A. General Standards: All stormwater facilities shall incorporate the following design standards:
1. All conveyance facilities shall be designed to accommodate a 25-year storm event.
2. When on site facilities must accommodate drainage from off site, such conveyance facilities shall be designed to accommodate a 50-year storm event.
3. Peak flows shall be calculated by the rational method for areas ten (10) acres or less. Peak flows shall be calculated by the soil conservation service (SCS) method TR-55, for areas greater than ten (10) acres. Other methods may be approved by the city.
4. The intensity-duration curves from the Idaho transportation department shall be used for the rational method.
B. Erosion, Sediment And Construction Site Waste Control: The design, testing, installation, and maintenance of erosion, sediment and construction site waste controls, as detailed in the stormwater management plan, shall be in accordance with city BMPs.
C. Infiltration Basins: All infiltration basins shall incorporate the following design standards:
1. Infiltration basins shall be designed either to retain and treat a volume equal to one-half inch (1/2") of runoff over the tributary impervious area, including roofs or to infiltrate a storm event of 0.1 inch per hour.
2. Infiltration basins designed to detain the treatment volume shall be a maximum of six inches (6") from the lowest point of the swale to the inlet of the overflow structure.
3. Infiltration basins shall have a minimum infiltration rate of 0.5 inch per hour.
4. Infiltration basins shall be planted and maintained with grass and/or other vegetative cover approved by the city. An encroachment permit issued by the city pursuant to title 12, chapter 12.44 of this code must be obtained before starting any landscaping work in infiltration basins located in city rights of way.
5. Infiltration basins must be renovated when they do not meet the minimum infiltration rate or when the vegetative cover dies.
6. Infiltration basins shall contain dry wells, or an equivalent approved by the city engineer, to accommodate overflow.
7. Where infiltration basins will be located between curb and sidewalk, both curb and sidewalk shall be considered an integral part of the storm management system and shall be installed with the infiltration basin. (Ord. 3455, 2012)