| Miami
Beach 32nd Street Hot Spot: Construction
& Short -Term Monitoring
ABSTRACT
The shoreline in the vicinity of 32nd Street in Miami Beach,
Florida has been determined to be an erosional hotspot within
the federal Miami Beach renourishment project. Earlier engineering
studies and numerical modeling determined that the use of
three artificial headlands was the most effective way of stabilizing
the shoreline. The structures were designed as a demonstration
project, which included the use of recycled materials and
implementing sand backpassing from down-drift areas to “pre-fill”
behind the structures.
This paper presents observations from the construction of
this project in the Spring of 2002. Specifically, the logistics
of the construction of nearshore rock structures are described,
including the staging, sequencing, and construction quality
control for the project. Additionally, the logistics of sand
recovery and transport on one of Florida’s busiest sections
of beach are discussed. Finally, the performance of the structures
is analyzed through short-term post construction monitoring
data.
INTRODUCTION
Erosional hot spots are defined as areas within a littoral
cell that experience higher than average levels of erosion.
Therefore, the erosional activity at hot spots can govern
the frequency of beach re nourishments for a stretch of shoreline.
The mechanisms for the localized levels of higher erosional
activity, while not fully defined are thought to include irregularities
in the shoreline, offshore bathymetry, coastal development,
etc. The study of the causes of hot spots is essential to
provide design criteria to increase the performance of beach
renourishment projects.
The Miami Beach littoral cell extends from Baker's Haulover
Inlet to Government Cut, a distance of approximately 13 miles.
The 32nd Street shoreline is part of the Beach Erosion and
Hurricane Protection Project for Dade County-a federally sponsored
project. A study of the Miami-Dade county regional sediment
budget (Coastal Systems, 1997) that considered the performance
of beach nourishments since the inception of the federal project
determined the existence of several hot spots within the county.
One of the more severely eroding areas within the county was
the 32nd Street area of Miami Beach, where the shoreline receded
an average of 17 feet per year from 1980 to 1996. It was concluded
that the higher localized levels of erosion of the shoreline
near 32nd Street were due to a protrusion of the shoreline
resulting from post-war development beyond the historical
dune line. Furthermore, it was concluded that an overall change
in the shoreline orientation near 32nd Street could be partly
responsible for the increased erosion rate.
A more detailed study of the 32nd Street hot spot (Coastal
Systems, 2000) examined possible stabilization alternatives
based on predicted performance, construction cost, and potential
downdrift and environmental impacts. The study used numerical
models including GENESIS and REF-DIF to predict shoreline
response to the various stabilization schemes. The results
of the REF-DIF modeling demonstrated that offshore bathymetry
coupled with the change in shoreline orientation did promote
the focusing of wave energy in the 32nd Street area. These
factors were predominantly responsible for the presence of
the hot spot. In addition, the study concluded that the protrusion
of the shoreline would potentially cause any unprotected beach
fill to be 'sheared off' rapidly. The use of structures to
'step' the change in shoreline orientation would result in
better beach fill performance. The study recommended the construction
of three artificial headlands coupled with beach fill as the
best alternative for meeting the project goals.
Finally (Sasso and Shah, 2001, the possible impacts of headland
construction on the waves and currents were examined to determine
the effects of the structures on the existing longshore currents
and the corresponding littoral drift in the region). The MIKE
21 wave and hydrodynamic modeling package developed by the
Danish Hydraulic Institute (DHI) was used to simulate waves
and currents at the hot spot. The current model demonstrated
the headlands did not block or significantly redirect longshore
currents. However, the testing of alternate configurations
permitted the optimization of the headland configuration,
minimizing the interference of the structures on the longshore
drift, while enhancing the protection at the hot spot.
CONCEPT
The concept, which was designed in 2000 and constructed in
2002, incorporates artificial headlands and sand backpassing
to stabilize the shoreline in the vicinity of 32nd Street.
The project was designated a demonstration project as it incorporates
several innovative approaches to sediment management including:
1. Regional Sediment Management: The control of hot
spots within littoral cells and nourishment projects is one
of the primary objectives of regional sediment management.
Ultimately, the control of hot spots will increase the overall
efficiency and benefit of renourishment activities by reducing
the erosional stress at the hot spot and adjacent areas.
2. Recycled Materials: To
reduce construction costs, the use of recycled material armor
units constructed from waste concrete was implemented. These
tetrahedronal shaped units, which have been successfully used
as artificial reef materials, were designed to be used as
an underlayer in the structures. These units have the additional
advantage of being of high density relative to local limerock.
3. Sand Backpassing: Conceptually,
this process reverses the direction of the natural drift,
by recirculating sand from accreting downdrift areas to the
project site. The area to the south of the project is an extensively
wide area of beach (200-300’+), and is accreting. This
process represents a cost-effective way of maximizing sand
resources, eliminating the need to inject costly new sand
into the system.
The resulting project controls the accelerated erosion at
the Hot Spot through a system of nearshore headland structures
. These artificial headland structures will gradually step
the shoreline around the severe change in shoreline orientation,
and transition the shoreline into a relatively stable section
of beach. In addition, the structures were designed to be
prefilled in order to minimize the disruption to the longshore
sediment budget. The concept is presented in Figure
2.
CONSTRUCTION
The project was constructed between March and June 2002, with
the project sponsors being Miami-Dade Department of Environmental
Resources Management (DERM) and the City of Miami Beach. Table
1 summarizes the overall parameters of the project.
Table 1: Summary of Artificial Headland
Construction
| Construction Cost |
$1.6 Million |
| Armor Stone |
7,200 Tons |
| Sand Prefill |
125,000 cubic yards |
| Structure Crest Lengths |
225’, 180’ and 150’ |
| Depth of Water |
-3’ to -5’
NGVD |
| Crest Elevation |
+5’ NGVD |
Artificial Headlands:
Construction of the artificial headlands and sand prefill
occurred concurrently. Due to the shallow depths of the structures,
upland construction was the most cost effective and most practical
method of construction. Initially, sand access roads were
constructed from the shore to the center of the structures.
This road was used to transport bedding stone and armor stone
out to the structure. Bedding stone was placed and immediately
armored with stone to prevent washout. A summary of the construction
techniques and issues is presented below.
- Geotextile:
Installed from the water by a team of divers. Sections
of geotextile were shackled to preset stakes, and
these sections were overlapped three feet at the seams.
The geotextile was then anchored with 12” of
bedding stone.
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- Tetrahedrons:
Constructed from recycled concrete, and with the use
of these units intended to be used as an underlayer
to limerock armor stone. Problems included cold joints
and damage, poor quality aggregate, and supply time.
As a result, 239 were used in the smallest breakwater
only.
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- Sand Tightening:
The tetrahedron core had large voids due to angular
shape of the units. Therefore, bedding stone was used
to chink the voids to tighten the structures, and
prevent sand losses to offshore.
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- Armor Stone:
Local limerock stone with a mean density of 130 pcf
and size ranging in diameter from five to six feet
was trucked to the site and placed with a long-reach
grapple.
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Sand Backpassing:
Sand was transported by truck from the recovery area,
which was approximately 1.5 miles to the south. Five articulated
all-terrain dump trucks were used to transport the sand, and
were able to achieve 7,000 cubic yards of material hauled
per day.
Sand was stockpiled at the 32nd Street construction area
and was graded seaward to conform to the headlands and bays.
A summary of the construction techniques and issues follows.
- Borrow Area:
Sand was recovered from 200’ by 200’ sections
of the beach, and these areas were restricted from
public access. An excavator mined the sand starting
at the high water line and worked landward excavating
to a depth of approximately four feet. Each 200’
by 200’ section was graded to match the natural
beach profiles prior to being re-opened to the public.
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- Sand Filling:
Continuous wave action made prefilling of the structures
difficult. Experience proved that stockpiling and
mass filling was the most effective way of prefilling
behind the structures. Large volumes of sand, approximately
15,000 cubic yards, were graded by several bulldozers.
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Construction was completed in June 2002, whereupon the monitoring
of the performance of the artificial headlands in controlling
the Hot Spot commenced.
PERFORMANCE MONITORING
Due to the concern for downdrift impacts, the permit conditions
for this project require that the shoreline response in t
he vicinity of the structures be closely monitored for four
years. The adopted monitoring plan requires the monitoring
of xx profiles updrift, downdrift and within the structures.
The results of the 1st quarterly survey, performed in October
2002, are presented for four representative profiles. The
four profiles chosen present the updrift, within structures,
and downdrift performance of the project, and the locations
of the profiles are identified in Figure 6. The pre-construction
(Jan 2002), design profile, post-construction (June 2002)
and the results of the quarterly survey (October 2002) are
shown in Figure 7.
A summary of the monitoring results is
presented below:
- Station 2+00: This profile is 200
feet south of the start of filling, and is within
the taper updrift of the first structure. At this
location the beach berm was extended approximately
50 feet. The profile adjustment since construction
was minor, demonstrating only a small amount of re-equilibration.
Offshore a small bar is noted and likely a response
to wave action.
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- Station 8+00: This profile is immediately
north of the first structure, at this location the
berm was extended approximately 170 feet during construction.
In planform, the shoreline has taken the expected
fillet shape typical of this type of structure. Since
construction the shoreline has receded largely through
reequilibration to a shallower more natural profile.
Consistent with the reequilibration process, the overall
shape of the profile is tending towards that surveyed
pre-construction, however the beach is approximately
75’ wider as a result of the structures downdrift.
It is expected that some additional accretion will
occur updrift of the structures as the structures
impound sand, however excessive accretion is not desired
due to the associated downdrift erosion.
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- Station 12+25: This profile is within
the first parabolic bay formed between structures
1 and 2. Between the structures the shoreline was
extended seaward approximately 100’ during construction.
The shape of the bay has been stable since construction,
and consistent with expectations, the slope of the
profile has adjusted to one that is less steep. According
to parabolic bay theory (Hsu & Silvester, 1993),
the planform of the bay should adjust according to
the immediate wave conditions while remaining anchored
by the artificial headlands.
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- Station 12+25: This profile is within
the first parabolic bay formed between structures
1 and 2. Between the structures the shoreline was
extended seaward approximately 100’ during construction.
The shape of the bay has been stable since construction,
and consistent with expectations, the slope of the
profile has adjusted to one that is less steep. According
to parabolic bay theory (Hsu & Silvester, 1993),
the planform of the bay should adjust according to
the immediate wave conditions while remaining anchored
by the artificial headlands.
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- Station 21+00: This profile is downdrift
of the southernmost headland, therefore no fill was
placed in this location. This area is most susceptible
to downdrift impacts as it is in the shadow zone of
the last structure. The area is one of the primary
trigger locations and will be monitored to ensure
downdrift impacts are not significant. Since construction,
a slight reshaping of the profile has occurred, however
the volume of material in the profile is approximately
constant.
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CONCLUSIONS
- The construction of near shore structures
in the surf zone is possible, if staging and construction
sequence is properly planned.
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- Sand backpassing along a busy stretch
of beach by trucks is possible and is not a public
relations problem if correctly planned and implemented.
Thus the Hot Spot was renourished without drawing
on dwindling offshore sand reserves.
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- In the short time frame since construction
the artificial headlands have controlled the Hot Spot
at 32nd Street.
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- The first quarterly performance surveys
demonstrate that the impact to longshore currents
is minimal, as predicted in the numerical modeling
phase. Downdrift erosion has not occurred south of
the structures. Within the structures the parabolic
bays and stabilize a wide recreational beach
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- Further monitoring will be of great
interest to determine the long term impact of the
structures in controlling the Hot Spot.
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|
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| The 32nd Street Breakwaters as the currently
stand. |
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| Figure 1: Project Shoreline 1997 Looking
South |
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| Figure 2: Conceptual Plan - 32nd
Street Hot Spot Project |
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| Figure 3: Headland Construction in Progress |
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| Figure 4: Stockpiling of Sand Prior to
Grading |
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Figure 5: Completed Project, 2002 Looking
North |
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| Figure 6: Location of Cross Shore Monitoring
Profiles |
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| Figure 7: Monitoring Survey Profiles |
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REFERENCES
Coastal Systems International, "Dade County Regional
Sediment Budget," Submitted to the Department of
Environmental Resources Management, Dade County, Florida,
1997.
Coastal Systems International, "City of Miami Beach
Erosional Hot Spots," Submitted to the Department
of Environmental Resources Management, Dade County,
Florida, 2000.
Hsu, J.R. and Silvester, R. (1993), "Coastal Stabilization
- Innovative Concepts," New Jersey: Prentice Hall.
Sasso, R.H. and Shah, A.M. (2001). “ Miami Beach
32nd Street Hot Spot: Numerical Modeling and Design
Optimization,” Proceedings 14th Annual National
Conference of Beach
Preservation Technology, 2001. |
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