Reflection of Storm Surge and Tides in Convergent Estuaries with Dams,
the case of Charleston, USA
Abstract
Convergent coastal-plain estuaries have been shortened by dam-like
structures worldwide. We used 31 long-term water level stations and a
semi-analytical tide model to investigate the influence of a dam and
landward-funneling on tides and storm surge propagation in the greater
Charleston Harbor region, South Carolina, where three rivers meet: the
Ashley, Cooper, and Wando. Our analysis shows that the principle tidal
harmonic (M2), storm surge, and long-period setup-setdown
(~4–10 days) propagate as long waves with the greatest
amplification and celerity observed in the M2 wave. All waves attenuate
in landward regions, but, as they approach the dam on the Cooper River,
a frequency dependent response in amplitude and phase progression
occurs. Dam-induced amplification scales with wave frequency, causing
the greatest amplification in M2 overtides. Model results show that
funneling and the presence of a dam amplify tidal waves through partial
and full reflection, respectively. The different phase progression of
these reflected waves, however, can ultimately reduce the total wave
amplification. We use a friction-convergence parameter space to
demonstrate how amplification is largest for partial reflection, when
funneling and wave periods are not extreme (often the case of dominant
tides), and for full reflection, when funneling and/or wave periods are
small. The analysis also shows that in the case of long period events
(>day), such as storm surges, dams may attenuate the wave
in funneling estuaries. However, dams may amplify the most intense storm
surges (short, high) more than funneling with unexpected consequence
that can greatly increase flood exposure.