How did SPU determine what the rate categories are?
SPU’s drainage rate categories group parcels with similar property characteristics which will, in turn, contribute similar amounts of stormwater runoff. SPU estimates the fraction of rainfall that becomes “runoff” from a parcel using standard runoff coefficients utilized in hydrological modeling. These runoff coefficients vary depending on how pervious (permeable) a surface is. More stormwater will run off of hard or impervious surfaces while pervious or permeable surfaces will absorb more water.
