COVID-19 Risk Level for Multnomah County: Extreme Risk

24.70.100 Drainage and Terracing.

(Amended by Ordinance No. 173270, effective May 21, 1999.)

A. General. Unless otherwise indicated on the approved grading plan, drainage facilities and terracing shall conform to the provisions of this Section.

B. Terrace. Terraces at least 6 feet in width shall be established at not more than 30-foot vertical intervals on all cut or fill slopes to control surface drainage and debris except that where only one terrace is required, it shall be at mid-height. For cut or fill slopes greater than 60 feet and up to 120 feet in vertical height one terrace at approximately mid-height shall be 12 feet in width. Terrace widths and spacing for cut and fill slopes greater than 120 feet in height shall be designed by the civil engineer and approved by the Director. Suitable access shall be provided to permit proper cleaning and maintenance.

A single run of swale or ditch shall not collect runoff from a tributary area exceeding 13,500 square feet (projected) without discharging into a downdrain.

C. Subsurface drainage. Cut and fill slopes shall be provided with subdrainage as necessary for stability. Adequate culverts shall be laid under all fills placed in natural watercourses and along the flow line of any tributary branches in such a manner that the hydraulic characteristics of the stream are not adversely altered. In addition, subdrainage shall be installed if active or potential springs or seeps are covered by the fill. All culverts/subdrainage shall be installed after the suitable subgrade preparation. Design details of culverts/subdrainage shall be shown on each plan and be subject to the approval of the Director and of other government/private agencies as may be required.

A subdrain system shall be provided for embedded foundation/ retaining walls and floor slabs where ground water or seepage has a potential to affect the performance of the structure. The plans shall indicate

1. subdrainage details with appropriate specifications,

2. location of footing subdrain/discharge lines and,

3. method of disposal.

In lieu of above, walls/floors may be waterproofed and designed to resist hydrostatic pressure.

D. Disposal. All drainage facilities shall be designed to carry waters to the nearest practicable drainageway or approved stormwater management facility, as approved by the Director and/or other appropriate jurisdiction as a safe place to deposit such waters. Erosion of ground in the area of discharge shall be prevented by installation of non-erosive downdrains or other devices.

Building pads shall have a drainage gradient of 2 percent toward approved drainage facilities, unless waived by the Director.

Exception: The gradient from the building pad may be 1 percent if all of the following conditions exist throughout the permit area:

1. No proposed fills are greater than 10 feet in maximum depth.

2. No proposed finish cut or fill slope faces have a vertical line in excess of 10 feet.

3. No existing slope faces, which have a slope face steeper than 10 horizontal to 1 vertical, have a vertical height in excess of 10 feet.

E. Interceptor drains. Paved interceptor drains shall be installed along the top of all cut slopes where the tributary drainage area above slopes towards the cut and has a drainage path greater than 40 feet measured horizontally. Interceptor drains shall be paved with a minimum of 3 inches of concrete or gunite and reinforced. They shall have a minimum depth of 12 inches and a minimum paved width of 30 inches measured horizontally across the drain. The slope of the drain shall be approved by the Director.

Search Code, Charter, Policy