| A series of estuary-lagoon systems are distributed along the eastern Hainan Island coastlines. Here, coastal development for tourism has become progressively intense, because of the clean air and water. However, this section of coastlines is also subjected to typhoons, which often result in sequential disasters. During a typhoon event, strong winds damage forests and houses, large storm waves erode the shoreline, typhoon induced heavy rain causes flood in the drainage basins, and combined action of storm surge and increased river flow leads to flooding in lagoonal areas. Boao Harbour, now being developed into a middle-sized coastal city, provides a good example. The sand spit that semi-encloses the lagoon to form an estuarine / tidal inlet system was breached at its northern section three times during storms in the past sixty years; these resulted from the storm waves, together with strong river flow inside the lagoon. The timing of the large river water discharge into the lagoon tends to match the storm-induced high water (because the rivers are relatively short); hence flooding in the low-lying lagoonal areas occurs once every two to five years. Human activities add further risks to flooding. For instance, at the Xiaohai inlet, the entrance channel was artificially narrowed for navigation purposes. This engineering work has hampered the river discharge into the open sea, causing an extremely severe flooding in 2000. Recently, artificial modification to the entrance channel to Boao Harbour has been proposed. For such an engineering scheme, however, additional flooding risk assessment must be carried out. Based upon analyses of the regional patterns of typhoon-induced sequential disasters and the related processes and mechanisms, the risk of flooding will be enhanced by inappropriate modifications to the entrance channel. Numerical studies indicate that the shape of the designed entrance channel for navigation is an important factor for the modified flood water levels. |
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