P8 watershed inputs include two classes of impervious area (swept and un-swept). Street sweeping parameters include frequency, season, and efficiency scale factor (normally = 1). Removal efficiencies for each particle class are specified in the particle file (default = NURP50.P8P). The removal efficiency used in the simulation is the product of the default value specified in the particle file and the efficiency scale factor specified for a given watershed. Efficiency estimates are described below.
Sweeper efficiency data reported by Bannerman ("Reducing the Uncertainty in the Calculations of Street Cleaner Performance for Wisconsin Municipalities", Wisconsin Dept of Natural Resources, Undated), based upon studies by the USGS are shown above.
Red Bars (added here) show approximate particle size ranges computed from Stokes Law, NURP settling velocity distribution, and assumed specific gravities ranging from 1.05 to 1.5. Unfortunately, direct measurements of particle size distribution or specific gravity were not made in the NURP study and are hard to find in the literature.
Diameter estimates for the highest particle class in the P8 NURP particle file (P80%) range from 60 to 200 microns. In this range, broom sweeper efficiency is negative (i.e. sweeping creates smaller particles by breaking up larger ones). Vacuum sweeper efficiency ranges from 10-20%.
Particle diameter estimates for second highest P8 particle class (P50%) range from 20 to 60 microns and vacuum efficiency is less than 10%.
Direct measurements of particle size distribution reported by Staher & Urbonas (1990) indicate a maximum particle diameter of ~40 microns (off the chart to the right), based upon studies of urban stormwater in Sweden. That value suggests an efficiency of ~0% for both sweeper types and all particle classes.
Based upon the above results, estimates of sweeper efficiency in the default particle files (NURP50.P8P, NURP90.P8P) in the current version of P8 have been modified relative to the values specified in P8 Versions <=3.2:
| Revised Estimates of Sweeper Efficiency in NURP50 and NURP90 particle files | |||||
| Particle Class | P0% | P10% | P30% | P50% | P80% |
| Previous Values | 0 | 4 | 8 | 12 | 16 |
| Revised Values | 0 | 0 | 0 | 5 | 15 |
The default sweeper efficiency values apply only to vacuum sweeping. It is recommended that broom sweeping not be considered as a BMP in P8 simulations. This does not mean that there are no benefits to broom sweeping, since it would tend to remove large particles that would otherwise accumulate in catch basins or other locations in the collection system before reaching site BMP's.