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UFL/COEL-95/024
SHORELINE RESPONSE AND SEDIMENT TRANSPORT
IN THE VICINITY OF LAKE WORTH ENTRANCE ON
PALM BEACH ISLAND
DECEMBER 1993 TO JUNE 1995
By
Robert G. Dean
and
Renjie Chen
November 1995
Prepared for:
Town of Palm Beach
UFL/COEL-95/024
SHORELINE RESPONSE AND SEDIMENT TRANSPORT IN THE VICINITY OF
LAKE WORTH ENTRANCE ON PALM BEACH ISLAND
DECEMBER 1993 TO JUNE 1995
November 1995
Prepared for:
Town of Palm Beach
Prepared by:
Robert G. Dean
and
Renjie Chen
Coastal and Oceanographic Engineering Department
University of Florida
Gainesville, Florida 32611
TABLE OF CONTENTS
LIST OF FIGURES ................... ................................... iii
INTRODUCTION ........................................................ 1
DESCRIPTION OF AVAILABLE DATA ........................................ 1
ANALYSIS AND INTERPRETATION OF RESULTS .............................. 1
December 1993 to March 1994 ......................................... 4
March 1994 to November 1994 ...................................... ... 4
November 1994 to December 1994 ..................................... 4
December 1994 to M arch 1995 ...................................... ... 4
March 1995 to June 1995 .................. ........................... 4
December 1993 to June 1995 .......................................... 6
DISCUSSION ................... ...........................................6
SUMMARY AND CONCLUSION ............ ................................. 6
REFERENCE ............ ............................................ 6
LIST OF FIGURES
FIGURE PAGE
1 Survey Plan Immediately South of Lake Worth Entrance ........................ 2
2 Placement Distribution of 1993-1994 Beach Nourishment ....................... 3
3 Placement Distribution of 1994-1995 Beach Nourishment ....................... 3
4 Longshore Distribution of Longshore Sediment Transport, Various Intersurvey
Intervals ....................................................... 5
SHORELINE RESPONSE AND SEDIMENT TRANSPORT IN THE VICINITY OF
LAKE WORTH ENTRANCE ON PALM BEACH ISLAND
INTRODUCTION
This report interprets shoreline changes and sediment placement volumes in the vicinity of the
north end of Palm Beach Island in terms of sediment transport in the general region placed and
transported toward the south.
DESCRIPTION OF AVAILABLE DATA
The dates for the survey data available for the analysis presented in this report are: December
1993, March 1994, November 1994, December 1994, March 1995 and June 1995. In addition to the
surveys, there were two nourishments which occurred from December 1993 to March 1994 and
November 1994 to February 1995. The volumes placed during these two periods were 169,700
cu.yds. and 196,620 cu.yds., respectively. The approximate distributions of the material placed were
provided by the U.S. Army Corps of Engineers.
The survey plan comprises 24 profiles commencing 200 ft. south of the Lake Worth Entrance
south jetty as shown in Figure 1. The placements during the 1993-1994 and 1994-1995 beach
nourishments are presented in Figures 2 and 3, respectively.
ANALYSIS AND INTERPRETATION OF RESULTS
The volumes placed in the two nourishment periods and the profile changes from the various
surveys were interpreted to determine sediment transport and redistribution toward the south in the
following manner. The continuity equation can be expressed as
S+ =S(x) (1)
ax at
in which x is the distance south from the south jetty at Lake Worth Entrance, t is time, Q is the net
longshore sediment transport, S is the average rate of nourishment placement per unit beach length
and V is the sediment volume per unit length at a particular location along the beach. Integrating this
equation from the south jetty where the longshore sediment transport is considered to be zero to the
limits of the surveyed area yields
Q(x) = at + f S(x)dx (2)
Jo At (2)'
Profile Number
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Figure 1. Survey Plan Immediately South of Lake Worth Entrance
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1993-1994 Nourishment 169,700 yd3
0 1000 2000 3000 4000 5000 6000 7000
Distance South of South Jetty (ft)
Figure 2. Placement Distribution of 1993-1994 Beach Nourishment
1994-1995 Nourishment 196,620 yd3
0 1000 2000 3000 4000 5000
Distance South of South Jetty (ft)
6000 7000
Figure 3. Placement Distribution of 1994-1995 Beach Nourishment
The transports associated with the various surveys are shown in Figure 4. These results interpreted
in terms of net annual longshore sediment transport are discussed below for the various intersurvey
periods.
December 1993 to March 1994
It is seen that the transport during this period increased fairly uniformly from the south jetty
to the end of the survey area. The net transport rate during this period was directed to the south at
a rate of approximately 400,000 cu.yds. per year. It is noted that this is a Winter period and also this
high rate may be explained partially by the fact that the nourished material may have created an
anomaly or "bulge" in the shoreline which tended to induce transport to the south.
March 1994 to November 1994
During this period, the transport over the 7,000 ft. monitored shoreline was actually directed
to the north at a rate of 54,000 cu.yds. per year. This period encompasses the entire Summer of 1994
during which northerly transport would be anticipated and also portions of the Winter seasons both
preceding and following the Summer of 1994. Although the effects are somewhat surprising, when
combined with the previous period, the net longshore sediment transport encompassing both
intersurvey periods is still to the south.
November 1994 to December 1994
This is an extremely short period. Additionally, some material from this second nourishment
effort was placed during this period complicating its interpretation. However, if strictly interpreted,
it would yield a very large longshore sediment transport rate to the south on the order of 450,000
cu.yds. per year; however in interpreting the figure, it should be noted that this rate is for an
extremely short period.
December 1994 to March 1995
During this four month period, the net longshore sediment transport was quite small,
amounting to an equivalent rate of approximately only 50,000 cu.yds. directed to the south.
March 1995 to June 1995
This three month period extends up to the most recent monitoring period and is equivalent
to 150,000 cu.yds. per year directed to the south.
500000
400000
300000
1000 2000 3000 4000 5000 6000
Distance from R76 (feet)
Figure 4. Longshore Distribution of Longshore Sediment Transport, Various Intersurvey Intervals
7000
Nourishment periods:12/93-03/94 and 11/94-02/95
| . . . . . . . . . . . . . . . . . . . . .
---- 12/93-03/94 .................. .................................. ................. ........
03/94-11/94
.. .11/94-12/94 .... ............................... ........ ..... ............
.-.*- 12/94-03/95
.--.:................ ---------- ................. : ................... :.................
03/95-06/95
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-U-- 12/93-06/95
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December 1993 to June 1995
This period encompasses all surveys for the entire 18 month period monitored. The total net
longshore sediment transport to the south is 156,000 cu.yds. which equates to 104,000 cu.yds. per
year.
On the basis of the results presented earlier, two results are evident: (1) the material placed
immediately south of the jetty does provide nourishment to the beaches to the south, and (2) the net
longshore sediment transport to the south is on the order of 100,000 cu.yds. per year.
DISCUSSION
There are some uncertainties associated with the results presented above that merit discussion.
First, the time period of 18 months, although significant, is relatively short and could have represented
a period during which the southerly net longshore sediment transport was not representative. It is
believed, based on calculations using wave data and wave direction, that there can be substantial
variabilities in longshore sediment transport including years when the net sediment transport direction
is even opposite to the average direction. A second possibility for uncertainty would be if some of
the sediments placed were carried back into the inlet. However, it is believed that the material placed
from maintenance dredging of the inlet was of such a size that it was not easily suspended and carried
back into the inlet. If sediments were transported back to the inlet, the effect on the calculated
longshore sediment transport would be to reduce the values below the approximate 100,000 cu.yds.
described above. This is because the volumetric reductions would be into the inlet rather than to the
south as concluded here.
SUMMARY AND CONCLUSION
Based on the results provided herein, the predominantly northeast waves and the material
placed during the Winters of 1993-94 and 1994-95 have resulted in sediment transport to the south
over the 7,000 ft. distance monitored. These values are approximately 100,000 cu.yds. per year and
compare reasonably well with an estimate (Dean, 1995) of 132,000 cu.yds. per year based on volumes
of material placed and associated shoreline changes occurring over a 44 year period.
REFERENCE
Dean, R.G. (1995) "Historical Shoreline Changes in the Vicinity of the PEP Reef Installation and
Reef Effects, Palm Beach, Florida", UFL/COEL-95/008, Coastal and Oceanographic Engineering
Department, University of Florida, Gainesville, Florida, 17 pages.
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