Low Impact Development (LID) Guidance
OVERVIEW AND BACKGROUND
The purpose of these “cut sheets” is to provide Caltrans Landscape Architects the information needed to design the landscape components of sustainable streets and parking lots. Sustainable and Low Impact Development (LID) design strategies can be successfully applied to main streets and parking lots to improve water quality and enhance livable communities. A sustainable strategy to address water quality considers designing stormwater runoff systems that are cost effective and integrated with the transportation feature. Landscape Architects must work closely with their project development teams (PDT) to identify opportunities to include LID techniques in main street and parking lot designs.
Biofiltration is Caltrans’ most cost-effective and most commonly used roadside stormwater treatment best management practice (BMP). In urbanized settings there often isn’t enough space for conventional biofiltration strips and swales. Instead, the best urban solutions are often biofiltration BMPs that have been adapted for main street and parking lot applications. These non-proprietary LID techniques include sidewalk stormwater planters, sidewalk stormwater tree trenches, parking lot stormwater planters, and permeable paving*. Though these BMPs are non-standard for Caltrans, they are appropriate for urbanized conditions because they provide stormwater treatment benefits in limited space and provide communities with desirable landscape amenities.
These BMPs are composed of conventional landscape materials familiar to Landscape Architects. Soil, plants, and gravel are combined to create small-scale stormwater treatment devices that mimic natural processes. These BMPs are very effective in treating runoff from “first flush” storms (1). A series of connected small-scale LID solutions provide treatment comparable to stand-alone biofiltration strips and swales, removing pollutants primarily by filtration through plants, infiltration through the soil, and evapotranspiration. Infiltrating stomwater runoff volume and treating it near its source can reduce or replace conventional stormwater conveyance systems (e.g. drain inlets and underground pipes).
Successful LID techniques can be seamlessly integrated in multi-modal transportation environments, providing communities sustainable streets and parking lots that benefit everyone.
General Design Considerations:
- Plants, soil, gravel, and geotextiles are used in LID design techniques that infiltrate, filter, store, evapotranspire, and detain stormwater in planters and trenches
- Landscape Architects must work with the Project Development Team Hydraulic Engineers and Project Engineers to determine the infiltration rate of the underlying soil, the groundwater table elevation, and to properly size the stormwater storage capacity needed in stormwater planters, trenches, and permeable paving base courses.
- Excess runoff should be diverted to overflow pipes connected to a stormwater system or diverted to downstream outlets where it can flow to a traditional storm drainage system. Planters can be designed for the detention and slow-release of runoff when underlying soils do not infiltrate adequately.
- Plant materials should be selected for the conditions unique to each project. The root structure of plants (including grasses, groundcovers, shrubs, and trees) help keep soils from becoming compacted, allowing for maximum infiltration.
- Healthy soils are an integral part of sustainable LID techniques. Soils may be amended with compost and other materials to improve infiltration and sustain healthy plants.
*The Office of Stormwater Management is currently developing guidance and specifications for the use of pervious concrete pavement.
(1) US EPA, Low Impact Development and Other Green Design Strategies,