An assessment of sedimentary equilibrium at a Florida estuary: The Loxahatchee

Abstract

Estuaries are increasingly dredged to maintain navigable depths in the channels. Their morphology has also been altered by sea level changes over timescales on the order of a century and longer. In that regard, estuaries in sedimentary equilibrium over a mid-timescale of a few decades are best suited to long-term sediment management protocols based on pre-set dredging and placement of sediment in designated zones. Three generic approaches, underpinned by the well-known concept of non-silting, non-scouring, sedimentary equilibrium, are used to assess the state of equilibrium of the Loxahatchee River estuary on the Atlantic coast of Florida. Relying on field observations and numerical simulations of flow and sediment transport, the first approach considers grain transport, the second is based on tidally averaged sand loads, and the third explores channel morphodynamics. The assessment indicates that the Loxahatchee remains close to mid-timescale equilibrium. Well-planned dredging brings about disequilibrium, limited to timescales between a few months and about a year. An analytical approach is used to estimate the duration of adaptation to new equilibrium after a channel is narrowed to increase the navigation depth. The main takeaway from this study is that successful dredging projects must ensure mid-timescale estuarine equilibrium.