To illustrate how values of Q^,
listed in Table 4-11 were
calculated, the value of Qa
is here calculated for
= 1 and
the north direction, the top value in the first column on Table 4-11.
The direction term, F(ao), is averaged over the sector from a = 67.5°
to a = 90°, i.e., from NNE to N in the example. The average value of
Ffa^) is found to be 0.261.
to the 5/2 power is simply 1 for this
case. The frequency given in Table 4-10 for Ho = 1 and direction =
north (NW to NE) is 9 percent or in décimal terms, 0.09. This is multiplied by 0.5 to obtain the part of shoreward directed waves from the
north sector (i.e., N to NE) resulting in f = 0.09 (0.5) = 0.045. Putting ail these values into Equation 4-42 gives
= 1.373 X 105 (0.045) (1)5/î (0.261)
= 1,610 yd?/yr.
(See Table 4-11)
Table 4-11 indicates the importance of rare high waves in determining the longshore transport rate. In the example, shoreward moving 8-foot
waves occur only 0.5 percent of the time, but they account for 12 percent
of the gross longshore transport rate. (See Table 4-11.)
Any calculation of longshore transport rate is an estimate of potential longshore transport rate. If sand on the beach is limited in quantity, then calculated rates may indicate more sand transport than there
is sand available. Similarly, if sand is abundant, but the shore is
covered with ice for 2 months of the year, then calculated transport
rates must be adjusted accordingly.
The procedure used in this example problem is approximate and
limited by the data available. Equation 4-42, and the other approximations listed in Table 4-11, can be refined if better data are available. An extensive discussion of this type of calculation is given by
Walton (1972).
Although this example is based on shipboard visual observations of
the SSMO type (Section 4.34), the same approach can be followed with
deepwater data from other sources, if the joint distribution of heig t
and direction is known. At this level of approximation, the wave period
has little effect on the calculation, and the need for it is bypassed as
long as the shoaling coefficient (or breaker height index) reasonably
satisfies the relation (Ks)= 1.14. (See Assumption 2d, Table 4-9.)
For waves on sandy coasts, this relation is reasonably satisfied. ( .g.,
Bigelow and Edmondson, 1947, Table 33; and Goda, 1970, Figure 7.)
4.534 Empirical Prédiction of Gross Longshore Transport Rate (MethodJl.
Longshore transport rate dépends partly on breaker
At
breaker height increases, more energy is delivered
•
the same time, as breaker height increases, breaker P^^ion moves
'
shore widening the surf zone and increasmg the cro
which sédiment moves.
4-107
listed in Table 4-11 were
calculated, the value of Qa
is here calculated for
= 1 and
the north direction, the top value in the first column on Table 4-11.
The direction term, F(ao), is averaged over the sector from a = 67.5°
to a = 90°, i.e., from NNE to N in the example. The average value of
Ffa^) is found to be 0.261.
to the 5/2 power is simply 1 for this
case. The frequency given in Table 4-10 for Ho = 1 and direction =
north (NW to NE) is 9 percent or in décimal terms, 0.09. This is multiplied by 0.5 to obtain the part of shoreward directed waves from the
north sector (i.e., N to NE) resulting in f = 0.09 (0.5) = 0.045. Putting ail these values into Equation 4-42 gives
= 1.373 X 105 (0.045) (1)5/î (0.261)
= 1,610 yd?/yr.
(See Table 4-11)
Table 4-11 indicates the importance of rare high waves in determining the longshore transport rate. In the example, shoreward moving 8-foot
waves occur only 0.5 percent of the time, but they account for 12 percent
of the gross longshore transport rate. (See Table 4-11.)
Any calculation of longshore transport rate is an estimate of potential longshore transport rate. If sand on the beach is limited in quantity, then calculated rates may indicate more sand transport than there
is sand available. Similarly, if sand is abundant, but the shore is
covered with ice for 2 months of the year, then calculated transport
rates must be adjusted accordingly.
The procedure used in this example problem is approximate and
limited by the data available. Equation 4-42, and the other approximations listed in Table 4-11, can be refined if better data are available. An extensive discussion of this type of calculation is given by
Walton (1972).
Although this example is based on shipboard visual observations of
the SSMO type (Section 4.34), the same approach can be followed with
deepwater data from other sources, if the joint distribution of heig t
and direction is known. At this level of approximation, the wave period
has little effect on the calculation, and the need for it is bypassed as
long as the shoaling coefficient (or breaker height index) reasonably
satisfies the relation (Ks)= 1.14. (See Assumption 2d, Table 4-9.)
For waves on sandy coasts, this relation is reasonably satisfied. ( .g.,
Bigelow and Edmondson, 1947, Table 33; and Goda, 1970, Figure 7.)
4.534 Empirical Prédiction of Gross Longshore Transport Rate (MethodJl.
Longshore transport rate dépends partly on breaker
At
breaker height increases, more energy is delivered
•
the same time, as breaker height increases, breaker P^^ion moves
'
shore widening the surf zone and increasmg the cro
which sédiment moves.
4-107