The Role of River Discharge and Geometric Structure on Diurnal Tidal Dynamics, Alabama, USA. Issue 3 (28th March 2022)

• Record Type: Journal Article
• Authors: Dykstra, Steven L.; Dzwonkowski, Brian; Torres, Raymond
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-28
• Subjects: fluvial‐marine -- Mobile Bay -- longitudinal -- geomorphology -- tidal river -- estuary; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC018007
• Languages: English
• ISSNs: 2169-9275

Abstract

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 diurnal 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 ∼40–50 km inland of the bayhead and seaward of the largest bifurcation. 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), allowing more tidal energy to propagate inland. Tidal currents were attenuated and delayed more by 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 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 geometric convergence of the system. Our simple analytical theory provides a pathway for understanding the tide‐river‐geomorphic equilibrium along increasingly dynamic coasts. Plain Language Summary: Coastal cities and ports are affected by tides and rivers, concentrating industry, infrastructure, and economies in regions where tide‐river interactions affect flooding, erosion, and water quality. Tide‐river interactions are commonly studied in stable rivers with large tides, limiting the variability of tide‐river interactions, leaving the effects of river discharge on tides poorly understood, and the risks for coastal communities during large river discharge events difficult to determine. To study the effects of river discharge on tide‐river interactions, observations in Alabama were investigated, spanning the Tombigbee River through Mobile Bay to the Gulf of Mexico. Results show that channel geometry and river discharge affected the tides. As river discharge increased, flood‐ebb tidal currents became seaward only, causing a region up to ∼180 km (110 miles) long to change from estuarine to riverine. The shift was captured in a simple equation using the river‐tidal energy ratio and generalized geometry, suggesting the affected region increases with the range of river discharge and that the greatest flooding and erosion risks to coastal communities can change between the sea and rivers. This simple equation provides a new tool for managers to understand tide‐river interactions now and under future changes from human development and sea level rise. Key Points: Along the fluvial‐marine transition, river discharge attenuated tides in landward regions and amplified tides in seaward regions Regions of bidirectional tidal flow became unidirectional as river discharge increased, shifting the tidal river ∼180 km seaward Planform channel geometry (e.g., width convergence) may not be a robust indicator of the transition in tide‐river hydrodynamics