TIDAL INLET MANAGEMENT AT JUPITER INLET: FIFTH PROGRESS REPORT
A. J. Mehta M. J. DelCharco and
E. J. Hayter
Jupiter Inlet District 400 North Delaware Boulevard Jupiter, FL 33458
REPORT DOCUMENTATION PAGE
1. Report so. 2. 3. Recipient's secession No.
4. Title sod Subtitle 5. Report Dte
TIDAL INLET MANAGEMENT AT JUPITER INLET: October 1991
FIFTH PROGRESS REPORT 6.
7. Author($) 8. Performing Organization Report No.
A.J. Mehta, M.J. DelCharco and E.J. Hayter UFL/COEL-91/012
9. Performing Organizatio Ne and Address 10. Project/Tak/vork Unit no.
Coastal and Oceanographic Engineering Dept. University of Florida
336 Weil Hall 11. contract or Grant No.
Gainesville, FL 32611 C89-002
13. Type of Report
12. Sponsoring Organization me and Address
Jupiter Inlet District Commission Fifth Progress Report
400 North Delaware Boulevard
Jupiter, FL 33458
15. Supplementary Notes
This progress report on the Jupiter Inlet Management study includes results based on physical and mathematical model studies, and tentative conclusions therefrom germane to possible improvements or modifications to the present system. These include the effects of jetty modifications on navigational safety/access, beach erosion and sand influx into the inlet; sedimentation in the proposed offshore channels; design of the JID trap; and construction of a possible new trap near (east of) the Florida East Coast Railroad
17. OrISiaator's Key words 18. Availability Statement
Hydrodynamics Jupiter Inlet
19. U. S. Security Classif. of the Report 20. U. S. Security Classif. of ThIs Page 21. No. of Pages 22. Price
Unclassified Unclassified 77
TIDAL INLET MANAGEMENT AT JUPITER INLET:
FIFTH PROGRESS REPORT
A. J. Mehta
M. J. DelCharco and
E. J. Hayter
Jupiter Inlet District
400 North Delaware Boulevard
Jupiter, FL 33458
TABLE OF CONTENTS
LIST OF FIGURES...........................3
LIST OF TABLES.................................5
1.2 Jetty Design...........................6
1.3 Navigation Channel....................8
1.4 Sand Traps............................8
II. PHYSICAL MODEL TEST RESULTS....................9
2.3 Test Criteria.........................9
2.4 Test Conditions..........................14
2.4.1 Jetty Modifications................14
2.4.2 Elevated Jetty Modifications..........14
2.4.3 Dredged Ebb Shoal..................16
2.5 Test Results......................................16
2.5.1 Jetty Modifications..................16
2.5.2 Example Calculation................22
2.5.3 Ebb Shoal Dredging..................25
2.6 Evaluation of Test Results................27
2.6.1 Jetty Length Modifications...........27
2.6.2 Elevated Jetties...................29
2.6.3 Dredged Ebb Shoal................31
III. EFFECTS OF POTENTIAL MODIFICATIONS ON FLOW AND SAND
TRANSPORT IN THE LOXAHATCHEE RIVER................54
3.2 Salinity Distribution.....................55
3.3 Model Application......................56
3.4 Model Results........................59
3.4.1 Modification 1...................60
3.4.2 Modification2.................. 61
3.4.5 Modification 5...................62
IV. RELATIVE INFILLING RATES IN OFFSHORE CHANNELS.......72
4.1 Introduction .. .. .. .. .. .. .. .. .. ..72
4.2 Basic Model.......................72
4.3 Beach Profile.......................72
4.4 Coefficient Cf.......................73
4.5 Filling Time .. .. .. .. ... .o...o.....74
V. REFERENCES .. ....... ... ... .. ........76
LIST OF FIGURES
2.1 Results from numerical simulation of shoreline
evolution with a jetty length of 220 feet .....
2.2 Jetty length vs. erosion distance with corresponding
scale values (0, -1, -2, -3).... .. .. .. .....
2.3 Inlet flow patterns for normal condition and
modification M: flood a
2.4 Jetty modification A
2.5 Jetty modification B. 2.6 Jetty modification C. 2.7 Jetty modification D. 2.8 Jetty modification E. 2.9 Jetty modification F. 2.10 Jetty modification G. 2.11 Jetty modification H. 2.12 Jetty modification I. 2.13 Jetty modification J. 2.14 Jetty modification K. 2.15 Jetty modification L. 2.16 Jetty modification MN 2.17 Jetty modification N. 2.18'Jetty modification 0.
nd ebb tides.............34
. . . . 35
. . . . 36
. . . . 37
. . . . 38
. . . . 39
. . . . 40
. . . . 41
. . . . 42
. o . . 43
. . . . 44
. . . . 45
. o . . 46
. . . . 47
. . . . 48
. o . . . 49
2.19 North jetty and south jetty elevation views and height
2.20 Physical model data collection points and dredged ebb
shoal area...................... ..
3.1 Proposed sand trap east of Florida East Coast Railroad
bridge . ....... .. .. .. .. .. .. .. ....
3.2 Original finite element grid for the Loxahatchee River
estuary . . . . . . .. .. .. ... .. ..68
3.3 Finite element grid for Modification 1 extension of
the jetties. only the ocean part of the grid is shown .69
3.4 Finite element grid for Modification 2. only the ocean
part of the grid is shown.................70
3.5 Predicted tides at Jupiter Inlet for March 1-30, 1991,
used for ocean boundary condition ............71
2.1 Shoreline in the vicinity of Jupiter Inlet, March 11,
2.2 Shoreline in the vicinity of Jupiter Inlet, March 8,
LIST OF TABLES
2.1 Thirty year erosion volume calculations ..........13
2.2 Jetty modifications....................15
2.3 Raw data from physical model. TEST #41: NE, MSL,
EBB FLOW, NORMAL CONDITIONS................18
2.4 Scales for navigational safety and erosion potential
ratio calculations ............................19
2.5 Example of data table for NE testing of Modification D .20 2.6 Results of jetting modification tests ......... ....22
2.7 Results of jetting modification tests without SE sand
supply factor related to jetty length ..............23
2.8 Comparison of wave heights (from NE and SE at MSL)
with dredged and normal conditions. ........ ........26
2.9 Dredged ebb shoal condition wave height ratios at
several points from numerical and physical models . o 27
3.1 Estimated net sediment transport rates for existing
3.2 Estimated gross sediment transport rates for existing
3.3 Results from model runs for Modifications 3, 4, and 5 62
3.4 Results from model runs for Modifications 6, 7, and 8 o65
4.1 Typical beach profile shape ............ ......73
4.2 Volumes and times (days) of offshore channel infilling .74
In this Fifth Progress Report of the Jupiter Inlet Management Study, we have examined several issues related to possible
improvements or modifications to the present system, including the ef fects of jetty modifications on navigational saf ety/access, beach erosion and sand influx into the inlet; and sedimentation in the
proposed offshore channels. In addition, modifications to the design of the JID trap and the creation of a new trap in the vicinity of the Florida East Coast Railroad bridge have been considered. These examinations are based on both mathematical and physical model studies. The physical model was also used to study
the impact of ebb shoal mining on beach erosion. In this case, however, test results seem to parallel those from the mathematical
model noted in the Fourth Progress Report; hence they are not discussed further; see sections 2.5.3 and 2.6.3. In the sequel, within the suite of options considered, the following issues are briefly highlighted in this chapter: improvements in jetty design, offshore channel creation, modification to the JID trap, and the creation of a new trap upriver.
1.2 Jetty Design
Our examination of options suggests the following tentative improvement alternatives to be noteworthy:
A. Physical model tests indicate that an L-shaped extension to
the south jetty, with each arm of L 50 ft in length (Fig.
2.14), would be advantageous from the point of view of reducing (e.g. 15 to 20 %) the return of the sand placed on the south beach by dredging the JID trap. Furthermore, this
extension would reduce wave action in the area immediately south of the south jetty, without materially increasing the
erosion of the beach in terms of reduced sand supply.
B. Raising the south jetty to +8 ft elevation would further
insure the above reduction in the return of sand, by reducing
the water f low and associated sand transport during storminduced water rises.
C. A 400 ft curved extension to the north jetty (see Fig. 2.16;
do not consider south jetty extension shown there) would measurably improve navigational safety by reducing wave
heights in the channel, together with reducing the risk of vessel wave attack abeam. Such a north jetty would also partly shelter the area immediately south of the south jetty.
Initially at least, before the beach north of the north jetty reestablishes an equilibrium profile compatible with the
modified jetty, the influx of sand could reduce by about 25 %.
Eventually the sand inf lux may return to the present value. At the same time, there would be significantly greater erosion of the south beach in the vicinity of the inlet as a result of
the extended north jetty; sand def icit could increase over the present by at least 700,000 to 800,000 cubic yards over long term. Note that this deficit is not a rate; it is the total
sand volume deficit due to the presence of the jetty. At present this amount is believed to be (depending on the method of calculation) around 200,000 to 250,000 cubic yards. If this jetty option is adopted, a sand bypassing pump (or dredging
system) to catch the sand in the area immediately north of the modified jetty, coupled with dredging in the interior (which
will have to continue at a reduced rate, e.g. 50 %), will have to be installed to mitigate the effects of erosion downdraft.
As a possible alternative to the bypassing system, the ebb shoal could be dredged on a regular (e.g. yearly) basis to replenish the sand-starved region downdraft. This ebb shoal dredging option can be coupled with the dredging of an offshore navigation channel as noted later. As a second
alternative a different offshore source of sand not integrally
associated with the ebb shoal may be used.
D. Raising the seaward 300 ft portion of the north jetty by 2 ft
would reduce the influx of sand during storm events; the
overall reduction could be 5 to 10 % over the present.
1.3 Navigation Channel
E. Dredging a shallow (-10 ft) channel in the eastern direction
could improve navigation by preventing vessels from grounding there. Such a channel could yield as much as 30,000 to 40,000
cubic yards of presumably beach quality sand on an annual basis. This channel should be dredged during the pre-summer window, i.e. in April-May; it should serve during the summer months but will close in Fall. Note however that the dredging
equipment required for this purpose will be different in general from that used to dredge the JID trap, the cost per
cubic yard will be higher, and the dredging work will not necessarily be a mere extension of the work being done in the interior. Note also that dredging of the ebb shoal for this purpose involves an a priori unquantifiable risk. Therefore, after the first dredging is carried out, the beaches in the vicinity of the inlet must be monitored for adverse impact by
surveys covering the dredged area and the beaches. Surveys should be obtained at least twice one six months later and
another a year later.
1.4 Sand Traps
F. Enlarging the existing JID trap to areal dimensions suggested
in Fig. 6.27 in Mehta et al. (1991b) and to a depth of -19 ft
should increase the trapping efficiency by about 20 %, and reduce the transport of sand upriver from the trap by 15 to 2 0 %. This should assist the marinas in terms of their shoaling problem and reduce the infilling rate of the region west of
the railroad bridge.
G. A new trap east of the railroad bridge, of areal dimensions
shown in Fig. 3. 1 and dredged to -12 ft, could reduce the inf lux of sand past (westward) of the railroad bridge by 25 to
II. PHYSICAL MODEL TEST RESULTS
In order to study fully the hydrodynamic conditions of the Jupiter Inlet system, and any changes made to it, a physical model
representing the inlet near field was created, as discussed in previous progress reports to the Jupiter Inlet District (Mehta et al., 1991a, and Mehta et al., 1991b).
The tasks of this portion of the study, which have already been documented in Mehta et al. (1991a) will be restated here:
1. Obtain data on wave heights and wave-induced currents along the shoreline for an assessment of alongshore sediment transport capacity.
2. Obtain data on the change in flow pattern, current magnitude and eddies caused by implementation of construction and dredging proposals (jetties, ebb shoal mining).
3. Obtain data on the change in wave heights as a result of implementation of construction and dredging proposals.
4. Evaluate relative merits and demerits of different alternatives.
2.3 Test Criteria
To be able to make an evaluation of the proposed jetty options and ebb shoal dredging, a standard must be set. This standard is the 'normal' conditions at Jupiter Inlet, i.e. the existing
conditions that simulate the hydrodynamics at Jupiter without modifications to the system.
The physical model was concerned with improving navigational
safety, sheltering the south beach from erosion due to wave action, reducing sediment influx into the inlet mouth, and analyzing the effects of dredging the ebb shoal. When analyzing the effects of
jetty improvements, it is also necessary to check the erosion potential of extending the jetty length.
A protocol for studying the above factors due to changes in the inlet system was established. By measuring wave heights, currents, and flow patterns in the model it was possible to achieve the objectives listed in section 2.2. The first three parameters, wave height and current magnitude and direction, were measured at several locations, 4 for each jetty design and 21 for the dredged condition. Flow patterns for the inlet nearfield were also observed for each test. Primarily the tests were done for 'as is' conditions to which the modified tests could be compared. Each jetty configuration was compared to the normal conditions to determine whether or not the changes were beneficial overall.
Criteria for navigational safety were based on wave height measurements, and are discussed in section 2.5.1. Next, the
relationship between wave heights, energy, and sediment transport will now be discussed. Wave energy is a function of the square of wave height; this emphasizes the importance of wave height on wave energy. This relationship can also be seen in the equation for predicting longshore sediment transport rates, namely:
k p Hb2 V sin 2 ab (
16 (Ps p) a'
where Q = longshore sediment transport rate, k = an empirical coefficient determined by comparing calculated values of Q with measured rates, p = density of water, ps = density of sediment, a b = angle wave crest makes with shoreline at breaking, a' = the solids fraction of the in situ sediment deposit (1-porosity), Hb = height of wave at breaking, db = depth of water at breaking, and g = gravity (Weggel and Perlin, 1988). By assuming the wave angle to be constant, and combining all other constants, this equation can be reduced to:
Q, = A H(2.2) This Q, value will represent a longshore sediment transport factor as related to wave energy.
Weggel and Perlin (1988) modified Eq. (2.1), and with an expression for longshore current developed by Lounget-Higgins (1970) developed another equation for longshore transport rates: k 2kCfp H (2.3)
=6 5 P (PB p) a b Xb [V]
in which 6 = the ratio of mean current velocity to a reference velocity, Cf = friction coefficient, xb = distance from the shore of the breaking waves, and [v] = mean longshore current velocity. This equation has a first order wave height (Hb) term and a longshore velocity term (v). Combining constants reveals the simplified equation:
Q2 = B Hb xb [v] (2.4)
This equation will represent longshore sediment transport as related to longshore current velocities.
The simplified Eqs. (2.2) and (2.4), will be used to relate the wave and current data taken from the physical model to sediment transport potential factors.
It is well known that the effects of extending jetties on a beach can increase erosion on the down drift side, and accretion on the other. It is then necessary to account for these adverse or beneficial effects of extending the jetties. A study by Dean and Grant (1989) generated a numerical model which modeled the effects of extending a littoral barrier (such as a jetty) on a coastline. This model was used to simulate end-line beach profiles at Jupiter for several cases of extended jetty lengths. The model allows for the historical background erosion and accretion rates to be used in calculating the new (due to jetty modification) profiles. A
historical shoreline map of Palm Beach County, generated by the Department of Natural Resources, was used to estimate erosion/accretion rates in the Jupiter Inlet vicinity. These rates were then used in the model. The result is a profile, a certain number of years (30 selected here), of the shoreline near Jupiter Inlet. A typical output of the model is shown in Fig. 2.1. Note
that since the jetties are very close with respect to the scale used in the plot, they are represented by a single line. After several different jetty lengths were tested, a plot relating downdrift erosion distance and jetty length was created, see Fig. 2.2. This graph could then be used to relate erosion potential to
jetty length, and assign integer values. Positive sign implies beneficial impact, while negative sign means an adverse impact. Increasing value of the integer implies increasing impact. A scale of 0 to 3 in steps of one, i.e. 0, 1, 2, 3, was chosen in general
for a quantitative evaluation of all impacts considered in this chapter. only negative values were assigned with reference to jetty impact on the downdrift beach stability, since extending the
jetties will increase erosion of the south beach, as seen from Fig. 2.1. Note that the 200 ft jetty length shown in the figure is the existing condition at Jupiter Inlet. So, for example, a 100 foot
jetty extension would be a 300 foot jetty. Since the erosion
caused by a littoral barrier is due to the cut of f of sediment supply, this factor is called the sediment supply f actor as related to jetty length. The range of values for this factor, from 0 to 3, were assigned by erosion distances. Erosion distances less than 27,500 feet were assigned a value of zero, between 27,500 and 28,500 ft were assigned -1, between 28,500 and 29,500 ft were assigned -2, erosion distance more than 29,500 ft were given a -3.
The results from this model were selected for a thirty year time period, as noted. From the erosion depth and distance values
calculated it was possible to calculate an erosion volume for several jetty length tests. By calculating the area of erosion for distance from a straight shoreline (no erosion) to the erosion line, with a slope of 1:10, and multiplying by the distance of erosion, for points every one thousand feet, an estimate of beach erosion was made for three jetty lengths. The slope of 1:10 was
chosen as typical, from a beach-slope profile south of Jupiter Inlet, near the Hilton hotel at Jupiter, Florida on 5/29/91 (Mehta
et al., 1991b) The thirty year erosion volumes for the jetties as they exist, for a 100 foot extension (300 ft.jetties), and for 200
foot extension (400 ft. jetties) were calculated and shown in Table 2.1.
Table 2.1: Thirty year erosion volume calculations
200 ft8 300 ft 400 ft
jetty jetty jetty
6.4 M ft3 14.0 M ft3 25.0 M ft3
(0.24 M yrd3) (0. 5 14 yrd3) (0. 9 M yrd3)
Sediment influx into the inlet mouth from the south beach when waves are f rom the northeast and southeast has been previously established by the sand tracer studies documented in Nehta et al.
(1991a) and Mehta et al. (1991b) This sediment influx is an important consideration in jetty modification designs. In order to quantify this influx, the flow patterns around each jetty
modification were video-recorded and then sketched (see Fig. 2. 3 as an illustration). Analysis of these flow patterns allowed
determination of the effects each jetty modification had on the sediment influx factor.
The ef fect of shore structures on the beaches in the proximity of the inlet have been noteworthy at Jupiter Inlet. Plate 2.1 shows the shoreline when the inlet was closed in 1945. Note the effects
of the single jetty, which are qualitatively similar to those shown in Fig. 2.1. Note also the virtual absence of the ebb shoal.
Plate 2.2 shows the shoreline in 1971. Note the fillet north of the north jetty, and the erosion south of the south jetty. The
bulging shoreline immediately south of the eroded shoreline appears to reflect the effects of prior nourishment. Note also the
characteristic, arcate ebb shoal recognized by the breaking wave pattern, and the natural channel gap through the shoal in the southeastern direction.
The sediment inf lux factor described above was for conditions at mean sea level (MSL). It has been noted elsewhere that
significant sediment transport into the inlet takes place during
storm events due to increased water levels and wave conditions. In order to study this activity, elevated water levels and increased
wave heights were tested in the physical model. These tests,
however, did not readily lend themselves to the previously discussed analysis. It was determined that the best analysis for these conditions would be a qualitative discussion of each test. This is done in section 2.6.2 of this report.
2.4 Test Conditions
In order to test changes in the hydrodynamic conditions at Jupiter Inlet due to modifications in the system, it is necessary
to recreate the dominate wave and current conditions. The dominate wave directions, water levels, wave heights, wave periods, and strength of flood and ebb flows have been discussed in Mehta et al. (1991b).
2.4.1 Jetty Modifications
Various jetty designs were tested in order to evaluate their
effectiveness at reducing erosion of the south beach, reducing sediment influx, and improving navigational safety. The jetty designs are listed in Table 2.2 and shown in Figs. 2.4 through
2.4.2 Elevated Jetty Modifications
The sediment flux into Jupiter Inlet is found to occur under normal flood tides (Mehta et al., 1991b) but is increased
considerably during storm events. A "northeaster" storm typically increases wave height, and therefore sediment transport capability, and the water level. Buckingham (1984) has shown that a 1.5 meter storm surge causes the flood tidal flow velocity in Jupiter Inlet to increase to one and one-half times the normal velocity. This
velocity increase increases sediment transport into the inlet. Overtopping of the existing jetties, when water and waves flow over
Table 2.2: Jetty modifications
North Jetty Extension
South Jetty Extension
50 straighta 100 straight 50 diagonalb 100 diagonal 100 tear dropc
0 0 0
0 0 0
400 curved 200 curved 200 tear drop
0 0 0 0 0
200 curvedd 100 straight 100 straight +
50 southe 50 south 50 straight 50 straight +
50 tear drop 100 curved 50 curved 50 tear drop
ai.e. along the direction (offshore) of the existing jetty. b,c,d,eSee corresponding Figures.
the jetties, can occur during storm conditions and wash nearby beach sand into the channel. As one observer put it, "the sand just flows over the (north) jetty" (Reynolds Miller, personal communication, 1991).
A practical approach to decrease this sediment influx during storm events is to increase the height of one or both of the jetties. The north jetty concrete cap is made up of four steps, each at different elevations. The two most seaward steps are the areas of concern. A U.S. Geological Survey marker (Y305) on the third step of the jetty has an elevation of 10.905 feet (NGVD -
which is mean sea level in 1929). The steps are approximately two
feet high so the elevation of the eastern most (seaward) step is 7 feet, and the next step (step 2) is 9 feet above NGVD. The seaward
step (step 1) is approximately 100 feet long and step 2 is approximately 200 feet long. The south jetty cap is 525 feet long
with the eastern most 100 feet being approximately 5 feet high, the next step being approximately 6.5 feet high.
Four tests were run with different jetty elevations. They are:
1) +2 feet on step I of north jetty, NE waves
2) +2 feet on steps I and 2 of north jetty, NE waves
3) south jetty raised to + 8 feet, NE waves 4) south jetty raised to +8 feet, SE waves.
Figure 2.19 shows the different configurations, including the existing one for the north jetty (a). The test conditions were that of storm condition, discussed in Mehta et al. (1991b) namely, +8 foot storm surge (100 year storm), 8 foot, 10 second waves and
increased flood velocity. The reason that the north jetty was raised by 2 feet in each test (b and c in Fig. 2.19) was so that the concrete cap steps of 2 feet could just be extended. A 3 foot elevation was used on the first step of the south jetty and a 1.5
foot elevation on the remaining part to bring the overall total jetty height up to that of a predicted 100 year storm surge elevation, 8 feet.
2.4.3 Dredged Ebb Shoal
Dredging of the ebb shoal is discussed fully in section 3.5 of the Mehta et al. (1991b). The dredged region in the physical model is shown in Fig. 2.20.
2.5 Test Results
2.5.1 Jetty Modifications
The jetties were modified to better protect the channel from
wave action and/or reduce wave action on the south beach. These are the reasons for modifications to the north jetty (to protect the
channel), and to the south jetty (to protect the south beach), and combinations of both.
The test criteria are given in section 2.3 and its application is given here. As stated previously, the equations selected for analysis relate the data to factors such as navigational improvement and sediment transport potential. The result of
employing these equations is shown in Table 2.3, but it is
difficult to interpret each number without carrying out further analysis. By dividing each modified test by the "normal" test, for the corresponding tide and wave conditions, a percentage relative
to the normal condition was calculated. This would show, for example if wave heights were reduced at certain locations or not.
If the wave height ratio was greater than 100% then the wave height of the modified test were obviously higher than the normal conditions. As noted previously, a scale was developed that would allow these percentages to be converted to a positive or negative number to represent a desired or undesired effect, respectively.
The numbers assigned show whether an effect is strongly positive or negative by using a + or 2, while + or 1 represents a slightly positive or negative effect. A zero is assigned if the effect is
minimal. Two scales were developed, one for wave height ratios and one for the sediment transport rates and are shown in Table 2.4. The scales were employed on the calculations to generate a table that reveals the positive or negative effects of wave height (for
navigation safety) or sediment transport (erosion potential) ratios at four locations for flood and ebb tides, and the other effects
noted in section 2.3. An example of the table is shown in Table 2.5. Since four data points (locations) were used, their average
values were calculated and stated at the bottom of each column. This allows the assigned numbers to be seen as overall effects of the modification, instead of the effects at certain locations.
The next step in analysis is to weigh each of the columns shown in Table 2.3 according to their relative importance. Note
first that there are five columns. Two of the columns are f or sediment transport values, two are for sediment supply factors, and only one is for navigation. In order to weigh navigation safety
Table 2.3: Raw data from physical model.
TEST #41: NE, MSL, EBB FLOW, NORMAL CONDITIONS
Position Wave Height Wave Height
1.0 0.7 0.9
0.6 1.5 1.1
1.6 1.0 2.5 1.9 1.6 1.4
Current Magnitude (ft/s)
2.1 2.9 3.7 3.6 2.4 0.6 1.0
Current Direction (deg. North)
115 94 100
Longshore Velocity (ft/s)
2.8 3.6 3.5 2.2 0.5 0.7
542 291 687 536 1324 615 127 148
Scales for navigational safety and erosion potential ratio calculations
Wave Height Ratios:
125 -149 75 -124 50 -74
Q, and Q2 Transport Ratios:
Value Ratio of
Assigned Qts or Q2tS
-1 125 199
0 75 -124
+1 25 -74
+2 < 25
Table 2.5: Example of data table for NE testing of Modification D
Flood Cl 0 +1 0
Tide J3 +2 +2 +1 -2 +2
J4 +2 +2 +2
J5 0 0 0
(Average) (1.0) (1.25) (0.75)
Cl J3 J4 J5
* Sand supply influx is not a factor during ebb tide
Sand Supply (Jetty Length)
Sand Supply (Influx)
evenly with the other factors its weighting should be doubled. Since the northeast and southeast wave directions were tested separately they had to be combined. Since the predominant wave direction at Jupiter is from the northeast, approximately 8 months of the year, and southeast wave action is the remaining 4 months, factors of 8/12 and 4/12, respectively, were multiplied to the wave height ratios. The same "monthly" ratios were multiplied to the sediment supply influx factor. When wave action from the southeast is dominant, erosion potential need not be considered. This is because southeast dominant waves usually occur in the summer, a time when beaches are naturally accreting due to moderate to light wave action and south to north longshore sediment transport. For this reason the Q, and Q2 factors were omitted for southeast waves. The result is a formula which combines the assigned positive and negative values for the southeast and northeast wave directions:
kQd ____o___(Hexi)R(2) (8) + (Qlei ) + ( emod ) + (S.S.jetty length)
Hit 12 Qiexist Q2ex.it
+ (S.S. influx) 8 + (Hod )xsE(2) (--) + (S.S. jetty length) 12 H., ~ 12
" (S.S.influx) (4) = V (2.5)
Where V is the resulting value assigned to the jetty modification, and S.S. denotes sediment supply factor with respect to jetty length or influx into the inlet mouth. This equation was used for each modification for both the ebb and flood tides. The results are shown in Table 2.6 along with the total value, V. The letters represent the jetty modification listed in Table 2.2. In Table 2.6 it can be noticed that several letters of the alphabet are missing (i.e. A, D, F, and H). This is because when these tests were initially run, with waves from the northeast, their analysis showed predominantly negative effects, therefore further testing was not merited. The results for these tests are not shown.
The sand supply factor due to jetty length has been included for both the northeast and southeast directions in the above calculations. This factor, it can be argued, may be negligible during the summer months when wave action is from the southeast.
Table 2.6: Results of jetty modification tests
Modification Flood Ebb Total
B 2.6 -3.8 -1.2
C 2.5 -2.1 0.4
E 2.8 -2.7 0.1
G 2.2 -3.8 -1.6
1 3.0 -2.3 0.7
J 2.7 -2.5 0.2
K 3.0 -1.3 1.7
L 2.4 -1.9 0.5
14 0.9 0.3 1.2
N 1.1 -3.1 -2.0
0 0.7 -4.2 -3.5
Because southeast dominant waves occur for a shorter period, 4
months, with considerably lighter wave action, and because the beach is not starting with an eroded profile, erosion of the north
beach can be considered minimal. As a test of sensitivity of weighting to the final results, the data in Table 2.6 were
recalculated so that the sand supply factor related to jetty length was only used for the northeast tests, and put in Table 2.7.
2.5.2 Example Calculation
In order to fully understand how the values in Tables 2.6 and
2.7 were calculated, it would be instructive to go through the steps and analysis it took to get them. The values for
modification "L"', a 50 foot tear drop on the south jetty will be calculated. The first step is to list all the test experiments done with the L configuration, and then the corresponding "normal" tests.
1) Test 58 NE,NSL,ebb tide, L
2) Test 60 NE,MSL,flood tide, L
3) Test 87 SE,MSL,ebb tide, L
Table 2.7: Results of jetty modification tests without SE
sand supply factor related to jetty length
3.7 2.5 3.8
3.2 3.0 2.7 3.0 2.4 2.9
2.1 0.3 0.7 0.2 1.7 0.5 5.2 2.0 0.5
4) Test 98 SE,MSL,flood tide, L
5) Test 41 NE,MSL,ebb tide, normal
6) Test 45 NE,MSL,flood tide, normal
7) Test 80 SE,MSL,ebb tide, normal
8) Test 92 SE,MSL,flood tide, normal
These data were obtained from the data recorded in the physical model tests and using Eqs. (2.2) and (2.4) in section 2.3 so that tables such as Table 2.3 were generated for each test. As an example, the data for position J3 (see Fig. 2.20) in test number 87 will be carried out. The wave height found in the model was 1.2 cm. To convert this to prototype feet:
1.2cm -L ift 2.ift
The sediment transport factors are:
Q, = A Hb2 V- = (1) (2.1 ft)2 V(32.2 ft/s2) (4ft) = 48
where 4 is used as the approximate breaking depth of 4 foot waves, and A, the sediment characteristic constants, are set at 1, and
Q2 = B Hb xb [v]=(1) (2.1 ft) (150 ft) (1.39 f) =432 ft3
Now we divide the Q, and Q2 values found in this test (#87) by the normal conditions found in test #80, which has the corresponding conditions of MSL, southeast waves, and ebb tide. Therefore at J3:
Hxod 2.1 .t.6 ft 100 = 81% H exist 2
Qlmod sec 100 = 64% Qlexis't 75 ft3
Q2mod sec 100 66%
Q2exist 652 ft3 sec
02mod sec 100 = 66%
Q2exist 652 ft3 sec
Thus by using the scales given in Table 2.4 the corresponding values for the ratios at J3 are: H/Hexist = 0, QImo/Q1exist = +1, Q2mod/Q2exist = +1. The sand supply factor related to jetty length is found by using Fig. 2.2. Since the modified jetty length is 50 feet, the corresponding factor is zero (0). On an ebb tide no
sediment will pass into the inlet, so a value of zero (0) is assigned to the sand supply factor related to influx.
This same procedure is used for each of the data collection
points on both ebb and flood tides. The assigned numerical values, such as those generated above, for Hmoc/Hexistf Qimoc/Qiexistf and Q2mod/Q2exist are averaged together. The last two columns, sand supply factors, remain constant for each data point (J2, J3 etc.) so no averaging is necessary.
The complete set of calculated values is presented in Table 2.5. The weighing factors have already been discussed, and will
now be used to combine northeast and southeast wave directions. The flood tide values for modification L will be considered first. The formula is given as Eq.(2.5). Using this equation for
modification L gives:
(0.5) (2) (0.667) + (0.5) + (0.25) + (0) + (2) (0.667)
+ (-0.5) (2) (0.333) + (0) + (0) (0.333) = 2.4
The same method is employed for the ebb tide data and gives a value of -1.9. Combining these two gives 0.5 (2.4 1.9 = 0.5). This number shows an overall positive impact of jetty modification IL"I.
2.5.3 Ebb Shoal Dredging
The ebb shoal dredge area and data station locations are shown in Fig. 2.20. To be consistent with the numerical model results shown in Nehta et al. (1991b), the wave ratios were calculated with respect to an offshore reference wave height and are presented in Table 2.8. Note that for northeast waves station B1 was used for the reference wave height, and for southeast waves station 06 was used. The normal and dredged condition wave heights are shown as percentages of the reference wave height for both northeast and southeast waves. The results of the ebb shoal dredge physical model study are compared to the results of the numerical
model study given in Mehta et al. (1991b) These results are shown in Table 2.9. Both tests were preformed with 4 foot 8 second waves, but the water level in the physical model was at MSL while the numerical model used +8 foot storm surge. The flood tidal
Comparison of wave heights (from NE and SE at MSL) with dredged and normal conditions
al Dredge Normal Dredge
Ebb Flood Ebb Flood Ebb Flood Ebb
93 60 100 86 38
14 12 19 52 38
43 24 51 31 93
40 43 40 19 31 26 10
48 100 91 32 16 11 27 70
14 84 70 23 25
34 30 25 25 23
34 100 116
40 101 57 31 15 105
94 23 33 58
34 49 33
125 38 100 107 69
14 23 57 106 35 56 16
21 94 99 18 35 53 33 52
14 57 95 81 86 16
64 93 100
61 27 29 27 26
44 51 25 58
84 84 76
40 45 59
49 77 100 27
14 27 16
42 30 33
94 132 75 77
14 24 23 75 60 100 37 97
24 27 31 38
55 62 10
42 114 66 72
40 35 65 100
20 23 32
Table 2.9: Dredged ebb shoal condition wave height ratios at several points from numerical and physical models
Model Wave Height Ratios, Hffo/Hexist
A2 C4 S2 S3 S4 S6
Computer 1.7 1.4 0.8 0.4 1.1 1.3
Physical 0.4 1.3 0.7 0.8 0.9 1.3
velocities were also slightly different; 3.6 fps for the numerical model and 5.4 fps for the physical model. Even with these
differences in test conditions, because the wave heights were similar, the results compare favorably, with the exception of station A2, and S3. These results are shown as ratios of fldHxs for each station shown.
2.6 Evaluation of Test Results
The test results presented in the last section will now be discussed. The first section will discuss the modifications to jetty lengths, the next raised jetties, and lastly the dredged ebb shoal.
2.6.1 Jetty Length Modifications
The initial jetty modification tests were listed in Table 2.2. This list included 15 tests with modifications to the jetties. Table 2.6 shows the results of the tests run and gives a "total" for each test. This total shows the overall performance of the jetty, based on the evaluation process described in sections 2.3 and 2.5.1. All of the tests were initially run for northeast waves. The results of these tests were analyzed and several tests
were omitted f rom, further testing. Tests that were omitted include A, D, F, and H. These tests were omitted because their results showed significant overall negative effects. The remaining 11 test modifications were carried out for southeast wave directions.
Table 2.6 can be further narrowed by eliminating the negative values, which are test modifications B, G, N, and, 0. These tests were negative due to the ebb tide results. The reasons for ebb tide conditions giving such strongly negative numbers was two-f old. First, the elimination of the sediment supply factor related to influx lowered the total result by removing this, primarily
positive, number from analysis. Second, the wave conditions during ebb tide can be increased by increasing the flow velocity out of
the inlet, which can happen if a jetty modification constricts the inlet width. So if a jetty modification increases the exit velocity of the water, or channels it in another direction, the effect can be wave heights higher than normal conditions. The remaining positive modifications range from 0.1 to 1.7 with five of the seven being less than 1.0. These five modifications should not be ruled out as possible solutions because combinations or
derivations of these options could prove very beneficial, but only the two modifications which are greater than 1.0, K and M, will be further discussed.
Modification K is a 50 foot straight with 50 south extension on the south jetty (Fig. 2.14). This modification greatly sheltered the south beach when wave action was from the northeast, and also cut off sediment influx into the channel, as measured by
tests using floats. This modification also had a high rating because it was a short extension, and would not greatly alter the
littoral drift pattern. Because it did not seem to change the flow conditions very much in the ebb tide situation, the ebb tide value
was not very low, when compared to other modifications. Again, the best advantage of modification K was its relatively short length.
The next highest modification, M, is a 400 foot curved extension on the north jetty and 100 foot curved extension on the south jetty (Fig. 2.16). This modification greatly sheltered the
inlet channel and the south beach, and therefore the analysis showed it to be a very favorable result related to navigational safety. The effect of modifying the jetties by such great distances, though, must be approached carefully. The time scale
used for the sand supply factor was 30 years; perhaps a longer time
scale would show more detrimental long range effects. it is
possible that the extended jetties may only increase the erosion
problems at Jupiter by offsetting the coastline even more than predicted, and as well alter the position of the ebb shoal by shifting it southwards. Therefore, although this modification came up with very positive results for reasons of navigational safety, careful thought should be given to the long term affects.
Table 2.7 shows the results for jetty modifications with the jetty length factor only being included once, for northeast waves, as noted. The results shown in this table reveal the sensitivity
of the weighing factors in analyzing the present data. Notice that there are now no negative numbers, showing that the jetty length factor can greatly influence the outcome of the results. In this table modification M is significantly the highest number, with E and N being the next best results. This table is a useful way to
visualize the effects that the jetty length factor played in calculating the results. Note too that these modifications could
be considered if somehow sediment supply to the south beach was not a problem, i.e. if a pumping station were installed or if dredged material was continually placed on the south beach.
2.6.2 Elevated Jetties
The desired result of raising the jetties is so that the sediment influx into the inlet would be reduced during storm events. A qualitative description is now given as an evaluation of the tests results.
When a normal (i.e. no modifications to the existing system)
storm condition was tested, the flood tidal velocity increased and seemingly everything (bits of styrofoam, wooden balls, dye, etc.) was transported into the inlet if placed on the north beach. The
severe wave action caused a strong littoral drift which "pushed" material down the coast. When the jetties were, for the most part, submerged under the +8 foot storm surge, they could not block the entrance of material into the inlet. Although most of the
transport was due to wave action, the increased flood velocity had
a strong effect on pulling material into the inlet as well. It was
noticed that when wave action was from the northeast, most of the
material set on the south beach did not enter the inlet, but rather got pushed down the coast by the wave action, and vice versa as well.
The first test was a +2 foot elevation on the first step of the north jetty with northeast waves (Fig. 2. 19b). Even with the
added height the f irst step was still only 9 feet above MSL, so with a +8 foot storm surge and 8 foot waves, the ef fect was negligible. This +2 foot rise might be beneficial for other storm surge elevations, such as a 50 year (+7 foot) storm or less.
The second test raised the first and second steps of the north jetty by 2 feet (Fig. 2.19c) The second step was then 11 feet above MSL. This test certainly showed the beneficial effects of raising the jetty to an elevation substantially above the 100 year storm elevation. Even though large waves overtopped the second step, material did not "flow" over it. Wooden balls that were
placed on the north beach rapidly moved down the coast to the inlet, but did not pass over the second step of the jetty. In fact most of the floats, and even the dye, would flow around the second step to the (submerged) first step, and then into the inlet.
The third test raised the south jetty to an overall height of +8 feet (NGVD) (Fig. 2. 16d) When the wave action was from the northeast, the modification had almost no effect on influx into the inlet, because as previously noted, material placed on the south beach did not enter the inlet even with normal storm surge conditions. This test did show that wave heights on the immediate
south beach were reduced, since the waves broke on the jetty. From this test it was seen that testing modifications to the north jetty with southeast waves was not necessary, since virtually no effect would be seen.
The last test was the raised south jetty with waves from the southeast. The longshore current that was generated by the waves
was so strong that it seemed to wash any material placed on the south beach into the inlet. The angle of the waves, combined with
the topography of the area just south of the jetties, seemed to enhance the effect of overwash into the inlet. Although it would
take the wooden balls and dye a few seconds longer to pass into the inlet than they did without modifications, the effects of raising the south jetty only 3 feet were minimal.
The second test run, with 2 feet added on both the first and
second steps, showed that raising the north jetty height would reduce sediment from flowing over it, but it also revealed the possibility that sediment may flow around the jetty and still enter the inlet mouth. Since all the tests done with raised jetty heights showed that the materials continued to enter the inlet, flow of sediment around the jetty and into the inlet during storm
events should be taken into account for any jetty modification. The effects of raising the jetty height and that of any length modification could be combined to, perhaps, reduce the sediment influx over and around the jetties.
2.6.3 Dredged Shoal Condition
Mehta et al. (1991b) discuss the ebb shoal dredge in detail, so the results shown here are meant merely to support all observations already made. The results in Table 2.8 show that, as
stated in Mehta et al. (1991b), "wave heights will decrease or increase depending upon wave refocusing due to bottom modification."
In Table 2.9 most of the positions show almost the same wave height ratio between the numerical and the physical models. The strong similarities between these two models gives confidence in
the results of both. Position A2 does not seem to match very well, probably because this location was at the shallowest part of the ebb shoal, in the physical model, and the waves broke before they reached the wave height gage.
W Accretion 7 ,IJetty (les)
z 100 4 :I
Shoreline W 100 without Jetty
U) ~Erosion LL -200
O 0 Years
-300 ------------.5 Years
-..... 10 Years
-400 .. 30 Years
-50o I I I I I I
0 20000 40000 60000 80000 100000
DISTANCE ALONG SHORELINE (ft)
Fig. 2.1 Results from numerical simulation of shoreline evolution with a jetty length
of 220 feet.
30 -3 29 -2 28 -1 27- 0 26
9A, I I I I
JETTY LENGTH (ft)
Fig. 2.2 Jetty length vs. erosion distance with corresponding
scale values (0, -1, -2, -3).
JETTY MODIFICATION M
FLOOD FLOW /
5 ft/s >
JETTY MODIFICATION M
Inlet flow patterns for normal condition and modification M: flood and ebb tides.
. . so.
0 100 200 scale In feet
Fig. 2.4 Jetty modification A.
0 100 200 scale in feet
. . 00
Fig. 2.5 Jetty modification B.
-~a a yey a
0 100 200
scale In feeto ....
0 100 200".' s c a l e I n f e e t" .-.'- -
Fig. 2.6 Jetty modification C.
0 100 200
scale in feet .
I I I ;,,"-r
Fig. 2.7 Jetty modification D.
*UOTqPDTJTPOUI KqqrD 8*Z I5-r
Jel ul 8i83S
O0Z O0 L 0
* ** . 0
-lII-.-. . .. . . . .. .
a a a ofa a
l.,.s a. s a
" ... .
o doo Por.:,.
0 100 200
scale In feet 4.
Fig. 2.9 Jetty modification F.
O 100 200 .
scale in feet .
Fig. 2.10 Jetty modification G.
0 100 200 ',"
scale In feet
Fig. 2.11 Jetty modification H.
0 100 200 scale in feet
Fig. 2.12 Jetty modification I.
0 100 200 scale In feet
Fig. 2.13 Jetty modification J.
ftft AN* fte
'Nuo UO4PTJTPOui 1 4ep IT*Z *5BTdi
,, ;Joel u! 8leos OO O0t 0
.5.. --., .., ...-. . . ..
V % *.,. %
~am ~ ms~mm
0 100 200 scale In feet
Fig. 2.15 Jetty modification L.
"% "t 1 af dU p I T E R fIN L e T
n--r-mm-s i e
scale in feet
_ _ _. .4"
Fig. 2.16 Jetty modification M.
0 100 200 scale in feet
Fig. 2.17 Jetty modification N.
JUPR INLE i>0
" "12"/oo4 I
aft %a %f ft f f f f a t f f f
0 100 200 scale in feet
Fig. 2.18 Jetty modification O.
(a) ,-< 200' >1 < 100'-3
North Jetty with + 2' on Step 1
North Jetty with + 2' on Steps 1 and 2
I Step I
South Jetty Raised to + 8'
(di I*100, -->*I
North jetty and south jetty elevation views and height modifications.
-- - - - - - ' I l l ,
VF/F/-/ ; / 7 /- 7
0 0 J4 C1 A 0
Fig. 2.20 Physical model data collection points and dredged ebb shoal area.
Plate 2.1 Shoreline in the vicinity of Jupiter Inlet, March 11, 1945.
Plate 2.2 Shoreline in the vicinity of Jupiter Inlet, March 8, 1971.
III. EFFECTS OF POTENTIAL MODIFICATIONS ON FLOW AND
SAND TRANSPORT IN THE LOXAHATCHEE RIVER
This chapter presents the results from the sediment transport modeling (using a two-dimensional depth-averaged finite element model) of the Loxahatchee River estuary. The sediment transport model used in this study, described in Mehta et al. (1990), predicts the two-dimensional (depth-averaged) velocity and salinity fields, and the change in bottom elevation due to scour or deposition resulting from sand transport at the nodes comprising the finite element grid.
The objectives of the modeling effort described herein were to analyze the effects of eight potential modifications to the Jupiter Inlet-Loxahatchee River estuarine system on the sedimentary and hydrodynamic regimes. The purposes of the potential modifications were to (a) reduce the flux of sediment into Jupiter Inlet and hence into the Loxahatchee River and Intracoastal Waterway
(Modifications 1 5), and (b) reduce the flux of sediment west of the Florida East Coast Railroad bridge (Modifications 6 8). The eight modifications analyzed are:
(1) Extension of both the north and south jetties by 15.2 m (50
(2) Extension of the north jetty by 122 m (400 ft) and the south
jetty by 30 m (100 ft).
(3) Increase the dredged depth of the Jupiter Inlet District (JID)
sand trap, located in the inlet channel, to 5.8 m (19 ft).
(4) Enlargement of the JID sand trap to the dimensions shown in
Fig. 6.27 in Mehta et al. (1991b). Two dredged depths were
considered: 5.8 m (19 ft) and 4.1 m (13.5 ft).
(5) Increase the length (but not the width) of the JID sand trap
to the dimensions shown in Fig. 6.27 in Mehta et al. (1991b)
and increase the dredged depth to 5.8 m (19 ft).
(6) Dredging of a new sand trap to -3.7 m (-12 ft) immediately
west of the Florida East Coast Railroad bridge (see figure on page 39 in Mehta et al. (1991c)). The areal dimensions of the
trap are 175 m (575 ft) wide (centered laterally across the
Loxahatchee River) and 46 m (150 ft) long (along the
longitudinal axis of the river).
(7) Dredging of a 2.4 m (8 ft) deep, 22.9 m (75 ft) wide
navigation channel starting at the center of the railroad bridge and running 390 m (1280 ft) westward along the
longitudinal axis of the river (see figure on page 39 in Mehta et al. (1991c)). Two cases were considered: one with the navigation channel and new sand trap described in Modification
6 above, and one with only the navigation channel.
(8) Dredging of a new sand trap to -3.7 m (-12 ft) immediately
east of the Florida East Coast Railroad bridge (Fig. 3.1).
The areal dimensions of the trap are 152 m (500 ft) wide (centered laterally across the Loxahatchee River) and 61 m (200 ft) long (along the longitudinal axis of the river) Two different median sediment sizes (0.20 mm and 0.50 mm) were used with this modification, whereas only a 0.50 mm sand size
was used with the previous seven modifications.
These modifications were made to the original finite element grid of the Loxahatchee River estuary shown in Fig. 3.2, and then the sediment transport model was run for a period of one month for
each of the cases described above. The results from each model run were compared with the sediment transport modeling results reported in Mehta et al. (1991b) A description of how the model was applied to each of these cases, followed by a discussion of the results obtained from the modeling effort, is given later in this
chapter. A review of a study performed by Environmental Consulting & Technology, Inc. (ECT) on the longitudinal salinity distribution in the Loxahatchee River is given next.
3.2 Salinity Distribution
one of the management issues to be included in the Loxahatchee River estuary comprehensive management plan is improvement of the navigation channel "to meet the recreation demand of the area and
to improve boating safety" (ECT, 1991). Using the results of a study by Applied Technology and Management, Inc. (ATM) to determine
"the optimal locations and configuration of the proposed channel modification of main channels as well as access channels" (ECT, 1991), the computer model RECEIV-II was used to estimate the changes in salinity intrusion caused by the recommended channel improvements. The recommended channel improvements, which would be implemented by dredging, proposed by ATM are given in the Task 6.1, Access Channel Assessment, and Task 6.2, Main Channel Assessment, reports, and are shown in Fig. 4-2 in ECT (1991).
ECT used four bathymetric conditions for the salinity modeling, each for two flow conditions, average low flow and extreme low flow. The four bathymetric conditions used were (1) existing bathymetry, (2) channel improvements recommended by ATM,
(3) dredging of a shallow segment of the Northwest Fork near Tequesta (shown in Fig. 4-3 in ECT (1991)) to -4 ft NGVD in addition to the channel improvements recommended by ATM, and (4) dredging of a segment in the lower reach of the Northwest Fork to a depth of -6 ft NGVD and filling a deeper segment to -6 ft NGVD (see Fig. 4-4 in ECT (1991)) in addition to the channel improvements recommended by ATM (ECT, 1991). The results from the salinity modeling using the RECEIV-II model showed that the maximum salinity increase due to the proposed channel modifications was approximately 0.5 ppt, a conclusion that is in complete agreement with that reached in the present study when considering jetty and sand trap modifications (see Mehta et al., 1991c). ETC concluded that the impact on the salinity intrusion caused by the dredging alternatives contained in the second, third, and fourth bathymetric conditions listed above would be insignificant. Figures 4-5
through 4-7 in ECT (1991) show the predicted longitudinal salinity distributions for both average and extreme low flow conditions for the second, third, and fourth bathymetric conditions.
3.3 Model Application
The original finite element grid (see Fig. 3.2) was composed of 963 quadrilateral elements and 3193 nodes. The size of the elements was varied such that the highest density of nodes occurred in the areas of expected high spatial velocity gradients (e.g., in
the proximity of the jetties). The same water boundaries in the
Loxahatchee River indicated in Fig. 6.6 in Mehta et al. (1990) were used in the original grid. The ocean boundary consists of the outer semi-circular arc. These water boundaries were chosen
because of their proximity to tide gage stations used during a study in 1976 by the University of Florida (Chiu, 1975) and because of their proximity to NOS secondary tide stations.
Calibration of the hydrodynamic module in the sediment transport model, described in Mehta et al. (1991la), was performed
by matching predicted and measured (Chiu, 1975) water surface elevations and velocities at several locations throughout the estuary. Calibration of the sediment transport module, described
in Mehta et al. (1991b), was performed by matching predicted sedimentation rates in the JID trap and the U.S. Army Corps of Engineer (USACQE) trap with the available dredging records.
For Modification 1, the original finite element grid was altered to extend both jetties at Jupiter Inlet by approximately 15.2 m (50 ft). The altered portion of the original grid is shown in Fig. 3.3. To extend the jetties, four additional elements and 18 additional nodes were used.
For Modification 2, the lengths of the north and south jetties shown in Fig. 3.3 were increased to approximately 122 m (400 ft)
and 30 m (100 ft), respectively. The modified portion of the
original grid is shown in Fig. 3.4. The modified grid is composed of 988 quadrilateral elements and 3286 nodes.
For Modification 3, the dredged depth of the existing JID sand trap was increased to 5.8 m (19 ft) in the original gird.
For Modification 4, the enlarged JID sand trap shown in Fig. 6.27 in Mehta et al. (1991b) was scaled onto the original finite element grid. Then the bottom elevations of the nodes inside the
enlarged trap were set equal to first -4. 1 m (-13.5 ft) and then 5.8 m (-19 ft). The side slopes of the enlarged sand trap were taken to be lV:3H. This slope was used to calculate the bottom elevations of the nodes immediately surrounding the trap.
For Modification 5, the increased JID trap length used in Modification 4 and the original width were used. The grid used for
Modification 3 was modif ied to account for the increased trap length and 5.8 m (19 ft) dredged depth.
For Modification 6, the new sand trap immediately west of the railroad bridge was scaled onto the original finite element grid. Then the bottom elevations of the nodes inside the new trap were
set equal to -3.7 m (-12 ft) The side slopes of the new sand trap were again assumed to be lV:3H. This slope was used to calculate
the bottom elevations of the nodes immediately surrounding the trap. The 'same procedure was used to modify the original grid and the grid for Modification 6, respectively, for the navigation channel only case and the navigation channel and new sand trap case contained in Modification 7.
For Modification 8, a new sand trap immediately east of the railroad bridge was scaled onto the original finite element grid. Then the bottom elevations of the nodes inside the new trap were set equal to -3.7 m (-12 ft). The side slopes of the new sand trap were again assumed to be 1V:3H.
Initial conditions used in the hydrodynamic module for all model runs reported herein were zero velocity and constant water surface elevation at all nodes. The initial nodal salinity values of the 1970 nodes inside the inlet were set equal to the average of
the high water slack and low water slack salinity distributions measured in the Loxahatchee River from Station 1 to Station 15 on
February 26', 1975 (Chiu, 1975) (station numbers refer to those used in the 1975 study). That is, the initial salinities at the 1970
interior nodes were taken to decrease approximately linearly (with distance upstream from the inlet) from 34.5 ppt at the inlet mouth to 31.5 ppt at the grid boundary in the northwest fork. The
initial nodal salinity values in the ocean part of the grid were set equal to 35 ppt.
Boundary conditions used for the hydrodynamic module for all
model runs consisted of the NOS predicted water surface elevations at the specif ied water boundaries for the period March 1 30, 1991. The predicted tide for the ocean boundary nodes for this period is shown in Fig. 3.5. Salinity boundary conditions for the
ocean boundary nodes were taken to be a constant 35 ppt, while
those at the five interior open water boundaries were assumed to vary sinusoidally between the low water salinity and high water salinity measured during the 1975 study at the longitudinal positions in the estuary corresponding to the locations of the boundaries. For all the model runs, except for Modification 8 as noted previously, a single grain size of 0.50 mm was used for sediment transport calculations.
3.4 Model Results
The estimated net sediment transport rates, repeated from Mehta et al. (1991b), into Jupiter Inlet, west of the railroad bridge, and west of the US 1 bridge, and the gross transport rate at the inlet mouth for D5= 0.50 mm, predicted using the existing (unmodified) finite element grid are given in Tables 3.1 and 3.2, respectively. These are included here for comparative purposes.
Table 3.1: Estimated net sediment transport rates for existing system
Location D50 (mm) Net Sediment Flux (m3/yr)
Jupiter Inlet 0.50 60,000
US 1Bridge 0.50 1,400
Railroad Bridge 0.50 800
Table 3.2: Estimated gross sediment transport rates for existing system
Location D50 (mm) Gross Sediment Flux (m3/yr)
Jupiter'Inlet 0.50 94,000
A discussion of the effects of each of the modifications on the hydrodynamic and sedimentary regime in the Loxahatchee River estuary is given next. The one general conclusion that applies to all eight modifications analyzed is that, as noted, negligible
effects (less than 0.5 ppt difference) on salt intrusion was noted between the salinity field obtained with the original grid and the modified grids. This result was found by comparing the predicted salinity fields from the original and modified grids at critical locations, e.g., at locations where changes in bottom elevation were made, at the grid boundaries in the northwest and southwest forks. The reason for this finding is that the use of sinusoidally varying salinity boundary conditions at the five interior open water grid boundaries, as discussed in Mehta et al. (1991b), controlled the salinity variation in the grid interior to such an extent that the relatively small changes made in the four grid modifications did not cause any differences in the predicted salinity fields. The artificial sinusoidally varying salinity boundary conditions were used because no measured salinities were available.
3.4.1 Modification 1
Comparisons discussed in this section are between the original grid configuration and Modification 1. The predicted current pattern in proximity of the extended jetties varies, as would be expected, from that with the original grid. The biggest change occurs in the current direction, as it is strongly controlled by the configuration of the jetties. However, the current speed at nodes around the jetties which did not change location between the original grid and Modification 1 did not differ by more than five per cent at any node. In the inlet channel, the predicted
velocities at six nodes were compared. The difference in current speed was never more than four percent at any of the six nodes. Negligible differences were noted in current directions at the six nodes. The net sediment flux into Jupiter Inlet was predicted to be seven per cent less than the value given in Table 3.1. Considering the margin of error typically associated with sediment transport calculations, the seven per cent difference is definitely not significant.
3.4.2 Modification 2
Comparisons discussed in this section are between the original grid conf iguration and Modif ication 2. The predicted current pattern in proximity of the extended jetties varies significantly f rom that with the original grid. The biggest change occurs in proximity of the north jetty due to the major modification in jetty length and, configuration. In the inlet channel, the predicted velocities ~at six nodes were compared. The difference in current speed was never more than eight percent at any of the six nodes. Negligible differences were noted in current directions at the six nodes. The net sediment flux into Jupiter Inlet was predicted to be 27% less than the value given in Table 3.1. Even considering the marginiof error typically associated with sediment transport calculations, the 27% difference is significant, and indicates that less sediment would be transported into the inlet with this jetty modification. The last statement is true in the short run after initial jetty modification, but once sediment built up along the north side of the north jetty, it is thought that more sediment would be transported into the inlet, thus possibly increasing the net sediment flux into the inlet.
3.4.3 Modification 3
comparisons discussed in this section are between the original grid configuration and the two cases considered with Modification 3. The predicted current pattern in the proximity of the deepened JID sand trap varies very little from that with the original grid. The biggest difference occurs with current speed within the trap. The decrease in the predicted current speed over
the deepened trap causes the increase in the sand trap filling rate given in Table 3.3 for Modification 3. The percentage changes in net sediment flux and trap filling rates are with respect to those
values calculated using the original grid. The percentage increase in net sediment flux into the inlet shown in Table 3.3 is primarily the result of less sediment being transported out of the inlet on
an ebb tide due to the presence of the deeper trap, which traps more sand during both flood and ebb tides than the existing trap.
Table 3.3: Results from model runs for Modifications 3, 4, and 5
change in Change in
Sediment Change in Sediment
Flux Trap Flux
Mod. Case into Filling past RR
No. Identification Inlet Rate Bridge
3 -5.8 m (-19 ft) + 5% + 12% 9%
4a -4.1 m (-13.5 ft) + 3% + 5% 4%
4b -5.8 m (-19 ft) + 11% + 28% 23%
5 -5.8 m (-19 ft) + 8% + 21% 16%
3.4.4 Modification 4
Comparisons discussed in this section are between the original grid configuration and the two cases considered with Modification 4. The predicted current pattern in the proximity of the enlarged
JID sand trap varies somewhat from that with the original grid. The biggest difference occurs with current speed within the trap (as would be expected) between the two depths considered in conjunction with enlargement of the trap. The difference in
predicted current speed in proximity of the trap results in the rather substantial differences noted in the sand trap filling rates given in Table 3.3. The percentage changes in net sediment flux and trap filling rates are with respect to those values calculated using the original grid. The percentage increase in net sediment flux into the inlet shown in Table 3.3 is insignificant. However, the percentage change in the net sediment flux to the west of the
Railroad Bridge with Case 4b (see Table 3.3) is significant. This result is obviously tied to the percentage increase in the trap filling rate predicted to occur for Case 4b.
3.4.5 Modification 5
Comparisons discussed in this section are between the original grid configuration and the two cases considered with Modification
5. The predicted current pattern in the proximity of the deepened
and lengthened JID sand trap varies little from that with the original grid. The biggest difference occurs with current speed within the trap. The decrease in the predicted current speed over the deepened and lengthened trap causes the increase in the sand trap filling rate given in Table 3.3 for Modification 5, as compared with that given for Modification 3. The predicted
increase in net sediment flux into the inlet for Modification 5 compared with Modification 3 is obviously due to the increased length of the trap used in Modification 5.
Comparison of the results given in Table 3.3 for Modifications 3 5 shows the expected conclusion that the largest change in net
sediment flux into the inlet, change in trap filling rate, and change in net sediment flux past the RailRoad bridge occurs for Modification 4b, which used the deepest and largest trap. Comparison of the results for Modifications 3 and 5 with 4b show
that deepening and lengthening the trap had more affect than widening the trap on the predicted sediment transport rate. This finding was expected since the predicted increase in trap filling
rate would be more or less linear with increase in trap width, whereas the increase in trap filling rate caused by trap deepening would be expected to be greater than a linear increase. The
justification for this statement is that sediment transport capacity is proportional to the cube of the velocity; thus as the depth-averaged velocity over the trap decreases more or less linearly with increase in the trap depth, the sediment transport rate will decrease at a much higher rate.
3.4.6 Modification 6
Comparisons discussed in this section are between the original grid configuration and Modification 6. The original JID sand trap was used with this modification. The new sand trap simulated in
the modified grid did not cause any noticeable effects on the tides upstream of the trap. This is not surprising considering the relatively small size of the trap. However, the trap reduced the
predicted net upstream (past the trap) sediment flux by 55 per
cent. Thus, a sand trap at the proposed location would be very effective at reducing the movement of sand further upstream, i.e., westward.
3.4.7 Modification 7
Comparisons discussed in this section are between the original grid configuration and the two cases considered with Modification
7. The effects of dredging a navigation channel to the west of the Railroad Bridge, both in conjunction with the new sand trap considered with Modification 6 and without the new sand trap are summarized in Table 3.4. The significant difference in the
predicted filling rate for the two cases again emphasizes the effectiveness of the proposed new sand trap in reducing the movement of sand further westward. The addition of the navigation
channel and new sand trap (Case 7b) resulted in an additional (with respect to Modification 6) 8 per cent decrease in the net movement of sand further upstream. The navigation channel by itself (Case 7a) caused a 15 per cent decrease in the net upstream movement of sand. Neither case considered with Modification 7 caused any noticeable effects on the predicted tides upstream of the
trap/navigation channel. Again, this is not surprising considering the relatively small size of the trap and navigation channel.
3.4.8 Modification 8
Comparisons discussed in this section are between the original grid configuration and the two cases considered with modification 8. The ef fect of dredging a new sand trap east of the Railroad Bridge are summarized in Table 3.4. The significant difference in
the predicted net sediment flux upstream of the Railroad bridge for Cases 8a and 8b is caused by the different sediment sizes. Less sediment is transported past the trap with the 0.50 mm sand than
with the 0.20 mm sand, because the lower velocities which occur when a trap is present are not able to transport as large a quantity of 0.50 mm sand as 0.20 mm sand. Thus, one can conclude
that the trap is more "efficient" with larger sediment sizes. Neither case considered with Modification 8 caused any noticeable
Table 3.4: Results from model runs for Modifications 6, 7, and 8
Filling Rate of Navigation Channel (Mod 7)
Change in Net Sediment Flux West
(upstream) of Railroad Bridge
without new trap with new sand trap
d5 0.20 mm d50 =0. 50 mm
ef fects on the predicted tides upstream of the trap/navigation channel.
The main conclusions reached from the sediment transport modeling effort described in this report are summarized below.
Modification 1: Modification 2: Modification 3:
No significant change occurred in either the hydrodynamic or sedimentary regimes as a
result of extending both jetties 15.2 m (50 ft).
The net sediment flux into Jupiter Inlet was
rather significantly reduced (27%) as a result of extending the north jetty by 122 m (400 ft) and the south jetty by 30 m (100 ft). Deepening the existing JID sand trap to 19 ft resulted in an increase in the net sediment flux into the inlet caused by a predicted decrease in the sediment transport rate out of the inlet during ebb tides. The latter is
attributable to the predicted increase in the trap filling rate.
Modification 4: Modification 5: modification 6: Modification 7: Modification 8:
Case 4b (-19 ft bottom elevation in the enlarged sand trap) resulted in a significant
increase in the predicted trap f killing rate and decrease in the net f lux of sediment to the west of the Railroad bridge.
Deepening and lengthening the JID sand trap again resulted in an even greater (compared with Modification 3) increase in the net sediment flux into the inlet. The longer
length of the trap compared with that used in
Modification 3 caused the increase in trap filling rate, and therefore in the net sediment flux into the inlet. The new sand trap located immediately to the west of the Railroad bridge results in a
predicted decrease in the net movement of sand further to the west of 55 per cent.
The addition of the navigation channel and new sand trap resulted in an additional 8 per cent decrease in the net movement of sand further upstream. The navigation channel by itself caused a 15 per cent decrease in the net upstream movement of sand. The new sand trap east of the Railroad bridge
results in a predicted decrease in the net movement of sand upstream of the bridge of 51 per cent for the 0.50 mm sand and 29 per cent for the 0.20 mm sand.
Fig. 3.1 Proposed sand trap east of Florida East Coast
Fig. 3.2 Original finite element grid for the Loxahatchee River estuary.
Fig. 3.3 Finite element grid for Modification 1 extension
of the jetties. Only the ocean part of the grid is
Fig. 3.4 Finite element grid for Modification 2. Only
the ocean part of the grid is shown.
Predicted NOS Tides at Jupiter Inlet
- used for ocean b.c.'s 100
60 I 40
0 6 10 15 20 25 30
Fig. 3.5 Predicted tides at Jupiter Inlet for March 1-30, 1991,
used for ocean boundary condition.
IV. RELATIVE INFILLING RATES IN OFFSHORE CHANNELS
As a part of this study, two offshore channel alternatives have been considered; as shown on p. 33 of Mehta et al. (1991c), these are: a southeast channel (to be designated as Channel A), and an eastern channel (Channel B). In order to determine which of these two options is likely to be longer lasting from the point of view of refilling of the dredged region by littoral sand transport, a very simple approach involving littoral sand transport modeling was attempted, as noted below.
4.2 Basic Model
A condensed description of the generic stress model, for shallow water, is that it is proportional to h vt 2 where iL is local longshore transport sediment rate per unit offshore distance (vol/time/length), h is water depth, vL is longshore current given by Longuet-Higgins (1970). By substituting vL, the following model can be obtained:
il = c-h3 (-Lh)2 (sina)2, (l>x>0) (4.1)
where a is the angle between wave crest and shoreline, x is the distance from shoreline, c is wave celerity, cf is the constant coefficient, and 1 is the width of surf zone. Subscript b refers to breaking condition.
4.3 Beach Profile
The data (x,h) for a typical beach profile needed to evaluate it in the study area are presented in Table 4.1
The beach profile has been assumed to follow the equilibrium relation, h = A xn. By plotting ln h against ln x, the coefficients A and n can be obtained easily. In this case, A is found to be 0.13 and n is 0.59.
Table 4.1: Typical beach profile shape
x(m) 22 43 69 112 146 292 318
h(m) 0.6 1.2 1.8 2.4 2.7 3.1 3.7
x(m) 481 537 588 730 820 902 962
h(m) 4.3 4.9 5.5 6.1 6.7 7.3 7.9
4.4 Coefficient Cf
The total transport rate It can be expressed as the integral
By carrying out the integration, I, is obtained as
As5n2( sina )21sn-1
C \ c nI1 =C 5n-1
Then, the coefficient cf can be obtained as
c = J
As 2 sina 2sn-1
hb 1 HococosO
1 4 2
K = 0.78
I1 =f 21ldx
where hb is the water depth at breaking line, H0, C0 and a0 are wave height, wave speed and wave angle at deep water, respectively. Based on data relevant to the study area from the Corps of
Engineer's Wave Information Studies (WIS) (for the year 1956, 1957, and 1958), Cf = 305559 X 106.
4.5 Fillingi Time
Let us consider two cases: case 1 with time (in a typical year) starting from the month of May, case 2 with time starting from December. In other words, case 1 assumes dredging in May, and attempts to look at the subsequent refilling rate. Case 2 assumes dredging in December. The results based on above model are tabulated as following:
Table 4.2: Volumes and times (days) of offshore channel infilling
Channel A B
Vol. (yard 3) 72,000 37,000
Case 1 137 days 133 days
Case 2 54 days 26 days
The results of Table 4.2 indicate that under the pre-summer (May) dredging window (case 1), both channel would refill in about four months. Note that the capital dredging volume of Channel A is
72,000 cubic yards as opposed to 37,000 cubic yards for the shorter Channel B. Thus the rate of infilling of Channel A would be about twice that of B. These results seem somewhat surprising in view of the fact that Channel A is designed to be along the direction of a natural channel through the ebb shoal. It must be recognized
however, that one reason for the existence of a natural channel along the general alignment of Channel A is the tidal currents associated with the inlet, whose effect is not included in this model.
Table 4.2 indicates that dredging either channel during the winter window (e.g. in December) would be unproductive, especially at Channel B, which would refill in less than a month.
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