The Impact of Reclamation on Tidal Flat Morphological Equilibrium

Abstract

Reclamation is one of the most prominent anthropogenic activities affecting tidal flat morphology and the related ecosystem service. Two representative types of reclamation are upper-flat enclosure and lower-flat enrockment. From a historical perspective, different type of reclamation was adapted in different areas in ancient China. As previous studies on reclamation are often site-specific, the reason that leads to such a difference is unclear. The intertidal dynamic equilibrium theory (DET) provides a comprehensive framework for this quest. Here, we extend the DET with additional effects of reclamation. A model based on DET (DET-ESTMORF) was applied to investigate the impact of reclamation on tidal flats. The model was validated by comparing our results against previous records and observations. Results show that both types of reclamation induce morphological adjustment by enhancing local accretion, which upset the previous equilibrium. Specifically, upper-flat enclosure drives tidal flats into evolution toward wave dominance, whereas lower-flat enrockment causes adjustment toward tidal dominance. Next, the impact of reclamation on tidal flats in different states (prograding or retreating) was investigated. We show that both enclosure and enrockment can induce accretion despite the varying wave climate and sediment supply, with the mean accretion rate raised by 1.8 cm/year and 1.2 cm/year, respectively. However, the resulting profiles are different in shape, especially on retreating flats. Finally, we point out that tidal range and sediment supply are the potential reasons affecting the choice of reclamation types in practices.

The shape of fringing tidal flats in engineered estuaries

“…In contrast to previous studies on tidal flat shape and evolution, we focused on fringing tidal flats, characterized by dominant longshore currents. A substantial part of previous studies focused on the equilibrium tidal flat shape for tidal flats along open coasts (Roberts et al, 2000;Pritchard and Hogg, 2003;Friedrichs, 2011;Winterwerp et al, 2013a;Zhong and Hu, 2021). For these flats the cross-shore shape is determined by the relative contribution of waves and the crossshore current.…”

“…As a result, observations of the impact of a dike relocation do not exist. Using a cross-shore profile model, Zhong and Hu (2021) concluded that tidal flats become accreting and steeper when reducing the tidal flat length because the cross-channel velocity gradients become smaller. On the other hand, a reduction of the tidal flat length also reduces the gross sediment transport and therefore sediment supply.…”

Tidal Flat Erosion Processes and Their Dynamic Mechanisms on the South Side of Sheyang River Estuary, Jiangsu Province

“…The geomorphology and sediment composition of tidal flats are controlled by hydrodynamics, sediment supply, and biological activities [1]. The area corresponding to the tidal flat is gradually decreasing [3] due to sea-level rise, land subsidence, and intensifying human activities in river basins and coastal zones, of which human activities are the main driving factor [10] causing most tidal flats in the world to experience erosion [42][43][44]. Coastal erosion generally manifests as shoreline retreat and bed erosion [13,42].…”

Dynamics and drivers of tidal flat morphology in China

“…To obtain the tidal-induced bottom bed shear stress, we used the same method as the original DET-ESTMORF 14 . Concerning the bottom-bed shear stress induced by wave propagation, we did not utilize the SWAN model as in the original DET-ESTMORF model to compute wave height distribution but instead calculated the along-range distribution of wave height by computing the energy loss due to bottom -bed friction 59 . The attenuation of wave energy can be described using the continuum equation for wave energy flux as follows 60 :…”