HydroCAD® Stormwater Modeling - Since 1986

## Analyzing the "Water Quality Volume"

### What is the Water Quality Volume?

Some stormwater regulations include special requirements for handling of the Water Quality Volume, or WQV.  Although the specific definition will vary, the WQV is commonly considered to be the runoff volume that includes 90% of all rainfall events in a given year.  Since the majority of all rainfall typically occurs in relatively small events, managing the discharge of the WQV is considered to be a cost-effective standard for minimizing overall pollutant discharge.

### How is the WQV calculated?

The determination of the WQV is typically based on local rainfall data and percent imperviousness.  Specific details will be provided by the applicable stormwater regulations.

### How do I model the WQV in HydroCAD?

Since HydroCAD runoff calculations are based on the SCS runoff equation, rather than the local WQV procedure, we must work backwards to determine the rainfall depth that will produce the same runoff volume as the pre-determined WQV.

In the case of a homogeneous watershed, we could rearrange the SCS runoff equation to calculate the rainfall depth directly.  But since different curve numbers will produce differing amounts of runoff, the WQV from the overall site is not as easily determined.  The easiest approach is generally to adjust the HydroCAD rainfall depth until the total runoff volume is equal to the pre-determined WQV.

Note: To determine accurate hydrograph volumes, it is critical that the proper time span be used.  The span must begin before the earliest runoff, and end after the latest runoff, otherwise the volume will be limited to the flow within the specified time span.  This will typically require a time span of at least 5-25 hours.

Once we know the rainfall depth which produces the specified WQV, we can store this as a separate "WQV".  This makes it easier to evaluate the various requirements that may apply to the WQV.

### Detaining the WQV

One of the more common regulations associated with the WQV is to "detain the WQV" for a specific period of time, such as 24 hours.  But exactly how is the time period measured?  It makes no sense to measure from the start of a 24-hours SCS storm, since the runoff frequently doesn't begin until about 10 hours.  A more meaningful approach would be to measure the time lag between the peak inflow and outflow of the detention pond, although using the detention time will give a more consistent result.