220-660-250  <<  220-660-260 >>   220-660-270

Outfall structures in freshwater areas.

(1) Description: Outfalls move water from one place to another, typically to a water body. They may convey irrigation water, stormwater, or other waste materials. The department recommends that a person construct energy dissipation structures landward of the riparian zone whenever feasible so discharged water can infiltrate into the soil or to sheet flow through the riparian zone into the stream.
(2) Fish life concerns: Outfalls can cause scouring or erosion of the bed. This can increase sediment supply to downstream reaches of rivers and streams. Outfalls can also cause bank erosion. This can cause a direct loss of bank side riparian vegetation. Riprap and other energy dissipation structures can bury instream habitat and riparian vegetation. In addition, outfalls can entrain fish.
(3) Limit of department authority over stormwater outfall projects:
(a) The department may not provision HPAs for stormwater discharges in locations covered by a National Pollution Discharge Elimination System municipal stormwater general permit for water quality or quantity impacts. The HPA is required only for the actual construction of any stormwater outfall or associated structures.
(b) In locations not covered by a National Pollution Discharge Elimination System municipal stormwater general permit, the department may provision HPAs to protect fish life from adverse effects, such as scouring or erosion of the bed of the water body, resulting from the direct hydraulic impacts of the discharge.
(i) Before prescribing specific discharge rates in an HPA under this subsection, the department must:
(A) Find that the discharge from the outfall will cause harmful effects to fish life;
(B) Send the findings to the applicant and to the city or county where the project is being proposed; and
(C) Allow a person to use local ordinances or other ways to avoid the adverse effects from the direct hydraulic discharge. The forty-five day requirement for issuing HPAs under RCW 77.55.021 is suspended when the department is meeting the requirements of this subsection.
(ii) After following the procedures in (b) of this subsection, the department may issue an HPA that prescribes the discharge rates from an outfall structure that will prevent adverse effects to the bed or flow of the waterway. The department may recommend, but not specify, the measures required to meet these discharge rates. The department may not require changes to the project design landward of the mean higher high water mark of marine waters or the ordinary high water mark of fresh waters of the state.
(4) Outfall design and construction:
(a) Before designing and constructing an outfall consider alternatives such as tying into existing municipal stormwater lines to avoid multiple stormwater discharge points and low impact development techniques utilizing pervious pavement, infiltration galleries, green roofs, etc., to minimize discharge impacts.
(b) To prevent the entry of adult or juvenile fish, construct the outfall structure according to a design approved by the department.
(c) To prevent scouring, protect the watercourse bank and bed at the point of discharge using biotechnical techniques or other department-approved methods.
(d) Design and locate outfalls so that outflow or any associated energy dissipaters do not cause a loss of habitat that supports fish life. The department may require that energy be dissipated using one or more of the following methods, or other effective method approved by the department:
(i) Existing natural habitat features (such as large logs, rootwads, natural large rocks, and rock shelves) without degrading the habitat function or value of the features;
(ii) Pads of native plants (live willow or dogwood stakes or other native shrubs) and biodegradable fabric;
(iii) Imported habitat components (large woody material);
(iv) Manufactured in-line energy dissipaters, such as a tee diffuser;
(v) Rounded rock energy dissipation pads; or
(vi) Angular rock energy dissipation pads, if the department determines other options are not feasible.
[Statutory Authority: RCW 77.04.012, 77.04.020, and 77.12.047. WSR 15-02-029 (Order 14-353), ยง 220-660-260, filed 12/30/14, effective 7/1/15.]
Site Contents
Selected content listed in alphabetical order under each group