south of the structure. Analyses of these data show that the transmission coefficients resulting from
the absence of the reef are in the range of 0.85 to 0.95, which are much larger than determined from
the analysis of the two long-term gauges alone (0.65 to 0.85). Apparently, wave energy dissipation
occurs between the locations of the two gauges, even in the absence of the reef
VOLUMES
The total volumetric change over the 356,735-sq-m (3,840,000-sq-fi) area from July 1992 through
July 1994 was -38,230 cu m (-50,000 cu yd) (University of Florida 1994b), which translates to an
annualized volume change of - 19,115 cu m. Except for the 4-month period following installation of
the first 57 P.E.P. reef units (the net volume change within the project area was +9,372 cu m
(+12,257 cu yd) (Coastal Technology Corporation 1992)), net volumetric changes within the project
area have been erosive. The volumetric changes measured from July 1992 to July 1994 for the six
regions defined above are presented in
Table 1.
Historical volumetric changes were also computed, in order to more accurately determine the effect
of the P.E.P. reef on the sediment budget of the project area. Analysis of beach profiles surveyed in
1987 and 1992 shows that the area generally accreted material naturally, prior to installation of the
P.E.P. reef. The total volumetric change over the entire study area during the 1987-1992 time
period was +8 1,812 cu m (+107,000 cu yd), which translates to an annualized volume change of .
+16,362 cu m (+21,399 cu yd). These results indicate that the general trend throughout the study
area was accretionary prior to placement of the P.E.P. reef, turning to erosional following the
placement of the structure. Volumetric changes during the period 1987-1992 for the six regions of
the project area are presented in Table 1.
ANALYSIS
As noted in the 12-month monitoring report, an overall erosional trend occurred within the project
area. All areas of the study area eroded except for the region south and landward of the reef In
addition, some minimal accretion was noted near the north end of the reef This pattern of
volumetric change suggests that the physical mechanism of longshore current generation caused by
wave mass transport over the reef as described in the 6-month monitoring report may be valid. This
phenomenon is described as "ponding," since mass transport over the reef due to wave action creates
a rise in water surface elevation, which is relieved by longshore flow. This process transports
sediment from the region landward of the reef toward the north and south ends of the reef In the
presence of a longshore current, this localized circulation near the reef will be superimposed on the
longshore current. Since the direction of peak wave energy is from the northeast, localized
circulation near the reef is skewed to the south, resulting in the high rates of accretion observed in
recent surveys, while much less accretion is noted to the north. This "ponding" phenomenon was
demonstrated in a laboratory study performed at the University of Florida (University of Florida
1996).
4
Previous Page
