California Phase II LID Sizing Tool (2024)

The Area Needed column is how large the BMP must be to accomplish 100% of the treatment. The Area Available column is how much area or volume you allocate to the BMP.

1. Decide which BMP or combination of BMPs you want to use.
2. Allocate area or volume to the BMPs using the boxes in the Area Available column.
3. Adjust the values so that the percent accomplished is 100% or more. If you reach this goal proceed to the next step.
4. If it is not feasible to reach 100% accomplished you may allocate some area or volume to the BMPs and proceed to the next step.

To obtain more information regarding any particular LID BMP type, click the BMP name in the table.

The CA Phase II LID Sizing Tool provides LID BMP areas for three different sizing methods allowed by the CA Phase II Permit and one method allowed by the Central Coast RWQCB (Region 3):

1. Design Storm Method
   
   
   

   The Design Storm Method is based on Section E.12.e.ii.c.1.a of the Phase II Permit, which allows LID stormwater retention and treatment facilities that evapotranspire, infiltrate, harvest/use, and biotreat stormwater to be designed as follows (SWRCB 2013):

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   “The maximized capture storm water volume for the tributary area, on the basis of historical precipitation records, determined using the formula and volume capture coefficients in Urban Runoff Quality Management, WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87 (1998) pages 175-178 (that is, approximately the 85th percentile 24-hour storm runoff event).”

   Further information about this method is provided in the documentation manual.

   
   
2. Percent Capture Method
   
   
   

   The Percent Capture Method is based on Section E.12.e.ii.c.1.b of the Phase II Permit, which allows LID BMPs to be designed as follows (SWRCB 2013):

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   “The volume of annual runoff required to achieve 80 percent or more capture, determined in accordance with the methodology in Section 5 of the CASQA’s Stormwater Best Management Practice Handbook, New Development and Redevelopment (2003), using local precipitation data.”

   Further information about this method is provided in the documentation manual.

   
   
3. Baseline Bioretention or Equivalent Performance
   
   
   

   The Baseline Bioretention or Equivalent Performance Method is based on Section E.12.e.ii.f of the Phase II Permit. This permit section allows use of a stormwater treatment measure designed to: 1) infiltrate, evapotranspire, and/or bioretain runoff based on the sizing criteria from Section E.12.e.ii.c.1; and 2) be as effective as a bioretention system with the following permit-specified design parameters (SWRCB 2013):

   
   

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   Maximum surface loading rate of 5 inches per hour, based on the flow rates calculated. A sizing factor of 4% of tributary impervious area may be used. Minimum surface reservoir volume equal to surface area times a depth of 6 inches. Minimum planting medium depth of 18 inches. The planting medium must sustain a minimum infiltration rate of 5 inches per hour throughout the life of the project and must maximize runoff retention and pollutant removal. A mixture of sand (60%-70%) meeting the specifications of the American Society for Testing and Materials (ASTM) C33 and compost (30%-40%) may be used. Subsurface drainage/storage (gravel) layer with an area equal to the surface area and having a minimum depth of 12 inches. Underdrain with discharge elevation at top of gravel layer. No compaction of soils beneath the facility, or ripping/loosening of soils if compacted. No liners or other barriers interfering with infiltration. Appropriate plant palette for the specified soil mix and maximum available water use.

   
   

   The equivalence standard is found in Section E.12.e.ii.g of the Phase II Permit and allows designs to differ from the E.12.e.ii.f specification if all of the following may be demonstrated (SWRCB 2013):

   
   

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   Equal or greater amount of runoff infiltrated or evapotranspired. Equal or lower pollutant concentrations in runoff that is discharged after biotreatment. Equal or greater protection against shock loadings and spills. Equal or greater accessibility and ease of inspection and maintenance.

   
   

   The CA Phase II LID Sizing Tool’s areas reported for the Baseline Bioretention or Equivalent Performance Method are based on a conservative interpretation of the second requirement concerning concentrations. Instead of quantifying the pollutant removal of any filtration or sedimentation mechanisms within an equivalent LID BMP, the tool only accounts for pollutant removal via evapotranspiration and infiltration losses. The assumption is that these latter mechanisms result in pollutant losses that are superior to the filtration mechanism in the permit-specified bioretention. This approach also means that all equivalent LID BMPs are sized to retain on site the same volume of runoff that would be discharged after biotreatment through the permit-specified bioretention.

   
   

   Further information about this method is provided in the documentation manual.

   
   
4. Central Coast Simple Method
   
   
   

   In lieu of adopting the methods listed in the Phase II permit, the Central Coast Regional Water Quality Control Board (RWQCB) has adopted other methods for sizing LID BMPs. One of the methods, the Simple Method, is included in the CA Phase II LID Sizing Tool. The Simple Method is similar to the Design Storm Method, except that for LID BMPs with an underdrain, any storage volume above the underdrain is not credited. The LID BMP areas resulting from using the Central Coast Simple Method are, therefore, larger than those resulting from the Design Storm Method due to the decreased storage. The Central Coast Simple Method is documented in the post-construction requirements of Resolution R3-2013-0032 (Attachment 1, Appendix D).

   
   

   Further information about this method is provided in the Documentation Manual.

   
   

The tables listed on these pages provide LID BMP areas for the various sizing methods described above. Note that the area reported for the Design Storm Method and Central Coast Simple Method is based on a default 85th percentile, 24-hour design storm as specified in the Phase II permit. Some areas of the state require use of a different design storm, such as certain areas of the central coast, where the 95th percentile design storm is a common requirement. For such cases, the user may override the default design storm (i.e., the 85th percentile storm) with an alternative precipitation depth.

To obtain more information regarding any particular LID BMP type, click on a BMP name in the SDM table. You will be directed to a summary page that provides the following information:

• Tabulation of the project information (input values, resulting areas, and sizing method)
• Description and schematic of the LID BMP
• Notes on the design assumptions and references
• Links to evaluate other selection criteria (design, construction, maintenance, costs, water quality performance, and feasibility assessments)