The Role of Tropical Cyclone on Changjiang RiverSubaqueous Delta Geomorphology: A Numerical Investigationof Tropical Cyclone Danas (2019)

Subaqueous deltas are facing growing threats of erosion. Tropical cyclones (TCs), which arepredicted to affect mid-latitude estuaries more frequently with higher intensity in the future, can play animportant role in subaqueous delta geomorphology change. Nevertheless, direct in situ measurement ofTC-induced bed elevation change (BEC) was rarely reported before. In this study, four tripods were deployedat different parts of the Changjiang River subaqueous delta (CRSD) to capture BEC during TCs in July2019. Rapid erosion (20 mm day −1 ) during TC Danas (2019) was observed by high precision bed elevationmeasurement, over 10 times faster than that during calm weather conditions. Moreover, the recorded rates ofBEC varied substantially among these stations. A coupled three-dimensional wave-current-sediment model wasthen developed for the CRSD to help understand the spatial variability of observed BEC. The model resultsshowed two striking features of BEC during TC Danas: (a) enhanced sediment accretion near the sharp bottomsalinity front offshore of the north channel; (b) marked erosion near the front of CRSD. Further analysis showedthat the converging bottom currents in concert with enhanced sediment resuspension during TC Danas led tosignificant convergence of sediment flux near the bottom salinity front. In addition, the diverging isobaths alongthe front of the CRSD resulted in the onshore deviation of bottom current and diverging sediment flux underthe downwelling-favorable winds of Danas. Many TCs in the CRSD generate similar downwelling-favorablewinds and currents, growing TC threats in the future may therefore exacerbate erosion in this region.