- Calculation of Stormwater Management Credits: This spreadsheet allows you to calculate the amount of credit available for stormwater mitigation applied to a property. The factors used on the worksheet are taken directly from the City of Portland’s Simplified Approach for Storm Water Management.
The amount of stormwater area mitigated by the various recommended methods is calculated in the above spreadsheet. Here's how to fill it out:
- 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.
- Add all effective mitigated impervious areas to determine total effective mitigated impervious area.
- 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.
- Determine the amount of credit based on number of ERUs mitigated.
Credits may be given for up to 1/3 of the total number of ERUs -- this means that 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.