Retention & Detention Systems
Retention & Detention Systems
A retention/detention system is an underground pipe system that stores surface runoff. In newly developed areas, these systems are typically sized to maintain the runoff rate prior to development so that existing storm sewers, ditches and other waterways stay within their capacity. Underground pipe networks allow the land above the system to be used for recreation, parking and other purposes.
The decision to select a retention system or a detention system is usually determined by local environmental regulations or the groundwater situation. A retention system typically uses perforated pipe so that the stored runoff can recharge groundwater, and a discharge outlet is designed to limit the flow rate into the receiving sewers or channels. A detention system typically uses non-perforated pipe and water tight joints so that the stored runoff exits only through the regulated discharge outlet.
Retention/detention systems are unique designs based on the storage needs, parcel dimensions/configuration, environmental regulations (e.g. maintaining a certain distance above groundwater), and other restrictions. They can be complex to design, but corrugated plastic pipe producers simplify the process by supplying components, such as headers and fittings, made specifically for retention/detention systems. Many producers have staff who can assist in laying out a system based on engineering criteria, which greatly simplifies product selection.
Components are specially made for retention/detention systems so they fit together easily. Few, if any, field modifications are required. The lightweight feature of the pipe and easy-to-assemble components help speed handling and placement. Contractors spend their time completing the system, not making field modifications to the product. When necessary, field modifications can be easily made.
In retention/detention applications, runoff may stay in the system for extended periods. If the runoff contains harsh chemicals, the pipe material must be able to tolerate them. Plastic pipe is highly resistant to chemical corrosion, unlike concrete or metal systems where chemicals, even in dilute concentrations, can shorten the system’s life.
Most retention/detentions systems are installed as shallow as possible to limit excavation costs and maintain clearance between the system and the groundwater. This allows for driveways, access roads, parking lots, and similar structures to be built above the system.
Grit and debris often enter a retention/detention system and can gradually impact its effectiveness by taking up storage volume or sealing off perforations. The system may need to be flushed and debris pumped out. The smooth interior of plastic pipe makes it easier to clean. Access ports can be designed into plastic pipe at regular intervals so that inspection can be easily conducted. These ports can also serve as entry ports for pumping and flushing equipment. Many pipe producers offer large diameter pipes with interior baffle plates to trap debris and sediment ahead of a stormwater management system. Isolating debris and sediment collection in these pipes can simplify routine maintenance.