Current through Register Vol. 24-18, September 15, 2024
(1)
Description: Outfalls move
water from one place to another, typically to a water body. They may convey
irrigation water, storm water, 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
storm water outfall projects:
(a) The
department may not provision HPAs for storm water discharges in locations
covered by a National Pollution Discharge Elimination System municipal storm
water general permit for water quality or quantity impacts. The HPA is required
only for the actual construction of any storm water outfall or associated
structures.
(b) In locations not
covered by a National Pollution Discharge Elimination System municipal storm
water 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 storm water
lines to avoid multiple storm water 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.
