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?