Publications

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.
  • 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.
  • 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.
  • 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.

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.
  • 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.

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.
  • 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.
  • 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.
  • 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.
  • 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.

CONCLUSIONS

  • The construction of near shore structures in the surf zone is possible, if staging and construction sequence is properly planned.
  • 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.
  • In the short time frame since construction the artificial headlands have controlled the Hot Spot at 32nd Street.
  • 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
  • Further monitoring will be of great interest to determine the long term impact of the structures in controlling the Hot Spot.
The 32nd Stret Breakwaters as the currently stand.
The 32nd Street Breakwaters as the currently stand.
Figure 1: Project Shoreline 1997 Looking South
Figure 1: Project Shoreline 1997 Looking South
Figure 2: Conceptual Plan – 32nd Street Hot Spot Project
Figure 2: Conceptual Plan – 32nd Street Hot Spot Project
Figure 3: Headland Construction in Progress
Figure 3: Headland Construction in Progress
Figure 4: Stockpiling of Sand Prior to Grading
Figure 4: Stockpiling of Sand Prior to Grading

Figure 5: Completed Project, 2002 Looking North

Figure 5: Completed Project, 2002 Looking North

Figure 7: Monitoring Survey Profiles
Figure 6: Location of Cross Shore Monitoring Profiles
Figure 7: Monitoring Survey Profiles
Figure 7: Monitoring Survey Profiles
 

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.

Adam M. Shah P.E.-Project Manager, CSI
R. Harvey Sasso, P.E.-Principal, CSI

Download a printable PDF version of this publication
 
www.coastalsystemsint.com