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Coastal Management

Hurricanes and Rising Tides – Can Living Shorelines Protect Us?

Smith CS, Puckett B, Gittman RK, & Peterson CH (2018) Living shorelines enhanced the resilience of saltmarshes to Hurricane Matthew. Ecological Applications. 28(4): 871-877. (https://doi.org/10.1002/eap.1722)

Hurricane Schmurricane

We love living near the water. 39% of Americans lived in shorefront counties in 2010, and that number is projected to rise to 47% by 2020. Many waterfront property owners install some sort of shoreline structure to protect their investment from coastal erosion and flooding. This is usually done with so-called ‘armored’ shoreline solutions, like rip rap or bulkheads (also known as seawalls). These structures erase the natural flux of the system, decreasing overall resiliency and removing important habitat in the process…but, then again, they’re cheap, they’re quick and easy, and they prevent erosion and protect private property…. Right?

Not exactly. Bulkheads require frequent, costly repair, and because wave energy is deflected, any sediment (and associated habitat) in front of the bulkheads is quickly scoured and removed, sometimes to deposit on top of unlucky systems nearby. It’s no surprise then that there is an increasing call for the removal of armored shorelines to replace them with greener, or softer, “living” ones1.

A bulkhead changes water flow and sediment transport patterns. This one, at MacKay Island, shows clear signs of storm damage, and resulting damage to the flora behind it. USFWS

A recent paper out of eastern North Carolina compared bulkheads (the most common type of armored shoreline in the region), rock sill living shorelines, and natural marshes to see how the three handled hurricanes, one of the Atlantic’s biggest, most costly shoreline stressors. Hurricane-force winds and storm-related flooding routinely cost shore communities billions of dollars in property damage, not to mention the risk of injury and loss of life. Under current climate change trajectories, hurricane models predict a 45-87% increase in the frequency of strong storms (Category 4-5) due to increased sea surface temperatures2. Sea level rise will compound the problem, amplifying storm surge. Waves from severe storms can also cause major salt marsh erosion. This erosion is actually an important ecosystem service the marshes provide to upland areas, but when marshes are degraded, poorly maintained and diminished in size, their ability to bounce back is impaired.

Oyster reefs are also increasingly being used in living shoreline installations, as see here in Rhode Island, though not in this study. USFWS

So what’s the damage?

Storm related marsh erosion can mean both the catastrophic destruction of marsh edge, but also the loss of elevation in what remains. This elevation loss means increased salinity, a physical parameter that plays a huge role in determining fauna and floral distribution within the marsh itself. Armored and living shorelines are each designed to prevent both of these processes, so both needed to be considered. The researchers therefore measured the stem density and surface elevation, as well as structural damage of a saltmarsh grass (Spartina alterniflora), at 12 sites each summer from 2015 to 2017. This gave them a data set covering the time period before and after Hurricane Matthew blew by in the fall of 2016, with an extra visit a few weeks after the storm. The immediate results were clear. When comparing the pre-storm data point to the post-storm survey, natural marshes and bulkheads lost elevation after Hurricane Matthew while living shorelines generally maintained their elevation. Moreover, while 3 out of the 4 bulkheads surveyed had some sort of damage, none of the shorelines sustained any visible damage from the storm. So if you are a new waterfront homeowner looking to install new shoreline protection, the scoreboard is:

Living shorelines – 1, bulkheads – 0.

When looking at the full study period, from 2015-2017, the trend at first appears upended: the data show bulkheads and rock sill living shorelines preserving landward elevation equally as compared to natural marsh, which lost elevation overall. However, the authors point out that the landward elevational gain is due to homeowner repair (bulkhead restoration and resodding) after the extensive damages caused by Hurricane Matthew. The living shorelines held their own without additional resources.

Living shorelines – 2, bulkheads – 0.

When you consider the marsh grass stem density data, the comparison is even starker. Spartina alterniflora stem density at the living shoreline edges increased, while stem density at the margins of the natural marshes decreased. Bulkheads couldn’t even be compared, as there was never any Spartina grass to begin with.

Living shorelines – 3, bulkheads – 0.

The authors are quick to note that this was just one category 1 hurricane, and that there are plenty of locations where living shorelines are not feasible (e.g., man-made canals). Moreover, not all living shorelines are the same, and this study only provides evidence for one type.

Takeaway

While the most common and most effective types of hardened shorelines differ from region to region, the scientific consensus is in: armored shorelines are bad policy: bad both economically and environmentally. And what’s more, the evidence is building that they might not be as cost effective as was previously thought, costing more money in repair long-term than they save in installation costs. This last point is crucial. If policy makers want to turn the tide of private property protection from armored shores towards greener, softer methods, homeowners need to be convinced that the environmentally friendly methods are as effective at protecting their investments. Scientific evidence is just the first step; the information needs to get into the right hands in order to bring about real world change.

References:

  • Barry, Aidan. 4 September, 2018. https://envirobites.org/2018/09/04/building-barriers-to-stop-sea-level-rise-whats-at-stake/
  • Knutson, TR, et al. 2013. Dynamical downscaling projections of twenty-first-century Atlantic hurricane activity: CMIP3 and CMIP5 model-based scenarios. Journal of Climate, 26(17)6591-6617. https://doi.org/10.1175/JCLI-D-12-00539.1

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