A fence is a physical barrier that can be set up around the perimeter of an asset. Fences often consist of individual pieces (such as individual pickets in a wooden fence, or individual sections of a wrought iron fence) that are fastened together. Individual sections of the fence are fastened together using posts, which are sunk into the ground to provide stability and strength for the sections of the fence hung between them. Gates are installed between individual sections of the fence to allow access inside the fenced area.

Many fences are used as decorative architectural features to separate physical spaces from each other. They may also be used to physically mark the location of a boundary (such as a fence installed along a property line). However, a fence can also serve as an effective means for physically delaying intruders from gaining access to a water or wastewater asset. For example, many utilities install fences around their primary facilities, around remote pump stations, or around hazardous materials storage areas or sensitive areas within a facility. Access to the area can be controlled through security at gates or doors through the fence (for example, by posting a guard at the gate or by locking it). In order to gain access to the asset, unauthorized persons would either have to go around or through the fence.

Fences are often compared with walls when determining the appropriate system for perimeter security. While both fences and walls can provide adequate perimeter security, fences are often easier and less expensive to install than walls. However, they do not usually provide the same physical strength that walls do. In addition, many types of fences have gaps between the individual pieces that make up the fence (i.e., the spaces between chain links in a chain link fence or the space between pickets in a picket fence). Thus, many types of fences allow the interior of the fenced area to be seen. This may allow intruders to gather important information about the locations or defenses of vulnerable areas within the facility.

There are numerous types of materials used to construct fences, including chain link, iron, aluminum, wood, or wire. Some types of fences, such as split rails or pickets, may not be appropriate for security purposes because they are traditionally low fences, and they are not physically strong. Potential intruders may be able to easily defeat these fences either by jumping or climbing over them or by breaking through them. For example, the rails in a split rail fence may be removed, and the pickets in a picket fence may be able to be broken easily.

Important security attributes of a fence include the height to which it can be constructed, the strength of the material comprising the fence, the method and strength of attaching the individual sections of the fence together at the posts, and the fence's ability to restrict the view of the assets inside the fence. Additional considerations should include the ease of installing the fence and the ease of removing and reusing sections of the fence. More specific discussions of each fence type are provided below.

The effectiveness of a chain link fence can be increased by reducing the link size. The smaller the link size, the more material per fence area, and thus the more material that must be cut through to get through the fence. In addition, it is more difficult for an intruder to get his foot into a smaller mesh size, which reduces his ability to climb the fence. Burying several inches of the chain link mesh into the ground can also help prevent intruders from lifting or digging under the fence.

Iron fences can be considered security fences, although they are not as common in security applications as chain link fences. They are strong and can be difficult to climb, particularly if there are no cross-supports that can serve as footholds. The tops of the pickets can also be pointed to increase the difficulty of climbing the fence.

Wood fences are not typically considered to be security fences, because they are vulnerable to being broken. However, wood fences may be more difficult to climb than are other types of fences, because they do not provide good footholds. Wood fences have been combined with other types of fences (typically chain link or small-gauge wire) to create a hybrid fence that reduces the vulnerabilities of either fence type alone. Typically, the wood pickets are interwoven between the wire or attached behind the chain link; this makes the fence more difficult to climb (intruders cannot get a foothold in the wire/chain link because of the pickets behind it) as well as more difficult to break (the chain link/wire adds strength the wooden pickets).

Wire security fences consist of hardened steel wires that run horizontally between the posts. These wires are resistant to cutting and breaking, making this a very strong type of fence. Riverdale Mills' Wirewall product is manufactured such that every intersection of wires in the fabric is individually welded, reducing this as a potential weak point.

Wire fences are also typically designed so that the space between wires is very small. This prevents intruders from getting a handhold or a foothold in the fence. It also prevents potential intruders from using wirecutters to cut the fence, because the wirecutters cannot fit between the wires to cut them.

Table 1 provides a comparison of the important security and usability features of these fence types.

Some fences can include additional measures to delay, or even detect, potential intruders. Such measures may include the addition of barbed wire, razor wire, or other deterrents at the top of the fence. Barbed wire is sometimes employed at the base of fences as well. This can impede a would-be intruder's progress in even reaching the fence. Fences may also be fitted with security cameras to provide visual surveillance of the perimeter. Finally, some facilities have installed motion sensors along their fences to detect movement on the fence. Several manufacturers have combined these multiple perimeter security features into one product and offer high security fences that incorporate stronger materials, sensors, alarms, and other security features. These specialized fences will be covered in other product guides.

The correct implementation of a fence can make it a much more effective security measure. Security experts recommend the following when a facility constructs a fence:

Costs for fences will vary depending on the type of material chosen, the height of the fence, and the length of fence to be installed. Table 2 provides cost information for several different kinds of fences, including capital costs plus estimates of the number of manhours required to install the fence.

The cost for Wirewall will depend on the mesh size and the wire gauge. For example, a typical 8-foot high security fence of 9 gauge galvanized wire and 2.4-inch by 2.4-inch mesh would cost $17-$18/linear foot, installed. A medium security fence of 8.5 gauge galvanized wire and 2-inch by 2-inch mesh would cost $22-$24/linear foot, installed. A high security fence of 8.5 gauge galvanized wire and 1/2-inch by 3-inch mesh would cost $28-$30/linear foot, installed.

Some security fencing systems include enhanced security features, such as razor wire at the top of the fence. However, additional security features can be installed along with the fence at additional cost. Two examples are listed below: