Mt. Woodson Reservoir Floating Cover Replacement - Ramona, California
Tensioned Cable Floating Cover removal and replacement for municipal potable water treatment and storage
45 mil CSPE geomembrane
The City of Ramona Municipal Water District had planned to replace an existing 29-year-old 45mil CSPE floating cover in the next two to three years. However, as part of a strategic risk management review of their existing water storage facilities, they decided to move forward with this replacement in 2020. As the material was reaching the end of its 30 year warranty period, the City had budget capacity and determined it important in terms of long term preventative maintenance of this reservoir to proceed with a new cover replacement a year or two early even though it was still functioning without tears or damage.
The project required demolition and removal of the existing 29 year old CSPE geomembrane cover and the supply and installation of a new replacement 45 mil beige/tan CSPE geomembrane floating cover. The project faced several logistical challenges, including limited working area and not being able to take the exiting reservoir out of service during installation of the new cover system to prevent the stored potable water from being exposed to environmental contamination during construction. To fulfill this requirement, the existing geomembrane cover had to be removed simultaneously and in sequence with installation of the new geomembrane cover. This required welding the leading edge of the new geomembrane cover directly to the end of the old geomembrane cover material. As the old geomembrane was pulled off the reservoir, the new geomembrane was floated into place. The irregular shape of the reservoir, coupled with the limited working area, prevented deployment and welding of large prefabricated geomembrane panels along the edge of the reservoir, which would have accelerated construction. To address the tight constraints, Layfield constructed a 14,000 ft2 scaffold supported wood platform located at the northwest corner of the reservoir as a staging area for the geomembrane panels. This wood platform provided the additional work area required to stage, unroll, and weld the prefabricated panels for the new floating geomembrane cover systems.
The geomembrane cover system was tensioned around the perimeter with 5 ft (1.5 m) high mechanical weighted tensioning towers installed on 8 ft (2.4 m) centers for the original installation. These steel tensioning towers were installed 29 years ago and required some refurbishing of components prior to being reused for the new cover system. The tensioning towers include pulleys and weights that incorporate steel cables connected to a reinforced designed CSPE geomembrane strip attached to the cover geomembrane. This perimeter cable system provides the required tensioning and buoyancy that allows the cover geomembrane to fluctuate with the water freeboard levels and support personnel on top of the geomembrane for maintenance and cleaning.
This was a challenging floating cover system because of the irregular shape of the reservoir, tight working space around the reservoir, and environmental conditions that had to be dealt with, such as not being able to expose the potable water to open air at any time during the construction. The project team demonstrated creativity and engineering in their approach to resolve these challenges. This installation would not have been possible without using a flexible geomembrane that allowed large factory/prefabricated panels to be supplied to the site to minimize the amount of field seaming and geomembrane handling. The design and construction of the 14,000 ft wood scaffoldplatform was critical to allow sufficient workspace to unroll and weld the newCSPE geomembrane to the existing cover geomembrane before installation. The project also proved that the new CSPE geomembranecould be welded to the 29 old CSPE geomembrane allowing the installation to becompleted without exposing the underlying potable water to the environmentduring construction.
HOW THE USE OF FABRICATION IMPROVED THIS PROJECT
The 45 mil (1.14 mm) thick CSPE geomembrane for the cover system was prefabricated in Layfield’s Lakeside, California factory and shipped to the project site in custom size panels that are 35 ft wide without folds by various lengths allowing for the irregular shaped configuration of the reservoir to be covered. Additional cover accessories, including float tubes, sandbags, and hatch doors, were also prefabricated in the factory. The factory fabrication of large geomembrane panels significantly reduced the amount of field welding, field testing, patching, time, and project costs.
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The FGI Pond Leakage Calculator Geomembranes v. Compacted Clay Liners
Fresh water is a precious resource with demands rising daily and supply greatly fluctuating. Only two (2) percent of all water on Earth is fresh water with the other 98% being salt water. This 2% of fresh water is comprised of: 87% ice, 12% groundwater, and 1% rivers and lakes. Thus, only 13% of the available freshwater is readily accessible. Therefore, it is imperative that we capture and hold these limited water resources for agriculture, domestic use, and industry and also protect valuable groundwater from surface or subsurface contamination.
The FGI’s Pond Leakage Calculator is a Microsoft EXCEL spreadsheet based on Darcy's Law of Seepage and provides a comparison between leakage rates from a canal, pond, or reservoir constructed with compacted fine-grained soils and a geomembrane liner system. The Leakage Calculator allows the user to input the size of the containment basin (including length, width, depth, side slope angle and freeboard), the anticipated level of hydraulic conductivity of the compacted soil or geomembrane liner, and the relative cost of water in dollars per acre-foot of water.
The Calculator then calculates the volume of the basin in gallons, a comparison of leakage rates between the compacted soil and geomembrane liner systems in gallons, and the cost of the leakage based on the cost of water per acre-foot to replace it. This Calculator is designed to help consultants, engineers, architects, and end users decide how to line their canals, ponds, reservoirs, and basins to capture and/or protect valuable fresh water. The Calculator does not consider variances in construction quality and operational techniques on the long-term effectiveness of the chosen liner system. The FGI has additional research and publications to help with other aspects of successful water containment applications.
Types of Geomembranes Four (4) popular types of geomembranes are available for pond liner systems. These four (4) geomembranes in ALPHABETICAL order are: (1) EPDM, (2) reinforced polyethylene (RPE), (3) Polypropylene (PP), and (4) Polyvinyl Chloride (PVC). EPDM (Ethylene Propylene Diene Monomer) geomembranes are unreinforced and have been used for the construction of ponds of varying kinds. EPDM geomembranes are made from rubber and can be welded together with tape and primer. EPDM can be reinforced or unreinforced. RPE geomembranes have a high tensile strength and puncture resistance because they are reinforced. RPE geomembranes also can be welded with heat. PP geomembranes can be unreinforced or reinforced depending on the application. Reinforced PP geomembranes also have a high tensile strength and puncture resistance because they are reinforced. PVC geomembranes are also unreinforced and have been used successfully for decades in water canals, ponds, and reservoirs. PVC geomembranes can be welded with heat and/or solvents.
Please click below to access FGI’s Pond Leakage Calculator.
On 10 June 2022 during a trip to Brisbane, Australia for the GeoANZ#1 Conference (a geosynthetics conference organized by the Australasian Chapter of the International Geosynthetics Society (ACIGS), Tim Stark visited the offices and factory fabrication facility of Fabtech in Adelaide, Australia. Tim met with Graham Fairhead, CEO; Mark Bennett, Construction Manager; and Daren Noonan, Factory Manager of Fabtech (see Figure 1) to discuss current and future FGI activities, including upcoming FGI webinars, MQA and CQA training, welder certification, and current University of Illinois research on action leakage rates, carbon footprint calculator, and various operation and maintenance guidelines. Tim also met with other Fabtech employees, SA Water representatives, and toured Fabtech’s factory fabrication facilities. Graham showed Tim some current fabrication projects and new fabrication capabilities including the ability to factory fabricate, test, and roll large tank liners (see Figure 1), floating cover system floats, hatches, and air vents, and development of panel layout diagrams to minimize field seaming. Mark Bennett also took Tim to observe installation of a new bottom liner system and floating cover for the Potts Hill Water Reservoir near Sydney, Australia on 15June 2022. Fabtech is designing and installing the 367,000 square meter bottom liner system and the accompanying floating cover for the Sydney Water Board, which makes it the largest in Australia.
Dr. Stark hopes to visit the facilities of all FGI members to better understand each member’s business and how the FGI can provide additional benefit to them. The FGI is an exciting advancement in the geomembrane industry and allows the Institute to focus on all geomembranes that can be factory fabricated.