Office of Water Programs, California State University Sacramento

Basin Sizer

Version 1.3

Office of Water Programs, California State University Sacramento

New Features in Version 1.3

Several new features have been added to Basin Sizer Version 1.3.

Water Quality Flows - Sizing information for BMPs designed as flow through devices is now available in Basin Sizer.

Export to Web Page - This feature located in the File Menu replaces the Export to Excel command. Clicking on this menu item will export the water quality volume data, water quality flow results, and rainfall station information to your web browser. The tables can be copied and pasted into any spreadsheet or other document. In addition, the file can be saved to any location on your computer by clicking on Save in the File Menu of your web browser.

Removed Storm Water Quality Task Force Method - New a new method is described in the New Development and Redevelopment Handbook at The old methodology was removed from Basin Sizer to avoid confusion between the methods.

Changes from Version 1.1 to 1.2.

Updated Layer Controls - Layer controls have been moved to the Layers Menu.

Unit Conversions - The results can be displayed in standard or metric units by using the Units Menu.

About Basin Sizer

This program provides a tool for deriving, at any site in California, the following data:

1.“Water Quality Volume” (WQV) information for sizing storage-type storm water BMPs, as allowed in Appendix B.5.3 of the California Department of Transportation (Caltrans) Storm Water Management Plan (SWMP), using the following method:

Maximized Volume Method - The 85th percentile storm runoff capture ratio determined using the methods provided in Chapter 5 of the Urban Runoff Quality Management WEF Manual of Practice No 23, 1998, published jointly by the Water Environment Federation and the American Society of Civil Engineers.

2.Pre-set Water Quality Event Depth is a depth that has been established by certain Regional Water Quality Control Boards for areas within their jurisdictions, such as the Lake Tahoe Basin and the Los Angeles metropolitan area.

3.85th percentile 24-hr storm is the water quality event depth required by some jurisdictions and agencies for dischargers within a specific area. Using historical data, it is calculated to capture up to the 85th percentile 24-hour storm. Note that the 85th percentile 24-hr storm method is not one of the approved methods listed in the Caltrans SWMP for determining WQV for Caltrans projects.

4.“Water Quality Flow” (WQF) data for sizing BMPs based on flows is automatically displayed when a site is selected. The WQF numbers were developed in cooperation with Caltrans, the SWRCB and the nine RWQCBs. For details see the Caltrans Storm Water Quality Handbooks, Project Planning and Design Guide, September 2002.

Who Should Use This Program?

The intended users of this program are Caltrans planners and designers, and consultants working for Caltrans. The intended use is to size permanent infiltration basins, detention basins, and flow based BMPs constructed for water quality treatment purposes.

For non-Caltrans projects, this program also calculates the 85th percentile 24-hour storm rainfall depth, which is used by the California Coastal Commission (CCC) and is also specified in some of the newer NPDES municipal discharge permits and SUSMP (standard urban storm water management plan) documents around the state as the Water Quality Event Depth.

Limitations

By using the software, the user agrees that:

The software and any related documentation is provided "as is" without warranty of any kind, either express or implied, including, without limitation, the implied warranties or merchantability, fitness for a particular purpose, or non-infringement; the entire risk arising out of use or performance of this software product remains with the user.

In no event shall California Department of Transportation (Caltrans) the California State University, Sacramento (CSUS) Foundation, the CSUS Office of Water Programs (OWP) or any other unit of the California State University, Sacramento be liable for any consequential, special, incidental, or indirect damages of any kind (including, without limitation, damages for loss of business profits, business interruption, loss of business information, or other pecuniary loss) arising out of the delivery, performance, or use of the software.

Using Basin Sizer

Basin Sizer contains data from over 1,000 rainfall stations throughout California. The information from these stations can be used to size storm water infiltration and detention basins using the WQV for a project site anywhere in California. If a project site is very close to a particular rain gauge, data from that gauge can be used directly, assuming the location of that rain gauge is otherwise representative of the project area, particularly with respect to elevation. If the project site is located some distance away from the nearest rain gauge location, the Water Quality Event Depth is determined by interpolating data from representative station(s) nearest the site as selected by the user.

Locating the Project on the State Map

After opening Basin Sizer, use the map on the left side of the screen to locate a project site within the state by using the tools discussed below.

Select the approximate regional limits for the project by using the Magnifying Glass; move the magnifying glass over the area of interest. Click and drag the mouse to draw a rectangle around the site. To zoom out, hold the shift key down and click on the map. To further refine the map's location with respect to the project's location, use the Hand Tool to pan around the map by clicking and dragging. To aid locating the project, the Layers Menu may be used to add/remove and label highways and water bodies from the map. Another means of aiding in project locations, the Identification Tool (marked with an arrow and a question mark) may be clicked on rainfall stations and highways with the identification tool, and a pop-up box will provide the identity of the feature in question. After selecting the regional limits, use the Arrow Tool to represent the site (click when the arrow is at the location desired and an orange rectangle will be inserted onto the map). If desired, to return to original map of the entire state, click on the Zoom to Extents button (shown as a small map of the state).

Selecting rainfall stations

Once the project site has been located on the map, a list of nearby rainfall stations will appear on the upper right side of the screen. This list has four columns and a check box to the left of each station. The first column shows the distance from the site to the station in kilometers. The second shows the elevation of the station in meters. The third column contains the length of recorded data for the station. The fourth column lists the name of the station. These stations are sorted by distance from the project site. Scroll down this list to find rainfall station(s) near the proposed basin site. As a general rule, look for stations with the same elevations and the longest periods of record data (“years”). Click the box(es) on the left side of the station list to include one or more stations. Consult with your District Hydraulics Engineer if addition guidance is required.

Water Quality Volume

Infiltration basins and detention devices (usually placed as basins) are storm water Treatment BMPs that are designed to treat a volume of runoff, rather than a flow rate; Caltrans refers to this volume the Water Quality Volume (WQV).

The Basin Sizer program offers four methods for determining WQVs. For all methods, the WQV required by the basin is the product of three terms:

WQV = Water Quality Event Depth x runoff coefficient x tributary area

Basin Sizer output

For the Maximized Volume method the output is given as the product: water quality event depth x the runoff coefficient

For the Pre-set Water Quality Event Depths and the 85th percentile methods the output is given as the Water Quality Event Depth only.

Additional comments for each method

a) Maximized Volume Method - The maximized volumes are calculated by the method described in Guo and Urbonas (1996). In brief, this method analyses historical rainfall to estimate how much water can be treated by a small-sized basin. Then progressively larger basins are used in the calculation until a point of diminishing returns is found. At this point the WQV is determined from the basin size. These steps were repeated with runoff coefficients varying from 0.3 to 1.0 and “basin-full” drainage times from 12 to 72 hours.

To calculate the WQV using this method: one a project is located on the map, and rainfall station(s) selected, enter the output table, finding the cell intersecting the runoff coefficient for the area tributary to the basin and the basin's design drainage time. The value in this cell is equal to the product of: Water Quality Event Depth x runoff coefficient. Multiply this number by the tributary area to the basin to obtain the required volume of the basin, the WQV.

If runoff coefficient is unknown at the time Basin Sizer is run, take the output value under C = 1.0, at the appropriate drainage time, as the Water Quality Event Depth and calculate the WQV after the runoff coefficient and tributary area have been determined.

Single or double interpolation may be employed for runoff coefficients and/or drawdown times not shown on the output table.

b) Pre-setWater Quality Event Depth - Several Regional Water Quality Control Boards have established water quality event depths to be used when calculating the WQV for basins within their jurisdictions. The areas where these stipulated values were set are shaded in gray on the map.

To calculate the WQV using this method: locate the project site on the map, and select the nearest rainfall station, and the water quality event depth will be indicated in a text message toward the bottom of the screen (“A water quality depth of [value] has been set by [RWQCB].”). While the utmost effort has been used to ensure the accuracy of the limits of these zones, the user should verify by other means that the project is subject to these criteria, especially at the margins of the gray area.

c) 85th percentile method - To calculate the WQV using this method: locate the project on the map, and select appropriate rainfall station(s), and the water quality event depth (calculated as the statistical event depth that would generate 85% of the average annual runoff) appears on the output screen.

Saving and Opening Project Sites

Once a project site has been chosen and the nearby stations selected, this information can be saved by selecting the Save or Save As in the File menu. Saved files can be restored with the Open command and the unneeded files can be removed with the Delete command.

References

Guo, James C. Y., and Ben Urbonas. “Maximized Detention Volume Determined by Runoff Capture Ratio.” Journal of Water Resources Planning and Management.

122(1996):29-33.

SWQTF (Storm Water Quality Task Force), 1993. California Storm Water Best Management Practices Municipal Handbook. Appendix D, 1994.

Caltrans Storm Water Quality Handbooks

Basen sizer-