11-1-27: DRAINAGE AND STORMWATER MANAGEMENT:
   A.   General Requirements. General Requirements. An adequate drainage system, separate and independent of any sanitary sewer system and including any necessary ditches, pipes, culverts, intersectional drains, drop inlets, bridges, etc., shall be provided for the proper drainage of all surface water for all subdivisions and land developments. The surface water discharge rate shall not exceed the historical rate prior to development. Sizing and location of all drainage structures shall be the responsibility of a registered professional engineer. The City may require the use of on-site control methods such as retention or detention to mitigate the stormwater and drainage impacts of the proposed subdivisions and land developments. The Planning Commission shall not approve any preliminary plat of subdivision that does not make adequate provision for storm and flood water runoff channels or basins as determined by the City Administrator. No building permit shall be issued for any building within a subdivision or development permit issued for the development of land, if there is not present throughout the subdivision or to the land development an adequate system of drainage and stormwater management.
   B.   Method of Design and Capacity. Storm sewers, where required, shall be designed by the Rational Method, or other methods as approved by the City, and a copy of design computations shall be submitted along with required plans. Drainage improvements shall accommodate potential runoff from the entire upstream drainage area within the site and shall be designed to prevent increases in downstream flooding. Capacity for a 10-year storm or rain shall be provided for all street drainage structures such as catch basin, inlets cross drains, etc. Capacity for a 100-year frequency storm event shall be provided for all main drainage structures such as retention basins, principal storm sewers, and all types of flood protection works.
   C.   Location. Drainage facilities shall be located in the road right-of-way where feasible, and shall be constructed in accordance with standards and specifications of the City. Catch basins shall be located at low points of streets. Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within road rights-of-way, perpetual, unobstructed easements at least fifteen (15) feet in width for drainage facilities shall be provided across property outside the road right-of-way and with satisfactory access to the road.
   D.   Discharge. Drainage shall be designed so as to avoid concentration of storm drainage water from each lot or land development site to adjacent lots, land development sites, or vacant properties. Storm water shall not be discharged directly to perennial streams. It shall be directed toward natural drainages. If water must be discharged to a stream, the water quality flowing into the stream must meet or exceed the water quality in the receiving waters. The water quantity flowing into the stream must be evaluated to ensure the stream channel can accommodate the increased flows and not disrupt or degrade the ecology of the water body.
   E.   Grading and Site Drainage. Lots or land development sites shall be laid out so as to provide positive drainage away from all buildings, and drainage for individual lots or land development sites shall be coordinated with the general storm drainage pattern for the area. Buildings and parking lots shall be appropriately drained so as to prevent damage to abutting properties or public streets. All disturbed or graded ground areas of a building site not used for buildings or open storage areas shall be appropriately stabilized and grassed or covered with plants or landscaping materials.
   F.   Cross-Drainpipes. Where a watercourse separates the buildable area of a lot from the street by which it has access, provisions shall be made for installation of a culvert or other structure, the design of which shall be approved by the City. Cross-drains shall be provided to accommodate all natural waterflow, and shall be of sufficient length to permit full-width roadways and the required slopes. Cross drainpipes shall have head walls of an approved type on inlet and outlet ends of the pipe. Pipe installed within the right-of-way shall be reinforced concrete pipe. All storm drainpipes shall be minimum 18 inches in diameter. Storm sewer slopes shall be equal to or greater than one percent.
   G.   Drop Inlets. Drop inlets shall be generally three-foot by three-foot boxes with two-foot by three-foot grates unless otherwise specified by the City.
   H.   Easements. Where an irrigation ditch or channel, natural creek, stream or other drainage way crosses a subdivision or land development, the subdivider or developer shall provide an easement sufficient for drainage and maintenance. Easements shall be provided for all drainage facilities as approved by the City. When a subdivision or land development is traversed by a watercourse, drainage way, channel, or intermittent stream, a stormwater or drainage easement of at least twenty (20) feet shall be provided. (Ord. 437, - - 2020; amd. Ord. 453, 3-28-2023)