16.32.050 Drainage requirements.
The subdivision shall be protected from inundation, flood, hazards, sheet overflow and ponding stormwater, springs and other surface waters. The design of improvements shall be such that water occurring within or flowing onto the subdivision will be carried off such subdivision without injury to any improvements, building sites and structures to be installed on sites within the subdivision, or to adjoining areas. Waters occurring within the subdivision shall be carried to a storm drainage facility or to a natural watercourse by such improvements as may be required by the city engineer. Drainage design within the subdivision shall accommodate anticipated future development within the drainage area. Any off-tract outlet drainage facility required to carry stormwater from the proposed subdivision to a defined channel, or conduit, shall be made adequate for ultimate development in the drainage area.
A. Design Flows. Runoff quantities shall be determined by the modified rational method using basic data supplied by the city engineer for the frequency or occurrence stipulated herein. Drainage facilities directly affecting the proposed subdivision shall have the following minimum capacities:
1. Major drainage channels and conduits shall have sufficient capacity to contain a one-hundred-year frequency of occurrence runoff;
2. Secondary drainage channels and conduits shall have sufficient capacity to contain a twenty-five-year frequency of occurrence runoff;
3. Minor drainage facilities shall have sufficient capacity to contain a twenty-five-year frequency of occurrence runoff.
B. Design Standards. Design of drainage channels, conduits and appurtenances, shall conform with design standards approved by the city engineer.
C. Culverts, gutters, and appurtenances shall be as follows:
1. Minimum Sizes. All storm sewers shall be of a size adequate to carry the design flow, but shall not be smaller than fifteen inches in diameter;
2. Erosion Control. Drainage within the street right-of-way but outside the surfaced area shall be controlled to prevent erosion by such methods as approved by the city engineer;
3. Catch Basins. Catch basins shall be of a design approved by the city engineer;
4. Allowance for Flood Stage. Drainage structures shall account for tidal action and flood stage where applicable.
D. Open Channels, Conduits, and Appurtenances. Open channels, conduits and appurtenances shall be designed to the satisfaction of the city engineer.
E. Widths of drainage easements shall be as follows:
1. Closed Conduits. Minimum widths of drainage easements for closed conduits shall be ten feet. For conduits having a diameter or width greater than four feet, the minimum width of drainage easement shall be equal to the diameter or width of the conduit plus ten feet;
2. Open Channels. The width of drainage easements for natural channels, excavated earth channels and channels lined with concrete shall contain the full width of the channel and the required adjacent access strips.
F. Subdrainage. Subdrain facilities shall be provided when recommended in the soils report, or as required by the city engineer, and shall be shown on the improvement plans, where applicable.
G. Specifications. All construction materials, methods, tests, and workmanship shall comply with the requirements of the “Atherton Standard Specifications” as recommended by the city engineer. (Ord. 441 § 1 (part), 1988)