Ordinance Text
8-13D-9-34: STORM DRAINAGE SYSTEM:
General Description: The natural drainage flows are altered by development of an area in three (3) basic ways. First, drainage patterns are redirected and concentrated by grading and the construction of roads. Second, flows are increased by the installation of more impervious surfaces than previously existed. Finally, the quality of the water is impacted by the runoff from pavement surfaces and fertilized landscaped areas. The storm drainage system proposed for Hidden Springs will safely transport concentrated flows to management facilities strategically located to filter nutrients and attenuate increased flows.
The diagrammatic drainage plan depicts internal drainage shed boundaries and possible locations of major management features.
Design Criteria: The design of drainage facilities associated with the Ada County road system falls under the jurisdiction of the Ada County Highway District. The design of the drainage facilities for Hidden Springs is depicted in the proposed road section designs.
Conveyance Systems: Redirection and concentration of drainage flows will primarily be a result of construction of the road network serving the project. A combination of rural and urban road sections are proposed, depending on the development intensity of a particular area. In rural section areas, grassed roadside swales will be the primary conveyance with culverts crossing at low points and other critical locations. Curb and gutter will convey drainage to a more formal storm sewer system of inlets and culverts in areas where urban road sections are proposed. In each case, the conveyance systems will be designed to deliver the storm water runoff to the management facilities while minimizing erosion.
Management Facilities: The storm water management facilities will be the primary drainage feature for mitigating the increased flows generated from Hidden Springs. These features will range from level swales graded parallel to the contour where flows are small to detention areas where higher densities will create larger flows. Other examples of these facilities include seepage trenches, dry wells, and ponds. A number of site-specific factors will affect the type of facility chosen. Less dense development, as in the northeast portion of the development, will generate lower flows requiring smaller structures in local applications. More dense development areas will include regional facilities for management due to higher anticipated flows and site availability. Soils and slope conditions, proximity to existing drainageways and size of the drainage area will also dictate the selection of facility.
These management facilities will serve a second purpose of water quality management, which is discussed below.
Water Quality Management: The quality of storm water runoff has become one of the most sensitive issues associated with land development. Residual materials from pavement surfaces, excess nutrients from fertilized landscaped areas and sediments carried with higher velocity flows contribute to the degradation of surface water quality. A number of strategies will be employed at Hidden Springs to minimize that degradation and to treat the water prior to discharge to the natural drainageways.
Other sections of this application address management of landscaping. Relatively narrow road sections are being proposed to minimize pavement areas while still providing safe travel ways. Rural road sections are being emphasized to allow the treatment of runoff as close to the source as possible, specifically in roadside grassed swales.
As previously discussed, all surface water runoff will be directed to management facilities for the control of runoff quantities. Hydrologic studies and management facilities will be updated and/or expanded as land use changes occur. Detaining runoff in these facilities will also allow suspended sediments and nutrients to settle. Between the source and these management facilities, erosion and sediment control techniques will be utilized to "pretreat" the runoff. Vegetated roadside and culvert discharge swales will provide nutrient uptake. Small check dams will slow channel flow velocities. Riprapped aprons will slow velocities and disperse concentrated flows from storm drainage systems. The emphasis will be on keeping the concentration of flows to a minimum and dispersing those concentrated flows in vegetated swales at every opportunity. Not only does this promote nutrient uptake but it also promotes infiltration and ground water recharge.
Surface water quality will be addressed both during and after construction of development improvements. In addition to the methods previously discussed, temporary features employed during construction will include silt fences, strawbale barriers, siltation basins, sediment traps and other proven and appropriate best management practices. The temporary mitigation program will focus on limiting the area of disturbance and treating the surface water as close to the source as possible.
(Ord. 1002, 10-28-2025)
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boisestandard.org
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Ada County, Idaho — United States
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Nearby Sections
8-13D-9-29: WATER SUPPLY:8-13D-9-30: WATER SYSTEM:8-13D-9-31: WATER QUALITY MANAGEMENT:8-13D-9-32: WATER CONSERVATION:8-13D-9-33: SEWER SYSTEM:8-13D-9-34: STORM DRAINAGE SYSTEM:8-13D-9-35: SHERIFF, FIRE AND EMERGENCY MEDIC8-13D-9-36: LANDSCAPE STRATEGY:8-13D-9-37: ESSENTIAL PUBLIC SERVICES AND ANT8-13D-9-38: ZONING MAP:8-13D-9-39: HIDDEN SPRINGS LEGAL DESCRIPTION:8-13D-10: DESIGN REQUIREMENTS:8-13D-10-1: PURPOSE AND INTENT STATEMENT:8-13D-10-2: ARCHITECTURE:8-13D-10-3: SITE DESIGN:
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