The effect of large debris damming on scour behind a seawall due to tsunami

Debris damming can happen when the debris stuck on buildings or trees. The damming causes many problems such as an increase of the tsunami force on the buildings or the trees and an increase of the scour around the buildings or the trees. When large debris such as ship is stuck on a seawall that is aimed at tsunami protection, part of the water flow sideways of the debris. Such condition may affect the intensity of the flow on both sides of the debris while on the other hand change the scour capacity of the water behind the seawall. It is expected that the scour depth at exactly behind the debris reduces but, the scour behind both sides of the debris increase. Such conditions are important to be considered when designing a seawall for tsunami protection. A model of a seawall was constructed in a hydraulic laboratory. Tsunami attack on the seawall was simulated both with and without debris. The bed material was made of medium to coarse gravel. The scour behind the seawall was measured after each simulation. The tsunami flow was simulated based on the tsunami hydrograph recorded during the East Japan Tsunami in 2011. A variation of flows, debris sizes, seawall heights and the bed material were employed to study the effect of the debris damming on the scour more fully. The experiment showed that the scour behind the seawall due to tsunami without debris was significantly different to those with debris. As expected, more significant difference of scour depth was observed for larger debris size. The scour are deeper and longer at both sides of the debris. The increase of scour depths are 54, 43 and 10 for debris length to seawall height ratio (ld/hb) equals 1.0, 0.66 and 0,33, while the maximum scour length increase are 17, 14, and 8. The flow discharge over the wall, the wall height and the material are also found to be important variables to be considered in the design of seawall as tsunami protection. © 2020 22nd Congress of the International Association for Hydro-Environment Engineering and Research-Asia Pacific Division, IAHR-APD 2020: "Creating Resilience to Water-Related Challenges". All rights reserved.