Depth of Closure and the Equilibrium Beach Profile – A Case Study from Sand Key, West-Central Florida

Five hundred and fifty-five beach profiles surveyed repetitively during 5 years (3 years for the Indian Shores) along the 20-km beach at Sand Key, west-central Florida were analyzed to understand the regional characteristics of closure depth and equilibrium beach profile. In areas absent of direct control of the regional hard bottom, the closure depth was found at 4.9 m below NGVD in the northern portion of the study area. The hard bottom has direct control on the closure depth in the southern portion of the study area, resulting in a southward decrease from -4.6 m (-16 ft) NGVD to -3 m (-10 ft) at the southern end of the study area, following the trend of the hard bottom exposure. Using a 6-yr average of 12-hr extreme wave conditions obtained from a local WIS station, the Birkemeier (1985) formula reproduced the measured closure depth at the northern part of the study area (without the direct control of the hard bottom) well, whereas the Hallermeier (1978, 1981) formula over-predicted the measured value. The average beach profile, although significantly reducing the bar and trough features, represents a general shape of the nearshore bathymetry in the study area. The average profile was reproduced well by both the 2/3-power function and the exponential function. The offshore portion of the profile seaward of the bar was predicted better than the nearshore portion. The nearshore profile, from seaward of the shoreline to the bottom of the trough, was more closely reproduced by the 2/3-power function than the exponential function. The greatest discrepancy between predicted and average profile was found at the bar and trough.