The role of river discharge and geometric structure on diurnal tidal
dynamics, Alabama, USA
Abstract
As tides propagate inland, they become distorted by channel geometry and
river discharge. Tidal dynamics in fluvial-marine transitions are
commonly observed in high-energy tidal environments with relatively
steady river conditions, leaving the effects of variable river discharge
on tides and longitudinal changes poorly understood. To study the
effects of variable river discharge on tide-river interactions, we
studied a low-energy tidal environment where river discharge ranges
several orders of magnitude, the diurnal microtidal Tombigbee
River-Mobile Bay fluvial-marine transition, using water level and
velocity observations from 21 stations. Results showed that tidal
attenuation was reduced by the width convergence in seaward reaches and
height convergence of the landward backwater reaches, with the channel
convergence change location ~40-50km inland of the
bayhead and seaward of the largest bifurcation (~rkm
90-100). River events amplified tides in seaward regions and attenuated
tides in landward regions. This created a region of river-induced peak
amplitude seaward of the flood limit (i.e., bidirectional-unidirectional
current transition) and passed more tidal energy inland. Tidal currents
were attenuated and lagged more with river discharge than water levels,
making the phase lag dynamic. The river impacts on the tides were
delineated longitudinally and shifted seaward as river discharge
increased, ranging up to ~180 km. Results indicated the
location and longitudinal shifts of river impacts on tides in alluvial
systems can be estimated analytically using the ratio of river discharge
to tidal discharge and the geometry convergence. Our simple analytical
theory provides a pathway for understanding the tide-river-geomorphic
equilibrium along increasingly dynamic coasts.