Reservoirs are used to store raw or treated water. They can be located underground (buried), at ground level, or on an elevated surface. Reservoirs can vary significantly in size; small reservoirs can hold as little as 1,000 gallons, while larger reservoirs may hold many millions of gallons.

Reservoirs can be either natural or man-made. Natural reservoirs can include lakes or other contained water bodies, while man-made reservoirs usually consist of some sort of engineered structure, such as a tank or other impoundment structure. In addition to the water containment structure itself, reservoir systems may also include associated water treatment and distribution equipment, including intakes, pumps, pump houses, piping systems, chemical treatment and chemical storage areas, etc.

Drinking water reservoirs are of particular concern because they are potentially vulnerable to contamination of the stored water, either through direct contamination of the storage area, or through infiltration of the equipment, piping, or chemicals associated with the reservoir. For example, because many drinking water reservoirs are designed as aboveground, open-air structures, they are potentially vulnerable to airborne deposition, bird and animal wastes, human activities, and dissipation of chlorine or other treatment chemicals. However, one of the most serious potential threats to the system is direct contamination of the stored water through dumping contaminants into the reservoir. Utilities have taken various measures to mitigate this type of threat, including fencing off the reservoir, installing cameras to monitor for intruders, and monitoring for changes in water quality. Another option for enhancing security is covering the reservoir using some type of manufactured cover to prevent intruders from gaining physical access to the stored water. Implementing a reservoir cover may or may not be practical depending on the size of the reservoir (for example, covers are not typically used on natural reservoirs because they are too large for the cover to be technically feasible and cost effective). This document will focus on drinking water reservoir covers, where and how they are typically implemented, and how they can be used to reduce the threat of contamination of the stored water. While covers can enhance the reservoir's security, it should be noted that covering a reservoir typically changes the reservoir's operational requirements. For example, vents must be installed in the cover to ensure gas exchange between the stored water and the atmosphere (vent security will be discussed in a future Product Guide). This document will not focus on these types of additional operational requirements; however, utilities should be aware of them when determining whether installation of a reservoir cover is a feasible option for enhancing reservoir security.

A reservoir cover is a structure installed on or over the surface of the reservoir to minimize water quality degradation. The three basic design types for reservoir covers are:

A variety of materials are used when manufacturing a cover, including reinforced concrete, steel, aluminum, polypropylene, chlorosulfonated polyethylene, or ethylene interpolymer alloys. There are several factors that affect a reservoir cover's effectiveness, and thus its ability to protect the stored water. These factors include:

For example, it may not be practical to install a fixed cover over a reservoir if the reservoir is too large or if the local soil conditions cannot support a foundation. A floating or air-supported cover may be more appropriate for these types of applications.

In addition to the practical considerations for installation of these types of covers, there are a number of operations and maintenance (O&M) concerns that affect the utility of a cover for specific applications, including how different cover materials will withstand local climatic conditions, what types of cleaning and maintenance will be required for each particular type of cover, and how these factors will affect the cover's lifespan and its ability to be repaired when it is damaged. These O&M considerations will be discussed under each reservoir cover type below.

Floating covers can be designed to cover all or major portions of a reservoir. They consist of flexible membranes constructed from semi-rigid flat sheets that float on the water's surface. These sheets are typically made from multiple plies of polypropylene, chlorosulfonated polyethylene, high density polyethylene, or ethylene interpolymer alloys. The membrane is typically at least 36 millimeters thick, and the combination of the membrane's thickness and its use of multiple plies of material helps minimize the chance that holes that could allow contaminants to infiltrate into the reservoir will develop in the cover.

One major advantage of floating covers is that they incorporate flexible sections that can fold in on themselves, thus allowing the cover to adapt to changing water levels in the reservoir. For example, as the reservoir fills or drains, these sections can fold in or out to decrease or increase the cover's surface area, respectively. These flexible sections also act as natural channels to drain rainwater and/or wash water off of the cover, ensuring that this water does not drain back into the reservoir.

As discussed above, there are a number of different materials that can be used to design floating covers. Ideally, floating cover materials should be resistant to damage caused by sunlight, ultraviolet light, ozone, harsh climatic conditions, environmental stress cracking, and common chemicals used in maintaining water quality. However, each flexible-membrane material has unique physical properties that make it more or less resistant to different stresses, and the specific type of material chosen for any given application will at least partially depend on that material's ability to withstand local climatic conditions (i.e., snow, rain, sun), as well as other environmental stresses.

Prolonged exposure to extreme weather conditions can cause some cover material types to deteriorate and become brittle or cracked. For example, floating covers implemented in cold, icy conditions will be subject to bending and twisting caused by ice movement and reservoir level fluctuations. At the other extreme, floating covers on reservoirs in warmer climates will be exposed to high amounts of UV light and intense heat. While many early floating covers were constructed from ethylene-propylene-diene monomer (EPDM) rubber or polyvinyl chloride (PVC), these materials exhibited a low tolerance to the physical stresses associated with harsh climatic conditions and long term exposure to the elements. In recent years, flexible-membrane covers have been constructed from reinforced elastomeric membrane materials that have shown an increased resistance to long term weathering effects. Specific materials presently used in the design and construction of flexible-membrane floating covers include reinforced chlorosulfonated polyethylene (CSPE-R), commonly referred to as Hypalon?; reinforced polypropylene (PP-R); ethylene interpolymer alloys (EIA), also known as XR-5? and XR-3?; and high density polyethylene (HDPE). HDPE is composed from microbiologically-resistant polyethylene resins with moderate resistance to chemicals, damage by the elements, and cracking due to stress. However, CSPE-R and PP-R have an increased chemical stability and are more flexible than HDPE. More specifically, CSPE-R is constructed from a synthetic rubber that is very flexible and highly resistant to damage caused by oxidation and ultraviolet radiation, and thus is known as a very durable material that can provide a long service life. PP-R is a moderately flexible cover that offers optimal resistance to environmental stress cracking and to chemicals commonly used to maintain the quality of stored water. XR-5?, which is a composite fabric of DuPont Dacron polyester fibers that have been molecularly coated with complex compounds, has been found to experience minimal degradation in adverse environments. XR-3? is a geomembrane that has high resistance to damage caused by ultraviolet radiation and extreme weather conditions.

Some cover materials may be susceptible to rot due to the accumulation of debris or other materials on the cover, and therefore these cover types would require regular cleaning. Regular maintenance of the cover is also important,and requires inspecting and identifying any tears or breaks in the cover and repairing any holes. The frequency of cleaning and/or maintenance is generally dictated by site-specific conditions and previous experience; however, proper and timely cleaning of the cover will reduce future expenditures by increasing the lifespan of the cover. O&M schedules are often coordinated with water quality monitoring schedules to increase operations efficiency.

The type of material chosen for the floating cover will also affect its ability to be repaired. Repairs to floating covers are usually performed by properly trained personnel employing methods recommended by the manufacturer. Repair methods commonly used to field-seam geomembrane materials include adhesive, chemical fusion, and thermal fusion methods. Adhesive seaming is the oldest method and involves coating the surfaces of the geomembrane panels to be joined with an adhesive. The two pieces are then joined together and pressure is applied by a roller, creating a bond and permanently attaching the pieces together. Chemical fusion welding utilizes a chemical solvent to dissolve the surfaces of both sections of material to be joined. The chemical agent is placed between the geomembrane sheets to be fused; when the surface of the material is mollified, pressure is applied to mate the two surfaces together. When the chemical fusion agent evaporates, a tight, unified bond is formed. Thermal fusion welding uses a device that travels the length of the seam and heats the surface of the material to a molten state. Pressure rollers that follow the heating device are used to compress both pieces of material together, forming a taut, cohesive bond. Each of these methods has advantages and disadvantages. For example, seams requiring adhesives or solvents generally use products previously recommended by the membrane manufacturer, and it is important that no adhesive or solvent products contain extractable ingredients that, when dissolved in water, would result in an exceedence of drinking water standards. As a second example, CSPE-R can be thermally welded only when both pieces are new. Thus, after prolonged exposure to the elements, CSPE-R membrane material cannot be patched using thermal welding because its existing material is incompatible with new patching material. Instead, patches must be made using adhesive or chemical welding processes, which can be more difficult and can risk contamination of the reservoir. In contrast, PP-R, EIA, and HDPE can all be thermally welded regardless of the years of use and exposure to sunlight. PP-R can also be chemically welded.

Finally, users will want to consider the lifespan of a given cover/cover material when determining which type of cover to implement. Most floating cover systems have a warranty of 20 years. However, in many cases, the life expectancy of a floating cover is directly related to how well it is maintained. While a well-maintained floating cover can last 25 to 30 years, poor maintenance of the cover can reduce its lifespan to 15 years or less. When a cover exceeds its predicted lifespan there will be an increased risk of environmental stress cracking in the form of punctures or tears, and a potential decrease in the impermeability of the cover material.

Fixed reservoir covers are permanent structures that can be flat, conical, or dome-shaped, and can be constructed from steel, aluminum, fiberglass, or concrete (although concrete and steel are rarely used in current applications - see below). Fixed covers can provide good security and may have a long lifespan (depending on the type of construction material chosen - see below); however, some types of fixed covers may not have good resistance to the elements, and they must be designed to withstand the large amounts of stress imposed by their own weight. In addition, due to the high cost per square foot of area protected relative to other types of reservoir covers, fixed covers are usually limited to small to medium-sized reservoirs, with a typical maximum of 10 to 20 million gallons (MG) or a surface area of about 100,000 square feet.

A specific discussion of each fixed cover type is provided below.

Historically, concrete covers have been utilized for reservoirs ranging in size from 1 to 10 MG of storage volume. As mentioned above, concrete covers are generally outdated and are not considered the optimal choice for most facilities. Over time, environmental stresses cause concrete to degrade and crack, decreasing the security of the cover. Specifically, concrete is susceptible to damage and spalling caused by the elements, and thus concrete often requires a high degree of maintenance. Finally, the weight of concrete makes it very difficult to install and remove; therefore, replacing a concrete cover would require that the reservoir be out of service for an extended period of time.

Steel covers have traditionally been used for reservoirs containing 10 to 20 MG, although steel is not typically used in current or new applications. Like concrete, the high weight of steel makes it difficult to install and remove in a timely manner, and therefore replacing a steel cover would require that the reservoir to be out of service for an extended period of time. In addition, specialized coatings and paints are required for steel covers used in drinking water applications, and these must be reapplied to the cover on a regular schedule. Finally, although stainless steel is rust-resistant, it is susceptible to corrosion from caustic chemicals, and thus it is not appropriate for some applications where chemical spills may be a problem.

Aluminum is the material of choice for most new fixed reservoir covers because of aluminum's advantages relative to steel and concrete. For example, aluminum covers are more lightweight than are concrete and steel. In addition, they are also more resistant to the elements than are concrete or steel covers. Aluminum covers are coated with a thin layer of aluminum oxide to increase their resistance to corrosion caused by mercaptans, chlorine, and other corrosive vapors found in water and wastewater treatment plants. While this coating must be reapplied periodically, aluminum covers do not require re-coating as frequently as do steel covers.

Aluminum covers have two basic design types - geodesic domes and flat covers. Aluminum geodesic domes can be used for circular reservoirs only, whereas flat covers are used for rectangular and square-shaped designs. Both geodesic domes and flat covers have low painting and maintenance requirements, and thus have a long lifespan (50 to 100 years). In addition, because both the aluminum geodesic dome and the flat cover are lightweight, they can be constructed quickly - sometimes even while the reservoir is in operation. However, these two cover types also have differences. Aluminum geodesic domes have a self-supporting clear span design that does not require interior columns or supports built into the reservoir. Conversely, flat covers require interior columns and supports that must be built into the reservoir walls.

Fiber-reinforced plastic (FRP) (more commonly known as fiberglass) is a composite consisting of a plastic resin matrix reinforced by embedded glass fibers. The glass fibers give the fiberglass its strength (the strength of the material is dependent on the type, orientation, quantity, and location of the fibers within the composite), while the resin adds rigidity. Fiberglass is lightweight and corrosion-resistant, and thus it can make an excellent material for reservoir covers. It also does not require painting for use in drinking water applications.

An air-supported reservoir covers consist of a moderately flexible fabric that is inflated over the surface of the water. This type of cover requires that blowers operate continuously to maintain approximately 2 pounds per square inch (psi) (14 kPa) more pressure inside the cover than outside, thereby keeping the cover inflated. Manufacturers recommend that utilities keep a backup blower and an emergency generator available in case the first blower fails or the power goes off. Air-supported covers can be constructed from polyester-reinforced vinyl, CSPE, or Teflon-coated fiberglass. These materials have service lives of 5 to 10, 10 to 20, and 25 or more years, respectively. Air-supported covers can be designed for circular, square, or rectangular reservoirs.

The primary feature affecting the security of a reservoir cover is its ability to maintain its integrity. Any type of cover, no matter what its construction material, will provide good protection from contamination by rainwater or atmospheric deposition, as well as from intruders attempting to access the stored water with the intent of causing intentional contamination. The covers are large and heavy, and it is difficult to circumvent them to get into the reservoir. At the very least, it would take a determined intruder, as opposed to a vandal, to defeat the cover.

As discussed above, proper O&M is the most critical factor in ensuring that the cover maintains its integrity and continues to protect the stored water from contamination. The major O&M requirements for each reservoir cover type have been discussed in detail above, and utility managers can evaluate these requirements to help determine the cover type that best meets their requirements and their O&M capabilities.

Because O&M is so important for reservoir covers, utilities may choose to add optional features that make maintaining the cover easier for facility personnel. These options include installing walkways, guardrails, handrails, access panels, access doors, and hatches at appropriate locations on the cover. Walkways are needed in order for personnel to conduct routine inspections, sampling, and necessary repairs. Guardrails and handrails ensure the safety of personnel accessing the cover and also restrict access to specific areas. Access panels are used for inspecting the cover, taking samples, and making occasional repairs. Access hatches and doors can be installed to allow direct access to inlets, outlets, drains or other structures that may require inspection or service.

Floating cover systems can also be equipped with features such as vents, insulation, and surface water removal systems. Due to the impermeability of the floating cover material, vents are critical for releasing any gases that may become built up under the cover. They also aid in preserving the surface tension bond that exists between the cover and the surface of the water. As mentioned above, vents and vent security will be discussed in a separate Product Guide. Some cover systems offer insulation to regulate the water temperature and further assist the floating cover material to withstand extreme climatic conditions. Insulation is rated by its R-factor, which is the measure of the insulation's ability to reduce heat transfer with the atmosphere. Covers with higher R-factors have better insulating properties; typical cover insulation provides R factors ranging from 5 to 25. Surface water removal systems use pumping, siphoning, or gravity-flow systems to decrease the amount of surface water accumulating on the cover. This can help reduce standing water and debris that can cause the cover to rot.

Fixed cover systems offer options such as skylight panels and lightning protection. Skylight panels offer interior light into the reservoir and are useful for conducting visual inspections. Lightning protection helps to mitigate possible safety issues for personnel accessing the cover.

In addition to the security provided by the reservoir cover itself, there are other security features that can be added to a reservoir in order to safeguard the cover and the stored water. One measure is to incorporate perimeter berms or other barrier mechanisms that are high enough to prevent potential vandals or terrorists from being able to see into the reservoir area. Another option is to place a security fence at a sufficient distance away from the reservoir such that nothing can be thrown into the reservoir from outside the fence. A security fence will also reduce the risk of wildlife or other potential nuisances entering the reservoir grounds. For those areas subject to frequent vandalism, or areas considered high risk, it may be necessary to incorporate increased security measures, such as intrusion alarms, video surveillance, or periodic patrols by security personnel. For more information on these topics, please refer to the Fences, Alarms, and Visual Surveillance Monitoring Security Product Guides.

The costs associated with purchasing and installing a reservoir cover are dependent on the cover design, and also on the material from which the cover is constructed. Fixed covers have high initial costs when compared to floating or air-supported covers - in part because a fixed cover is an actual roof constructed over an existing open air reservoir. However, these high initial costs may be mitigated by the fact that the lifetime of the fixed cover is usually 2 to 3 times that of a floating or air-supported cover.

The cost to purchase and install a floating cover ranges from about $1.65 to $4.00 per square foot of covered reservoir area. Fees associated with designing a floating cover, which can be considerable depending on the design, are not included in these costs. As discussed above, the cost of maintaining a floating cover depends on the level of maintenance required.

Due to the high costs per square foot of protected area relative to other types of reservoir covers, fixed covers are generally limited to small- to medium-sized reservoirs, with a maximum storage volume of 10 to 20 MG or a maximum surface area of approximately 100,000 square feet. The cost to purchase and install a fixed cover ranges from $5.00 to $50.00 per square foot of covered reservoir area, depending on the material type (steel, concrete, aluminum, fiberglass). Aluminum and fiberglass covers require little maintenance, and thus these types of covers would have relatively low upkeep costs.

Installed costs for air-supported covers can range from $2.00 to $5.50 per square foot of covered reservoir area. Costs are not significantly different for different material types (polyester-reinforced vinyl, CSPE, and Teflon-coated fiberglass). Costs for additional equipment (capital costs for blowers and a generator, as well as the maintenance costs for upkeep of this equipment) are not included in these figures.

Other important cost considerations for any type of reservoir cover include costs for construction and installation. These additional costs may include the rehabilitation of the existing reservoir; upgrading existing storm water drainage facilities to account for additional runoff from the new cover; and/or adding partition walls or baffles to the reservoir to ensure that the reservoir continues to function properly with the new cover. Numerous other factors (such as size, complexity, or time of year) can also affect construction and installation costs.