Galvin (1972) showed that when field values of longshore transport
rate are plotted against mean annual breaker height from the same locality,
a curve
Q = 2 X 105
,
(4-44)
forms an envelope above almost ail known pairs of (Q, H&), as shown in
Figure 4-40. Here, as before, Q is given in units of cubic yards per
year;
in feet.
Figure 4-40 includes ail known (Q, H/,) pairs for which both Q and
are based on at least 1 year of data, and for which Q is considered
to be the gross longshore transport rate, Qg, defined by Equation 4-21.
Since ail other known (Q, H^) pairs plot below the line given by Equation
4-41, the line provides an upper limit on the estimate of longshore transport rate. From the defining équations for Qg and Q^, any line that
forms an upper limit to longshore transport rare must be the gross transport rate, since the quantifies Qrt»
Qn» as defined in Section
4.531, are always less than or equal to Qg.
In Equation 4-44, wave height is the only independent variable, and
the physical explanation assumes that waves are the prédominant cause of
transport. (Galvin, 1972.) Therefore, where tide-induced currents or
other processes contribute significantly to longshore transport, Equation
4-44 would not be the appropriâte approximation. The corrections due to
currents may either add or subtract from the estimate of Equation 4-44,
depending on whether currents act with or against prevailing wave-induced
transport.
4.535 Method 4 Example (Empirical Prédiction of Gross Longshore Transport
Rate. Near the site of the problem outlined in Section 4.533, it is desired to build a small craft harbor. The plans call for an unprotected
harbor entrance, and it is required to estimate costs of maintenance
dredging in the harbor entrance. The gross transport rate is a first
estimate of the maintenance dredging required, since transport from
either direction could be trapped in the dredged channel. Wave height
statistics were obtained from a wave gage in 12 feet of water at the end
of a pier. (See Columns (1) and (2) of Table 4-12.) Heights are available as empirically determined significant heights. (Thompson and Harris,
1972.) (To facilitate comparison, the frequencies are identical to the
deepwater frequencies of onshore waves in Table 4-10 for the problem of
Section 4.533. That is, the frequency associated with each H„ in Table
4-12 is the sum of the frequencies of the shoreward
on the corresponding line of Table 4-10.)
The breaker height, H^, in the empirical Equation 4-44 is related
to the gage height, Hg, by a shoaling-coefficient ratio, (Kg)^/(Ks)g,
where (Ks)^ is the shoaling coefficient (Equation 2-44), (H/H' in
4-108
rate are plotted against mean annual breaker height from the same locality,
a curve
Q = 2 X 105
,
(4-44)
forms an envelope above almost ail known pairs of (Q, H&), as shown in
Figure 4-40. Here, as before, Q is given in units of cubic yards per
year;
in feet.
Figure 4-40 includes ail known (Q, H/,) pairs for which both Q and
are based on at least 1 year of data, and for which Q is considered
to be the gross longshore transport rate, Qg, defined by Equation 4-21.
Since ail other known (Q, H^) pairs plot below the line given by Equation
4-41, the line provides an upper limit on the estimate of longshore transport rate. From the defining équations for Qg and Q^, any line that
forms an upper limit to longshore transport rare must be the gross transport rate, since the quantifies Qrt»
Qn» as defined in Section
4.531, are always less than or equal to Qg.
In Equation 4-44, wave height is the only independent variable, and
the physical explanation assumes that waves are the prédominant cause of
transport. (Galvin, 1972.) Therefore, where tide-induced currents or
other processes contribute significantly to longshore transport, Equation
4-44 would not be the appropriâte approximation. The corrections due to
currents may either add or subtract from the estimate of Equation 4-44,
depending on whether currents act with or against prevailing wave-induced
transport.
4.535 Method 4 Example (Empirical Prédiction of Gross Longshore Transport
Rate. Near the site of the problem outlined in Section 4.533, it is desired to build a small craft harbor. The plans call for an unprotected
harbor entrance, and it is required to estimate costs of maintenance
dredging in the harbor entrance. The gross transport rate is a first
estimate of the maintenance dredging required, since transport from
either direction could be trapped in the dredged channel. Wave height
statistics were obtained from a wave gage in 12 feet of water at the end
of a pier. (See Columns (1) and (2) of Table 4-12.) Heights are available as empirically determined significant heights. (Thompson and Harris,
1972.) (To facilitate comparison, the frequencies are identical to the
deepwater frequencies of onshore waves in Table 4-10 for the problem of
Section 4.533. That is, the frequency associated with each H„ in Table
4-12 is the sum of the frequencies of the shoreward
on the corresponding line of Table 4-10.)
The breaker height, H^, in the empirical Equation 4-44 is related
to the gage height, Hg, by a shoaling-coefficient ratio, (Kg)^/(Ks)g,
where (Ks)^ is the shoaling coefficient (Equation 2-44), (H/H' in
4-108