Basin Sizer
Version 1.3
Office of Water Programs, California State University Sacramento
Copyright 2001
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 www.cabmphandbooks.com. 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 24hour 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-set Water 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
Journal of Water Resources Planning and Management.
122(1996):29-33.
SWQTF (Storm Water Quality Task Force), 1993. California Storm Water Best
Handbook. Appendix D, 1994.
Runoff
Capture
Ratio.”
Management Practices Municipal
Caltrans Storm Water Quality Handbooks http://www.dot.ca.gov/hq/oppd/stormwtr/PPDG-stormwater-2002.pdf
Basen sizer- http://www.stormwater.water-programs.com/BasinSizer/Basinsizer.htm