III. C. Sediment Transport Processes |
The combined actions of waves and currents, as well as gravity, transport beach sediment in the longshore and cross-shore directions. The physical processes that contribute to sediment transport caused by waves are known in concept, but the details are difficult to determine because of the statistical nature of the problem. Waves arrive in a spectrum of heights and periods, and the sediment is composed of a complex distribution of sizes and densities. Most sediment transport estimation techniques relate wave power (longshore wave-energy flux) to the immersed weight of the sediment. A great deal of national attention has been given to determining transport rates, and the U.S. Army Corps of Engineers maintains a laboratory and scientific staff that is continually improving the technology. Computer prediction methods are available from the Corps as well as from the private sector for predicting transport on beaches. However, these may not be directly applicable to the beaches on Bainbridge Island for a number of reasons:
Sediment transport formulations are based on laboratory experiments or field observations of sand-sized material of single grain size or a small range of grain sizes. Most field studies have been conducted on open-ocean beaches. The predictions do not apply to poorly sorted sands, gravels, and cobble material common to Puget Sound beaches (Komar 1998a, p.399).
The prediction methods assume constant sediment characteristics in the cross-shore direction. This is not the case on Puget Sound beaches, where low-tide terraces are composed of fine material and the high-tide beach is composed of a wide range of grain sizes from coarse sand and gravel.
Sediment transport predictions depend critically on knowing the direction of the predominant wave energy along the beach, as well as the height of the breaking wave. These factors are influenced by the local bathymetry, which is seldom sufficiently known and is, itself, influenced by the sediment motion.
The rate of longshore transport is a complex function of wave energy, angle of wave attack, beach slope, current magnitude, sediment size (and size distribution), material density, and availability of beach materials. Not all of these factors are well known in most situations.
For the above reasons, predictions of beach sediment transport rates should be viewed with caution. The net direction of transport can often be reliably determined from other factors; the general processes are described below.
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