Salt marshes protect the coast – but not where it is needed most

Newswise — Salt marshes offer diverse ecological benefits, one of which is safeguarding the coast from floods. This is notably crucial in countries with low-lying landmasses, such as the Netherlands. A team of researchers from the University of Groningen and the Royal Netherlands Institute for Sea Research (NIOZ), working together with the local water management body, observed wave run-up throughout storms for a span of three years. The outcomes, published on 10 May in the Journal of Applied Ecology, assist the water management body in gauging the protective impact of salt marshes.

For a duration of three years, Beatriz Marin-Diaz, an ecologist, constantly kept track of the weather forecast during the storm season. "Following a storm, we needed to evaluate the wave's effect on our research area," she clarifies. The study area was located in the Wadden Sea, a shallow region between a series of barrier islands and the northern shoreline of the Netherlands, which consists of tidal flats and marshes. "The majority of our understanding of the protection that these marshes provide originates from theoretical models. Nevertheless, limited data exists on the real-life situation," she adds.

Observations

Marin-Diaz has conducted diverse measurements along the shoreline, such as evaluating the height of mud flats, the breadth of salt marshes, and the vegetation that grows there. The impact of storms was evaluated using wave loggers, and also by gauging the location of flood indicators on the dikes. In addition to her personal measurements, Marin-Diaz received data on wind intensity and direction from the Royal Dutch Meteorological Institute (KNMI). Furthermore, she examined charts going back roughly twenty years to scrutinize changes in the extent of salt marshes. The article in the Journal of Applied Ecology is founded on the observations and measurements gathered throughout three storm seasons.

The outcomes demonstrate that salt marshes truly lower the wave run-up on dikes. "The critical message is that in comparison to uncovered tidal flats, marshes were more successful in decreasing run-up on the dikes," Marin-Diaz elucidates. "This discovery is not reliant on the storm's wind direction, its location with respect to the barrier islands, the grazing status of the marsh, or the vegetation variety." The main factors seemed to be the elevation of the foreshore and the breadth of the marsh: greater foreshores and wider marshes offer increased protection.

Moderating influence

In addition, Marin-Diaz discovered that in regions where the mud flat elevation is relatively low, salt marshes do not exist. Her extended analysis indicated that marshes spread in regions where the adjacent tidal flats were higher (over 0.5m above the local sea level average), while salt marshes mainly receded in places where the mudflats in front of them were affected by erosion. "The areas where marshes are not developing are, in turn, the ones that would require greater protection. Thus, our conclusion is that in certain locations, we will still rely on hard engineering solutions as these nature-based defenses are not adequately effective," she notes.

This is a significant finding for the nearby water authority. "The research furnishes information that permits us to gauge the decrease in wave height and run-up contingent on the presence and attributes of the salt marshes in front," remark Kornelis de Jong and Jan-Willem Nieuwenhuis, two co-authors from the water authority. "We can now differentiate between protective necessities of dike sections with and without salt marshes. This will exert a moderating influence on future reinforcement efforts and their expenses along these sections."

Wading birds

In locations where marshes do not arise naturally, human interventions can aid in their establishment. Marin-Diaz says, "Constructing sedimentation fields or adding sediment to encourage the growth of new salt marshes may be a potential solution, but creating new marshes may also have disadvantages. It would come at the cost of tidal flats, which are essential for wading birds. Moreover, in certain areas, marshes may not grow even with human interventions. In such situations, reinforcing the dikes may be necessary."

Reference: Marin-Diaz, B., van der Wal, D., Kaptein, L., Martinez-Garcia, P., Lashley, C., de Jong, K., Nieuwenhuis, J.-W., Govers, L.L., Olff, H. and T. Bouma: Using salt marshes for coastal protection: effective but hard to get where needed most. Journal of Applied Ecology, 10 May 2023