STORM DRAINAGE SYSTEMS

Gutters, which are discussed in the previous section, are just one component of the complete storm drainage system. However, important economies can be achieved in the construction of entire storm drainage systems.

Following are guidelines for storm drainage systems:

• Use performance requirements in place of prescriptive standards in all components of storm drainage design.

• Consider detention/retention basins, especially when regional management is preferred.

• Use less expensive alternatives to corrugated metal and concrete pipe.

• Consider precast structures if available from local suppliers.

• Reduce the use of manholes and inlets by increasing spacings between structures, or by replacing them with curved pipe sections, tees, and wyes where appropriate.

Traditional stormwater systems were usually "closed": that is, once water entered the system, it passed through nonporous pipes and channels, sometimes for substantial distances, until it was finally discharged into a moving stream or river. More recently, the undesirability of removing a significant portion of > runoff from local areas where precipi­tation falls has become increasingly clear. Consequences can include: inadequate recharge of groundwater supplies; increased potential for contamination of groundwater; soil subsidence, such as the formation of sinkholes that occurred in central Florida; and downstream flooding.

Modern systems increasingly emphasize retention of rainfall in the local area where it falls. Parts of the con­veyance system can be left "open," substituting grassy swales and natural drainage for closed piping. Detention and/or retention basins can also accommodate excess stormwater, enabling the gradual recharge of local groundwater supplies.

Open portions of drainage systems cost less than equivalent closed piping. Environmental considerations and cost savings therefore go hand in hand. Additional savings can be achieved through regional stormwater manage­ment serving the entire drainage basin or several specific sites within a basin. Regional control of stormwater generally requires less construction by developers, and the local jurisdiction achieves savings in operational and management costs. Improved effi­ciency is another benefit over individual site controls, since the need for "piecemeal" planning can be reduced.

A ten-year design storm is the typical standard for the "minor" stormwater system in a residential development. However, major channels or culverts with large contributing areas require special consideration. Design storm frequency is based on convenience and economics. A community decides how much to pay to insure against the possibility of flooding. The merits of each proposed site plan must be considered, since each site adapts differently to various designs. Performance requirements, which generally encourage innovative and less costly alternatives, should be used over prescription standards.

Two effective methods for carrying excessive stormwater use detention and/or retention basins and "overland relief." Retention/detention facilities can take a variety of forms. Manmade lakes and subsurface absorption are two of the more popular systems. Although each has its own pros and cons, both achieve the intended objective of effective stormwater management. Lakes contribute to aesthetic value but require more land area. Soil absorption systems can be installed on "tighter" sites but are limited to the capacity of the soil.

Culverts and open concrete channels can be reduced in size by grading the surrounding land to direct stormwater on an overland path to the stormwater system downstream if the design storm is exceeded. Grassy swales provide overland relief in a residential neighborhood. Larger "flood banks" are used in major drainage areas.

Materials	In recent years, less expensive, more durable plastics have begun to replace traditional corrugated metal pipe (CMP) and reinforced concrete pipe (RCP). Polyvinyl chloride (PVC) and polybutylene (PB), both exhibit excellent resistance to corrosion and are currently used for stormwater pipe. PVC, a relatively inexpensive plastic, can be installed throughout most of the "minor" storm drainage system. PB is manufactured in sizes small enough for water supply systems, large enough for highway culverts, and in most sizes in between, usually in a corrugated configuration to provide the appropriate structural qualities. Due to their relatively light weight, PVC and PB pipe do not generally require special equipment for place­ment in the trench.
Stormwater Structures	Components of a sewer system -­manholes, sewers, inlet and outlet structures — must be examined for possible cost savings in installation methods, choice of materials, and use of new designs. Where available, precast manholes and inlets generally provide a less costly alternative to abor intensive, site-built structures.
Manholes/Inlets	An average of $1,000 to $1,500 can be saved by eliminating a single manhole, depending on depth and local cost factors. Although many communities allow manholes or inlets to be spaced a maximum of 600 to 800 feet apart, some standards limit maximum spacings to as little as 200 feet. These shorter spacings are carry-overs from an era when clean-out capabili­ties and construction techniques were inferior to those today. Officials must periodically review such standards to encourage state-of-the-art construction.

Manholes can also be eliminated by installing a curved section of pipe at nonabrupt changes in direction. Many communities also recommend installing a cleanout or other access within 50 feet of a bend to clear possible obstructions. The need for such access is questionable, since there is little risk of stoppage in the curved storm sewer alignment.

Manholes can be eliminated where smaller pipes join larger storm "mains." For example, a prefabricated tee or wye section can ioin a building roof drain (downspout) with the public storm drain, thereby avoiding the added cost of a manhole.

Endwalls, commonly installed at the end of a drainage pipe, can also be eliminated. With proper grading at the terminal end of the pipe, a flared end section will provide the needed transition at a much lower cost than an endwall.

Multiple use of drainage structures should be encouraged when possible. A yard inlet, combined with a curb type inlet, can achieve greater efficiency at less cost, receiving runoff from two or more directions.

83	І4

In some cases, inlet structures can be completely eliminated and replaced with flared end sections. If the surrounding area is graded properly, an end section can be used as an inlet in place of a drop type structure.

Since the cost of an end section is similar to a standard section of pipe, overall savings would be equivalent to the cost of any eliminated inlet structure.

Inlet/Outlet Rip-rap, grouted stone, or other Controls erosion controls can often be replaced with one of the commercially available fabrics designed for soil stabilization. The fabric is placed at the end of the channel or pipe after the area has been graded and seeded. Fabrics can be installed at less cost than concrete or stone erosion controls, and provide a more appealing site.