The calculation of area mitigated by the various recommended methods is shown in the spreadsheet below. The determination of the discount is shown below:
1) Determine the total surface area of each mitigation technique and enter into the appropriate cell. Calculate the effective mitigated impervious area by dividing by the corresponding factor.
2) Add all effective mitigated impervious areas to determine total effective mitigated impervious area.
3) Determine number of ERUs mitigated by dividing the Total Mitigated Area by 2,750. The allowable number of ERUs will be rounded down to the nearest whole number.
4) Determine the amount of credit based on number of ERUs mitigated.
The factors used on the worksheet are taken directly from the City of Portland’s Simplified Approach for Storm Water Management.
Credits may be given up to 1/3 of the total number of ERUs; therefore, a property owner must have at least 3 ERUs (8250 square feet of impervious surface) to qualify for a credit. Additional credit may be given to property owners who completely eliminate impervious surfaces on their property.