Rapid formation of marsh-edge cliffs, Jiangsu coast, China

Abstract

The artificial introduction and rapid expansion of Spartina alterniflora Loisel (S. alterniflora) have greatly changed the natural evolution of tidal flats, especially along the Jiangsu coast, China. The occurrence of an elevation gap at the seaward marsh margin (marsh-edge cliffs) manifests that tidal flats have experienced severe regional erosion and the state transformation from the smooth slope to the stepped profile. However, the mechanisms controlling the formation of marsh-edge cliffs and the state transformation of tidal flats remain elusive while the future evolution of a marsh-cliff-flat system is unclear. Here we show the rapid formation process of marsh-edge cliffs within less than five years in Yancheng Nature Reserve, Jiangsu coast, China, as a fantastic example for the first time, via examining the cross-shore topographic changes during 2008–2014 and investigating sediment characteristics, vegetation biomass, marsh deposition rate, currents and waves during 2013–2014. A one-dimension cross-shore model was used to estimate the relative predominance of waves and currents in response to the variations of tidal flat cross-shore profile and help interpret the formation of marsh-edge cliffs. The fast accretion of the marsh area after the introduction of S. alterniflora (approximately 4.3–5.3cm/yr) accelerated the seaward shift of marsh margin, narrowing the width of bare flat; meanwhile, reduced sediment supply from offshore triggered the down-cutting of the lower bare flat (e.g., the maximum of about 0.5m at the mean sea level from 2010 to 2011), strengthening wave actions to erode bare flat landward. These two processes jointly led to the predominance of waves (0.43–0.90N/m2) over currents (mostly <0.40N/m2) on the upper bare flat, which is consistent with the numerical modeling outcomes. As a consequence, the bed slope increased from 0.6‰ in 2008 to 1.0‰ in 2014, being vulnerable to wave attacks. The vegetation-root bonding of underlying sediments and the wave breaking at the marsh margin finally facilitated the formation of marsh-edge cliffs during 2011–2013. The cliffs were roughly estimated to retreat 20–110m during 2013–2014. The future evolution of tidal flat accompanied by the continuous backward shifting of cliffs, the initiation and seaward spread of new marsh area, and the formation of new marsh-edge cliffs, resemble the hysteresis effect, as seaward advance and backward erosion will take almost equivalent time of nearly 9–50years. This hysteresis effect of tidal flat evolution might further alter the local biological community structure and the pathway of coastal organic carbon flows.