§ 13.36.080. General design and construction standards

Unless otherwise provided, it shall be the developer's and property owner's responsibility to design, construct and maintain a system which complies with the minimum requirements included in this chapter and the public works standards.

A. Surface water entering the subject property shall be received at the naturally occurring locations, and surface water existing [on] the subject property shall be discharged with adequate energy dissipaters within the subject property to minimize downstream damage into the naturally occurring drainage basins to the maximum extent possible, as determined by the official.

B. Where open channel construction is used to handle drainage within the subject property, the following standards shall apply:

1. In open channel work, the water surface elevation from the ten- and one hundred-year design storms shall be indicated at upstream and downstream points which cross the subject property boundaries, at sudden grade changes, and/or other locations as deemed necessary on the plan and profile drawings. The configuration of the finished grades constituting the banks of the open channel, typical cross-sections of channel geometry, and hydraulic gradelines as appropriate shall also be shown on the drawings.

2. The proposed cross-section of the channel will be shown with stable side slopes at minimum horizontal to vertical ratio of three to one or flatter or as approved.

C. When a closed system is used to handle drainage within the subject property, the system shall be a minimum of ten feet from all structures. Closed drainage systems or culverts on an existing, defined watercourse, stream or creek shall be designed to convey flows from a one hundred-year recurrence storm event. All other closed drainage systems shall be designed to convey flows from a twenty-five-year recurrence storm event.

D. All drainage easements granted to the city within the subject property shall be at least twenty feet in width for operation and maintenance of open channel or closed system installation.

E. Where drainage facilities discharge to natural drainage ways and/or watercourses, approved energy dissipation facilities shall be provided to minimize erosion and deterioration of the stream bed or banks. Materials such as broken concrete slabs, pipe, tires, scrap metal or debris are prohibited for use as energy dissipaters.

F. All nonmetallic underground drainage facilities installed during construction shall have tracer tape placed a minimum depth of one foot above the facility.

G. In addition to any other applicable requirements, all drainage systems directing water from single-family residential rooftops and footing drains shall be either (1) tight-lined to a storm drainage system, (2) discharged to a dry well, (3) infiltrated through a French drain system, (4) dispersed to vegetated ground using a splash block, or (5) other similar handling as approved.

H. Surrounding properties shall be protected from stormwater runoff from the development. This shall be accomplished by constructing private drain lines on the perimeter of the development and discharging the storm runoff into a public storm sewer system. Open space tracts and buffers shall not be used for this purpose, unless otherwise approved. Private drain lines shall be contained in private drainage easements with appropriate statements placed on approving documents.

I. Drainage conveyance shall be provided to all impervious areas of the development. Each lot or other developed area shall be provided with direct connection to the main public stormwater conveyance system; sidewalk drains will only be allowed with approval of the official. Adequate provisions, to the satisfaction of the official, must be provided for directing stormwater runoff away from all neighboring lots. This may be accomplished by providing private drainage easements on common lot lines and/or construction of drainage swales, curtain drains, or perforated pipe and connecting to the public storm conveyance system.

J. Verification shall be provided to the city engineer that the as-built detention/retention facility is in compliance with the project requirements. This shall be certified by the design engineer.

K. All site-generated storm runoff shall be directly conveyed to a public storm conveyance system, unless otherwise approved by the official. Easements and/or hold-harmless agreements shall be obtained from affected property owners to allow passage of all site-generated storm runoff that is not directly conveyed to a contiguous public storm conveyance system.

L. Maintenance of all drainage facilities constructed or modified by a proposed project is the responsibility of the property owner, except the city, will inspect and may assume maintenance of, drainage facilities constructed for formal plat subdivisions, planned unit developments, and conditional use permits as specified in S5C4 of the Phase II permit. (Ord. 2441 § 7, 1997)

(Ord. No. 2827, § 1(Exh. A), 6-21-2010)