A. All development subject to this chapter shall be conducted so that:
1. Stormwater runoff is not accelerated or concentrated in greater quantities than the pre-development levels beyond the exterior property lines or project boundaries. There shall be no increase in the peak rate of runoff from the site when compared to the un-developed state, based on a 25-year design storm event, as defined. Un-developed shall mean the natural soils and vegetation in place prior to the start of any clearing, excavation, or construction.
2. Peak run-off volumes shall be calculated to determine storage requirements for retention and detention facilities. Volumes shall be calculated based upon the maximum accumulated volume over a twenty four (24)-hour period. Each storm duration interval shown on the intensity-duration- frequency curve shall be evaluated by the project engineer. Volumes shall be included on the plans. Volumes and design methodology shall be shown when a drainage report is required by the City.
3. No stormwater shall be collected or concentrated except within a channel or other stormwater feature protected against erosion and containing energy dissipation measures to prevent further erosion on adjoining lands. Any existing, unprotected channels shall be protected against further erosion.
4. Natural topography and existing vegetation are incorporated into the stormwater/erosion control plan, where possible, to accommodate additional stormwater flows.
5. Best Management Practices are implemented to filter pollutants and control erosion and sedimentation.
6. All existing stormwater/erosion control features shall be preserved and protected to ensure they continue to function.
7. All exposed soils or any soil or overburden stockpiles shall be protected to prevent sedimentation and mass movement.
8. Off-site transport of soils from construction sites shall be minimized through the stabilization of access points and installation of proper BMPs at site entrances.
9. Construction sites shall be managed to prevent the discharge of pollutants, such as petroleum products, paints, solvents, construction washings, and other materials.
10. Surface water shall not be diverted to adjoining properties nor shall berms, grading, or fill be placed to cause run-off to be diverted to adjoining properties. The City has no express duty to resolve disputes between neighbors over drainage issues.
B. Stormwater runoff shall be managed through compliance with the design standards and Best Management Practices by implementation of measures shown by a design professional to have an effective design capability that equals or exceeds the standards of this chapter.
C. For the design and review of plans, the applicant or design professional may use as guidance the Handbook of Best Management Practices for Stormwater Management and Erosion and Sedimentation Control; the Stormwater Management Plan Criteria and Engineering Standards prepared for the Panhandle Health District and the Interagency Stormwater Committee by Kennedy Engineers, Spokane, WA, April 1992; the latest edition of the International Code Council Stormwater Management Regulations; and the Environmental Protection Agency SWPPP Standards. These documents may be used for guidance purposes only, not as a regulatory standard.
D. Kootenai is underlain by clay and has a high ground water table. Rates of infiltration must be verified based on the soil types and ground saturation of the subject site. Generalized, assumed calculations are not acceptable.
E. Grass infiltration areas or other acceptable alternatives shall be sized to hold and treat the first one-half inch (1/2") of stormwater runoff from all impervious surfaces. The overall stormwater disposal system shall have a capacity to handle, without damage to the stormwater management system or adjacent land and improvements, a 25-year design storm event, based upon the Idaho Transportation Department Intensity- Duration-Frequency Curve, Zone C.
F. Certification in the Panhandle Stormwater and Erosion Education Program (SEEP) is encouraged for all contractors, developers, and excavators.
G. For all culvert installations, the standard drawings adopted by resolution of the City Council shall apply. For all building permit applications where the building lot fronts a stormwater drainage ditch, the property owner shall first obtain a driveway culvert permit and install the driveway culvert in conformance with this chapter and the conditions of the culvert permit prior to issuance of a building permit. (Ord. 214, 5-2-2017)