As planning attitudes change from building conventional stormwater systems to low impact development (LID) approaches, design standards for site planning and identification of best management practices are needed to ensure codes are properly implemented. Stormwater codes are discussed elsewhere in this Toolbox.
LID practices require site planning and hydrologic analysis. Site design that incorporates LID practices includes elements such as water catchments immediately downstream of impervious surfaces, curving flow paths to reduce runoff velocity, and basins interconnected by pipes or spillways that reduce the flow volume and rate of flow. In addition to slowing flow volume and velocity, LID practices provide an additional flood control mechanism by directing runoff to landscapes, bufferyards, and riparian areas.
Site planning should strive to result in a development design that functions with the smallest possible footprint to convey runoff through LID distributed infrastructure. LID sites are designed to meet “first flush” retention volume requirements. The first-flush carries the highest pollutant load with required detention volumes dependent on an area’s climate. Runoff above this amount flows to other conveyance systems.
Stormwater systems may also be designed to provide multiple-uses, and design standards should also provide guidance for multiple-use concepts.
Detention basins and stormwater harvesting basins can be a vegetated amenity, provide passive and active recreation, enhance habitat, and support landscape bufferyards.
Though LID practices are not required for stormwater control, Pima County is a leader in promoting LID. Pima County and Flagstaff may be the only jurisdictions in Arizona to incorporate these practices in stormwater planning. The recent revisions to the County’s stormwater design manual to include LID guidelines, standards, and incentives should provide a stimulus for implementation of LID practices.
Pima County modified its Stormwater Design Standards to include a section on Low Impact Development in order to encourage implementation of LID projects. Among the general requirements are that site planning should minimize disturbance and enhance areas of high permeability, and that LID practices should be distributed throughout the site. Inspection and maintenance protocols are required, and if maintenance is by a private entity, like a homeowners association, this must be included in the association’s Covenants, Conditions and Restrictions.
The manual incentivizes the use of stormwater harvesting basins by providing a method to reduce the required volume of detention facilities when basins are distributed throughout a site. Other LID practices are also incentivized when flood control benefits can be measured – for example, protection of high permeability areas and maximizing the amount of time that water is retained through use of swales and increased lengths of flow paths. Applicants for LID projects are also encouraged to consult with the County early in the review process, including during rezoning, to determine if LID practices can satisfy other County conditions, such as native plant and landscaping requirements and preservation of riparian habitat.
Sites must be designed to meet “first flush” volume requirements of retaining the runoff volume from the first ½ inch of rain that falls on a disturbed or impervious area of a new development. The manual describes the required construction standards with examples of designs for various types of basins, including bio-retention basins as shown in the drawing. Bioretention basins are engineered to collect more water and enhance infiltration via a constructed soil medium planted with vegetation. Using bioretention basins instead of standard basins can reduce the overall extent of required basin area. The manual also describes different types of pervious pavements that may be installed (porous gravel, concrete grid, etc.) and general standards for their use.
Model Design Standards
Pima County Stormwater
The Pima County Low Impact Development and Green Infrastructure Guidance Manual can be found here.
The Riverside County Design Handbook for Low Impact Development Best Management Practices can be found here.
The Flagstaff Stormwater Management Design Manual, Chapter 9 also contains useful information, and can be found here.