Tropical Cyclone-Induced Waves and Storm Surge at four locations along the Southern African East Coast

[摘要] ENGLISH ABSTRACT: Tropical cyclones are synoptic scale rotating storms that form over oceans with warm water, producing powerful winds and rainfall. The characteristic low pressure of the system and the extreme winds generate high levels of storm surge, ocean waves and currents, which pose a threat to a coast and its infrastructure. The social and economic consequences of these storms can be devastating, often resulting in human fatalities. Quantification of the risk exposure to these storms is therefore crucial for human safety and the design of infrastructure along the coast of a tropical cyclone-prone region, such as the Southern African East Coast.Considerable research into the risk presented by tropical cyclones, has been conducted using numerical models, although very little has been done for the South-West Indian Ocean. The present research focusses on the tropical cyclone-induced waves and storm surge, along the Southern African East Coast. The primary results of this thesis are estimates of the 50-, 100-, 200- and 500-year significant wave height and storm surge levels, expected to be produced by tropical cyclones, at four locations along the Southern African East Coast.Third-generation numerical models were used to generate the wave and storm surge estimates at the four locations, namely; Durban, Maputo, Beira and Pemba. Historical tropical cyclone data from the Best Track data, as well as the results of other studies, were used to develop input parameters for the numerical models. Sensitivity tests of these parameters were conducted in order to see how each parameter influences the model results. The results of the sensitivity tests were used to determine the design storm parameters for the proposed numerical model tests.The model tests comprised of simulating a tropical cyclone varying in time and space, resulting in estimates of the significant wave height and storm surge levels at the location of interest. A total of four return periods were simulated for each of the four locations, resulting in a total of 16 simulations. The output of the simulations were specified at points along the 20 m contour for the waves and the 10 m contour for storm surge. These depths were chosen in order to determine estimates before certain coastal processes such as refraction and diffraction, have a major influence on the results.The numerical model was calibrated by simulating Hurricane Ike, which occurred over the Gulf of Mexico in September 2008. The model produced reasonably good results when compared to the measured data, although the model did under-estimate the storm surge. It is advisable to take this into account when using the estimates of the expected storm surge levels.The results of the model tests indicate that the expected waves produced by tropical cyclones along the Southern African East Coast, do pose a major threat and need to be taken into consideration in the planning and design of coastal infrastructure. The largest waves are expected to occur at Beira and the smallest at Durban. At the 100-year return period, Durban, Maputo, Beira and Pemba are expected to produce significant wave heights of 2.1 m, 4.6 m, 6.3 m and 4.4 m respectively. Estimates of the expected tropical cyclone-induced storm surge indicate that Durban and Pemba are not at risk of flooding, with maximum expected storm surge levels of roughly 0.3 m at the 500-year return period. Beira and Maputo on the other hand, are potentially at risk with maximum storm surge levels of 2.1 m and 1.1 m respectively, at the 500-year return period.

[发布机构] Stellenbosch University