Evolution of a Swash Zone Berm Nourishment and Influence of Berm Elevation on the Performance of Beach-Nearshore Nourishments along Perdido Key, Florida, USA

A nourishment was placed within the swash zone along eastern Perdido Key, Florida, in 2011-2012 using maintenance-dredged material from nearby Pensacola Pass, referred to here as a "swash zone berm nourishment." The study area was divided into three sections, the swash zone berm project and two adjacent areas to the west and east, and was monitored with time series beach surveys. The performance of the 2011-2012 nourishment with a constructed berm elevation of +0.91m North American Vertical Datum 1988 (NAVD88) was compared with two previous nourishments in 1985 and 1989-1991, with +3.0 m NAVD88 and +1.2 m NAVD88 elevations, respectively. The low elevation for the 2011-2012 nourishment allowed natural overwash processes to occur frequently, which resulted in net onshore sediment transport and growth of the active berm. The swash zone berm evolved back to the natural equilibrium profile shape maintained in the study area within 8 months. The high-wave energy conditions associated with the passages of Tropical Storm Debby and Hurricane Isaac accelerated the equilibrium process. Sediment volume gain west of the project area due to longshore spreading of the nourishment occurred mostly in the trough between the shoreline and the bar, rather than on the dry beach. In terms of rate of shoreline retreat, the short 1.2-km 1985 nourishment performed the poorest with a rate of 40 m/y. The long 7.3-km 1989-1991 nourishment performed the best with a retreat rate of 11 m/y. This suggests that high berm elevations do not necessarily lead to better nourishment performance. Instead, longshore extent of a nourishment may dominate project performance. Furthermore, the very high nourishment density of 1550 m(3)/m did not improve nourishment longevity.