Morrison Rob Barr Engineering Using Mixing Zones Higher in the Watershed MECC Kansas City May 11-13, 2016 Overland Park www.mecconference.com

1. Using Mixing Zones Higher in the
Watershed
Rob Morrison, P.E.
Senior Water Resources Engineer
Barr Engineering Co.

2. 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…

3. How is a mixing zone applied?


4.  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

5. 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

6. Regional Regression Equations
 Streamflow-variability Index
 Drainage Area
 Longest Flow Length

7. 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

8. Regression Equation
Drainage Area and Longest Flow Length

9. Final Calculation

10. Drainage-Area Ratio (DAR) Method


11. 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

12. Regional Regression-Equation Method


13. Approaches Compared


14. 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

15. 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

16. 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?