12.08.120 Road construction standards – Drainage.
A. Drainage Plan. The developer shall submit a written drainage plan for all proposed development, roadways, and existing drainage facilities immediately downstream of the tributary being analyzed. The drainage plan should include drainage improvements or facilities relative to site grading, lot development, roadway improvements, drainage structures, ditches, swales, detention ponds, above or below-grade retention facilities, and other surface water control facilities. The following table of minimum return periods shall be used for the design of critical drainage facilities described in the drainage plan:
Type of Drainage Structure | Return Period (Exceedance Probability) |
|---|---|
Ponds, Stormwater Retention/Detention Facilities | 100-year (1%) |
Category II or III Roadway Culverts | 50-year (2%) |
Category I Roadway Culverts | 25-year (4%) |
Design return periods other than those listed above may be specified for any type of drainage structure at the written direction of the director of public facilities.
Drainage plans that require critical drainage structures, as listed in the table above, shall be prepared by a civil engineer licensed to practice in the state of Alaska and submitted to the director of public facilities for review and approval.
1. Driveway Culverts.
a. Shall be a minimum diameter of 18 inches. Larger diameter culverts may be specified by the director of public facilities.
b. Pipe slope shall match the flowline of the adjacent ditch.
c. Minimum cover shall be 12 inches.
2. Drainage Ditches, Swales.
a. Shall be constructed to prevent ponding.
b. Shall outlet into an established drainage channel within the corporate limits of the borough on property within its jurisdiction, unless otherwise approved by the director of public facilities.
c. Foreslopes shall have a maximum slope of one and one-half to one.
d. Backslopes shall have a maximum slope of one to one, except in cases where bedrock is present.
3. Storm Water Conveyance Systems, Feeder Lines, Culverts of Any Type.
a. Shall be constructed to prevent excessive siltation of riparian habitats, channel erosion or drainage to public or private property.
b. Shall outlet into an established drainage channel within the corporate limits of the borough on property within its jurisdiction, unless otherwise approved by the director of public facilities.
c. Shall extend two feet beyond the outer edge of road/driveway.
d. All plastic pipe, coupling bands and fittings shall meet AASHTO M294 Type S or Type D for corrugated polyethylene pipe (CPP) or noncorrugated polyethylene pipe (PE), ASTM D2680 for polyvinyl chloride (PVC) or acrylonitrile-butadiene-styrene (ABS) pipe, and AASHTO M230 Type S or Type D for polypropylene (PP) pipe. Alternative pipe materials must be approved by the director of public facilities. (Ord. 22-05-617 § 4)