Case study of toe scour and deposition at seawalls in the Qiantang River estuary

In tidal estuarine regions, toe scour and deposition at seawalls, which are influenced by runoff and tidal flow, present complexities not found in inland rivers and wave-dominated coasts. Existing methods for predicting toe scour at seawalls are often inaccurate in estuarine environments and can be computationally demanding, limiting their practical application. Hence, there is a need for a simplified, reliable approach tailored to these dynamic settings. This study investigated the characteristics of toe scour and deposition through a case study in the Qiantang River Estuary on the basis of field data. We analyzed continuous monthly riverbed topography data from five cross-sections collected between 2011 and 2018 to examine riverbed profiles and their temporal variations in front of the seawall. In addition, we elucidated the underlying scour and deposition mechanisms. The results indicate that the riverbed slope at the seawall toe ranges from 14° to 27° over 50 m, whereas the riverbed flattens and significantly fluctuates between 50 and 100 m. Toe scour frequently occurs when the maximum daily average runoff discharge exceeds 5000 m3/s or when the average daily runoff discharge surpasses 2200 m3/s. Conversely, deposition is primarily observed with larger tidal ranges and lower runoff conditions. Furthermore, we developed a process-based prediction method (PPM) based on the sediment transport capacity to predict scour depths during various flood events. This method achieves a relative error within 20 %, showing improved accuracy over existing approaches. This study provides a reliable method to predict toe scour at seawalls in tidal estuaries, supporting safer seawall design and more effective maintenance strategies.