A. Applicability: The following standards shall apply to all development and subdivisions where one or more acres of land is disturbed by development activity.
B. Drainage Facilities: The applicant shall provide suitable drainage facilities for any surface runoff from within or upstream of the development. Natural drainage channels shall be used whenever possible. Where drainage in natural channels may negatively impact an irrigation or drainage district's ability to comply with the clean water act, alternative drainage methods as approved by said district shall be used. To avoid soil erosion and manage nutrients, sediment catchment basins may be required.
C. Easement: Where a development contains a stream, watercourse, drainageway, channel or ditch, an easement shall be provided adequate to contain that watercourse and any further width necessary for maintenance or reconstruction.
D. Discharge: Drainage systems shall not discharge into any sanitary sewer facility.
E. Mosquito Abatement District: When the proposed development or any part thereof falls within the boundaries of a mosquito abatement district, all drainage plans and systems shall follow the guidelines of such district.
F. Design: All drainage features for the development shall be designed by an Idaho registered professional engineer. Stormwater drainage systems shall be designed so stormwater runoff from the site does not exceed the water quality nor water discharge rate that existed prior to the proposed development.
1. Hydrologic Procedures: The rational method shall be used for drainage areas up to fifty (50) acres. For other drainage areas, the SCS TR-55 method shall be used.
2. Design Storm: The following design storm return periods shall be used:
Facility Type | Return Period |
Conveyance system (ditches, pipes, inlets, and curb and gutter) | 25-year |
Detention basins | 25-year |
Retention basins/subsurface disposal systems | 100-year |
3. Peak Runoff: The peak runoff rate (Qp) when determined by the rational method.
The intensity shall be determined from the Idaho transportation department's intensity-duration-frequency curves for zone A based on the time of concentration (duration) and frequency (return period).
4. Runoff Volume: Runoff volume (V) shall be determined using the triangular SCS unit hydrograph.
The runoff volume shall be analyzed using the calculated Tc and sixty (60) minutes (and corresponding Qp values) and whichever produces the greater volume shall be used for design. (Ord. 2009-05, 9-28-2009)