1.4.5 Minimum Requirement 5 - Onsite Stormwater Management

Projects shall employ Stormwater Management BMPs to infiltrate, disperse, and retain stormwater onsite to the extent feasible without causing flooding or erosion impacts.

1.4.5.1 Compliance Options by Project Type

All projects that require Minimum Requirement #5 must employ Stormwater Management BMPs as detailed below. The compliance options for the project depend on the amount of improvements proposed, the location of the project, the size of the project parcel, and whether or not the project is Flow Control Exempt.

Note that the site may contain multiple parcels. The designer may choose difference compliance methods for different parcels. The applicant may choose different compliance methods for different parcels, depending on the proposed design and the options for each parcel as detailed below.

Projects that Trigger Only Minimum Requirements #1 - 5 - Not Flow Control Exempt

Projects triggering only Minimum Requirements #1 through #5, except Flow Control Exempt Projects shall either:

Use BMPs from List #1, Table 1 - 1: The List Approach for MR #5 Compliance, for all surfaces within each Surface Type in List #1; OR

Use any Flow Control BMPs to achieve the LID Performance Standard AND apply BMP L613 - Post-Construction Soil Quality and Depth.

Projects that Trigger Minimum Requirements #1 - #9 - Not Flow Control Exempt

Projects triggering Minimum Requirements #1 through #9, except Flow Control Exempt Projects, shall either:

Use BMPs from List #2, Table 1 - 1: The List Approach for MR #5 Compliance, for all surfaces within each Surface Type in List #2; OR

Use any Flow Control BMPs to achieve the LID Performance Standard AND apply BMP L613 - Post-Construction Soil Quality and Depth.

Flow Control Exempt Projects

Flow Control Exempt Projects (triggering Minimum Requirement #1 - #5 Only or Minimum Requirements #1 - #9) shall either:

Use BMPs from List #3, Table 1 - 1: The List Approach for MR #5 Compliance, for all surfaces within each Surface Type in List #3; OR

Use any Flow Control BMPs to achieve the LID Performance Standard AND apply BMP BMP L613 - Post-Construction Soil Quality and Depth.

If a project has multiple threshold discharge areas (TDAs), all TDAs must be Flow Control Exempt to use the options listed above.

1.4.5.2 Compliance Methods

Low Impact Development (LID) Performance Standard

The LID Performance Standard allows for the use of any BMPs sited throughout the project site that when modeled meet the LID Performance Standard. The LID Performance Standard allows for a more flexible site design. The LID Performance Standard requires modeling by a Washington State Licensed Professional Engineer. The modeling outputs shall be included in the Stormwater Site Plan Report. See Volume 2 - Documentation for additional information on what is required for modeling outputs.

Utilize one or a combination of Flow Control BMPs to meet the flow reduction modeling standards below. Rain Gardens cannot be used to achieve the LID Performance Standard though Bioretention may be used provided it is designed to provide flow control.

Using an Ecology approved continuous simulation model (assuming a 15-minute timestep) for design, stormwater discharges shall match developed discharge durations to predeveloped discharge durations for the range of predeveloped discharge rates from 8% of the 2-year return period flowrate to 50% of the 2-year return period flowrate. Projects required to comply with Minimum Requirement #7 must match developed discharge durations to predeveloped discharge durations for the range of predeveloped discharge rates from 8% of the 2-year return period flowrate up to the full 50-year return period flowrate.

The predeveloped condition to be matched shall be a forested land cover condition unless:

Reasonable historic information is provided that indicates the site was prairie prior to settlement.

Note: an applicant may submit this historical information as part of the Stormwater Site Plan Report for any given project.

In this case, the predeveloped condition to be matched can be modeled as pasture.

The project is located within an area which has been verified to have 40% Total Impervious Area as of 1985 (the drainage area of the immediate stream and all subsequent downstream basins have had at least 40% total impervious area (TIA) since 1985). Figure 1 - 4: 40% Total Impervious Area as of 1985 depicts those areas meeting the criteria. The City of Tacoma tMap also has a layer showing these areas.

Note: only the approved map determines these areas. Additionally areas cannot be added on a case by case basis.

In this case, the predeveloped condition to be matched can be the existing land cover condition.

Figure 1 - 4: 40% Total Impervious Area as of 1985

The List Approach

The List Approach requires evaluating the BMPs in Table 1 - 1: The List Approach for MR #5 Compliance. For all surfaces within each Surface Type, evaluate the feasibility of the BMPs in the order listed, and use the first BMP that is feasible. The "or" indicates that either BMP may be used to satisfy the requirement though all BMPs must be evaluated for feasibility.

The project proponent must document why a BMP is considered infeasible. If a BMP is feasible and used for all surfaces within that Surface Type, no other BMP from the list is required for those surface areas within that Surface Type. The infeasibility analysis shall be included in the Stormwater Site Plan. See Volume 2 - Documentation for additional information.

Feasibility shall be determined by evaluation against:

Design criteria, limitations, and infeasibility criteria identified for each BMP in this manual; AND

Competing Needs Criteria as listed below:

Requirements of the following federal or state laws, rules, and standards:

Historic Preservation Laws and Archaeology Laws as listed at https://dahp.wa.gov/project-review/preservation-laws

Federal Superfund or Washington State Model Toxics Control Act

Federal Aviation Administration requirements for airports

American with Disabilities Act

When found to be in conflict with special zoning district design criteria adopted and being implemented pursuant to a community planning process.

Public health and safety standards (ex. active zone of a skate park, bike park, or sport court where permeable pavement violates safety standards).

Transportation regulations to maintain the option for future expansion or multi-modal use of public rights-of-way.

City of Tacoma Land Use Regulatory Codes that provides protection of tree species or other critical areas

1.4.5.3 Objective

To use stormwater management practices on individual properties to reduce the amount of disruption to the natural hydrologic characteristics of the site.