What is Low Impact Design (LID)?

May 27th, 2014 by Fiedler Group

First introduced in the 1990’s in Prince George’s Co, MD with the use of bioretention cells, and then following years of subsequent development, the EPA issued an Executive Order and final guidance addressing Low Impact Development (LID) requirements in late 2009.

Central to the design objective of LID requirements is maintenance of the pre-development hydrology – the retention of rainfall on-site through infiltration, evaporation/transpiration, and reuse to the same extent as occurred prior to the development.

LID design principles follow four basic steps:

  • Determining pre-development conditions and identification hydrologic goal
  • Assessment of treatment goals
  • Identification of a process that addresses the specific needs of the site
  • Implementation of a practice that utilizes the chosen process and that fits the site’s constraints

The LID strategies used for maintaining or restoring the natural hydrologic functions of a site include the following elements:

  • Natural or man-made strategies
  • Site design strategies and decentralized source control techniques to limit runoff (Integrated Management Practices)
  • A primarily focused on the infiltration, evaporation, transpiration, and reuse of rainwater
  • A revised philosophy intended to reduce runoff rate and volume, and improve quality at the source

The cornerstone of the LID strategy are the Integrated Management Practices (IMPs).

The IMPs are integrated into the site design and provide multiple features throughout the site to help mimic pre-development hydrologic conditions and infiltrate storm water at the site rather than treating it as a waste and discharging it to existing water bodies.

A typical IMP’s includes five categories:

1. Site utilization

– Reducing the impervious footprint
– Mimic predevelopment hydrologic conditions
– Increase Tc

2. Filtration

– Natural water quality treatment

3.  Interception/Infiltration

– Key strategies to compliance

4. Retention of the 95th percentile rainfall event
5. Structural solutions

– The last line of defense
– Come with a higher cost

Many IMP design strategies are available:

  • Bioretention
  • Soil Amendments
  • Filter Strips
  • Vegetated Buffers
  • Grassed Swales
  • Dry Wells
  • Infiltration Basins/Trenches
  • Inlet Pollution Removal Devices
  • Rainwater Harvesting (Rain Barrels and Cisterns)
  • Tree Box Filters
  • Vegetated Roofs
  • Permeable Pavers

For further understanding of LID application and requirements contact the Civil Design experts at Fiedler Group today.