Observations of tidal and storm surge attenuation in a large tidal marsh

Abstract

Tidal wetlands are increasingly valued for their role in coastal defense. Nevertheless, in situ observations of storm surge attenuation within wetlands are still scarce. We present water level measurements along a 4 km intertidal channel and on the surrounding marsh platform for regular spring to neap tides and two major storm surge tides, showing the effects of flood wave height and marsh geomorphology on the amount of flood wave attenuation. Undermarsh tides with peak water levels below marsh platform elevation are mostly amplified (up to 4 cm/km) within the channels. Overmarsh tides with peak water levels above the marsh platform are generally attenuated along the channels, with maximum attenuation rates of 5 cm/km for tides that inundate the marsh platform by 0.5–1.0 m. For lower or higher flood waves, including storm surges, attenuation rates decrease. Furthermore, the observations show that the maximum attenuation occurs along narrow channel transects where the width of the platform is larger, whereas attenuation rates are lower along wider channels with smaller adjacent marsh platforms. These observations are confirmed by an analytical approximation of tidal wave propagation through convergent channels. The analytical model indicates that differences in attenuation rates are induced by variations in the cross‐channel averaged friction between channel sections and between tides with varying peak water levels. Finally, the highest attenuation rates of up to 70 cm/km are observed over short distances on the vegetated marsh platform. We conclude that this study provides an empirical basis for the wider implementation of nature‐based flood defense strategies.

Potential of marshes to attenuate storm surge water level in the Chesapeake Bay

“…These observations are in concordance with our findings and the lowest rates or water level amplifications were computed during the highest storm tides. We also observed that these effects could be a result of an increase in the storage area, a reduced bottom friction for higher water depths and lower friction exerted over the flow by the vegetation submergence in accordance with Stark et al ().…”

“…Similarly, moderate and major events revealed also a wide range of attenuation rates, especially at the MGB site. This wide variation was also observed by Stark et al () across their short non‐channeled transects (50–100 m) on the marsh platform in the Netherlands. Other studies performed on the East Coast of the U.S. such as van der Molen () found that the astronomical tide elevations were attenuated from −2 cm/km (amplification) to 11 cm/km over a larger tidal marsh (Item 10 in Table ).…”

“…The largest variability on attenuation rates found on the ES1–ES2 reach might be associated with the largest variation of water level at ES1 and to the short distance between sensors. Stark et al () also analyzed three short transects (50–100 m) located at the marsh platform and the marsh channel, and similarly, observed higher variability in the attenuation rates (the equivalent to a range from −0.005 cm/m to 0.07 cm/m). The higher attenuation capacity of this reach is also related to the impact of the edge (and therefore, abrupt change in water depth) and the contribution of wave set‐up to each station.…”

“…Additionally, water level amplification occurred at this section of marsh for certain events with water levels over 1.2 m depth at ES4 (Winter Storm [2016] and Hurricane Hermine [2016] events). Stark et al () observed that water levels were attenuated only during events that resulted in water levels between 0.5 m and 1 m above the marsh platform and minimum attenuation or amplification was found on the highest recorded storm tides. These observations are in concordance with our findings and the lowest rates or water level amplifications were computed during the highest storm tides.…”

Establishing vegetated foreshores to increase dike safety along lake shores

“…Vegetated foreshores, such as salt marshes and mangroves are commonly occurring in deltaic systems where tides play a role in the sediment dynamics of the location [1,2]. These foreshores create additional safety against flooding, as they are able to reduce incoming wave energy and withstand erosive forces [3,4].…”