ERDC/CHL CHETN-IV-61
December 2003
physical model results discussed later will illustrate this. Field data from Table 1 were used to
compose the plot in Figure 8. A S/L ratio (S is spur length and L is distance from spur to the
local average shoreline) is plotted for each field site. A shoreline response ratio was determined
from the ratio of distance from the local average shoreline (defined as the shoreline within 300 m
of the jetty) to the immediate shoreline at the jetty, divided by the distance from local average
shoreline to the spur. Therefore, a shoreline response of 1.0 means the shoreline has reached the
spur. This is seen for Bakers Haulover and Shark River, with S/L ratios greater than 0.4. The
others have S/L less than 0.4 and shoreline responses much less than 1.0. In this simplified ap-
proach that may neglect other important parameters, such as beach slope and wave height, a line
was drawn at S/L = 0.4 to divide from full shoreline response and a partial shoreline response.
Typically, one would not want the shoreline to reach the spur in order to keep the potential for
sand transport to the sea side of the spur minimal. If the wave climate is not too energetic or if
the spur is on the downcoast side of a jetty system where sediment is bypassed to, it might be
acceptable. On the other hand, one needs to have a long enough spur to create a deflection of the
longshore currents as seen in physical model experiments discussed later.
Spur Construction. Care must be taken in the structural design of a spur. Little design guid-
ance is available. Issues involved would include the effect of a mach stem wave increasing wave
height at the junction of the spur and the breakwater, wave focusing at the junction, the place-
ment of stone at the transition region between jetty and spur, and the possibility of scour at the
spur tip. Careful structural design would be required, especially in energetic wave climates. The
site-specific nature of the local bathymetry and structure-wave interaction might require a model
investigation.
PHYSICAL MODEL EXPERIMENTS OF SPURS: The field monitoring study of the
Siuslaw Inlet project by Pollock et al. (1995) indicated good agreement with physical model
results of Bottin (1981, 1983). The field monitoring indicated that at high water the flow patterns
were circular eddies (Figure 9a) and there was a strong seaward-flowing rip current along the
jetty. At lower tide stages, and dependent on wave height, there might be an "S"-shaped flow
pattern (Figure 9b). The results were similar in the physical model study of Bottin. Based on this
information it may be noted that wave height, tide stage, and water depth are probably significant
design parameters for determining the hydraulic response of spur jetties and most likely the
sediment circulation response.
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