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Nearshore Sediment Transport and Beach Profile Characterization

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Open File Report: Second West-Central Florida Coastal Studies Workshop
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Morphodynamics
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Applications to Sediment Budget Study for West-Central Florida Coasts

Ping Wang, Department of Geology, University of South Florida, Tampa, Florida
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, Florida

Introduction

Nearshore sediment transport and beach profile characterization are essential for a coastal sediment budget study. Nearshore sediment transport, especially longshore sediment transport and its cross-shore distribution, is the dominant mechanism for sediment re-distribution. Realistic beach profile characterization is critical to define landward and seaward boundaries of the budget area, as well as for reliable estimation of the cross-shore sediment transport and morphological change. The following is a summary of results from a nearshore sediment transport study and the quantitative beach profile characterization along Sand Key beaches and their applications to the sediment budget study.

Nearshore Sediment Transport

Streamer sediment trap and short-term impoundment techniques were used to measure longshore sediment transport and its distribution in the field. Horizontal sediment flux in the water column decreases logarithmically upward. The greatest rate of decrease was found in the trough and smallest rate of decreasing in the swash zone. The cross-shore distribution of longshore sediment transport is not uniform. Six types of distribution patterns were found from field measurements on barred and non-barred coasts. The distribution patterns can be modeled reasonably well based on distribution patterns of energy dissipation and excess shear stress. The measured total rates were compared with various empirical predictions. It was found that the total rate of longshore sediment transport tends to be greatly over-predicted by the empirical predictions (Fig. 1).
graph

Figure 1: Comparison between measured and predicted rates of longshore transport.

Depth of Closure

The depth of closure is defined here as the seaward limit beyond which the profile does not exhibit measurable change in depth during the period of investigation. The closure depth can be used as the seaward boundary for sediment budget calculations. In the present study, closure depth is determined by comparing repetitive surveys over a 5-year period (Fig. 2). The closure depth was found to be at -4.9 m at the northern end of Sand Key and decreased toward the south to -3.0 m at Redington Beach. Three factors are believed to be important in controlling the closure depth: 1) the time interval during which the depth is defined, 2) the wave climate, especially the extreme wave climate, and 3) the elevation and slope of the bedrock.
graph

Figure 2: An example of repetitive surveys.

Beach Profile Characterization

Several models have been developed to quantitatively describe the shape of beach profiles. Among them, the simple Dean (1977) equilibrium beach profile model is by far the most commonly used one. It was found that Dean's model can reproduce the average profile well (Fig. 3). The average profile represents the general shape of individual profile with the common bar/trough features significantly reduced. The offshore portion displays less variation during the averaging than the nearshore portion. A new two-segment model, developed based on the analysis of Inman et al. (1993), is proposed. It is demonstrated that the new model is capable of reproducing the actual bar/trough beach profiles to the depth of closure (Fig 4).
graph

Figure 3: Dean's (1977) profile model, notice the average profile is reproduced better than the actual profiles.

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Figure 4: An example of the modified Inman et al. (1993) model, notice the bar/trough feature is reproduced well.

Summary

Field measurements indicate that the commonly used empirical predictions, e.g. the CERC formula, tend to over predict the total rate of longshore sediment transport for the west-central Florida coasts. The seaward boundary of significant morphological changes caused by sediment transport can be determined from repetitive beach profile surveys. A model which is capable of reproducing the bar/trough profiles was developed and calibrated for the west-central Florida coast. All the results can be used directly for comprehensive sediment budget study.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Morphodynamics > Nearshore Sediment Transport and Beach Profile Characterization--Applications to Sediment Budget Study for West-Central Florida Coasts


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