(A) General. Unless otherwise noted on the approved plans, drainage facilities and terracing of graded slopes shall conform to this section, to the currently adopted city design standards and to the city's Standard Plan No. 220-4.
(B) Terraces.
(1) Terraces at least eight feet in width shall be established at not more than 30 foot vertical intervals on all cut or fill graded slopes in order to control surface drainage and debris. Where only one terrace is required, it shall be at the mid-height of the slope. Terrace widths and spacing for cut and fill slopes greater than 120 feet in height shall be designed by the civil engineer based upon recommendations of the soil engineer and approved by the City Engineer. Suitable access shall be provided to all terraces to permit proper cleaning and maintenance.
(2) Terrace drains shall have a minimum gradient of 2% unless waived by the City Engineer. Terrace drains shall have a minimum depth at the deepest point of no less than one foot and a minimum paved width of at least three feet and shall be designed to accommodate all runoff created by the cut or fill slope as well as any tributary runoff which enters the terrace drain.
(C) Subsurface drainage. Cut and fill slopes shall be provided with subsurface drainage as necessary for stability and as recommended by the soil engineer or the engineering geologist.
(D) Storm water discharge. All drainage facilities shall be designed to carry storm water runoff to the nearest practicable drainage way approved by the City Engineer and any other appropriate jurisdiction as an acceptable and safe location to deposit such runoff. Erosion of the ground in the area of discharge shall be prevented by installation of non-erosive down drains, energy dissipaters or other devices approved by the City Engineer.
(E) Interceptor drains. Concrete interceptor drains (brow ditches) shall be installed along the top of all cut slopes where the tributary drainage area above the cut slope drains toward the cut slope, unless waived by the City Engineer. The slope gradient for the interceptor drain shall be the same as for terrace drains or as approved by the City Engineer.
(F) Storm water runoff. Storm water runoff shall not be allowed to flow over cut or fill slopes which are greater than a five to one (5:1) vertical to horizontal ratio, but shall be provided for as follows:
(1) Wherever practicable, each lot shall be graded so that storm water will drain from the backyard through the side yard and front yard directly to the abutting street or toward approved drainage facilities at a gradient of not less than 1%. Wherever practicable, drainage shall not be directed across other lots or over cut or fill slopes;
(2) When the provisions in the above subsection are not practicable, as determined by the City Engineer, storm water shall be collected along the top of slopes or at the rear of graded lots by means of paved gutters and/or French drains and carried to properly sized outfall or area drains which shall also serve as erosion control devices. Such drainage shall not be allowed to drain across the surface of sidewalks or parkways. Asphalt concrete may not be used for any drainage device. Down drain ditches shall be a minimum of 18 inches deep;
(3) Where slopes are terraced at 30 foot intervals, drainage shall be provided in paved ditches a minimum of 36 inches wide and 12 inches deep. Construction of the ditches shall be as described below and shall be located on the terraces with one side of the ditch two feet from the toe of the slope. Where a terrace is constructed to conform to slope requirements, but is intended to be of a temporary nature, the City Engineer may waive the drainage ditch requirements, if a satisfactory surety bond or other means to guarantee the improvement is posted with the city;
(4) Down drains, interceptor drains and terrace drains shall be connected together to collect and transport all storm water runoff entering the drains. They shall be of sufficient depth, as verified by hydraulic calculations, to allow for unimpeded flow when terraces are crossed. Down drains, interceptor drains and terrace drains shall be constructed of Portland cement concrete or air blown mortar. They shall be reinforced with wire mesh and/or other appropriate concrete reinforcement as determined by the project engineer and approved by the City Engineer. If pipe is used for down drains to transport runoff from terrace ditches, it shall be either reinforced concrete pipe (RCP), plastic pipe (PVC) or other pipe material approved by the City Engineer. Anchor lugs or collars may be required by the City Engineer if the pipe slope is equal to or greater than a two to one (2:1) horizontal to vertical ratio. Pipe specifications shall be approved by the City Engineer. Special design features shall be provided for abrupt changes in direction of terrace ditches and down drains;
(5) The discharge from any down drain, ditch or pipe shall be controlled so as to prevent erosion of the adjacent grounds. Velocities shall be reduced by means of adequately sized aprons of rock, grouted rip-rap, box-type energy dissipaters or other materials approved by the City Engineer.
(G) Maintenance of drainage facilities. Where the continuous functioning of a drainage facility is essential to the protection and use of more than one lot within the site of a development project, a mutual and reciprocal covenant or deed restriction shall be recorded by the owner of the lots on which the drainage facility is located, imposing on each such lot owner the responsibility for maintaining that portion of the drainage facility located on each lot owner's respective lot.
(H) Off-site drainage easements. All easements necessary for the construction of permanent off-site drainage facilities shall be acquired by the permittee. The easements shall be subject to the approval of the City Engineer and the City Attorney and recorded prior to the issuance of the grading permit.
(Ord. 3378 § 5 (part), 2023; Ord. 2568 § 1, 2002.)