Evaluation of Report "SURVEY-BASED SEDIMENT BUDGET ANALYSIS FOR SEBASTIAN INLET, VOLUMES I, II, and III" by William R. Dally and Kathy FitzPatrick
Evaluation Conducted By:
Robert G. Dean
January 14, 1998
Evaluation Conducted For: Bureau of Beaches and Coastal Systems Marjory Stoneman Douglas Building Tallahassee, Florida
"SUVEYBASD SDIMNT UDGT ANALYSIS FOR
Evaluation of Report
"SURVEY-BASED SEDIMENT BUDGET ANALYSIS FOR
SEBASTIAN INLET, VOLUMES I, II, and III"
William R. Dally and Kathy FitzPatrick
January 14, 1998
Evaluation Conducted For:
Bureau of Beaches and Coastal Systems
Marjory Stoneman Douglas Building
Evaluation Conducted By:
Robert G. Dean
Department of Coastal and Oceanographic Engineering
University of Florida Gainesville, Florida
TABLE OF CONTENTS
LIST OF FIGURES .....................................................1iii
LIST OF TABLES...................................................... iv
BACKGROUND ........................................................ 2
Brief H1istorical Review............................................... 2
Discussion of Features Relevant to This Evaluation ...........................3
Study Elements Included in the D-F Report ................................. 3
REPORT REVIEW AND EVALUATION ..................................... 5
Data Analysis ...................................................... 5
Initial Evaluation of Report and Findings ................................... 5
ADDITIONAL ANALYSIS TO THAT PRESENTED IN THE D-F REPORTS ......... 8
Introduction ....................................................... 8
Alternate Plots and Analysis of Data Presented in the D-F Reports .................8
Evaluation of Historical Shoreline and Volume Data.........................11
Volumetric Changes.............................................. 16
Even/Odd Analysis .............................................. 16
Results From the Pelnard Considere Solution ............................... 16
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .....................21
Summary ........................................................ 21
Conclusions and Recommendations..................................... 22
REFERENCES ........................................................ 23
APPENDIX A BASIS FOR THlE PELNARD CONSIDERE SOLUTION FOR A
PARTIAL LITTORAL BARRIER................................... A-i
LIST OF FIGURES
1 Configuration of Sebastian Inlet Including Historical Shoreline Changes. (Wang, et
al, 19 9 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 The Seven Sub-Areas Selected by Dally and FitzPatrick for Analysis. (Dally and
F itzP atrick, 1996) ..................................................... 6
3 Volume Changes for Each of the Seven Sub-Areas ............................. 9
4 Volume Changes for Each of the Seven Sub-Areas Adjusted for Different
Individual Survey A reas ................................................ 10
5 Cumulative Sum of Volume Changes for all Seven Sub-Areas ................... 12
6 Cumulative Sum of Volume Changes for all Seven Sub-Areas but Excluding South
B each . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7 Cumulative Sum of Volume Changes for North Beach and South Beach ........... 14
8 (a) Longshore Distribution of Shoreline Changes, 1972-1986, Vicinity of Sebastian
Inlet. (b) Shoreline Changes Accumulated Away From Inlet ..................... 15
9 Longshore Distribution of Total Shoreline Changes, 1972-1986, Vicinity of
Sebastian Inlet and Decomposition Into Even and Odd Components .............. 17
10 Longshore Distribution of Odd Component of Shoreline Changes, 1972-1986,
V icinity of Sebastian Inlet .............................................. 18
11 Results of Even/Odd Analysis of Sebastian Inlet Vicinity Shoreline Changes 19461970. (D ean and W ork, 1993) ........................................... 19
11 Longshore Distribution of Non-Dimensional Deposition Rate .................... 20
A-1 Longshore Distribution of Non-Dimensional Deposition Rate ................... A-3
LIST OF TABLES
I Chronology of Improvements at Sebastian Inlet (Coastal Tech., 1987) .............. 4
2 Summary of Average Annual Volume Changes for Seven Sub-Areas ............... 7
Evaluation of Report
"SURVEY-BASED SEDIMENT BUDGET ANALYSIS FOR SEBASTIAN INLET, VOLUMES I, II, and III"
Sebastian Inlet is located at the boundary between Brevard and Indian River Counties, Florida. Downdrift property owners (south) of this inlet contend that the inlet has historically caused and continues to cause erosion of their beaches. See Figure 1 for a layout of Sebastian Inlet and historical shoreline positions. In order to provide a more thorough basis for evaluating the potential impact of the inlet and also to provide guidance for modification of the program for management of the sand resources at the inlet, if shown to be appropriate, the Sebastian Inlet Tax District (SITD) has carried out a four year study with one of the major objectives being the development of a "modem" sediment budget for the inlet. The study was conducted under the direction of Professor William R. Dally of the Florida Institute of Technology and Ms. Kathy FitzPatrick of the SITD and comprised detailed surveys over three years of the study and incorporated the prior three years of available data that had been collected by the SITD. That study resulted in the report "Survey-Based Sediment Budget Analysis for Sebastian Inlet, Volumes I, II, and III" by William R. Dally and Kathy FitzPatrick. The objective of the present report is the evaluation of the Dally and FitzPatrick reports, hereafter referred to as the "D-F" reports".
I S-COiQ in Ftirt I
- OLD JETTIES (1924)
- JETTY EXTENSIONII955)
~ RIPRAP (1959.1972)
JETTY EXTENSIONS (1970)
Figure 1. Configuration of Sebastian Inlet Including Historical Shoreline Changes.
(Wang, et al, 1991)
There are both legal and technical issues associated with the question of management of Florida's inlets. One of the legal questions related to the present issue of concern is whether any liability extends beyond the six year period considered in the D-F study for impoundment of sand resources within the influence of the Sebastian Inlet and associated downdrift erosion. Although it is clear that the inlet/jetty system has had a deleterious effect on the downdrift shoreline since the inlet was originally established, the question addressed in the reviewed reports concerned the modern-day time frame: that is whether or not the inlet as it is functioning today (specifically over the past six years) is continuing to exacerbate the downdrift erosion. Although the present study was tasked only with the technical evaluation of the reports by D-F, limited efforts were made here to supplement the data and interpretation presented in the D-F reports.
Brief Historical Review
Historically, inlets in the vicinity of modern-day Sebastian Inlet have formed but have not been stable over the long-term. Inlets formed by storms, high water levels inside Indian River Lagoon or other processes have resulted in migration and closure and are evidenced by the presence of relict flood tidal shoals inside the lagoon. In 1886, local interests attempted to open an inlet; however, this opening was closed soon by a hurricane. Several other efforts to develop an inlet were made with similar results until the Sebastian Inlet Tax District was formed by the Florida Legislature in 1919. The District designed an inlet located slightly north of the present inlet and construction commenced in 1924. The inlet channel was partially excavated when a hurricane completed opening the inlet in August 1924. Although this inlet required maintenance dredging, it remained open until 1942 when, due to wartime fuel shortages which limited maintenance dredging, the inlet was closed by a northeaster.
Sebastian Inlet was opened in its present position in 1948. Since that time, a combination of maintenance dredging, sand trap construction and operation and jetty modifications have resulted in a stable and navigable entrance. In 1955, the north jetty was extended 175 ft in a southwesterly (landward) direction and the south jetty was raised. Both jetties were extended southwesterly in 1959. In 1962, a channel was excavated with the following dimensions seaward of the bridge: 200 ft wide and 11 feet deep. Landward (east) of the bridge, the channel was 150 ft wide and 11 ft deep. The 1962 modifications included construction of a sand trap and the northwest channel was excavated. In 1963, the Coastal Engineering Laboratory of the University of Florida was contracted to conduct a model and field study and to develop recommendations that would improve navigation and reduce shoaling in the inlet (CEL, 1965; Chiu, 1966). The three main recommendations resulting from that study were: construction of a sand trap to impound sand entering on the flood currents this sand would later be pumped to the downdrift (south) beaches, a lengthening by 500 ft of the north jetty, and a lengthening by 75 ft of the south jetty. These recommendations were later implemented. More recently, (in the Fall of 1996 and in early 1997), approximately 200,000 cubic yards of compatible sand were placed a short distance south of the inlet. This sand was excavated from an inland borrow pit. The suitability of this borrow pit sand had been evaluated by Professor
Randall Parkinson of the Florida Institute of Technology (1995) who found that the sand "appears to be an acceptable source for beach nourishment." The mean diameter of the borrow sand was 0.28 mm which was stated to be "equivalent to the mean grain size of the sand trap and flood shoal sediment..." and "slightly finer than the sediment obtained from the feeder and control beaches." This volume of sand is sufficient to offset some of the deficit which appears to be occurring on the downdrift beaches. A chronology of construction and maintenance activities at Sebastian Inlet through 1988 as developed by Coastal Tech (1988) is provided in Table 1.
Discussion of Features Relevant to This Evaluation
One feature of the Sebastian Inlet system is the predominance of rock along the south shoreline. We were shown July, 1997 aerial photography taken when the water was exceptionally clear. These photographs showed reef systems oriented parallel to the shoreline and these reef systems appeared to occupy a substantial portion of the nearshore zone. These reefs are also evident in the profile plots of Volume III by D-F. In considering the shoreline change trends on the updrift and downdrift beaches, it is relevant to note that a particular volume chang per unit beach length on the beaches downdrift of Sebastian Inlet would cause a greater shoreline chang than the same volume change per unit beach length on the updrift beaches. The reason is the shallow rock that is present along and serves to "perch" much of the downdrift beaches. Considering an ideal profile that translates without change of form, a berm elevation of 6 feet and a depth to rock of 10 ft on the south side of the inlet and a depth of closure of 16 ft on the north side of the inlet, the change in horizontal position, Ay, on the north and south sides for a volumetric change of 1 yd 3/ft of beach is
J1.7 ft (South Side)
tAl=1.2 ft (North Side)
Study Elements Included in the D-F Report
The D-F report culminates with the quantification of a "modem" sediment budget for Sebastian Inlet based on surveys dating from Summer 1989 through Summer 1995. From 1989 to 1991, surveys were collected on an annual basis. Starting in 1991, survey data were collected semiannually and were extended from 3,000 feet to 14,000 feet north of the inlet and from 15,000 feet to 20,000 feet south of the inlet and on the ebb and flood tidal shoals. The surveys were conducted by Morgan and Eklund, a professional hydrographic surveying company, and it is apparent that a considerable effort was made to ensure the quality of the data although, as will be noted later, some of the results still raise questions regarding data quality.
Table 1. Chronology of Improvements at Sebastian Inlet (Coastal Tech., 1987) Date Improvements
1886 First cut attempted by hand.
1895 First cut completed, inlet closed by storm.
1918 First dredge cut completed, inlet closed by northeaster.
1919 Florida Legislature establishes the Sebastian Inlet Tax District, a special tax
district, with responsibility to maintain the inlet. 1923 Permit obtained and dredging begins.
1924 100 feet wide and 6 feet deep channel with rock jetties extending 400 feet
offshore are completed.
1939 Approximately 72,000 cubic yards of sediment are removed at a cost of $6,000.
194 1/1942 Inlet closed by northeaster. 1947 Approximately 70,000 cubic yards are dredged to open an 8 feet deep 100 feet
wide channel at a cost of $35,000.
1948 Channel is realized in current configuration. About 202,000 cubic yards of sand
and 660 cubic yards of rock removed at a cost of $57,625.
1950 Maintenance dredging removes 140,840 cubic yards of sand and 4,843 cubic
yards of rock at a cost of $38,000.
1952 A 3 00 foot extension of the north jetty was completed at a cost of $24,3 00.
Channel dredging removes 18,000 cubic yards at a cost of $13,000.
1955 Maintenance dredging removes 60,000 cubic yards of material at a cost of
$15,580. Two new jetties are completed. The north jetty was extended 250 feet
and the south jetty was extended 175 feet.
1958 Maintenance dredging removes 65,000 cubic yards.
1962 Channel dredging and sand trap excavation removes 282,400 cubic yards at a
cost of $247,138.50.
1970 North and south jetty extensions completed.
1972 New 37 acre sand trap excavation totaling 420,000 cubic yards completed at a
cost of $195,998.
1978 786,500 cubic yards excavated from sand trap and 187,600 cubic yards placed
on downdrift beach at a cost of $599,900.
1985/1986 13 3,290 cubic yards excavated from sand trap and 110,03 8 cubic yards placed
on downdrift beaches at a cost of $287,779.
1987/1988 Navigation channel located west of sand trap is dredged.
REPORT REVIEW AND EVALUATION
Volume changes were analyzed and presented in seven different geographic zones: (1) inlet throat, (2) navigational channel, (3) sediment trap, (4) flood tidal shoal, (5) ebb tidal shoal, (6) north beach, and (7) south beach, see Figure 2. The longshore extent of the north beach data was limited to 3,000 feet because that was the extent of the available data from 1989-1991. The analysis and interpretation of the data was reasonably straightforward and included the annual quantification of the volume changes within the seven geographic regions noted above and presented in Figure 2. These changes were presented in Tables 1-7 for the measured volume changes and Tables 8-14 for the normalized volume changes which are the measured volume changes adjusted for the plan areas of the surveyed areas. Since different surveys encompassed different areas, the normalized volume changes were adjusted to a common area for all surveys for each sub-area.
Initial Evaluation of Report and Findings
The sediment budget developed in the D-F reports is independent of the longshore sediment transport rates. This budget was based solely on the volume changes in the seven sub-areas defined; however, the method applied and the results presented do address the issues of interest to the SITD and the Florida Department of Environmental Protection (FDEP).
In addition to presenting the intersurvey volume changes, Tables 8-14 present the "net annual rates" of volume change for each sub area which is the total volume change divided by the associated time interval. These net annual rates are reproduced in Table 2.
Table 2 shows that the most active components of the system are the inlet throat (-23,327 yd3 Iyr), the sediment trap (- 18,033 y& /yr), the ebb tidal shoal (+24,663 y& /yr), the North Beach (+26,086 yd3 /yd) and the South Beach (-131,833 yd3/yr). Relevant to this discussion is that sand was removed from the sediment trap twice (between the 8/89-7/90 and 7/92 7/93 surveys) for a total of 365,000 y&3. Incorporating this into the results shown in Table 2 would result in an annual accumulation rate in the sediment trap of 42,800 yd3 /yr as contrasted to a net erosion in Table 2 of 18,033 yd& /yr. Although the net annual change in the ebb tidal shoal is reasonably large 24,663 ycf /yr, it is small compared to the interannual. changes ranging from -261,266 yd& to +219,035 yd3.
Flood hoal Ib Shoal
Figure 1 Plan View of 7/95 Survey Showing Domains Used in Budget Analysis
Figure 2. The Seven Sub-Areas Selected by Dally and FitzPatrick for Analysis. (Dally and FitzPatrick (1996)
Summary of Average Annual Volume Changes for Seven Sub-Areas
Sub Area No. of Years for Net Annual Cmet
Sub Area Changes Chney'/rom nt
Calculated Cag d y
Inlet Throat 6 -23,327
Navigation Channel 6 4,973
Sediment Trap 6 -18,033
Flood Tidal Shoal 6 -2,149
Ebb Tidal Shoal 6 24,663
North Beach 5 26,086 Longshore Extent of Survey =
South Beach 4 -131,823 Longshore Extent of Survey =
____ ____ ___ __ ____ ___ 17,000 ft
sumJ________1 -132,3151 1____________The D-F report is organized clearly and the volume changes in the seven sub-areas analyzed and presented relatively straightforwardly. The average annual volume changes in these subareas have been summarized in Table 2. Tfhe summary changes reported are the average annual values over the period of survey documentation. It will be shown later (as is also evident from Tables 8-14 of the D-F report), that there is substantial variability in some of the volume changes and this variability should be considered in interpretation of these average values. Also, no attempt was made to account for the limited length of the north shore segment included in the analysis.
The report develops an "Average Annual Budget" for Sebastian Inlet based on the six years of data incorporated into the analysis. It is concluded (p.47) based on the analysis that extends 3,000 ft north of the inlet that "during the six years that viable survey data are available, the analysis indicates that the inlet has, in total, mechanically and naturally bypassed 12,486 yd' more sediment than it has trapped." This statement presupposes that that no volume storage occurred north of the limited (3,000 feet) shoreline to the north of the north jetty included in the analysis and is regarded as problematic.
The interpretive model adopted by D-F for the sediment transport system in the vicinity of Sebastian Inlet is that the potential for sediment transport from the north and toward the south of the inlet exceeds the supply of sediment from the north. Thus, less sediment accretes to the north than erodes to the south.
With regard to survey accuracy of the South Beach, it is stated (D-F, p. 46) "It is stressed that the average bed changes for both North and South Beaches are well-within the vertical tolerance of the hydrographic survey techniques ( 0.5 ft), and that due to the wide spacing of the profiles and the abundance of rock reef south of the inlet, these volume computations are not as reliable as those for the other domains." However, the normalized volume South Beach volume change for the intersurvey period 7/91 to 7/92 is -634,962 y&3 which, for a shoreline length of 17,000 ft is equivalent to 37 yd3 Ift, a substantial change representing in magnitude, the lower limit of a beach nourishment project. Although the survey accuracy may be 0.5 ft, if this error is unbiased, the integrations of profile changes to produce volumes should reduce the error significantly. Although the full data set has not been examined, it may be possible to improve the accuracies of the volumes, perhaps by reducing the seaward distance over which the volumes are calculated. The following sections attempt to address further the intersurvey variability and the limited length of data analysis on the north beach, and thereby provide additional information and understanding of the sediment budget.
ADDITIONAL ANALYSIS TO THAT PRESENTED IN THE D-F REPORTS
The supplementary analysis effort presented here includes three components: (1) alternate plots and analysis of data presented in the D-F reports, (2) limited analyses of earlier DNRIDEP data, and (3) discussion of longshore distribution of depositional patterns updrift of the inlet as predicted by the Pelnard Considerd approach.
Alternate Plots and Analysis of Data Presented in the D-F Reports
This section considers volume changes somewhat differently than in the D-F report. In order to examine the volumetric changes further and to provide a basis for visually recognizing the trend of changes, the variability with time of the cumulative volumetric changes for each of the seven subareas were calculated from Tables 1-7 and are plotted in Figure 3. As discussed previously, because the surveys in the seven sub-areas did not encompass the same area each time, Dally and FitzPatrick "normalized" the changes for all surveys by assuming that for those surveys which encompassed areas smaller than the normalized areas, the volume change per unit area (elevation change) for the area less than the normalized area was equal to the average in the surveyed areas. The normalized area was the average of the areas for the six summer surveys. Dally and FitzPatrick applied this method only to the summer surveys, rationalizing that this would remove seasonal bias and it appears that the quality and extent of the summer surveys may have been better and greater, respectively than the winter surveys. In particular, the areas encompassed in the February 1991 survey on the north and south beaches were 0.05 and 0.09, respectively of the normalized values for these two areas. Figure 4 presents the variation with time of the normalized cumulative volume changes for all seven sub-areas. This plot includes the winter data. Because of the limited coverage of the February 1991 survey on the north and south beaches, the starting time for these data is February 1991 .1n interpreting the results in Figures 3 and 4, it is helpful to note that bypassing from the sediment trap
I Flood Shoal
o Sed. Trap
..... ... .. . . . . - . .. - In le t T h ro a t .. . . . .
~0.8 _----O0_ Ebb Shoal
.. . . . .. .. . . --- --- N orth B each -
0~~I 0. ------006 .South Beach'
Figure 3. Volume Changes For Each of The Seven Sub-Areas
1 .0u - --- I I I I IIIII CO) Inlet Throat
.20.6 South Beach
0 0.4 -----0
M 0 .2 - - - - -:- - -
~ 0.4 ----..6 .......................... ... .. -- -- --
> -1.0 I I
0 1 2 3 4 5 6
Time (Years) Figure 4. Volume Changes for Each of the Seven Sub-Areas
Adjusted for Different Individual Survey Areas
in the amounts of 249,000 yd 3 and 1 16,5 00yd 3 were conducted during the 8/89-7/90 and 7/92-7/93 intersurvey periods, respectively.
Figure 5 presents the cumulative stum of the summer to summer volume changes for all seven sub areas. It is seen that the interannual changes are very large and South Beach will be demonstrated in subsequent plots to be the dominant contributor to this large variability. Figure 6 excludes South Beach volume changes from the cumulative changes presented in Figure 5. It is seen that the changes are quite gradual and always positive. Comparison of Figures 5 and 6 documents that South Beach changes dominate those of all seven sub-areas.
Although it is clear at this stage that the South Beach volumetric changes are dominant, it is of interest to examine a simple model in which the transports are the same on two sides of a partial littoral barrier but the transports may vary in magnitude and direction. If no sand is lost to the system and if the surveys extended sufficiently far on both sides of the littoral barrier, summing the volumes on the two sides of the barrier should always result in zero volume change. The results of summing the north and south beach volume changes are presented in Figure 7 where it is seen that although the net changes are quite negative, the changes are somewhat less in magnitude than the cumulative volumetric changes for North Beach alone, thus demonstrating some features of this simple model.
The analyses reported in this section suggest that one or a combination of three possibilities is responsible for the large negative cumulative volume changes: (1) the surveys did not extend a sufficient distance updrift (north) to capture all volume changes in the north beach sub-area, (2) there were accuracy problems in the survey data, and/or (3) there is a deficit of sand flow toward Sebastian Inlet from the north. The purpose of examining historic data in the next section is to attempt to determine whether the surveys extended sufficiently far to the north to capture the volumetric change and to provide additional quantification to the volume changes.
Evaluation of Historical Shoreline and Volume Data
In order to examine whether there was additional sand storage on the updrift side of the inlet along segments not included in the D-F study, data available from the FDEP were examined. The available survey data were collected in 1972 and 1986, thus representing a time interval of 14 years. The analysis of these data and the results obtained are described below.
Shoreline Changes The longshore distribution of the shoreline changes on both sides of Sebastian Inlet are shown in Figure 8a and the cumulative areas of shoreline changes progressing away from the inlet in both directions are presented in Figure 8b. It is seen that on the north side, there was generally shoreline advancement and on the south side, recession is generally present. In analyzing these data, there were three locations where the changes appeared inconsistent with the main body of the data. In such cases, the closest aerial photographs to the times of the surveys were examined and for two locations (R-214 and R-2 19, Brevard County), the change was deleted from the data Examining Figures 8a and 8b, it appears that the accretion and erosion north and south of the inlet were still occurring in the period 1972 1986 at the limits of the approximately 30,000 feet segments
/ .' / :\ ,/ :~
. . . . . . . . . . .
. . . . . - - -\ -
. .. .. ... .. ..\
. . . . . . . . . . .
... .. ... .. .. ... .. ..
Figure 5. Cumulative Sum of Volume Changes
For All Seven Sub Areas
. . . . . . . . . . .
1 2 3 4 5
Figure 6. Cumulative Sum of Volume
For All Seven Sub Areas But Excluding South Beach
0.2 0.1 0.0
0 ,1 ,1.. IMt
' If IIfL
- 8 0 i i i i i i i I i . . l l l l l l l l l l l l l l
-30000 -20000 -10000 0 10000 20000 30000
Longshore Distance (Ft)
Figure 9. Longshore Distribution of Total Shoreline Changes, 1972-1986,
Vicinity of Sebastian Inlet and Decomposition Into Even and Odd Components.
/0 1 /\ r\
0 e 0 \ C\2 ^' I 1
-- oI\ 1 \ IL/ / ^
8 0 . . I' i l si l I , , ,, I ,
-30000 -20000 -10000 0 10000 20000 30000
Longshore Distance (Ft)
Figure 10. Longshore Distribution of Odd Component of Shoreline Changes, 1972-1986, Vicinity of Sebastian Inlet.
o 0.2 I
o 0 .. . . .. . .. . .
- 0 2 - - - - - . . . . . . . . . . - - - - - . . . . . . . . .
a ) -0 3 -- -- -- -- -- . .. .. . :- - - . . . .. . .-- --- --- -aI)
E -0 .5 .. . .. .. . . . . . . . . . . . .
0 1 2 3 4 5 6
Figure 7. Cumulative Sum of Volume Changes
For North Beach and South Beach
100 1" l 'l I l " "
I. 0 f ,~~i: I I I I I I I I I I I I . . ,
500 1 -1 " '1 " T I I I
-30000 -20000 -10000 0 10000 20000 30000
Longshore Distance in Feet
Figure 8. (a) Longshore Distribution of Shoreline Changes, 1972-1986, Vicinity of Sebastian Inlet.
(b) Shoreline Changes Accumulated Away From Inlet.
incorporated in these plots. It is of interest that the cumulative shoreline change areas within the + 30,000 feet are approximately +400,000 ft2 on the updrift side of the inlet and 350,000 ft2 on the downdrift side of the inlet.
Volumetric Changes If the conversion ratios developed earlier (Page 3) are employed, this results in volumetric change rates of+ 23,800 yd/yr on the north side and 14,700 yd3/yr on the south side of the inlet.
Even/Odd Analysis As a brief effort to evaluate by a different approach, the effects of the inlet on the beaches to the north and to the south, an "even/odd" method of analysis was applied to the shoreline change data from 1972 to 1986. These results are shown in Figure 9 where it is seen that overall, the odd function is somewhat greater than the even function and extends a considerable distance from the inlet. In various applications, the odd function has been interpreted as representing the blocking effect of the sediment transport system by the jetty. The odd function shows that there has been a buildup to the north of the inlet and an associated erosion to the south. The odd function is presented separately in Figure 10 and, by definition, has the same magnitude on both sides of the inlet but has different signs on the two sides. Applying the same conversion factors from shoreline changes to volumes (Page 3), similar volumetric changes are obtained from the odd function to those presented under "Volumetric Changes" above.
Dean and Work (1993) had previously developed an even/odd analysis of Sebastian Inlet for the period 1946 to 1970 with the results presented in Figure 11. These results were interpreted as the inlet impounding approximately 75,000 yd3/yr.
Results From the Pelnard Considere Solution
In some respects, it is somewhat surprising that after more than two decades of the North Jetty extension, an average annual deposition rate of some 26,000yd 3 was found in the D-F analysis within 3,000 ft north of the north jetty. Yet, in examining Figure 3, this rate appears to be reasonably steady. The Pelnard Considere solution for a littoral barrier may assist in interpreting this result. The Pelnard Considere method of shoreline analysis and the resulting solutions provide idealized frameworks for evaluating more realistic cases. Although the situations represented in the Pelnard Considere methodology are considerably more simplified than those in nature, they assist in gaining insight into the processes.
The Pelnard Considere solution of interest here is that for a partial littoral barrier which here is represented by the Sebastian Inlet jetties. It is noted that the two main approximations present in this solution are that the profile translates without change of shape in response to a change in volume and that the wave direction is constant. The details of the solution are presented in Appendix I. The type of information that one can hope to derive from this solution is some idea of the depositional patterns updrift of the jetty and thus whether the 3,000 feet of documented change affects the sediment budget developed.
4 ---40- Odd
wU 0 Y
z % '
-6,000 -2,000 0 2,000 6,000
LONGSHORE DISTANCE (M)
" Inlet (Present Alignment) Cut In 1948
" Jetties Constructed: 1950's to 1970
" Net Longshore Sediment Transport Rate Estimates (Southerly)
Range From 120,000 m3/yr to 230,000 m3/yr
" Bypassing From Interior Sand Trap
* Above Shoreline Change Values From 1946-1970
Figure 11. Results of Even/Odd Analysis of Sebastian Inlet Vicinity Shoreline Changes 1946-1970.
(Dean and Work, 1993).
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
. . . . . . . . . .
- - - - - - - - -
Z U.U0 1 2 3 4 5 6 7 8 9 1
Non-Dimensional Distance Updrift of Jetty, -x'
Figure 12. Longshore Distribution of Non-Dimensional
S l I I I
----------- ------............ - - -
---------. . . . . . . .
. . . . . - - -
.. . . . . . 7 . . . .
I i I I i I i I I
........ t' = 5.0
- - - - - - - - - - -... . . . t' = 2 0 . . . . .
. . . . . . . .. . . . . .. - - - - - --.. .. .
.. .. . . . . ... . . . . . . . . . . . . . . . . . . . .. . . . ..
. .. . . . ... . . . . . .. . . .. . . . . . . . . . . . . . . .
. . ..... . . . . ... . . . . . . . . ... . . . . ... . . . ..
I i i I I I
Figure 12 presents the longshore distribution of the depositional patterns on the updrift side of the littoral barrier as predicted by the Pelnard Considere solution. In Figure 12, x' and t' represent nondimensional shoreline position and time, respectively and it is seen that for small times, the deposition occurs close to the littoral barrier and for later times, the depositional peak migrates updrift and becomes more diffuse. The reduction in the areas under the curves with increasing time is due to the bypassing that commences at t' = 1. According to the Pelnard Considere solution, the updrift shoreline builds out to the length of the littoral barrier with large time. Two questions arise in relating these results to Sebastian Inlet: (1) what is the present appropriate value oft', and (2) to what degree are these results representative of the beaches updrift of Sebastian Inlet? Although further analysis based on the Pelnard Considere approach would appear to be enlightening, it is considered beyond the scope of this evaluation. However, it is considered likely that sand continues to deposit beyond the 3,000 feet extent of the north beach surveys employed to develop the sediment budget.
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
The reports by Dally and Fitzpatrick (D-F) present, document and interpret the results of sixyears of survey data in the general vicinity of Sebastian Inlet. The results are presented for seven sub-areas of the inlet complex, see Figure 2.
The results are interpreted in terms of average annual volume changes for each of the seven sub-areas. These results were presented in Table 2. Regardless of the reason, it is seen that the 17,000 ft South Beach segment included in this analysis appears to continue to lose sediment at a substantial rate. The 3,000 ft segment included in the North Beach segment demonstrates reasonably slow growth at a rate of 26,000 yd3 /yr.
A supplemental analysis, conducted as part of this evaluation has included alternate examination of the data presented in the D-F reports, a limited analysis of DEP historical shoreline position data including examination of the patterns of longshore distributions of shoreline change north and south of the inlet and examination of results from an idealized analytical model. Surprisingly, although the analysis presented here extends approximately 30,000 ft north and south of the inlet, the accumulation on the north side of the inlet is approximately 24,000 yd3/year, reasonably close to the value of 26,000 yd3/year determined in the D-F report. The downdrift erosion determined here within the 30,000 ft is approximately 15,000 yd3/year compared to the erosion of 132,000 yd3/year found in the D-F report. Changes appear to extend beyond the 30,000 feet distances from the inlet considered here.
Conclusions and Recommendations
1. The sediment budget components, determined in the D-F report should be interpreted in
terms of the intersurvey variability, whether the surveyed areas likely encompassed the areas
of inlet-induced changes, and the human-actions within the time period examined.
2. The 3,000 ft North Beach segment included in the analysis does not appear to encompass the
full updrift influence of Sebastian Inlet. Additionally, a six year period may be too short to
develop a valid sediment budget.
3. The large intersurvey changes of the South Beach volumes should be examined to assess
whether or not the data are valid.
4. The sediment budget should be adjusted to include the two sand placement operations which
occurred during the period of analysis.
Bruun, P. J. A. Battjes, T. Y. Chiu and J. A. Purpura (1966) "Coastal Engineering Studies of Three Coastal inlets", Bulletin No. 122, Florida Engineering and Industrial Experiment Station, University of Florida, Gainesville, Florida.
Coastal Engineering Department (1965) "Coastal Engineering Hydraulic Model Study of Sebastian Inlet, Florida" Report No. 65/006, Florida Engineering and Industrial Experiment Station, University of Florida, Gainesville, Florida.
Coastal Technology Corporation (1988) "Sebastian Inlet District Comprehensive Management Plan", Vero Beach Florida.
Dally, W. R. and K. FitzPatrick (1997) "Survey-Based Sediment Budget Analysis for Sebastian Inlet, Volumes I, II, and III", Reports Prepared for the Sebastian Inlet Tax District.
Dean, R. G. and P. A. Work (1993) "Interaction of Navigational Entrances With Adjacent Shorelines", Journal of Coastal Research, Special Issue No. 18, pp.91-110.
Mehta, A. J., W. D. Adams and C. P, Jones (1976)" Sebastian Inlet Glossary of Inlets, Report No. 3", Coastal and Oceanographic Engineering Laboratory Report No. COEL-76-01 1, University of Florida, Gainesville, Florida.
Parkinson, R. (1995) "Grain Size Analysis of Fisher and Sons Samples", Memorandum Report with Attachments to Ms. Kathy FitzPatrick of the Sebastian Inlet Tax District.
Wang, H., L. Lin, H. Zhong and G. Miao (1991) "Sebastian Inlet Physical Model Studies: Part I Fixed Bed Model", Coastal and Oceanographic Engineering Department Report No. UFL/COEL91/001, University of Florida, Gainesville, Florida.
BASIS FOR THE PELNARD CONSIDERE SOLUTION
FOR A PARTIAL LITTORAL BARRIER
The Pelnard Considere Solution for a Partial Littoral Barrier
One of the predominant impacts of ajettied inlet is the simple blocking of the net longshore transport by a barrier of length, V, in which case the amount of sediment accumulated on the updrift side of the inlet is equal to the volume eroded on the downdrift side. If it is assumed that the profile responds without change of form and that diffraction effects are negligible, the solution for the case of a partial littoral barrier is
y(x,t) =]+ exp --Ixl erfc |x| 1), ttbp
y(x,t)= erfc (x| t tbp
in which partial bypassing of the barrier commences at t' = tbp, X and y are in the longshore and offshore directions, respectively, erfco0 is the complementary error function, o, is the azimuth from which the breaking wave is propagating, 3o is the azimuth of the unperturbed outward shore normal, and the plus and minus signs apply for negative and positive x respectively in the first equation and to the updrift and downdrift sides of the barrier, respectively in the second equation. The time that bypassing commences, tbp, is given by x 2
tbp = 4Gtan2ob (2)
Equation 1 can be expressed in non-dimensional form as
y I(,x [ V 't7e x') )
-x'Iv Ierfc ,xl t 1
y'(x',t')=+ erfc x) t' 1
x tan(P-0b) t' t
y V-71P tbp
in which in this idealized solution, bypassing at t' = 1 commences. This solution is highly idealized and considers unidirectional waves and a profile that is displaced seaward and landward without change of form due to deposition and erosion, respectively. Nevertheless, some insight into the problem of concern can be gained through examining the above solutions. In particular, the patterns of updrift deposition are of interest. Figure 12 in the main text and reproduced here as Figure A-i, presents the longshore distribution of non-dimensional deposition rate as a function of various nondimensional times, V. It is seen that initially as illustrated for the smallest t' presented (t' = 0.1), the deposition rate is highest near the jetty. As time progresses and the shoreline segment near the jetty becomes impounded to capacity and commences bypassing, there is an updrift migration of the location of the peak migration rate. Additionally, the length of the deposition pattern becomes broader. At any particular time, the total depositional rate updrift of the updrift jetty must equal the difference between the ambient transport rate into the updrift region and the bypassing rate.
I I I I I I I I
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Non-Dimensional Distance Updrift of Jetty, -x'
Figure A-I. Longshore Distribution of Non-Dimensional
... .. . ". . . . .. t = .
...... t = 1.0
........ t' = 5.0
. . . . . . . . . . . . . . . .
. . i . . . . . .. . . . . . ,.. . . . .. ..
. . . . . - - - .I . I
1 2 3 4 5 6 7 8 9 10
. . . . . . . . .
- - - - - . . . . .
. . ... . . . . . . . . .
. . . . . . . . . .
I I I I I