Low Impact Development Approaches (LIDAs) such as rain gardens, vegetated swales, porous pavement, and other facilities can help maximize a site’s potential to treat stormwater.
Clean Water Services and its partners originally the Low Impact Development Approaches Handbook to promote sustainable development practices for Tualatin River Watershed in 2009. Low Impact Development Approaches (LIDAs) can maximize a site’s potential by reducing or incorporating the area needed for stormwater management and water quality treatment while protecting natural resources and habitat.
The LIDA Handbook (PDF, 13.7) is a supplement to Clean Water Services’ Design and Construction Standards and is to be used in conjunction with the Standards and other applicable regulations and local codes. It is updated periodically as codes and policies change and new techniques and best practices emerge. The latest update is from June 2021.
This colorful handbook details the benefits of LIDA, site planning, and the design process. It includes inspection checklists, detail drawings, a glossary, and links to additional resources. The design fact sheets illustrate rain gardens, vegetated swales, porous pavement, and the other types of facilities with detailed information about effective design, planting, and maintenance.
The LIDA Handbook is a collaborative product, which includes input from the land use jurisdictions in urban Washington County and Clean Water Services, Tualatin Hills Park & Recreation District, and Metro. It is a reference for all residents and jurisdictions within the Tualatin Basin. Users are encouraged to consult with the local jurisdiction, such as cities and counties, before undertaking a project.
Why Use LIDAs?
Typically, LIDA facilities are vegetated landscape elements such as planters, vegetated filter strips, and swales that filter and/or infiltrate stormwater. Other types of LIDAs are porous pavements and green roofs that reduce impervious area and runoff volume. LIDAs are integrated with the site landscaping to provide stormwater management, visual amenities, and habitat benefits. Low impact site design may preserve trees and vegetation, and conserve and reuse water. Site design approaches may include lot size averaging, density transfers, and clustering or placement of buildings and parking areas to avoid impacts to habitat, vegetation, and drainage courses. Low impact design may also reduce the need for and/or sizing of practices addressing hydromodification.
In addition to aesthetic and habitat benefits, LIDAs may:
- Meet Clean Water Services’ stormwater quality requirements for new development and redevelopment sites.
- Reduce area needed for water quality and/or hydromodification facilities by integrating LIDAs into landscaping, buildings, and pavements, which may result in more buildable land.
- Reduce and slow stormwater runoff for better water quality and less hydromodification via erosion.
- Cut project costs by eliminating piping and other engineered structures.
- Reduce the piping and excavation needed to manage stormwater runoff because it is conveyed and treated above ground.
- Use the same areas for stormwater management and landscaping (e.g., a flow-through planter may count toward required site landscaping).
- Qualify for credits for green building, site design, etc.
- Qualify for development credits such as allowable building height increases, reduced setbacks or reduced lot sizes.
- Preserve trees and significant vegetation by incorporating them into LIDA facilities or protecting them in the site design.
- Provide mitigation of heat island effects and related climate change outcomes.
Site Analysis: The first step in using LIDAs is a thorough site analysis to learn how water moves through the site and how natural hydrologic functions could be preserved. Inventory conditions on and adjacent to the site, including topography, soils, hydrology, and vegetation. The site analysis includes site visits, topographical and vegetation/habitat surveys, review of maps and reports, and development of a site base map.
In the site analysis, the physical attributes of the development or redevelopment site should be reviewed before placing streets, parking lots, and buildings to optimize stormwater management and habitat protection. Existing features should be incorporated into the site design by working with rather than against site attributes and constraints. A site layout that integrates site amenities to manage stormwater and protect habitat may reduce permitting delays.
Site Planning: After completing the site analysis, prepare a site plan for permit submittal that addresses the five LIDA objectives listed below, in order of importance:
- Conserve existing resources.
- Minimize disturbance.
- Minimize soil compaction.
- Minimize imperviousness.
- Direct runoff from impervious areas onto pervious areas.
Selecting LIDAs to Match Site Conditions: LIDA facilities can be constructed on and adjacent to buildings and integrated into site landscaping and hardscape such as parking lots and along streets. LIDA facilities can be used singly to manage rainfall and runoff from a drainage area or constructed in a series of multiple facilities. The site analysis helps identify the types of LIDAs best suited to the site.
Find additional details and definitions in the LIDA Handbook (PDF, 13.7MB).
Design Steps for LIDA Facilities
For most development sites, LIDA facilities may be designed using CWS sizing factors. Complete stormwater plan submittal requirements are detailed in the Design and Construction Standards, and local jurisdictions may have additional requirements. For sites less than one acre, the impervious area requiring treatment may be reduced if LIDAs are used. This LIDA Sizing Form (PDF) can assist in sizing. These are steps to the sizing process:
- Determine impervious area requiring treatment.
- Deduct impervious area LIDA credits.
- Size LIDA facilities for remaining impervious area.
- If needed, design water quality facilities for large impervious areas or remaining untreated impervious area.
Find additional details and definitions in the LIDA Handbook (PDF, 13.7MB).
LIDA Design Sheets (Fact Sheets)
These design sheets provide example photos, design layout sketches, use and design criteria, planting, and maintenance information for the types of LIDAs allowed by CWS Standards.
- Porous Pavement (PDF)
- Green Roof (PDF)
- Structural Infiltration Planter (PDF)
- Rain Garden/Non-Structural Infiltration Planter (PDF)
- Structural Flow-Through Planter (PDF)
- Landscape Filter Strip (PDF)
- Vegetated Swale (PDF)
- Extended Dry Basin (PDF)
- Constructed Water Quality Wetland (PDF)
- Conveyance and Stormwater Art (PDF)
- Planting Design and Habitat (PDF)
- Planting Templates (PDF)
- Public and Private Facility Plant Lists (PDF)
- Operation and Maintenance Plans (PDF)
- Private Stormwater Facilities Agreement (PDF)
Terminology & Definitions
Below are general terms and their definitions used in the handbook. Additional definitions of terms are included throughout the handbook.
Best Management Practices (BMPs) are techniques used to control stormwater runoff, sediment control, and soil stabilization, as well as management decisions to prevent or reduce nonpoint source pollution. The EPA defines a BMP as a “technique, measure or structural control that is used for a given set of conditions to manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner.”
Low Impact Development Approaches (LIDA) mitigate the impacts of increased runoff and stormwater pollution using a set of planning, design, construction techniques, and stormwater management approaches that promote the use of natural systems for infiltration, evapotranspiration, and reuse of rainwater. LIDA can occur at a wide range of landscape scales (i.e., regional, neighborhood, and site) and include, but are not limited to, green roofs, porous pavement, and vegetated stormwater management approaches.
Green development practices: Stormwater management techniques that utilize the processes of retention, infiltration, and evapotranspiration to treat runoff and reduce the volume of stormwater. (Gresham Development Code)
Low impact design: An approach for site development that protects and incorporates natural site features into erosion and sediment control and stormwater management plans. (Low Impact Design Manual for the Auckland Region 2000)
Low impact development aims to mimic natural hydrology and processes by using small-scale, decentralized practices that infiltrate, evaporate, and transpire rainwater. Specifically, LID aims to:
- Minimize impervious surfaces.
- Disconnect hydrologic elements (roofs, downspouts, parking areas).
- Maintain/increase flow paths and times.
- Utilize decentralized treatment practices. (NAHB Research Center Toolbase Services)
Also, a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of onsite natural features integrated with engineered, small-scale hydrologic controls to more closely mimic predevelopment hydrologic functions. (Low Impact Development – Technical Guidance Manual for Puget Sound)
Retrofit Project: A project that addresses existing stormwater management issues where there were previously no requirements.