Investigating Typhoon Induced River-Surge Interactions in the Tamsui Estuary, Taiwan.

It is increasingly important to understand the combined influence of the main drivers of coastal risk due to sea level rise and the potential increase in extreme weather events. An Asian Basin stochastic typhoon set was used to force a storm surge model of Taiwan to investigate the interaction between storm surge and high river discharges (50, 100 and 200 year return period discharges) in the Tamsui River. Taiwan is a mountainous country leading to the combined risk of surge and high river discharge occurring simultaneously in estuary regions. The typhoon tracks were selected using a Hurricane Surge Index (Kantha, 2006) and cross the northern tip of Taiwan with maximum sustained winds (V_max) between 51 m/s and 75 m/s (Cat 3-5). Peak surge elevations in the Tamsui River range from 5.7 m to 10.3 m. The surge interacts with the river flow to induce changes in the water elevation between -8 m and 4 m depending on the surge elevation and river discharge and increases the inundated area in the range 37 km to 204 km. Significant positive interactions occur in the Tamsui Estuary (Fig. 1a) but do not have implications for increased inundation and occur at the start of the flood phase and the end of the ebb phase as previously shown in idealized test cases (Maskell et al., 2013). Current vectors in the estuary show that at the time leading up to high water the river outflow starts to become dominant in the mid-channel reducing maximum water levels by up to 10% in the combined surge and river solution. However, surge inhibits downstream propagation of the flood wave in the upper river channels increasing water levels by up to 2 m. The maximum inundated area (1330 km²) is caused by the combination of defence overflow due to the maximum surge (10.27 m) and increased river levels (RP100) in the upper channels leading to significant inundation either side of the Keelung River (Fig. 1b). The Erchung floodway is effective in diverting some of the flow (up to 10,443 m³/s) reducing inundation elsewhere in the river network.