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A Comparison of Four Types of Sewage Drop Structures

through Physical Modeling

Tony Margevicius, P.E. (OH) 13-February-2018

WEAT – North Texas Section February Seminar

Agenda Why should we care?

What is a sewage drop structure?

Challenges of deep tunnel sewage drop structures

Hydraulics of sewage drop structures

Pneumatics of sewage drop structures

Comparison of types of sewage drop structures

Summary, questions and answers

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Why Should We Care?

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Why Should We Care?

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Are these recent inventions?

What are the main components of a sewage drop structure?

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What is a Sewage Drop Structure?

Design for wellhole (an extremely deep drop structure), Cleveland, Ohio, circa 1914. Source: Leonard Metcalf and Harrison P. Eddy, American Sewerage Practice, Vol. 1: Design of Sewers, 1st edition (New York: McGraw-Hill, 1914), p. 544.

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What is a Sewage Drop Structure?

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Challenges of Drop Structure Two miscible fluids

Three phase flow regime

As flow falls, it accelerates

Four dimensional flow

Analysis is difficult; two common approaches:

Mathematical models, including Computational Fluid Dynamics (CFD)

Physical models

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Challenges of Drop Structures How to dissipate the energy of falling water / solids?

How to prevent undesirable effects of high velocity?

How to prevent undesirable effects of air flow?

How to maintain a stable, predictable flow regime?

How to fit into site, geology, etc.?

How to handle debris, solids, & grit?

How to analyze?

The biggest challenges are hydraulic and pneumatic.

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Drop Structure Hydraulics “Terminal” drop velocity can exceed 50

fps (15 m/s)

What is terminal velocity of a brick?

At these speeds, several concerns arise:

Noise

Cavitation

Erosion

Vibration

Impact loads

Drop Structure Pneumatics Viscous drag pulls air

with flow

Nearly full tunnels are largest concern

Air is compressible

Air pressure can be up to ten times the water pressure

Vacuum conditions are also possible

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WASTEWATER

Pescod, M.B. and Price, A.C. “Major Factors in Sewer Ventilation”; Journal Water Pollution Control Federation, Vol. 54, Number 4 (April) 1992, pp 385-397

Drop Structure Pneumatics

Problems with air:

Capacity

Surface eruptions

Noise

Effective friction factor

Surge creation

Flow instability

Odor stripping

Corrosion

Pescod, M.B. and Price, A.C. “Major Factors in Sewer Ventilation”; Journal Water Pollution Control Federation, Vol. 54, Number 4 (April) 1992, pp 385-397

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WASTEWATER

AIR VELOCITY CONTOUR CURVE

IN PERCENT OF

WASTEWATER VELOCITY

WASTEWATER SURFACE

DIRECTION

OF FLOW

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Comparisons of Drop Structures Most popular drop structures

Plunge drop

Vortex drop

Helicoidal ramp

Baffle drop

Others (DC WASA, Portland model, drill drop, Chicago “boot,” etc.)

“Apples to apples comparison”

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Plunge Drop Oldest drop

Simple and cheap

Common for small flows and shallow drops

Not usually used for most tunnel systems, due to: Flow instability

Air entrainment

Poor energy dissipater

Releases odors

Noisy

Prone to vibration

Plunge Drop

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Vortex Drop

Structure • Commonly used on CSO

tunnels

• Inlet can be of several types (scroll, circular, tangential, etc.)

• De-aeration chamber is recommended to reduce air entrainment

• Hydraulically efficient

• Proven

• Limited access for people and equipment

Type H-4 Vortex Drop

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Helicoidal Ramp

Drop Structure

• Hydraulically, similar concept to vortex drop

• Can use continuous ramp or intermittent ramp

• Less common for CSO tunnels

• Limited access for personnel and equipment

• Adit design is critical for hydraulics / air entrainment

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Type B Helicoidal Ramp

• Provides access for

people and equipment

• Can introduce several

flows into one drop

• Provides limited ability to

arrest surge and geysers

• With proper design,

provides good de-aeration

• Gaining in popularity

(Cleveland, Toronto area,

London, New Zealand,

China, etc.)

Baffle (Cascade)

Drop Structure

Baffle Drop Structure

The Baffle Drop Manhole has been used for almost a century.

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Baffle Drop Structure

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Comparison of Drop Structures Feature Vortex Plunge Helicoidal Baffle

Total number of sewage installations ++ +++ - +

Commonly used in sewage tunnels +++ + + ++

Hydraulically efficient ++ +++ ++ +

Provides access for equipment/people + + - +++

Surge mitigation built-in + ++ + +++

Costs less to build ++ +++ + ++

Commonly built over top of tunnel + +++ ??? ++

Accommodates multiple inlet sewers - + - +++

Reduces air entrainment +++ - + +++

Reduces odors ++ - ??? +++

Self-cleaning ++ - + +++

Minimizes need for flow conditioning + +++ - +++

Suitable for a wide range of flows + +++ ++ +++

Comparison of Drop Structures

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Comparison of Drop Structures

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Summary Dropping sewage into tunnels presents many

challenges and risks.

There is no such thing as an empty pipe. For sewage drop structures, focus on what you cannot see - air.

There are many types of sewage drop structures. Make sure you know when to use which.

Lots of opportunities to improve the body of knowledge.

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Questions and (Hopefully!) Answers…

Tony Margevicius, P.E. (OH) 1300 East 9th Street, Suite 500 Cleveland, OH 44114 Tony.Margevicius@AECOM.com +1.216.622.2300