3.2. Worst-case Scenarios
The 5180 synthetic storm tide events simulated using ADCIRC are used as boundary conditions to drive inland flood modeling using FloodMap. Given the high level of floodwall protection, there are only a few storms that can cause inundation in Shanghai under the current sea level. Fig. 3a-c shows the spatial pattern of the storm tides and the resultant inundation footprints for three worst-case scenarios in Shanghai in the synthetic simulation under the NCAR/NCEP current climate condition. Typhoon A is a large sized storm that moves northwestward and makes landfall in Jinshan with a high intensity. Due to the wind field’s asymmetry, the largest wind forces to the right of the track push surge into Yangtze River Estuary and produce the highest storm tide (over 9 m above Wusong Datum) at the northernmost end of Shanghai. Another relatively strong northwestward-moving storm (Typhoon B) makes landfall in Chongming and induces a comparable storm tide along the northern coast of the island. A smaller-size storm (Typhoon C) which makes landfall farther from the city also generates an extreme sea level during high tide in Hangzhou Bay (8.7 m above Wusong Datum) due to its higher intensity. The three worst-case scenarios have very low probabilities of occurrence (> 5000 years, based on their storm tide return period at the location of the maximum storm tide for each storm, shown by the black star in Fig. 3a-c) and are much higher than the largest storm tides in recorded history (6 ~ 7 m above Wusong Datum).
The synthetic events cause varying degrees of coastal flood inundation in Shanghai. The highest storm tides due to Typhoon A overwhelm low-lying and poorly-protected coastal floodplains and islands in the Yangtze River Estuary (Fig. 3a). When subject to Typhoon B induced coastal flooding, the inundation area covers about 564 km2 in the northeastern part of Chongming Island (Fig. 3b). Particularly, the waterfront areas of western (Typhoon A) and eastern (Typhoon B) Chongming are flooded with maximum water depths higher than 3 m. In contrast, only marginal inundations with shallow water depth (mostly lower than 0.5 m) are observed in the coastal region along Hangzhou Bay under the Typhoon C scenario (Fig. 3c). This relatively small inundation depth can be explained by the presence of lateral topographic confinement and the relatively high crest heights of the seawalls (7 ~ 8 m above Wusong Datum). Pudong New Area and Changxing Island appear to be immune to these extreme flood scenarios, because of their high standard seawalls (8 ~ 11 m above Wusong Datum) and comparatively low storm tides in the lower Yangtze River Estuary. Mainland Shanghai seems to be generally safe from the storm-induced flooding except a small portion of dike-protected hinterland in Baoshan (Fig. 3a). The results presented here highlight the vulnerability of the areas in the upper Yangtze River Estuary where larger storm tides develop and which have relatively low levels of seawall protection, such as Chongming Island and Baoshan.