Alteration in tides and flood dynamics caused by channel deepening: case study of the Saint Johns River, Florida

In this contribution, we show that channel deepening can amplifiy tide and storm surge--while simultaneously decreasing the river slope during both normal conditions and during floods.  We investigate the Saint Johns River Estuary, Florida, an example of a hyposynchronous, strongly frictional estuary with a landward decay in tidal amplitudes. Records since the 1890s and numerical modeling show that tidal range doubled in Jacksonville, Florida (40 km from coast), while tidal discharge approximately doubled everywhere. Overall, an increase in channel depth from 5 to 10m drove the observed changes, with width and length changes comparatively minor factors. Tidal amplitude evolved in a spatially variable way--negligible at the coast and inland, maximal 20-30km from the ocean.  The change in the M2 constituent is approximated by the equation x * exp(mu*x), where x is the distance from the ocean and mu is a damping coefficient that depends on depth, drag coefficient, and other system properties.  The observed tidal evolution is similar to storm surge:  Numerical modeling of hurricane Irma (Sept. 2017) under 1898 and 2017 bathymetric conditions confirms that both tidal and storm surge amplitudes have increased over time, with a maximum change about 20-25km from the inlet. Nonetheless, hurricane Irma produced overall high water levels in the historical bathymetric configuration. The reason is that the mean water level slope required to move water out of the modern estuary has decreased. An analytical model confirms that reduced slope is caused primarily by channel deepening.  However, greater tides and storm surge imply an increased vulnerability to a worst-case scenario hurricane.