Potential morphological responses of an artificial beach to a flood in extreme events: field observation and numerical modelling

Conch Island is a typical artificial island at the Tanghe Estuary in Bohai Sea, China. To improve natural environment and boost local tourism, beach nourishment will be applied to its north-western shore. The projected beach is landward and opposite to the Jinmeng Bay Beach. Nowadays, with climate changes, frequent heavy rainfalls in Hebei Province rise flood hazards at the Tanghe Estuary. Under this circumstance, potential influences on the projected beach of a flood are investigated for sustainable managements. A multi-coupled model is established and based on the data from field observations, where wave model, flow model and multi-fraction sediment transport model are included. In addition, the impacts on the projected beach of different components in extreme events are discussed, including the spring tides, storm winds, storm waves, and sediment inputs. The numerical results indicate the following result. (1) Artificial islands protect the coasts from erosion by obstructing landward waves, but rise the deposition risks along the target shore. (2) Flood brings massive sediment inputs and leads to scours at the estuary, but the currents with high sediment concentration contribute to the accretions along the target shore. (3) The projected beach mitigates flood actions and reduces the maximum mean sediment concentration along the target shore by 20%. (4) The storm winds restrict the flood and decrease the maximum mean sediment concentration by 21%. With the combined actions of storm winds and waves, the maximum value further declines by 38%. (5) A quadratic polynomial relationship between the deposition depths and the maximum sediment inputs with flood is established for estimations on the potential morphological changes after the flood process in extreme events. For the uncertainty of estuarine floods, continuous monitoring on local hydrodynamic variations and sediment characteristics at Tanghe Estuary is necessary.