An Engineered Approach
Mitigation for hardbottom impacts is a complicated, costly
component of a beach nourishment project. After beach nourishment
project impacts have been avoided and minimized to the greatest
extent possible, regulatory permits generally require mitigation
of hardbottom impacts from the fully equilibrated beach profile
by constructing artificial reefs. These mitigation sites are
typically required to be placed as close as possible to the
impact areas. Therefore, artificial reef construction in the
nearshore zone requires innovative construction methodologies
due to the proximity of sensitive
hardbottom marine resources and shallow water.
The Broward County (Segment III) Shore Protection Project
is predicted to impact 7.6 acres of nearshore hardbottom habitat.
Regulatory agencies required 10.1 gross acres of artificial
reef mitigation in the form of limerock boulders placed between
existing reefs in 15-foot water depths at three sites in the
vicinity of the beach fill. The project was constructed from
June through September 2003, and involved the precise placement
of approximately 66,000 tons of 4 to 6 foot diameter (nominal)
limestone boulders acquired from a quarry on Grand Bahama
Due to the construction in narrow, environmentally sensitive
areas, a designbuild approach was used by the contractor to
plan and implement the project. Components of the construction
consisted of an innovative pilebased crane barge mooring system
designed to efficiently operate in between hardbottom areas
in shallow water and not impact marine resources with cables
and/or anchors. Underwater video mapping techniques were utilized
to confirm sandy bottom conditions for boulder placement and
to monitor construction operations. Precise DGPS positioning
equipment and software was employed to control boulder placement
by the crane. This approach to artificial reef construction
ensured the mitigation project was completed on schedule with
no impacts to adjacent hardbottom resources.
The Broward County (County) Beach Restoration Project is planned
to place 2.5 million cubic yards (cy) of sand on approximately
11.8 miles of shoreline. The resulting widened beaches are
expected to bury approximately 13.5 acres of nearshore hardbottom
during equilibrium of the beach fill. The hardbottom substrate
provides habitat for many benthic and fish communities, despite
the shallow water and dynamic wave environment. Mitigation
of these unavoidable impacts was required.
The Broward County Beach Restoration Project is divided into
three segments as follows in Table 1 and as illustrated in
Construction of the Segment III Beach Restoration was prioritized
with construction planned for the summer of 2004. The monitoring
and performance of the Segment III mitigation project is a
requirement prior to the issuance of environmental permits
for construction of Segment II.
The nearshore reef is generally characterized by low topographic
relief, and the marine resources are of lower biological diversity
and density as opposed to the offshore reefs in Broward County.
Mitigation for these impacts is required to be constructed
as close as possible to the impact areas. The mitigation reefs
were to be constructed by September, 2003, followed by coral
transplantation from the predicted impact nearshore reefs.
This program schedule, negotiated over two years with federal
and state regulatory and resource agencies, ensures that the
mitigation would be in place and functioning prior to the
construction of Segment III Beach Restoration.
The Segment II and III Beach Restoration projects require
the construction of 11.9 acres of substrate within a permitted
13.5 acre footprint. Specifically for Segment III, 10.1 acres
were required to compensate for 7.6 acres of predicted impacts.
The requirement is based on a 1.2:1 mitigation ratio. The
Segment III mitigation was prioritized for initial construction,
with Segment II mitigation to follow at a later date.
Limestone boulders were specified in order to provide a substrate
suitable as mitigation since the variable relief of local
nearshore hardbottom is difficult to replicate. Boulders ranging
from 4 feet to 6 feet were analyzed for stability in the nearshore
wave environment, and allow for predicted settlement in sandy
bottoms adjacent to hard bottom areas. The selected sandy
bottom areas consisted of approximately 3 feet of sand over
hardbottom to prevent excessive scour or settlement. The distribution
of limestone boulders will provide a rugosity of 1.56, which
represents a 44% increase in mitigation reef rugosity over
natural nearshore hard bottom.
Mitigation Construction Constraints
The County permitted 5 sites as follows for Segment III mitigation:
Due to the proximity of the marine resources and nearshore
hardbottom, the County required a 50-foot buffer between the
mitigation areas and adjacent hardbottom. Furthermore, construction
operations over offshore reefs and the shallow water (15 feet)
in the mitigation construction areas as shown in Figure
3 presented significant challenges for marine construction.
The County procured the mitigation construction through a
qualificationsbased selection process as opposed to advertising
for bids. A Request for Proposals (RFP) was advertised, and
contractors were "short-listed." Contractors presented
means and methods to complete the construction based on the
constraints and schedule established for the project. The
County was especially concerned about marine construction
in close proximity to nearshore hardbottom and impacts to
resources during past mitigation projects in Florida. Coastal
Systems Development (CSD) based in Coral Gables, Florida was
selected, and a contract to place 66,000 tons of limestone
boulders over 10.1 acres of mitigation areas was negotiated
Construction Operational Plans
Prior to mobilization, Coastal Systems International, Inc.
(CSI) conducted towed underwater video mapping in the Spring
of 2003 of the 5 mitigation sites to confirm the proposed
construction footprints. The tow vehicle is positioned with
DGPS in real time from navigation software to provide project
horizontal coordinates on the video feed. The application
of this technology provided precise limits of the nearshore
hardbottom for mapping as shown in Figure 4 utilizing the
County's LADS data as a base map.
After the onsite hardbottom edge mapping effort, the updated
limits of the hardbottom were overlaid with the proposed mitigation
areas, and it became apparent that there was a shortage of
available acreage within the permitted mitigation areas for
Segment III. The dynamic nature (movement of sand) of the
nearshore hardbottom habitat in conjunction with the 50-foot
required buffer reduced the available acreage. Additional
area was available offshore of Segment II beaches north of
Port Everglades, however it was preferable that mitigation
be located adjacent to the areas of impact.
Additional towed underwater video mapping conducted by CSI
located suitable mitigation areas adjacent to Areas 7 and
11. Jet probes were conducted to confirm the depth of sand
in these new areas for compliance with the mitigation requirements.
Modifications to the environmental permits were submitted
for the revised areas prior to construction.
Construction operational plans were prepared to combine Mitigation
Areas 7 and 8 and Areas 10 and 11 into two main construction
areas along with the additional mitigation areas mapped. Access
corridors were mapped, and all coordinate geometry was established
for project construction. The plans included details for the
crane barge mooring, a temporary single-pointmooring (SPM),
ingress and egress corridors, and the locations of reef areas.
The plans were submitted to the County for review and approval
prior to construction.
Marine construction in the shallow-water nearshore zones requires
careful planning for scheduling of the construction and selection
of the equipment profile. Wave Information Study (WIS) statistics
were reviewed for the project site prior to construction,
and a statistical analysis was performed. The analysis provided
wave characteristics (height, period, direction) in the form
of exceedance probability tables to determine optimum months
for acceptable wave conditions for construction. This combination
of coastal engineering along with local marine construction
experience indicated that optimum construction period was
from mid-May through mid-September (summer months). Conventional
floating barge equipment could be utilized to meet the production
schedule, although 24-hour/7 days a week construction was
required to complete the construction based on estimated production
rates and the large quantity of boulders to be placed. The
coastal engineering analysis was also used to provide design
wave characteristics for both the crane barge mooring system
and offshore SPM for temporary supply barge mooring.
Moorings were designed for both the crane barge that would
be used for placing boulders as well as the temporary single-point
mooring (SPM) used for the storage of loaded/unloaded barges.
The mooring design took into account following construction
- Provide safe mooring during normal
- Use of available mooring hardware,
chains, cables, and anchors
- Provide maximum safe operating conditions
(winds, waves) for project managers to make informed
decisions on when to bring floating equipment into
- Protection of marine resources
Crane Barge Moorings: The safe
mooring of floating equipment in close proximity to the shoreline
and reefs presented challenges for the marine construction
operations. Statistical wave/wind data and tidal current data
were used to provide design conditions for the mooring of
the crane barge coupled with a supply barge fully loaded with
boulders. Multiple conditions of waves, winds and currents
were simulated to provide design conditions for the system.
While placing boulders from floating equipment was not possible
in wave conditions greater than 3 to 4 feet in height, construction
managers needed the option to suspend operations temporarily
without the need to demobilize equipment to safe port at considerable
expense in towing costs and delays in the construction schedule.
A dynamic mooring analysis was conducted to determine mooring
forces. A system of pile anchors placed in the 50-foot buffer
was selected as the anchoring system as opposed to conventional
anchors. Steel H-piles were designed for the calculated mooring
loads. The use of pile anchors avoids the potential for dragging
anchors, and negates the need for a separate anchor handling
barge. In addition, the system provided a stable, stiff anchoring
system for precise positioning of boulder placement. However,
the placement of anchors requires careful planning including
review/approval by the construction managers as well as superintendents
to ensure crane reach for all reef placement areas. The pile
anchor locations were established to provide continuous coverage
at optimized space intervals to allow for the barge system
to winch itself with deck-mounted winches to the appropriate
location without tug assistance, and to adjust the heading
for sea conditions. Refer to Figure 6.
Steel cables are typically used for the winch lines of a barge
mooring system. However, floating lines with equivalent strength
of the steel cable were utilized by conventional deck winches.
The floating lines minimized the risk of cable-dragging in
close proximity to reef edges. In addition, all barge towing
operations used floating lines exclusively.
Single Point Mooring (SPM): The
SPM was placed at a convenient location to both reef construction
sites utilizing the LADS data. Placing the SPM in 70 feet
of water was chosen to avoid wave shoaling effects and to
be in between the second and third reefs. Towed underwater
video mapping was conducted to confirm adequate sandy area
for anchors and chain. Similar to the crane barge, a dynamic
mooring analysis was conducted based on combinations of wave,
wind and current loads for the design loading of a fully loaded
boulder supply barge. The SPM was designed for wave heights
up to 8 feet, giving the construction managers flexibility
in planning for various sea state conditions.
The limestone boulders in the specified size range are difficult
to find in large quantities in South Florida. Sourcing 66,000
tons of boulders from quarries would have required over 3,000
truck loads of boulders to be staged at a waterfront site,
then loaded onto barges for transport to the placement sites
offshore. There were no convenient staging areas close to
the construction areas, and this upland source of boulders
would have required doublehandling of the boulders. The expansion
of the port facilities in Freeport on Grand Bahama Island
presented an ideal opportunity to barge boulders directly
to the reef sites without any upland staging area or double-handling
of materials. Most of the boulders had been quarried during
aggregate production, and a temporary barge loading area was
established in the Bahamian port. Several initial loads of
rock contained boulders which exceeded the 4 to 6 foot (nominal)
size range. Though the biologists at the regulatory and resource
agencies did not regard the oversized boulders as problematic
and no permit violations were issued, the contractor's quality
control program was refined to ensure that the remaining boulders
conformed with the nominal specifications.
The construction specifications required precise placement
of boulders to provide a single layer of boulders. Furthermore,
precise positioning was required due to the complex geometry
of the reef sites and proximity of marine resources. A system
of Differential GPS (DGPS) receivers was used to provide the
heading of the barge and positioning of the crane boom in
real time. All of the receivers were interfaced with marine
construction positioning software operating on computers on
aboard the crane barge. CAD files from the reef construction
plans were input directly into the positioning software to
form a "honeycomb" template for the crane operator
to place each boulder in a planned location. A computer located
in the crane cab enabled the crane operator to verify the
location of the crane boom and barge relative to the placement
area. Once a boulder was placed, the crane operator marked
the position. This technology allowed construction to progress
on a 24/7 basis. The positioning equipment also assisted the
superintendent with planning the sequence of construction
and optimizing each barge move. The equipment provided sub-meter
positioning accuracy in real time, and also produced as-built
surveys as the construction progressed. Quality control diving
operations were still required to ensure the continuous layer
due to the boulder size variability and dynamic motion of
the crane barge system.
The engineered approach to this complex marine construction
ensured that the Broward County Segment III Beach Restoration
Project mitigation was completed on schedule, on budget (with
no change orders), and most importantly with no impacts to
marine resources. The pre-construction underwater video mapping
confirmed reef placement areas and protective buffers. Mooring
designs provided safe mooring systems for floating equipment
utilizing coastal engineering analysis results. Precise positioning
of the crane boom provided accurate placement of boulders
within complex geometrical templates.
The resulting reef construction has produced immediate mitigation
results, with juvenile fish and other marine resources that
recruited shortly after boulder placement. The mitigation
was also completed prior to beach construction, allowing for
the transplantation of coral resources from the predicted
nearshore impact areas.
T.K. Blankenship, P.E.-Engineering Department Head, CSI
R. Harvey Sasso, P.E.-Principal, CSI
Stephen Higgins, Kenneth Banks-Broward County, D.P.E.P
|Table 1 - Beach Restoration Project Segments
|Figure 1: Limits of beach restoration,
Broward County, Florida.
|Figure 2: Mitigation requirement for nearshore
|Table 2 - Proposed sites for Segment III
|Figure 3: Mitigation construction areas.
|Figure 4: Towed underwater video mapping
|Figure 5: Monthly wave statistics.
|Figure 6: Crane barge mooring configuration.
|Figure 7: Completed artificial reef configuration.
Boone, W. M., “Bulkheads: Materials of
Construction,” Public Works, May 1985
Underwater Investigations Standard Practice
Manual, ASCE Manuals and Reports on
Engineering practice No. 101, 2001.
MIL-HDBK-1025/4, Seawalls, Bulkheads and
Quaywalls, Department of the Navy, Naval
Facilities Engineering Command,
MIL-HDBK-1025/6, General Criteria
Waterfront Construction, Department of the
Navy, Naval Facilities Engineering Command,
Disclaimer: The material presented in this perspective
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