Abstract
Foreshores consisting of both bare tidal flats and vegetated salt marshes are found worldwide and they are well studied for their wave attenuating capacity. However, most studies only focus on the small scale: just some isolated locations in space and only up to several years in time. In order to stimulate the implementation of foreshores serving as reliable coastal defense on a large scale, we need to quantify the decadal wave attenuating capacity of the foreshore on the scale of an estuary. To study this, a unique bathymetrical dataset is analyzed, covering the geometry of the Westerschelde estuary (The Netherlands) over a time-span of 65 years. From this dataset, six study sites were extracted (both sheltered sites and exposed sites to the prevailing wind direction) and divided into transects. This resulted in 36 transects covering the entire foreshore (composed of the bare tidal flat and the vegetated salt marsh). The wave attenuation of all transects under daily conditions (with and without vegetation) and design conditions (i.e. events statistically occurring once every 10,000 years) was modelled. Overall, the spatial variability of the geometry of a single foreshore was observed to be much larger than the temporal variability. Temporal changes in salt marsh width did not follow the variability of the entire foreshore. Both under daily and design conditions, vegetation contributes to decreasing wave energy and decreases the variability of incoming wave energy, thereby decreasing the wave load on the dike. The southern foreshores, sheltered from the prevailing wind direction, were more efficient in wave attenuation than the exposed northern foreshores. A linear relation between marsh width and wave attenuation over a period of 65 years was observed at all marshes. The present study provides insights needed to calculate the length of a salt marsh to obtain a desired minimum wave attenuating capacity.
| Original language | English |
|---|---|
| Article number | 103628 |
| Journal | Coastal Engineering |
| Volume | 156 |
| DOIs | |
| Publication status | Published - 1 Mar 2020 |
Funding
This work is part of the research program BE SAFE, which is financed by the Netherlands Organization for Scientific Research ( NWO ) (grant 850.13.010 ). Additional financial support has been provided by Deltares , Boskalis , Van Oord, Rijkswaterstaat , World Wildlife Fund , and HZ University of Applied Science . Bas W. Borsje was supported by the Netherlands Organization for Scientific Research ( NWO-STW-VENI; 4363 ). Data and scripts in support of this manuscript are available at https://doi.org/10.4121/uuid:4c25347f-f71e-466b-be3c-1fe8f8d8784c . Appendix A Table 3 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Zuidgors. Table 3 Zuidgors (ZUI) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 0 −160 120 −30 15 1–10 years 0 −210 280 −65 40 11–20 years −5 −225 310 −105 100 21–30 years 10 −165 365 −100 175 31–40 years 25 −135 350 −84 235 41–50 years 15 −160 375 −100 230 Table 4 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Baarland. Table 4 Baarland (BAA) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 0 −35 295 −5 95 1–10 years 5 −35 915 −5 194 11–20 years 50 −20 930 5 715 21–30 years 30 −15 945 15 725 31–40 years 40 10 955 20 740 41–50 years 60 20 965 40 800 Table 5 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Zimmermanpolder. Table 5 Zimmermanpolder (ZIM) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 5 −55 45 −5 10 1–10 years 10 −75 105 −15 50 11–20 years 45 −65 195 −5 105 21–30 years 75 −25 225 30 160 31–40 years 105 30 265 55 185 41–50 years 130 30 285 65 210 Table 6 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Hoofdplaat. Table 6 Hoofdplaat (HOO) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 0 −25 50 −5 15 1–10 years 5 −45 90 −5 35 11–20 years 20 −40 115 −15 65 21–30 years 20 −45 125 −15 80 31–40 years 20 −50 125 −20 90 41–50 years 33 −50 130 −25 110 Table 7 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Paulinapolder. Table 7 Paulinapolder (PAU) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 0 −100 130 −15 25 1–10 years 0 −215 215 −80 45 11–20 years −10 −285 170 −135 25 21–30 years −55 −305 45 −200 10 31–40 years −95 −325 20 −255 10 41–50 years −115 −340 20 −260 10 Table 8 Marsh width change over a period of a single year, periods between 1 and 10, 11 and 20, 21 and 30, 31 and 40 and 50 years, for Hellegatpolder. Table 8 Hellegatpolder (HEL) Median (m) Minimum (m) Maximum (m) 10th percentile (m) 90th percentile (m) 1 year 0 −35 65 −10 5 1–10 years 0 −155 215 −35 15 11–20 years −10 −160 270 −85 10 21–30 years −35 −215 115 −95 10 31–40 years −45 −205 115 −125 0 41–50 years −55 −205 110 −140 15 Table 9 Average width (m) of the marsh and bare tidal flat over the assessed period and all transects per foreshore. Table 9 Location Average width marsh (m) Average width bare tidal flat (m) Zuidgors (ZUI) 483 634 Baarland (BAA) 152 1977 Zimmermanpolder (ZIM) 205 1253 Hoofdplaat (HOO) 69 275 Paulinapolder (PAU) 258 552 Hellegatpolder (HEL) 125 677 Appendix B Table 10 Coefficients for determining relation between the vegetated marsh width and wave attenuation. The relation can be determined using y = ax + b, where a and b are linear coefficients, y is the wave attenuation and x is the width of the marsh. Table 10 Location Coefficient a Coefficient b Zuidgors (ZUI) 0.0273 0.8726 Baarland (BAA) 0.0235 0.9051 Zimmermanpolder (ZIM) 0.0222 1.8731 Hoofdplaat (HOO) 0.1232 1.3113 Paulinapolder (PAU) 0.0453 3.3875 Hellegatpolder (HEL) 0.0643 3.0042
Keywords
- Building with Nature
- Coastal protection
- Foreshore
- Nature Based Flood Defense
- Salt marsh
- SWAN
- Wave attenuation