Stormwater regulations to change

Local jurisdictions ultimately choose which aspects of the new manual to adopt

Tim Leavitt, P.E. is the manager of Engineering Services for PBS Engineering and Environmental and a Vancouver City Councilmember
The Washington Department of Ecology (Ecology) is the chief environmental management agency for our state. As noted on their website (, the mission of this state agency is to "protect, preserve and enhance Washington’s environment, and promote the wise management of our air, land and water."

The management of surface water generated from land development activities is a component of Ecology’s goal to prevent pollution of our coveted natural resources. In 2001, Ecology produced a comprehensive document entitled the Stormwater Management Manual for Western Washington. The purpose of the manual is to provide local governments with acceptable standards for setting new rules and regulations, to provide development consultants with design criteria for stormwater and erosion control, and to provide businesses with guidance for producing pollution prevention plans.

After public input, Ecology recently updated the 2001 Manual to 2005. The purpose of this update was to clarify and correct language, incorporate recently recognized technologies and modernize design criteria and design procedures. Many of the updates were substantively insignificant, however, several key changes were incorporated in the manual. Property owners, developers and land development consultants must be aware of these key revisions, as they may have significant impacts to stormwater management associated with new and re-development projects.

The following are several of the important revisions in the 2005 manual for Southwest Washington:

• Flow control exemption for discharge to certain rivers and large lakes, under certain conditions. This includes the Columbia River.

• Allowance for consideration of existing pre-developed conditions in areas that have been highly urbanized for an extended period of time. The 2001 manual required modeling of existing conditions to historic, or pre-European, settlement conditions.

• Discontinuing allowance for single-event hydrograph approach to sizing of flow control and most treatment facilities. In short, the single-event hydrograph was determined to inadequately model the extended storm events realized in Western Washington. Ecology is now requiring the use of continuous simulation modeling.

• Updated design procedures for sizing of infiltration and filtration systems. Additional criteria and modeling is required.

• Requirement for Certified Erosion and Sediment Control Lead for disturbance of sites one acre and larger. Previously required only for sites of five acres and larger.

Although Ecology has updated the manual, the impact on development of properties in SW Washington has yet to be determined. Ultimately, the local agencies – Clark County and each city – have the ability to determine which aspects, if any, of the manual they may adopt. Consulting with a professional civil engineer or land use attorney for details specific to a particular property is recommended.