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.  


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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