Fast coastal tsunami amplitude forecasting along the French Mediterranean shoreline based on a transfer function method

In the framework of operational conditions, the real time coastal modeling in near field is challenging to obtain accurate and reliable tsunami warning products for flooding hazard. Maps of inundation and impacts for planning community response can be produced through coastal predictions with run-up computation by solving numerically high-resolution forecast models in real time, taking into account all local effects. However, these runs are too time consuming in near field and operational context. An alternative approach is based on early prediction tools of the coastal wave amplitude calculated from empirical laws or transfer functions derived from these laws. Such tools are suitable in near field context (almost ten times faster than the high-resolution runs), but all local effects are not well taken into account and the assessment of run-up is missing. The linear approximations of coastal tsunami heights are provided very quickly using the maximum wave heights from a computationally cheap regional forecast, with global and conservative estimates.

Within the French Tsunami Warning Center (CENALT), a forecasting tool based on a transfer function method is being implemented. This fast prediction technique is based upon a recently extended version of the usual Green's Law (Giles et al., 2022[1]), which introduces local amplification parameters with the aim of capturing the neglected localized effects. The method includes an automated approach which optimizes for these local amplification parameters by minimizing a cost function.

Local amplification parameters are calculated for the entire French Mediterranean coastline at 25 m resolution from a data set of 12 scenarios (high-resolution simulations). The forecasting results capabilities are analyzed, and shown for several coastal sites. The local tsunami wave heights modeled from the transfer function present a good agreement with the time-consuming high resolution models. The linear approximation is obtained within 1 min and provides globally estimates within a factor of two in amplitude. Although the resonance effects in harbors and bays are not reproduced and the horizontal inundation calculation needs to be studied further, this tool is well suited for an early first estimate of the coastal tsunami threat forecast.

[1] Giles, D., Gailler, A., & Dias, F. (2022). Automated Approaches for Capturing Localized Tsunami Response—Application to the French Coastlines. Journal of Geophysical Research: Oceans, 127(6), e2022JC018467.