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

TLDR

High incident wave heights (up to 1.5 m) at this exposed macrotidal coast are much greater than those recorded in other salt marshes worldwide. The study aimed to quantify how introduced Spartina alterniflora attenuates waves by measuring wave parameters and water depth at 13 consecutive tides across nine sites from 10 m seaward to 50 m landward of the marsh edge. Measurements revealed that wave attenuation over the marsh was 1–2 orders of magnitude higher than over adjacent mudflats, with vegetation responsible for over 80 % of the reduction; waves were fully damped within ~80 m (requiring ≥100 m for peak waves), a distance longer than in American marshes, and attenuation correlated inversely with the water‑depth‑to‑plant‑height ratio, confirming that tall introduced shoots are more effective and that published models can predict attenuation when adjusted for site‑specific drag.

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).