Coastal tomographic mapping of nonlinear tidal currents and residual currents

• Published in: Continental shelf research Vol. 143; pp. 219 - 227
• Main Authors: Zhu, Ze-Nan, Zhu, Xiao-Hua, Guo, Xinyu
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2017
• Subjects: Coastal acoustic tomography; M4 and M6 generation mechanisms; Nonlinear tidal currents; Residual currents; Nonlinear tidal currents; M4 and M6 generation mechanisms; Coastal acoustic tomography; Residual currents
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2016.06.014

Depth-averaged current data, which were obtained by coastal acoustic tomography (CAT) July 12–13, 2009 in Zhitouyang Bay on the western side of the East China Sea, are used to estimate the semidiurnal tidal current (M2) as well as its first two overtide currents (M4 and M6). Spatial mean amplitude ratios M2:M4:M6 in the bay are 1.00:0.15:0.11. The shallow-water equations are used to analyze the generation mechanisms of M4 and M6. In the deep area, where water depths are larger than 60m, M4 velocity amplitudes measured by CAT agree well with those predicted by the advection terms in the shallow water equations, indicating that M4 in the deep area is predominantly generated by the advection terms. M6 measured by CAT and M6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20m, indicating that friction mechanisms are predominant for generating M6 in the shallow area. In addition, dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents. This is the first ever nonlinear tidal current study by CAT. •M4 and M6 tidal currents were mapped by Coastal Acoustic Tomography.•M4 and M6 tidal currents were predicted using the shallow water equation.•M4 currents in the deep area are mainly caused by the advection term.•The quadratic friction term is the primary mechanism generating M6 in shallow area.