Tidal Bore Dynamics of a Mixed Estuary: The Hooghly River, India

The study establishes, with the aid of field measurements and numerical modeling, that the upper reach of the Hooghly estuary (India), though damped, registers an intense tidal bore. The lower estuary is amplified but does not show signs of bore formation. Limited quantitative studies are available on bores forming in damped estuaries. This study attempts to fill this gap using tidal gauge and velocity observations supplemented with simulation results obtained using the open-source numerical shallow-water solver TELEMAC 2D. New estimates of bore intensity in the Hooghly are presented confirming that the bore Froude number may reach 1.37 in the upper estuary for higher tidal ranges. The numerical model is used to gain further insights into the formation and evolution of the Hooghly bore. The model, despite using a vertically averaged velocity field, simulates tidal wave propagation and estimates of velocity magnitude during bore propagation with significant accuracy. The study demonstrates that the proposed model is an efficient and inexpensive tool for capturing the bore phenomenon and may be used to supplement field observations. Using the simulation results, key features of the Hooghly bore, such as the distance of the bore inception from the ocean mouth and the threshold tidal range for bore formation are investigated. The numerical simulations highlight the impact of riverbed asymmetry in bore appearances in natural estuaries. The analysis also reveals new information about bores in the Hooghly estuary, which may be useful for planning shipping and berthing operations in the port terminals along the river. A comparison with other estuaries witnessing tidal bores indicates that the Hooghly experiences a strong bore, despite the high dissipative forces owing to the rapid convergence of river width.