Mixing Zone Considerations: Outfall Discharge Studies & Design Improvements€¦ · ·...
Transcript of Mixing Zone Considerations: Outfall Discharge Studies & Design Improvements€¦ · ·...
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Mixing Zone Considerations: Outfall Discharge Studies &
Design Improvements
David Wilson
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Outfall Discharge Studies & Design Improvements
• Mixing Zones Background
• Oregon RMZ-IMD
• Outfall Mixing Zone Studies
• Design Improvements of Existing Outfalls & Diffusers, and Replacement Outfalls
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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)
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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
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Level 1 MZ Study Example
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Level 2 MZ Study Example
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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
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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
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RMZ-IMD Part 2 - Mixing Zone Study Elements
• Study Plan • Outfall & RMZ Characteristics • Ambient Receiving Water Conditions • Discharge Characteristics • Environmental Mapping • Dilution Modeling Analysis
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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.)
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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
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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)
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Instrumentation for Field Studies Stage/discharge measurements
Velocity measurements
Field tracer studies
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Field Dilution Measurements
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Mixing Zone
Current Direction
Diffuser
Fluorometer and CTD repeatedly lowered and raised
through plume
Receiving Water Tracer Sampling
Plume
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Field Tracer Measurements along ZID & RMZ
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Receiving Water Tracer Sampling - Profiles
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Receiving Water Tracer Sampling - Transects
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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
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Dilution Models
• Theoretical Models – Visual Plumes (UM3, DKHW & PDS) – UDKHDEN – RIVPLUME – Computational Fluid Dynamics
• Empirical Models – RSB – CORMIX
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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
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CORMIX—Output Examples
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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)
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Visual Plumes—Output Examples
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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)
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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Construction of 48” CC-Steel Outfall Diffuser Section - Tidal River
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Installation of 24” CC-Steel Outfall Riser with Tideflex Duckbill Valve
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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’)
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Installation of 42” HDPE Outfall with 3, 12” Risers into Sheet Pile Enclosure - Tidal River
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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
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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
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River Outfall Construction – Diffuser with Tideflex Duckbill Valves Installed
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River Outfall Diffuser with Tideflex Duckbill Valves Operating
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Questions?