Wave Attenuation at a Salt Marsh Margin: A Case Study of an Exposed Coast on the Yangtze Estuary |
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| Authors: | S L Yang B W Shi T J Bouma T Ysebaert X X Luo |
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| Institution: | 1.State Key Laboratory of Estuarine and Coastal Research,East China Normal University,Shanghai,People’s Republic of China;2.Netherlands Institute of Ecology (NIOO),Yerseke,The Netherlands |
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| 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). |
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