2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

9.4.5 Drainage and Terracing

  1. General – Unless otherwise indicated on the approved grading plan, drainage BMPs and terracing shall conform to the provisions of this chapter for cut or fill slopes steeper than 3H:1V.

  2. 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 sloughing. Where only one terrace is required, it shall be at approximately mid-height. For 3:1 or steeper cut or fill slopes greater than 60 feet and up to 120 feet in vertical height, one of the required terraces shall be located at approximately mid-height and shall be 12 feet in width.

    Example:

    1. A vertical slope of 36‑foot height is proposed. A minimum of one terrace, at least 6 feet in width, shall be provided at approximately 18‑foot vertical height.

    2. A vertical slope of 75 feet is proposed. One terrace, 12 feet in width, shall be provided at approximately 37.5‑foot vertical height. Two additional terraces, each at least 6 feet in width (one higher and one lower) shall be provided so that no vertical height greater than 30 feet is created without a terrace.

    3. Terrace widths and spacing for cut and fill slopes greater than 120 feet in height shall be designed by a geotechnical engineer and approved by the director. Suitable access shall be provided to permit proper cleaning and maintenance of the terraces.

    4. 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 down drain.

  3. Subsurface Drainage – Cut and fill slopes shall be provided with subsurface drainage as necessary for stability.

  4. Disposal:

    1. All drainage BMPs shall be designed to carry waters to the nearest practicable drainage course approved by the director 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 down-drains or other devices.

    2. Building pads shall have a drainage gradient of 2% toward approved drainage BMPs, unless waived by the director.

  5. Interceptor Drains – Paved interceptor drains shall be installed along the top of all graded slopes where the contributing drainage area uphill from the slope 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 with of 30 inches measured horizontally across the drain. The slope of drain shall be approved by the director.