Laboratory and numerical studies of a pile cluster groin

UFL/COEL-2000/006

LABORATORY AND NUMERICAL STUDIES OF A PILE
CLUSTER GROIN

by

Sean E. Mulcahy

Thesis

2000

LABORATORY AND NUMERICAL STUDIES
OF A PILE CLUSTER GROIN

By

SEAN E. MULCAHY

A THESIS PRESENTED 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

2000

ACKNOWLEDGMENTS

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

page

ACKNOWLEDGMENTS ............................................................. ii

LIST OF TABLES .......................................................................... vi

LIST OF FIGURES ......................................................................... viii

ABSTRACT .................................................................................. xiv

CHAPTERS

1 INTRODUCTION .............................................................. 1

Purpose of this Study............................................................. 1
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........................................... 111

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

Table page

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 (Deep Water).......................................................... 52

4.1 Values for the Eddy Diffusivity, s, 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 W ave Heights.................................. ..................... 102

4.7 Hydrodynamic Characteristics of Multiple Groins for the Field
Environment with Varying Spacing........................................... 104

LIST OF FIGURES

Figure page

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
m iddle 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
drawn 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
com putations........................................................................ 72

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 m and extend 80 m 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
w ave 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
w ave 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 m for the four groin plot and H=1.0 m 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 m), '2 Groins far' (60 m and 210 m),
'4xW Groin' (60 m), '4 Groins' (40, 60, 80, and 100 m).......................... 107

4.22 Depositional patterns for the field environment with multiple groins
with 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

By

Sean E. Mulcahy

August 2000

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 (q7) 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 permitted the examination of different

combinations of input variables and provided the opportunity to establish general design

parameters for pile cluster groins.

CHAPTER 1
INTRODUCTION

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.

Descriptive Terms

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 WATER 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.

0.*.

... .-.-C
:*`~. ~ p

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

5

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.

CHAPTER 2
BACKGROUND

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

BAR TROUGH

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,

K, = 0.5(1 -B2)1/2(1+ cos2 /)

where,
K, = Transmission Coefficient
B = Complement of Groin Permeability; (1- Permeability)
p = 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

Distance [ml

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 Naples 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

0-1 75
1-2 4
2-3 9
3-4 9
4-5 2
5-6 0.5
6-7 0.1

Table 2.1 Wave height versus % occurrence from 1956 to 1975 WIS 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 ( 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
ao (0) Ho (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

QNORTH (yd / year) 128,300 132,550
QsoUTH (yd / year) 162,700 158,750
QGROSS (yd' / year) 291,000 291,300
QNET (yd3 / year) 34,400 26,200
(NTOS)
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 ( 1.52 meters)

increments. The diameter of the piles is one foot ( .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 -

2.18)

Even though the groins and pier could not stop the storm 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 yd3, 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-18-1 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

downdrift impacts (CEC, 1997).

22

+.6000oo0 -6 660+ N

rMnaD. AW o
R-70 I OO' Voo'
SCAE. 1 2000'

MEXCO

C EACH
+4650 0 +1
|" +NaPLe +

n- I,, U-80

fl,*-S. 1 1 lWICA
Ecop... Psu -us f o

u-n-
1- NAPLES J
.r 24P vie o STATE Wacaxis, fr

AU6 0 1 190b ST-E

........ GaM Gxi of

Ho, n-a M TR-E a cMTD lu

Figure 2.4 Plan view of Naples Beach with locations of existing groins, from CEC (1994)

P-ILA---

PLAN VIEW

12' METER luBr PILE (7PICAL)

.: o
* *

LtRS EtiMY WCaDR TI MIW

OHMIW UWNIW

W* LWLR O=FWILW

vAiCS

VARIES

PRFILE VIEW
I.S
PU VACW0 S'-O'

IT- w FER Mw pr (FnIcAL)

.0 FT. MGH. V.D.

I I S TATE
: \ I ,S
AUG 0 1 1994
SWFFO iC D
NlOe 1I. If OJur MIUts '-9' VyAe (cC.) miN AMisWimo Rows Or I Aw J fP
2 ANY RWAR W MOPOAMOI Am VA MVaINEWf Of r4 ftj LV/VP Ws
SWAUL Of WUe MRAW NA IIWM W PIUiAImOW 1 OW $WUCtRW(S

RECONSTRUCT EXISTING PILE CLUSTER GROIN

Figure 2.5 Design schematic of Naples pile cluster groins, from CEC (1994)

I _

I

PLAN VIEW

wuBE aBEAM (rIIcALM

PROFILE VIEW

1.
0.0 Fr. N.G.V.7

4. CoeCaTIM: r U OwaIp T mrw Ar vI *mAwa
S ANr rPArMS uMnCA "S uA UAWi Mw OF A
PrVorMcu" fM4W t 9ew4 r(ViOAo o 1 fi

RECONSTRUCT EXISTING

Nr .

D afEs SpW TO Ms
T o PU
WASMneW AD NUT (WC DtML)
MW WaOgN X144 K
MvC',a

TIMB&~~iROIN

AU6 0 11994

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 downdrift 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 ofbaitfish.

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

u1.aW:

"Ism"

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

""C~*M 4~4i~'~i~llslok9

-Sam

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

Ak

Wr

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

I Y~ 'I ''

L

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

CHAPTER 3
LABORATORY EXPERIMENTS

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 1x2 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

3.6).

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

4 Incident Waves
C0 C, 0

Wave Gauge Position
Relative to Groin (TYP.)
Groin

0 0

0 0 0

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.(EC,)m FD u = S*(EC,)our

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 = CpDh u2
F'ui = 4 24 4u
u = uCoscrt(cos2t)dt =
T g 3/"

(ECg) = Hpg (8)
8
where, CD = Drag Coefficient, z 0.64**
D = Diameter of Pile
h = Still Water Depth
Hacoshk(h+z) .
u = Hcoshk(+ z) Horizontal Orbital Velocity
2 sinh k(h + z)
uo = Time Averaged Magnitude of the Orbital Velocity
H = Wave Height
g = Acceleration of Gravity
SDerived from Drag Force Experiments.

Assuming shallow water conditions and simplifying,

2 2H CDD
HOUT = HM 3 sh

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 m long,

2.46 m wide, and 0.75 m 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,

Q=1.594H2.514

where Q is in (m3/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 m3/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.

Q, 0.074
v = 0.074 0.73 m/s
A, 0.48 x 0.21
Q2 0.114
V2 = 0.59 m/s
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 1/16th

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 EFriction = EouT

(hm +) =E (houT b2h QUU 2
h' 2g b2 h 2 2g

ET'r, = 0.01102 meters
EFrrT n = 0.00869 meters
Ssee 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

3.2.

Quiescent Depth (m) Channel Width (m) Volumetric Flow Rate Flow Velocity
(m3/sec) (m/sec)
0.2095 0.47625 0.0737 0.7387
Location No 1 2 3 4 5 6 7 8

27.50 .2270 .2476 .2603 .2667 .2786 .2865 .2953 .3024 .3104
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 (m) Channel Width (m) Volumetric Flow Rate Flow Velocity
(m3/sec) (m/sec)
0.4032 0.47625 0.1144 0.5958
Location No 1 2 3 4 5 6 7 8
(m) 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 .4127
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
18.50 .4056 .4040 .4032 .4024 .4024 .4024 .4024 .4008 .4008

Table 3.2 Recorded water depths (meters) for Trial 2

WaterDepth vs. Position for Tial 1 (shallow water)

28 27 26 25

24 23 22
Position (m)

20 19 18

--No Piles
--- 1 Sect.
2 Sect.
-... 3 Sect.
-x-4 Sect.
----5 Sect.
--6 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 forTrial2 (deep water)

49 ---No Piles
48
48 Sect.
47
*-46 j 2 Sect.
45 -- Sect.
_----- -- -44 .

-2 -*-5 Sect.
S41 6 Sect.
40
39 7 Sect.
28 27 26 25 24 23 22 21 20 19 18 8 Sect.
Position (m)

Figure 3.4 Plot of recorded water levels along the flume channel during Trial
2 for a varying number of pile sections

V~-

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

7yQ.

where,

Differentiating with respect t

pgqb h(, q
9qx1 h3 g1

Sh
Sx

3E F, v
9 = rPwv+ AS2
Ex |As
pfv2
8
F = CDPDv2h
FD C, D 2 h
2
y = Specific gravity of water
Q = Volumetric flow rate
P, = Wetted perimeter
f = Friction factor of the channel
As = Unit spacing of piles & 0.1524 m

o (x) and substituting variables,

pf q3 CDDq3 hb]
= 4(2h + b) + CD qhb
8h3' 2h3 As2

q 2 [f 2+b C+ Db

This equation can be integrated numerically if h is approximated as an

average, h.

Ax q 2 b2 CD Db
houT = hm + + 2AS2
8 h 2 As
gh2 b 1_~ J
h g

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 fields the

following expression,

8bgh2 q--
f = (h -hour)
q2 AX 2+

Using the data from the flume when there were no piles present produces the

following values forf

Trial 1 Trial 2
0.0218 0.0251

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,

gh b l--
CD = (h -ho) 2 2+b 2s2
Axq- 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- ql 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 Co 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
Sections
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 Co for Trial 1 (Shallow Water) and Trial 2 (Deep
Water)

Beach Profile Response 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
- r -j I
\u WAVE RAY
WAVE GUIDE

z I
oI
Z \I WAVE GUIDE-.
Srn
2 I
2r I Approximate Location
5 of Pile Cluster Groin During the
8 I Beach Profile Response Experiment

0o I \
I'
I |
*
,- I

z
-a

rn
1a
37
37,

west-side east-side
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 4.88 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 yar 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 yam 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 (0-

hour) 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 swashzone. However, it was visually evident that

changes did occur in the swashzone, 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 yar

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
-s-ioo'kJ
--8.00 ao~o
a. 0C5 .00 --

-1.o. -10.00
ci12. M

SGroin Footprint
UPDRIFT SIDE OF GROIN

^^^^O'Oo-^4.00-^

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

S. DOWNDRIFT SIDE OF GROIN UPDRIFT SIDE OF GROIN

___ --- Groin Footprint
-___"_-____ .oo__ _. _- -. .._.*. 0-. -

,, --I. -. ,

S10.0

,I zo 1..0o;

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

UPDRIFT SIDE OF GROIN

4.00o y Groin Footprint
0.00 1. .0
7.0 -00 0.00
n9004.0'-6.00 -
'4c

1
/ rf
'/

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)
b P
g I 0

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)
Ul .L I P .b 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

I-W

-O OdN

4A W*-:
---- ^l -B '

Groin Footprint

C-sm- ObU

Groin Footprint

"', I l l 1 II,

o~-
`'~~b; ~.~es*

Alongshore Position (m)

- - - - - -- '- -- --

0-

Groin Footprint
a

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 Position (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

7.00 cm

6.00 cm

5.00 cm

4.00 ancm

3.00 cm

2.00 ancm

1.00 cm

0.00 ancm

-1.00 cm

-2.00 cm

-3.00 cm

-4.00 cm

-5.00 cm

9.00 cm

8.00 cm
7.00 cm
6.00 cm
1.0 5.00 cm
4.00 cm
i 2. 3.00 ac
o 2.00 on
o3, 1.00 cm
0.00 acm
-1.00 cm
4.00
-2.00 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
Alongshomre Pos'tion (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

3.00 cm

0.00

"1.00 cm
o2

0 0.00 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)
-3.00 cm
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:

The formation of an oblique offshore bar on the updrift side of the groin.

The seaward migration of the shoreline.

The net rise in elevation of the submerged profile.

The location and magnitude of the updrift and downdrift deposits.

The presence of a relatively low lying return channel.

The presence of an offshore flow.

The very small scour produced around the piles.

CHAPTER 4
NUMERICAL MODEL

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, (q), 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.

Along-shore X

Computational Grid

AX

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+1 = x At 1, qx,+ qyi,;+1 qyij
Ax Ay

k+1
qxi -

qxA At g + Ay + GG, h-2 FP
ql At) Ax Ay Ay

1y = 1 [ qy,1 At gh + GG0)
f At L I Af y)
1 + -2
+ 8 h

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, qr, 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:

= 4CDDh
f = fbed+
As2

where, D is the pile diameter, h 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 loop 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 cross-

shore 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 for 7, 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.

62 i
The lateral diffusion term, ,- contains the horizontal eddy diffusivity
Sy2

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 = NxJ-gh

where 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,j (Longuit-Higgins, 1970).

8irmN
P =8mN
Kf

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,

b Pg P f
81Tm

where,

1 HC" coso 5
hb -

hb
m

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, s, 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:

-1000 / 1
DEP = -10 Aqx + Aq )

qxSED -2
h

qySED -2
h

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 (x/xb)

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 run. 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.

Laboratory Trials

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, = 0.035m, T= 1.lsec, and 0o = 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

Deposition

5
Cross-shore (m)

x 103

4.

oL
10
104

qx Vx

55
010
5 0 0 5

0.1 -

I 0.05 -.

04
10

0 0

Vy

-1
10

0 0

Figure 4.3 Hydrodynamic characteristics and depositional pattern calculated by the
numerical model for the laboratory environment without the presence of a groin

1

Water Velocity Vectors
I . . . . . .

... .. .... .. . ... . ...... ... . . . . . . . .
. . . . . . . . . . . . . .. . . . . . . . . . . .
. .. . . . . . .I .. . . .. . . . . . . . . . . . .

. . . . . . . . . . . . . . . .. -. -.. . . . . .

2 4 6 8 10
Alongshore (m); Groin centered at 6m

12 14

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 cross-

shore 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.

3.

1 -

2
0
0

. .7 7.. .. ... ... . . . . . - . . . ..7
-:::::> -------------- ---------- -- ---------- --> - > -""- -"*

x10-4

Cross-shore (m)

x 10'

"0
EO

-2
10

10

5

E 0

-5
10

5
0 0 Alongshore (m)

qx

,...:. :... l n

x 104

0 0 0 0

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.

Deposition

Vx

Water Velocity Vectors

- - - -- -^J-^J'^^J'^^^^^---

-~~ ~ ~ -- -
----------- -------- ------------------
11 1 1--1 1- -----------------f *--^- jj . j. .r----
_ __r__-- .. .. . -r-... .~ j w m r-r-r j__ ____ _ ~j- 1-r/~~~ r- i

r-r----.---L*s^^" i.-- r-->^^HrLI,..... ^*

2 4 6 8 10
Alongshore (m); Groin centered at 6m

12 14

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 set-

down 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).

. . . . , ,,.. , .
.. o. . o.. . l i .

. . . . . . . . . . . . . .i . . .

. . . . . . . . p l l l ~ l . .
. ..o ,.. . . ., o . .
. . . . . . . , , o ~ . .

..............................

. . . . . . . .

......................
. . . . . . . . . .
......................

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 MAX miN A r VxMPXb VxmIN % Red.in Vx
(m) (m) (m) (m/s) (m/s) _
No groin 0.0 0.0 0.0 .0788 .0788 0.0
groin .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.

84

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.

E
x 103
3
2

-1

-2

50
100
Alongshore (m)

ta, H=0.5m

: . .

50
0 frnccchr

Eta, H=1.Om

0.01...

0.005 .

-0.005 ..

-0.01

3 50 -" 0Q
050

1r0 m 0

Eta, H=2.0m

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

MILLISECOND	CLASS.METHOD	MESSAGE
0	sobekcm_page_globals.constructor
0	sobekcm_page_globals.constructor	Application State validated or built
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.constructor	Navigation Object created from URI query string
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.display_item	Retrieving item or group information
0	sobekcm_page_globals.get_entire_collection_hierarchy	Retrieving hierarchy information
0	sobekcm_assistant.get_entire_collection_hierarchy
0	cached_data_manager.retrieve_item_aggregation
0	cached_data_manager.retrieve_item_aggregation	Found item aggregation on local cache
0	item_aggregation_builder.get_item_aggregation	Found 'all' item aggregation in cache
0	system.web.ui.page.page_load (ufdc.page_load)
0	sobekcm_page_globals.constructor.on_page_load
0	html_echo_mainwriter.add_style_references	Adding style references to HTML
0	html_echo_mainwriter.add_text_to_page	Reading the text from the file and echoing back to the output stream
34	html_echo_mainwriter.add_text_to_page	Finished reading and writing the file