on year by year in the various Northshore watersheds and towns, in order to stay ahead of the moving target of stormwater management. We have shown how the wetland loss data are accessed and presented, and the agencies and governments need to use these data to continuously update their flood risk plans and modeling.
The continued filling of wetlands in these Parishes also brings up the need for restoration of floodplain function along the river floodplains upstream of populated areas along the I-12 corridor. There have been large sand and gravel mining operations over the last 80 years along the major Northshore rivers: Bogue Chitto, Tangipahoa, Amite and Comite have which been left with open pits and spoil piles where bottomland hardwood riparian (riverside) forests once stood. Many gravel pits and ponds are inactive and need restoration. Some that are close to river channels fill with water when a river floods. When a river leaves its banks during a flood and enters a nearby mined area in its floodplain, it encounters deep ponds and lots of easily transported material such as silt, sand, clay and gravel. These gravel pit-capture events during floods can affect a river’s slope and sediment load characteristics. This altered hydrology results in restricted flow when the river returns to a normal flow after floods have passed.

Restoring the floodplain forests on old mine sites could be accomplished by re-contouring and replanting the mine areas. Creating incentives for landowners to pursue this restoration could lead to a new variety of mitigation for wetland loss in Louisiana’s Florida Parishes. In the presentation, some restored gravel mines were shown as examples of what this kind of restoration could do along the rivers. After restoration, river channels become more stable, even during flood periods. Stable stream channels and better functioning forested wetland acres along them, storing water in river floodplains upstream of population centers, should make flood conditions downstream less dire and ease water management problems for Parishes.

Healthy Gulf will host another meeting in St. Tammany Parish in the coming months, and then it will take the information from the meetings, along with the impressions of community members about them, to the state agencies comprising the Louisiana Watershed Initiative (LWI). The LWI was formed by Governor John Bel Edwards after the disastrous 2016 floods that devastated many areas of Louisiana. The continued wetland filling in areas of Louisiana that have already lost many acres of functioning wetlands, underscores the need to continuously update plans and models for flood risk management. We will ask the agencies leading the LWI effort to acknowledge and account for the moving target of wetland loss within the larger picture of flood risk modeling and water management project implementation.

Andrew Whitehurst is Healthy Gulf’s water program director.

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