Calculating Credits and Debits for Compensatory Mitigation in

Wetlands of Western Washington

Patricia JohnsonWashington State Department of Ecology

Debits and Credits

Debits = area-based units representing the loss of wetland functions at an impact site

Credits = area-based units representing the gain of wetland functions at a mitigation site

Background

Creates functional currency to compare functions and values lost to those proposed

Based on the Washington State Wetland Rating System for Western Washington

HGM based

Function Groups

Improving water quality – Water Quality

Flood storage and flow reductions – Hydrologic

Habitat for plants and animals – Habitat

Photo courtesy of King County, WA

Score for each function based on a qualitative rating of:

The potential of the site to provide the function

The potential of the landscape to maintain each function at the site scale

The values each function may have for society

What is the potential of the site to provide the function?

Based on indicators of structure.

Site Potential:

What is the potential of the landscape to affect the function at the site scale?

Landscape Potential:

What is the value of the function to society?

Value:

Scoring for each function

Site potential

+Landscape potential

+Value

High = 3Medium = 2Low = 1

H,H,H = 9H,H,M = 8

H,H,L = 7 H,M,M = 7

M,M,M = 6H,M,L = 6H,L,L = 5

M,M,L = 5M,L,L = 4L,L,L = 3

Example Wetland Impact (Debit):

Before After

Example Wetland Impact (Debits)Rating of Wetland Unit BEFORE impact

Function Improving Water Quality Hydrologic Habitat

Site Potential (H, M, L)

M = 2 L L

Landscape Potential (H, M, L)

H = 3 H M

Value (H, M, L)

L = 1 L L

Score for Wetland Unit 2 + 3 + 1 = 6 5 4

Losses (Debits) and Gains (Credits)

are estimated by multiplying the score

by the area and

by the appropriate Modifier

Photo courtesy of King County, WA

Calculating DebitsModifier: Temporal Loss Factor

Calculating Debits

Score (Water Q.) x Area x TLF = Debits (Water Q.)

Score (Hydrologic) x Area x TLF = Debits (Hydrologic)

Score (Habitat) x Area x TLF = Debits (Habitat)

Example: DEBITS from ImpactDELAYED Mitigation

Improving Water Quality Hydrologic Habitat

Score for Wetland Unit 6 5 4

Acres of impact (non-forested)

1

Basic mitigation requirement (BMR) 6 x 1 = 6 5 4

Temporal loss factor (Delayed) 3

DEBITS 6 x 3 = 18 15 12

Total Debits: 18 + 15 + 12 = 45

Permittee purchases 45 credits from ILF Program

Calculating Credits

Basic Credits = Increase in Score [Score at “maturity” of site – Score before]

Creation : Score before = 0 Re-establishment: Score before = 0 Rehabilitation: Score before = (from form)Enhancement: Score before = (from form)

Example ILF Mitigation Site (Credits)Before After

Example ILF Mitigation Site (Credits)

Rating of Unit BEFORE mitigation

Improving Water Quality Hydrologic Habitat

Site Potential (H, M, L) M = 2 M M

Landscape Potential (H, M, L) H = 3 H L

Value (H, M, L) H = 3 H H

Score 2 + 3 + 3 = 8 8 6

Improving Water Quality Hydrologic Habitat

Site Potential (H,M,L) H = 3 H H

Landscape Potential (H,M,L) H = 3 H L

Value (H,M,L) H = 3 H H

Score 3 + 3 + 3 = 9 9 7

Example ILF Mitigation Site (Credits)

Rating of Unit AFTER mitigation

Calculating CreditsModifier:

Risk of Failure

Type of Mitigation Risk Factor

Advance mitigation The site meets criteria in Charts 1 and 3 of the site selection guidance [i.e. identified in a local plan and is sustainable] AND meets the criteria in Charts 4-11 for the appropriate functions. (Ecology publication #09-06-032) Advance means that at least two years has passed since plantings were completed or one

year since “as-built” plans were submitted to regulatory agencies.

1.0

Advance mitigation without meeting criteria in Ecology publication #09-06-032 0.83

Concurrent Mitigation Mitigation site meets criteria in Charts 1 and 3 of the site selection guidance [i.e. identified in a local plan and is sustainable] AND meets the criteria in Charts 4-11 for the appropriate functions. (All worksheets for Chart 3 and in Appendix D of Ecology publication #09-06-032 are submitted)

Risk factor applies to all types of mitigation.

0.9

Mitigation site chosen meets the criteria in Charts 2 and 3 of the site selection guidance [i.e. identified as a site with potential and that is sustainable] ; AND meets criteria in Charts 4-11 for the appropriate functions. (All worksheets for Chart 3 and in Appendix D of Ecology publication #09-06-032 are submitted)

Risk factor applies to all types of mitigation.

0.80

Site does not meet criteria in site selection guide, or guide was not used. Re-establishment, rehabilitation, or enhancement of an aquatic bed,

shrub, or forest community Re-establishment, rehabilitation, or enhancement of an emergent

community Creation of an aquatic bed, shrub, or forest community with data showing

there is adequate water to maintain wetland conditions 5 years out of every 10.

Creation of an emergent community with data showing there is adequate water to maintain wetland conditions 5 years out of every 10.

Creation of an aquatic bed, shrub, or forest community without adequate hydrologic data.

Creation of an emergent community without adequate hydrologic data.

0.67

0.5

0.67

0.5

0.5

0.4

Calculating Credits

Increase in Water Q. Score x Area x RF = Credits (Water Q.)

Increase in Hydrologic Score x Area x RF = Credits (Hydrologic)

Increase in Habitat Score x Area x RF = Credits (Habitat)

Improving Water Quality Hydrologic Habitat

Rating of Unit AFTER mitigation 9 9 7

Rating BEFORE mitigation

0

Increase in Score (A-B)

9 - 0 = 9 9 7

Acres CREATED 1

Basic mitigation Credit

9 x 1 = 9 9 7

Risk Factor 0.9

CREDITS 9 x 0.9 = 8.1 8.1 6.3

Example ILF Mitigation Site (Credits)

Proposed Credits for CREATED Wetland Areas

Example ILF Mitigation Site (Credits)

Proposed Credits for Enhanced Wetland Areas

Improving Water Quality Hydrologic Habitat

Rating AFTER mitigation 9 9 7

Rating BEFORE mitigation

8 8 6

Increase in Score (A-B)

9 – 8 = 1 1 1

Acres ENHANCED 12

Basic mitigation Credit

1 x 12 = 12 12 12

Risk Factor 0.9

CREDITS 12 x 0.9 = 10.8 10.8 10.8

Improving Water Quality Hydrologic Habitat

CREATED Credits 8.1 8.1 6.3

ENHANCEDCredits 10.8 10.8 10.8

TOTALCREDITS 8.1 + 10.8 = 18.9 18.9 17.1

PROPOSED Total Credits for the Project = 54.9

Example ILF Mitigation Site (Credits)

Proposed TOTAL Credits

Total Credits: 18.9 + 18.9 + 17.1 = 54.9

Comparing Debits to Credits

Wetland Impact Debits with Delayed TLF: 45 total Debits 18 debits for Water Quality 15 debits for Hydrologic 12 debits for Habitat

PROPOSED Credits for ILF Mitigation Site: 54.9 total Credits 18.9 credits for Water Quality 18.9 credits for Hydrologic 17.1 credits for Habitat

An ILF mitigation site must fulfill at least as

many credits (released) as the number

sold to permittees (debits).

Photo courtesy of King County, WA

Credit – Debit Tool is Guidance

SCORES from the Tool provide a STARTING POINT

Tool may not capture site-specific factors on debit or credit end

Do not want Tool to drive the design

Questions?

31

Patricia JohnsonWA Dept. of Ecology(360) 407-6140Patricia.johnson@ecy.wa.gov

http://www.ecy.wa.gov/programs/sea/wetlands/mitigation/creditdebit/index.html