Flood safety

Many modern trends facilitate the need for the further development of systems that mitigate impacts of coastal flood hazards and promote resilience of nearshore communities and ecosystems. Mangroves and other natural coastal defenses have the potential to augment traditional engineered coastal structures in preventing events such as wave overtopping. In order to effectively design infrastructure combining hybrid engineered systems, costly testing at 1:1 scale is required. This study addresses a knowledge gap in defining the nature of the interactions between "green" and "gray" coastal defenses, focusing on overtopping and scaling experimental results. The study aims to analyze data from tests at different scales and compile a methodology for designing prototype scale tests from small scale experiments to identify the relative importance of friction and scaling effects moving from prototype to small scale experiments. With such a methodology, these reduced-scale physical models would become better suited to provide design feedback.

Coastal flood risk is projected to increase significantly in the future due to climate change (sea level rise, storm surge) and ongoing land subsidence. In these usually highly urbanized regions, flood defenses are typically hard 'grey' structures such as levees. Levees require regular maintenance and periodic strengthening to meet safety standards. The structural and economic limit of strengthening levees is reaching limits. Instead, more sustainable methods are explored (Nature based Solutions). One of these solutions are tidal marshes. Tidal marshes attenaute waves, reducing levee failure probability, but also reduce flood impact in the event of a levee brach. In order to quantify this secondary effect, levee breach models should include foreshore erosion as well. In this study we performed seven large-scale experiments with and without a foreshore. Our preliminary results show a distinct difference in inflow hydrodynamics which should be considered for modelling efforts.

The combination of physical laboratory experimentation and the coupling of two CFD numerical models was employed to study the hydraulic behavior and run-up in different typologies of hybrid solutions for coastal flood protection. The results obtained reveal significant discrepancies between the run-up measured in the laboratory and the run-up that can be obtained through traditional engineering equations after propagating the waves through vegetation using a numerical model. This highlights the importance of reconsidering the approach typically used to analyze such solutions.

The Qiantang River is a typical strong tide estuary, the world-famous tidal bore have huge turbulent energy causes damage to the sea wall. Spur dike is an important engineering to prevent the seawall foundation from tidal bore. But the tidal bore occurs at the low tide level, strong turbulence, high flow velocity, water level rises sharply. The spur dike height, length, inclination angle need to be determined by physical model according to the specific bore dynamic conditions. In the tests, the fixed-bed model was used to study the tidal current velocity reduction rate and region, the falling current circulation characteristics in dike field. The movable-bed model test is used to study the effect of each test on protecting the beach. This physical model study reveals the rule of spur dike influence on tidal bore dynamics so the effect of spur dike on protecting the beach is clear.