MECHANISM OF SEAWALL DESTRUCTION DUE TO TSUNAMI
Science of Tsunami Hazards
Tsunami that happened along Sunda Strait on 22 December 2018 was generated by the collapse of the flank of Anak Krakatau volcano in Indonesia. The tsunami hit Lampung and Sunda beaches areas. The hydrodynamic force of the tsunami hit everything on its course severely including seawalls that are used to protect the beach from erosion due to wind waves. The stability of such structure is limited to the design wind wave force. Tsunami has a significantly different nature compare to wind waves.
... to wind waves. When tsunami arrives at the coastline it becomes surge where, the water particle movement is significantly faster than that of the wind waves. The force of such high speed of water particles at a seawall may destroy and drag away the material far from the original location. Field survey was conducted following the tsunami event along Sunda Strait in Banten area. Based on the survey, a hydraulic simulation of seawall destruction due to tsunami was carried out in the laboratory. It was found that there were two distinct mechanisms of seawall destruction. When the land support of the seawall is relatively weak, the seawall may be dragged landward. However, when the land support behind the seawall is relatively strong, for example more than 40% of the seawall's total height that stabilize the seawall against the turning moment, the seawall may be dragged seaward by the tsunami return flow (run down). These mechanisms are relevant to tsunami mitigation. Consideration of seawall collapse due to return flow of tsunami should be taken into account since, normally tsunami is a wave train that attacks the beach in a series.