To protect facilities and properties under the increasing threat of coastal disasters due to climate change and human interventions, or to meet the demands of economic development, maritime transportation, resource utilization, and recreational activities, a mass of coastal structures, including seawalls, breakwaters, inlet jetties, wharves, and artificial islands, have been built along the coastline. Approximately 40% of the world’s population lives within 100km of the coastline and the trends of coastward urbanization and population growth will continue in the future.
A high proportion of artificial coastlines has already been considered an important factor affecting coastal erosion vulnerability. Adverse impacts posed by these hard structures have been recognized. Specifically for sandy coastlines, the erosion of adjacent beaches is of great concern, which may threaten coastal properties and lead to the loss of natural landscapes.
Coastal managers and scientists are seeking more nature-based solutions to the adverse impacts of hard structures. Beach replenishment, including beach, dune, and shoreface nourishments, has been widely used to mitigate beach erosion and rebuild landscapes. Sand bypass systems have also been implemented to copy with the littoral drift interruption by transporting the updrift sand through a pipeline network to the downdrift of coastal structures. Besides the above well-recognized approaches, a potential yet loess studied alternative option is the direct removal of unsuited coastal structures, i.e., removing the original cause of disturbance to trigger natural recovery. This has recently emerged as an increasingly attractive option being considered and implemented in coastal restoration engineering.
While this approach could be promising for natural beach morphology recovery, little understanding or estimation of the recovery ability remains a bottleneck for evaluating its effectiveness, and creates uncertainty for its planning and application in different sites. More specifically, there are key questions related to the process of beach morphological response to structure removal, including whether and to what extent the shoreline will recover to the state without features caused by the structures, how long the recovery will take, and which factors will influence the recovery ability. These questions are more complex considering that some unique morphological features developed near the structures, e.g., the sharply discontinuous shoreline on the two lateral sides of an attached breakwater or the tombolo behind a detached breakwater, will be exposed to very inadaptive hydrodynamic environment after structure removal, and thus exhibit dramatic evolution with possibly shoreline discontinuity, spit formation, and shoreline merging processes. However, previous studies have focused on the physical mechanism of beach morphological evolution after structure removal has rarely been documented.
As an exploratory work, this study focuses on sandy shoreline evolution after the removal of an attached breakwater, a classic coastal structure extending from the shore, blocking longshore sediment transport and causing significant downdrift beach erosion. The recently developed ShorelineS model was used to reveal the shoreline recovery ability under simplified and generalized conditions. Sandy shorelines have the ability to naturally recover after the removal of an attached breakwater, that is, the accretion and erosion caused by the breakwater will be flattened after breakwater removal and shoreline can recover to the original straight shape. The recovery duration was found to range from years to centuries, and the recovery ratio can range from full recovery to partial recovery (or even none recovery), both of which depend on the site-specific structure and environmental conditions. Overall, a higher shoreline recovery ability (i.e., shorter duration and larger ratio) is related to shorter effective breakwater length, downdrift inclined breakwater direction, lower angle of wave incidence, and greater longshore sediment transport rate. The behavior of the sand spit evolution, which initially forms at the tip of the breakwater deposition, is the dominant mechanism that leads to distinct shoreline recovery processes. Preliminary formulas were proposed to quantitatively estimate shoreline recovery duration and recovery ratio, which account for the important mechanisms for recovery ability. They are simple and allow for a quick evaluation of shoreline recovery ability supporting coastal management and project planning.
The relevant research results have been published in the academic journal Frontiers in Marine Science (Volume 10, 2023), with Dr. Chi Shanhang and Professor Zhang Chi as the first and corresponding authors, respectively. This study has been supported by national key research and development projects, key projects of the National Natural Science Foundation of China, and basic research business expenses projects of central universities.