What's with all the Street Sweeping?

The City sweeps streets and parking lots every two weeks during the summer. This is done because there is some reduction in the discharge of chemical constituents, sediment, and litter to stormwater from street surfaces and parking lots with an intensive (at least twice weekly) street-cleaning program. Street sweeping has been found most effective for stormwater quality improvement in commercial business districts and intensely developed areas (Washington State Dept of Ecology, 1992). The reduction in solids and other materials resulting from an aggressive street sweeping program can reduce the maintenance requirements of structural runoff controls and provide aesthetic benefits to area residents. Solids collected by street sweepers must be disposed of properly, commonly in municipal landfills.
Improvements in the design and use of street sweepers may offer hope for additional reduction in stormwater loads. The four types of sweepers currently being used include:

1. Mechanical street sweepers
2. Vacuum street sweepers
3. Regenerative air street sweepers
4. Advanced high efficiency sweepers

Mechanical broom sweepers are more effective for removing litter and other large particles. On the other hand, vacuum sweeper inlets must be close to the ground to provide sufficient suction and consequently are not effective for litter removal, but collect more of the smaller particles responsible for much of the chemical constituent load in stormwater runoff. Regenerative air sweepers are similar to vacuum sweepers, except that they have a larger pickup head, and the air is recycled. Advanced high efficiency sweepers are used in industrial applications and are designed to remove the smaller particles.
Tandem street sweeping offers an opportunity to exploit the strengths of both mechanical and vacuum type sweepers. Tandem operations involve the combined use of mechanical and vacuum sweepers in successive cleaning passes. A new type of vacuum-assisted dry sweeper also has been developed that provides the important components of tandem sweeping in a single unit.
Chemical constituent, litter and sediment removal rates are also directly related to frequency of sweeping (particularly vacuum sweeping), the rate at which sediment and other debris accumulates on paved surfaces, and the average interval between storms.
The rate at which sediment accumulates depends on a number of factors, including traffic count, adjacent land use, and site design. Sediment is also continuously being removed by wind and traffic-generated turbulence. Consequently, the maximum accumulation (equilibrium between accumulation and removal) can occur in just a few days on highways lacking curbs or other roadside barriers. Structures that help retain sediment on shoulders and the road surface such as concrete guardrails or curbs allow more material to accumulate; therefore, the maximum accumulation might not occur for several weeks.