(a) The Stormwater Runoff Management System shall be composed of:
(1) A primary system;
(2) A secondary 100 year flood system; and
(3) A secondary 10 year flood system.
(b) These regulations shall limit any activity which will adversely affect the hydraulic function of detention facilities, open channels, drainage swales or enclosed stormwater conveyance systems.
(c) Stormwater runoff retention from sites in excess of one acre shall be limited to a factor of one and shall conform to requirements established by the City Engineer. Extra retention may be required on sites abutting ditches or storm sewers not designed for a ten year flood.
(d) When the City Engineer determines that proposed development will cause or increase downstream flooding conditions during the 100 year storm, provisions shall be made to minimize such flooding conditions and shall be included in the design of storm drainage improvements. Such provisions may include downstream improvements and/or temporary controlled detention of stormwater runoff and its regulated discharge to the downstream storm drainage system.
(e) The design and construction of drainage systems shall be such that watercourse traversing the project and natural water emanating from within the project shall be carried through and off the project without injury to improvements, building sites or buildings existing or to be installed within or adjacent to the project. Drainage water entering the project shall be received and discharged at locations and as nearly as possible in the manner that existed prior to construction of the drainage facilities within the project. The design of drainage facilities within the project shall be such that they shall conform to the ultimate drainage requirements of the land uses within the project watershed.
(1) The primary system shall be constructed and maintained under Chapter 1338, Flood Damage Prevention Standards.
(2) The secondary 100 year flood system shall be designed to allow uphill stormwater to flow onto and through the project whenever the ten year flood system is filled to and overflowing its capacity. It shall be designed to prevent the flooding of the main floor of buildings existing or to be constructed within the project and to further prevent the flooding of the main floor of the building downhill and uphill from the project. It may utilize open ditches, existing drainageways, drainage swales, open channels, roadways, public rights of way, temporary and permanent detention basins and temporary detention of stormwater runoff in wet bottom basins, dry bottom basins, parking lots, rooftops, percolation storage or other means approved by the City Engineer.
(3) The secondary ten year flood system shall be designed to prevent the possible flooding against building walls of existing and future building construction on the site and to prevent the flooding against building walls of existing and future buildings on adjacent sites.
It shall be designed to drain standing stormwater from the site within six hours after the termination of any ten year storm. When swales are not steep enough to accomplish this, a drain tile and stormwater inlet shall be constructed in the low point of yards and drained through tile to a public storm sewer or ditch.
It shall utilize storm sewer, yard slope and drainage swales to contain uphill and on site stormwater within the site which shall be discharged to a public storm sewer or ditch and not allowed to discharge onto adjacent property not part of a public ditch or storm sewers.
Yards shall be graded and maintained to drain away from buildings within an eight foot perimeter of buildings to swales or catch basins channeled to prevent stormwater damage to adjacent properties. Minimum grade over sod or grounds shall be one fourth inch per foot. Minimum grade over concrete or pavement shall be one-eighth inch per foot.
(f) When a ditch or storm sewer drains across private property an easement shall be obtained and/or provided.
(g) Driveways may not intersect the pavement higher than the pavement edge. This is to prevent a snow plow from catching the edge of the driveway.
(h) Culverts under driveways shall be installed large enough and steep enough to prevent flooding of the pavement during any ten year flood and shall be constructed in anticipation of any downstream blockage removal and/or replacement. Culvert tile shall be corrugated metal pipe or equivalent and be a minimum size of twelve inches unless a storm sewer already exists along the street which is designed for ten year flooding, thereby allowing an eight inch culvert tile.
(i) The top of a culvert pipe shall be located at least five inches below the pavement edge to allow a swale for roadway to drain to in the event the culvert tile are extended across the lot. This may mean that the bottom of the tile is located below the bottom of the ditch or other adjacent tile. Tile shall be installed with a downhill grade of at least four inches in every 100 feet of length.
(j) Parking lots for commercial, industrial or multifamily buildings shall not drain onto any City street pavement.