Abstract
Deriving reliable estimates of design water levels and wave conditions resulting from tropical cyclones is a challenging problem of high relevance for, among others, coastal and offshore engineering projects and risk assessment studies. Tropical cyclone geometry and wind speeds have been recorded for the past few decades only, therefore resulting in poorly reliable estimates of the extremes, especially at 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), to create thousands of realizations representative for 1,000 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. The estimates of the extreme values derived for wind speed, wave height and storm surge are shown to converge within the 1,000 years of simulated cyclone tracks. Comparing second-order hazard estimates based on historical and synthetic tracks show that, for this case study, the use of historical tracks (a deterministic approach) leads to an underestimation of the mean computed storm surge 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 modelled storm surge and wave heights were computed. An increase in tropical cyclone frequency of +25.6 % and wind intensity of +1.6 %, based on literature values, could result in an increase of storm surge and wave heights of +11 % and +9 % respectively. This suggest that climate change could increase tropical cyclone induced coastal hazards more than just the actual increase in maximum wind speeds.
Highlights
Tropical cyclones (TCs) are among the most destructive natural hazards worldwide
Since the synthetic tracks were based on historical data, statistical properties for the synthetic tracks for the BoB for current (STCC) should be similar to those of the Historical Tropical Cyclones (HTC)
5 Conclusions In this study, estimates of extreme storm surge and significant wave heights induced by tropical cyclones were derived along the Bay of Bengal, both based on historical and synthetic tracks
Summary
Tropical cyclones (TCs) are among the most destructive natural hazards worldwide. Over the last two centuries, it is estimated that 1.9 million people have lost their lives as a result of TCs worldwide (Shultz et al, 2005; Nicholls et al, 1995). The estimation of design values resulting from TCs is often based on a limited number of recorded Historical Tropical Cyclones (HTC) at a given region (for the BoB see e.g. Chiu and Small, 2016; Dube et al, 2009). This results in a large statistical uncertainty in estimating the firstorder hazards resulting from TCs (e.g. wind speeds) due to the limited number of observations at a certain location. The Tropical Cyclone Wind Statistical Estimation tool (TCWiSE; Nederhoff et al, 2021) can, for example, be used to generate numerous synthetic tracks This allows for the creation of a much longer dataset to perform extreme value analysis on than otherwise available through HTC tracks only, and which can be used to calculate more reliable estimates of first-order hazards. Some work has been done in this direction but with focus on different regions than the BoB (e.g. Australia (Haigh et al, 2014) or USA (Lin et al, 2012, Marsooli et al 2019)) and/or without taking waves into 60 account, which is found to be an important factor leading to flooding in the northern BoB (Krien et al, 2017) and arguably worldwide
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