transport rate, only the (Hj, aj) data and Figure 4-38, or the (Ho, a?)
data and Figure 4-39 are needed. If the shoaling coefficient is significantly different from 1.3, multiply the Q obtained from Figure 4-39 by
the factor 0.88 Æs. (See Table 4-9, Assumption 2d.)
Figure 4-39 applies accurately only if clo is a point value. If a0
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 a0 for NE may be 12 percent higher than the value obtained by averaging over the 45° sector implied by NE. The more accurate approach is
given in the example problem of the next section.
The unit 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 sédiment transport. Use of an immersed
weight unit does eliminate the différence 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 (spécifie gravity 2.65), volume is directly
proportional to immersed weight. On some beaches the sand may be calcium
carbonate which has a spécifie gravity ranging from 2.87 (calcite) to
2.98 (aragonite) in pure form. Naturally occurring oolitic aragonite
sand with a spécifie 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 hâve
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 spécifie 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
4-10. 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 transport, but often they are the only data available. It is assumed that
shipboard Visual observations are équivalent to significant heights.
(Cartwright, 1972; and Walton, 1972.)
4-102
data and Figure 4-39 are needed. If the shoaling coefficient is significantly different from 1.3, multiply the Q obtained from Figure 4-39 by
the factor 0.88 Æs. (See Table 4-9, Assumption 2d.)
Figure 4-39 applies accurately only if clo is a point value. If a0
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 a0 for NE may be 12 percent higher than the value obtained by averaging over the 45° sector implied by NE. The more accurate approach is
given in the example problem of the next section.
The unit 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 sédiment transport. Use of an immersed
weight unit does eliminate the différence 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 (spécifie gravity 2.65), volume is directly
proportional to immersed weight. On some beaches the sand may be calcium
carbonate which has a spécifie gravity ranging from 2.87 (calcite) to
2.98 (aragonite) in pure form. Naturally occurring oolitic aragonite
sand with a spécifie 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 hâve
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 spécifie 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
4-10. 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 transport, but often they are the only data available. It is assumed that
shipboard Visual observations are équivalent to significant heights.
(Cartwright, 1972; and Walton, 1972.)
4-102