EM 1110-2-1100 (Part III)
30 Apr 02
e. Littoral drift roses.
(1) The littoral drift rose is a potentially useful tool for interpreting littoral drift trends along a section
of shoreline where the shoreline curvature is mild and bottom contours are reasonably parallel to the shoreline
(Walton 1972, Walton and Dean 1973). The littoral drift rose shows how the littoral drift changes with
change in shoreline orientation relative to prevailing wave climatology. The drift rose is constructed using
standard prediction techniques for calculating littoral drift given that the shoreline is oriented in a direction
sn, where sn is the azimuth angle of the perpendicular to the shoreline in the seaward direction (see Figure III-
2-14 for an example measurement of sn at Ponte Vedra Beach, FL). The principle behind the littoral drift rose
is that a range of shoreline orientations is considered. These orientations correspond to the range that exists
at the study site. For each possible shoreline orientation, as described by sn, the total positive and negative
littoral drifts along the shoreline are calculated for a given time-averaging interval (i.e., 20 years, annual,
monthly, etc.). These calculated drift values are plotted in a polar plot, as shown in Figure III-2-15 for the
Ponte Vedra Beach, FL area. From the plots, the littoral transport rate for any given shoreline orientation can
be determined by entering the plot with the seaward directed normal of the shoreline orientation "sn" and
reading off the total positive, total negative, and net littoral drift values. As the littoral drift for a given wave
angle is proportional to sin (2αb), the net drift rose average for a real wave climatology has lobes that cause
(2) Littoral drift roses so constructed may be utilized in helping to identify tendencies of the shoreline
toward stability or instability. As an example, for a long shoreline with variations of shoreline orientation,
it is possible that there is a null (also termed nodal) point along the shoreline, which is defined as the location
for which the averaged positive and negative littoral transport have the same magnitudes, yielding zero net
drift. The littoral drift rose can be utilized in helping to identify this null (nodal) point. On the littoral drift
rose, this null (nodal) point is reflected as a crossing of the positive and negative littoral drifts on the total
littoral drift rose, or, as the point at which the net drift is zero on a net drift rose. Walton (1972) and Walton
and Dean (1976) identified potential null points on barrier islands along the Florida coastline using the littoral
drift rose and by integrating the longshore sand transport equation using changes in historical
shorelines along with a known boundary condition of sediment accumulation at the end of the islands. Along
the East Coast of the United States, there are several well-documented null points, such as just north of the
Delaware-Maryland state boundaries and in New Jersey near Barnegat Inlet. At both of these locations the
shoreline orientation changes significantly near the sites of the null (nodal) points and the net drift is to the
south, south of the null (nodal) points, and to the north, north of the nodal points. Mann and Dalrymple
(1986) examined the location of a null point along the Delaware coastline and concluded that there was a
significant variation in its annual location, corresponding to the variation in the annual littoral drift rates.
Such a shifting should be reflected in littoral drift roses for different averaging periods.
(3) Walton (1972) and Walton and Dean (1973) examined the stability of many shorelines using the
littoral drift rose concept. Figure III-2-16 shows an "unstable" littoral drift rose and a barrier island
orientation such that the initial island is oriented at the angle of the "unstable" null (nodal) point. Applying
this rose to the island in the figure, it can be seen that if a negative perturbation (recession of shoreline due
to mining, barrier overwash, etc.) is initiated on the island (away from the ends of the island where the
transport scenario would not conform to the assumptions of the calculated littoral drift), the induced transport
is away from the null point resulting in an erosional feature growth and potential island breakthrough (hence
the term "unstable"). In a similar manner, for the "unstable" littoral drift rose an induced transport response
to a positive shoreline perturbation in the same location (perhaps due to dredge spoil placement) would induce
further transport of sand toward the perturbation, causing the perturbation to grow (a self-sustaining
"positive" feedback mechanism). Walton (1972) has postulated that cuspate features such as shown in
Figure III-2-17 (often found in bays, rivers, etc.) may be the result of "unstable" littoral drift roses for those
sites due to the wave climatology experienced in long narrow fetch-enclosed locations.
Longshore Sediment Transport
III-2-33
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