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Director:
J.E. “Sam” Ryan, CBO

5280 Northwest Drive
Bellingham, Washington 98226
Telephone: (360) 676-6907
E-mail: pds@co.whatcom.wa.us

	Developments in Regulated Watersheds:
		How to Determine the Appropriate Residential Stormwater Facilitie

Whatcom County Code Chapter 20.80.636 determines when a stormwater facility is required for development projects within the Lake Whatcom, Lake Samish, Lake Padden, Birch Bay, or Drayton Harbor watersheds. If a stormwater facility is required, the criteria listed in Chapter 2, Section 221 of the Whatcom County Development Standards should be applied. For projects located in Sudden Valley, please contact watershed staff or the Sudden Valley Architectural Control committee for their requirements 360-734-6430.

To determine the appropriate type of stormwater facility for a particular project, the following methods shall be applied:

1. Is the project part of a larger plat or development which has existing stormwater management facilities which can accommodate additional stormwater volume?
2. To determine whether your project is part of a larger plat or development, please contact the Engineering Division (360-676-6730)
3. If yes, development may hook up to existing stormwater system (Usually retention/detention pond).
4. If the project is not part of a larger plat or development, move on to Question 2).
5. If 1) does not apply, are the site soils suitable for infiltration?
   Please refer to Section 5.1 of the Whatcom County Development Standards to determine proper documentation of suitable soils.
   If yes, staff looks at setback requirements in Section 5 of Development Standards.
   If site has suitable soils for infiltration and can meet setbacks, applicant can opt to use:

   • An infiltration trench (with attached catch basin) - Drawing 5.2.A
   • A drywell - Drawing 5.3
   If the site cannot accommodate an infiltration system, move on to Question 3).
6. If 1) or 2) does not apply, can the site accommodate a dispersion system?
   Staff looks at setbacks requirements in section 4 of the Development Standards.
   If the project/site can meet the required setbacks, the applicant can opt to use:

   • Splashblocks - Drawing 4.1.1
   • Dispersion trench - Drawing 4.3.1.B
     • A dispersion trench where there is a high probability for erosion - Drawing 4.3.1.B
   • Sheet flow dispersion through vegetation (for steep driveways only) - Drawing 4.2.1
     • Sheet flow dispersion through vegetation (driveways with varying or moderate to low slope) - Drawing 4.4.1
   If the project cannot accommodate a dispersion system, move on to 4).
7. If 1), 2), or 3) does not apply, the project will require an engineered stormwater report.
   If the project cannot accommodate an infiltration or dispersion system, then the applicant must submit an engineered stormwater report. As stated in the Development Standards, should projects not comply or elect not to use the development standards, "...a stormwater design report to be signed and stamped by a professional engineer registered in the state of Washington to address drainage issues not covered by these standards. In many cases, an engineering analysis will demonstrate a more innovative, appropriate and economic size or type of stormwater facility.
8. Pollution Generating Impervious Surfaces (PGIS) shall be required to incorporate treatment alternatives. These surfaces include impervious areas that are subject to vehicular use (i.e. driveways); industrial activities; storage of erodible or leachable materials, wastes, or chemicals and which receive direct rainfall or the run-on or blow-in rainfall.

The PGIS treatment alternatives are described in Section 6 of the Development Standards, as follows:

• Filter Strip - Drawing 6.2.1
• Biofiltration Swale - Drawing 6.2.2
• Bioretention Facility - Drawing 6.2.3
• Wetpond.
  

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