Tide-modulated river discharge division in the Ganges-Brahmaputra-Meghna delta channel network, Bangladesh

Abstract

Study RegionThe Ganges-Brahmaputra-Meghna (GBM) delta in Bangladesh Study FocusThe flow distribution in a deltaic channel network is complicated by tidal wave propagation. We use a 2DH numerical model to investigate the variability of the tide-modulated seasonal, subtidal, intratidal and residual divided discharges. New Hydrological InsightsSimulations show that the divided subtidal discharges and correlated subtidal water levels display remarkable fortnightly fluctuations in dry months, whereas they are heavily distorted by the rapid rising & falling limbs of main stream flood waves in wet months. A subtidal water circulation that net water is transported from the tributaries to the main stream maintains at lower upstream discharges but is reversed by larger upstream discharges. With a series of increasing upstream riverine discharges routing downstream, the subtidal inflows experience a phase shift in the fortnightly cycles, in which the inflow peaks gradually move from spring tide to neap tide; moreover, the residual inflows demonstrate a non-monotonic variability, i.e. enlargements to the river-only case occur at two particular upstream discharges, compared to the reduced residual inflows at the majority of upstream discharges. These characteristic processes are explained by the mean amplitudes and phase relationships of tidal species (D1,D2,D4,D1/14) and the increasing dominance of river discharge over tidal wave. Our findings deepen the understandings of the complex nonlinear river-tide interactions in a deltaic channel network.