Salty about coastal walls
Keim, R.F., M.G.T. Lemon, and E.C. Oakman. 2019. Posthurricane salinity in an impounded coastal wetland (Bayou Sauvage, Louisiana, USA). Journal of Coastal Research 35(5):1003-1009.
Featured Image: Looking up from a coastal storm protection levee south of New Orleans, LA
Many coastal communities are investing heavily in various types of hard infrastructure as the last line of defense against sea level rise and increased coastal storm damage. These investments are predominantly in the form of some type of wall either to prevent flood water from entering a community (levees) or to prevent erosion of land from wave action (sea walls, bulkheads, Fig. 1). Although these structures provide important temporary protection from rising sea levels for coastal communities, many of these structures are notoriously subject to catastrophic failure (see history of Mississippi River Levees). This is especially true in the present day reality of climate change and the increasing frequency of extreme events.
Just as ecological communities change drastically as you travel up a mountain from base to summit (thank you Alexander von Humboldt for first communicating this phenomenon), coastal vegetation changes less noticeably as you move from the ocean, inland. The distance over which this change occurs depends in large part on the elevation gradient (slope) which controls the amount of water and salinity exposed to plants. The continuum of salt tolerant, brackish, and freshwater wetland communities is ever changing following the dynamic nature of water and sediment along the coast. Artificial barriers such as storm protection levees or sea walls disrupt the continuum of coastal ecological community gradients, leading to situations where hardwood forest can be directly adjacent to a salt marsh with just a levee in between (Fig. 2). Research has shown that such a discontinuity can increase the vulnerability of coastal systems to sea level rise by limiting the system’s ability to migrate upslope (Enwright et al. 2016). Similarly, new research adds to a growing body of literature that indicates artificial barriers can increase the vulnerability of an impounded system when infrastructural failure eventually occurs.
Coastal communities with little to no other options continue to rely on walls for protection. Nowhere is this more true than in southeastern Louisiana where levees have been a long relied on partner against the wraith of Mississippi River flood water. Levee construction along the Mississippi river started with the French in 1717 and has since expanded in size and areal extent all the way north to Missouri. Storm protection levees were mainly a response to flooding from Hurricane Betsy in 1965. Storm surge flowed into the city from backwater bodies of water, those not protected by mainline Mississippi River levees. This flood event prompted the congressional authorization of the Lake Pontchartrain and Vicinity Hurricane Protection Project which resulted in the impoundment of a majority of southeastern Louisiana south of Lake Pontchartrain (Fig. 3). What happens when the walls surrounding large impounded areas fail and water can no longer follow natural flow paths to drain? Hurricane Katrina provided an informative case study into this question that is explored in a recently published paper, Keim et al. (2019).
Bayou Sauvage National Wildlife Refuge is a 25,000 acre refuge that is located entirely within the New Orleans city limits. More than half of this refuge was impounded during the 1970s for potential suburban development. These plans were abandoned and the refuge was established in 1990. Prior to Hurricane Katrina (2005) the refuge consisted of 200 acres of bottomland hardwood forest along the Bayou Sauvage Ridge, a 2000 year old distributary of the Mississippi River. During Hurricane Katrina, a levee breach flooded the entire impounded area with saline water, resulting in the death of 68% of the forest occupying the Bayou Sauvage Ridge (Howard 2012). Targeted replanting efforts have mostly failed along the historically forested ridge most likely due to elevated soil salinities that still persist even after 14 years of recovery.
Keim and others used salinity sampling, long term monitoring data, and modeling to explore the recovery time of impounded forest soils under current management conditions. The focal unit of the study is completely impounded with the exception of two outlets that allow for gravity drainage into a canal. Water level and salinity was measured for a little over a year at the 2 outlets and soil salinity was measured at various locations within the lagoon area and along the ridge during winter 2016. They found that higher elevation areas (ridge) corresponded with lower soil salinity than sites within the lagoon and that deeper soils had higher salinity than shallower soils. In addition, salinity at the outlets was relatively stable and only decreased slightly in response to rain events.
To determine the recovery rate of the unit following the Katrina surge, post hurricane soil salinity was needed. Unfortunately this exact information was not known but estimates ranged from anywhere between 3 parts per thousand (ppt) to potentially matching the salinity of surface water which was reported to be anywhere from “7 ppt to the high 20s” (USFWS 2009; ocean water is generally 35 ppt). The team used soil salinity data from three long term monitoring stations established two years after the storm in order to estimate the recovery rate. One of the stations was located in an impounded marsh and two were located outside the storm protection levee adjacent to the refuge impoundment. The initial soil salinity of the impounded station was slightly higher than the salinity of the outside stations but all stations showed declines over the past 12 years and are currently all around a salinity of 3 ppt, slightly higher than the average ridge salinity of 1.7 ppt. The authors used the salinity time series of the impounded site to estimate an initial soil salinity of 20 ppt after the storm. This estimate is on the higher end of the reported post storm salinity range. Outside stations showed higher variability in salinity indicating exposure to higher frequency drought and high tide events. However recovery to lower salinity was rapid after these events.
So what does all this salinity data mean for ecosystem recovery of impoundments following structure failure? Lower initial salinity of the impounded site relative to sites located outside the levee indicate a slower initial recovery of impounded sites post levee failure. The high initial estimate of 20 ppt for the post storm soil salinity indicates that saline water sat in the impounded area unable to drain for weeks. This water most likely became increasingly salty over time as water evaporated and salinity increased. This process killed trees on the ridge and enabled salty water to soak into soil pores. Because the site remained impounded after the storm, normal flushing of soils by rain could only occur at high elevations where gravitational gradients allows for water to drain through salty soils into the lagoon where it by in large remained (Fig. 4). Because of constant water levels in the impounded area, flushing of soils is not as efficient. Soils outside the levee are subject to higher water level variability and natural drainage paths. This allows the soil to flush more efficiently but also exposes the soils to an increased frequency of small, high salinity events. There is a trade off. Impounded soils take much longer to recover after an infrastructural failure due to inefficient flushing but non-impounded soils may have a higher frequency of salt exposure. This difference in hydrology translates to big implications for ecosystem resilience.
Implication for coastal ecosystems
Vegetation communities located inside impounded areas are artificially fresh. These communities are not adapted to the environmental conditions that would prevail with the absence of an impoundment. When structures fail, the community is subject to extremely adverse conditions that result in high mortality like that observed on the Bayou Sauvage Ridge after Hurricane Katrina. The ridge ecosystem completely changed due to the impoundment holding salty water for weeks. Recovery to post storm conditions was prolonged and it in fact may never be functionally obtained due to inefficient flushing. Meanwhile, ecosystems that are exposed to natural environmental conditions are inherently more resistant to large disturbances and recovery times are generally shorter for large disturbance events due to the maintenance of natural processes such as drainage. The natural system is more resilient to large storm surge events because the vegetation community has adapted to this type of variability, whereas the impounded community is adapted to artificial conditions. We increase the vulnerability of ecosystems by implementing these types of protections in the same way that we give coastal communities a false sense of protection by building levees and encouraging development in areas that are subject to flooding.
Coastal communities face difficult decisions as sea levels continue to rise. Short term solutions like walls may only end up being more harmful in the long run. Adaptive strategies that incorporate nature rather than trying to avoid it such as living shorelines and lifting houses may prove less damaging over the long term then potential catastrophic failure of walls and the ecosystems and communities hiding behind them.
Enwright, N.M., K.T. Griffith, and M.J. Osland. 2016 Barriers to and opportunities for landward migration of coastal wetlands with sea level rise. Frontiers in Ecology and the Environment 14(6):307-316.
Howard, J. 2012. Hurricane Katrina impact on a leveed bottomland hardwood forest in Louisiana. American Midland Naturalist, 168(1), 56-69.