Understanding Your Outlet Works

Types of Outlet Works for Small Dams

Gary Fischer, P.E. Hydrometrics, Inc. and Carroll College

•Should I slip line or replace the conduit?

•What is the best way to control releases?

•What is the best to control the reservoir level?

•What materials should I use for repair/replacement?

•What permits might be required?

•Where do I go to get more information?

Your small dam outlet needs repair/replacement. Questions you might have:

Outlet Works Components

Intake Structures Control

Mechanisms Energy Dissipation

Conduits

INTAKE STRUCTURES

INTAKE STRUCTURES

Depends on function and layout Uncontrolled riser Glory hole Weir control Low level gate Gate and riser Siphon Tower w/ multiple ports

Depends on size of reservoir and drainage area

Pros and Cons of Various Intake Configurations

Reservoir

Vertical Slide Gate

Divider

Pros Easy access to gate for operation Gate is protected from ice damage “Spilled” water is conserved (in delivery system) Easy to operate Cons Can be hydraulically inefficient if tailwater is present (high losses) Complicated hydraulics Expensive at larger diameter installations

Drop InletVertical Slide Gate

Outlet conduit

Reservoir

Gangplank or Walkway

Pros Easy access to gate for operation Relatively easy installation “Spilled” water is conserved (in delivery system) Cost effective Hydraulically efficient gate position Cons Stem and riser can be damaged by ice Need to drain reservoir to work on gate

Drop Inlet

Gate Outlet conduit

Reservoir

GateStem

Pros Hydraulically efficient gate position “Spilled” water is conserved (in delivery system) Cons Gate stem must be buried to protect from damage Easy to damage by mis-operation – operator must be careful not to bend stem Susceptible to clogging with debris – trash rack important

Divider

Reservoir

Pros “Spilled” water is conserved (in delivery system) Great for remote locations Less expensive than systems with gate & gate operators Difficult to mis-operate Cons Cannot make low level releases unless all flashboards are removed Only reasonable for small low head dams

Gate in tower, no drop inlet, auxiliary spillway

Pros Easy access to gate for operation Straight forward hydraulics Cons Venting required Spilled water may be lost Need to construct auxiliary earthen spillway – can be expensive, high maintenance

GATED INLET

GATED INLET

GATED INLET

GATED INLET

DROP INLET

DROP INLET

GLORY HOLE

Divider

Reservoir

INTERNAL WEIR TOWER

INTERNAL WEIR TOWER

INTERNAL WEIR TOWER

INTERNAL GATE IN TOWER

Reservoir

Vertical Slide Gate

Divider

INTERNAL GATE IN TOWER

COMBINATION

Drop InletVertical Slide Gate

Outlet conduit

Reservoir

Gangplank or Walkway

COMBINATION

SIPHON

TRASHRACKS

TRASHRACKS

TRASHRACKS…NOT

Control the flow of water through the dam

Either stops water from flowing or throttles

the amount of water flowing

Types and sizes depend on purpose and the size of the dam and outlet

CONTROL MECHANISMS

GATE TYPES SLIDE (SLUICE) GATES:

Normally fully open but can be used in a sluice (underflow) situation as flow control

GATE PLACEMENT

Wall mount (in riser tower or bulkhead wall):

GATE PLACEMENT Upstream inlet (in reservoir) – vertical gate

Upstream inlet (in reservoir) – angled gate, parallel with upstream slope of dam

GATES – SEATING VS. UNSEATING HEADS

Seating head – when water pressure is pushing gate onto supporting structure

Unseating head – when water pressure is pushing gate away from supporting structure

VALVE TYPES

Gate valve (guard valve, emergency valve, isolation valve:

Butterfly valve or globe valve (used for control but not as ideal for venting as a gate valve):

HYDRAULIC GATES

Remote actuators eliminate long gate stems – reduce ice damage. Used in wilderness dams

VENTING For non-pressurized systems, venting is needed to prevent negative pressures and possible cavitation

Venting occurs on downstream side of gate/valve

Important to size vents correctly to avoid inadequate air flow

Water exiting an outlet has a large amount of energy. This energy can cause erosion, which can lead to instability of the outlet or dam. An energy dissipation feature is needed to rapidly reduce the flow energy and minimize or eliminate erosion.

ENERGY DISSIPATION

ENERGY DISSIPATION

Important to keep control of flow away from the downstream end of the outlet Try to avoid tailwater control (if possible) Keep flow control in the pipe, not downstream of the pipe

ENERGY DISSIPATION

IMPACT BASIN

IMPACT BASIN

ENERGY DISSIPATION

PLUNGE POOL

PLUNGE POOL

ENERGY DISSIPATION

STILLING BASINS

ENERGY DISSIPATION

STILLING BASINS

ENERGY DISSIPATION

BAFFLED CHUTE

ENERGY DISSIPATOR

PRESSURIZED:

NON-PRESSURIZED:

CONDUITS

OPERATING CONDITIONS PRESSURIZED FLOW: WATER/AIR TIGHT INTERNAL WATER PRESSURE EXTERNAL PRESSURE FROM EMBANKMENT

NON-PRESSURIZED FLOW: WATER TIGHT TO PREVENT LEAKAGE NO INTERNAL PRESSURE EXTERNAL PRESSURE FROM EMBANKMENT

PIPE MATERIALS PRESSURIZED FLOW: REINFORCED CONCRETE PRESSURE PIPE STEEL DUCTILE IRON POSSIBLY PLASTIC (HDPE) IN LOW HAZARD DAMS ONLY

NON-PRESSURIZED FLOW: PRECAST GASKETED CONCRETE PIPE CAST IN PLACE CONCRETE POSSIBLY PLASTIC (HDPE) IN LOW HAZARD DAMS ONLY POSSIBLY CORRUGATED METAL IN LOW HAZARD DAMS ONLY

Pressure Pipe vs. Non Pressure

Pipe?

PIPE MATERIALS

DONEY LAKE DAM - ASTM C361 REINFORCED CONCRETE LOW HEAD PRESSURE PIPE PIPE WILL GENERALLY FLOW NON PRESSURIZED, EXCEPT IN EXTREME STORM CONDITIONS

PIPE MATERIALS

ACKLEY LAKE DAM – PRECAST CONCRETE GASKETED PIPE HYDAULICS DESIGNED SO PIPE IS NON-PRESSURIZED IN ALL CONDITIONS

PIPE MATERIALS

PIPE MATERIALS SLIP LINING OR REPAIR: HDPE CURED-IN-PLACE PIPE

POTENTIAL PROBLEMS

Piping of embankment or foundation material

around conduits Settlement and deformation Separated joints or loss of joint material Erosion or abrasion of internal surfaces Material deterioration Misalignment

JOINTS STEEL: WELDED JOINTS DUCTILE IRON: FLANGES MECHANICAL JOINTS HDPE: FUSED JOINTS

CAST IN PLACE CONCRETE: WATER STOPS REINFORCED CONCRETE PIPE: INTEGRAL O-RING GASKET CORRUGATED METAL: WATER TIGHT GASKETED BANDS

FOUNDATION / SUPPORT DIFFERENTIAL SETTLEMENT IS BAD FOR PIPES IN DAMS SOLID FOUNDATION IS NECESSARY IF FOUNDATION MATERIAL IS WEAK, OTHER SUPPORT IS NEEDED PIPE CRADLES FLOWABLE FILL OR ENCASEMENT COMPACTION OF FOUNDATION MATERIAL CRITICAL

SEEPAGE / PIPING PREVENTION

FILTER MATERIAL REQUIRED ON OUTSIDE OF PIPE INTEGRATION WITH CHIMNEY FILTER COMPATABLE MATERIALS WITH EMBANKMENT OR FOUNDATION CUTOFF COLLARS NO LONGER USED

FROM ‘FILTER DIAPHRAGMS’, NRCS, PART 628 DAMS, NATIONAL ENGINEERING MANUAL, CHAPTER 45

3XDo 3XDo

1.5XDo

FILTER DIAPHRAGM AROUND CONDUIT

More Information?

Questions?

Gary Fischer, P.E.

Hydrometrics, Inc. and Carroll College

(406) 443-4150

Gfischer@hydrometrics.com