(a) Intervening terraces. Paved (reinforced concrete or gunite) intervening terraces shall have a minimum width of six (6') feet and may be installed on the face of all cut and fill slopes at intervals not to exceed thirty (30') feet measured along a vertical plane.
The longitudinal slope of intervening terraces shall not be less than two (2%) percent or more than twelve (12%) percent, and any change in the rate of grade within these allowable slopes shall increase the grade in the direction of the flow.
A single run of an intervening terrace shall not exceed two hundred (200') feet to a downdrain.
If soil in paved terrace areas is termed "expansive", the paved terraces shall be designed by a registered civil engineer to resist the expansive characteristics of the area.
(b) Diverter terraces. Where recommended by a soils engineer, paved (concrete or gunite) diverter terraces, a minimum of thirty (30") inches in width and one (1') foot in depth, shall be installed at the top of all cut slopes where the tributary drainage area above has a slope exceeding ten (10) horizontal to one vertical and a horizontal projection greater than fifty (50') feet. The diverter terrace design shall be shown on each plan for City approval, based on the recommendations of the soils engineer and engineering geologist to the satisfaction of the City Engineer.
(c) Vee channels. Where a slough wall is required at the toe of the slope, the City Engineer may require a vee channel to be constructed behind the wall to carry off the slope waters.
(d) Inlet structures, downdrains, and outlet structures.
(1) Inlet structures. Inlet structures shall be of concrete, galvanized iron hot-dipped in asphalt, or equivalent. The inlet shall be grated or grilled or of such entry shape as to prevent the entry of objects of greater than four (4") inches dimension and permit objects of a maximum of two (2") inches dimension. Inlet structures shall be placed on the bench and shall be so shaped as to provide small entry losses. An overflow structure into the "vee" down-drains shall be provided.
(2) Downdrains. Downdrains shall be of concrete, corrugated galvanized iron hot-dipped in asphalt, or equivalent. Pipe downdrains shall have a diameter of a size required by runoff calculations but not less than twelve (12") inches. Open channel downdrains shall be designed by a civil engineer and shall have a minimum capacity equal to four (4) times the capacity of the required pipe size. The alignment of downdrains shall be such as to maintain a constant velocity head.
(3) Cleanouts. Cleanouts shall be provided at all points of severe change in grade and at points of entry to public rights-of-way.
(4) Outlet structures. Outlet structures shall be of concrete, galvanized iron hot-dipped in asphalt, or equivalent.
Where outletting into streets, the structure shall be of City standards or a design approved by the City Engineer. Where outletting into natural watercourses or other approved locations, the structure shall be provided with adequate velocity reducers, diversion walls, riprap, concrete aprons, or any similar energy dissipator. All slope drainage shall be collected and disposed of in the drainage device.
(e) Runoff computations. Runoff shall be based upon the proper fifty (50) year isohyetal map, and the runoff calculation shall be based upon the latest methods adopted by the City Engineer.
(f) Drainage dispersal walls. A drainage dispersal device shall be constructed whenever it is necessary to convert channel flow to sheet flow, and the structure shall be of a design approved by the City Engineer.
(g) Subdrains. Subdrains shall be installed to collect any active or potential springs or seeps which will be covered by the fill. Subdrains shall be installed after any watercourse has been excavated to firm material in preparation for receiving the fill. Individual design shall be shown on each plan for the City Engineer's approval. Upon the recommendation of the soils engineer and engineering geologist, and upon the approval of the City Engineer, the installation of subdrains may be eliminated.
(h) Site drainage. All building pads with cut or fill shall be constructed to carry surface waters to the nearest practical street, storm drain, or natural watercourse approved by the City Engineer and/or appropriate governmental agency as a safe place to deposit such waters. At least a two (2%) percent grade toward the approval disposal area shall be required for building pads. Where recommended by the soils engineer, eave or ground gutters shall be provided to receive all roof water and deliver it through a non-erosive device to a street or watercourse. Compacted fill berms shall be required to be constructed at the top of all slopes where diverter terraces are not required by the City Engineer.
(§ 2, Ord. 156-NS, eff. August 27, 1970; as amended by § I, Ord. 534-NS, eff. June 19, 1975)