CETN II-30
(3/92)
E. Groins interrupt longshore currents, trapping some of the drift. Sand accumulates on the updrift (left)
side of each groin and erodes from the downdrift side.
F. Jetties, along with the currents which flow in and out of an inlet, interrupt longshore drift. In this
example, the updrift (left) side has prograded more than the downdrift side. Accumulation on the right
side suggests that occasional drift reversals may occur.
G. The seawall has protected a stretch of the shore and produced an effect similar to a headland.
Although this part of the shore may be generally eroding, the shoreline loss is most pronounced on the
downdrift (right) side of the structure.
H. An shore connected breakwater interrupts longshore drift similar to groins or jetties. Eventually,
when the shore on the updrift side has prograded to the seaward end of the shore-perpendicular portion
of the breakwater, sediment bypassing will be reinitiated and shoaling within the harbor may become a
problem.
I. An detached breakwater can effectively reduce longshore currents because a portion of the shore is
protected from waves. As a result, currents deposit their sediment load in the lee of the breakwater,
allowing the shore to prograde. The shore may erode downdrift of the breakwater or the salient behind
the structure may be asymmetric with respect to the breakwater due to dominant wave direction.
N A T U R A L A N D / O R M A N - M A D E I N F L U E N C E S O N D R I F T INDICATORS: Several recent erosion
problems have been studied using morphologic indicators to evaluate the complex circulation and sand
transport patterns. Two such studies illustrate the application of morphologic indicators as well as other
clues to interpret the circulation and how it affects coastal engineering solutions.
Bethune Beach. FL: Shoreline erosion has placed upland development at risk along a section of the east
central Florida coast. In order to make decisions on the type of erosion control needed, field
observations, historical shoreline trends, and wave hindcast data were used to refine hypotheses on sand
transport along the coast and reasons for the resulting erosion patterns. The net drift direction was
originally thought to be from north to south, with a possible influence of the jetties at Ponce DeLeon inlet
causing erosion to downdrift beaches. A look at physical characteristics of the shoreline from the south
jetty of Ponce DeLeon Inlet to Cape Canaveral gave clues to the actual coastal
processes (Figure 3a). The beach was widest, with a fine-grained flat slope, adjacent to the south jetty,
and became progressively more narrow, steeper and more coarse grained to the south. This is opposite.
to the typical flattening of beach slope and decreasing grain size in the downdrift direction. The barrier
island morphology indicated a narrow barrier with historic breakthroughs and transient inlet formations..
Erosion conditions have been active in this area in the geologic past. These processes continue into the
present with evidence that undeveloped areas contain natural overwash features, while developed, areas
include scarped dunes or seawalls with end flanking. The recent historical shoreline trends, determined
from profiles and aerial photography, demonstrate this erosional trend. An examination of offshore depth
contours shows a shoal outlined by the 18 m contour that tends to focus waves at the beach inshore of
the shoal, increasing wave heights and energy, Using these morphology data and along with additional
data on suspected cell compartments, and Ponce DeLeon Inlet's sediment budgets and evolution, a model
for circulation along this coast was developed (Stauble and Da Costa, 1987). The model had seasonal
circulation patterns, with nodal points at the area of highest erosion and localized drift reversals (Figure
3b). The area of the coast in need of protection was at the erosional nodal point and indicated that shore
protection options need to account for this localized erosional cell circulation.
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