Generating reliable estimates of tropical-cyclone-induced coastal hazards along the Bay of Bengal for current and future climates using synthetic tracks
Authors: Leijnse Tim Willem Bart , Giardino Alessio , Nederhoff Kees , Caires Sofia 
Published: June 2022

Abstract. Deriving reliable estimates of design water levels and wave conditions resulting from tropical cyclones is a challenging problem of high relevance
for, among other things, coastal and offshore engineering projects and risk assessment studies. Tropical cyclone geometry and wind speeds have been
recorded for the past few decades only, thus resulting in poorly reliable estimates of the extremes, especially in regions characterized by a
low number of past tropical cyclone events. In this paper, this challenge is overcome by using synthetic tropical cyclone tracks and wind fields
generated by the open-source tool TCWiSE (Tropical Cyclone Wind Statistical Estimation Tool) to create thousands of realizations representative of
1000 years of tropical cyclone activity for the Bay of Bengal. Each of these realizations is used to force coupled storm surge and wave
simulations by means of the processed-based Delft3D Flexible Mesh Suite. It is shown that the use of synthetic tracks provides reliable estimates of
the statistics of the first-order hazard (i.e., wind speed) compared to the statistics derived for historical tropical cyclones. Based on estimated
wind fields, second-order hazards (i.e., storm surge and waves) are computed that are generated by the first-order hazard of wind. The estimates of
the extreme values derived for wind speed, wave height and storm surge are shown to converge within the 1000 years of simulated cyclone
tracks. Comparing second-order hazard estimates based on historical and synthetic tracks shows that, for this case study, the use of historical
tracks (a deterministic approach) leads to an underestimation of the mean computed storm surge of up to −30 %. Differences between the use of
synthetic versus historical tracks are characterized by a large spatial variability along the Bay of Bengal, where regions with a lower probability
of occurrence of tropical cyclones show the largest difference in predicted storm surge and wave heights. In addition, the use of historical tracks
leads to much larger uncertainty bands in the estimation of both storm surges and wave heights, with confidence intervals being +80 % larger
compared to those estimated by using synthetic tracks (probabilistic approach). Based on the same tropical cyclone realizations, the effect that
changes in tropical cyclone frequency and intensity, possibly resulting from climate change, may have on modeled storm surge and wave heights was
computed. As a proof of concept, an increase in tropical cyclone frequency of +25.6 % and wind intensity of +1.6 %, based on literature
values and without accounting for uncertainties in future climate projection, was estimated to possibly result in an increase in storm surge and
wave heights of +11 % and +9 %, respectively. This suggests that climate change could increase tropical-cyclone-induced coastal hazards
more than just the actual increase in maximum wind speeds.