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30.63B.140 Standards for drainage and terracing.
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(1) Cut or fill slopes steeper than one unit vertical to three units horizontal (33 percent slope) shall be designed to comply with the following criteria:

(a) Terraces at least six 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 of the slope. 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 a civil engineer. Suitable access shall be provided to permit proper cleaning and maintenance of drainage facilities, if any;

(b) Swales or ditches on terraces shall have a minimum gradient of 0.5 percent;

(c) Cut or fill slopes shall be provided with subsurface drainage as necessary to ensure slope stability and proper conveyance of groundwater;

(d) All drainage facilities shall be designed to carry waters to the nearest practicable drainage way in a safe manner and shall comply with chapter 30.63A SCC. Outfalls or points of discharge shall be designed using BMPs and construction procedures that prevent or minimize erosion pursuant to volumes II and III of the Drainage Manual;

(e) Building pads located on cuts or fills shall have a drainage gradient of two percent toward approved drainage facilities except that the gradient from a building pad may be one percent if all of the following conditions exist throughout the project site:

(i) No proposed fills are greater than 10 feet in depth;

(ii) No proposed finish cut or fill slope faces have a vertical height more than 10 feet; and

(iii) No existing slope faces that are steeper than one unit vertical to 10 units horizontal (10 percent slope) have a vertical height more than 10 feet; and

(f) Paved interceptor drains shall be installed if the drainage flow path is greater than 40 feet above the cut slope. Interceptor drains, if required, shall be paved with a minimum of three inches of concrete or gunite and may be reinforced. They shall have a minimum depth of 12 inches and a minimum paved width of 30 inches, measured horizontally across the drain.

(2) The department may approve an alternative slope or retaining structure when it is determined that the documentation on the construction plans and land disturbing activity site plan demonstrates that the alternative slope or retaining structure will be stable and will not adversely impact downstream properties. (Added by Amended Ord. 10-023, June 9, 2010, Eff date Sept. 30, 2010; Amended by Ord. 15-103, Jan. 11, 2016, Eff date Jan. 22, 2016).

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