Introduction to L-THIA Low Impact Development

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Low Impact Development (LID) practices aim to reduce the impacts of stormwater and pollutants from land development.

The goal of LID is to maintain, as closely as possible, the predevelopment hydrologic regime for new developments or move toward the original hydrologic regime in existing urban areas.

L-THIA/LID is an easy to use spread-sheet screening tool that evaluates the benefits of LID practices. The Long-Term Hydrologic Impact Assessment (L-THIA) model estimates the average annual runoff and pollutant loads for land use configurations based on more than 30 years of daily precipitation data, soils, and land use data for an area. In this model of the L-THIA, users need only to input the follow:

• location (state and county are used to gather rainfall data)
• type of soil in the area where the land use change is to occur (available online if unknown)
• type and size of land use change that will occur (e.g., 100 acres of agricultural land converted to 50 acres high-density residential and 50 acres commercial).
• LID practice(s) to screen.

The L-THIA/LID model consists of two screening levels for the LID approach.

Basic screening allows the users to adjust the percent of imperviousness for particular landuses.

Lot-level screening consists of a suite of LID practices such as bio-retention (rain gardens), porous pavement, narrowing impervious surfaces (streets, sidewalks and driveways) and vegetated rooftops. These practices intercept, redirect, and slow the movement of runoff and pollutants moving through a watershed.

L-THIA/LID will generate estimated runoff volumes, depths, and expected nonpoint source pollution loadings to waterbodies, based on the information provided by the user. Results can be displayed in tables, bar graphs, and pie charts.

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