LABORATORY AND NUMERICAL STUDIES OF A PILE CLUSTER GROIN
Sean E. Mulcahy Thesis
LABORATORY AM) NUMERICAL STUDIES
OF A PILE CLUSTER GROIN
SEAN E. MULCAHY
A THESIS PESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
I heartily appreciate the guidance and patience that my advisor and supervisory committee chairman, Dr. Robert G. Dean, showed me during these past two years at the University of Florida. I also want to extend my thanks to the other members of my supervisory committee, Dr. Robert J. Thieke and Dr. Dan M. Hanes. If any other future students are reading this, sign up for Dr. Thieke's surfzone hydrodynamics class. If it is not being offered, convince him to do otherwise.
Dr. Michael Stephen, P.G., Mr. Michael Poff, P.E., and the rest of the staff at Coastal Engineering Consultants, Inc. provided invaluable information describing the existing pile cluster groins and beach history at Naples, Florida. Their perspectives, materials, and help are greatly appreciated.
I would like to thank my parents, Mike and Rita Mulcahy for their support, and I would like to assure them that I take full responsibility for my motto, "Question Everything." Unfortunately, I sometimes lack the gift of diplomacy.
I would be remiss if I did not acknowledge and thank an individual who, while I was at the University of California at Santa Barbara, helped convince me that graduate school was the correct path for me and whose recommendation afforded me this opportunity. Thank you, Dr. Sally Maclntyre.
Lastly, I want to thank my friends--the OSU and Cincy crowds, the triad in D.C., and the few I have met here at UF. Although we don't see each other as often as we would like, know that I consider us family and you all have helped me reconfirm what is really important in life.
TABLE OF CONTENTS
ACKNOWLEDGMENTS .................................................................. ii
LIST OF TABLES ........................................................................... vi
LIST OF FIGURES ......................................................................... viii
A B STR A C T .................................................................................. xiv
I INTRODUCTION .................................................................. I
Purpose of this Study ............................................................... I
Descriptive Terms .................................................................... 2
2 BACKGROUND ..................................................................... 6
Characteristic Behavior of a Permeable Groin ................................... 6
Previous Studies on Permeable Groins ............................................ 7
The Naples Experience .............................................................. 14
Past Performance of Naples Pile Cluster Groins ................................. 19
3 LABORATORY EXPERIMENTS ................................................ 38
Wave Height Reduction Experiments ............................................. 39
Drag Force Experiments ............................................................. 42
Beach Profile Response Experiments .............................................. 52
4 NUMERICAL MODEL ............................................................. 71
Numerical Model Ideology .......................................................... 71
Numerical Model Results ........................................................... 78
Laboratory Trials ............................................................ 78
Wave Height Results ........................................................ 84
Incident Wave Angle Results ............................................... 91
Groin Permeability Results ................................................. 95
Groin Width, Placement, and Multiple Groins Results ................. 101
5 DESIGN RECOMMENDATIONS ................................................ 109
Groin Length .......................................................................... 109
Pile Depth .............................................................................. 109
Groin Spacing ......................................................................... 109
Groin Permeability and Groin Width .............................................. III
6 SUMMARY AND CONCLUSIONS .............................................. 115
APPENDIX DERIVATION OF NUMERICAL MODEL EQUATIONS AND THE NUMERICAL MODEL COMPUTER CODE .............................. 120
Continuity Equation .................................................................. 120
Conservation of Momentum Equation .............................................. 121
Numerical Model Computer Code ................................................. 124
REFERENCES ................................................................................. 130
BIOGRAPHICAL SKETCH ................................................................. 132
LIST OF TABLES
2.1 Wave Height versus % Occurrence from 1956 to 1975
WIS Station 43, CERC (1989) ..................................................... 17
2.2 Deep Water Wave Conditions, WIS Station 43, CERC (1989) .................. 18
2.3 Longshore Sediment Transport Rates ............................................... 19
3.1 Recorded Water Depths (meters) for Trial 1 ....................................... 47
3.2 Recorded Water Depths (meters) for Trial 2 ....................................... 47
3.3 Values for the Flume Friction Factor, f, as Calculated
from Trial 1 and Trial 2 Data ........................................................ 50
3.4 Calculated Values of CD for Trial 1 (Shallow Water) and
Trial 2 (D eep W ater) ................................................................. 52
4.1 Values for the Eddy Diffusivity, e, Given the Incident Wave
Conditions and P Value Used in the Numerical Model ......................... 76
4.2 Hydrodynamic Characteristics of the Trials Representing
The Laboratory Environment ....................................................... 83
4.3 Hydrodynamic Characteristics of the Field Environment
with a Single Groin for Varying Wave Heights ................................. 85
4.4 Hydrodynamic Characteristics of the Field Environment
with a Single Groin for Varying Incident Wave Angles ......................... 91
4.5 Hydrodynamic Characteristics for the Field Environment
with a Single Groin for Varying Groin Permeability ............................ 96
4.6 Hydrodynamic Characteristics for the Field Environment
with a Single Groin of Standard and Double Widths
for Varying Wave Heights .......................................................... 102
4.7 Hydrodynamic Characteristics of Multiple Groins for the Field
Environment with Varying Spacing .............................................. 104
LIST OF FIGURES
1.1 Definition sketch of the beach profile,
from (Larson & Kraus, 1989).............................................. 3
1.2 Two types of permeable groins found on Naples Beach; Pile Cluster
Groin (Background) & Slotted Timber Groin (Foreground)................. 4
2.1 Schematic of the beach profile & the associated typical
longshore current velocity, from (Raudkivi, 1996)......................... 9
2.2 Typical beach profile response from permeable pile groins
along the Baltic Coast; note the raised profile within the groin
but the loss of sand at the groin's toe, from (Raudkivi, 1996) ..............12
2.3 Quartz sand and shell constituents of Naples Beach ......................... 16
2.4 Plan view of Naples Beach with locations of existing groins,
from (CEC, 1994)......................................................... 22
2.5 Design schematic of Naples pile cluster groins, from (CEC, 1994)........... 23
2.6 Design schematic of Naples timber groins, from (CEC, 1994) ............... 24
2.7 A photograph of three pile cluster groins (pcg) south of the Naples Pier;
note the vegetation line, the shoreline, and the offshore bar near
the pcg's, Also note the oblique bar on the southern side of the
middle groin............................................................. 29
2.8 Aerial photograph of the southern end of Naples Beach; note
Gordon Pass at the top of the photo, the three pile cluster groins (pcg),
the multiple timber groins, the offshore bars near the pcg's, the
oblique secondary bars on the southern side of the pcg's, and the
vegetation and dry beach prominences at the pcg's ........................ 30
2.9 Aerial photo of Naples Pier; note the location of the dry beach
prominence slightly below (south) of the pier, the vegetation
prominence slightly north of the pier, and the seaward turn of the
offshore bar at the pier ................................................... 31
2.10 Pile cluster groin features; note the dry beach prominence
(southern side of the groin/ towards the top of the photo) and
the vegetation line prominence (northern side of the groin/ towards the bottom of the photo), also notice the slight turn
seaward of the offshore bar near the groin .......................................... 33
2.11 A combination pile cluster groin and timber groin at Naples Beach;
note the shoreline prominence along the left side of the groin and
the incident wave angle, the missing piles in the ocean section
of the groin, and the sea birds attracted to the schools of bait fish
draw n to the groin ..................................................................... 34
2.12 Another pile cluster groin along the southern stretch of Naples Beach
(looking north); note the low pile section for pedestrian travel, the high
water line, the vegetation line, and the landward extent of the groin
which is susceptible to flanking during storm events ............................ 34
2.13 A view northwards from Naples Pier; recall that the pier acts as a
permeable groin and notice the shoreline bulge in its region of
influence; Also note the dunes and the vegetation that now obscure
the seaw all .............................................................................. 35
2.14 A view southward from the pier; again, note the shoreline and
vegetation line bulge in the groin's region of influence .......................... 35
2.15 A photograph of structural damage caused by the 'No Name Storm'
of 1982 .................................................................................. 36
2.16 A photograph of Naples Beach after the 'No Name Storm'; note that
this area is free from any pile cluster groins and the shoreline had
retreated all the way to the seawall .................................................. 36
2.17 A photograph of the shoreline in the vicinity of a pile cluster groin;
note the presence of a dry beach on either side of the groin and the
vegetation line prominence on the northern (left) side of the
of the groin in response to the SW waves during the storm ..................... 37
2.18 A photograph of the shoreline in the vicinity of Naples Pier; note the
dry beach on either side of the pier and the dry beach prominence on
the northern (left) side of the pier, Also note the shoreline rapidly
transitions to the seawall at the far left of the picture ............................ 37
3.1 Schematic of the wave gauge positions relative to the groin for
the wave height reduction experiments ............................................ 40
3.2 Schematic of the modified flume used in the drag coefficient
experim ents ............................................................................ 44
3.3 Plot of recorded water levels along the flume channel during
Trial 1 for a varying number of pile sections .................................... 48
3.4 Plot of recorded water depths along the flume channel during
Trial 2 for a varying number of pile sections .................................... 48
3.5 A schematic of the Departmental wave basin used in the beach
profile response experiments ....................................................... 53
3.6 A photograph of the 1:10 scale model groin used in the experiments ........... 55
3.7 Topographic surface plot of the initial beach for Trial 2; this beach
had been subjected to twelve hours of wave action; The groin was
placed in the most uniform region of the beach profile ......................... 62
3.8 Topographic surface plot of the beach after 4 hours of wave action
during Trial 2; note the seaward migration of the shoreline
especially near the groin, the formation of an oblique offshore bar
on the updrift side of the groin, the finger-like projections of the
submerged profile immediately downdrift of the groin, and the
fluvial style deposit at the toe of the groin ........................................ 63
3.9 Topographic surface plot of the beach after 8 hours of wave action
during Trial 2; note the advancement of the shoreline, the
continuing presence of the updrift and oblique offshore bar, and
the finger-like projections immediately downdrift of the groin ................ 64
3.10 Plot of the SWL and the +4 cm contour for the initial beach and for
the 4-hour interval; note the large seaward gains in the SWL's cross-shore position immediately updrift and downdrift of the
groin after 4 hours of the groin's presence ........................................ 65
3.11 Plot of the SWL and the +4 cm contour for the initial beach and for
the 8-hour interval; note the continuing seaward gains of the shoreline
immediately updrift and downdrift of the groin and the recession
of the +4 cm contour downdrift of the groin producing a milder
beach slope ........................................................................... 65
3.12 Plot of the SWL and the +4 cm contour for the 4-hour and 8-hour
intervals; note that the SWL shows some fluctuations but little net transition while the +4 cm contour receded downdrift of the
groin resulting in a milder beach slope ........................................... 66
3.13 Deposition and erosion contour plot (4-hour minus the Initial Beach);
note the formation of the oblique offshore bar updrift of the groin, the
heavy deposition inside of the surfzone immediately updrift of the
groin, the heavy deposition immediately downdrift of the groin, the
less drastic deposition between the offshore bar and the shoreline and underneath the groin, the deposition in deeper waters underneath the groin, and the overall increase in elevation of the beach profile in the
region of study .......................................................... 67
3.14 Deposition and erosion contour plot (8-hour minus Initial Beach); note
the region surrounded by the offshore bar, the immediate updrift shoreline, the downdrift finger-like deposition, and the offshore
deposition underneath the groin .........................................68
3.15 Deposition and erosion contour plot (8-hour minus 4-hour); note the
expanse in white suggesting that the profile may be reaching
equilibrium, the slightly erosive regions underneath the groin, the
smoothing of the offshore bar, and the continuing deposition
downdrift of the groin.................................................... 69
4.1 A schematic of the grid used in the numerical model
4.2 Plot of the effects of P on the cross-shore distribution of the
alongshore current velocity; P = 0.25 was used in the model .............. 77
4.3 Hydrodynamic characteristics and depositional pattern calculated
by the numerical model for the laboratory environment without the
presence of a groin....................................................... 79
4.4 Water velocity vector plot for the laboratory environment without
the presence of a groin................................................... 80
4.5 The hydrodynamic characteristics and depositional pattern calculated
by the numerical model with the presence of a groin centered at
6 meters and extending 8 meters seaward ................................ 81
4.6 Water velocity vector plot for the laboratory environment with
the presence of a groin ................................................... 82
4.7 Set-up and set-down induced by the presence of a groin for three
different wave heights- 0.5, 1.0, and 2.0 meters........................... 84
4.8 Surface plots for the alongshore flows and water velocities for varying
wave heights (groins centered at 60 mn and extend 80 mn seaward); note
the influence of the drag force of the piles at the toe of the groins
especially for the smaller wave heights................................... 86
4.9 Plots of the cross-shore water velocities and the depositional patterns
associated with the field environment with a single groin for varying
wave heights ............................................................ 87
4.10 Water velocity vectors for the field environment with a single groin
for varying wave heights; note the eddies for the 0.5 meter wave and
the dominance of the longshore current as the wave height and
surf zone width increase ................................................. 89
4.11 Plots of Eta and the depositional patterns associated with the field
environment with a single groin and varying incident wave angles ......... 92
4.12 Surface plots of the alongshore and cross-shore water velocities
for the field environment with a single groin for varying incident
wave angles .............................................................93
4.13 Water velocity vectors for the field environment with a single groin
for varying incident wave angles.......................................... 94
4.14 Surface plots of the cross-shore water velocities in a field
environment with a single groin with varying permeabilities............... 97
4.15 Water velocity vector plots for the field environment with a
single groin for varying groin permeability...............................98
4.16 Plot of Vx at the centerline of groins of varying permeability............... 98
4.17 Plots of the depositional patterns for the field environment with a
single groin for varying groin permeability ............................... 99
4.18 Plots of the set-up and set-down for the field environment with
multiple groins spaced closely and far apart; it should be noted that
H=2.0 mn for the four groin plot and H=1.0 mn for the two groin plot ....... 103
4.19 Plot of the set-up and set-down associated with the field environment
for two groins spaced sufficiently close to interfere with each other........ 103
4.20 Surface plots of longshore current velocities for the field environment
with multiple groins and varied spacing .................................. 106
4.21 Water velocity vectors for the field environment for multiple groins at
varied spacing; the groins are centered at: '2 Groins close'
(20 m and 100 in), '2 Groins far' (60 m and 210 in),
'4xW Groin' (60 in), '4 Groins' (40, 60, 80, and 100 in) ........................ 107
4.22 Depositional patterns for the field environment with multiple groins
w ith varied spacing .................................................................... 108
5.1 Plan view of Naples Beach in proximity to PCG-18-1; note the
mean high water lines at 6/96 (post construction) and 6/99 and
the effective length of the longshore influence of the structure ................. 113
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science LABORATORY AND NUMERICAL STUDY OF A PILE CLUSTER GROIN
Sean E. Mulcahy
Chairman: Dr. Robert G. Dean
Major Department: Civil and Coastal Engineering
This study was conducted to develop an improved understanding of the
hydrodynamics and sediment transport characteristics in the vicinity of a pile cluster groin. A scale model of a permeable pile cluster groin designed for construction in Naples, Florida was fabricated. Laboratory studies were performed to measure the effects of the pile cluster groin on incident wave height reduction, longshore current head loss, and sediment depositional patterns. A numerical model quantifying the hydrodynamics of the pile cluster groin and related effects on sediment transport was developed to provide a comparison with laboratory results. The versatility of the numerical model also provided the opportunity to examine the effects of the groin when subjected to a widely varying set of input conditions. The numerical model also permitted the examination of the effects of multiple groins and facilitated the characterization of design parameters for these pile cluster groins.
After performing the wave height experiment, it became apparent that the wave
height decay was so small it could not be recorded by our instruments. For this particular pile cluster groin, the effects on wave height decay were small. The flume experiment measured the hydraulic head loss induced by the groin on a steady and uniform flow. Based on these measurements, a realistic drag coefficient was computed for the piles. The beach profile response experiment was conducted to map the sediment deposition and erosion patterns caused by the groin. The model groin was placed on an equilibrium beach profile and subjected to eight hours of waves. Profile surveys were taken at an initial time and then again at both four and eight hours. Topographic maps were created that showed the depositional and erosional features within the vicinity of the groin.
The numerical model developed was based on discrete representations of the
continuity and momentum equations. The model operated on a simple planar beach. The input variables included the incident wave conditions, the groin location and characteristics, beach slope, and time step parameters. By changing the boundary conditions along the grid boundary, the model could represent both an enclosed basin and an open beach. In both versions, the free surface elevation (il) and the long-shore and cross-shore flows per unit length were calculated. These values were then used to quantify sediment transport in the vicinity of the groin and determine areas of deposition and erosion along the beach profile.
The flexibility of the numerical model perinitted the examination of different
combinations of input variables and provided the opportunity to establish general design parameters for pile cluster groins.
Purpose of this Study
The retention of sand in beach systems has been and will continue to be of paramount interest in the field of coastal engineering. Beaches offer protection to property and structures and are also a favorite recreational site for the public. They also provide their own unique ecosystem. The state of Florida has large private and public economic interests in its beaches, and the understanding of the processes of sand retention and transport along its shores is of grave importance. Since the birth of the coastal engineering field, the influence of natural and man-made structures on both the adjacent and distant shorelines has been investigated. This thesis discusses the role and effects of a pile cluster groin on the longshore shoreline position and the submerged beach profile. A more thorough understanding of this type of structure will provide the opportunity for better management of Florida's shorelines.
The hydrodynamics and influences on sediment transport of the pile cluster groins are currently understood on an empirical level. Groins that have been constructed in the past have been subjected to monitoring surveys to evaluate their performance. Also, laboratory studies have been conducted on the influence of these groins on incident waves and longshore currents. This thesis examines the role that a specifically designed pile cluster groin has on sediment transport and deposition. Laboratory studies were
performed to observe the effects of a pile cluster groin on both the planform of the beach and the beach profile. Experiments were also performed attempting to quantify certain hydrodynamic characteristics of the groin. A numerical model of the hydrodynamics associated with the groin was developed with the attempt of corroborating the laboratory results and aiding in identifying optimal design parameters for this type of groin.
It was hoped that certain questions about the groin effects could be answered. How does the longshore current respond to a permeable barrier? Does the structure create cross-shore flows? Does the structure influence the incident wave conditions in its proximity? How does the structure affect sediment transport in the region? Does the groin have a negative impact on the surrounding shorelines? How does the groin perform during high energy storm events? What effects does the structure have on the underlying profile? What are the basic design parameters for this style of groin? And lastly, are pile cluster groins an effective tool for beach preservation? This study was designed to answer these questions.
The shoreline along the ocean or other large bodies of water frequently consists of many distinct regions and features. A cross-section taken perpendicular to the shoreline is called a beach profile (Figure 1. 1). The profile is usually divided into four sections: the offshore, the nearshore, the beach, and the coast. Waves, propagating from the offshore region, break in the nearshore zone and mold the beach profile. The sloping bottom of the profile gradually shoals the waves, and the waves break when the wave height is approximately 0.78 times the water depth. The submerged profile seaward of wave breaking is denoted as the offshore while the landward submerged profile is called
HWL: HIGH WATER LEVEL LWL. LOW WATES LEVEL
Figure 1.1 Definition sketch of the beach profile, from Larson and Kraus (1989)
the surfzone. Bars often form in the region of wave breaking. Broken waves continue to propagate landward, and the remaining wave energy is dissipated in the swash zone, or foreshore, as the waves rush up this steep portion of the profile. The backshore may include depositional features known as berms which are created by sediment deposited by the wave runup. The landward boundary of the shore is usually delineated by a row of sand dunes. These form from the accretion of wind-blown sand from the beach. The dunes are frequently habitats for various forms of vegetation that trap and collect the wind-blown sand. The vegetation line often lies in this region. The terms updrift and downdrift refer to the regions adjacent to the structure or any other point along the beach. These regions are delineated by the predominant incident wave direction and the
direction of the longshore sediment transport. Viewing from the beach, if the longshore current is traveling to the right, then the left side of the structure would be referred to as the updrift side. Conversely, the right hand side of the structure would be referred to as the downdrift side.
Figure 1.2 Two types of permeable groins found on Naples Beach; Pile Cluster Groin
(Background) & Slotted Timber Groin (Foreground)
The pile cluster groin is a permeable (to both water and sand) structure that
usually extends perpendicular to the shore seawards through the nearshore zone. A single row or multiple rows of piles are driven into the seabed. Pile density refers to the number of piles per unit area. The permeability of the groin describes the groin's resistance to flow through it. High permeability infers a very low resistance to longshore flow, whereas, low permeability denotes a very high resistance to flow. The permeability of a pile cluster groin is a function of the pile density and the arrangement of the piles. Here
permeability is quantified by the ratio of open area over the total cross-sectional area of the groin when viewed from the side, expressed in percent.
Characteristic Behavior of a Permeable Groin
There are many designs and modes of operation of permeable groins. Some
permeable groins simply consist of piles or metal sheeting driven into the sand at spaced intervals. Others have more complicated designs with interior holes that provide their permeability. One design is based on a low level top that permits the swash to constantly overtop the groin. These permeable groins function much like weirs. In all cases, properly functioning permeable groins allow, to some degree, water and sediment to pass through them.
Non-permeable groins act as a complete barrier to longshore sediment transport. They are often employed at the ends of a littoral cell and are termed "terminal structures", but they have also been used along the open beach. Since they stop the longshore flow of sand, they cause heavy accretion along their updrift shoreline and an equivalent amount of erosion along their downdrift shoreline. Because of this negative downdrift effect, they must be employed very carefully along the open beach.
Since permeable groins permit both water and sediment to sluice through, they retard the longshore current in their vicinity and decrease the ability of the current to transport sediment. They provide a much more uniform shoreline than the saw-tooth pattern associated with their non-permeable counterparts, and they do not contribute to
the severe erosion associated with the complete blockage of sand from the downdrift shoreline. Nevertheless, permeable groins are not immune to unfavorable effects on the littoral system. Depending on the sea conditions and the groin's design, strong seaward flows can form along their updrift side, and the longshore current velocity can be significantly greater at their seaward end than if the groin were not present. Both of these can cause seaward sediment transport and deposition in deeper waters.
Previous Studies on Permeable Groins
Even though permeable groins have been constructed for centuries along some of the world's coastlines, there is very little literature discussing their effects on the neighboring littoral system. Some sites receive significant benefits and beach accretion while others continue to erode after the installation of the groins. Much of the government's and the public's disapproval of groins can be attributed to highly publicized cases where the construction of groins had the opposite effect than that for which they were intended (Kraus, et al., 1994). A major reason for these failures is the lack of a full understanding of the groin's hydrodynamic effects on the adjacent beach. It is thought that groins are site specific structures, and their implementation requires a thorough study of the local wave climate, beach sediment transport characteristics, and beach profile bathymetry at the site.
It has been shown that with careful design, permeable groins can have many
beneficial qualities for shoreline protection. In the laboratory, the effects of permeable groins on tidal and wave induced currents has been explored (Bakker, et al., 1984). In the field, systematic surveying of numerous beach profiles has shown shoreline accretion in the presence of these groins (Trampenau, et al., 1996, Price, et al., 1972). The major
benefits of permeable groins include: low construction and maintenance costs, reduction in both tidal and wave induced currents, decrease in longshore sediment transport, more uniform shorelines (no saw-tooth patterns), decreased intensity of rip currents along the updrift side of the structure, and reduction in erosion on the leeward side of the groin (Bakker,et al., 1984; Raudkivi, 1996).
In most cases, groins are implemented as a means to control sediment transport in the nearshore. Permeable groins do not impound sand directly, but their influence on the water column causes significant changes in the column's ability to entrain and transport sediment. Pile cluster groins result in a hydraulic resistance to the longshore current, thereby retarding the current's velocity. This reduction in velocity inhibits the production of turbulence at the seabed which reduces the amount of sediment in suspension and the thickness of the suspended sediment layer. The net result is a significant reduction in the capacity of the longshore current's ability to transport sediment (Raudkivi, 1996).
Laboratory studies were performed by Bakker, et al. (1984) to observe the resultant longshore current profile in the presence of a permeable pile groin. The experiments used separate trials to isolate the effects of groin spacing and groin geometry. Bakker also examined the influence of a groin on a uniform current and a combination of a uniform current superimposed with regular waves. The groins consisted of individual piles arranged perpendicular to shore with increasing permeability towards the seaward end of the groin (approximately 5%-50%) (Bakker,et al., 1984).
Current velocities, without the presence of waves, were reduced to 50% inside the length of the groins. The current velocities produced by both a uniform current and regular waves decreased to 65% of their original values. These results are obviously
Figure 2.1 Schematic of the beach profile & the associated typical longshore current velocity, from Raudkivi (1996)
dependent on the arbitrary pile permeability chosen by Bakker et al., but some valuable general conclusions can be inferred from his uniform current experiments. While the groin significantly reduces the longshore current velocity within its reach, it causes an increase in the longshore current velocity just seaward of its tip. The longshore current velocity is the smallest at the beach and increases seaward (this is most likely due to the increasing pile spacing). Lengthening the groins will extend the zone of the longshore current velocity reduction, but this is partly at the expense of the decrease in reduction of the current velocity closer to shore. Groins consisting of two rows of piles are more effective than single row groins whose longshore spacing is half the longshore spacing between the double row groins. The presence of the groin causes a seaward directed flow along its updrift side (Bakker, et al., 1984).
The introduction of waves onto the uniform current produced some different key observations inside the surfzone. Because the regeneration of the uniform current downdrift of the groin results from a slow diffusion of mass and momentum, a "transition distance" is required before the effects are felt at the shoreline. However, if breaking waves are present, the turbulent energy and momentum transfer produced quickly regenerate the longshore current inside the surfzone. Therefore the reduction in the current velocity inside the surfzone is not as great as when only a uniform current was present. However, outside of the surfzone, the reduction in the longshore current velocity is greater in the presence of waves. This is due to the orbital motion in the plane of the pile screens providing an added resistance to the longshore current (Bakker, et al., 1984).
Permeable pile groins can also affect the wave energy in their vicinity. Very low permeability groins behave as oblique breakwaters and can significantly alter the wave climate along the shore. The waves propagating near a permeable groin can be divided into four categories: incident wave, transmitted wave, reflected wave, and diffracted wave. For most engineering concerns, this wave climate can be simplified greatly. The permeable pile groin's effectiveness as a breakwater is highly dependent on the incident wave direction and the spacing between the piles. As long as the obliquity of the incident waves to the groinis not large, the diffracted and reflected waves are negligible. Since most groins are constructed near the shore and the refracted waves are shoaling ever more parallel to the shoreline, most pile groins contribute little to the reflection and diffraction of the incident waves. Using Raudkivi's transmission coefficient,
KtV = 0.5(1 B2 )112 (1 + cos 2 /)
K = Transmission Coefficient
B = Complement of Groin Permeability; (1 Permeability) /3 = Incident Wave Angle to the Normal to the Groin's Side
a 10% groin permeability leads to halving the transmitted wave height when the waves approach parallel to the groin (Raudkivi, 1996). The high permeability of the Naples' pile cluster groins and the local wave conditions support the assumption that the incident wave propagates relatively unhindered through the pile cluster groin.
The sparse literature discussing the performance of permeable pile groins in the field has been favorable. The shorelines along the southern coast of the Baltic Sea have seen a large seaward migration of the mean still water line (msl) and a significant rise in the elevation of the submerged profile inside the reach of the groins. There was noticeable erosion just seaward of the ends of the groins, but this was expected due to the increased velocity of the longshore current at this point. Low permeability groins accumulated sand at faster rates than their higher permeability counterparts, but there was large sediment deposition everywhere within the groin field. The downdrift beach did not suffer any erosion due to the retained sand within the groins (Raudkivi, 1996; Trampenau, et al., 1996).
Permeable pile groins have been implemented along the Netherlands coast for
hundreds of years. Monitoring of existing and more recently constructed permeable pile groins has shown their effectiveness (Bakker, et al., 1984). Some stretches have shown significant gains in beach volume while others have eroded. This raises a valid point that permeable groins do not always stop beaches from eroding. Sediment transport is
Figure 2.2 Typical beach profile response from permeable pile groins along the Baltic coast; note the raised profile within the groin, but the loss of sand at the groin's toe; the groins were installed in 1991, from Raudkivi (1996)
dominated by the local wave climate, and if the wave energy is too severe, permeable groins will not be able to counteract the erosive capabilities of the waves. They do, however, reduce the erosion below that rate which would be present if the groins were not in place along the beach.
Permeable pile groins are susceptible to certain dangers that require some design considerations. Even though they are permeable, the groins inevitably create a set-up along their updrift boundary. This set-up is smaller than if they were non-permeable structures, but the set-up still causes an offshore flow of water. This seaward flow can be substantial enough to erode a deep return channel along the updrift side of the groin and present a threat to human safety (Bakker, et al., 1984). This return flow can also cause a cross-shore transport of sediment out of the littoral system, at least temporarily. The greater the permeability of the groin, the less set-up is created, but this is at the expense of the reduction in the longshore current velocity. It is suggested that the groin have moderate permeability inside the swash zone to eliminate excessive set-up at the groin (Raudkivi, 1996).
Although pile groins reduce the longshore current velocity, the flow constrictions, due to pile spacing, create a localized acceleration of the water through the piles (Bakker, et al., 1984). This acceleration can be large enough to produce significant scour around the base of the piles. Groins along the Baltic coast routinely had 0.5 meter deep scour holes extending 1.5 meters to either side of the groin (Raudkivi, 1996). Mussle growth, or other marine life, can exacerbate this problem (Bakker, et al., 1984). The augmented longshore current velocity at the seaward end of the groins presents scour problems of its own. The Netherlands has experienced scour to a depth that the piles were unstable (Bakker, et al., 1984). These scour problems can be combated by driving the piles deep enough into the underlying seabed. Raudkivi suggests at least 60% of the pile should extend farther than the lowest expected seabed level.
Major storms can present problems for the groins also. With storm surge and high energy waves, the landward end of the groin may present problems. If the high waterline extends past the landward end, outflanking can occur (Bakker, et al., 1984). During outflanking, the landward end of the groin can become a hotspot for erosion. Piled up water can form a channel around the landward end of the groin, and severe localized erosion of sediment can occur. Practicality is ultimately the decisive factor, but the landward end of the groin should extend well inland of the expected high water line (hwl). Another problem with excessive storm surge is the overtopping of the groins. If the groins are submerged, they lose some of their effectiveness (Raudkivi, 1996). However, the practicality of cost and aesthetics will normally determine the top elevation of the groin.
The Ngples Experience
Naples is located along the southwestern coast of the State of Florida. Naples' beach lies on an approximately north-south azimuth along the Gulf of Mexico. The region is micro-tidal. The northern and southern boundaries of the littoral cell are delineated by Doctors Pass and Gordon Pass, respectively. These inlets are held fixed by rock jetties or terminal groins which serve as impermeable boundaries inside the surfzone. However, there is some sediment transported around the ends of the jetties and deposited inside the inlets.
Naples' residential boom occurred after World War Two, and heavy development ensued along its shoreline. The city constructed a seawall in the early 1950s which is still present today. When the wall was first constructed, it was in the active beach zone. Permeable timber groins were built in late 1952 and the beach accreted within this area. The groins are present in aerial photographs dating from the 1950's and are common in many old pictures in City Hall. Many local long-time residents contend that the groins created a usable dry beach and continue to serve to stabilize the shoreline today. This view is also held by city officials, beach committee members, and local coastal engineers. The groins were originally constructed before strict shoreline construction regulations were implemented by the State. Due to their age and timber construction, some of the groins have since fallen into disrepair. Local residents and city officials believe that the groins provide a strong backbone protecting their beach, and they wish to construct new groins in place of the old. There has been little quantitative explanation on how the groins work, but history at this site suggests their beneficial side effects. This study was
performed to more fully understand the physical influence of the groins on the littoral system and to provide design guidance.
In May 1996, Naples conducted a beach restoration project. Presently, the majority of the Naples beach is 90-140 feet wide, with an average of 120 feet, as measured from the seawall to the mean high water line (mhw). The beach progressively narrows south of the timber groin field until the swash of the waves impinges on the seawall just north of Gordon Pass. The Naples' beach consists of quartz sand and shell. The sand mean diameter is 0.20 millimeters with a range from 0. 16 to 0. 33 mm (See Figure 2.3). There is an offshore submerged bar that runs parallel to the shoreline approximately 150-200 feet from the mean high water line (mhw). The landward position of the beach is marked by the seawall constructed in the early 1950's. Where there is a significantly wide dry beach, the back beach contains dunes and dune grass vegetation. A relatively level berm extends from the dunes to the active beach face where the profile steepens due to the incoming breaking waves.
The wave climate at Naples is normally fairly benign, and the wave direction can vary daily. Major storm events usually create waves from the southwest because that is the most likely location for tropical lows. Cold fronts, however, usually generate waves from the northwest. The Coastal Engineering Research Center, CERC, U.S. Army Corps of Engineers conducted a wave information study, (WIS), from 1956 to 1975 to generate a wave climate for the Gulf of Mexico. This study produced a time series of wave height, period, and direction for multiple stations throughout the Gulf of Mexico. Station 43 is in close proximity to Naples and serves as a good record of deep water wave heights for the Naples region. The WIS shallow water wave model was then used to compute the 20
Figure 2.3 Quartz sand and shell constituents of Naples Beach
year wave statistics including wave height, wave period, and direction. Table 2.1 includes the wave height and percent occurrence for the 20 year WIS data for Station 43. Coastal Engineering Consultants (CEC), a local coastal engineering firm in Naples sponsored this study on behalf of Collier County and used this information to generate wave heights, wave periods, and their percent occurrences along radial directional increments of 22.5 degrees (See Table 2.2). The first value for each wave angle represents "typical" wave conditions. The second value corresponds to low frequency "storm" events (CEC, 1994).
Because of the typically benign wave conditions and the varying wave direction, Naples experiences a low net littoral transport. The net transport is directed from the north to the south and is due to the more frequent occurrence of northwest waves. CEC generated the wave information presented in Table 2.2 and the applied energy flux method to calculate general longshore transport rates (CEC, 1994). Information from beach surveys and dredging records were also used to calculate the longshore transport.
Wave Height (FT) % Occurrence
Table 2.1 Wave height versus % occurrence from 1956 to 1975 V/IS Station 43, CERC (1989), Table data taken from CEC (1994)
These different sources of information provided values that agreed in order of magnitude and direction (CEC, 1994). It should be noted that the longshore sediment transport is sensitive to site specific characteristics such as local shoreline orientation, wave interaction, and seabed constituents. Longshore sediment transport information is provided in Table 2.3.
Specific historical information on the original design basis and construction of the groins is limited. There are four pile cluster groins (see Fig. 2.4 and 2.5) along the Naples' beach. One pile cluster groin lies north of the Naples Pier. The other three are fairly evenly spaced along the southern stretch of beach. The pier itself also behaves as a large permeable groin. Permeable timber groins (see Fig. 2.6) are also present along the southern half of the shoreline. Timber groins north of the pier have been removed due to disrepair. Both types of groins are dilapidated and suffering from decay.
It appears that the timber groins were constructed as secondary structures to influence the sand within each reach of the pile cluster groins. They are much less permeable than the pile cluster groins, but at approximately 50% permeability, they are
still highly porous. Their individual length varies, and they are much shorter than the pile cluster groins. Most of the timber groins do not extend through the surfzone. The Naples' pile cluster groins consist of wooden piles driven to stable depths in the sand. Rows of two or three piles are staggered alternately throughout the length of the groin. The groins are ten feet wide and extend to varying lengths out into the sea. All of them originally extended past the surfzone, but due to decay some have fallen into the sea and some interior sections have collapsed. The rows of piles are spaced at five feet (ze 1.52 meters) the surfzone, but due to decay some have fallen into the sea and some interior
Wave Wave Wave%
Angle Height Period Occurrence
a. (0 H0 (ft) T (sec)
-90.0 2.66 4.33 2.826
-90.0 5.08 7.15 0.097
-67.5 2.72 4.42 2.215
-67.5 4.99 7.21 0.103
-45.0 2.53 4.44 3.740
-45.0 4.59 7.13 0.240
-22.5 2.56 4.32 3.029
-22.5 4.56 7.41 0.674
0.0 2.62 4.19 2.369
0.0 3.84 7.61 1.097
22.5 3.02 4.32 2.983
22.5 4.69 7.67 0.470
45.0 2.99 4.36 2.481
45.0 4.99 7.31 0.060
67.5 3.18 4.41 2.321
67.5 5.05 7.34 0.017
Table 2.2 Deep water wave conditions, WIS Station 43, CERC (1989), Table data taken from CEC (1994)
Longshore Transport Naples North of Pier Naples South of Pier
QNORT. (yd3 / year) 128,300 132,550
Qso,0r (yd3 / year) 162,700 158,750
QGROSS (yd3 / year) 291,000 291,300
QNET (yd3 / year) 34,400 26,200
Table 2.3 Longshore sediment transport rates, Table data taken from CEC (1994)
sections have collapsed. The rows of piles are spaced at five feet (Z 1.52 meters) increments. The diameter of the piles is one foot (Z .305 meters), and they are fashioned out of timber. The groins have a permeability of 80% (CEC,1994) Past Performance of Naples Pile Cluster Groins
A beach is a dynamic environment. The beach profile continually tries to adjust toward an equilibrium state depending on the incident wave climate. Coastal structures are used to alter the wave climate, to influence the dissipation of wave energy, and to control sand flows. As the wave climate changes, the beach is forced to respond. In the early 1950's, Naples' residents constructed a seawall to protect their property from the ocean. At that time, there was not a dry beach. Witnesses testify that after the groins were constructed, the beach began to accrete and build out. The groins retarded the longshore current enough to permit the deposition of suspended sediment along the shoreline. In time, a dry beach was created. The permeability of the groins enabled sediment to travel freely along the shoreline. This was the primary cause for a uniform shoreline and the absence of severe downdrift erosion that would have threatened the integrity of the seawall.
Permeable groins do not directly catch and trap sand. They only decrease the ability of the ocean to transport it. If the wave energy is great enough, erosional trends and events will persist over the effects of the groin field. Such was the case for Naples' beach during the 'No Name Storm' of 1982. Prior to the storm, there was a usable dry beach approximately 75 to 100 feet wide. After the storm, during which the waves were six feet high and the storm surge was five feet, long sections of Naples' beach were submerged up to the seawall (CEC, 1994). This was true along the whole stretch of shoreline except within the vicinities of the pile cluster groins and the Naples' Pier. At these locations there was still a dry beach- why? (Stephen, 1982) (See Figures 2.15
Even though the groins and pier could not stop the storms erosion, they could mitigate its effects. The beach in their proximity still suffered significant losses from erosion, but not enough to completely deplete the sand reserves of the dry beach (See Figures 2.16, 2.17, and 2.18). This was proof that not only did the groins act as sand accretion agents during mild wave conditions- they provided a reservoir of sand and served as erosion control structures during severe wave conditions. They also accomplish their task in such a manner as to not adversely affect the downdrift shoreline as greatly as their non-permeable counterparts. The 'No Name Storm' served as a powerful testament to the usefulness of such structures despite the common negative connotation often associated with groins along the open beach.
Collier County conducted a beach restoration project from November 1995 to
May 1996. The region of restoration extended from Doctors Pass southward to just north of monument R-78 (See Fig. 2.4). The beach was nourished with sand dredged from four
offshore borrow areas. The average fill density was 42.6 cubic yards! linear foot) and provided an additional dry beach width of 80 to 120 feet. The total fill volume was 759,150 yd 3, and the project length was 17,800 feet. Dune vegetation was planted along most of the landward boundary of the beach.
Annual surveys have been performed to monitor the beach profile and the
project's performance. Special interest was taken in the surveys near PCG-1 8-1 which is the only pile cluster groin north of the pier and within the project area. This groin was restored so its effects on the beach fill could be monitored. In May 1997, the beach updrift of the groin experienced an average recession of 7.5 feet while the downdrift beach had an average accretion of 0.9 feet. The average shoreline adjustment for this reach was 3.3 feet of recession. This was an approximate 30% reduction in the shoreline adjustment compared to the entire Naples Beach fill. This average adjustment was also a decrease from the previous monitoring period of 9.9 feet recession. This indicated that the structure continued to stabilize the shoreline within its region of influence during beach profile equilibration. A 1,276 and 1,390 cubic yard gain were experienced by the updrift and downdrift beaches respectively. This equated to a 2.2 cubic yard per linear foot accretion for the region near the groin. The entire project area experienced a loss of 8,427 cubic yards which equaled an erosion rate of 0.5 cubic yards per linear foot. These results indicate the pile cluster groin has benefited the adjacent beaches within its influence. Surveys at monuments R70 and R72 (see Figure 2.4) showed no adverse downdrifi impacts (CEC, 1997).
+s4sooo STATE LAN seooo
+CNo acx ano sum McAW conots
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NQE: ChaNS ARE 10T DRAM TD SCMLE
0 Zon- 2000 o
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Figure 2.4 Plan view of Naples Beach with locations of existing groins, from CEC (1994)
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12- DIAMETER TIMBER PILE (17PICAL)
* & ^S @M *M
Figure 2.5 Design schematic of Naples pile cluster groins, from CEC (1994)
t V RAFIL0 VIEW
rirA 11MKR PSi (FTPCAL)
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aggg 1. 12' DJAWM FUS gu (qq) ard A.PPwA MO Ros6 OF ? AMP J PU5
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RECONSTRUCT EXISTING PILE CLUSTER GROIN
S"RAUL ,Va a VAIAS
TWIMER EAM (rYICAL)
1.0 WW. OAQRAg CRON 4NG VARIS
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PRFatur nrW TW W QIGWAL OUOA W OF nC 5
RECONS TRUC T EXIS TING
Wag atLES SVp TO SVC f70 PE
WA5WI4R5 AN NVT (3C OCffL) T nugge"arws SWAR UC 7RVCFUR(S
AUG 0 1 1994
Figure 2.6 Design schematic of Naples timber groins, CEC (1994)
Surveys from June 1998 showed the updirft and downdrift beaches experienced average accretions 4.3 and 14.1 feet respectively. The average of 9.2 feet accretion is significantly positive compared to averaged 0.2 feet recession for the entire Naples Beach. This accretion is in contrast to the previous two monitoring periods recessions of
3.3 and 9.9 feet. In this period, the profile near the groin gained approximately 2,978 cubic yards. This was approximately equivalent to 2.5 cubic yards per linear foot accretion. The entire beach had an accretion rate of approximately 0.4 cubic yards per linear foot. Surveys south of the structure did show losses equivalent to 1. 1 cubic yards per linear foot. This rate is negative compared to the entire beach average but positive compared to other reaches within the fill. Some of the mild downdrift losses recorded during this monitoring period probably can be attributed to the sand retention at the groin, but once a new equilibrium was reached, the natural sand bypassing rate was restored (CEC, 1998).
The most recent surveys from June, 1999, showed updrift and downdrift shoreline gains of 13.9 and 6.9 feet respectively. This average of 10.4 feet was slightly greater than the entire Naples Beach average of 9.0 feet. The 10.4 feet accretion was also slightly greater than the previous monitoring period adjustment of 9.2 feet. The beach in proximity to the pile cluster groin had returned to its adjusted post construction position. Besides the shorelines within the Doctors Pass Nearshore Berm Disposal and immediately adjacent to the Naples Pier, this was the only stretch of shoreline to achieve this positive change. The beaches adjacent to the groin experienced a gain of 1,410 cubic yards. This was less than the gain of 2,980 cubic yards for the previous monitoring period, but the net volume change for three years was an increase of approximately 5,400
cubic yards. The gain for this period equated to 1.0 cubic yards per linear foot. This was less than the average 2.4 cubic yards per linear foot experienced by the entire project length. Unlike the surveys from May 1997 and June 1998, profiles at R70 to R72 showed gains of 2,840 cubic yards equal to 1.8 cubic yards per linear foot. This probably indicated that the beach within the structure's influence was in equilibrium, the fillets of sand on both sides of the structure were filled, and more bypassing was occurring. This idea was supported by the shoreline returning to its post- construction position. For three years, the area adjacent to the groin had experienced gains of 1.5 cubic yards per linear foot per year. The entire Naples fill area averaged 1.0 cubic yards per linear foot per year. This suggested that the structure had increased the accretion rate 50% versus the entire fill. PCG-18-1 has benefited the adjacent beaches with no downdraft impacts observed (CEC, 1999).
The offshore bar runs parallel to the beach along most of the Naples' coastline. However, at the pier and the pile cluster groins, the bar makes a mild shift seaward (See Fig. 2.7, 2.8, and 2.9). One explanation for the bar's seaward shift is based on the observation that not only does the shoreline exhibit a seaward migration in the vicinity of the groins, but the submerged beach profile also extends seaward. If the whole profile is shifted seaward, it would be a natural result that the offshore bar is also transported seaward. This explanation would support the observation that the offshore bar shifts seaward along both sides of the groin. The offshore bar also may be influenced by mild return flows induced by the groin, but this hypothesis better explains the presence of the large secondary bars that form oblique to the shore.
The resistance to the longshore current caused by the groin results in a set-up on its updrift side. This combined with the net mass transport of the waves towards the shore creates a hydraulic gradient directed through the groin and also offshore. A return flow along the seabed may be generated, and the secondary bar may form and act as a funnel to guide the backwater offshore and through the groin at a more efficient seaward location (See Fig. 2.7 and 2.8 ). High energy wave events would be more conducive to this type of bar formation, and this would support the observation that the secondary bars only appear to form along the southern side of the pile cluster groins since the tropical lows approach Naples from this direction.
It should be noted that no long-term observations of the offshore and secondary bar geometries near the groins have been conducted. The highly variable wave directions at the Naples coast can alter the locations and forms of the bars on daily timescales, but the typically mild wave heights can also leave storm formations present for extended periods of time. Bar formation is an important highlight of the laboratory experiments, and will be discussed further later.
The elevated submerged profile in the vicinity of the pier and the pile cluster groins may be an important reinforcing mechanism in the abilities of the structures to accrete and retain sediment. Once the submerged profile has been elevated, waves will begin to refract around this feature. Since waves travel faster in deeper water than they do in shallow water, the wave crests tend to conform to the outline of the submerged profile. There is a potential for a null point to exist in the longshore transport. The elevated profile acts as a focusing lens to the incoming wave trains whose longshore components tend to cancel each other.
The raised profile also causes the waves to break farther offshore thus reducing the wave energy at the shoreline. These two features are conducive to low sediment transport and may serve as a reinforcing agent for the stabilization of the bathymetry. This reasoning led CEC to develop a wave refraction model to see if the model could reproduce the physical bathymetry seen in the field. The model worked well for regions in close proximity to the groin and supports the belief that the raised beach profile near the groin serves to be self-reinforcing (CEC, 1995).
The shoreline and the vegetation line also provide insight to the hydrodynamics of the groins. Even though Naples experiences incident waves from a variety of directions and the seaward migration of the shoreline is fairly symmetric and centered at the groins, the maximum seaward prominences of the dry beach and the vegetation line most often lie on opposite sides of the groin. (See Fig. 2.7- 2.14 ) The vegetation line prominence is usually found on the northern side of the structure while the dry beach prominence is usually located on the southern side. The dry beach prominence is a product of the higher frequency wave directions. For the Naples' region, this corresponds to northwesterly waves. The wave energy is typically mild from this direction excluding strong seasonal cold fronts.
This results in the dry beach prominence being located slightly downdrift of the groins when viewing typical wave directions. The dry beach is dependent on the daily high water line and incident wave conditions, and can vary on shorter time scales. Any extended period of southwesterly waves will transport the dry beach prominence to the northern side of the groins. The main observation is that the dry beach prominence lies on the downdrift side of the structure given the local wave conditions.
Figure 2.7 A photograph of three pile cluster groins south of the Naples Pier; note the vegetation line, the shoreline, and the offshore bar near the PCG's, Also note the oblique bar present on the southern side of the middle groin
The vegetation line provides a more permanent record than the volatile shoreline. The vegetation line is only affected by large storms which are low frequency events. The scars in the vegetation line and dunes take more time to disappear. The fact that the vegetation line prominence usually lies on the northern side of the groins is a result of the large tropical storms generating waves arriving from Naples' southwest. The vegetation prominence lies downdrift of the groin relative to these storm waves.
Figure 2.8 Aerial photograph of the southern end of Naples Beach; note Gordon Pass at the top of the photo, the three pile cluster groins (PCG), the multiple timber groins, the
offshore bar near the PCG's, the oblique secondary bars on the southern side of the
PCG's, and the vegetation and dry beach prominences at the PCG's
Figure 2.9 Aerial photo of Naples Pier; note the location of the dry beach prominence slightly below (south) the pier, the vegetation line prominence located slightly north of
the pier, and the seaward turn of the offshore bar at the pier
As discussed, the dry beach prominence and vegetation prominence are downdrift of the groins relative to the waves that created them. This slightly asymmetric shoreline geometry may be due to additional reduction in the wave energy in the lee of the groin. It may also be influenced by the settling time of the sediment particles falling out of suspension. The observations made during the laboratory experiments may provide better insight to the cause of this asymmetry.
In summary, the Naples' experience with pile cluster groins has been
overwhelmingly positive. The groins after their initial construction soon retained enough sand to form a usable dry beach. This not only serves as an effective barrier against future property damage, it provides the public with a highly desirable recreational area. The groins even behave as artificial fishery habitats attracting large schools of baitfish. The groins have proven to be effective builders of the beach during mild wave conditions and protective mitigators against erosion during more severe seas. They stabilize the shoreline and significantly increase the lifespan of beach restoration projects. The reconstructed PCG- 18-1 has positively affected the adjacent shorelines within its reach. Pile cluster groins accomplish all of this without negatively affecting the adjacent shorelines.
Figure 2. 10 Pile cluster groin features; note the dry beach prominence (southern side of
groin/towards top of photo) and the vegetation line prominence (northern side of
groin/towards bottom of photo), also notice the slight turn seawards of the offshore bar near the groin
Figure 2.11 A combination pile cluster groin and timber groin at Naples Beach; note the shoreline prominence along the left side of the groin and the incident wave angle, the missing piles in the ocean section of the groin, and the sea birds attracted to the schools of bait fish drawn to the groin
Figure 2.12 Another pile cluster groin along the southern stretch of Naples Beach
(looking north); note the low pile section for pedestrian travel, the high water line and the
vegetation line, and the landward extent of the groin which is susceptible to flanking during storm events
Figure 2.13 A view northward from Naples Pier; recall that the pier acts as a permeable groin and notice the shoreline bulge in its region of influence; also note the dunes and vegetation that now obscure the seawall
Figure 2.14 A view southward from the Naples Pier; Again, note the shoreline and
vegetation line bulge in the pier's region of influence
Figure 2.15 A photograph of structural damage caused during the 'No Name Storm' of 1982
Figure 2.16 A photograph of Naples Beach after the 'No Name Storm'; note that this area
is free any pile cluster groins and the shoreline had receded all the way to the seawall
Figure 2.17 A photograph of a shoreline in the vicinity of a pile cluster groin; note the presence of a dry beach on either side of the groin and the vegetation line prominence on
the northern (left) side of the groin in response to the SW waves during the storm
Figure 2.18 A photograph of the shoreline in the vicinity of Naples Pier; note the
presence of a dry beach on either side of the pier and the dry beach prominence on the northern (left) side of the pier, also note that the shoreline rapidly transitions back to the seawall at the far left of the picture
The purpose of the laboratory experiments was to examine the dominant
hydrodynamic mechanisms responsible for the performance of pile cluster groins in the field and to observe the bathymetry generated by a pile cluster groin in a controlled environment. Two mechanisms were hypothesized to contribute to the effectiveness of the groin: 1.) Reduction in the incident wave height at the shoreline due to the drag force and potential wave reflection of the piles, and 2.) Reduction in the longshore current velocity induced by the groins. Both of these effects would significantly decrease the sediment transport capacity of the water within the groins' region of influence.
For the purpose of the laboratory experiments, a scale model (1: 10) of the Naples pile cluster groins was constructed. The model consisted of two and three piles arranged in alternating rows spaced at 6 inch intervals. The piles were represented by 1-1/8 inch diameter wooden dowels. These were the nearest sized mass produced dowels to the exact 1.2 inches that a 1:10 scale would require. The dowels were anchored in 1-1/4 inch plywood decking for stability. The decking was far above the water line and did not interact with any of the hydrodynamics of the system. The decking also provided a base to place weights to keep the structure immobile. The groin was constructed in lx2 foot sections, and with eight independent sections, the full model groin extended 16 feet. The
entire structure was heavily coated in water sealant to prevent warping and rot (see Figure
Wave Height Reduction Experiments
Despite literature describing the negligible effects of permeable pile groins on the transmission of waves (Raudkivi, 1996), it was decided to attempt to record the wave height reduction as the waves passed through the model groin. A capacitance wave gauge was placed at a center location inside the department's wave basin far from the physical boundaries of the basin. This water depth at this location was uniform, and thus, was not an area of active shoaling. A monochromatic wave train was generated and recorded without the presence of the model groin. This was done at the start of every trial, and this record served as the base record for comparison. The wave period was 1. 1 seconds and the height was 3.5 centimeters at the test location without the presence of the model groin. In all cases, the waves traveled parallel to the longitudinal axis of the groin.
The groin was then placed in the basin, and sixty second records were taken with the wave gauge in different locations relative to the groin (see Figure 3. 1). It is important to note that the wave gauge itself was not moved, but rather, the groin was moved around the wave gauge. This was done because the recorded incident wave height appeared to vary slightly with location in the wave basin.
Multiple trials were conducted, but the results were disappointingly erratic and inconclusive. A consistent reduction in the recorded wave height was expected relative to the absence of the groin or at the stations in front of the groin with those records measured from behind the groin. The results were inconsistent from the beginning and
I Incident Waves
Wave Gauge Position
Relative to Groin (TYP.)
Figure 3.1 Schematic of the wave gauge positions relative to the groin for the wave height reduction experiments
appeared to not depend on wave absorption by the groin. Some trials showed increases in wave heights behind the groin. Some trials showed unexpected differences between measurements taken at adjacent positions, and sometimes, the exact same position recorded different results in consecutive sixty second records. These differences were on the order of several millimeters which was a significant portion of the total wave height. Trials with larger wave heights and longer period waves did not prove any more useful. The last idea was to construct a standing wave channel bordered by cinder blocks to magnify the amplitude of the wave and double the effective length of the groin. This experiment also produced inconclusive results.
The fickle nature of the wave gauge's position, the physical limitations of the wave gauge's capabilities, and the imperfect generation of monochromatic waves produced errors larger in magnitude than the observation that we were attempting to isolate. This is not to say that wave height reduction does not occur as waves travel
through piles- it undoubtedly does. However, given this model groin length and pile density, the small reduction in wave height proved impossible to measure reliably and consistently. Structures with higher pile densities could have a significant effect on the transmitted wave height (Dean, 1978). The results of these experiments supported Raudkivi's claim that under most circumstances permeable pile groins have a negligible effect on the transmitted wave height.
Using the conservation of energy equation,
s.(ECg)m F.u = s.(ECg)ouT
where s is the spacing between piles, ECg is the depth integrated energy flux in/out of the control volume, and FD u is the time averaged energy dissipated by the drag force of one pile, an expression for wave height decay was derived.
FDu = 2
iuiu2 =4 V4 4u
- Ju est(cos2ot) dt T o3/" (ECg) = H8Pg ( gUh
where, CD = Drag Coefficient, z 0.64** D = Diameter of Pile h = Still Water Depth U Hcrcoshk(h + z); Horizontal Orbital Velocity
2 sinh k(h + z)
u0 = Time Averaged Magnitude of the Orbital Velocity H = Wave Height g = Acceleration of Gravity Derived from Drag Force Experiments.
Assuming shallow water conditions and simplifying,
2 2 HCD
HOUT = H 3N s h
Calculations using the parameters found in the wave basin predicted a wave height reduction of 0.086 millimeters (0.246%) for a wave encountering a single pile. For a wave crest traveling the length of the groin (17 piles per unit width, s), the cumulative reduction in the wave height would be 1.434 millimeters (4.096%). These calculations did not account for any diffraction reinforcement from the adjacent wave crests which would lessen the wave height decrease. These calculations were also representative of a wave crest propagating along the longitudinal axis of the groin. In the field, this is not typical, and depending on wave obliquity, a specific width of the wave crest may only travel through a fraction of the piles.
The performance of the Naples pile cluster groins may be a result of reduced
wave heights and energies at the shore, but these are not directly the result of the groins. The wave height reduction most likely stems from wave refraction around the raised submerged profile and the breaking of waves farther offshore due to the seaward migration of the offshore bar.
Drag Force Experiments
The other major hydrodynamic mechanism associated with pile cluster groins is the retardation of the longshore current. As discussed earlier, the piles act as a hydraulic resistance to the longshore flow. The numerical model simulates the presence of the groin by incorporating a separate friction factor in the cells in which the groins are
located. In order to determine an appropriate value for this friction factor, experiments were conducted inside a flume. These experiments were designed to measure the hydraulic head losses as the flow passed through the piles. The goal was to establish a value for the drag coefficient (CD) of the wooden piles. The drag coefficient could then be incorporated in the numerical model formulation.
These experiments were performed in the Hydraulics Research Flume located in the Civil Engineering Department of the University of Florida (see Figure 3.2). The horizontal bottom of this recirculating flume is lined with fiberglass and is 30 mn long,
2.46 mn wide, and 0.75 mn deep. A 100 hp pump drives the flow. The flow rate is controlled by a vertical gate 30 inch bypass. The volumetric flow rate is measured by a V-notch Thompson weir. The inflow passes through two arrays of 2 inch diameter PVC pipe diffusers to produce a more uniform flow. At the downstream end of the flume, a sluice gate controls the water level. The flow is conveyed back to the main holding tank (Gosselin, 1997).
For these experiments, the main channel was narrowed considerably by placing cinder blocks along the diffusers and the length of the test channel. This restricted the flow to a channel only slightly wider than the groin. The modified flume provided a channel that was 0.48 meters wide and 13.0 meters long. Flow conditions in the flume were controlled by the head and tail gates. The volumetric flow rate was measured via a manometer located upstream of the V-notch weir in the holding area. The Civil Engineering Department had calibrated the volumetric flow rate over the weir resulting in,
where Q is in (m 3/sec) and H is in meters. The value of H, the hydraulic head at the weir, was found by reading the manometer, h*. The manometer had a 5.62 cm offset from the bottom of the weir so the value of H used was, H = (h 5.62)/100.0, (Gosselin, 1997).
Figure 3.2 Schematic of the modified flume used in the drag coefficient experiments (Modified from Gosselin, 1997)
Two separate trials were conducted. The target velocity for both trials was 0.61 m/s (2 ft/s). The water depth of the first trial was relatively shallow compared to the water depth of the second trial. In this manner, one could look at the frictional losses
caused by the piles with two differing submerged lengths. This provided confirmation of the generality of the drag coefficient. The flow conditions in the flume were dependent on upstream and downstream sluice gates, and achieving the desired flow rates and velocities required a trial-and-error procedure. The target velocity value was arbitrary, and some flexibility in the flow parameters was allowed. The main criterion was measurements at two different water depths with approximately the same flow velocity.
The modified channel width was 0.48 meters (1.56 ft). The volumetric flow rates for Trial 1 and Trial 2 were 0.074 mn3 /sec (2.60 cfs) and 0. 114 M3 /sec (4.04 cfs) respectively. The water depths measured in a quiescent region of the flume were 0.21 meters (0.69 ft) during Trial 1 and 0.40 meters (1.32 ft) during Trial 2. Dividing the volumetric flow rates by the cross-sectional areas yielded the flow velocities.
_, = _, 0.074 =0.73 m/s
1A, 0.48 x0.21
- 2 0.114 =0.ms
A2 0.48 x 0.40
The approach in the experiments was to measure the hydraulic head losses
induced by the piles on the flow and to interpret the results in terms of drag coefficients. The flow of water overcame the frictional losses of the piles by increasing the hydraulic head in the channel upstream of the piles. This hydraulic head gradient provided the needed energy to sustain a constant flow through the piles. Measurements of the water depth at thirteen locations in the channel were taken and recorded. These locations were spaced at 0.75 meter intervals. A fixed tape measure provided a reference for location along the channel. The modified channel extended from the diffusers at the 30 meter
location to the 17 meter location. Measurements were conducted from 27.50 meters to 18.50 meters. When the piles were placed in the flume, the first section started at the 26.70 meter location. When all eight sections were in place (16 ft in length), the piles extended to the 21.75 meter location. This allowed two water depth measurements to be taken upstream of the piles and at least five measurements to be taken downstream of the piles. Measurements were not taken within the reach of the piles because the channel was too narrow. Water depths were measured by hand with a scale to the nearest l/16th inch.
The flume itself was a source of friction to the flow. The added cinder blocks that constructed one side of the channel's boundary dissipated a significant amount of energy. These frictional losses not related to the piles were taken into consideration. To isolate the ambient friction of the channel, measurements of the water depth were taken along the channel without the presence of any piles. Using the energy equation, the hydraulic head loss due to friction was determined as follows.
EIN EFricton =EouT
(hN + ) -EFr, = (houT + b2hoUT )
h 2g b 2h 2 2g
EFr, = 0.01102 meters
Errn = 0.00869 meters
see Table 3.1 and Table 3.2 for the numerical values of variables
For each of the trials, measurements of the water depth were taken without any piles present. Then measurements were recorded after single sections (1x2 foot) of piles were successively added to the channel. Since there were eight sections total, nine sets of
measurements were collected for each trial. Those results are presented in Tables 3.1 and
Quiescent Depth (in) Channel Width (in) Volumnetric Flow Rate Flow Velocity (m3/sec) (ml/sec)
0.2095 0.47625 0.0737 10.7387
Location No 1 2 3 4 5 6 7 8
27.50 .2270 .2476 .2603 .2667 .2786 .2865 .2953 .3024 .3 104 26.75 .2222 .2469 .2580 .2667 .2762 .2762 .2929 .2992 .3080 26.00 .2222 .2286
25.25 .2222 .2254 .2254
24.50 .2191 .2222 .2222 .2286
23.75 .2191 .2222 .2222 .2230 .2222 23.00 .2191 .2199 .2199 .2199 .2199 .2222 .2286 22.25 .2191 .2199 .2199 .2199 .2199 .2199 .2254 .2127 21.50 .2159 .2191 .2183 .2199 .2191 .2191 .2215 .2199 .2199 20.75 .2159 .2191 .2159 .2167 .2183 .2191 .2183 .2167 .2191 20.00 .2159 .2167 .2159 .2167 .2159 .2167 .2159 .2167 .2159 19.25 .2143 .2167 .2159 .2159 .2151 .2159 .2159 .2151 .2159 18.50 .2127 .2127 .2127 .2127 .2119 .2072 .2103 .2127 .2127
Table 3.1 Recorded water depths (meters) for Trial 1
Quiescent Depth (mn) Channel Width (mn) Volumetric Flow Rate Flow Velocity
0.4032 0.47625 0.1144 10.5958
Location No 1 2 3 4 5 6 7 8
() Piles Section Sections Sections Sections Sections Sections Sections Sections 27.50 .4127 .4286 .4381 .4477 .4548 .4635 .4699 .4770 .4802 26.75 .4096 .4262 .4350 .4437 .4508 .4604 .4667 .4739 .4762 26.00 .4088 .4 127
25.25 .4072 .4127 .4127
24.50 .4064 .4120 .4120 .4120
23.75 .4096 .4120 .4120 .4120 .4104 23.00 .4072 .4104 .4104 .4104 .4096 .4104 22.25 .4088 .4104 .4104 .4104 .4096 .4089 .4096 .4127 21.50 .4072 .4104 .4089 .4089 .4089 .4089 .4072 .4064 .4064 20.75 .4072 .4072 .4089 .4072 .4072 .4072 .4072 .4056 .4056 20.00 .4064 .4072 .4072 .4064 .4064 .4056 .4064 .4056 .4040 19.25 .4064 .4064 .4056 .4056 .4056 .4056 .4056 .4040 .4032
1.50 .4056 .4040 .4032 .4024 .4024 .4024 .4024 .4008 .4008
Table 3.2 Recorded water depths (meters) for Trial 2
WaterDepth vs. Position forTrial 1 (shallow water)
28 27 26 25
24 23 22 21 20 19 18 Position (M)
- No Piles I1 Sect.
- 4 Sect. 5 Sect.
-7 Sect. 8 Sect.
Figure 3.3 Plot of recorded water levels along the flume channel
during Trial 1 for a varying number of pile sections
WaterDepth vs. Position for Tiial 2 (deep water)
-No Piles 1 Sect.
-*-4 Sect. e 5 Sect.
- 8 Sect.
22 21 20 19 18
28 27 26 25 2'
Figure 3.4 Plot of recorded water levels along the flume channel during Trial
2 for a varying number of pile sections
4 23 Position (m
From these data, values for the drag coefficient (CD) can be determined for each of the specific sets of measurements. An average of these values would be used as a parameter describing the groin in the numerical model. From the Bernoulli equation, Q2 q2
E(x) = h(x)+ = h(x)+
b2 h2(x) 2g h2(x) 2g
where q is the volumetric flow rate per unit width. The energy loss per unit length can be expressed by
Differentiating with respect t
9gb ( h3 g
=E ~[P FDb7]
9X =- wV+ AS2
FD CoPD v2h
y = Specific gravity of water Q = Volumetric flow rate Pw = Wetted perimeter f = Friction factor of the channel As = Unit spacing of piles &0.1524 m
o (x) and substituting variables,
(pfhq3 +b) CDDq3 hb]
- 3(h3b) 2h3 As2 j
q 2 [ f(2+ b + ___,.
This equation can be integrated numerically if h is approximated as an average, h.
hou = h+ + 2AS2
g 2 b 1_ _2
There are only two unknowns-f and CD. One can solve forfby using the water depth data collected when there were no piles present in the channel and the only frictional dissipation of energy is from the channel itself. Solving for fyields the following expression,
8b gh 2 1_ 2
f = (h,-houT)
Using the data from the flume when there were no piles present produces the following values forf
Trial 1 Trial 2
Table 3.3 Values for the flume's friction factor, f, as calculated from Trial land Trial 2 data
These values forf are in relatively good agreement and are used in calculating the values of the drag coefficients.
At this stage, CD is the only unknown. Using the recorded data, a value for the drag coefficient can be calculated for each pile condition for each trial. Solving the numerically integrated energy equation for CD yields,
g bl- JCD = (hN-houT) 2 2+b 2s2
Ax q 8, h) Db
Table 3.4 shows the corresponding calculated values for CD. The water depths used in the calculations are those recorded in Tables 3.1 and 3.2. The channel friction factors for each trial were those recorded in Table 3.3. Ax was found by subtracting the two positions of the recorded water depths. The channel width, b, is fixed at 0.48 meters. The volumetric flow rate per unit width is known for each trial- q, and q2 are 0.155 m2/sec and 0.240 m2/sec respectively. The pile diameter, D, is constant at 0.029 meters, and the pile spacing, As, is fixed at 0.15 meters.
This average value of CD provides an essential pile parameter, and therefore, also of the groin. The fact that this value was calculated from data recorded through experiments lends validity to the final results. It should be remembered that, in nature, the diameter and roughness of the piles could change significantly with age due to marine growth along their surfaces (Bakker, 1984), thus altering the value of the drag coefficient. The engineer needs to be aware of these changes and determine whether they will affect the performance of the groin.
# of Pile Trial 1, CD Trial 2, CD
1 Section 0.710 0.772
2 Sections 0.660 0.645
3 Sections 0.544 0.626
4 Sections 0.701 0.607
5 Sections 0.546 0.616
6 Sections 0.634 0.592
7 Sections 0.681 0.655
8 Sections 0.695 0.639
Average for Trial: 0.647 0.644
Overall Average: 0.645
Table 3.4 Calculated values of CD for Trial 1 (Shallow Water) and Trial 2 (Deep Water)
Beach Profile ReMonse Experiments
A major step in fully understanding and predicting the effects of a pile cluster groin on the littoral system was through physical experiments conducted in the Coastal and Oceanographic Engineering Department's wave basin (See Figure 3.5). The groin was subjected to a variety of exploratory experiments. The sections of piles were arranged in different patterns, and the wave climate included both normally incident waves and waves arriving at approximately 10 degrees to the shoreline. The still water line was recorded at zero, four, and eight hours to indicate shoreline change.
These first trials reflected potential errors and lacked an appropriately controlled environment. The region of shoreline observed only extended 1.5 meters on either side of the centerline of the structure. This was far too small to capture the full effect of the structures. Another problem was that only the position of the still water line was measured and recorded. This meant that the remainder of the beach profile was not
SNAKE WAVE MAKER
l Nll lll Il II 1 1 1j I ll l Il 1 1
I \ WAVE RAY
rn I Approximate Location
- I of Pile Cluster Groin During the
I Beach Profile Response Experiment
UPDRIFT SIDE DOWNDRIFT SIDE
Figure 3.5 A schematic of the departmental wave basin used in the beach profile response experiments
documented. While the shoreline change induced by a structure is an important feature, the response of the submerged profile more fully represents the full effect of the structure. Another difficulty was the supply of sand to this closed system. A constant supply of sand at the updrift boundary of the basin was not maintained for all of these experiments. This caused the basin to take on pocket beach characteristics instead of the more field representative infinitely long beach system.
The main goal of the wave basin experiments was to observe the effects of the model groin on the sediment transport and depositional characteristics of the system. These observations would hopefully match and help explain the littoral transport
phenomena documented along Naples Beach. The laboratory results would also provide an empirical foundation to compare and calibrate the numerical model being developed.
As noted previously, the model pile cluster groin was reproduced at an
approximate 1: 10 scale model of the pile cluster groins found presently along the Naples coast. The wave climate used in these experiments was also chosen to represent that typically found at Naples Beach. The monochromatic waves generated by the wave maker had an approximate breaking angle of 10 degrees. The deepwater wave height was 0.035 meters and the breaking wave height was approximately 0.053 meters. Transforming these heights by the 1: 10 length scale, the corresponding wave heights would be 0.35 meters and 0.53 meters respectively at the Naples site. These agree with typical mild wave conditions at Naples, Florida. The period of the waves in the laboratory was 1.1 seconds. The beach consisted of quartz with a mean diameter of 0.25 millimeters. The sand was well sorted with grain sizes deviating only slightly from the mean.
Two eight-hour trials were run with the groin positioned perpendicular to the beach. The groin was located in a central section of the beach face to maximize the distance between it and the lateral basin boundaries. The landward end of the groin was positioned well above the reach of the swash zone, and the 4.8 8 meter (16 feet) groin extended beyond the seaward reach of sand. A supply of sand was introduced at the updrift boundary of the basin far away from the groin. Sand was added and raked into the system at zero and four hours of each trial.
Figure 3.6 A photograph of the 1: 10 scale model groin used in the experiments
At the beginning of each trial and at the end of each four hour interval, the wave maker was stopped to allow documentation of the topographic features of the full movable bed. The area under study was also expanded. A rectangle extending approximately 5 meters both updrift and downdrift of the groin and 5 meters seaward from the landward end of the groin defined this region. It was believed that this area would be large enough to capture all of the influences of the groin. Black yarn was then placed along the still water line, thus defining the 0.0 centimeter elevation datum. The water elevation in the basin was then lowered by two centimeter increments, and the black yam was placed along all the exposed water lines. This process was repeated through the -12.0 cm elevation. Below this elevation, the beach profile transitioned quickly and uniformly down to the concrete floor of the basin. The floor represented the
-24.0 cm elevation, and the last black yam marking was placed along its edge.
The black yam represented lines of constant elevation that provided high contrast to the white quartz sand. After all of the elevations and features were marked, a digital
photograph was taken from a catwalk above the basin. This photograph was then downloaded onto a computer and served as the basis for a digital topographic map of the beach profile bathymetry. Precise locations of multiple landmarks placed in the basin were known. These landmarks formed the grid that was superimposed onto the digital photograph. Aligning the base grid with the landmarks in the photograph provided the means to accurately digitize the yarn location in the basin. This data were then used by the software SurferTm to produce a topographic map of the beach profile. SurferTM implemented the Kriging method to interpolate elevations for the points located between the known lines of constant elevation. In areas of complex topographic features, some manual data editing was necessary to establish the final topographic map, and this was done with considerable care.
The first trial was a learning experience. The initial beach was groomed to be as uniform as possible, but in doing so, a large error was introduced. The results showed that the profile had lowered significantly almost everywhere. It was then realized that raking the beach caused the sand to be loose and unconsolidated. The subsequent wave action compacted the sand and lowered the profile. This loosened sand was most likely more easily mobilized and transported. The first trial also did not include an initial (0hour) base topographic map for comparison with future maps. This meant that only the 4-hour and 8-hour intervals could be compared. This was unsuitable. Lastly, the elevations in the first trial were only taken from the still water line to the concrete floor, thus not defining changes in the swashizone. However, it was visually evident that changes did occur in the swashizone, and it was decided that elevations above the still
water line needed to be recorded. All of these concerns resulted in the decision to perform a second, more controlled, trial.
The second trial followed the same procedure as the first with the addition of the following improvements. First, the initial beach was not raked or groomed in any way. It was desired to have an initial beach that had reached equilibrium under the incident wave climate. Without the presence of the groin, the beach was subjected to 12 hours of wave action. Figure 3.7 shows the topography of the initial beach. Secondly, black yam was placed along the +1.0 centimeter and +4.0 centimeter elevations. This was accomplished by raising the water level in the basin when the wave maker was first turned off. The addition of these two elevations would capture the sediment transport above the still water line.
For the second trial, the groin was placed in a central location again but not exactly centered on the grid formed by the landmarks. The groin was centered at approximately 0.45 meters (see Figure 3.7) in the center of a smooth and uniform section of the beach. The topographic features seen in Figure 3.7 were those found at the beginning of the second trial. The groin was located in the sand at the center of the most uniform region in the beach profile. This system was then subjected to four and then an additional four hours of wave action. Figures 3.8 and 3.9 show the topography of the beach profiles at these times.
The entire beach profile changes are informative, however, the four key features are as follows. Three of these features are all in close proximity to the groin which suggests that their formation may be attributed to the presence of the groin. These key features include: the seaward migration of the shoreline along approximately the entire
area of study, the formation of a pronounced oblique offshore bar on the updrift side of the groin, the regions of heavy deposition on the updrift side of the groin inside the surfzone and immediately downdrift of the groin, and the positive elevation change of the entire submerged profile landward of the depth of closure.
Figures 3.10, 3.11, and 3.12 show the still water line and the 4.0 cm contour line for the initial beach and four the four and eight hour intervals. From Figure 3. 10, one can see that the still water line at the four hour interval migrated significantly seaward near the groin. In fact, the still water line moved seaward along the whole region of study except between the 2.0 and 3.0 meter alongshore locations. In this region, it remained relatively unchanged. The still water line at the eight hour interval still maintained noticeable seaward gains even though it had receded slightly from the four hour position (See Figure 3.11 and 3.12). The region between the 2.0 and 3.0 meter locations showed recession of the still water line from its original initial beach position. The 4 cm contour also receded downdrift of the groin after eight hours of wave action. These recessions may be attributed to a lack of sand reserves at the updrift boundary of the basin or to the beach establishing a new equilibrium profile.
It is important to note that the region between the 2.0 and 3.0 meter locations repeatedly showed signs of localized high wave energy. This could be attributed to the groin, however, this embayment was present during every laboratory experiment performed. The embayment formed after the beach had been groomed and leveled independent of whether or not the piles were in place. The embayment was first noticed during the wave height reduction experiments which were performed in the middle of the basin. All of these observations support the interpretation that the presence of the groin
did not cause the embayment, but rather, its creation is probably due to the physical characteristics of the basin. Examining Figures 3.10, 3.11, and 3.12, it is evident that the still water line in the embayment receded only slightly after eight hours. The embayment was not exacerbated by the presence of the groin, and the significant accretion at both of its ends is what tends to make the embayment appear to grow in magnitude with time. Figure 3.8 shows a major deposit of sand just updrift of the embayment, and the refraction of waves around this feature may be a cause for its persistence.
A major feature created during this trial was the formation of an oblique offshore bar off the updrift side of the groin with its most seaward end closest to the groin. There were irregular bathymetric features in the same vicinity of the bar in the initial beach profile, but the presence of a distinct and sloped bar at the four and eight hour intervals is consistent. Figures 3.8 and 3.9 clearly show the bar; however, clearer pictures of the formation and evolution of the bar are displayed in Figures 3.13 and 3.14. These figures show the elevation changes of the beach profile between each of the different time intervals. These changes are representative of the deposition and erosion patterns during the experiment.
The oblique offshore bar also formed in wave basin experiments in which the
piles were present. This bar always formed outside of the surfzone and updrift of the pile configuration and always ran oblique to the shore with its most seaward end closest to the groin. The bar never extended through the groin and was usually accompanied by a depression in the profile landward of itself. The formation of the oblique offshore bar appears to be directly related to the presence of the groin since it never formed when piles were not present in the basin.
Unlike the Naples beach, the laboratory beach never formed a bar that ran parallel to the shore for the length of the coastline. However, secondary bars that bear a strong resemblance to the oblique bar formed during the laboratory experiments are found at Naples Beach near the pile cluster groins and the pier. It is believed that these bars are a direct result of the set-up and the offshore directed flow induced on the updrift side of the groins. These oblique bars may act in conjunction with the orientation of the updrift shoreline and form a natural path for the seaward return of the wave generated mass transport. The bars never extend through the groin because of the offshore directed flow along the groin's updrift side.
Curiosity led to impromptu dye injections updrift of the groin. The dye traveled along the shoreline until it reached the region between the bar and the shore. At this point, the dye divided almost evenly. Half continued to travel along the shore dominated by the longshore current. The other half turned sharply seaward and traveled along the updrift side of the groin until it diffused. These dye injections could not be interpreted quantitatively, however, they did document the presence of an offshore flow along the updrift side of the groin. This offshore flow was expected and is an important basis for comparison with the numerical model.
The heavy deposition in the relatively small area inside the surfzone and on the updrift side of the groin is another important feature (see Figures 3.13 and 3.14). It is thought that this area may be a low energy region. The seaward flow of the mass transport, the groin, and the shoreline may act as boundaries isolating this region. More importantly, this area is at the updrift extent of the groin's influence on the longshore transport. This region is analogous to backwater regions in open channel flow. The
longshore current velocity feels the retarding effects of the groin and begins to lose its capacity to carry sand.
From Figures 3.13 and 3.14, one can see that deposition continues through the groin inside the surfzone. The downdrift side of the groin is an area of deposition that extends well seaward of the surfzone. The finger like projections (see Figures 3.8 and
3.9) in the submerged profile constitute one of the largest areas of deposition. This may be because of the settling time of the suspended sediments. The longshore current is retarded through the groin causing the suspended sand to begin to settle, but it takes time for the sediment to fall out of suspension thus finally settling on the downdrift side of the groin. Another more likely reason is that this is another area of low energy. Like impermeable groins, permeable groins can create an onshore flow along their downdrift sides. The area along the shore and downdrift side of the groin is susceptible to low energy eddies that are conducive to sediment deposition.
A second possibility is that this asymmetry in the buildup of the submerged
profile is due to the offshore directed flow along the updrift side of the groin. The energy levels at the shoreline may be conducive to a symmetric building of the submerged profile, but the seaward flow induced by the groin takes suspended sediment in the updrift water column and carries it far offshore. It is probable that some of this suspended sediment travels through the groin and deposits in lower energy regions downdrift of the groin and outside of the surfzone. The continuous offshore flow keeps the updrift submerged beach profile from building out as far as the downdrift submerged beach profile. Evidence for the offshore flow carrying sediment to deeper waters can be
DOWNDRIFT SIDE OF GROIN
-0.00 -1.000 S-4.000 -"-.
UPDRIFT SIDE OF GROIN
-; .oo Q
Figure 3.7 Topographic surface plot of the initial beach for Trial 2; this beach had been subjected to twelve hours of wave action; the groin was placed in the center of the most uniform region of the beach profile
DOWNDRIFT SIDE OF GROIN UPDRIFT SIDE OF GROIN
. 7-,- Groin Footprint
~ 00 ------~ 1.0
Figure 3.8 Topographic surface plot of the beach after 4 hours of wave action during Trial 2; note the seaward migration of the
shoreline especially near the groin, the formation of an oblique offshore bar on the updrift side of the groin, the finger like projections
of the submerged profile immediately downdrift of the groin, and the fluvial style deposition at the toe of the groin
DOWNDRIFT SIDE OF GROIN
.0) 1.0 00.000
- 00 .0.00
-1 2.00 -.-
UPDRIFT SIDE OF GROIN
- Groin Footprint
Figure 3.9 Topographic surface plot of the beach after 8 hours of wave action during Trial 2; note the advancement of the shoreline, the continuing presence of the updrift and oblique offshore bar, and the finger like projections in the submerged profile immediately downdrift of the groin
longshore Position (m)
9o o o
Figure 3.10 Plot of the SWL and the +4 cm contour for the initial beach and the 4-hour interval; note the large seaward gains in the SWL's cross-shore position immediately
updrift and downdrift of the groin after 4 hours of the groin's presence
Alongshore Position (m) S-- 0 r.
. _~o.Om tj
,J ~ ~ ~ O ,- W ..--- + ..
- - - -
Figure 3.11 Plot of the SWL and the +4 cm contour for the initial beach and the 8-hour
interval; note the continuing seaward gains of the SWL immediately updrift and downdrift of the groin and the recession of the 4 cm contour downdrift of the groin producing a milder beach slope
Alongshore Position (m)
S I I' J. ] U 1-i
Figure 3.12 Plot of the SWL and the +4 cm contour for the 4-hour and 8-hour intervals; note that the SWL shows some fluctuations but little net transition while the +4 cm
contour receded downdrift of the groin resulting in a milder beach slope
found in the deposits centered around the groin between the 3.0 and 4.0 meter cross-shore marks (See Figures 3.12, 3.13, and 3.14).
Some or much of this deposition may have been caused by the along-shore
transport of sand in the deeper waters. Figure 3.12 shows deposition at the bottom right of the figure. It is thought that this was caused by the sand placed at the updrift boundary of the basin. Figure 3.12 might give the impression that this placed sand fed the deposition seen at the groin at the same depths, but Figure 3.7 shows that there was a major depression separating these two deposits. In fact, in Figure 3.7 one could trace where the water would flow if it were poured into the embayment. It would run into the depression landward of the bar and spill out underneath the groin. This is strong
-5.00 -4.00 -3.00 -2.00 -1.00 0.00
1.00 2.00 3.00
Alongshore Posiion (m)
Figure 3.13 Deposition and erosion contour plot (4-hour minus Initial beach); note the formation of the oblique offshore bar updrift of the groin, the heavy deposition inside the surfzone immediately updrift of the groin, the heavy deposition immediately downdrift of the groin, the less drastic deposition between the offshore bar and shoreline and underneath the groin, the deposition in deeper waters underneath the groin, and the overall elevation of the beach profile in the region of study
6.00 cm 5.00 cm 4.00 cm 3.00 cm
2.00 cm 1.00 cm 0.00 cm
9.00 cm 8.00 cm
7.00 cm 6.00 cm 1.0 5.00 cm
4.00 cm 2.0 3.00 cm
-o 2.00 cm
o 1.00 cm
-5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 -3.00 cm
Alongshore Position (m) -4.00 cm
-5.00 cm Figure 3.14 Deposition and erosion contour plot (8-hour minus Initial beach); note the definitive region surrounded by the offshore bar, the immediate updrift shoreline, the downdrift finger-like depostion, and the offshore deposition underneath the groin
o .. 111 1.00 CM
-1 .00 CM
-5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 -2.00 cm
Alongshore Position (m)
Figure 3.15 Deposition and erosion contour plot (8-hour minus 4-hour); note the expanse of the area in white suggesting the profile may be reaching equilibrium, the slightly erosive regions underneath the groin, the smoothing of the offshore bar, and the continuing deposition downdrift of the groin
evidence that the fluvial style deposit centered underneath the groin at the -8,-10, and 12 cm contours was caused by the offshore flow.
It is important to note that this sand is still in the littoral system, that the lower
lying areas still experienced significant net gains in elevation, and the shoreline migrated seaward everywhere except in the embayment. The offshore flow is not a sink for sand, but rather, a potential explanation for the asymmetry in the rise of the submerged profile. This may also be a reason for the dry beach prominence being located on the downdrift side of the groin. Lastly, the scour observed around the piles for this specific design and wave conditions was very small (averaging approximately 0.5 cm). If the piles are driven to a sufficient depth, scour should not be a problem even for storm events. However, scour modeling was not a goal of this model effort. The important results from the beach profile response experiment are:
0 The formation of an oblique offshore bar on the updrift side of the groin.
0 The seaward migration of the shoreline.
0 The net rise in elevation of the submerged profile.
* The location and magnitude of the updrift and downdrift deposits.
0 The presence of a relatively low lying return channel.
* The presence of an offshore flow.
* The very small scour produced around the piles.
Numerical Model Ideology
The numerical model was developed to gain a quantitative understanding of the
groin hydrodynamics and to serve as a basis for comparison for the observations recorded during the laboratory experiments. The model calculates the water surface elevations, the alongshore and cross-shore flows, and the sediment deposition for a rectangular area specified by the user. The numerical model uses discretized forms of the Continuity and the Conservation of Momentum equations. The user inputs the number of time steps, the size of the time step, the size and resolution of the grid, the wave period, the wave height, the incident wave angle, the beach slope, and the location of the primary groin. Values for other variables, including the presence and locations of multiple groins and the hydraulic friction imposed by the groin(s), can be set by the user.
Figure 4.1 is a schematic representation of a typical grid region and the associated subscript nomenclature implemented by the numerical model. The values for water surface set-up or set-down, (q7), and the depth, (h), are defined at the grid nodes. The along-shore and cross-shore flows, qx and qy respectively, are defined at the interfaces between each of the grid nodes. For a particular grid node (i, j), the qx(i, j) and qy(i, j) values are denoted by the flows entering the rectangle centered around (i, j). Other variables used in the numerical model may be centered on varying locations. The
Appendix discusses the derivations of the equations used by the numerical model and the expressions used to calculate the values of individual variables.
Figure 4.1 A schematic of the grid used in the numerical model computations.
The final forms of the discretized Continuity and Momentum Equations used by the numerical model are:
k k k k
k+1 = Atqxk+1, qx + qyit,+1 qyi,.j
S Ax + Ay
k+1 Atigh + + GG 6h -FP I q At) Ax Ay Ay2
k + 1~~~~~~ 1 1 q 7 A f g A + G G Y ,~ j J
For their derivations, please see the Appendix.
The numerical model reads in the input data specified in the input file. Pile
characteristics are assigned in the early portions of the program. These values may be adjusted by the user before the program is executed. The output variables, q7, qx, and qy, are initialized at value zero. A planar beach is created from the input slope, number of cross-shore grid nodes, and the A y value. For larger wave heights, the five most shoreward rows are held at a constant shallow depth not equal to zero. This improves the model stability. Friction values are assigned to each cell based on the given value. During these trials,f = 0.08. This value was chosen as a realistic value given the sandy bottom of the basin and expected water velocities. The cells that represent the groin(s) location are assigned a given hydraulic friction factor. This number is stipulated in the body of the program, but was calculated from the following formula: f = fd+4CDDh
where, D is the pile diameter, hi is the average cross-shore depth, As is the pile spacing, and CD (0.64) is the drag coefficient calculated from the flume experiments. Inserting the appropriate values for the lab experiment yields a pile friction factor of 0.647. The corresponding Naples pile hydraulic friction factor is 0.643. The net drag force of the piles on the incident waves is also calculated. The model shoals the incident waves and applies the spilling breaker criterion, H=.78 h, as the waves travel towards shore.
The main ioop of the program imposes the boundary conditions: no cross-shore flow at the shoreline, no gradients in the alongshore flow and the along-shore water surface elevation at the updrift and downdrift boundaries, and no gradients in the crossshore flow and the cross-shore water surface elevation at the off-shore boundary. These boundary conditions represent an open beach system. A closed system can be stipulated by forcing no flow conditions along the edges of the grid. The numeric model calculates the water surface elevation, alongshore flow, and cross-shore flow for every grid node. This process is then repeated for the next time step until the number of specified time steps has been reached.
The nonlinear and viscous terms in the momentum equations are calculated in subroutines. These subroutines implement an alternating grid scheme to calculate the finite differences. Values from grid nodes on alternating sides of the current grid node are used to compute the new values for qx and qy. This helps insure the stability of the computations. The stability of the model is also reinforced by the subroutine 'UPDATE' which is called at the end of every time step. 'UPDATE' insures that the correct preceding values forq7, qx, and qy are used to calculate the new values. 'UPDATE' also computes intermediate values for the water surface and current flows to smooth the transition between time steps. All of these values are stored in a three dimensional array and their averages are used to evaluate other expressions.
The lateral diffusion term, e ,52f contains the horizontal eddy diffusivity coefficient, E. This parameter is not well understood, but a realistic cross-shore distribution of the along-shore current velocity was needed. Longuet-Higgins expressed the eddy diffusivity by:
e = Nx.Jwhere 0.0 < N< 0.016 and Nis a dimensionless constant, x is the effective mixing length (distance to the offshore breakpoint), and h is the depth (Visser, 1984). Longuet-Higgins also defined the parameter, P, which is the ratio of the eddy diffusivity to the bottom friction (Longuit-Higgins, 1970).
The effects of P on the normalized cross-shore distribution of the along-shore
velocity are shown in Figure 4.2. Comparisons with laboratory data show that values of P between 0.1 and 0.4 are realistic. Therefore, P was assumed to be 0.25. For the assumed values of P, the beach slope, and the bottom friction factor, N equals 0.0124 which is within the realistic limits. Solving for N in the previous two equations, then equating, and then solving for e yields,
1 H Ccoso5 hb x hb
Table 4.1 shows the values of the eddy diffusivity for each of the incident wave conditions used in the numerical model.
Ho (m) e (m2/s)
0.035 (lab) 0.019
0.5 (field) 1.139
1.0 (field) 2.616
2.0 (field) 6.010
Table 4.1 Values for the eddy diffusivity, e, Given the incident wave conditions and P value used in the numerical model
The last time step calculates the final values for 17, qx, and qy. From the qx and qy
values, the deposition of sand at each grid node is calculated from the equation:
DEP = -1000 (AqxsE + Aqys) Ax AY
Longshore Velocities on a Planar Beach for Different p-values
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 X (xlxb)
Figure 4.2 Plot of the effects of P on the cross-shore distribution of the along-shore
current velocity; P = 0.25 was used in the numerical model
The arbitrary factor '1000' serves to magnify the deposition values. The model is based on sand depositing when there is a convergence in the sediment transport vector and sand eroding when there is a divergence. For purposes here, it is assumed that the sediment transport components are proportional to the bottom shear stress. No attempt is made to quantify the exact amount of sand that deposits or erodes at a location. The model attempts to identify areas of deposition and erosion and their relative magnitudes with respect to each other. The values for the water surface elevation, alongshore and cross-shore flows, and the corresponding deposition/erosion are then written to the output file.
Numerical Model Results
Many trials of the numerical model were ran. Each trial used a specific set of parameters to represent particular environments including: the laboratory beach profile response experiment and the typical conditions at Naples Beach. The versatility of the model also provided the opportunity to isolate the effects of individual variables, such as groin permeability, incident wave angle, incident wave height, groin width, and multiple groins and their spacing. From these trials, general design considerations were derived.
Only two trials that mimicked the laboratory environment were performed. The first trial represents the case of zero groins present in the basin. The second trial represents the case of a single groin present in the basin like the beach profile response experiment. The same incident wave conditions present during the laboratory experiment were input into the model: H, = 0.03 5m, T = 1. 1 sec, and 0, = 10 degrees. Originally, the laboratory trials implemented boundary conditions that represented a closed system, but since the width of the computational grid was 50% smaller than the width of the basin, placing impermeable boundaries on the grid would not mimic the laboratory settings. Therefore, an open beach computational grid was used. The dimensions of the grid were 60 cells by 60 cells with a resolution of 0.2 meters square. The time step was
0.02 seconds, and the total number of time steps was 10,000. This proved to be sufficient time to provide system equilibrium. The results follow in Figures 4.3, 4.4, 4.5, and 4.6 and Table 4.2
' 5 oO0
Figure 4.3 Hydrodynamic characteristics and depositional pattern calculated by the numerical model for the laboratory environment without the presence of a groin
Water Velocity Vectors
. . . . . . . . . . . . . . . . . .
. . . . . . . . . I . I . . . . . . . .
. . . . . . . . . . I . . . . . . . . . .
7 .... .. ... . .. .. .... ... ... .. ...... .. ..... .... ..... .... ....
4 6 8 10
Alongshore (m); Groin centered at 6m
Figure 4.4 Water velocity vector plot for the laboratory environment without the presence of a groin
Most of the plots for the case when no groins are present are trivial, but they serve as foundations for comparison with other plots with different input variables. Figure 4.3 shows the steady-state hydrodynamic system. Figure 4.4 shows the water velocity vectors for the case of no groin present. The velocity does not return to zero at the shore because the model imposes a finite depth at the shoreline. The alongshore and crossshore velocities are easily computed by dividing the alongshore and cross-shore flows by the sum of the water depth and set-up or set-down at each point.
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
0 0 Alongshore (m)
C) (N1 E 0
x 10. 10 ...
Figure 4.5 The hydrodynamic characteristics and depositional pattern calculated by the numerical model with the presence of a groin centered at 6 meters and extending 8 meters seaward.
Water Velocity Vectors
. .... .. .. .. ... . . .-, .. ... .. ....,, L ... ..
...4,-'...-..-. . -.. -. .
92 4 8 10 12
Along shore (m); Groin centered at 6m
Figure 4.6 Water velocity vector plot for the laboratory environment with the presence of a groin
From the laboratory environment trial, the basic effects of the groin can be seen (see Figure 4.5). Landward of the breakpoint, there is a decrease in the longshore flow rate and velocity. The groin induces a nearshore hydraulic gradient. The set-up and setdown are on the order of a few tenths of a millimeter, which is not visible, but are sufficient to generate cross-shore flows. The magnitude of the cross-shore velocities is an order of magnitude lower than the along-shore velocities. This is a direct result of the high permeability of the Naples pile cluster groins and is a desirable trait. However, these cross-shore flows are strong enough to manifest themselves as subtle redirections in the water velocity vector plot (see Figure 4.6).
I . . . . . . . . . .
. . . . . . . . . . . . . I . . .
. . . . . . . . . . . . . . I . .
. . . . . . . . . . .
. . . . I . . . . . . .
. . . . . . . I . . . .
The numerical model does not capture the complex depositional patterns observed in the laboratory experiment (see Figure 4.5). This particular trial shows mild accretion along the updrift beach and underneath the groin near the shore and mild erosion downdrift of the groin. This may have been a disappointment, but some of the major depositional features observed in the lab may be present in environments with more complicated hydrodynamics. Table 4.2 quantifies some of the major hydrodynamic characteristics. The Vx values represent breakpoint values.
Status MM fumN A r7 Vxm~xb Vxmb % Red. in Vx
() (in) (i) (m/s) (m/s)
No groin 0.0 0.0 0.0 .0788 .0788 0.0
lroin, .000234 -.000149 .000383 .0785 .0496 36.8
Table 4.2 Hydrodynamic characteristics of the trials representing the laboratory environment
The rest of the trials represent the Naples field environment. The size of the
computational grid usually remains at 60 cells by 60 cells, but the grid is now at 2 meters by 2 meters. The wave heights used are 0.5, 1.0, and 2.0 meters. The wave period is held fixed at 5 seconds. In all cases, the groins extend 40 cells (80 meters) from the shore. The pile variables are at their field dimensions. In the following trials, all parameters are held fixed except the one that is under study. In the trials where a particular parameter is held fixed, it is held at the typical Naples' value except for the wave height, which is held at 1.0 meter.
Wave Height Results
In these trials, the model contains a single, identical groin. The incident offshore wave angle is 10 degrees. The groin is two cells wide which is slightly wider than the corresponding Naples groin but as close as the grid resolution would permit. Other groin characteristics follow the design of the Naples pile cluster groins.
100 Alongshore (in)
0.01, 0.005, 01
100 50 100
50 100 50
0 Cross-shore (m)
Figure 4.7 Set-up and set-down induced by the presence of a groin for three different wave heights- 0.5, 1.0, and 2.0 meters
Figure 4.7 shows the calculated set-up and set-down for three different wave
heights. The mean water elevation difference between the updrift and downdrift sides of