lagoons and deep water in the longshore direction comprise the principal
known natural sinks for beach sand. Of potential, but usually unknown,
importance as either a source or a sink is the offshore zone seaward of
the beach.
The works of man in beach replenishment and in mining or dredging
may provide major sources or sinks in local areas. In a few U.S. localities, submarine canyons or wind may provide major sinks, and calcium carbonate production by organisms may be a major source.
************** EXAMPLE problem ***************
GIVEN:
(a) An eroding beach 4.4 miles long at
long. Beaches on the remainder of
Figure 4-50a.)
root of spit that is 10 miles
the spit are stable. (See
(b)
A uniform recession rate of 3 feet per year along the eroding
4.4 miles.
(c) Depth of lowest shore parallel contour is -30 feet MSL, and
average dune crest élévation is 15 feet MSL.
(d) Sand is accumulating at the tip of the spit at an average rate of
400,000 cubic yards per year.
(e) The variation of y along the beaches of the spit is shown in
Figure 4-51. (y = Qu/Qrt; Equation 4-23.)
(f) No sand accumulâtes to the right of the érosion area; no sand is
lost to the offshore.
(g) A medium-width jettied inlet is proposed which will breach the
spit as shown in Figure 4-50a.
(h) The proposed inlet is assumed to trap about 15 percent of the
gross transport, Q$.
(i) The 1.3-mile long beach to the right of the jettied inlet will
stabilize (no érosion) and realign with y changing to 3.5.
(j) The accumulation at the end of the spit will continue to grow
at an average annual rate of 400,000 cubic yards per year after
the proposed inlet is constructed.
4-134
known natural sinks for beach sand. Of potential, but usually unknown,
importance as either a source or a sink is the offshore zone seaward of
the beach.
The works of man in beach replenishment and in mining or dredging
may provide major sources or sinks in local areas. In a few U.S. localities, submarine canyons or wind may provide major sinks, and calcium carbonate production by organisms may be a major source.
************** EXAMPLE problem ***************
GIVEN:
(a) An eroding beach 4.4 miles long at
long. Beaches on the remainder of
Figure 4-50a.)
root of spit that is 10 miles
the spit are stable. (See
(b)
A uniform recession rate of 3 feet per year along the eroding
4.4 miles.
(c) Depth of lowest shore parallel contour is -30 feet MSL, and
average dune crest élévation is 15 feet MSL.
(d) Sand is accumulating at the tip of the spit at an average rate of
400,000 cubic yards per year.
(e) The variation of y along the beaches of the spit is shown in
Figure 4-51. (y = Qu/Qrt; Equation 4-23.)
(f) No sand accumulâtes to the right of the érosion area; no sand is
lost to the offshore.
(g) A medium-width jettied inlet is proposed which will breach the
spit as shown in Figure 4-50a.
(h) The proposed inlet is assumed to trap about 15 percent of the
gross transport, Q$.
(i) The 1.3-mile long beach to the right of the jettied inlet will
stabilize (no érosion) and realign with y changing to 3.5.
(j) The accumulation at the end of the spit will continue to grow
at an average annual rate of 400,000 cubic yards per year after
the proposed inlet is constructed.
4-134