Coastal restoration as a climate change adaptation strategy

By Meg Sutton, Environmental Defense Fund

Global climate change has induced an increase in global mean sea level with a 3.1 mm/year average rate of increase since 1991^[1]. Climate projections indicate a widespread increase of more intense precipitation events, with an associated increased risk of flooding. Similarly, climate scientists also predict an increase in hurricane wind speed and total volume.

The low lying, coastal Mississippi River Delta region is particularly vulnerable to the climate change threats of sea level rise, increased flood risk and more intense hurricanes. The area is additionally plagued by human-induced environmental degradation that has occurred over the past 200-300 years. The region has lost 1,900 square miles of land since the 1930s and is losing the wetland areas that are crucial to the region’s ecosystem function, economy and character.

Predicted land change along the Louisiana coast over the next 50 years if we do nothing more than we have done to date. Red indicates areas likely to be lost, and green indicates areas of new land. This map is based on assumptions about increases in sea level rise, subsidence, and other factors. (Estimate based on less optimistic scenario of future coastal conditions.) Map provided courtesy of the Louisiana’s Coastal Protection and Restoration Authority.

The numerous threats to the region set up a potential dilemma of competing interests. Should resources and attention be focused on immediate restoration or longer term climate change adaptation? Fortunately, no such choice has to be made. Climate change adaptation and coastal restoration do not constitute a zero sum game. Restoration of coastal Louisiana reduces the vulnerability to the major risks posed by climate change and therefore can be seen as a climate change adaptation strategy.

The global rise in mean sea level — termed eustatic sea level rise — is further complicated in the Mississippi River Delta region by subsidence (sinking land). The sum of the two is referred to as relative sea level rise. The Gulf of Mexico has one of the highest rates of subsidence in the nation due to sediment compaction and the extraction of groundwater, oil and natural gas^[2]. These encroaching sea levels increase mean water levels in boundary regions, accelerate coastal erosion and alter the salinity levels of sensitive coastal habitat systems. These factors have contributed to the high rate of land loss in the region.

Restoration of the deltaic system can help stabilize shorelines and reduce the associated risks with rising sea levels. Deltas are formed by the constant inflow of sediment from rivers. However, the Mississippi River Delta has been cut off from this natural process through the construction of extensive levee systems for navigation and flood protection. Through planned sediment diversions, the natural deltaic process can be restored and help increase the resiliency of coastal areas. This will combat the effects of both eustatic sea level rise and subsidence.

The projected increase in the intensity of precipitation events due to global climate change will exacerbate flood risk in the Mississippi River Delta region. Research has shown that coastal wetlands can greatly reduce flooding and storm damage. A one-acre area of wetland can store up to one million gallons of water^[3], providing a significant buffer between flood waters and populated areas. In addition, wetland vegetation acts as a natural flood barrier by reducing the speed of flood waters. Healthy wetlands therefore have the potential to reduce both the volume and speed of floodwaters that reach surrounding areas. Restoration efforts seek to improve the condition of surviving wetlands in the Mississippi River Delta region as well as strategically reestablish historical wetland areas with sediment diversions. The restoration efforts to augment total healthy wetland area in the region will simultaneously reduce flooding risk associated with climate change.

As highlighted by Hurricanes Katrina and Rita in 2005, the coastal Mississippi River Delta region is highly vulnerable to hurricanes and their associated storm surge. Climate change is predicted to increase hurricane wind speed and total precipitation, further amplifying this threat. Coastal wetlands have been shown to reduce both wave energy and wave height when storm surge passes through them^[4]. These wetland regions introduce a frictional drag that reduces the intensity of waves, and restoration will therefore help protect surrounding regions from storm surge now and into the future.

The restoration of ecosystem function in the Mississippi River Delta would provide significant benefits for both the short and long-term future of this crucial economic and ecologic zone. The overlapping interests of restoration and long-term climate change adaptation serve to strengthen the case for large-scale immediate restoration of the region.