Stormwater Design Overview

Last updated May 10, 2022
By Ian Story

Drainage Design Overview

This guide applies to projects in our typical areas of work (located in western Washington). Most stormwater design in this region is based on the same guidance code: the Stormwater Management Manual for Western Washington (SWMMWW), developed by the department of ecology. Under this, each county has developed their own stormwater manual. Since these are all based on the same parent document, they tend to be substantially the same. The three we work with most frequently are the King County Surface Water Design Manual (KCSWDM), the Pierce County Stormwater Management and Site Development Manual (PCSWMSSD), and the Snohomish County Drainage Manual (SCDM).

Local jurisdictions adopt their own stormwater design manuals and codes. These are usually based on the county manual with local amendments.

We’ll be focusing here on the King County manual (KCSWDM) for simplicity, but the other manuals are substantially similar.

This is a guide for small-scale stormwater design, focused on projects that do not require a full engineered design and determining the thresholds for what does and does not need an engineer.

Thresholds for Drainage Design

Regarding drainage design for small-scale projects, the first question is always whether it is required. This section discusses the typical thresholds for determining whether drainage design is required, and whether you will need to get a civil engineer involved in the project.

Threshold 1: Is Drainage Design Required?

Typically, projects above 2,000 square feet of New Plus Replace Hard Surfaces require drainage design and review by the permit department, and projects below 2,000 square feet do not. This is the threshold for the KCSWDM. Please note that some jurisdictions have adopted stricter thresholds (for example: Seattle’s threshold is 750 square feet).

A secondary threshold applies for projects that disturb a large land area, even if you aren’t installing lots new hard surfaces. Projects that disturb at least 7,000 square feet of land require drainage review, regardless of hard surface area. This doesn’t come up very often in practice.

Additionally, any project that requires Targeted Drainage Review (below) requires engineered drainage design for any new or replaced hard surface areas, regardless of size.

Projects that require Drainage Design must check the infiltration capacity of soils on site (see below). If your project will require Drainage Design, I recommend checking this as early in the project as possible.

Threshold 2: Is Engineered Drainage Design Required?

Most small scale and single-family projects do not require an engineered drainage design. The basic level of drainage design is called “Simplified Drainage Review” (SDR). SDR plans and documentation can be prepared by anyone familiar with the code. If the project’s size or complexity exceeds the limits of Simplified Drainage Review, then you will need to hire a civil engineer to complete the drainage design for the project, which is beyond the scope of this guide.

The following items trigger engineered drainage design:

  • 5,000+ sq ft of New Plus Replaced Pollution Generating Impervious Surfaces.
    • Secondary check: if the project exceeds this threshold and is located inside the Urban Growth Area (ie, is not a rural lot), you can apply one of the alternative thresholds below (based on the soil type determined in your soils report). Note that these options also have additional limits on total land disturbance area not listed here – see the manual for additional details (this doesn’t usually come into play for single-lot residential projects).
    • 7,947+ sq ft (till soils – tends to be a densely packed hardpan including fine silts, sand, and gravel; poor drainage)
    • 6,872+ sq ft (outwash soils – tend to be clean sands and gravels; good drainage)
  • 3/4+ acres (32,670+ sq ft) of New Pollution Generating Pervious Surfaces.
  • Projects that require Targeted Drainage Review (below).

Measuring Hard Surfaces

Most of the drainage design requirements are based on the square footage of hard surfaces created by the project. For purposes of drainage design, Hard Surfaces* are defined as any surface that prevents or slows the entry of water into the soil or causes water to run off the surface faster than under natural conditions. This definition includes roofs and pavement, but it also includes things like spaced wood decks, packed gravel, dirt roads, and grasscrete.

Also, all of the drainage thresholds and design requirements are based on New Plus Replaced Hard Surfaces. You do not get any credit for existing hard surfaces – if you replace a patio with a roof or replace your concrete walkway with pavers, these are considered just the same as brand new hard surfaces. I get a lot of questions asking why this is. As I understand it, the current regime of drainage rules came into effect in 1992. The Washington Department of Ecology updated their Puget Sound Water Quality Management Plan to require that all jurisdictions to adopt stormwater management plans, and published a technical manual (Stormwater Management Manual for the Puget Sound Basin – the forerunner of the current Stormwater Management Manual for Western Washington) as a model code for use by local jurisdictions (the language required either adoption of DOE’s manual or a manual with substantially equivalent technical standards). While existing hard surfaces were grandfathered in (according to KCSWDM, the official grandfathering date is January 8, 2001), the intent was to improve stormwater quality over time by requiring new projects to meet the standards and also to slowly update existing surfaces over time. They do this by requiring any replaced surfaces to be brought up to current standards, ratcheting the total inventory of hard surfaces towards compliance.

Finally, changes to hard surfaces are cumulative over time. If you complete multiple projects within a short time period, you have to count the aggregate change in hard surfaces, not just the hard surfaces for the current project.

Note for land subdivision: When applying for short plats and other land subdivision, you must complete drainage design and improvements for the full anticipated buildout of those lots before the plat will be finalized. The full buildout is based on the local jurisdiction’s hard surface limit (for example, 50% of the lot), or 4,000 square feet per lot, whichever is less. If desired, you can establish a lower limit by recording a covenant on the lot. Because of the thresholds above, short plat will usually require an engineered drainage design.

Targeted Drainage Review

Targeted Drainage Review (TDR) means there are special site and environmental conditions that need to be addressed in the drainage design. These come into play most frequently for projects with Environmentally Critical Areas (ECAs). If a project requires Targeted Drainage Review, the drainage design must be completed by a licensed civil engineer.

TDR Checklist

Does the project contain, or is the project adjacent to, a Flood Hazard Area, Erosion Hazard Area, or Steep Slope Hazard Area?No
Is the project located within a Critical Drainage Area or Landslide Hazard Drainage Area?No
Will the project construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth or that receives surface and stormwater runoff from a drainage pipe/ditch that is 12 inches or more in size/depth?No
Is the project a Redevelopment Project on a High-Use Site with at least $100,000 of proposed improvements?No

If the answer to any of the above is ‘Yes,’ then TDR is required.

Here are some resources to check the above items:

Critical Drainage Areas (CDAs)

Definition: An area where the Department of Natural Resources and Parks [or a local jurisdiction] has determined that additional drainage controls (beyond those in the KCSWDM) are needed to address a severe flooding, drainage, and/or erosion condition that poses an imminent likelihood of harm to the welfare and safety of the surrounding community.

The KCSWDM Reference Section 2 (at the back of the manual) includes a section of adopted CDAs. At the time of this article, there are no officially adopted CDAs.

Local jurisdictions can designate their own CDAs. For example, the City of Sammamish designates the Beaver Lake and Pine Lake watersheds as CDAs.