5.3.3
If MR #5: On-site Stormwater Management is triggered, it can be met by using the List approach or the LID Performance Standard approach. See Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management for a detailed discussion of these approaches and their applicability based on project size and thresholds.
For both approaches, selection of BMPs shall build upon site assessment and planning information described in Volume I, Chapter 2 and the feasibility analysis described above in Vol II–5.3.1 Determine Dispersion Feasibility and Vol II–5.3.2 Determine Infiltration Feasibility. If the project triggers MR #7: Flow Control and MR #6: Runoff Treatment requirements in addition to On-Site Stormwater Management Requirements, see Vol II–5.3.4 Select BMPs for Runoff Treatment and Vol II–5.3.5 Select BMPs for Flow Control for additional discussion of BMP selection.
5.3.3.1 On-Site List Approach
If the MR #5: On-site Stormwater Management Requirement is triggered and the On-Site List Approach is selected as the method for compliance, follow the steps presented below to select the appropriate BMP(s) for a given project.
Step 1: Determine if Dispersion and Infiltration are Feasible
See Vol II–5.3.1 Determine Dispersion Feasibility and Vol II–5.3.2 Determine Infiltration Feasibility.
Step 2: Calculate Areas by Surface Type
For each project type, divide the project area into lawn and landscape areas, roof areas, and other hard surface areas (i.e., driveways, walkways, sidewalks, etc.) with distinct drainage pathways.
Step 3: Identify the Applicable Onsite List
Identify whether List #1, List #2A, List #2B, or List #3 applies to the project based on the minimum requirements that are triggered. See Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management for details on determining which list shall be applied.
Step 4: Evaluate BMPs for Each Surface Type in Order
The onsite BMP lists provided in Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management include potential onsite BMPs for different surface types (lawn and landscape, roofs, and other hard surfaces) as identified in Step 2. For each surface type, the BMPs shall be considered in the order listed and the first BMP considered feasible shall be used.
See Vol II–5.4 BMP Design for additional requirements that may affect the design and placement of BMPs on the site.
5.3.3.2 LID Performance Standard
If the MR #5: On-site Stormwater Management is triggered and the LID Performance Standard is the method used for compliance, follow the steps presented below to select the appropriate BMP(s) for a given project.
Step 1: Determine if Dispersion and Infiltration are Feasible
See Vol II–5.3.1 Determine Dispersion Feasibility and Vol II–5.3.2 Determine Infiltration Feasibility.
Step 2: Select BMP(s)
Select a BMP, or multiple BMPs, to meet the LID Performance Standard. Refer to Vol II–5.4 BMP Design for BMP applicability, site suitability, and design criteria.
Step 3: Model BMP Design
Model the selected BMPs to determine the required sizing to meet the LID Performance Standard using approved hydrologic modeling methods (see Volume III, Section 2.6 in the Ecology Manual).