Influence of Grain Size on Sediment Transport Rates With Emphasis on the Total Longshore Rate

Abstract

PURPOSE: The calculation of longshore sediment transport (LST) rates is a key component of most coastal engineering studies. While the LST process is conceptually simple, in practice the development of reliable rates is made difficult by problems associated with collecting accurate field data, by limitations to model predictions, and by substantial variations of the rates in time and space. This Coastal and Hydraulic Engineering Technical Note (CHETN) summarizes the state of understanding of the influence of grain size on surf zone sediment transport and is a companion to Smith et al. (2004). This CHETN discusses details of bed-load and suspended load transport, and the classical bed-load regime is shown to encompass two distinct modes of transport. Four LST models with varying levels of complexity are discussed to show how they incorporate the physics of grain size variation and its effect on the transport rate. In addition, a relationship between the K coefficient in the CERC formula (Coastal Engineering Manual (2002), Section III-2-3-a) and grain size is presented. Finally, some inconsistencies between theory and data are discussed in the context of the interrelationship between grain size and beach slope. The final conclusion is repeated here. In general, an increase in the median grain size will decrease LST rates in the surf zone. If a simple exponential relationship between transport rate and grain size (D) is needed (Equation 7: LST rate ∝ D n ), the most appropriate value for the exponent n should be of the order of -1, as seen from Equation 23. However, this tech note argues that this is clearly a simplistic view of surf zone sediment dynamics. A more realistic (though still highly simplified) approach would be that, for fine grain sediments (D50 on the order of 0.15 to 0.30 mm), suspended load transport should dominate and n should be somewhere within the range of -0.5 to -3.0. For coarse sands (D50 around 1.0 mm), sheetflow bed-load transport should dominate (Figure 6), and the transport rate should be nearly independent of grain size (n = 0). For large gravel and shingle (D50 > 20 mm), the dominant transport should be in the IM (Initial Motion) bed-load regime, with n within the range of -0.5 to -2.0. To state this another way, the exponent n in Equation 7 is itself a function of grain size. BACKGROUND: Though most researchers recognize the median grain size as a parameter of first order importance in its effects on the magnitude of sediment transport, its variation is not as easily studied as the variation of many of the other primary parameters. In the field, it is easy to obtain data for a range of values for wave heights and periods, for instance, as these vary constantly in time. However, the grain size on a beach typically shows no (or insignificant) variations in time. Similar problems can occur in the laboratory. Changing the wave height or period in a laboratory study is usually as simple as reprogramming a wave paddle. However, usually, changing the grain size in a large model is infrequently done because of the large cost in both time and money.