Drawing a line in the sand: How much overwash can a marsh handle after a hurricane?

Walters, D. C. and Kirwan, M. L. 2016. Optimal hurricane overwash thickness for maximizing marsh resilience to sea level rise. Ecology and evolution6(9), 2948-2956.

DOI: 10.1002/ece3.2024

Why are salt marshes important?

Salt marshes have historically received the short end of the stick. They’ve been discounted as smelly swamps, unsightly breeding grounds for mosquitoes, and unusable land in desirable coastal locations. It is only more recently that they have received attention for the many beneficial functions they serve. Recent work has valued marsh services, including storm protection, at $10,000 per hectare. Left in a natural state, marshes act as sponges, absorbing flooding and brute force from wave action during storms and hurricanes. Humans have made these areas less resilient by fortifying coastal areas against storms. When areas are fortified and shorelines are hardened with paving, flooding during hurricanes is able to breach further inland since the pavement acts as waterslides. Ensuring salt marshes persist is important for protecting coastal communities.

Photo of a backbarrier marsh. Together, marshes and barriers protect coastlines from storm events. Resource: Katelyn Szura.

Hurricanes and salt marshes: how do the two interact?

A common type of salt marsh is a backbarrier marsh in which a beach and dune provide a protective front against wave and storm action, allowing for the creation of marshes behind them. When hurricanes hit these areas, they pick up sand from the beach and deposit it on the marsh side of the dune when vegetation traps the suspended sediment from the flooding waters. This is called overwash. Salt marshes are adaptive and can benefit from this added elevation with increased plant growth. However, too much overwash has shown to lead to plant mortality. Without healthy plants, marshes become vulnerable to erosion and lowered resilience. Hurricane intensity and sea level are expected to increase with climate change, which could threaten marshes with an increased risk of harm from wave force, sand additions, and heightened flooding.

What did the study find?

This study examined the combined influence of sea level rise and overwash on plant growth in a backbarrier salt marsh. Various depths of overwash were tested on marsh plants, representing thicknesses from small storms to large hurricanes. The study found that the addition of overwash actually improved plant growth, up to even a surprising 120% over the control with no sand additions. However, the benefits to plant growth occurred only to an extent. Plants responded the best to 5-10 cm of burial with sand. Outside of this range the plants did not grow as well. Deep burial led to plant mortality. Adding in sea level, the results found that overall plants grew the best when exposed to less flooding at the shallower overwash depths. The trend was reversed for 10 and 15 cm sand thicknesses where increased flooding led to greater plant growth.

Overwash thickness experimental setup. Overwash thickness varied, increasing front to back. A comparison between the beginning and end of the growing season can be seen in photos A and B. Photo C shows plant growth decreases when overwash depth increases. Resource: Walters and Kirwan, 2016.

How can we use this information?

It is helpful to have a better understanding of how overwash impacts marshes since it can be used to manage and promote healthy and more resilient salt marshes. Results from this study can be used to help promote plant growth in marshes that are experiencing increased flooding from sea level rise. In recent years, sediment additions have been used to restore low lying marshes, helping them to gain elevation in order to survive increased flooding from sea level rise. This study provides a target depth for sediment additions, not only for restoration but also for when hurricanes strike. The threat of increased hurricane frequency and intensity could have negative impacts to marsh plant health, but understanding how to manage overwash to an optimal depth when hurricanes strike could help marshes to be more resilient.

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Katelyn Szura

I am currently completing my Masters in Biological and Environmental Science at the University of Rhode Island. My research focuses on examining how nitrogen inputs affect greenhouse gas fluxes from salt marshes, ultimately linking this work to how it impacts carbon storage in coastal wetlands. When not knee deep in marsh mud I enjoy running, hiking, sailing, and spending time with my pup, Bailey.

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