Basin Sizer
Basin Sizer is a software tool developed for the California Department of Transportation (Caltrans). This software computes water quality volumes (WQVs) and water quality flows (WQFs) by methods approved for Caltrans use to meet the requirements of the California State Water Resources Control Board. This application is being updated for digital accessibility and will continue to function while updates are in progress.
Welcome to Basin Sizer
Center the crosshairs on an area of interest to get sizing information.
Use the buttons to the left to turn the map layers on and off.
Calculations on water quality volumes and flows will appear to the right of the map.
Accessibility Assistance: Caltrans makes every attempt to ensure our documents are accessible. Due to variances between assistive technologies, there may be portions of this document which are not accessible. Where documents cannot be made accessible, we are committed to providing alternative access to the content. Should you need additional assistance, please contact Joshua Gualco at (916) 956-4732. For additional information visit https://dot.ca.gov/request-ada-compliant-documents.
Select a method:
Caltrans Water Quality Volume
Select sites from the list above.
Trash Sizing
Querying...
Trash sizing uses a map from NOAA Atlas 14 and does not depend on the list of sites above.
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The 85th Percentile 24-hr Storm Method is based on the cumulative frequency of daily rainfall totals. The water quality volume is determined as the 85th percentile 24-hour storm (daily rainfall total) for the applicable area. To use this method follow these steps
- Identify the "BMP Drainage Area" that drains to the proposed BMP. This includes all areas that will contribute runoff to the proposed BMP, including pervious areas, impervious areas, and off-site areas, whether or not they are directly or indirectly connected to the BMP.
- Calculate the composite runoff coefficient "C" for the area identified in Step 1.
- The 85th percentile 24-hour storm will be automatically calculated above.
- Calculate the required capture volume of the BMP by multiplying the "BMP Drainage Area" from Step 1 by the "C" from Step 2 and the "85th percentile 24-hour storm" from Step 3 to give the BMP volume. Due to the mixed units that result (e.g., ac-in., ac-ft) it is recommended that the resulting volume be converted to cubic feet or meters for use during design.
According to the California State Water Resources Control Board, a BMP designed to treat trash must meet the following criteria.
- Appropriately sized to treat not less than the peak flowrate resulting from a 1-year, 1-hour storm event (design storm) or at least the same peak flows from the corresponding storm drain;
- Do not bypass trash below the design storm under maximum operational loading conditions; and
- Trap all particles that are 5 mm or greater up to the design flow or at least the same peak flows from the corresponding storm drain; and do not have a diversion structure present upstream such that a portion of the peak flow is not treated to trap all particles 5 mm or greater.
The 1-year 1-hour design storm for trash sizing is from NOAA Atlas 14.
Maximized Volume Method
Drawdown Time (hours)
Runoff Coefficient
Select sites from the list above.
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The Maximized Volume Method analyses historical rainfall to estimate how much water can be treated by a basin. Small basins will be able to treat frequently occurring small storms, but will only partially treat large storms. To find the maximized volume, basins from small to large are plotted against the amount of water they treat. This produces a graph that rapidly rises and levels out as the basins get larger and can treat more storms. The maximized volume is the knee of this curve. After the knee of the curve, larger basins give diminishing returns on the amount of water they treat. The calculations are done automatically in Basin Sizer. To use this method follow these steps.
- Identify the "BMP Drainage Area" that drains to the proposed BMP. This includes all areas that will contribute runoff to the proposed BMP, including pervious areas, impervious areas, and off-site areas, whether or not they are directly or indirectly connected to the BMP.
- Calculate the composite runoff coefficient "C" for the area identified in Step 1 and select it from the drop down box.
- Select the drawn down time of the basin (12, 24, 48, or 72 hours). This is the time it takes for the basin to drain from completely full to empty.
- The unit basin storage volume will be automatically calculated above.
- Calculate the required capture volume of the BMP by multiplying the "BMP Drainage Area" from Step 1 by the "Unit Basin Storage Volume" from Step 4 to give the BMP volume. Due to the mixed units that result (e.g., ac-in., ac-ft) it is recommended that the resulting volume be converted to cubic feet or meters for use during design.
California Stormwater Handbook Water Quality Volume
Drawdown Time (hours)
Runoff Coefficient
Capture (% Runoff)
Urban Runoff Quality Management (ASCE) Water Quality Volume
Drawdown Time (hours)
Watershed Imperviousness Ratio
California Stormwater Handbook Water Quality Flow
Percentile
Safety Factor x
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California Stormwater Handbook Water Quality Volume
The CASQA California Stormwater BMP Handbook approach is based on results of a continuous simulation model, developed by the Hydrologic Engineering Center of the U.S. Army Corps of Engineers. The Storage, Treatment, Overflow, Runoff Model (STORM) was applied to long-term hourly rainfall data at numerous sites throughout California. STORM translates rainfall into runoff, then routes the runoff through detention storage. The results of the STORM model are incorporated into the California Stormwater BMP Handbook approach. This approach is simple to apply, and relies largely on commonly available information about a project. The following steps describe its use.
- Identify the "BMP Drainage Area" that drains to the proposed BMP. This includes all areas that will contribute runoff to the proposed BMP, including pervious areas, impervious areas, and off-site areas, whether or not they are directly or indirectly connected to the BMP.
- Calculate the composite runoff coefficient "C" for the area identified in Step 1 and select it from the drop down box.
- Select the drawn down time of the basin (24 or 48 hours). This is the time it takes for the basin to drain from completely full to empty.
- Determine the applicable requirement for capture of runoff (Capture, % of Runoff). This is normally set by a regulator and is commonly 75-85%.
- The unit basin storage volume will be automatically calculated above.
- Calculate the required capture volume of the BMP by multiplying the "BMP Drainage Area" from Step 1 by the "Unit Basin Storage Volume" from Step 5 to give the BMP volume. Due to the mixed units that result (e.g., ac-in., ac-ft) it is recommended that the resulting volume be converted to cubic feet or meters for use during design.
Urban Runoff Quality Management (ASCE) Water Quality Volume
The Urban Runoff Quality Management approach is based on volume-based BMP sizing methodology described in Urban Runoff Quality Management (WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87, (1998). It is based on two regression equations. The first equation determines the runoff coefficient. The second regression equation relates mean annual runoff-producing rainfall depths to water quality volumes determined by the "Maximized Volume Method". The following steps describe the use of the Urban Runoff Quality Management Approach.
- Identify the "BMP Drainage Area" that drains to the proposed BMP. This includes all areas that will contribute runoff to the proposed BMP, including pervious areas, impervious areas, and off-site areas, whether or not they are directly or indirectly connected to the BMP.
- Calculate the "Watershed Imperviousness Ratio", which is equal to the percent of total impervious area in the "BMP Drainage Area" divided by 100. The resulting value of the runoff coefficient "C" is shown to the right.
- Select the drawn down time of the basin (24 or 48 hours). This is the time it takes for the basin to drain from completely full to empty.
- The unit basin storage volume will be automatically calculated above.
- Calculate the required capture volume of the BMP by multiplying the "BMP Drainage Area" from Step 1 by the "Unit Basin Storage Volume" from Step 5 to give the BMP volume. Due to the mixed units that result (e.g., ac-in., ac-ft) it is recommended that the resulting volume be converted to cubic feet or meters for use during design.
California Stormwater Handbook Water Quality Flow
The California Stormwater BMP Handbook approach for water quality flows is based on the cumulative frequency of hourly rainfall intensities. Typically, the water quality flow is defined as the flow of runoff produced by a rain event equal to at least two times the 85th percentile hourly rainfall intensity for the applicable area. The percentile and safety factor can vary by region and regulator. The following steps describe the application of the California Stormwater BMP Handbook approach.
- Identify the "BMP Drainage Area" that drains to the proposed BMP. This includes all areas that will contribute runoff to the proposed BMP, including pervious areas, impervious areas, and off-site areas, whether or not they are directly or indirectly connected to the BMP.
- Calculate the composite runoff coefficient "C" for the area identified in Step 1.
- Select the cumulative hourly rainfall intensity percentile. This is usually 85%.
- Choose a safety factor for the rainfall intensity. This multiplier is usually 2, but it may vary by region.
- The rainfall intensity for the selected percentile will be calculated automatically above. The modified intensity has the safety factor applied.
- Apply the Rational Formula to calculate the water quality flow. Multiply the "BMP Drainage Area" from Step 1, the "C" from Step 2, and the "modified intensity" from Step 5 together. The result is the water quality flow.