View of salt marsh from bridge over Wolfe Glade on Junction & Breakwater Trail, Delaware. Photo by R.A. Kroft
If you live near the sea, chances are your life, home, and/or way of life have already been saved at least once by salt marshes that protect against coastal erosion and mitigate the effects of extreme weather. The daily rhythm of these precious coastal wetlands that are flooded and drained with the tides usually goes unnoticed by humans. They are teeming with life, providing “essential food, refuge or nursery habitat for more than 75% of U.S. fisheries species, including shrimp, blue crab and many finfish.”1
Unsung Heroes of Coastal Flood Defense
It has long been known that salt marshes protect shorelines from erosion and flooding, acting as a buffer for wave action and absorbing rainwater.1 An international team of researchers led by the Royal Netherlands Institute for Sea Research (NIOZ), Delft University of Technology, Deltares and Antwerp University have published results of their analysis of two devastating floods that breached Holland’s critical dikes.2 They found that “the value of nature for flood defense has actually been evident for hundreds of years.”3
According to a NIOZ press release, the team consulted records going back 300 years and found, “Salt marshes have reduced the number of dike breaches during the well-known 1717 historic flood disaster. More interestingly, the 1953 flood disaster also tells us that salt marshes are not only ‘wave absorbers’ that ease wave attacks on the dike, but are also ‘flood fighters’ that lower the flood depth by limiting the size of breaches when the dike would fail during severe storms. And having smaller and shallower breaches because of salt marsh protection can save many lives.”3
Salt Marshes in the Context of Climate Change
As scientists and engineers consider strategies to mitigate the threats posed by rising sea levels and increasingly stronger coastal storms, they are looking at ways to combine the “natural” defenses of salt marshes with engineered protections like seawalls and dikes. One idea suggested by the NIOZ paper is that it may “be possible to enhance coastal safety by creating salt marshes in between double dikes, where a secondary more landward dike is present and the most seaward primary dike is opened to allow natural processes to ensure marsh development. Despite no longer useful for wave reduction, such marshes are still very helpful for flood protection by making the landward dike more stable during extreme storms and buffer the effects of the rising sea in the long run.”3
Cities like Miami (considered “ground zero for sea-level rise in the U.S.) and New York City (inundated during Hurricane Sandy) are undertaking projects to enhance seawalls, build movable flood barriers, and look for other ways to harden their infrastructure against future storms, including creation of new sand dunes.4 The U.S. Environmental Protection Agency has pointed out the value of wetlands to urban areas stating, “Wetlands within and downstream of urban areas are particularly valuable, counteracting the greatly increased rate and volume of surface-water runoff from pavement and buildings. The holding capacity of wetlands helps control floods and prevents water logging of crops. Preserving and restoring wetlands together with other water retention can often provide the level of flood control otherwise provided by expensive dredge operations and levees.”5
An EPA factsheet provides the example of The Great Flood of 1993 in the upper Mississippi River Basin that was responsible for 38 deaths and billions of dollars in property damage to call attention to what is really lost when wetlands are developed. “Historically, 20 million acres of wetlands in this area had been drained or filled, mostly for agricultural purposes. If the wetlands had been preserved rather than drained, much property damage and crop loss could have been avoided”—not to mention preventing the loss of life.6
Wetland protection is a complex subject. Developers must seek permits to develop projects on wetlands and ensure that they mitigate the impact of their project by building a new wetland in the same watershed or by purchasing credits with a “Mitigation Bank” that is “a wetland, stream, or other aquatic resource area that has been restored, established, enhanced, or preserved for the purpose of providing compensation for unavoidable impacts to aquatic resources permitted under Section 404 of the Clean Water Act or a similar state or local wetland regulation.”7
Photo: R.A. Kroft
As the photo shows above, it can be startling to see instances where developers have been permitted to build on coastal wetlands. The project above uses landfill to create a homesite on wetlands separated by a wooden retaining wall.
The photo below shows that the wetlands surrounding that housing development have been designated as a Conservation Easement that is to remain in its natural state. The program is administered by the U.S. Department of Agriculture’s Natural Resources Conservation Service for private landowners to protect or restore wetlands on their property.8
Photo: R.A. Kroft
The U.S. is just beginning to grapple with the grave dangers posed by sea-level rise for our coastal towns and cities. On a sunny day when the grasses are green and the salt marsh is a quiet neighbor, it is easy to take for granted “hidden value of natural defense”3 offered by these wetlands. The Dutch are light-years ahead of us in developing ways for almost half of their inhabitants to live below sea level.9
The NIOZ paper concludes that “saltmarshes can reduce both the chance and impact of the breaching of engineered defences….provid[ing] new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defence functions.”2
In an interview with DutchNews, climate scientist, Dewi le Bars, expressed optimism about the Netherlands’ expertise and infrastructure in managing rising sea levels. He said, “A lot of countries will suffer much earlier…In Miami, sea level rise means more flooding. But in the Netherlands, sea level rise means more cost in protecting the coast.”9
—R.A. Kroft
Sources:
1NOAA. What Is A Salt Marsh? National Ocean Service, National Oceanic and Atmospheric Administration. U.S. Department of Commerce. 9 October 2019.
2Zhu, Z., Vuik, V., Visser, P.J. et al. Historic storms and the hidden value of coastal wetlands for nature-based flood defence. Nature Sustainability (2020).
3NIOZ. Royal Netherlands Institute for Sea Research. Historic floods reveal how salt marshes can save lives in the future. Press Release. 29 June 2020.
4Guterl, Fred. These Five Cities Are Vulnerable To Rising Seas, Including Miami and New York. Newsweek. 11 February 2020.
5EPA. Why Are Wetlands Important? U.S. Environmental Protection Agency website.
6EPA. Functions and Values of Wetlands. Factsheet. U.S. Environmental Protection Agency. EPA843-F-01-002c, March 2002.
7EPA. Mitigation Banks Under CWA Section 404. U.S. Environmental Protection Agency.
8Taylor, Ciji. Are Wetland Easements Right for You? USDA Natural Resources Conservation Service. 16 May 2018.
9Nicholls-Lee, Deborah. As Sea Levels Rise, How Long Until the Netherlands Is Under Water? DutchNews.nl 23 December 2019.
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R.A. Kroft writes about her day-to-day journey in living a smaller, more sustainable life and other topics that interest her.
This Site Was Inspired By An Interest in Protecting the Environment:
We had the privilege and joy of learning from Dr. Charlie Stine who instilled a love for the natural world through incredible field trips with the Johns Hopkins Odyssey Certificate program in Environmental Studies. At the time, the program was endorsed by the Maryland Department of Natural Resources. Sadly, after Dr. Stine retired, the program was phased out. We hope that we honor his legacy by shining a bright light on environmental issues and sharing good news about the success of various conservation programs when possible.
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