STORMWATER RETENTION / DETENTION SYSTEMS
A retention/detention system is an underground pipe system that stores surface runoff. These systems are typically sized to maintain the runoff rate
of newly developed areas to what existed prior to the development so that existing storm sewers, ditches, and other channels stay within their capacity.
Underground pipe systems provide the added benefit of being able to use the land above the system for recreation, parking, and other purposes.
The decision to select a retention system or a detention system is usually determined by the 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 designed to limit the flow
rate into the receiving sewers or channels. A detention system typically uses non perforated pipe and watertight joints so that the stored runoff exits
only through the discharge outlet.
Benefits of HDPE corrugated pipe in retention/detention applications
- Custom design and layout - Retention/detention systems are unique designs based on the storage needs, parcel dimensions, environmental regulations (e.g. maintaining a certain distance above groundwater), and other restrictions. As such, they can be complex and time consuming to design. However, HDPE corrugated pipe producers greatly simplify the process by providing components, such as headers and fittings, made specifically for retention/detention systems. Some producers have staff available to lay out a system in a parcel based on the engineer's criteria; this assistance greatly simplifies product selection.
- Streamlined installation - Because components are specially made for retention/detention systems, they fit together much like a puzzle. Few if any field modifications are required. The lightweight feature of the pipe and components help speed handling and placement. Quality joints that are easy-to-assemble help make installation a less time-consuming process. Contractors spend their time completing the system, not making field modifications to the product.
- Resistant to aggressive environments - 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. Chemical corrosion is not a problem for HDPE, but many chemicals even in dilute concentrations shorten the life of concrete or metal systems.
- Suitable in minimum cover situations - Most retention/detentions systems are installed as shallow as possible to limit excavation costs and maintain clearance between the system and the groundwater. Properly installed HDPE corrugated pipe can withstand AASHTO HS-25 loads with at least 1 ft (0.3m) of cover for pipe 48-inch (1200 mm) and smaller, or 2 ft (0.6 m) of cover for larger pipe diameters. This allows for driveways, access roads, parking lots, and similar structures to be built above the system.
- Simplified inspection and maintenance - Grit and debris often enter retention/detention systems and can gradually impact its effectiveness by taking up storage volume or sealing off perforations. The system may need to be flushed and the debris pumped out. The smooth interior of HDPE pipe makes it easier to get a cleaner system. Access ports can be designed into HDPE pipe at regular intervals so that inspection can be easily conducted. These ports can also serve as entry ports for pumping and flushing equipment.
- Cost-effective installations - HDPE corrugated pipe and components that are specifically engineered for retention/detention system allow for low installed cost and long life resulting in cost-effective installations.
Corrugated HDPE - On The Job
Chicago Cubs' players do their best to maximize the winning,
and a PPI-member corrugated drainage pipe manufacturer in has done its part to minimize the rain outs. The space between the batting area and the
backstop at historic Wrigley Field tended to flood during heavy rains, and that was an obstacle for team officials who wanted to install three
rows of premium seating directly behind home plate. Plans also called for an underground club level area to be directly under those premium seats.
The most sensible option for Osborn Engineering, a firm that has overseen most major renovations at Wrigley Field in the last 20 years,
was to install a drainage system underneath the playing field. Osborn specified a high-density polyethylene
) pipe retention/detention system.
"Our preference was to go with something lightweight, and we needed a whole lot of lateral lines to connect to the manifold" said Scott Vura
the engineer on the project from Osborn. "And we also needed all the fabrication to be done off site, so that hand-carrying the system into the stadium
would have to be possible."