The impact of salt marsh change on sediment accumulation and wave attenuation at the East Chongming Island

As a key part of substance and energy exchange between land and ocean, estuarine wetland is the hotspot and focus of multiple disciplines. Under natural and artificial influences such as sea level rising, human reclamation, nature conservation, and exotic species invasion, salt marsh in the East Chongming wetlands had undergone severe changes during the past decades. Based on field investigation, remotely sensed images were classified and corrected for the periods of 1980, 1990, 2000 and 2010 at the East Chongming Island. In combination with existing research results, the impact of salt marsh change on sediment accumulation and wave attenuation of the East Chongming wetlands were quantified. The results showed that: the area ratio of Phragmites / Spartina / Scirpus were 23:0:77, 73:0:27, 33:9:58 and 34:34:32 in 1980, 1990, 2000 and 2010, respectively, with the total area of salt marsh vegetation changed from 51 km² to 83 km², 37 km² and 44 km². Under constant sedimentation during 1980-1990, the total amount of suspended particulate matter adhered by vegetation increased by 2669 t, while the amount of sediment accumulation increased by 619×10⁴ m³. The length of seawall without enough vegetation protection under normal tidal heights and under storm surge decreased by 5 km and 14 km, respectively. Due to intensive human reclamation during 1990-2000, the total amount of suspended particulate matter adhered byvegetation decreased by 2453 t, the amount of sediment accumulation decreased by 1384×10⁴ m³. The length of seawall without enough vegetation protection under normal tides and under storm surge increased by 7 km and 21 km, respectively. With the influences of constant sedimentation and exotic species invasion during 2000-2010, the total amount of suspended particulate matter adhered by vegetation increased by 2503 t, and sediment accumulation increased by 91×10⁴ m³. The length of seawall without enough protection under normal tides and under storm surge decreased to 6 km and 20 km, respectively. Saltmarsh vegetation protection is of great importance for land accretion and for the safety behind the seawall, in addition to its importance for a number of other ecological functions such as bio-productivity, biodiversity, and purification.