mixing zone considerations: outfall discharge studies & design improvements€¦ · ·...
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Mixing Zone Considerations: Outfall Discharge Studies &
Design Improvements
David Wilson
Outfall Discharge Studies & Design Improvements
• Mixing Zones Background
• Oregon RMZ-IMD
• Outfall Mixing Zone Studies
• Design Improvements of Existing Outfalls & Diffusers, and Replacement Outfalls
Mixing Zones in Oregon
• Mixing zones are authorized by EPA’s regulations and state law, provided that the state rules ensure that designated beneficial uses are protected
• Mixing zone rules are a component of Oregon’s EPA-approved water quality standards
• Anti-degradation rules allow for permitted discharges to existing mixing zones - that follow MZ rules
• Anti-degradation rules also allow for new discharges with mixing zones if no WQ degradation and no TMDL issues - that follow MZ rules
• Existing MZ sizes have typically been assigned based on limited information and without consistency
Thermal Plume Limits in MZ Rules
Instantaneous Lethality - Limit maximum plume temperature of 32 deg C after two seconds of plume travel from discharge
Thermal Shock - Limit the cross-sectional area of river that can exceed 25 deg C, by control of plumes
Migration Blockage - Limit the cross-sectional area of river that can exceed 21 deg C to 25 percent
Spawning & Incubation Impacts - Limit temperatures near spawning redds areas so 13 deg C is not exceeded
Regulatory Mixing Zone – Internal Management Directive (RMZ-IMD)
Purpose: The purpose of the RMZ-IMD is to assist DEQ staff in allocating and evaluating regulatory mixing zones in NPDES permits. Guidance not rules. Part 1: Allocating Regulatory Mixing Zones •Defines steps for sizing and allocating a regulatory mixing zone •Clarifies documentation needed in the permit and fact sheet to support mixing zone allocation Part 2: Reviewing Mixing Zone Studies •Provides guidance for staff consistency when requesting and reviewing mixing zone study information •Clarifies what information should be provided in a mixing zone study prior to permit development
RMZ-IMD Part 1 Part 1 of the IMD is oriented toward the
permit writers developing mixing zones for existing (or new) discharges and it includes:
• Background on RMZ • Re-evaluating Existing RMZ and Allocating
New RMZ • RMZ Rule Requirements and Sizing
Guidelines • Assessing RMZ Allocations • RMZ Checklist for use with Permit and Fact
Sheet
RMZ-IMD Part 1 - RMZ Allocation Review 1. If without documented basis for the existing RMZ size
and orientation, then must be reviewed and documented in the permit renewal.
2. Re-orientation, relocation, or re-sizing RMZ based on the MZ Study results and DEQ review.
3. If RMZ is re-oriented, relocated, or resized this is not considered backsliding – as long as effluent limits do not increase.
4. Redefinition of ZID or addition of ZID may be necessary and is not considered backsliding – as long as effluent limits do not increase. (Anti-degradation review may be required)
5. RMZ for human health does not have to be same size as aquatic life RMZ (2012 RMZ-IMD)
RMZ-IMD Part 1 - RMZ Rule & Sizing
To allow a RMZ the Mixing Zone rules require: • point source discharges may not cause specified effects
to beneficial uses, • avoid overlap with other MZ (as possible) • allow space for fish passage in streams, • not allow acute toxicity outside of ZID, • free of deposits & nuisance materials • minimize impacts to critical benthic habitat • small as feasible to be effective and protective of aquatic
organisms and human health • limit thermal impacts in-stream
Mixing Zone Design Elements in Rivers
MIXING ZONE
FISH MIGRATION PASSAGE
Low Flow
High Flow
Outfall Diffuser
• River width limits • Accommodate fish
passage requirements • Low river flow plume
width • High river flow plume
length • Allow for lateral plume
movement in flow range
• Old MZ often circles
Mixing Zone Design Elements in Estuaries
• Site-specific currents • Seasonal & tidal water
column stratification • Accommodate fish
passage requirements • Allow for plume
movement during tidal reversal
• Old MZ often circles
RMZ-IMD Part 2
Part 2 of the IMD (Reviewing Mixing Zone Studies) defines expectations for the MZ studies and the level of effort required to produce such studies. Part 2 includes:
• Expected Effort and Mixing Zone Study Checklist
• Mixing Zone Study Components • Outfall Discharge Modeling
RMZ-IMD Part 2 - Level of Effort Level 1: Simple -- discharge has a low risk of impacts to
“ecological resources and public health”; no potential to exceed acute criteria at the end of pipe (or only due to chlorine and ammonia), and available dilution factor is >20 based on 25% of critical low flow, and discharge is not classified as “major”
Level 2: Moderate -- discharge has potential to exceed acute
criteria at the end of pipe, and available dilution factor is >20 based on 25% of critical low flow; or discharge meets acute criteria at end of pipe, but available dilution factor is <20 based on 25% of critical low flow.
Level 3: Complex -- potential discharge effects or to exceed
acute criteria at the end of pipe; and available dilution factor is <20 based on 25% of critical low flow. Applies to all major dischargers (> 1 mgd Design Flow)
Mixing Zone Study Efforts
Study Approaches: Level 1 - modeling based on available data
Level 2 - site-specific field measurements, inspection & dilution modeling
Level 3 - site-specific field measurements, inspection, tracer measurements of dilution & dilution modeling
Level 1 MZ Study Example
Level 2 MZ Study Example
Necessity of Outfall Mixing Zone Study?
• Requirement of DEQ for NPDES permit renewal
• Used in NPDES permits to define compliance with WQ Standards & WQ-based effluent limits
• Used to define and revise MZ & dilution factors - technically support updates and revisions
• If significant change in facility flows/operations
• Field validation of new outfall or diffuser changes
Valid dilution factors and MZ size are critical to permit compliance & facility operational costs
Framework for Mixing Zones in NPDES Permit Development
Water Quality-Based Effluent Limits Derivation
Receiving Water Data
Effluent Data
Mixing Zone Study Results
Reasonable Potential Analysis, Thermal Analysis &
RMZ Allocation Review
No Limits Needed
Limits Needed
Improve Discharge or MZ & No Limits Needed
Improve Discharge & Limits Needed
Outcomes
No Feasible Improvements & No RMZ Allowed*
* Potential Site-specific criteria or UAA
WET Data
Inputs
RMZ-IMD Part 2 - Mixing Zone Study Elements
• Study Plan • Outfall & RMZ Characteristics • Ambient Receiving Water Conditions • Discharge Characteristics • Environmental Mapping • Dilution Modeling Analysis
Considerations for Outfall Studies
• Outfall configuration and location • Mixing zone boundaries • Parameters of concern for discharger (i.e. temp.) • Influences on mixing processes:
– River hydraulics, local bathymetry – Effluent flows & density, discharge velocity – Ambient density gradients – Contaminant build-up (reflux), tidal effects – Tidal vs. uni-directional flow – Single port or multi-port diffuser – Orientation of discharge ports (angles, risers, etc.)
Why conduct field measurements to support outfall dilution modeling?
• Site-specific measurements of currents, water column are key input to dilution models
• Critical receiving water conditions assumed in modeling dilutions – best measured
• Field-measured dilution with tracers to calibrate & validate model results
• Basis of existing MZ or dilutions may not be technically supported or correctly defined
• Test new outfall or modified diffuser
Data Needed for Outfall Studies and Designs
• Effluent and receiving water quality & quantity
• Local bathymetry
• River flow & stage records
• Current velocities & directions
• Tidal stages & current velocity (estuarine/marine)
Instrumentation for Field Studies Stage/discharge measurements
Velocity measurements
Field tracer studies
Field Dilution Measurements
Mixing Zone
Current Direction
Diffuser
Fluorometer and CTD repeatedly lowered and raised
through plume
Receiving Water Tracer Sampling
Plume
Field Tracer Measurements along ZID & RMZ
Receiving Water Tracer Sampling - Profiles
Receiving Water Tracer Sampling - Transects
Model-predicted versus Actual Dilutions
• Mixing process is dynamic
• Mixing changes continuously
• Field data key for complex discharge sites
Instantaneous Time averaged
Laser-induced fluorescence images of merging buoyant jets in density-stratified crossflow
Dilution Models
• Theoretical Models – Visual Plumes (UM3, DKHW & PDS) – UDKHDEN – RIVPLUME – Computational Fluid Dynamics
• Empirical Models – RSB – CORMIX
CORMIX • Evaluates wide range of discharge types – surface,
submerged, single & multiport
• Uses rule-based system and empirical equations to make dilution predictions (flow classes)
• Simplifies many diffusers into a vertical equivalent slot – immediate dilution effect (added CORJET)
• Recognizes boundaries to plume
• Exercise caution for complex & marine discharges
• Cautious use for most diffuser designs
CORMIX—Output Examples
Visual Plumes - VP • Multiple models – UM3, DKHW & PDS
• Evaluates wide range of discharge types – submerged single & multiport
• 3-dimensional integral models, but user must track boundaries
• Can simulate time-series of ambient changes
• Agrees well with field data for most discharges
• Recommended for diffuser designs (also UDKHDEN)
Visual Plumes—Output Examples
CFD Image of Diffuser Port Jet & Plume in Section View
• Stage 1 – at
ports (red) • Stage 2 – at
end of port jet (green)
• Stage 3 – dilute plume (blue)
CFD Images of Diffuser Plumes
• Image depicts “port jet & plume slices” at three stages of development for a multi-port diffuser discharge
• Stage 1 – at ports (red)
• Stage 2 – at end of port jets (green)
• Stage 3 – dilute plumes prior to merging (blue)
Case Study #1 – Discharge to Small River
• Discharge to WQ limited river
• New outfall required field tracer study
• Modeling showed “unstable conditions” with CORMIX
• Dilutions assigned in permit were conservative (low)
Dilutions Measured Along Mixing Zone Boundary (1 ft depth)
0
100
200
300
400
500
600
700
800
0 5 10 15 20 25 30 35
Distance from West Bank (feet)
Inst
anta
neou
s M
inim
um D
ilutio
ns M
easu
red
Center of Plume
Min. Avg.207-302
Min. Avg.68 - 94
Min. Avg.90 - 135 94
Min. Avg.85 - 93
Measured versus Model-predicted Dilutions
1
10
100
1000
0 10 20 30 40 50 60 70 80 90 100 110
Distance from Outfall Port (feet)
Inst
anta
neou
s M
inim
um D
ilutio
ns M
easu
red
Zone of Immediate Dilution
Mixing Zone Boundary
n = 995 n = 166 n = 1249
5 (Centerline Dilution)
33 39(Average Dilution)Model-predicted Dilutions
Field-measured Instantaneous Dilutions
46
14
47
Field-measured Minimum Average Dilutions
Case Study #2 – Discharge to Medium River
• Discharge to WQ limited river, exceeding WQS, costly to add treatment & facing legal challenges
• Improved outfall diffuser & revised MZ size to meet needs
• Modeling differences between VP and CORMIX to resolve MZ dimensions
• Agency required field tracer study to validate dilutions and MZ size for permit
Vertical Profile of Plume
Profile at MZB
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Dye (ppb)
Dep
th (
feet)
• define plume location in water column
• continuous measurements to capture maximum and range of concentrations
Transect T-12 at Downstream MZB (246 ft or 75 m downstream of diffuser) - 8-9 ft Depth (1536-1542)
0
5
10
15
20
25
30
0.649 0.65 0.651 0.652 0.653 0.654 0.655
Time (PDT)
Dye
(p
pb
)
NorthMZB
SouthMZB
Transect Across Plume
• define plume location & width in river
•continuous measurements to capture range
Case Study #2 – Results • Field measurements at 7Q10 low river flow
documented improved outfall diffuser performance
• Dilution model calibrated with tracer study & field results showed VP correct model
• Enlarged MZ size and model-predicted dilutions validated
• Facility solved discharge problems & legal challenges
Case Study #3 – Discharge to Large River with Tidal Influence
• Discharge to WQ limited river, effluent NH3 limits probable & costly to facility
• Unusual outfall diffuser (parallel)
• Unique MZ size & limited by location
• Complex modeling to represent overlap of adjacent ports & tidal influence
• Permit required validation of dilutions
Continuous Dye Measurements Collected at ZID
-0.2
9.8
19.8
29.8
39.8
49.8
59.8
69.8
79.8
Time (PDT)
Dye
(ppb
)
Flood Tide Period (Upriver Current)
Flood Tide Period (Upriver Current)
Flood Tide Period (Upriver Current)
Case Study #3 – Results
• Field measurements at 7Q10 low river flow
• Dilution model calibrated with tracer study
• Field results showed VP model correct & dilutions validated
• No changes allowed to MZ size
• Facility avoided effluent NH3 limits & costly treatment additions
Outfall Design Improvements •Conversion of single-port outfall to diffuser
•Modify or replace ports to optimize dilution & hydraulics
•Extend outfall to deeper site with new diffuser
•New outfall to meet physical demands & wide range of effluent flows
Discharge Improvement - Examples
Willamette River Outfall Modification •Need fast solution
to improve dilutions at low river flows
•Need outfalls during construction
•Hydraulic constraints
•Minimize cost of modifications
Discharge Improvement - Examples Willamette River Outfall Diffuser Modification •High dilutions
needed
•Need existing diffuser thru construction
•Minimize cost of modifications
•Hydraulic constraints & port velocity limits
Discharge Improvement - Examples Columbia River Outfall Extension & New Diffuser •Need existing diffuser thru construction
•Utilize existing 48” outfall pipe
•Add 550’ extension and 125’ diffuser
•High dilutions needed
Construction of 48” CC-Steel Outfall Diffuser Section - Tidal River
Installation of 24” CC-Steel Outfall Riser with Tideflex Duckbill Valve
Discharge Improvement - Examples Columbia River Outfall Extension & New Diffuser
• River flow range 85k to +400k cfs
•Potential for large physical impacts to structure
•Large sand waves (+10’)
Installation of 42” HDPE Outfall with 3, 12” Risers into Sheet Pile Enclosure - Tidal River
Plan & Profile of 42” HDPE Outfall - Tidal River
Shallow Tidal River Diffuser
•Tidal-dominated small river with shallow depths
•Potential for physical impacts to structure
•Recreational users
•Wide range of future effluent flows
•Resized MZ for tidal currents
Discharge Improvement – Examples
Shallow River Diffuser
•River flow range 150 to +75,000 cfs
•Large physical impacts to structure
•Wide range of effluent flows
•Far-future capacity needed
River Outfall Construction – Diffuser with Tideflex Duckbill Valves Installed
River Outfall Diffuser with Tideflex Duckbill Valves Operating
Questions?