Credits & debits for wetland mitigation

We developed a tool for calculating when a proposed wetland mitigation project adequately replaces the functions and values lost when wetlands are impacted. It is called the Credit/Debit Method. It was originally developed for the King County in-lieu-fee program. However, it can be applied to other projects as an option for determining compensatory mitigation.

The tool is designed to provide guidance for both regulators and applicants during two stages of the mitigation process:

  • Estimating the functions and values lost when a wetland is altered.
  • Estimating the gain in functions and values that result from the mitigation.

We don't require this method be used. The guidance provides one method for determining the adequacy of compensatory wetland mitigation, but does not set any new regulatory requirements.

Western Washington

  • Calculating Credits and Debits for Compensatory Mitigation in Wetlands of Western Washington: Final Report (March 2012)
    • Debit/Credit spreadsheets: There are two Microsoft Excel sheets that can be used to perform the calculations associated with Appendix E: Worksheets for Estimating the Adequacy of Wetland Mitigation. The first spreadsheet calculates the "debits" generated at the site where impacts are incurred. The second spreadsheet calculates the "credits" for an increase in functions at a mitigation site. The spreadsheets require the same information described in Appendix E and also contain the table of temporal loss factors and risk factors.

Eastern Washington

Credit/Debit Method for temporary impacts

This information clarifies how to use the Credit/Debit Method to calculate the debits for temporary wetland impacts. If all the functions at a site are reestablished to their previous levels, compensatory mitigation would only be needed for the temporal loss of functions and the potential risk the re-establishment would fail. Temporary impacts are discussed in Section 3.2 of the Credit/Debit Method.

If temporary impacts will be restored on site, use the following process:

  1. Calculate the impacts (debits) using the Credit/Debit Method and use the temporal loss factor that matches the Cowardin Class being impacted (e.g., PEM, PSS, PFO)
    • If the long-term temporary impact is between one and two years, then the temporal loss factor will be a minimum of 1.5 or a maximum of 3.5 (use the concurrent temporal loss factor for the subject Cowardin Class from the Appendix table in the C/D Method).

      Note: For impacts to PEM wetlands that will be restored on site within one year, it has been agency policy (Corps and Ecology) that little to no compensation is required, depending on case-specific factors. However, if the wetland contains a native emergent plant community, compensation likely will be required.
    • If the long-term temporary impact is two years or more, then use the delayed temporal loss factor for the appropriate Cowardin Class (a minimum of 3 and a maximum of 7).
  2. Using the C/D method, calculate the credits proposed to be generated by the on-site restoration activities, inclusive of the risk factor (use the risk factor Appendix table in the C/D Method). For on-site restoration, this factor would typically be 0.67 for PSS and PFO and 0.5 for PEM.

    Note: The risk factor may need to be adjusted depending on the project and the extent of the temporary impacts.
    • It is possible that the proposed re-establishment could increase the functioning of the site if re-establishment involves replacing an invasive-plant dominated community with native plants.
  3. The number of credits the applicant needs to compensate for the temporary long-term impacts from the project is the difference between number 1 and 2 above.

If long-term temporary impacts won't be restored

If long-term temporary impacts won't be restored on-site, use the C/D method as if this was a permanent impact. For projects buying in-lieu fee credits, the temporal loss factor would be a minimum of 3 depending on the Cowardin class being impacted.