The energy reduction factor for flowslides: a new parameter to assess the energy used for landslide-tsunami initiation

ABSTRACT: Tsunami waves are generally produced either by the displacement of a fault when an earthquake occurs or by the movement of soils caused by an underwater landslide. Sometimes, landslides that originate inland will reach a body of water in their runout zone and may create a tsunami wave, also called impulse wave. This type of tsunami initiator is considered in this paper, more precisely flowslides in sensitive clays that reach a body of water. In the generation of tsunami waves, different factors from the landslide itself must be considered, such as the velocity of the landslide when it reaches the water body and its thickness. These parameters have to be obtained analytically or by using numerical modeling. Most studies on landslide mobility use a conventional approach based on apparent rheological properties calibrated in order to back-calculate the behavior of flowslides. However, apparent rheological properties are not the same as the rheological properties of the material as measured in the lab. This difference is attributed to the energy loss in remolding, which is not directly taken into account when using the conventional approach. In this paper, a new factor, using the destructuration index concept, is introduced in order to take into account the energy required to remold the material: the Energy Reduction factor (FER). In modeling the runout simulation it provides a more realistic simulation of the runout behavior. This factor was determined for a sensitive clay flowslide which took place in Quebec. For the flowslide studied here, the use of this factor reduces the maximum velocity of the landslide by a factor of 3.5, which will decrease the initial tsunami wave height by a 2.4 factor.