Tidal change in response to the relative sea level rise and marsh accretion in a tidally choked estuary

Abstract

Tidal range is a key factor for flooding, mixing, and transport in estuaries. Sea level rise is expected to change tidal range in the future. This change is complex, and has been showed to either increase or decrease, and vary along the estuary axis. Large changes are expected to occur in costal Louisiana, as it is experiencing some of the highest relative sea level rise (RSLR) rates in the continental USA (0.92 cm yr−1). A finite volume community ocean model was used to investigate how tidal dynamics in Barataria Bay estuary will be affected by future RSLR and marsh accretion scenarios. Under the present sea level condition, five major tidal choking areas – where tidal range reduces sharply, and phase lags develops – were identified. RSLR reduced tidal choking intensity and thus increased tidal range within the estuary. Contrary to previous modeling analyses in other estuaries suggesting that flooding of the low-lying land with sea level rise would increase frictional effects and thus reduce tidal range, this study suggested that tidal range in a choked tidal system like Barataria Bay increases even when accompanied by extensive land inundation. This occurs because the channel conveyance effects are larger than the frictional effects of the low-lying areas. In the lower and the middle bay, the largest increase in tidal range occurred when the marsh area was assumed to keep pace with RSLR. Under this condition, mean tidal range could increase by a maximum of 16 cm compared to the present-day sea level condition. However, in the upper bay the largest increase in tidal range occurred when no accretion was assumed. Under this condition, mean tidal range could increase by a maximum of 13 cm. RSLR also induced amplification of tides at the head of the estuary. A detailed momentum balance analysis indicated that sea level rise shifts tidal wave regime from a dissipative tidal wave to a progressive wave, which is more susceptible to tidal amplification.