INDEPENDENT REVIEWS OF REPORT:
"REVIEW OF SELECTED EAST COAST FLORIDA INLETS PHASE 1: SEBASTIAN INLET, FL
EVALUATION OF COASTAL PROCESSES AND MANAGEMENT PRACTICES AND DEVELOPMENT OF RECOMMENDED MODIFICATIONS"
Robert A. Dalrymple Scott A. Douglas
David L. Kriebel
Bureau of Beaches and Wetland Resources Department of Environmental Protection Tallahassee, FL 32399
September 10, 2003
INDEPENDENT REVIEWS OF REPORT:
"REVIEW OF SELECTED EAST COAST
PHASE 1: SEBASTIAN INLET, FL
EVALUATION OF COASTAL PROCESSES AND
MANAGEMENT PRACTICES AND
DEVELOPMENT OF RECOMMENDED MODIFICATIONS"
Robert A. Dalrymple, Johns Hopkins University Scott A. Douglass, University of South Alabama
David L. Kriebel, U. S. Naval Academy
September 10, 2003
Bureau of Beaches and Wetland Resources Department of Environmental Protection
Tallahassee, FL 32399
Reviews Assembled and Submitted by:
Department of Civil and Coastal Engineering
University of Florida
Gainesville, FL 32611 6590
This document presents the three independent reviews of the report "Review of Selected East Coast Florida Inlets. Phase 1: Sebastian Inlet, FL. Evaluation of Coastal Processes and Management Practices and Development of Recommended Modifications". The report reviewed was written by Robert Dean and the reviews presented here were carried out by: Professor Robert A. Dalrymple, Johns Hopkins University; Professor Scott A. Douglass, University of South Alabama, and Professor David L. Kriebel, U. S. Naval Academy and are reproduced fully herein as received.
Professor Robert A. Dalrymple
Johns Hopkins University
Robert A. Dalrymple, Ph.D., P.E
1902A Indian Head Road
Ruxton, MD 21204
Dr. Robert G. Dean
Department of Civil and Coastal Engineering Weil Hall
University of Florida
Gainesville, FL 32611-6590
Dear Dr. Dean:
This letter is a review of your report, Review of Selected East Coast Florida Inlets: Phase 1: Sebastian Inlet, FL, February 24, 2003. This report discusses the impact of the inlet on the adjacent shorelines and the sediment budget for the inlet.
Several techniques are used in the report to calculate the nature of the sediment transport around the inlet. The primary data sources are aerial photographs and beach profiles taken by the Sebastian Inlet Tax District both north and south of the inlet in respectively Brevard County (BC) and Indian River County (IRC.
The large set of aerial photographs in Appendix A provides an interesting visual history of the inlet's evolution in time, showing the complex interaction of the inlet with the shoreline. Clearly the presence of the inlet caused a significant downdrift impact that currently extends for miles south of the inlet. Much of the downdift erosion appears to have occurred in the time period 1949-1962.
The beach profiles extend (after 1999) up to 30,000 feet north and south of the inlet. Due to the presence of some survey errors, a novel technique was used to identify and to correct survey errors. (These kinds of errors are not unexpected. Survey errors are often encountered in studies of this kind). The author's technique is very reasonable--figures 3,4,5 and the results in fig 7 and 8 are convincing that there were survey errors and the corrections are reasonable.
Questions about the profiles: The survey of 2002 is used as a reference survey and other surveys are often corrected to that survey. Since the 2002 survey is arbitrarily chosen, what is the sensitivity of the results to the choice of 2002 survey? If another survey were chosen, how would the results change?
Analyses of the profile data. In Section 5 some profile analyses are carried out. In 5.2.2, the average shoreline position north and south of the inlet are determined. (Which of the 30 and 31 monuments were used, being common with the 2002 survey, and which were not?) Clearly there is a updrift shoreline
advance visible in the data and downdrift shoreline retreat, with the exception of the short period in 1999-2001 when the downdrift beach moved seaward.
In Fig 12 the volume changes north and south of the inlet are shown. How were these values calculated? Why are only the data from 1999-2001 used?
Figs 15 and 16 show the average shoreline change rate (in ft/yr) and the cumulative shoreline change rate for the periods of 1999-2002 and 1972-2002. Fig 16 indicates, as does Fig 11, that the IRC shoreline has retreated during the 1972-2002 time span.
This shoreline retreat can be related approximately to volume lost. Using the equation for the one-line shoreline model (as used in Appendix C), we have the shoreline position y(xt) given by the solution of the equation:
oy/a = G a2 y/ a x2
Integrating this equation with resp,:ct to x from 0 to L gives
f Jay/at dx = G o),lax IL- G ay/x jo
Introducing the equation for sedime nt transport, we have the final result that the integral of the shoreline change rate along the shore is related to the difference between the outgoing lo ngshore transport rate and the incoming longshore transport rate.
(h* + B) f oy/at dK = Q(L)- Q(O)
Therefore the ordinates of figs 15 and 16 can be considered to be change in longshore transport along the reach divided by the active profile dimension, or (Q(L)-Q(O))/(h*+B), where the au hor gives (h*+B) as 22.7' in Appendix C. Using this approach, the change ir alongshore transport on the BC beach for 1972-2002 is Q(L)-Q(O) =11, 350 (y/yr (22.7*13,500/27). For IRC, Q(L)Q(0)=8,400 cy/yr, leaving the reacli. This seems to give a factor of 10 difference with the volume calculations for the same time period, fig 17. However, this difference is not the same for the 1999-2002 time period, as the calculations are relatively close: this method yields 100,000 cy/yr, and fig 17 gives about 150,000 cy/yr. it is not clear how to reconcile these 1972-2002 numbers, which one would think should be relatively close.
In Fig 15, between R5 and RIO and for 1999-2002, there is a strong signal from the fill material and the onshore attachment of the bypass bar. In Figs 16, for average shoreline changes from 1972-2002, there is a very large area of accretion from R2-R1 0. Is this the influence of the bypass bar or the influence of the late beach fills dominating the data. How do the data for average shoreline change and volume look when plotted for 1972-1999? Would this give a truer
signal of the long-term change?
Note: Fig 19 sand trap volumes has no caption.
The surprising result of this study is that, despite the shoreline recession on the IRC beach, there is an increase in beach volume both upd rift and downdrift of the inlet. In large part, this can be ascribed to the nourishment, the bypass bar and the dredging, but none-the-less there is a positive volume increase on both sides of the inlet from 1972-2002 as seen in Table 6. For Indian River County, there is about 20,000 cy/yr (.67*30,000 ft) for 1972-2002, while for 1999-2002 it is 85,000 cy/yr. (Again It is not entirely clear why there is such a shoreline recession signal in the beach profiles in IRO, but a deposition in the volume calculations.) So the crux of the report's major recommendation is that the change in volume of the beaches should be the same for each side of the inlet. In other words, if the BC beach is accreting, then the IRC beaches should accrete at the same rate. This does assume that the influence of the updrift jetty impounding sediment is not the major reason for the accumulation in the beaches.
An even-odd analysis (developed by the author) using simply the Table 6 Rate of Average Volume Change would indicate that there is (from 1972-2002) an even signal V e of 70,000 cy for each reach. (V...e+V o=120,000 cy; V _e Va o 20,000). This would in fact give a higher nourishment requirement than suggested by the author.
However, the counter argument could be agreed to: that there should be no erosion of the IRC beaches. This implies that, using a volume calculation, the current situation is ok; however the shorelines are still retreating. Therefore a more aggressive stance would be required to maintain the shoreline positions.
1. Continuation of Semi-annual monitoring makes good sense.. The data are useful for studies of this kind.
2. Recommendations 10.2 and 10.3 are management. goals, which are sensible.
3. The recommendation that the south jetty be extended makes sense, although the report doesn't spend much time making the case. Clearly the jetty is short and it is being bypassed by the sediment. Is the current jetty sand-tight? Clearly it is desirable to make sure that sand bypassing the inlet naturally on the bypass bar (and fill material) does not return to the inlet channel by northward transport. This would reduce the need for dredging.
4. The possibility of changing the type of bypassing at the inlet with the intent of getting more coarse sediment should be explored.
Appendix C. Sediment Interaction with Littoral Barriers and Inlets
This theoretical section gives some indication of idealized barriers and inlets on a beach. The standard theoretical approach shows that the effect of the inlet after 50 years should persist for 8 miles updrift and downdrift of the inlet. For a novel time-varying wave direction, the effect of the inlet still extends that far away from the inlet. It is not clear from the profile data (nor is the argument made) that this is truly the case at Sebastian Inlet. If it were so, then the beach profile measurement program should be extended to 40,000 feet from the inlet.
Has any attempt been made to calibrate the one-line model results with the data?
The time-varying wave climate is an interesting extension of the model.
Appendix D. Sediment Budget
While it is a reasonable assumption on a straight coastline, it is not certain that Q IN is equal to QOUT, even in the absence of the inlet. This is not true in the vicinity of Cape Canaveral.
The onshore sediment transport rates obtained by setting Q_IN=Q_OUT seem very high; they are comparable in magnitude to the longshore transport rate. It is unusual to expect this large onshore transport and more work to verify this would yield important results for this region.
In summary, the report shows that the I RC shorelines are receding but the volumes are increasing. The recommendation is to nourish the IRO shorelines to ensure that the shoreline volume change is the same as for the BC beaches. This approach is a reasonable one.
Professor Scott A. Douglass University of South Alabama
Review of report entitled
"Review of Selected East Coast Florida Inlets: Phase I.- Sebastian Inlet, FL:
Evaluation of Coastal Processes and Management Practices and Development of Recommended Modifications"
Robert G. Dean
Review by: Scott L. Douglass K.IOverview:
The report being reviewed presents analysis on the coastal processes and management practices at Sebastian Inlet, Florida and develops recommendations concerning the future management of sand at the inlet. The report focuses on the volumes of sand at the inlet and on the nearby beaches. The sediment budget analyses in this report use available survey data.
This review provides comments, section-by-section, on each portion of the report followed by specific discussion on four areas as requested:
(1) The method of adjusting beach profiles for closure,
(2) The methods used to develop the overall sediment budget, (3) The idealized inlet/beach concepts and interpretations, and
(4) The overall conclusions and recommendations.
The "Introduction" to the report states that the purpose is to develop an
understanding of the sand movement in the vicinity of Sebastian Inlet (SI) including the sand's interaction with the inlet and to develop a sediment budget for the inlet. An earlier report by the same title (dated January 20, 2003) is mentioned. This reviewer has not seen that earlier report but has no indication that this review would be any different if the earlier report were available.
The "Background" section of the report briefly outlines some of the relevant
history of SI. The inlet has been stabilized and open in its present location since 1948. From 1924 to 1942 the inlet was maintained slightly to the north. The history of modifications to the inlet; including jetty extensions, creation of a sand trap in the throat of the inlet, and dredging of the inlet; is outlined. Table 1 summarizes these activities/modifications in a compact timeline. The most recent jetty modification was done in 1970. Since then, sand has been moved. There is no indication of sand bypassing prior to 1978. In 1978 and 1985 there was some partial bypassing of sand
1 Douglass' review of Dean (2003)
dredged from the inlet to the southern beaches. Since 1989, there appears to be complete bypassing of all dredged sand to the southern beaches. The locations of the downdrift placement of this sand are not specified. Since 1996, there has been additional, i.e. beach nourishment, sand added to the southern beaches from inland quarry sources. The total amount of sand placed on the downdrift beaches since 1970 is over 1.3 million cubic yards (MCY). Of this total, 0.9 MCY was sand bypassed ("transferred" from inlet including channel and sand trap) and about 0.4 MCY was new nourishment sand. The new nourishment sand includes essentially annual placements since 1998. At least 50,000 cubic yards of nourishment sand have been placed on the south beaches in each of the past 5 years.
This "Background" section of the report is a focused presentation of the relevant information used later in the report as well as an adequate overall explanation of the relevant history. It is not, however, a complete literature review of all the scientific literature discussing SI. The inlet management plan for SI is cited and presumably that document presents a more complete literature review through 1988 (the date of publication of the inlet management plan). This reviewer is not familiar with all of the literature and data available for SI and so this review is based on the information presented in this report. (Note: This reviewer did provide a report on the monitoring program to the SITD in the mid- I990's but did not review that report while preparing this review).
The "Methodology and scope" section of the report outlines the data used. This is primarily elevation surveys of the inlet (throat, ebb shoal, flood shoal/back bay) and the adjacent beaches (for up to almost 6 miles in both directions). There is also an appendix of 42 historical air photos of SI. The survey coverage of the inlet and adjacent beaches is impressive in terms of the spatial coverage (extent and density of beach profiles) and temporal coverage (semi-annually since 1990 with several other surveys dating back to 1972).
The "Data screening and adjustments to individual profile data" section of the report essentially discusses the quality assurance procedures used. This includes corrections to the data for obvious errors, monument relocations and lack of closure. Each of these corrections seems appropriate and correct. Analysis without these corrections would be flawed. The closure correction is discussed ftuther below.
The "Analysis and presentation methodology" section of the report presents results of the profile analyses. Averaged (or "composite") profiles are developed. Shoreline changes and volumetric changes are calculated for a longer time fr-ame (since 1972) and a shorter time frame (since 1999), respectively. These results are presented by profile line location and also averaged for the beaches to the north and south of the inlet.
The "Analysis results" section of the report presents and discusses some the
detailed results of the analyses on the changes in shorelines and volumes in the different portions of the system. There are some interesting, and somewhat curious to this outside reviewer, results concerning the details of the volume changes. For example, Figure 21
2 Douglass' review of Dean (2003)
indicates that the amount of sand stored in the back-bay, flood tidal shoals has decreased somewhat since 1989. Since there is no explanation given, it raises questions about whether there are survey coverage problems (is it growing beyond the limits of the 1989 survey where it is not being measured?) or whether the flood shoal had reached equilibrium prior to 1989 (is no sand from either side of the trap moving across the trap, or is sand driven by wave action in Indian River Lagoon toward the pass and then pulled back out to sea on ebb tides?). Similarly, the volume of sand in the sand trap has apparently decreased since 1989. It may have been full prior to that survey and was cleaned out shortly thereafter (from 1990 to 2002, the volume of sand in the sand trap increased). The ebb-tidal shoal volume shows a trend of increasing volume in the 19892002 time fr-ame. The regression line indicates an increase even though "end-point analysis" shows a slight decrease. These are fairly obvious questions that have probably been asked and answered by the author and others more familiar with the inlet and the data. However, since these values are part of the sediment budgets in this report, a related question (addressed below) concerns their impact on those budgets and conclusions.
The "Theoretical and numerical shoreline changes in the vicinity of a complete or partial littoral barrier" portion of the report summarizes the interpretations of models described in Appendix C. The effect of a littoral barrier on longshore sand transport is investigated theoretically. One result is that bypassing is never complete around a littoral barrier. The updrift shorelines continue to accrete for very long time periods with corresponding recession of downdrifi shorelines. The deleterious effects on downdrift shorelines are increased when the effects of sand storage in inlet shoals is considered.
The "Summary of results" section of the report summarizes the shoreline change and volumetric change results. The "Sediment budget considerations" section of the report presents separate sediment budgets for SI for both the longer (since 1972) and shorter (since 1999) time periods. The "Recommendations" portion of the report outlines five recommendations for the future management of the sand around SI.
Method of adjusting beach profiles for closure:
The survey data is adjusted for lack of closure in the offshore that is probably due to errors in assumptions about the water level at the time of the bathymetric survey. Such errors are common in bathymetric surveys. The method of adjustment used here is to compute the cumulative area difference between any survey and a selected "reference" survey starting from about 20 foot depths and moving seaward. The slope of that area value versus the distance offshore is an estimate of any possible vertical datum error. Elevation values below 5 foot deep were then adjusted for each profile.
The method for adjusting for lack of closure is appropriate and needed for substantive volume calculations. Basically, the method assumes that there were no significant changes in bottom elevation from about 20 foot depths to 40 foot depths. Thus, some physical logic is used to back out a reasonable correction for the assumed
3 Douglass' review of Dean (2003)
water surface elevation errors. While it is possible that the there could be real depth changes out this deep, it is more likely that the water surface elevation assumption was incorrect. This is particularly true for the examples shown that are so linear in the plot of area difference versus distance offshore (Figures 9 and 10). Other measurement techniques (e.g. repetitive measurements on vertical, embedded rods by divers) would be needed to address the unlikely possibility of significant fluctuations in elevation across the entire offshore portion of the profiles. This reviewer did wonder about the effect of the adjustment on the shape of the profiles near the 5 foot depth (i.e. was there reasonable overlap between the wading and offshore profile portions?).
Overall sediment budget methods:
The overall sediment budget methods are appropriate and enlightening. The survey data results are summarized into an overall sediment budget for the two time periods (1972- and 1999-2002). While that is not very enlightening due to the number of unmeasured inputs in any sediment budget (longshore sand transport and onshore sand transport from the shelf), some of the results shown in Appendix D are very interesting. The estimate of the amount of sand that would have been stored on the south beaches without any inlet is about an additional 70,000 cy/yr. This, compared with the actual nourishment placed on those beaches, about 30,000 cy/yr, leads to the conclusion that there has been an average annual deficit of about 40,000 cy/yr reaching those beaches due to SI.
Also, there were a few questions raised above in this review about the volumes of sand measured in the vicinity of SI (ebb-shoal, sand trap, and back bay). In particular, the question about the overall impact of these results on the sediment budget can be answered here. The changes in the volumes of sand in these three areas essentially cancel each other out in the sediment budget (see Table D.l1: 3 0-year time fr-ame). The monitoring data indicate that these volume changes are small compared with the volume changes on the adjacent beaches for the longer time frame but not necessarily for the shorter time frame. This makes physical sense in that these inlet features can store fluctuating amounts of sand from year to year. Their influence over the multi-decadal scales is probably less than the yearly scales because of the age of the inlet (and the lack of significant engineering modifications in the monitored time frame).
Idealized inlet/beach concepts and interpretations:
Appendix C presents some idealized inlet/beach concepts with direct application to SI. The concepts and theory appear to be similar to those proposed by the author over a number of years and in a variety of cases. The application to inlets shown here is new to this reviewer however. The principles employed are appropriate and the results lead to very interesting interpretations that make sense for SI. One interpretation is that the effects of any initial barrier on longshore sediment transport spread farther out and get less dramatic with time. Another is that natural bypassing around a littoral barrier
4 Douglass' review of Dean (2003)
essentially never reaches the ambient transport rate. The impacts on updrift and downdrift shorelines are also investigated numerically. This theoretical investigation provides results which match the results of the sediment budget qualitatively fairly well. Thus, it provides further explanation of the impact of SI on the beaches.
Overall conclusions and recommendations:
This reviewer concurs with the overall conclusions in this report. The north shorelines have accreted at the expense of the south shorelines. These effects are continuing through the time frame of these monitoring data and the theoretical analysis indicates why this would be. These effects extend for significant distances up and down the coast. The deficit, caused by the inlet, of sand getting to the 6 miles of south beaches is about 40,000 cubic yards per year.
Although, considering the age of this inlet, the overall conclusions of this report may be surprising and perhaps discouraging, several facts should be noted. One, the inlet is fairly young in geologic time scales and it is well understood that inlets have tremendous influences on adjacent beaches over decades and centuries. Two, the creation of the inlet and its maintenance caused trapping of beach sands in new floodand ebb-tidal shoals, the updrifl shorelines, and in probable upland disposals of dredged sands. The volume of these trappings clearly exceeds the volume of new nourishment sands brought to the area to date. The sediment budget and theoretical models presented in this report appear to describe and quantify the overall, ongoing impacts of SI on the adjacent beaches well.
This reviewer agrees in general with each of the five recommendations. They seem to be measured, appropriate recommendations in line with the findings of this report. Clearly the author's recommendations are based on more knowledge of the existing policy framework and possibilities than this outside reviewer can provide. However, the overall goal outlined in the second recommendation is appropriate for SI and many other engineered inlets:
"The goal should be to stabilize the downdraft beach systems equitably
through placement of adequate volumes of good quality sediment. "
Such a goal is consistent with modem engineering principles of sustainable development.
5 Douglass' review of Dean (2003)
Professor David L. Kriebel
U. S. Naval Academy
David L. Kriebel, PhD, PE
Professor of Ocean Engineering
United States Naval Academy
This review is based on an evaluation of the report Review of Selected East Coast Florida Inlets Phase 1: Sebastian Inlet, FL by Robert Dean, with regard to its stated goals as listed on Page 1:
" To develop an understanding of the sediment flows near Sebastian Inlet,
" To develop a sediment budget for Sebastian Inlet, and
" To make recommendations for sand management at Sebastian Inlet.
This review is based on the content of the project report and is not based on a review of other published documents or independent data concerning Sebastian Inlet.
In general, the report presents a thorough analysis of shoreline and bathymetric survey data for Sebastian Inlet and 30,000 ft of shoreline north and south of the inlet. The volume of data considered in the report is quite large and the report does a good job of utilizing, and making sense of, the vast amount of survey data available. Clearly, analysis of the large data set was difficult and required great care. The report provides a good description of the process used to check data and either cull poor-quality data or correct the data for suspected survey effors. Analysis of such data can be approached in several ways, and one notable feature of the report is the way in which a given set of data was analyzed in multiple ways. For example, most erosion or accretion rates are computed by both end-point methods and by least-squares methods. The analysis of beach profiles is also done in at least three ways: by considering all individual profiles, by averaging many profiles alongshore, and by using GIS software. Based on the thoroughness of analysis, it therefore appears that the author has made a goodfaith effort be as careful as possible in the data analysis and to glean as much information as possible from the available data.
Long-Term versus Short-Term Changes
In my opinion, inlet management should be based on data collected over as long a time period as possible to average out short term (one or two year) variability in shoreline positions or sand volumes. As a result, I would consider changes from 1972-2002 to be more representative of long-term inlet impacts than those based on 1999-2002. In this regard:
The author never really addresses which of the two time periods he considers more "representative" and both are given essentially equal weight. Perhaps some discussion
of this issue should be added.
It is not clear why volume changes in Figure 12 do not extend back to 1986 and 1972.
1 realize that profile spacing was greater then (3,000 ft) compared to more recent data.
But it would seem that a sufficient number of profiles could be found north and south
of the inlet (up to about 10 on each side) to permit at a reasonable estimate of volume
change in 1986 and 1972.
Similarly, the determination of volume changes based on averaged profiles (bottom p.
28) should be done using the 1986 and 1972 data sets. As noted above, there should be about 10 profiles that could be used to determine of the average profile form in 1972 and 1986, and the subsequent long-term changes in the average profile up to
Did the author's "scope of work" restrict analysis to data collected by the Sebastian Inlet Tax District and FDEP or could he have also considered older historic shoreline data such as the NOS "T-sheets"? If so, it may indicate longer-term preinlet trends (to be discussed in more detail below) that could help in the interpretation
of post-inlet beach response.
Extent of Inlet Impacts
The beach profile data used in the analysis are generally those that cover the greatest distance north and south of the inlet, i.e. 30,000 ft either side of the inlet. Some data collected over shorter distances from the inlet were discarded, presumably because the author believed that inlet impacts extended farter from the inlet and were not fully documented in some surveys.
The author should include some discussion or analysis about the longitudinal extent of inlet impacts. Some data in the report suggest inlet impacts may extend father than 30,000 ft south of the inlet and the numerical model results also suggest inlet impacts may extend farther. One of the study recommendations suggests increasing the future survey area to 40,000 ft either side of the inlet. This recommendation should be more fully or more strongly justified based on any evidence that inlet impacts extend
farther than 40,000 ft downdrift.
The report contains numerous aerial photographs of Sebastian Inlet. Despite the excellent photographs, the report contains little discussion of what is observed in the photographs or what can be concluded from them.
The photographs seem to show rock reef outcrops south of the inlet. These are not discussed in the report but must exert some influence over processes. The presence of these reefs raises some questions: How far south do they extend? Are they present north of the inlet too? To what extent do the reefs "armor" the downdrift shoreline
and perhaps shift inlet impacts farther south?
The photographs also seem to show a substantial pathway for sand to enter the inlet around the short south jetty. One of the recommendations is to lengthen the south jetty, to stop sediment losses into the inlet from the south. Some discussion of the sediment transport processes around the south jetty would strengthen this
Correction for Survey Error
The report makes much of the need to correct beach profile surveys for various survey errors and closure problems. While it is troubling that many beach profiles had to be discarded, it is certainly better to discard some beach profiles than to include erroneous data in the analysis.
Regarding the report's treatment of closure problems, there is always debate about whether such closure problems are an artifact of the surveying method or whether they are real and part of some physical process that is not yet fully understood. The report assumes a priori that the closure problems are due to survey error and then applies an innovative procedure to correct the profiles to achieve closure. Several comments may be made about this approach:
It would be appropriate on page 3 to discuss the survey methods and why there is reason to assume that surveys were biased. Were surveys were done by fathometer or sled? Were elevations were related to a tide gage reading (as implied on page 8), by differential GPS, or by direct reference to a local benchmark independent from a tide gage. Is there reason, based on the survey methods, to believe that surveys were in error, or is the error simply inferred from the lack of closure? [I note that some other reports on Sebastian Inlet prepared by other authors for the SITD also show the need
for large vertical "corrections" to survey data]
The method used to correct the profiles seems sound as long as the lack of closure is in fact due to a uniform vertical shift in the survey datum. If the survey errors are due to an error in orientation or bearing, then the cumulative error curve would not be linear but would have some curvature. In fact, there is some curvature in the curves shown in Figures 9 and 10 suggesting that some of the closure error may be due to misalignment of the profiles (as found in Figure 4) However, the degree of curvature is slight and the method used by the author may still offer a reasonable (though not
exact) means of correcting profiles for survey error.
One question that should be raised about the correction procedure relates to the effect of the corrections near the limit of wading depth (approx 5 ft depth) where the onshore and offshore surveys are joined. I would have expected that the wading and boat surveys would have included some overlap in this region. Once the bias errors were removed, how well did the wading and offshore survey data agree? If the offshore surveys were biased by a datum error, then I would have expected poor agreement with the wading surveys initially and improved agreement once the
correction had been applied. Is this what was found?
The bias errors found in Figure 9 seem to range from about 0.2 to 0.6 feet, with one being much larger. Later, in Table 5, it is shown that a much smaller I inch (0.08 ft) bias can have a very large effect on volume computations. In fact, the bias errors can be as large as the actual volumetric changes being measured in effect, the "noise"
due to the bias error can be as large as the "signal" caused by the inlet. Given the 0.2 to 0.6 :ft bias found in many offshore profiles, do you have reason to suspect similar bias errors in the back bay, ebb shoal, and sand trap? If so, how large are the suspected bias errors compared to the volumes measured? What impact would these
have on the sediment budget and on the recommendations of the report?
What is the basis for selecting a depth of 20 feet as the closure depth? The report shows some data, e.g. Figure 3, which support this choice but other data, e.g. Figure 5, which show beach changes to depths of 30 feet or more. This raises the question: to what extent would results of the report change if a different depth of closure were
The report concludes that there is significant onshore transport of sand originating from outside the 20 ft depth of closure. Given that this assumed onshore transport must cross the assumed closure depth, this raises the question: Why do profiles have to "close"? If you accept the large influx of sand from offshore, then isn't it possible that some of the observed lack of closure is a real phenomenon resulting from the
movement of sediment from depths exceeding 20 feet?
Analytical Shoreline Model
The analytical model discussed on page 34 and in Appendix C is useful for showing idealized shoreline changes expected up and down drift of an inlet. However, the results seem to be of little practical value in this report when the rest of the report is based solely on analysis of measured beach and bathymetric survey data. No conclusions or recommendations in the report are based on the analytical model results, and results in Appendix C are not really integrated into the rest of the report. For example:
The analytical model results are based on a net longshore transport rate of 260,000 cy/yr. This far exceeds the values given in the sediment budget of 137,300 to 164,350 cy/yr. As a result, it would be best to revise the analytical model results to, say,
150,000 cy/yr to agree more closely with values shown in the sediment budget.
Similarly, the analytical model shows (Fig C4) that bypassing rates after 50 years should be about 60% of the ambient longshore transport rate. Referring to the sediment budget, for longshore transport rates of 137,300 to 164,350 cy/yr, this would suggest bypassing rates of 95,000 to 113,000 cy/yr. This differs significantly from the
bypassing rate of 50,000 cy/yr assumed in the sediment budget.
Shoreline changes suggested by the analytical model do not explain any of the more complicated shoreline changes observed in the data in Fig 15 and 16. For example, the analytical model suggests that on the updrift side, the shoreline should advance by about 400 ft in 30 yrs, yet the data in Fig 16 show an accretion rate of about 2.5 ft'yr or about 75 ft in 30 years. On the downdrift side, the analytical model predicts shoreline erosion of about 400 ft in 30 years at the inlet, while the data in Fig 16 show a shoreline advance of about 1.8 ft/yr or 54 ft in 30 years (presumably due to nourishment or due to armoring by reefs). Fig 16 also shows a complicated downdrift pattern of accretion near the inlet and erosion farther away, whereas the model shows erosion that diminishes south of the inlet. Why are model results so poor compared to the measured shoreline change? This seems to require some explanation and
Sediment budgets constructed for two time periods, 1972-2002 and 1999-2002. While these sediment budgets were constructed carefully based on available data, they are not unique or definitive as is stated in the report. For example, the report clearly shows much variability and uncertainty in the data. Three to four different volume estimates are shown in Table 6, with some values differing by at least a factor of two. Bias errors were found to exist in the beach profile data, and Table 5 shows the potential for additional bias errors in other volume calculations. Given these uncertainties, one must expect similar uncertainty and variability in the sediment budget.
In addition, as discussed in Appendix D, completion of the sediment budgets requires numerous assumptions. While some values are based on survey measurements (volumes in sand trap, ebb shoal, etc), others cannot be measured directly and must be assumed. These include: the amount of onshore sediment transport, Qos, the amount of natural sand bypassing, QBp, as well the transport rates entering and leaving the north and south boundaries respectively, QIN and QOUT. Changing the assumed values, or changing the relationships among these four unknowns, will leads to different sediment budgets and perhaps different conclusions regarding future nourishment needs.
Given these difficulties, the "non-uniqueness" of the sediment budget should be addressed more thoroughly. In particular, a sensitivity analysis should be included to illustrate the sensitivity of the sediment budget (and of proposed additional nourishment requirements) to some of the assumptions and to the uncertainties in the data. For example:
The sediment budget is based on an average of volume changes from Table 6, which take the mean of end-point and least-squares results and of individual versus averaged profiles. How sensitive are the final conclusions to the range of values shown in
Conclusions are based on an assumed bypassing rate of 50,000 cy/yr. What evidence is there of this much bypassing? How would conclusions change if bypassing were actually much smaller, or larger? What effect would this have on sediment budget and
conclusions regarding the nourishment deficit?
Conclusions are based on the assumptions of substantial onshore transport and that.
QIN equals QouT. As shown in Appendix D, the assumption that QIN = Qour necessitates large rates of onshore sediment transport. One the other hand, one can assume no onshore transport and find that QIN must be much larger than QOUT, which may in fact be reasonable over the 11I to 12 mile region being considered. One could also imagine a case where there is some amount of onshore transport and also some difference between QIN and QOUT. The key question then becomes: would conclusions
and recommendations of the report change if these assumptions were changed?
In a recent study of the downdrift effects of Canaveral Harbor, Prof Rich Weggel completed several sediment budgets that encompassed the shoreline from Cape Canaveral to Sebastian Inlet. One of his findings is of interest here: he found that to balance the sediment budget for the region between Canaveral Harbor and Sebastian
Inlet required the assumption of offshore directed transport (sediment losses from the beach to the offshore). This is opposite of the findings in the present study of Sebastian Inlet. I am not sure what to make of these radically different conclusions, except they point to the difficulties and uncertainties that exist when trying to track relatively small sand volume changes over fairly long lengths of shoreline. Results are sensitive to any "noise"~ or uncertainty in the data, and to the assumptions that are made to "close" the sediment budget. This reinforces the need in the present study to include a discussion of the "sensitivity" of the sediment budget to changed input or
Comparison of Pre- and Post-Inlet Trends
One of the ways in which the Corps of Engineers estimates inlet impacts is to compare postinlet conditions to pre-inlet conditions. The final recommendations for additional nourishment of 41,800 to 42,950 cy/yr are, in effect, based on this type of analysis as shown in Appendix D. For example, given the assumption that QIN = QouT, consideration of the preinlet condition suggests that there was onshore transport resulting in beach accretion of 61,800 cy/yr (based on 1972-2002 data) south of the inlet. Consideration of the post-inlet volume of sediment stored from on the beaches from 1972-2002 of 20,000 cy/yr suggests a deficit of 41,800 cy/yr. The conclusion that can be drawn is that the region south of the inlet was accreting prior to inlet construction, and that from 1972-2002 it has also been accreting but at a reduced rate.
Is it possible to confirm this pre-inlet accretion trend independently through use of historic shoreline maps? As a quick estimate, onshore transport rates over the 30,000 ft length of shoreline are between 61,800 cy/yr (1972-2002) and 126,900 cy/yr (199920 02). Assuming QIN =QOUT, a berm elevation of about 7 ft, and a depth of closure of about 20 ft, these rates of onshore transport should have produced pre-inlet shoreline accretion rates of around 2 to 4 feet per year. These are rather high accretion rates and should be evident in pre-inlet shoreline change trends despite the questionable
accuracy of the data from that period.
Further evidence of a background accretion trend appears to show up in Figures 17 and 18, on page 30, and in Table 6 which all show that the beaches south of the inlet have gained sand volume at a rate that exceeds the nourishment rate. [At the same time, the shoreline from 1972-2002 eroded slightly on average]. The report should discuss this result in more detail because it has a strong bearing on the recommendation for additional beach nourishment. One individual may see that beaches south of the inlet have been accreting and conclude that additional sand is not needed that there is no apparent erosion to offset. Another individual may see that beaches are accreting at a rate less than if the inlet had not been constructed that there is "foregone" or "unrealized" accretion that has never occurred due to the inlet.
The report should address these issues more forcefully because it is not clear what the goal should be for future inlet management: (1) to prevent further erosion or (2) to
fully restore pre-inlet accretion (this is addressed more below).
The most significant recommendation on page 36 is that there has been an annual deficit of 41,900 cy/yr (based on 1972-2002 data) or 42,950 cy/yr (based on 1999-2002 data) and that future nourishment and bypassing should be increased by this amount.
As noted in the report (and in some of the above comments), the beaches south of the inlet have been gaining sand volume throughout the measurement period. Post-inlet shoreline changes in Table 6 show that the shoreline south of the inlet eroded at an average rate of about -0.5 ft/yr from 1972-2002 yet gained about 20,000 cy/yr of volume due in part to the combined nourishment and bypassing of 44,700 cy/ft. Given the very slight shoreline erosion rate coupled with a volumetric gain, it appears that nourishment and bypassing have maintained the beaches south of the inlet in a relatively stable, nearly neutral, state (not strongly accreting or eroding) for the past 30 years. From 1999-2002, both the shoreline and beach volume have increased, with volumetric accretion rates exceeding the rate of nourishment.
The recommendation to provide an additional 41,900 cy/yr of nourishment (or bypassing) would, when added to the 20,000 cy/yr of beach volume gain south of the inlet, restore the pre-inlet volumetric gain of 61,800 cy/yr due to onshore transport. As a result, beaches south of the inlet should accrete. If spread evenly over the 30,000 ft of shoreline, the added nourishment should cause the shoreline to advance at a rate of about 1.4 ft/yr, more than offsetting the -0.5 ft/yr of historic erosion from 1972-2002 and leading to net beach accretion.
Given these results, the recommendation for an additional 42,000 cy/yr of nourishment will no doubt be opposed by some as unnecessary. In my opinion, then, one of the major questions to be answered by the various parties is a philosophical one:
What is the goal of future beach and inlet management at Sebastian Inlet:
(a) to restore the pre-inlet accretion, or
(b) to simply prevent any future erosion and keep the beach in a long-term neutral
Which of these two scenarios is most appropriate for future beach management is a subject of discussion among the interested parties or perhaps a subject that should be addressed in FDEP state-wide regulations. Keeping the beach in a stable or neutral state, to prevent further erosion but not to cause beach accretion, should be the minimum goal of future beach management. To fully achieve that would require sand to offset the documented -0.5 ft/yr long term shoreline erosion rate between 1972 and 2002. This would require additional nourishment (or bypassing) of about 15,000 cy/yr above the existing volumes. Fully restoring the pre-inlet accretion conditions, balancing sand volumes north and south of the inlet, and reverting the system back to its "natural" state would then represent the maximum goal for future beach management. As noted in the report, this would require additional sand bypassing of about 42,000 cy/yr. Perhaps the report should address these minimum and maximum options and lay out a stronger case for why one is recommended over the other.
Comments on Final Recommendations
There are five recommendations specifically outlined in the last section of the report (p~p 3638), along with the sixth recommendation on p. 36 to bypass or nourish with additional sand. Specific comments on these recommendations are as follows:
Rec 10.1 Continuation of Semi-Annual Monitoring. The need for continued monitoring is well established by the variability in the data displayed in the report, and by the continued controversies over the effects of Sebastian Inlet. The recommendation to expand the monitoring to 40,000 ft north and south of the inlet may also be justified. The data in the report, the numerical model results, and evidence from other inlets on the US East Coast suggest that downdrift impacts may exceed the 30,000 ft distance now surveyed.
Rec 10. 2 Develop an Agreed- Upon Basis for Analyzing and Responding to Results and Rec 10. 3 Analyze Data in a Bipartisan Effort. I agree with Prof Dean's statements that data could be "analyzed, interpreted and acted upon in various manners." It would be useful to establish some guidelines that could be followed by all when analyzing the same data. For example, I noticed that some reports prepared for the SITD also applied corrections in the vertical datum of various bathymetric surveys similar to, but not identical to, those used in the present report. It would be useful for all parties to establish common guidelines for these corrections.
Rec 10.4 Consider Extending the South Jetty. This recommendation does not appear to be discussed elsewhere in the report and is not based on quantitative data. The report should discuss this in more detail. For example, the differences between net and gross longshore transport, and data showing reversals in transport direction, should be discussed. Also, the report should discuss the aerial photographs which appear to show sand flowing south-tonorth around the south jetty.
Rec 10.5 Initiate Discussions with State Division of Parks and Recreation. The state is fortunate to own land immediately north of the inlet and this provides opportunities to access sand that has been trapped by the north jetty. The report suggests use of permanent or semipermanent fixed sand bypassing. I would also note that recent experience at Canaveral Inlet has shown that sand bypassing can be achieved by mobile dredge and this may be an option that should also be considered.
Recommendation to Bypass More Sand. The recommendation to bypass 42,000 cy/yr of sand and to balance the shoreline and volume changes north and south of the inlet should also be openly discussed by the various stakeholders. As I discussed above, the data in the report appear to show that beaches south of the inlet were accreting prior to inlet construction. These beaches have been largely stabilized by past mitigation efforts, yet they have also not accreted to the extent one might have expected due to the blockage of the inlet. From a scientific standpoint, it is certainly desirable to return the beach system to its natural, preinlet condition and the report shows that this requires additional bypassing. From a public policy and economic standpoint, however, this may lead to beach accretion downdrift of the inlet which is not really necessary to achieve sound coastal management and protection of existing infrastructure (homes, building, roads, utilities, etc). As a result, the ultimate decision about shoreline management should be based on broad discussion among stakeholders and perhaps on development of new state guidelines.
David L. Kriebel, PhD), PB Professor of Ocean Engineering United States Naval Academy
Page ii 1ILPr I" Line Word "results" used twice, delete first one
Page ii 1 "LPara Last sentence I find it confusing to discuss results from an earlier draft of report. This seems irrelevant once this report is revised and corrected. Suggest deleting sentence and all references to findings in an earlier draft.
Page ii 3"' Para 3"' Line Wording "shorelines increased" does not make sense. Do you mean shorelines advanced more north of inlet than south? or shoreline change rates were larger north of inlet? Pagel 1 1"LPara 2" Line Add comma or semicolon after SI to read: "...interaction
with SI, and to..."
Page 1 3mO Para 2M" Line Add comma after word "operation" to read:
"and operation, and jetty..."
Page 1 3rd Para 6th Line The wording "Landward (east)..." is not clear as landward
should be to the west. Do you mean "Landward (west) ... "? Page 1 3r' Para Last line Merge prilsntence with partial sentence top of page 3 Page 2 Figure 1 should be re-drawn, or perhaps replaced by a
_______________________good map or an aerial photo.
Page 3 1"S Para Top 3 lines Sentence states that three recommendations are to follow, but only one is included after colon. Sentence beginning "The sand..." does not make sense, and remaining two recommendations appear to be stated in 3rd line. Need to re-write this section.
Page 3 4th Para 4Lfl Line Add -comma after word "shorelines" to read:
".adjacent shorelines, and aerial..." Page 3 3' Para This paragraph, discussing Appendix C, seems out of
place. All other text on page discusses a chronology of events related to construction of inlet or historic data so discussion of analytical model results is out of place.* Page 5 Table 1 Several entries do not state what happened to dredged sand
or where placed sand came from. Specifically: (1) where was 420,000 cy of sand disposed in 1972, on the beaches or in some offshore or upland disposal site? (2) where did the 200,000 cy of sand come from in 1996/1997, from upland sites or dredged from inlet system? Page 7 Table 2 What do percentages mean in columns for ebb shoal, sand
trap, and back bay?
Page 9 Table 4 What do numbers, such as 13226 listed for Figure A.3,
mean are they relevant?
Page 13 and others with bach Beach profile graphs are very small and it is hard to see profiles differences in profiles. Scale used from +40' to -60' or -80'
compresses profiles too much. A range from +20' to -40' would be better.
Page 15 Top para Text is missing from top paragraph before word "errors"
Page 15 Top Line Word "If' should be "In" and text should read "In the case
Page 20 Last Para Last 4 Incomplete sentence followed by repeated text. It appears Lines that all text after the word "approximately" should be
deleted and this paragraph merged with text top of page 26 Page 26 2"' Para Last Sentence I am not sure what this sentence means when it says "calculated volumes are proportioned..." Does this relate to the percentages listed in Table 2 that I asked about above? If so, need to define.
Page 27 Figure Figure number and caption missing
Page 28 2 and 3'" Para Sentences starting with word "Overall..." should probably
include the words "end-point" 'to define shoreline change rates. This will distinguish these rates from those found using least-squares methods.
Page 28 51' Para 3" Line Units of Yd3 /ft/year should be yd 3/ft Page 31 V para last 2 The statement starting with "The results demonstrate that
sentences the deeper the trap...." is not supported. No data has been
shown in the report correlating depth of trap to sand volume in each dredging.
Page 35 Table 6 I found use of yd3/ft/yr to be confusing in this table. Most
volume measures in the report (especially those used in the sediment budget) are in yd3/yr. Switching to a "per foot" basis makes it tough to follow earlier discussions of volumes over entire 30,000 ft distance and makes it tough to relate Table 6 to the sediment budget in App D. Page 37 Figure 24 Figure quality poor, should be replaced.
Page 37 Figure 24 It would be clearer to make two sediment budget graphs in
And Page D-9 Figure D.2 two separate figures: (1) one for 1999-2002 and (2) one for
Page A-4, A-20, A-24, A-3 8 Text blurred (maybe just a photocopy error in my copy).
Also, font size keeps changing in each figure would be nice to standardize.
Page C-3 2n6 Para 1st Line Figure C.2 not Figure 2. Page C-i10 Is some text missing? There is no text discussing Figures
C-il1 and C-12.
Page 37 Figure 24 and Numbers for Back Bay are reversed... .should be +37,700
Page D-9 Figure D.2 for 1999-2002 (upper number) and -11,100 for 1972-2002
(lower number). I have checked sediment budget computations and these changes do not effect computed sediment budget. They appear to have been used correctly in computations, just listed incorrectly on the figure.