Numerical investigation of the influence of coastal structure removal on sand spit morphology

The application of hard engineering solutions in coastal protection has been controversial because of their potential environmental impacts. Selective removal of intrusive coastal structures offers an alternative to address the coastal erosion problem. The key idea is to remove the erosion-causing structures on highly artificial coasts, triggering the natural recovery of eroded shorelines. However, the effectiveness and underlying mechanisms of this approach in real coastal environments remain poorly understood. This study investigates the influence of coastal structure removal on coastal evolution using a computationally efficient coastline model ShorelineS. The analysis focuses on the sand spit at the Fudu Estuary in Liaoning Province, China, which is a stretch of morphologically complex coast suffering from erosion due to intense coastal engineering. An estuary jetty, an attached breakwater, an artificial island, and cofferdams were constructed along the 2 km coast between 2010 and 2015. Validated by annual satellite-derived shorelines and hourly hindcast wave data, the model can well reproduce the morphological change of the sand spit both in the natural state and under the influence of man-made structures. Post-removal shoreline evolutions are quantitatively analyzed in nine coastal structure removal scenarios, so that the optimal removal scheme and the shoreline recovery mechanisms can be identified. The results demonstrate that rational removal of coastal structures can reverse shoreline erosion and landform degradation by changing nearshore wave conditions, regulating sediment transport paths, and increasing sediment inputs. The present study suggests that the general principle of removing coastal structures to enhance coastal resilience in a broader coastal setting is to restore the balance between sediment loss and input, basing on understanding of sediment transport mechanisms and impacts of structures on coastal morphology.