Low-impact development is a stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of on-site natural features, site planning, and distributed stormwater management practices that are integrated into a project design.
LID Principles
LID principles are land-use management strategies that emphasize conservation, use of on-site natural features and site planning to minimize impervious surfaces, native vegetation loss and stormwater runoff. LID principles are different from LID best management practices such as rain gardens and permeable pavement.
LID Requirements
LID Best Management Practices are required for new development and redevelopment of a certain size in Bellevue. As of Dec. 31, 2016, the city adopted the Department of Ecology 2012 Stormwater Management Manual for Western Washington (as amended in December 2014).
To comply with the city’s 2013-2018 National Pollution Discharge Elimination System (NPDES) Permit, the city adopted Ordinance No. 6323, which includes amendments to the Bellevue Land Use Code that address the following LID principles:
- Measures to minimize impervious surfaces.
- Measures to minimize loss of native vegetation.
- Other measures to minimize stormwater runoff
Beginning December 31, 2016, new development and redevelopment in Bellevue will be required to reduce impervious surfaces on proposed development sites, with the option to make up the balance of the site coverage with permeable surfaces, if technically feasible.
Development will also have the ability to use zero-lot-line configurations within planned unit developments, and the conservation of vegetation and native soils and minimization of hard surfaces will be a criteria for PUD approval.
New clearing and grading code amendments will require a permit for removal of more than five significant trees within any three-year period. Other code amendments will allow the use of LID best management practices within required landscaping and rights of way.
What is hard surface?
A hard surface is an impervious surface, a permeable pavement, or a vegetated roof. Hard surfaces include all surfaces that are considered impervious under the Bellevue Land Use Code, as well as permeable pavement surfaces (porous asphalt, pervious concrete, and permeable pavers)
What are the different surfaces on my lot and how do I calculate my coverage?
You may have several different types of surfaces on your lot. Your home is a structure, this is regulated by the first line in the table below. Conventional asphalt or concrete driveways, patios and walkways are impervious if they do not allow water to infiltrate into the soil below. Permeable surfacing techniques are considered hard surfaces because while they allow water to infiltrate through to the soil below, they still disturb the native conditions that were there prior to development.
When determining how much coverage of each type of surfaces you may have on your lot, you’ll need to consult the dimensional requirements table in Bellevue LUC 20.20.010. So, for example, if your lot is 10,000 square feet in the R-4 zone:
- You may have up to 3,500 square feet (35% of 10,000 square feet) for your building footprint, which includes your residence and any accessory structures (garages, sheds, etc.)
- Your total hard surface coverage may not exceed 7,500 square feet (75% of 10,000 square feet).
- Subtract out the coverage by structures and you are left with 4,000 square feet of additional hard surfaces.
- If infiltration is feasible, 1,000 square feet of this may be impervious and the remaining 3,000 sf would need to be constructed using permeable surfaces.
- If infiltration is not feasible, you’ll be left with 1,500 square feet of impervious surface coverage above your coverage by structures, unless an engineer is able to design a surface that meets the Department of Ecology’s requirements for infiltration (such as an underdrain or deep injection well).