passing gas - northwest hydroelectric association · tdg: a primer • tdg supersaturation can...

MASSACHUSETTS | COLORADO | WASHINGTON | OREGON

Passing Gas Science & Management of TDG at Hydro Facilities

Modeling TDG Abatement Measures


TDG: A Primer

• TDG supersaturation can cause gas

bubble trauma in fish

– Bubbles form in fish blood and tissues

– The bubbles can block blood flow – likened

to divers getting “the bends”

• TDG production in spillway discharge is

related to

– the depth of air entrainment

– the residence time of bubbles at depth

• Abatement measures typically try to

reduce the depth of air entrainment or

residence time of air at depth

Gorham, F. P. (1901) Gas-Bubble

Disease of Fish and Its Cause, Bulletin

of the United States Fish Commission,

vol. 19, 1899 (!!)


TDG Abatement: Modeling

• Range of modeling/analysis options for abatement

measures screening, selection and design

– Spreadsheet-based calculations

• Example: Mystery project, undisclosed location

– CFD model of proxy characteristic (plunge depth)

• Example: Cabinet Gorge Dam, Idaho

– Multi-model (physical + CFD) coupled with gas-transfer equations

• Example: Boundary Dam, Washington



• Outlets

– 4 spillway bays; 8 LLOs

• Abatement measures considered

– Block loading the LLOs/increasing

operating head

– Adding hydro power

– Spillway deflectors

• Tools

– MS Excel-based calculations

• Method

– Estimate the reduction in spillway use

as a proxy for TDG improvement

Mystery Project, Undisclosed Location

?



Cabinet Gorge Dam, Clark Fork River, Avista

Before and After Modifications

• Outlets

– 8 spillway bays; 4 unit powerplant


– Original concept – re-open

construction diversion tunnels as an

alternative outlet to spill

– Final concept – add “roughness

elements” to break up jet to reduce air

entrainment depth

• Tools – Original concept: Physical and CFD

modeling, spreadsheet based gas-

transfer model

– Final concept: Single-bay CFD




Proof of Concept Field Testing

• Staged approach

– Step 1: Use a single bay “prototype test” as a full-scale

model for proof of concept for roughness elements

– Step 2: Use CFD model to estimate relative difference in

plunge depth from a single bay for different designs

– Step 3: Modify additional bays and monitor performance



• CFD as a design development tool ‒ Compare relative TDG performance of different roughness

element geometries using air entrainment depth as a proxy

‒ Determine effect on spillway capacity during passage of the PMF

‒ Obtain hydrodynamic loads for structure design

• Did not attempt to quantify TDG or interaction of flow

released from multiple bays

Compare Jet Impact Area Compare Air Content with Depth Plunge Pool Geometry





Proof of Concept Field Testing



Boundary Dam, Pend Oreille River, Seattle City Light

• Outlets

– 2 spillway bays; 7 sluices, 6 unit

powerplant


– Throttle sluice gates

– Roughen sluice flow

– Spillway flow splitter/aerator

• Tools – Physical model

– Full project CFD

– Spreadsheet-based gas transfer

model (TDG predictive tool)

– Field data collection and analysis

Modeling and analysis by:

Alden Research Laboratory, Hatch, and Dr. John Gulliver




• Staged Approach

– Physical model for design development – simulate numerous

potential designs

– CFD model with particle tracking from upstream end of spillway to

point of particle exhaustion in tailrace

– Spreadsheet-based TDG predictive tool to convert particle histories

into estimated downstream TDG

– Field data for CFD model validation and performance verification



Challenges

• Changed conditions ‒ Power generation protocols

‒ Generating unit/spill bay outages

‒ Upstream TDG

• Data collection ‒ Low flow years

‒ Sonde failure

• Proof of performance ‒ Direct comparison pre- to post-modification can be challenging

due to the above factors

• Near-field spillway hydraulics ‒ Roughness element concept is based on supercavitating baffle

blocks (USBR); predicting potential areas at risk of cavitation

damage and designing to protect against it is challenging



Questions and Discussion

(after the last presentation)

passing gas - northwest hydroelectric association · tdg: a primer • tdg supersaturation can...

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