transport rate, only the (H^j, a^) data and Figure 4-38, or the (H^, a^) 

 data and Figure 4-39 are needed. If the shoaling coefficient is signifi- 

 cantly different from 1.3, multiply the Q obtained from Figure 4-39 by 

 the factor 0.88 vlCg. (See Table 4-9, Assumption 2d.) 



Figure 4-39 applies accurately only if o.q is a point value. If a^ 

 is a range of values, for example a 45° sector implied by the direction 

 NE, then the transport evaluated from Figure 4-39 using a single value 

 of a^ for NE may be 12 percent higher than the value obtained by aver- 

 aging over the 45° sector implied by NE. The more accurate approach is 

 given in the example problem of the next section. 



The imit for Q is a volume of deposited quartz sand (including 

 voids in the volume) per year. Bagnold (1963) suggests using immersed 

 weight instead of volume in the unit for longshore transport rate (Section 

 4.521), since immersed weight is the pertinent physical variable related 

 to the wave action causing the sediment transport. Use of an immersed 

 weight unit does eliminate the difference between lightweight material 

 and quartz that occurs if volume units are used. (Das, 1972.) However, 

 in coastal engineering design, it is the volume and not the immersed 

 weight of eroded or deposited sand that is important, and since beach 

 sand is predominantly quartz (specific gravity 2.65), volume is directly 

 proportional to immersed weight. On some beaches the sand may be calcium 

 carbonate which has a specific gravity ranging from 2.87 (calcite) to 

 2.98 (aragonite) in pure form. Naturally occurring oolitic aragonite 

 sand with a specific gravity of 2.88 (Monroe, 1969), has an immersed 

 weight 14 percent greater than pure quartz sand. Since the longshore 

 transport Equation 4-40, with one exception (Watts, 1953; and Das, 1971, 

 p. 14) , is based on quartz sand, then oolitic sand beaches may have 

 slightly lower longshore transport rates than is suggested by comparison 

 with data from quartz sand beaches. However, the scatter in the data 

 (Fig. 4-36) makes such a specific gravity effect difficult to detect. 



4.533 Energy Flux Example (Method 3) . Assume that an estimate of the 

 longshore transport rate is required for a locality on the north-south 

 coastline along the west edge of an inland sea. The locality is in an 

 area where stronger winds blow out of the northwest and north, resulting 

 in a deepwater distribution of height and direction as listed in Table 

 4-10o Assume the statistics were obtained from visual observations 

 collected over a 2-year interval at a point 2 miles offshore by seamen 

 aboard vessels entering and leaving a port in the vicinity. This type 

 of problem, based on SSMO wave statistics (Section 4.34), is discussed 

 in detail by Walton, (1972), and Walton and Dean (1973). Shipboard 

 data are subject to uncertainty in their applicability to littoral trans- 

 port, but often they are the only data available. It is assumed that 

 shipboard visual observations are equivalent to significant heights. 

 (Cartwright, 1972; and Walton, 1972.) 



4-102 



