(A) Streets and roadways.
(1) Cross-drainage structures shall be provided to pass flow under streets without diversion to adjacent properties. Street crossings shall be designed to convey the 10-year peak discharge under the road. Regardless of the size of the culvert, street crossings are to be designed to convey the 100-year peak discharge under and/or over the road. The maximum depth of flow over the road during the post-development or future condition 100-year event is one foot. The 100-year flow is to be directed to an area downstream of the crossing to which the flow would have gone in the absence of the crossing. Flows up to and including the 100-year frequency shall not cause increased flooding of private land, developable lands or buildings, unless a drainage easement is obtained for those areas. The ponded headwater elevation shall be delineated on the site topography map, or delineated by field survey, as required.
(2) If drainage is to be carried in streets, minimum pavement widths may be increased to handle flows or as required by the Public Works Department.
(3) Roadside drainage channels shall be in accordance with the drainage analysis and the drainage plan as approved by the Public Works Department. Roadside drainage channels are not allowed adjacent to streets with curb and gutter.
(4) Inverted crown streets will not be acceptable without written approval of a variance from the Public Works Department. Inverted crown streets shall be constructed using portland cement concrete.
(5) In general, "at-grade" or "dip" crossings are not permitted. Where the construction of a culvert is not practical or desirable, a grated concrete channel may be constructed to convey the storm water below the driving surface.
(6) Depth of flow in the roadside drainage channels for the design storm shall be lower than the adjacent subgrade elevation.
(7) Local soil conditions, flow depths, and velocities within the channel are usually the primary hydraulic considerations in channel geometric design; however, terrain and safety considerations have considerable influence. Steeper side slopes of rigid, lined channels may be more economical and will improve the hydraulic flow characteristics. The use of steeper slopes is normally limited to areas with limited roadway where the hazard to traffic can be minimized through the use of guardrails or parapets.
(8) Whenever possible, storm water shall be conveyed separately from the street system and defined drainageways shall follow existing alignments.
(9) Curb requirements for developments are based on zoning regulations, outlined in the most recent edition of the UDC.
(10) Runoff conveyed by the street section shall be limited as follows:
(a) Ten-year storm event or flow depth at the top of curb, whichever is less.
(b) Streets without curb and gutter shall convey drainage in bar ditches.
(c) The 100-year 24-hour storm peak discharge must be contained within the street right-of-way or within the tract designated for private streets.
(11) If curbs and gutters are to be installed, a detailed cross-section of the proposed curb and gutter shall be specified on the Project Plans for approval before construction. Construction and design shall be to the latest ADOT or M.A.G. Standard Specifications and approved by the Public Works Department.
(12) Drainage ditches behind curbs are not allowed.
(B) Parking lots.
(1) Parking shall be permitted in the floodplains of watercourses provided that the maximum depth of flow does not exceed two feet during a 100-year flood. Overnight parking will be limited to floodplains where the flooding does not exceed one foot in depth during the 100-year flood, unless otherwise approved by the town.
(2) Any parking lot that is located within a 100-year floodplain shall have a prominent sign posted at the entrance to the parking area that contains the information that the parking lot is subject to periodic flooding of depths up to "x" feet, and that overnight parking is not advised.
(3) Parking areas may be used as detention/retention facilities provided that maximum depths of ponding do not exceed one foot. The areas which have the maximum depth of ponding shall be located in the more remote areas of the parking lot, whenever feasible.
(4) Any portion of a parking lot that is located within a detention/retention facility shall have a prominent sign posted at the entrance to the parking area. The sign shall contain the information that the parking lot is subject to periodic flooding of depths up to "x" feet, and that overnight parking is not advised.
(C) Design procedures. Hydraulic calculations for flow in gutters and street sections, and design of storm drain inlets shall be done in conformance with Drainage of Highway Pavements, Hydraulic Engineering Circular 12, Federal Highway Administration (1984).
(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)