Using Mixing Zones Higher in the

Watershed

Rob Morrison, P.E.

Senior Water Resources Engineer

Barr Engineering Co.

Applicable Definitions

Mixing zone - An area of dilution of effluent in the receiving

water beyond which chronic toxicity criteria must be met

Low-flow conditions – the minimum amount of stream flow

occurring immediately upstream of a wastewater discharge and

available, in whole or in part, for attenuation of wastewater

pollutants

7Q10, 30Q10, 1Q10…

How is a mixing zone applied?

If you have upstream flow data…Calculate your low-flow

– DFLOW, Log-Pearson Type III Distribution

If you don’t have upstream flow data…

Methods for determining low flow values

How it works…

Computed Statistics at Streamgages, and Methods for Estimating Low-Flow Frequency Statistics and Development of Regional Regression Equations for Estimating Low-Flow Frequency Statistics at Ungaged Locations in Missouri

Number of

Gages on

Stream

Calculation Method

2+ Power Curve Equations

1 Drainage-Area Ratio Method

0 Regional Regression-Equation

Regional Regression Equations

Streamflow-variability Index

Drainage Area

Longest Flow Length

Streamflow-variability Index

Region 1

– Impacted by clays and shales

– Smallest Low Flows

Region 2

– Influence by underground reservoirs

– Losing streams possible

– Largest low flows

Region 3

– Man-made changes for agriculture

– Influenced by alluvial groundwater

Regression Equation

Drainage Area and Longest Flow Length

Final Calculation

Drainage-Area Ratio (DAR) Method

Drainage-Area Method

Goal: estimate low-flow conditions of an Ozark plateau stream in

urban watershed (southwest Missouri)

Limitation: need low-flow estimates for an ungaged location of

interest along stream

Flow data available: USGS gage located 4.3 miles downstream of

location of interest

Compared Drainage-Area Method to Regression Equation

Regional Regression-Equation Method

Approaches Compared

Conclusions

Methods are available to estimate critical low flow values on

ungaged streams

Mixing zones may become available and used in Reasonable

Potential Analysis or Water Quality Based Effluent Limits

Limitations

Regional Regression-equation method underestimated low-flow conditions

Compared DFLOW (EPA) 7Q10 analysis of streamgage data vs. regression equation output for gaged location – regression equation underestimated DFLOW analysis by ~80%

For drainage area ratios of 0.4 to 1.5, DAR method should be utilized before resorting to regional regression equation method

No guarantees you’ll get the flow you hope for

Mixing zones are approved at the discretion of the agency

Robert K Morrison, PE

Barr Engineering Company

1001 Diamond Ridge, Suite 1100

Jefferson City, MO 65109

573-638-5025

rmorrison@barr.com

Thanks and Questions?