Project Description

Terminus Wake Park – Shoreline Erosion Control

  • Project: Terminus Wake Park HydroTurf
  • Project Type: Wake Park Shoreline Erosion Control
  • Location: Emerson GA
  • Owner: Terminus
  • Completion: 2014
  • Engineer: NA
  • Size: 4 acres
  • Project Profile Terminus Wake Park Project Profile
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  • I. Introduction
  • The newly opened Terminus Wake Park is located within the massive LakePoint Sporting Community & Town Center Complex in Emerson, GA, a site right off Interstate 75, approximately 30 miles north of downtown Atlanta. The park represents one of the latest examples in a growing trend in extreme sports- Cable Wakeboarding, in which wakeboard riders are pulled, not by a boat, but by a moving cable system suspended by towers surrounding a lake. While wakeboarding parks are a relatively new concept, they often face a very familiar civil engineering challenge- controlling erosion of the lake banks, specifically from wave attack created both by wind and the wakeboard riders. The Terminus Wake Park chose to address this challenge with a relatively new technology that utilizes engineered synthetic turf with an integrated heat-welded polyethylene geomembrane, which is then infilled with a high-strength cementitious material.
  • II. Project Background
  • The Terminus Wake Park, covering roughly twenty (20) acres, is comprised of a total of three (3) lakes. The first lake, at roughly one (1) acre, is a “beginner” lake designed to introduce people to the sport of wakeboarding and teach them the basics. Lake number two is more of an intermediate lake, covering approximately five (5) acres and is designed to develop rider’s abilities through increased experience and exposure to jumps, rails and other stunt-style props. The largest lake, which covers approximately seven (7) acres, is the advanced lake, and will serve as the site for numerous national and international wakeboarding competitions. The park is located within the LakePoint Sporting Community, an enormous destination-resort for travel sports, including baseball, soccer, lacrosse, volleyball, softball and many others, spread across 1,300+ acres. Like the Terminus Wake Park, LakePoint is an extraordinarily unique facility, and will play host to literally thousands of athletes and their families in multiple sports. In between competitions, the athletes and their families will have at their immediate disposal numerous hotels, restaurants, and entertainment options, including of course, the Terminus Wake Park.
  • The nature of the design for the wake park’s three lakes offered some rather unique challenges. Controlling erosion on the banks was the top priority, but other criteria needed to be considered as well. For example, wakeboard riders entering and exiting the lake needed to be able to navigate the banks comfortably and safely. In addition, the high-profile location within the larger LakePoint facility meant that aesthetics would be critically important. Finally, maintenance of the protection system had to be minimized. After all, a lake that is shut down while the banks are being maintained means that no one is able to ride, which is bad news for Terminus and their clients.
  • III. Solution Evaluation and Selection
  • The potential for erosion on the banks of the three lakes was significant. The reality of wakeboarders circling the lake continuously for 8-12 hours per day, in season, plus the potential for wind-driven waves, year round, meant that wave attack on the banks would be an almost daily occurrence. Traditionally, wave-attack erosion applications are treated with some form of hard armor, such as rock riprap, concrete lining, or articulated concrete blocks (acb’s), which are all good examples of the more popular “go-to” methods. Each of these methods has their relative strengths and weaknesses, and each was considered for protecting the lake banks against erosion at Terminus. While any of these traditional methods might have likely done an adequate job of controlling erosion on the banks, each included some undesirable or unacceptable caveat, whether it be maintenance, aesthetics, financial or some combination of all of these.
  • Fortunately, a new system, HydroTurf® Advanced Revetment Technology (Patent Pending), by Watershed Geosynthetics, LLC (Alpharetta, GA), was presented to the project stakeholders and appeared to provide all of the positive attributes of the traditional methods with none of the limitations. HydroTurf® can perhaps best be described as an integrated, geomembrane-backed, fiber-reinforced, concrete lining system. Essentially, it is comprised of engineered synthetic turf, integrated with a polyethylene geomembrane to effectively eliminate water intrusion onto the subgrade and increase subgrade interface friction. Once in place, the turf is then infilled with a dry cementitious mix called HydroBinderTM, which includes a 5,000 PSI thin-set concrete technology, a dust suppressant, a color additive which can be customized to match existing soil conditions, as well as other proprietary ingredients. Once the infill is in place, it is hydrated and begins curing immediately. The integrated polyethylene geomembrane serves to keep water off of the subgrade. The 5,000 PSI HydroBinder infill serves both as ballast and as impact and scour protection for the system. The engineered synthetic turf is dual-purposed as well, serving an aesthetic value in the form of realistic-looking turf grass as well as a functional value in the form of continuous fiber-reinforcement and substrate anchoring for the cementitious infill. The team at Terminus selected a turf that was comprised of 100% green fiber, but the color options include green, tan and green/tan mixes to maximize an aesthetic match in any given climate.
  • Upon introduction to this new system, an evaluation of HydroTurf® by the Terminus team as a possible solution for their project ensued. Consistent with the approach taken with the more traditional hard armor solutions, the first analysis was evaluating the system’s suitability for controlling erosion of the banks from the primary application challenge- wave attack.
  • Extensive hydraulic testing on the HydroTurf® system had recently been completed at the Hydraulic Laboratory of the Engineering Research Center at Colorado State University (CSU) in Fort Collins, CO. Specifically the following tests were conducted: Steady State Hydraulic Testing, including Steady State Overtopping, Hydraulic Jump, Heavy Debris Load (Impact and Abrasion), and Intentional Damage. In addition, Wave Overtop Testing for Levee Landward-Side Slope Protection, and again intentional damage, but in a wave overtop condition, was conducted as well. In all tests, the full-scale flume and wave overtop flume were run to their maximum capacity, and the performance threshold of the system was not realized. Specifically, no erosion or instability of the system was observed at any time either during or upon completion of any of the tests.
  • Of specific interest to the Terminus team was the performance of the HydroTurf® system in the steady-state hydraulic jump and wave overtop testing. CSU calculated the maximum energy dissipation results of the system in the hydraulic jump test at 120 horsepower. Figure 1 illustrates the relationship between the power dissipation of the system as a function of the specific energy at the entrance to the hydraulic jump. In addition, the system was tested for 13 hours in the CSU Wave Overtopping Simulator, up to the maximum overtopping flows of 4 cubic feet per second per foot and a cumulative total volume of 165,600 cubic feet per foot. Given the fact that this maximized test volume equates to a 500-year hurricane event in New Orleans, LA, the team determined the HydroTurf® system was fully capable of providing the necessary protection of the banks from wave attack.
  • Chart showing Power Dissipation as a Function of Specific Energy, Hydraulic Jump for HydroTurf® at Terminus Lake

    Figure 1

  • Figure 1 – Power Dissipation as a Function of Specific Energy at the Entrance to the Hydraulic Jump for HydroTurf® (Hydraulic Laboratory at the Engineering Research Center, Colorado State University)
  • With the question of the technical suitability answered to their satisfaction, the team moved down the checklist to the next selection criterion – accessibility and safety of the wake boarders walking on the system while entering and exiting the respective lakes. At first glance, the team felt this would be a non-issue. However, upon further analysis, two concerns were raised that required answers- one, given that a significant portion of the system would remain in a submerged condition, would the synthetic turf surface potentially be too slippery for safe accessing on the lake banks, especially since the finished slopes were estimated to range from 4:1 to 6:1; and two, would the cementitious infill prove to be either too hard or too abrasive, or both, for comfortable trafficking in bare feet. It was decided that given the subjective nature of these concerns, the only effective method for evaluation would involve the installation of a test section on which these concerns could be fully vetted.
  • In September of 2013, a test section measuring approximately 20′ x 30′ was installed at the Terminus site. The installation was placed directly on the bank of lake number 3, the largest lake reserved for advanced riders and competitions. The water level was then raised so as to simulate what would be ultimate real-world conditions as closely as possible (See Photograph 1). The evaluation of accessibility and safety were initiated, and it was quickly determined that the concerns over the system being either too slippery or abrasive were quickly abated. In addition, the sample plot provided the team the additional opportunity of assessing the aesthetics of the system, an aforementioned selection criterion for the preferred solution. It was quickly determined that the appearance of the system not only far exceeded the aesthetics of the other traditional hard armor solutions, but would serve the prominent location within LakePoint most effectively
  • IV. Design and Construction
  • Now that the final bank protection material was selected, the design could be updated and installation of the HydroTurf® system could be inserted into the construction schedule. Up to this point, the wake park had been operating for a few months, albeit on a somewhat limited basis, without any bank protection in place. This afforded the owner the benefit of a revenue stream before final completion of the project. The multiple lakes allowed a phased approach to the construction schedule. The first lake to be treated with the final bank protection system was lake number two, the intermediate-level lake, which happened to be situated on the southern-most side of the Terminus site. The first step of the installation involved dewatering the lake, which simply involved transferring the water from lake number two to lake number three and lake number one. Once the lake was dewatered and the subgrade was allowed to dry, the banks were graded to final elevation. Lake numbers two and three both included an island in the center of each of the respective lakes. The HydroTurf® system was going to be installed on both the outer banks of these lakes as well as the island banks within each. (See Photograph 2 for Site Layout).
  • Once the banks were at final grade, preparation of the subgrade was undertaken, which simply entailed basic smoothing and finishing. The roll size of the material used on this project was 12′ x 300′. At the bottom of the banks, the system was anchored in an anchor-termination trench measuring approximately 12″ x 12″ (30cm x 30cm). The location of the bottom trench was below the minimum design water depth, so as to prevent erosion of the unlined lake bottom from potential headcutting underneath the HydroTurf® armoring layer. At the top of the banks, the system was anchored in a 6″ x 6″ (15cm x 15cm) anchor trench. Both the bottom and top anchor trenches were filled with grout. Seams between adjacent 12′ roll-widths were overlapped roughly 4″-6″ (10cm – 15cm) and heat-welded using a wedge-type welder or hand-held hot air blower. Both the engineered synthetic turf material and the coating on the back of the system are constructed of polyethylene, making it relatively easy to achieve an effective, water-resistant heat-bonded weld between the materials.
  • The installation was sequenced such that all of the turf installed each day would also be infilled with the cementitious material the same day, and the corresponding anchor trenches would be filled with grout, thereby minimizing the potential for problems due to weather changes or sudden storms. Average installation rates varied depending upon weather and labor, but typically fell within the range of 10,000 – 20,000 square feet per day. (Please see installation sequence photographs below).
  • Once the crew completed work on Lake Two, they switched to Lake One, the beginner’s training lake. The configuration of this lake was different from that of the other two. Instead of being oval-shaped with an island in the center, this lake was more long and thin, with approximate dimensions of 90′ x 350′. In addition, where the cable movement of the other lakes involved continuous, clockwise rotation, this lake allows for movement either in the south-to-north direction or vice-versa, but not both at the same time. The bank treatment for all three lakes was largely the same, albeit the depth of Lake One was somewhat shallower than that of Lake’s Two and Three, due mostly to the less aggressive activities planned for this lake.
  • The final treatment was applied to Lake Three, the advanced skills and competition lake. Again, this lake was installed in much the same manner as the other two, including the island banks in the center of the lake. Total installed quantities of the HydroTurf® system approximated 200,000 Square Feet, with the breakdown as follows: Lake One (Beginner Lake) –20,000 Square Feet; Lake Two (Intermediate) Lake and Island – 60,000 Square Feet; Lake Three and Island – 70,000 Square Feet. It was decided to install the system on the two man-made isthmuses, one between Lake One and Three, and the other between Lake Three and Two. The result was an attractive, durable and virtually maintenance-free armoring system that supports barefoot trafficking comfortably.
  • V. Conclusions
  • While cable-pulled wakeboarding parks may be a relatively new phenomenon, bank protection from wave attack has been a civil engineering challenge for a very long time. Historically, tradeoffs have had to be made between performance, economics, aesthetics and maintenance in the selection process for treating this common application challenge. The Terminus Wake Park perhaps serves as the first project in a new era where these tradeoffs are no longer necessary.