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California Phase II LID Sizing Tool

Welcome to the California Phase II Low Impact Development (LID) Sizing Tool. This is a web-based tool that assists stormwater practitioners in selecting and sizing LID Best Management Practices (BMPs) that meet the sizing requirements set forth in California’s National Pollutant Discharge Elimination System (NPDES) permit for stormwater discharges from small municipal separate storm sewer systems (MS4s). The tool allows users to input their location, soil type, and impervious areas, and then queries a database containing pre-solved sizing factors and design curves for a variety of LID BMP types, performs permit-based sizing calculations, and tabulates allowable sizes for each LID BMP type. Sizing results are provided based on three different sizing methods allowed by the Phase II permit: a Design Storm Method, a Percent Capture Method, and a Baseline Bioretention or Equivalent Performance Method. Sizing results are also provided for the Central Coast RWQCB (Region 3) simple sizing method adopted via resolution R3-2013-0032. Users are also provided references for considering LID BMP feasibility factors beyond sizing, such as site topography and geometry and LID BMP maintenance requirements and costs. The tool includes training videos to visually instruct users on various aspects of the tool’s interface, input, and output. Further information about the tool and its development is provided in the Documentation Manual.

Development of this tool was funded by the California State Water Resources Control Board’s (SWRCB) Proposition 84 Stormwater Grant Program (SWGP).

The tool consists of a website that is linked to a database through a server. The database stores precipitation and evaporation data for multiple geographic locations throughout California, pre-defined parameters for multiple LID BMP types and project soil types, and pre-solved design curves based on SWMM 5 modeling. Over 13,000 SWMM simulations were run to develop these curves. After the user enters project information into the tool’s website, the server queries the database, performs calculations, and tabulates the areas required for various LID BMP types.

Use these Layers to help you find the inputs for steps 1 to 3 below the map

Step 1

Climate Stations

Step 2

Soil Types

Step 3

Find your impervious area

Distance (feet):  0

Area (acres):  0

To ensure the most accurate values zoom in close to your site.

Other Layers


  Step 1 - Select a climate station

This tool provides results based on 91 climate stations throughout California. Use the drop down to choose the climate station that best represents your project site. You can view the stations on the map by checking the “Climate Stations” box under Step 1 of the Layers sidebar. Then click on any station on the map to learn its name, years on record, 85th percentile design storm, and other information. You will be able to override the design storm on the following page.

  Step 2 - Input a saturated hydraulic conductivity

  inches per hour

Enter your project site’s saturated hydraulic conductivity based on on-site field data..

If you don’t know it, check with your local regulator to see if it is acceptable to use estimates from the USDA NRCS. If it is, you can view the USDA NRCS hydrologic soil group on the map above by checking the “Soil Types” box in Step 2 of the Layers sidebar and clicking on the color covering your project location.

To see tables of saturated hydraulic conductivities for soil groups and textures click here Click for help

  Step 3 - Input the impervious area

The CA Phase II NPDES permit requires that the project site be divided into discrete Drainage Management Areas (DMAs). Runoff from each DMA must be managed using LID BMPs that meet specific sizing criteria specified in the permit. The tool uses a scenario where the DMA consists of a 100% impervious catchment draining to a LID BMP. Input the size of the impervious catchment of the DMA of interest for your project.

You can use your own measured area or calculate an area using the measure tool in Step 3 of the Layers sidebar.

After clicking submit you will be given a table of LID BMP sizes designed to treat the impervious area.