& Municipal Drainage
& Municipal Drainage
Storm sewers collect stormwater runoff through a surface inlet and drain it in a closed system, often inlet to inlet, to an appropriate outlet, such as a stream or other waterway. Storm sewer systems range from small and simple, such as for a modest housing or retail development, to large and complex networks in metropolitan areas serving a combination of residential, commercial and industrial developments.
When stormwater runoff discharges into waterways, it can be subject to strict regulations pertaining to quality and quantity. In the past, stormwater runoff was considered relatively clean and could be freely discharged into watercourses. It is now accepted that runoff may contain hydrocarbons and other chemicals, a wide range of suspended solids, as well as debris picked up from the ground surface during rainfall. Screening, filtration or hydrodynamic systems may be a necessary to improve runoff quality.
The quantity and the runoff rate are also significant. Storm sewers are sized for specific runoff rates; improperly sized systems may create a surcharged condition or upstream flooding. Corrugated plastic pipe is ideal for storm sewer systems, such as underground retention/detention systems as it has the structural strength and watertight joint systems to control runoff quantity and rates of release.
Many storm sewers are installed under pavement and must withstand vehicular loads, often under a minimum of cover. A system may have a wide range of cover requirements, especially when the terrain is hilly or rolling. Flexible corrugated plastic pipe outperforms its rigid counterpart, reinforced concrete pipe, in deep burials due to its ability to interact effectively with the backfill. Properly backfilled, corrugated plastic pipe can be buried at depths of 20 ft (6 m) or more.
Corrugated plastic pipe with a smooth interior allows for efficient flow through a closed storm drainage system. With such excellent hydraulics, and a low Manning’s ’n’ factor, pipe can often be reduced by at least one diameter from its corrugated interior counterpart. A smaller size can mean less initial pipe cost, narrower excavation and reduced backfill, all of which help to increase installation rates and savings. The smooth interior also allows sediment to be flushed out of the system so that it can maintain its capacity.
Depending on the diameter, corrugated HDPE pipe meets or exceeds broadly accepted standards such as AASHTO M252 or M294 and ASTM F667, F2306, or F2648. Similarly, corrugated polypropylene pipe meets or exceeds the standards set by AASHTO M330 and/or ASTM F2881. Both Type C (corrugated interior) and Type S (smooth interior) are widely available. Pipe is available with a range of joint options, including watertight joints, which are often specified for storm sewers. Corrugated plastic pipe is approved by most state Departments of Transportation, local municipalities, and regional agencies for use as culverts and in other storm drainage applications.