Wave Attenuation at a Salt Marsh Margin: A Case Study of an Exposed Coast on the Yangtze Estuary

Abstract

To quantify wave attenuation by (introduced) Spartina alterniflora vegetation at an exposed macrotidal coast in the Yangtze Estuary, China, wave parameters and water depth were measured during 13 consecutive tides at nine locations ranging from 10 m seaward to 50 m landward of the low marsh edge. During this period, the incident wave height ranged from <0.1 to 1.5 m, the maximum of which is much higher than observed in other marsh areas around the world. Our measurements and calculations showed that the wave attenuation rate per unit distance was 1 to 2 magnitudes higher over the marsh than over an adjacent mudflat. Although the elevation gradient of the marsh margin was significantly higher than that of the adjacent mudflat, more than 80% of wave attenuation was ascribed to the presence of vegetation, suggesting that shoaling effects were of minor importance. On average, waves reaching the marsh were eliminated over a distance of ∼80 m, although a marsh distance of ≥100 m was needed before the maximum height waves were fully attenuated during high tides. These attenuation distances were longer than those previously found in American salt marshes, mainly due to the macrotidal and exposed conditions at the present site. The ratio of water depth to plant height showed an inverse correlation with wave attenuation rate, indicating that plant height is a crucial factor determining the efficiency of wave attenuation. Consequently, the tall shoots of the introduced S. alterniflora makes this species much more efficient at attenuating waves than the shorter, native pioneer species in the Yangtze Estuary, and should therefore be considered as a factor in coastal management during the present era of sea-level rise and global change. We also found that wave attenuation across the salt marsh can be predicted using published models when a suitable coefficient is incorporated to account for drag, which varies in place and time due to differences in plant characteristics and abiotic conditions (i.e., bed gradient, initial water depth, and wave action).

Nature-based flood protection: The efficiency of vegetated foreshores for reducing wave loads on coastal dikes

“…wave periods) (Van Gent and Doorn, 2001). Three data sets were used for model calibration and validation: the data from Hellegat and Bath, described in this paper, and wave data from a salt marsh covered with Spartina alterniflora (smooth cordgrass) at East Chongming island, China, reported in Yang et al (2012). For the calibration and validation of the SWAN model, the bathymetry of the field sites (Fig.…”

Engineering with Nature to Reduce Wave Energy in Wetlands

“…The seasonal patterns of wave attenuation observed in the British saltmarsh and seagrass environments appear to correspond to seasonal growth cycles of the vegetation (Möller and Spencer, 2002;Möller et al, 2006;Paul and Amos, 2011). In the Yangtze Estuary, S. alterniflora demonstrated a greater capacity to attenuate waves than the shorter, more flexible native species Scirpus mariqueter (Ysebaert et al, 2011) and exhibited rates of wave attenuation that were 1 to 2 orders of magnitude greater than an adjacent mudflat (Yang et al, 2012).…”

Coastal wetland loss, consequences, and challenges for restoration

“…According to a study at the Florida coast, a 7 km wide strip of mangroves can reduce more than 70% of wave height during hurricanes (Zhang et al 2012). In salt marshes, the wave height decreased exponentially with the landward distance from the marsh edge (Yang et al 2012).…”

“…For example, how to compromise between the different functionalities of coastal wetlands, such as S. alterniflora? Its functions for coastal protection and sediment trapping are considered to be positive along the Chinese coastline (Yang et al 2008(Yang et al , 2012, but as an invasive species, it has negative effects on the native organisms (He et al 2012), especially in southern China (Gao et al 2014). In contrast, in the middle Atlantic coast of the US, the invasive Phragmites australis proved to be more effective in combating sea level rise with higher mineral and organic sediment trapping ability than the local S. alterniflora (Rooth and Stevenson 2000).…”