Hydrodynamics and Sediment Transport in Response to Seawall Removal: Yellow River Delta, China

Rapid development of oil exploitation and local industry and aquaculture, extensive seawalls have been constructed in the Yellow River Delta (YRD) in the past decades. There were 395.4 km of seawalls in the YRD in 2001. Seawalls may result in changes in hydrodynamic conditions, sedimentation and geomorphologic process in the intertidal zones. We applied a hydrodynamic and morphological model Delft3D to investigate the response of an intertidal zone at YRD to seawall removal. After model verification with the water level data recorded at tidal stations, comparison was made between model runs with and without seawalls. The results showed that seawall removal accelerated the water level response in the intertidal zones by a quarter of tidal cycle and increased water level by 2.1 folds compared with presence of seawall. Seawall removal released constraint on water exchange across the interface of the intertidal zone and open sea and thereby increased the current velocity by 3.65 folds. Meanwhile, bed shear stress, which is strongly correlated with velocity (r²= 0.88 and 0.75 in two components), increased by 3 folds after seawall removal. Consequently, erosion increased when the seawall was removed. On the other hand, seawall removal enhanced sedimentation due to restored hydrological connectivity between ocean and the intertidal zones, which brought in sediment from open ocean. Overall, seawall removal led to net sedimentation because sediment deposition greatly exceeded erosion. In contrast, there was no sedimentation occurring when seawall is present. Finally, our results also showed that the seawall removal has negligible impact on open sea areas indicating the impact is primarily focused on the intertidal zone at landward side of seawalls. In conclusion, seawall removal can restore hydrological connectivity between intertidal zones and ocean, which is critical to maintain ecologically function of intertidal zones as sediment traps and ecologic shelters. The results of this study can provide input to appropriate coastal management planning strategies to reduce negative effects of seawall and other similar coastal engineering structures.