(c) Slope of the foreshore tends to decrease with increasing wave
height, again with scatter.
(d) For design of beach profiles on océan or gulf beaches, use
Figure 4-32, keeping in mind the large scatter in the basic data on Figure 4-33, much of which is caused by the need to adjust the data to
account for différences in nearshore wave climate.
4.53 LONGSHORE TRANSPORT RATE
4.531 Définitions and Methods. Littoral drift is the sédiment (usually
sand) moved in the littoral zone under action of waves and currents. The
rate, Q, at which littoral drift is moved parallel to the shoreline is
the longshore transport rate. Since this movement is parallel to the
shoreline, there are two possible directions of motion, right and left,
relative to an observer standing on the shore looking out to sea. Movement from the observer*s right to his left is motion toward the left, indicated by the subscript £t. Movement toward the observer’s right is
indicated by the subscript rt.
Gross longshore transport rate, Qg, is the sum of the amounts of
littoral drift transported to the right and to the left, past a point on
the shoreline in a given time period.
Qj = Qr( + Q£< .
(4-21)
Similarly, net longshore transport rate, Q^, is defined as the
différence between the amounts of littoral drift transported to the right
and to the left past a point on the shoreline in a given time period.
Qn = Q,< - QS( •
(4-22)
The quantities Qrt, Qjj.t, Qn and
hâve engineering uses: for
example,
is used to predict shoaling rates in uncontrolled inlets;
Qn is used for design of protected inlets and for predicting beach érosion on an open coast;
and
are used for design of jetties and
impoundment basins behind weir jetties. In addition, (L, provides an
upper limit on other quantities.
Occasionally, the ratio
(4-23)
is known, rather than the separate values Q£4. and Q^. Then (K is
related to
m terms of y by
"
(1+T)
(1-7)
(4-24)
This équation is not very useful when y approaches 1.
4-88
height, again with scatter.
(d) For design of beach profiles on océan or gulf beaches, use
Figure 4-32, keeping in mind the large scatter in the basic data on Figure 4-33, much of which is caused by the need to adjust the data to
account for différences in nearshore wave climate.
4.53 LONGSHORE TRANSPORT RATE
4.531 Définitions and Methods. Littoral drift is the sédiment (usually
sand) moved in the littoral zone under action of waves and currents. The
rate, Q, at which littoral drift is moved parallel to the shoreline is
the longshore transport rate. Since this movement is parallel to the
shoreline, there are two possible directions of motion, right and left,
relative to an observer standing on the shore looking out to sea. Movement from the observer*s right to his left is motion toward the left, indicated by the subscript £t. Movement toward the observer’s right is
indicated by the subscript rt.
Gross longshore transport rate, Qg, is the sum of the amounts of
littoral drift transported to the right and to the left, past a point on
the shoreline in a given time period.
Qj = Qr( + Q£< .
(4-21)
Similarly, net longshore transport rate, Q^, is defined as the
différence between the amounts of littoral drift transported to the right
and to the left past a point on the shoreline in a given time period.
Qn = Q,< - QS( •
(4-22)
The quantities Qrt, Qjj.t, Qn and
hâve engineering uses: for
example,
is used to predict shoaling rates in uncontrolled inlets;
Qn is used for design of protected inlets and for predicting beach érosion on an open coast;
and
are used for design of jetties and
impoundment basins behind weir jetties. In addition, (L, provides an
upper limit on other quantities.
Occasionally, the ratio
(4-23)
is known, rather than the separate values Q£4. and Q^. Then (K is
related to
m terms of y by
"
(1+T)
(1-7)
(4-24)
This équation is not very useful when y approaches 1.
4-88