Phase I Municipal Stormwater Permit Modification

The proposed permit adopts the 2019 Stormwater Management Manual for Western Washington. Volume III, p. 462 of the manual addresses "Inflow from Areas that Don't Require Flow Control" to flow control facilities, and states that "The performance of Fl...↓ow Control BMPs can be compromised if the additional area, beyond the area that needs to be mitigated, is too large." This statement quoted from the manual is unsubstantiated, and no basis is provided in the manual for the limitation on additional inflow area to that which adds a maximum of 50% to the 100-year peak runoff to the un-detained target area runoff. In fact, it can be demonstrated using WWHM2012 that allowing additional inflow beyond the limitation specified in the manual can provide superior flow control compared to a facility sized for the target area only.

The current standard should be reconsidered, and possibly suspended until the question can be investigated further. Information regarding WWHM2012 analyses that led to this conclusion is available on request.

In relation to recent published research by Tien et al (2021) from the WSU salmonid. stormwater research group, it would be prudent to require that all runoff from surfaces that receive tire traffic be treated or infiltrated in such a way as to remov...↓e the tire exudate pollutants described in Tien et al 2021 and related subsequent research.

This treatment requirement should include not only retrofitting or redesigning existing direct stormwater outfalls, but also requiring effective infiltration or bioswale treatment prior to outfall release to salmon-bearing surface waters. In particular, this requirement should apply equally to those waters that are currently "exempt" (such as the Puyallup River), and thus allow construction of new direct outfalls after treating only for the 6-month storm. This level of treatment is inadequate to ensure removal of the tire exudate pollutant, and these rivers are becoming more polluted with greater salmonid mortality rates over time.

To better protect water quality as well as timing of flows to surface waters, all stormwater from new development should be infiltrated in uplands, only allowing overflow during extreme rainfall events (store and infiltrate at least up to the 2-yr storm).

As an aside, I note that infiltration is not recommended in many of our shallow glacial till soils, which do infiltrate effectively prior to development. In these soils, shallow broad infiltration facilities installed above the till surface do work the great majority of the time, and should be implemented. The overall philosophical goal should be to get water back into the ground onsite, and the practical goal should be to spread many small infiltration facilities across the site, and avoid the "one large pond" stormwater management systems.