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
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
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
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 downdrift
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(x,t) given by the solution of the equation:
oy/at = G 2 y/ a x2
Integrating this equation with resp ct to x from 0 to L gives
f ay/at dx = G o/ax IL G ay/x Jo
Introducing the equation for sedim 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 longshore transport rate and the incoming
longshore transport rate.
(h* + B) I dy/at d< = Q(L)- Q(0)
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(0))/(h*+B), where the author 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(0) =11, 350 cy/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 R10 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-R10. 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 updrift 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 IRC, 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
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 V_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
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
QIN 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 IRC shorelines are receding but the
volumes are increasing. The recommendation is to nourish the IRC 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
Robert G. Dean
Review by: Scott L. Douglass l ai-o.-
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-1990's but did not review that report while preparing this
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
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 further 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 frame (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 1989-
2002 time frame. 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
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 downdrift 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 longshoree 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.1: 30-year time frame). 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 flood-
and ebb-tidal shoals, the updrift 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 downdrift beach systems equitably
through placement of adequate volumes ofgood 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 errors.
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 good-
faith 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.
I 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 pre-
inlet 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 farther 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 1 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 Qour. 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 Qour, which
may in fact be reasonable over the 11 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 Qour. 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 post-
inlet 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 QI = Qour, consideration of the pre-
inlet 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 (1999-
2002). Assuming QIN = Qou, 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
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
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 (pp 36-
38), 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-to-
north 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 semi-
permanent 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, pre-
inlet 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, PE
Professor of Ocean Engineering
United States Naval Academy
Page ii 1" Para 1 Line Word "results" used twice, delete first one
Page ii 1 Para 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'r 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?
Page 1 1" Para 2" Line Add comma or semicolon after SI to read: "...interaction
with SI, and to..."
Page 1 3md Para 2"' 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 3rd Para Last line Merge partial sentence 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" 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 4h" 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 beach 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 2nd 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 2nd 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 5"' Para 3'" Line Units of yd /ft/year should be yd /ft
Page 31 2n 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-38 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 2nd Para 1" Line Figure C.2 not Figure 2.
Page C-10 Is some text missing? There is no text discussing Figures
C-l1 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.