B. Petry Lecture NotesN. Lukovac

Hydraulic Structures

Hydraulic Structures

Table of Contents:

1 GENERALITIES 1

1.1 ABOUT THESE LECTURE NOTES 1

1.2 INTRODUCTION 1

1.3 BRIEF LIST OF HYDRAULIC STRUCTURES (INCLUDING THOSE OUT OF THE SCOPE

OF THESE LECTURE NOTES) 2

2 COLLECTION AND EVALUATION OF BASIC DATA 5

2.1 TOPOGRAPHIC SURVEYS 5

2.2 GEOLOGY 6

2.3 HYDROGEOLOGY 7

2.4 SEISMOLOGY 7

2.5 METEOROLOGY AND CLIMATOLOGY 7

2.6 HYDROLOGICAL ASPECTS AND RELATED HYDRAULIC ASPECTS 8

2.6.1 RIVER DISCHARGE SERIES (FLOW SERIES) 8

2.6.2 FLOODS 8

2.6.3 ROUTING OF HYDROGRAPHS. 10

3 SPILLWAYS 13

CONCEPT – HYDRAULIC STRUCTURE DESIGNED TO RELEASE WATER IN EXCESS FROM A

RESERVOIR TO A RIVER STRETCH DOWNSTREAM OF A DAM 13

3.2 COMPONENT WORKS AND CLASSIFICATION 13

3.3 SPILLWAY TYPES 14

3.4 DATA FOR SPILLWAY DESIGN 17

3.5 DETAILED HYDROLOGIC DATA 17

3.6 DETAILED HYDRAULIC DATA – SUPPORT OF HYDRAULIC DESIGN 18

3.7 SELECTION CRITERIA AND PROCEDURE 18

3.8 DESIGN METHODOLOGY 19

3.9 HYDRAULIC PROBLEMS (SEE HYDRAULIC DESIGN CRITERIA) 19

3.10 CONTROL GATES 22

4 OUTLET WORKS 25

4.1 CONCEPT – HYDRAULIC STRUCTURES USED TO CONVEY WATER FROM A RESERVOIR

TO A POINT DOWNSTREAM OF A DAM. 25

CLASSIFICATION: COMPONENTS 25

4.3 INLET AND OUTLET CHANNELS 27

4.4 INTAKES 27

4.5 CONTROL STRUCTURE 27

4.6 CONVEYANCE STRUCTURE 28

4.7 TERMINAL STRUCTURES 29

4.8 HYDRAULIC PROBLEMS AND THEIR PREVENTION 29

ENERGY DISSIPATERS 31

ENERGY DISSIPATION ON SPILLWAYS 31

5.2 SKI-JUMP AND FLIP BUCKET 31

5.3 STILLING BASINS 32

5.4 DOWNSTREAM EROSION 34

5.5 DISSIPATION AT BOTTOM OUTLETS 36

6 NAVIGATION LOCKS 39

CONCEPT 39

6.2 TYPES AND CLASSIFICATION 39

6.3 LOCK CYCLE 40

6.4 RELEVANT HYDRAULIC ASPECTS 41

7 PUMPING STATIONS AND PIPELINE CONVEYANCE 45

7.1 PUMPING STATIONS 45

7.1.1 USAGE AND CLASSIFICATION 45

7.1.2 PUMP PARAMETERS 46

7.1.3 DESIGN AND SELECTION OF PUMP, SUMP AND MAINS 48

7.1.4 PRESSURE TRANSIENTS 49

7.2 PIPELINES 51

7.2.1 HYDRAULICS 51

7.2.2 LOADS 53

7.2.3 PLACEMENT CONSIDERATIONS 54

8 SPECIAL STRUCTURES 57

8.1 FISH LADDERS AND PASSAGES 57

8.2 SPAWNING CHANNELS 58

8.3 SELECTIVE INTAKES 61

1 Generalities

1.1 About These Lecture Notes

These lecture notes are written as brief guide to make it easier to follow the course on Hydraulic

Structures. They should also serve as remainder for future reference concerning the lectures as well

as references listed for each subject covered. Appendixes provided in form of handouts, mainly

selected excerpts from useful references, should serve as extension of lecture notes and guidance

for further more detailed studies.

1.2 Introduction

In various textbooks on Hydraulic Structures one can find different contents. That is mainly due to

different perceptions about what the hydraulic structures are. The broadest definition is that: these

are “all structures in contact with water” that would include structures such as bridges, hydraulic

tunnels, docks, coastal and offshore structures etc. However, in this course curriculum is limited

only to hydraulic structures of interest for River Engineering and River Basin Development, and

only those that are not given elsewhere. For instance: dams are given in “Engineering of Dams”,

and river diversion structures in “River Diversions and Headworks”. Most other structures that are

not related to River Engineering and River Basin Development are covered in other Masters

Programmes of IHE especially in Hydraulic Engineering. Therefore, in this course the emphasis is

given to structures that are, in one way or the other, related to Dams like: Outlets, Spillways,

Navigation Locks and the like. Part of these lecture notes will be repeated in “Engineering of

Dams” as a reminder, since some of the structures given here can not be neglected in that course as

they are inseparable parts of most of the dams.

Humankind built hydraulic structures, in different forms, since the earliest days of known history,

in order to solve problems that could not be solved otherwise. Hydraulic structures are as old as

Civilization. There could not be a developed civilization without water management, and if one

looks back, one can see that all major settled civilizations were using water supply systems and

irrigation. At first, small diversion dams were used (there are records about the dam built on the

Nile River before 4000 BC) with water conveyance lines and irrigation networks. The oldest known

aqueduct was built near Nineveh, the capital of Assyria in 703 BC. The first Roman aqueduct was

Aqua Appia opened in 312 BC and it was supplying the city of Rome with water. Well-preserved

remains and remnants of some of those structures can still be seen. Some hydraulic structures as

old as 400-500 years are still in use.

Need, for hydraulic structures in order to solve water management problems is ever present, and it

will not be exhausted in foreseeable future if ever. These lecture notes will provide some

references that may help in proper planning, investigation, design and construction. It is not

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intended here to provide a “recipe cook book” but rather basic considerations of major aspects,

giving the clue where and how to search for answers to questions that may arise in practice. Even

then all the answers will not be found. To try to find some of them, at least in special non-standard

cases, further research may be required.

1.3 Brief List Of Hydraulic Structures (including those out of the scope of

these lecture notes)

Dams (given in “Engineering of Dams”)

Intakes (partly given here partly elsewhere in the programme of this branch)

Outlets (given here)

Spillways (given here)

Energy Dissipaters: Stilling Basins, Plunge Pools, Flip Buckets, Ski Jumps, Aprons (given

here)

Navigation structures locks, ship-lifts and inclined planes (given here), inland ports

Pumping stations (briefly given here)

Canals, (navigation and water conveyance) (Spawning Canals given here)

Other conveyance structures like pipelines (briefly given here)

Drop structures, culverts and siphons

Steel structures like gates, valves, air-vessels, air vents, silt outlets etc. (partly given here)

Diversion work structures diversion dams and weirs, river intakes, settling basins, drop

structures etc. (given in “River Diversions and Headworks”)

Fish ladders and passes (given in “River Diversions and Headworks”)

Barriers weirs and barrages, bottom withdrawal or Tyrolean intakes (given in “River

Diversions and Headworks”)

Check dams

Hydro power stations of various types (given in “Hydropower Development”)

Earth retaining structures like sheet-piles, retaining walls, gabions, etc. (partly given

elsewhere in the programme)

Piers, jetties, groynes (groins) for river training and/or shore protection. (partly given

elsewhere in the programme)

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Other river training structures like sills, cascades etc. . (partly given elsewhere in the

programme)

Bridges, viaducts, aqueducts

Tunnels . (partly given elsewhere in the programme)

Drainage sluices

Irrigation structures

Levees and canal dikes (embankments)

Revetments

Docks

Caissons

Fendering and mooring structures

Dikes (sea-dikes, and flood control dikes in river training)

Coastal structures breakwaters, shore protection works

Sea outfalls and intakes

Offshore pipelines

Offshore structures

Man-made islands

Even this list is not exhaustive as one can think of even more structures that could be called

“hydraulic”. However, some of them, that are most important for program in River Engineering

and River Basin Development, are dealt with in this course. They are marked above, as well as

other structures that a taught elsewhere in the programme. Most of the others are covered in other

two programmes (branches) of Hydraulic Engineering at IHE.

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SOME IMPORTANT REFERENCES ON HYDRAULIC STRUCTURES

1. Hydraulic Structures P. Novak (and others)

2. Handbook of Applied Hydraulics C.V. Davis

3. Design of Small Dams – United States Bureau of Reclamation

4. Advanced Dam Engineering – Jansen

5. Hydraulic Design Criteria – U.S. Corps of Engineers

6. Proceedings of International Conferences – ICOLD

7. Proceedings of International Conferences IAHR

8. International Water Power and Dam Construction

9. Hydropower and Dams (International Journal on…)

10.Water Power Manual – U.S. Corps of Engineers

In addition to that there is:

A large variety of technical periodicals in a variety of languages with papers on hydraulic structures

(Russian Chertousov, Agroskin and Chugayev, then other books in English, German, Spanish,

Portuguese, Japanese, etc.)

A large variety of other texts (books, periodicals) on subjects related to hydraulic structures.

NOTE: Lists of good references can also be found in the appendices of several publications cited

above.

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2 Collection and Evaluation of Basic Data

In order to carry out reliable engineering activities of hydraulic structures there are major aspects that

could be regarded as basic “INPUT” data that must be carefully studied. Topography and

geomorphology, geology and hydrogeology, meteorology and climate, hydrology and hydraulics are

among those. “Raw” data must be collected, analyzed or investigated, tested and processed in other ways

to obtain suitable and reliable data for further activities. Extent of data collection and processing usually

depends upon current stage of the project. These would be discussed more into detail in the lecture notes

and course on Dam Engineering, but here just a brief list is included as a reminder. Most of those data

depend on the purpose of the structure, whether it is part of more complex structure (Dam or the like) or

“stand alone” structure, and they also depend on stage of the project. In a word: quantity and quality of

data depends on the aim of the present project stage, but they can also be limited by physical availability.

2.1 Topographic Surveys

No engineering work can be done without topographic maps. Most of the countries in the world have

ready-made maps for all or most of the area up to certain scale (usually 1:25000 and, for areas of

higher interest, even better maps). Those, if existent, can be used for preliminary studies. However, more

detailed maps are required for each particular project, and those are to be done on purpose, covering the

areas determined by a project team. They are required to present the landscape as accurately as needed

(and possible), so that future structures could be projected in “real world” terrain configuration.

Methods

Aerial surveying (used both for preparation of maps and for different analysis of the area such as:

geological, geo-morphological, topographical, etc.)

Ground surveying (scale maps, ground profiles – sections…)

Different scale maps are used in the course of different phases. They depend upon the phase (of planning,

design or construction), and sometimes upon the importance of the structure. In some cases there may be

limitations in time or in site accessibility (related to technological availability of sophisticated – laser

beam based – surveying instruments in “inaccessible” gorges). Generally, the following are the minimum

requirements for scale maps:

Masterplan 1:100000, 1:50000, 1:25000, 1:20000, 1:10000 (for presentation purposes

even 1:250000 or more can be used)

Pre-feasibility 1:10000, 1:5000, 1:2500, 1:2000, 1:1000

Feasibility 1:2500, 1:2000, 1:1000, 1:500

Final (detailed design) 1:500, 1:250, 1:200, 1:100, 1:50

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2.2 Geology

Apart from terrain shape, its geological and geomechanical characteristics should be known and

described in order to determine appropriate foundations and to study available natural construction

materials. In addition, relation of water and geological formations must be studied as well as possible

seismo-tectonic activities.

General – regional geological conditions (both plan view – maps, and elevation – profiles to be

presented)

Engineering Geology

Foundation considerations

Rock foundations

Soil foundations

Non-uniform foundations (combination of those above, gypsum, organic materials…)

Construction materials

Availability

Quantities

Quality (types – gradation and mineral content, properties and characteristics – shear

strength, permeability, workability, compressibility, penetration resistance).

Suitability for:

Exploration and Investigation methods (both for foundation and construction materials)

Surface explorations

Geophysical (surface) explorations

Subsurface explorations

Sampling methods

Logging Explorations

Field and Laboratory Tests

2.3 Hydrogeology

Ground water levels (piezometric stages for different seasons)

Ground water - surface water relationships (different seasons)

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Permeability (porosity, fissures, cracks, joints, faults, caves) and groutability

Ground water seepage paths and connections

Mechanical and chemical actions of water on geological formation

Springs, sink-holes, underground reservoirs

Inter-relation of different hydrogeological formations (barriers, conductors, anticlines, synclines…)

2.4 Seismology

In areas with higher seismological risk, special design and construction techniques must be applied in

order to meet required safety. For preliminary studies regional data, if any, can be used. However, for

feasibility study and onwards much more detailed seismic studies must be carried out to provide reliable

data for design.

2.5 Meteorology and Climatology

General type of climate in the area

Temperatures

Precipitation

Rainfall

Snow

Humidity

Solar radiation

Wind distribution and magnitude

2.6 Hydrological Aspects and Related Hydraulic Aspects

2.6.1 River Discharge Series (Flow Series)

Basic data – Streamflow records at various locations along the river (preferably at section of

interest). Area correlations

Record of precipitations (snowmelt) at different locations of basin.

Completing discharge series

7

Rainfall-runoff models; transformation of precipitation in run-off + routing (SSAR, HEC)

Snowmelt – runoff models; transformation of (Snow + ice) in run-off + routing (DAD, etc.)

Basin transposition techniques; correlations between adjacent basins.

Regression models; statistical correlation

Stochastic models, stochastic hydrology

2.6.2 Floods

Determination of spillway capacity and river diversion capacity

Risks

Let: TR - period of return of flood considered (years)

N - lifetime of structure (years)

dam – N = 50, 100, or larger

diversion – N = 1, 2, 3, years

R - risk = probability of exceeding a flood having a return period TR

R = 1 - (1 - 1/ TR)N

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TR N

10 20 50 100 1000 10000

1 10.0 5.0 2.0

2 19.0 9.7 4.0 Not usual

5 >> 22.7 9.6

10 >> >> 18.3 9.6 1.0 0.1

50 >> 4.9 0.5

100 Not usual >> 9.5 1.0

200 >> 18.1 2.0

(R in % )

Determination of design floods

Envelope curves for river basins – Myers, Creager, Crippen Qmax = CAn

Observation of floods – flood hydrographs

Statistical distribution – Gummbel, Log. Pearson, other distributions

Empirical methods based on runoff, precipitations, and basin characteristics.

Unit hydrographs techniques

Storm patterns, PMP/PMF techniques.

Usual design procedure – application of several methods. More and more widespread use of PMP/PMF

approach; checked against statistical methods.

PMP/PMF methodology

Divide drainage basin in meteorologically homogeneous sub-basins. Study applicable maximum moisture

content of atmosphere Evaluate worst antecedent condition (soil, moisture, base flow, etc..) Define

most possible storm pattern Route storm in each sub-basin with probable max. precipitation Route

through main channel system Analyze response sensitivity to different data and parameters Compare

with statistical methods.

Important factors to be taken into account

Antecedent conditions – moisture of ground, previous precipitations, base flow

Sources of runoff; rainfall, snowmelt

Intensity, duration, geographic distribution of rainfall

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Storm patterns, hydro-meteorological condition

Routing through channel system.

2.6.3 Routing Of Hydrographs.

Basic equations

Cross section Profile

Energy equation

f 0

2

S = S -y

x-

x

v

2g-

1

g

v

t

Continuity equation

Av

x+ vB

y

x+ B

y

t= q

Methods

Full hydraulic method – complete equations

Diffusion method

f 0S = S -y

x

Kinematic Wave

Sf = So

10

Storage routing

I(t)

O(t)

I - inflow O - outflow S - storage

O = f1(S) or S = f2(O)

Muskingum

O (t1) = O1I (t1) = I1

O (t2) = O2I (t2) = I2

2 1 1 1 1 2 2 1O = O + C ( I - O )+ C ( I - I )

12 1

2 1C =

2( t - t )

2K( - X)+ ( t - t )1

2

2

1C =

2( t - t ) KX

2K( - X)+ ( t - t )2 1

2 1

K - travel time parameter

X - storage in reach parameter

Averaging and lagging – empirical

11

S(t)

12

3 Spillways

One of the major aspects of dam safety during the operation of the dam is safe release (evacuation) of

excess water (mainly floods) from the reservoir behind the dam itself. The structures that are specially

designed and built to meet this goal are called spillways. Here they are referred to as complete set of

structures needed to convey the excess water from head water to tail water in safest possible way, having

in mind economical and other aspects such as optimized fitting in general lay-out of the dam with its

other auxiliary structures.

3.1 Concept – Hydraulic structure designed to release water in excess from a

reservoir to a river stretch downstream of a dam

3.2 Component Works and Classification

Classification according to use

Service spillways – frequent use, no damages

Auxiliary spillways – infrequent use, some damages

Emergency spillways – reserve protection, damages

Spillway capacity: (neglecting approaching velocity; H is spillway head, b is net

width or can be considered as length of the spillway crest, Cn is spillway coefficient), or:

According to shape:

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Sharp-crested NOTCH weir (C1=0.62; C2=0.413; C3=1.83)

Broad Crested (C1=1/3 for abstract case (for optimal practical shape it is about 94% of this, and it

can be taken down to 83% of that in worst case); C2=0.385; C3=1.707)

Practical profile OGEE spillway (C1=0.745 for design head; C2=0.497; C3=2.201). Here the head

measured from the crest is less compared to one from the corresponding notch! Shape (Creager), for

instance, can be expressed as:

For Hmax=1.65Hdesign cavitation occurs and actual head should never exceed this value. For this case C1=0.81.

According to flow conditions:

Overfall spillway from a reservoir (Ogee, Morning Glory…)

Control weir (flow measurements, water level maintenance, other regulating functions)

Side channel spillway (spilling from a water body into a side channel spatially varied flow in

channel)

Side weir (Spilling from channel laterally into another channel or basin spatially varied flow in

main channel and on the spillway crest)

Most of them can have free flow or submerged flow, affected from downstream by tailwater conditions.

Control structure – component of spillway providing partial or complete control of discharges – gated or

ungated control structures

Conveyance structure – conduction of flow

Terminal structures – structure at end of spillway providing adequate back flow of discharges to

downstream river channel

3.3 Spillway Types

Control

(regulation)

Control (inlet) Conveyance Terminal

A B C D

1 SLUICE GATE OVERFALL (ogee, notch, and sill…)

FREE FALL STILLING BASIN

2 RADIAL GATE COLLECTING CHANNEL CASCADE SKI JUMP3 FLAP GATE SHAFT SPILLWAY SPILLWAY CHUTE WATER CUSHION4 FUSE PLUG SIPHON FREE SURFACE5 UNREGULATED CULVERT PRESSURE TUNNEL

14

15

Table (and sketch) above represents only major representatives of each group and it allows to make 375

combinations out of which 190 are possible and “only” ca. 65 MEANINGFUL. Considering, say, different

types of stilling basins as separate groups, then D1 could be split into more groups allowing for more

combinations.

Some examples follow:

Control structures

Straight, curved – B1

Side channel, double side channel – B2

Morning glory (shaft spillway) – B3

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Drop

Labyrinth crest, orifice – B1, B5

Siphon, Stepped spillway – B4

Conveyance structure

Chute – C3

Conduit

Tunnel – C4, C5

Free fall – C1

Terminal structures (Energy dissipaters):

Hydraulic jump stilling basin – D1

Roller bucket, stilling basin – D1

Flip bucket, deflector bucket – D2

Plunge pool – D3

Combination flip bucket + jump – D2

Direct discharge

3.4 Data for Spillway Design

Topography – influence on type, layout, downstream inundation

Geological conditions – foundations, rock mass downstream

Hydrological data – floods, discharge series

Hydraulic data – flow conditions upstream, downstream

Project requirements, special requirements

Reservoir flood detention capacity

Downstream developments

Other data – structural, water quality, environment

3.5 Detailed Hydrologic Data

Stream flow records – discharges, volumes, peaks.

Flood studies

Floodplain inundation maps

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Tail-water curves. Backwater effects. Morphological studies

3.6 Detailed Hydraulic Data – Support of Hydraulic Design

Reservoir inflow, storage, sedimentation, trash load, ice problems, operation, water quality

Downstream requirements, releases, flow profiles

Upstream backwater

3.7 Selection Criteria and Procedure

Safety:

High operation reliability

Structural safety

Control of releases – dam safety

Adequate evaluation of downstream hazard

Adequate design flood

Function:

Adequate release capacity

Compatibility with type of dam

Satisfy project requirements

Compatibility with site topography and geology

Economic considerations

Frequency and magnitude of releases

Selection procedure

a. Determine outflow and surcharge (elevation of storage level) to accommodate design flood

b. Select alternatives

c. Combine components

d. Compare alternatives – technical, costs

e. Select best alternative

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3.8 Design Methodology

a. Allocate reservoir volume for sediment deposits, live storage, surcharge and freeboard

b. Define spillway crest elevation (trial and error)

c. Select design flood

d. Flood routing through reservoir, for different spillway alternative dimensions and types

e. Layouts – costs – cost comparison

3.9 Hydraulic Problems (see hydraulic design criteria)

Discharge capacity

Geometry of crest

Geometry of gates

Energy dissipation

Hydraulic pressures

Cavitation (see sketch on next page)

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Cavitation index:

=

p / - p / + h +hvgr

v / 2g

a v

2

2

cos

Where:

pa = atmospheric pressure

pv = vapor pressure

= specific weight of water

g = acceleration of gravity

r = radius of curvature

Cavitation criterion: > cr

cr = critical value for incipient cavitation

For fairly smooth surfaces cr = 0.25

aeration

If cr cavitation may occur. A good control of cavitation is aeration of flow

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Buckingham theorem:

F (qa /q, Fr, Eu, Re, We, tg, tg, t/h) = 0

/geometry \

qa- specific air discharge (m3/s/m)

q = vh - specific water discharge

Fr = v/gh - Froude number – Inertia/Gravity

Eu = v/p/ - Ëuler number – Inertia/Pressure difference

We = v/L - Weber number – Inertia/Surface tension force

= surface tension

Re = vh/ - Reynolds number – Inertia/Viscous forces

See numerical values pp. 630 - 632 Advanced Dam Engineering

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3.10 Control Gates

Plane:

Slide - low pressure

Roller - medium pressure - high pressure

Caterpillar - very high pressure (outlets)

Radial:

Tainter

Sector

Flap gates

Operation mechanisms

Cable

Chains

Pressurized hydraulic hoist (oil driven piston)

Rough estimation of weight for different type of gates can be done using correlation-derived formulae

compiled by Davis. Values can be used in very preliminary phases of projects and can be considered as

slightly conservative, but nonetheless useful in first assessment of the cost estimate.

Radial (tainter) gates:

Gives the weight of moving part of the gate, while weight of embedded parts like anchorage, sills and steel

plates can be taken as 35% of this (actually varying from 10% to 50% for small and large gates respectively).

Weight of fixed-type hoist can be roughly estimated as W (kg)=300Capacity (tons), where capacity may

vary from 75% to 150% of the gate leaf weight. For traveling type hoists W (kg)=167Capacity (t) 1.33.

Vertical-lift (sliding) gates:

Gives the weight of moving part of the gate, while weight of embedded parts like anchorage, sills and steel

plates can be taken as 35% of this (actually varying from 10% to 50% for small and large gates respectively).

Weight of two-drum hoist can be roughly estimated as W (kg)=225Capacity (tons), where capacity may

vary from 120% (for fixed-wheel gates) to 150% (for sliding gates) of the gate leaf weight. Capacity exceeds

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the weight of the gate leaf by 10-20 % for lifting beam and the rest is difference due to friction. For single

drum hoists W (kg)=68Capacity (t).

Rolling gates and Hoists:

Gives the weight of moving part of the gate, while weight of embedded parts like anchorage, sills and steel

plates can be taken as 20% of cylinder weight. In average loading condition (depending on submergence of

the gate) the weight of the fixed-hoist unit with lifting chains can be taken as 30% of the cylinder weight.

Drum gates:

Gives the weight of the gate including moving and embedded parts, operating mechanisms and piping.

Travelling Gantry Crane

(Given in Anglo-American system of units)

Enclosed TVA type: W = 59.5f (W) 0.74 (in tons)

Open utility type W = 28.9f (W) 0.456 (in tons)

Where: ;

C = maximum hoist pull, tons;

S = span runway rails, ft;

A and B = respective lengths of upstream and downstream legs of crane (ft), measured from runway rail to

hoist platform or trolley rails.

For more details on this matter consult “Handbook of Applied Hydraulics” by Davis, fourth edition 1993,

McGraw-Hill, New York.

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4 Outlet Works

As the spillways convey excess water from the reservoir in order to maintain safety of the dam, outlet

works convey required water to fulfill demand(s) downstream such as water supply, irrigation,

hydropower, etc… In other words, outlet works are “responsible for safe delivery of the project’s

product” which is water that should meet demand(s) in terms of both quantity and quality. In many cases

outlets are used for water evacuation, like during flushing operations or reservoir emptying, or can

contribute to increase evacuation capacity during floods.

4.1 Concept – hydraulic structures used to convey water from a reservoir to a

point downstream of a dam.

(Outlet works – embankment dams, sluices – concrete dams)

4.2 Classification: Components

According to function

Irrigation

Municipal (potable), industrial water

Flood control

Power generation

River flows

Additional spillway capacity

Diversion during construction

Emergency drawdown – emptying time of

the reservoir for given constant inflow:

A = reservoir area A(H)

a = Control section outlet area

Ha = steady reservoir level for given inflow

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c = discharge coefficient (can be assumed constant)

Here

Where f is friction head loss coefficient (can be obtained from Moody diagram or assuming highly

developed turbulent flow for hydraulically rough pipes quadratic region and equating Darcy-Weisbach

and Manning’s formulae for given n ). For details on this consult lecture-notes “Basic

Hydraulics or chapter 7 of these lecture-notes.

is sum of local head loss coefficients (such as trash rack, intake, bends, contractions, expansions,

branching, etc. – including exit loss coefficient which is equal 1.0 if outflow is to still or slow-flowing water

or air).

Combination of functions

Type of flow

Pressure flow

Free surface (gravity) flow

Combination

Components – all or some of the following:

Inlet Channel

1Intake or

Intake Structure

2Conduit

Waterway Tunnel

3Gate Chamber

or Downstream Gate

Structure

4

Chute

5

Energy Dissipator

6

Outlet Channel

7

Conveyance - (1), (3), (5), (7)

Control - (2), (4)

Energy dissipation - (6)

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4.3 Inlet and Outlet Channels

Consideration of following important points

Clogging (sliding or deposition of rock masses or Plugging sediment)

Adequate location

Stabilization of slopes

Adequate operation

Sediment transport (erosion, abrasion)

Channel lining

Channel stabilization

Traps

4.4 Intakes

Important points

Location with respect to water levels

Control or not (gates)

Special functions (for instance, selective withdrawal – multiple level intakes for water supply)

Provision of trash-racks (in most cases)

Shape of hydraulic passages

4.5 Control Structure

Important points:

Location of structure (intake, mid-structure, downstream)

Type of gates, valves

Plane gates:

Slide

Roller

Variations

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Radial gates:

Tainter

Top-seal radial

Valves:

Needle valves, tube valves (Can operate submerged, expensive, unstable for small openings cavitation)

Hollow jet, C=0.7 (not suitable for submerged outflow)

Butterfly

Howell-Bunger (Cone) up to 250 m of head, A=0.8Apipe, C=0.85-0.9; better dissipation with

ring (fixed large hollow cylinder) placed downstream of the cone ring jet valve C=0.75-

0.80

Gate valves

Spherical valves

Operational safety – redundancy operation gate, revision gate, operation, maintenance

Planning all operations with gates – assembly, erection, disassembly, removal

Structure:

Intake structure

Gate shaft or tower

Gate chamber

4.6 Conveyance Structure

Important points:

Cavitation (due to high velocities), aeration

Shape of transitions, slots

Lining

4.7 Terminal Structures

Important points:

Energy dissipation

28

Stilling basin (special case impact structure)

Dispersion of jet

Plunge pool, flip bucket

No energy dissipation

Shapes of hydraulic passages

4.8 Hydraulic Problems and Their Prevention

Cavitation:

Improvement of shape of water passages

Increase of pressure in affected areas

Aeration

Abrasion:

Special lining (concrete, steel)

Particular problem in stilling basins

Scouring:

Lining

Rockfill protection

Structural vibration:

Influence on supports of elements

Elastic properties

Masses (Ex. Trash-racks)

Vortices:

Design modifications of intakes

Anti-vortex devices

Other problems

Back current

Hydrodynamic loads

29

Uplift

30

5 Energy Dissipaters

Energy dissipation process can be

considered in 5 separate stages:

1. On the spillway (outlet) surface

2. In the free falling jet (if any)

3. At impact into the downstream

pool

4. In the stilling basin (bucket,

pool)

5. At the outflow into the river.

5.1 Energy Dissipation on Spillways

The energy loss on the spillway surface:

e = v’2/2g where v’ is velocity at the end of the spillway, is

the Coriolis coefficient, and is the head loss coefficient related to

the velocity coefficient (ratio of actual to theoretical velocity) as:

Relative head loss:

After Novak and Čabelka (1981) for S/H<30 and smooth spillways:

1 1 0.0155S/H

The value of can be increased ( decreased) by using rough

spillway surface (e.g., stepped spillway, or baffles). Aeration

should be provided to prevent cavitation damages.

5.2 Ski-jump and Flip Bucket

Ski-jump can be used at the end of chute or tunnel spillway. Most of energy dissipation is achieved along

13 (spillway surface, jet, impact) and if jet is conveyed far enough in geologically suitable condition,

“stilling basin” (usually plunge pool for ski-jump spillways) can be avoided by letting the jet to do pool

“excavation” (by erosion) as needed. Ski-jump is used in 1951 for the first time, and its use is growing

ever since.

Head loss in the jet up to 12%.

Can be enhanced by more jets colliding

31

Impact (phase 3) provides main benefit in energy dissipation

Intensively aerated jet before impact increases efficiency

Optimal dissipation in the jet itself is obtained for S’/S 0.6

Best results are for “disintegrated” jet, which occurs at distance L 6q1/3 from the crest.

Theoretical throw distance of the jet:

Where

H0 = S + H S’ y/2 energy to the middle of the off-taking jet.

is take-off angle

And y = depth at the off-take of the ski-jump

can be assumed approximately as 1.0

Flip bucket is special version of the ski-jump usually placed at river bottom. Main parameters are R

(radius) and (take-off) angle.

At low flows bucket acts as stilling basin downstream protection against erosion is necessary

Proper operation for high flows with a jet

For v < 20 m/s air resistance can be neglected

For v = 40 m/s throw distance reduction up to 30%

Theoretical throw distance L = (v2/g) sin 2

Major concern is to throw the jet as far as possible from the structure.

Protection against retrogressive erosion

3-D forms of flip bucket to skew jet into desired direction.

Tailor made hydraulic scale models

5.3 Stilling basins

Hydraulic jump stilling basins

Type I IV Stilling basins with chute blocks and baffles (USBR)

Plain or slotted roller buckets (USBR)

32

Spatial hydraulic jump basins (change in width, change in depth, flow from lateral channels)

Hydraulic jump stilling basin:

Depth of water entering stilling basin y1 can be obtained from:

and then second conjugate depth for rectangular basin:

Depth of the stilling basin respecting need for certain “submergence” as safety measure:

D = y2 y0

y0 is normal depth in the river downstream of the stilling basin usually obtained for computation from tail

water flow-rating curve.

is submergence coefficient and should be grater than 1.10, i.e., downstream conjugate depth should be

more than 10% submerged. This is stilling the jump surface and preventing cavitation on the apron slab.

As with computation of required depth, available energy for computation of y1 changes (increases)

computation should be iterated until all values fit.

Length of the stilling basin can be adopted as: L = K (y2 y1), where 4.5 < K < 5.5 for 10 > Fr1 3

respectively.

Above formulae are valid for rectangular basin with horizontal bed. At the end of basin simple end sill

can be provided with slope of 1:3, where the basin length includes this sloped sill. Basin has to be safe

for whole range of discharges (not only the high design flow).

Better efficiency for higher Froude Number.

Efficiency for low Froude number can be as low as 50%

Fluctuation of pressure in the basin (cavitation, forces on apron slab)

Structural concerns:

Uplift drainage, anchorage, weight

Abrasion

Vibration

Cavitation

33

Design flow for stilling basin computation need not necessarily be equal to that of the spillway (and/or

outlet). Lower flows can be considered for economical reason, allowing some damage of the basin itself

and just downstream in very exceptional cases. However, spillway structure (or dam) should be designed

for higher flows.

Above there are two examples of USBR stilling basins. Although those types allow shallower and

somewhat shorter basins therefore saving in terms of excavation and sometimes concrete as well, they

have some serious disadvantages compared with simple hydraulic jump stilling basins. Construction of

baffle teeth-blocks requires “filigree” work in terms of reinforcement and formworks. In operation,

however, these types of stilling basins have proven to be vulnerable to devastating cavitation effects

partly induced by teeth themselves. Extensive repair works might be required usually involving use of

expensive epoxy-materials.

5.4 Downstream Erosion

After stilling basin

No stilling basin can dissipate 100% of the incoming energy. Erosion downstream of stilling basins or

flip-buckets and ski-jumps is to be expected.

Control of the position and magnitude of erosion

Rip-rap

Concrete aprons

Expected erosion (scour depth) can be roughly estimated using Novak’s expression:

Where

H* is difference between upstream and downstream levels

34

Y0 is tailwater depth

q is specific discharge per meter width

d90 is 90% grain size of sediment in the river bed

Required length of downstream riprap (or apron) bed protection for low head structures after US War

Department:

, Ho = H+S (available energy see figure at the beginning of the chapter), yd =

tail water depth, and v is tail water velocity. (This formula gives rather high values)

After ski jump

Scour of (in the) plunge pools can be expressed in general by equation of Locher & Hsu:

C = coefficient 0.65<C<4.7

x, y, w, z = exponents

0.5 < x < 067; 0.1 < y < 0.5; 0 < z < 0.3; 0 < w < 0.1

Wild variation of coefficients could be simplified like in case of Martins formula:

for ski-jump

Neglecting the impact angle and elevation of the take-off, as well as composition of the riverbed can be

criticized. However, for most cases, according to experiments, major influence on scour hole is by unit

discharge, and then by total available head (which also represents jet’s velocity). In Russian practice

(Zamarin’s formula) more emphasis is given to position of the out-coming jet, angle of impact, jet’s

velocity and allowable velocity (the one that will not cause any scour which might be difficult to

determine in practice). Still major influence is by q in this formula as well.

35

After flip bucket

For flip buckets simplification by Tarjamovich’s expression could be considered:

1 = upstream angle of the scour hole as a function of flip bucket exit angle

For 100 < <400 140 <1 <240

One has to be aware that all these formulae can just give an idea about possible location and order of

magnitude of the scour hole, so that necessary precaution measures can be foreseen. They might be useful

in comparison of different alternatives showing differences taking into account equal assumptions.

5.5 Dissipation at Bottom Outlets

Size and position of the outlet (above or below tail water level), importance of the structure and

downstream conditions can influence type of energy dissipation for bottom outlets.

Aeration and dispersion of the jet above tail water, by means of gates or valves (e.g., hollow jet).

Reduction of specific discharge as it enters the stilling basin (gradual expansion, and/or deflectors).

Sudden expansion energy dissipaters possible cavitation effects that should be drifted away from

the boundaries of the structure.

By

direct impact of the jet against the wall or in the vertical stilling wells (for small-capacity outlets).

36

This figure shows comparison of the model and actual situation

Example of Spillway, Outlet Works and Stilling Basins (at Friant dam, USA):

37

Example of Spillway, Outlet Works and Stilling Basins (at Friant dam, USA)

38

6 Navigation Locks

6.1 Concept

Navigation locks are hydraulic structures that are provided

to allow navigable connection between two water bodies

having different water levels. In this way concentrated

heads on canalized rivers and canals are usually overcome.

They normally appear in association with dams or similar

structures in natural streams or man-made channels.

6.2 Types and Classification

According to the type of structure, height of lift and capacity, locks may be classified as follows:

Inclined planes

Ship elevators

Chamber locks for small to large barges of convoys

Small lift - up to approximately 20 m

Medium to large lift - from 20 to 35 m

In recent navigation practice, associated with current transportation requirements, most of the navigation

locks are of chamber type. Navigation locks of this type make it possible for ships to move from one part

(level) to the other by the operation of movable elements (gates, valves). These structures usually include

heads (at the ends) equipped with gates, and chamber(s) that can contain ships to be locked through.

There are filling (emptying) systems as well equipped by valves. Water level in the chamber is increased

or decreased to match upstream or downstream levels. Usual dimensions of chamber are in following

range:

Length - from 20 to 200 m

Width - up to 35 m

Lift - from a few meters to 35

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6.3 Lock Cycle

In a chamber lock, a typical operation to transpose a vessel(s) from low level to high level consists of the

following steps:

With the chamber empty, opening of downstream gate

Entry of vessel in the chamber, mooring (securing) and closure of downstream gate

Filling of the lock chamber

Opening of upstream gate and exit of the vessel

This is followed by descending operation (from upper level to lower level).

A complete cycle usually requires from 20 min to 2 hrs, depending on the chamber dimensions and lift

height.

The lock operation uses water! In each cycle the equivalent of the chamber useful volume is conveyed

towards downstream.

Figure 1The lower gates are closed; the drain valve is closed; the filling valve is open allowing the lock chamber to fill to the upper level; and the upper gates have been opened allowing the towboat to enter the lock chambers.

Figure 2Now the towboat is in the lock chamber; the upper gates and the filling valve are closed; and the drain valve is open allowing water to drain out into the lower level. The towboat is lowered as the water level lowers.

Figure 3When the water level reaches the lower level, the lower gates are opened allowing the towboat to leave the lock chamber and proceed on down the river to the next lock and dam where it will go through the same procedure.

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6.4 Relevant Hydraulic Aspects

From a general point of view, the main objective of an appropriate planning, design, construction and

operation of a navigation lock is to achieve an economic, safe and operationally reliable solution.

Filling time:

For instantaneous complete opening:

A is lock area in plan; a is filling system area (valve); c is outlet coefficient (function of time, but could

be taken as constant; h is head (difference in water levels); H is total lock’s head.

For gradual linear opening in time T1 (a=atT1/t)

And total filling time:

In this case Qmax. occurs at ht=4/9H if hT1<4/9H, or else it is at hT1.

Equalizing water levels between two locks of areas A1 and A2:

And for A1=A2

If the opening is gradual but not linear, step method has to be used:

41

for adopted t hi can be computed and from there discharge as well

Other types of overcoming head difference for navigation purposes:

Thrift locks (saving water, but expensive and slower, heads up to 30 m)

High head elevators (up to 100 m, horizontal water filled troughs)

Lifts (up to 100 m in length, low water usage, high travelling speed – relatively high capacity)

Inclined planes (as above but trough is mounted on special leveling undercarriage which travels

along or normal to trough’s axis)

Usually boat is settled at the bottom of the trough by releasing some water prior to lifting operation. Ac-

celeration and deceleration during operation must be kept within acceptable limits. Lifts (vertical or in-

clined) are more prone to damages and are more sensitive in operation than standard locks. They are usu-

ally more expensive to build and maintain. Their capacity per lifting operation is much lower. Neverthe-

less, if high head is to be overcome, alternative between single lift and multi-step locks should be com-

pared, and the former might have advantages (especially if the space is limited).

Example of part of the Navigation notice NO. 1-1997 (February 1997), from Ohio River

Division, North Central Division, Lower Mississippi Valley Division, regarding safety of navigation:

“SAFETY

1. Commercial and recreational craft shall use the locks at all times except for navigable pass dams, and authorized fixed weir passages. 2. Vessels shall not pass under gates in the dam when they are out of the water and the river is flowing freely through the gate opening. 3. Lockage of leaking or listing vessels may be refused. Leaking or listing vessels shall be moored in a location outside of the channel so as not to interfere with passing navigation. 4. All craft and tows approaching a lock, within a distance of 200 feet of the upper or lower lock gate, shall proceed at a speed not greater than two miles per hour (rate of a slow walk). 5. All tows entering the lock shall be properly aligned with the guide or lock wall. Tows may be required to stop prior to entering certain locks at which unusual conditions exist. 6. When an amber flashing light is displayed and approval is given by lock personnel, a descending or ascending vessel may approach and moor with a backing line to the guide wall; however, the head of the tow shall be no closer than 100 feet from the near end of the lock gate recess. 7. Burning fenders shall be dropped overboard immediately rather than being placed on the deck of a barge or towboat. Fenders shall not be secured to cleats or timberhead and left

42

unattended. 8. When tows are underway in the lock approaches or lock chamber and there is a potential for damage to the structure a minimum of two deckhands with fenders shall be stationed at the head end of every tow 100 feet or greater in width. One deckhand with a fender shall be required at the head end of tows less than 100 feet in width. Additional personnel shall be required at the aft end if the lock operator determines that it is necessary to protect the lock and guide walls from damage. 9. It is the responsibility of the vessel operator to provide adequate mooring lines. The lock operator may require mooring lines to be replaced with satisfactory lines before lockage is made if the lines appear to be of such quality, size, or condition that would make safe lockage questionable. 10. Mates and deckhands, when preparing to moor within the lock chambers, shall not throw heavy mooring lines onto the walls, but shall wait for a heaving line. 11. All towboat crews, while locking or moving a tow into or out of a lock chamber, must station themselves to preclude the possibility of being injured by the parting of a cable or line under strain. Single lines only will be used to check a moving tow. During inclement weather conditions (snow and ice) the working area of the tow where lines are used shall be free of snow and ice to prevent injury to towing industry personnel. Working lines shall be kept dry and in working condition (not frozen) to allow lines to be worked properly and to prevent injury to personnel. 12. Towboat crew members shall not jump between moving tows and lock or guide walls while preparing for lockage, locking, or departing lock. Use of lockwall ladder ways is permitted only after tows are securely moored and the chamber is at upper pool. 13. Tabulated below are the minimum number of vessel personnel required for handling lines during lockages. The captain/pilot can not act as a deckhand.

TYPE OF VESSEL MINIMUM MINIMUM MINIMUM OR TOW NUMBER OF NUMBER OF NUMBER OF PERSONNEL LINES USED EMERGENCY USE LINES

Towboats with up to 1 1 1 one barge length and all other vessels less than 65 feet All other vessels requiring 2 2 1 single lockage Tows requiring double 3 2 1 lockage (one deckhand to remain with first cut) Set-over tows 3 2 1 Knock-out tows 2 2 1

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14. All vessels, when in the locks, shall be moored and/or moved as directed by the lock operator. 15. Commercial towing companies shall ensure that vessel operators and boat crew members have received orientation and training in all aspects of deck work and lockage procedures to ensure the safety of personnel, floating plant, and structures. 16. All cylinders or containers holding gases or liquids under pressure or any other chemical or substance shall be securely fastened to the hull of the vessel to prevent their rolling overboard into the lock chamber. 17. All containers holding paint, gasoline, or other volatile materials shall be securely fastened with tight fitting covers.”

Other Instructions and data (such as operational aspects, etc.) are usually given in this kind of notices.

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7 Pumping Stations and Pipeline Conveyance

7.1 Pumping Stations

7.1.1 Usage and Classification

Most large pumping stations pump water from open surface sources (rivers, lakes, canals, and basins,

i.e., sumps).

Groundwater abstraction by smaller units (submerged pumps)

Usage:

Dewatering (drainage) behind a dike, or cofferdam

Lowering a water table (or groundwater table)

Pumping sewage or storm water (or sewer) flow to treatment plants

In water supply networks to supply to higher elevations, or (booster pumps) to boost pressure

heads

In Pump-Storage Hydropower schemes reversible pump-turbine units are used. In the past

separate units, for pumping and generation, were more common.

Abstraction from boreholes (or wells)

Different uses and purposes usually require different pumps. Common types of pumps are:

Type Discharge Head Application

Cen

trif

ugal

Rotodynamic pumpRadial-flow type

Low High>30m To pump water or sewage; Pumping

clean water with higher efficiency; Sewage pumps are usually with slow speed.

Axial flow type High Low<15m

Mixed-flow type Medium Medium2330m

Reciprocating pump Low Medium Viscous fluid pumping; Borehole pumping; leakage.

Air-lift pump Low Low Inefficient but used for GW recovery from skewed wells, sands and silt.

Table continues…

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… CONTINUED

Type Discharge Head Application

Jet pump Low Medium Combined with centrifugal pump, borehole abstraction inefficient

Screw pump High Low Archimedes’ screw principle; Low speed; Mud or liquids with silt.

Helical rotor pump Low Low Helical rotor and stator elements; for sewage or liquids with suspended matter.

7.1.2 Pump Parameters

Most common pump type is centrifugal rotodynamic. Most important parameter that characterizes this

sort of pumps is specific speed of rotation:

Where Q is discharge (l/s), H is manometric head (m) and N is the rotational speed (rpm).

Manometric head is gross head that includes difference in elevation of water levels in the sump and upper

basin plus head losses in suction part of the conveyance (from sump to the pump) and in distribution part

of the conveyance (from the pump on). Pump has to develop even higher head to overcome the impeller

(in)efficiency p. H = Hm / p

Required power to operate the pump would be:

[in kW]

For clean water and Hm (m), Q(m3/s), and =pmt<1.0 (overall efficiency including pump, motor

and transformer if needed).

Mostly pumps operate under varying conditions of discharge and head. Then:

Q2 = Q1(N2/N1), and H2 = H1(N2/N1)2

46

Pumps can be operated in “parallel” or in “series”. Later must be operated simultaneously. Pump with

impellers in series is called multistage or booster pump.

Pressure at the pump impeller inlet (ps) is usually below atmospheric pressure (pa). From Bernoulli’s

equation between the sump and this section:

ps/g = pa/g (hs + Hs + vs2/2g)

Here Hs is position of the pump (above sump water level), Hs is head loss in suction pipe, and vs is flow

velocity there.

If ps < pv (vapor pressure) cavitation occurs. This can be dealt with by increasing intake and pipe

dimension (decreasing losses and velocity) and by limiting suction head (pump position):

Net positive suction head (NPSH):

Thoma’s cavitation number is: = NPSH / Hm

NPSH values are supplied by pump manufacturers. Nss (suction specific speed) is:

Nss = NQ1/2/(NPSH)3/4; = (Ns/Nss)4/3; 4700<Nss<6700 for most centrifugal pumps.

Critical cavitation number (from model tests):

c = 0.103 (Ns / 1000)4/3

Most pumps, placed above water level, need priming (air must be expelled prior to pumping).

47

7.1.3 Design and Selection of Pump, Sump and Mains

To optimize the pipeline diameter for given pump, or to select a pump for given pipeline diameter a

graph like this should be used. Pump characteristic and efficiency is obtained from manufacturer. System

characteristics are obtained for different pipelines by adding head losses to static head. Optimization in

economic terms is possible taking into account desired maximum flow rate and most frequent one. Note

that reasonable operating range of selected pump is relatively limited.

In case of considerable variations of levels or demands, or if the pipeline is expected to change roughness

or cross section (clogging) with time, additional care should be taken in order to select proper equipment.

In this course most important pumping installations are those for water supply. Common intakes for this

kind of installations can be:

Horizontal belmouth entry type from the water body (river, sump)

Vertical, horizontal or turned-down intakes in dry or wet wells. These can be separate for each pump

(single-pump) or combined for multiple pumps.

Good sump design must avoid formation of vortexes.

Approach velocity should be kept below 0.3 m/s, avoiding sudden or abrupt expansions and large

stagnant zones.

Pump intake should be directed towards approaching flow.

Dividing walls should separate multiple intakes in the same sump.

Vortex suppression devices could be applied (floating rafts, curtain walls, grating cages, etc)

48

Belmouth intake provides good inflow conditions and minimizes the entrance losses. Minimum

submergence of the intake should be:

Where a = 0.51.5; and b = 2.02.5 (after Knaus)

Minimum sump volume depends on pumping flow rate, number of units and frequency of start-ups of the

pups. The later must be limited since in each start-up operation electric motor generates considerable

heat.

Most pumping stations (especially sewage and storm-water) need to be provided with bar screens (trash

racks) to prevent larger objects from entering the sump. For this purpose usually steel bars spaced at ca.

30mm, and blocking about 40% of the area, are used. They are often inclined 6090o to horizontal. Head

loss of at least 0.15 m should be accounted for. To diminish turbulence they should not be placed too

close to the pump. Anti-swirling devices may be required.

7.1.4 Pressure Transients

Pumping station and the pipeline must be protected against pressure transient phenomena

(surgeswaterhammer) due to sudden opening or closure of the valves or most commonly caused by

power failure (sudden stoppage). This is going to be analyzed into detail in the course Advanced

Hydraulics II later on in the Programme.

Change of head for quick closure/opening can be expressed by:

Where a is celerity of the pressure wave:

For water =1000 kg/m3, bulk modulus K20108 N/m2, k=1011/E

For steel E201010 N/m2, k=0.5; D is pipeline diameter, e is pipe wall thickness.

For other materials: k=1 (cast iron), k=5 (concrete, lead), k=10 (wood)

Opening or closure is considered to be quick if it’s shorter than time needed for pressure wave to travel to

the upper reservoir and back (0T, =2L/a).

49

If the opening or closure takes longer than pressure change is diminished: . If along the

pipeline cross section changes, each change generates transmission and reflection pressure waves that

superimpose with original ones and affect the results. For branching or looping networks these must be

taken into account, and computation becomes rather more complicated. In pumping stations it is often

difficult to control times of opening and (especially) closure. Thus different measures can be applied to

control the drop/rise of head:

Flywheels if coupled with the pump they provide additional inertia so that pump rotates a while

after power cut occurs. Suitable for small installations.

Bypasses and pressure relief valves Bypass with non-return valve “sucks” part of the original flow

mitigating the negative effects of sudden stoppage. Pressure release valves and air inlet valves could

be provided in the pipeline as addition or alternatively.

Surge tanks and air vessels have to be placed as close to the pump(s) as possible. Therefore, often it

is not practicable to use open surge tanks (for they would require enormous heights). Rather, close air

vessels with air compressors are more commonly used. They “convert” (or limit in space) more

severe waterhammer effects to milder (and longer) surge (mass oscillation) effects.

Air vessels serve both for sudden opening and closure. A check valve should be provided between the

pump and air vessel. Predetermined extreme levels in the air vessel trigger the compressed air delivery.

Neglecting head losses, simplified solution for sudden complete closure (in terms of head change) is:

From here Vmax can be computed:

Vmax1.2Hmin=V0

1.2H0

Period of oscillation is:

Including losses in the pipeline and (entrance into/exit from) the air vessel, computation gets somewhat

more complicated and is usually solved by finite difference equation or by using design graphs for given

(or assumed) head losses. For pipelines with changing diameters equivalent length (one diameter length

that would have same head losses as original pipe) can be used in simplified computations.

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7.2 Pipelines

Pipelines are used for water or sewer conveyance usually under pressure, but also with free flow. They

can be made of various materials such as: Steel, cast iron, concrete, wood (obsolete), vitrified clay

(obsolete), asbestos cement (recently considered environmentally dangerous), plastic materials (PVC) and

other materials for special purposes (brass, copper, lead, glass, rubber, etc.).

7.2.1 Hydraulics

Basic hydraulic problems for steady flow through pipelines can be solved by 2 formulae:

Continuity (mass conservation): Aivi = Constant

Bernoulli (energy conservation):

H is the sum of head losses between sections of interest. They include linear friction losses along the

pipe and local or “minor” losses (in bends, elbows, joints, valves, contractions, expansions, etc.).

Numerous formulae are available to compute linear friction losses. Probably the most universally used is

Darcy-Weisbach formula: (in USA practice Hazen-Williams’ expression is more

commonly used)

Here f is Darcy’s friction coefficient. Different researchers have determined its value in the past. There

are various experimentally obtained expressions used to determine f.

There are different flow regimes possible in the pipes, dependent on Reynolds number:

Re = vD/or ReR = vR/ R = D/4 is Hydraulic radius of the pipe. is kinematic coefficient of

fluid’s viscosity (for water: t = 20o = 1.01x10-6m2/s, and t = 10o = 1.3x10-6m2/s)

For Re<2320 there is laminar flow regime in the pipe, and then Darcy’s coefficient is:

In turbulent flow there are three regimes:

Hydraulically smooth pipes (Re<27(D/, approximately – is absolute average pipe wall

roughness)

Among many used formulae (like Nikuradze, Prandtl-Karman, etc.) Colebrook’s formula is given

here as probably the most practical:

51

Transition turbulent flow (27[D/ < Re < 21.6 CD/, where C is Chezy coefficient = [8g/f])

Then another formula of Colebrook can be used:

For even higher values of Re f = f() and does not depend on Re itself anymore (quadratic region

of flow resistance). Then for instance Nikuradze’s formula can be used:

For all regions with limited accuracy ( 5%) for /D<0.01 and f<0.05, Moody’s formula can be used:

Material and the state of pipe (10-3m)

Concrete – rough 1-3

Concrete – smooth 0.3-0.8

Steel (welded) – new 0.04-0.1

Steel (welded) – used, stained, incrusted 0.15-1.5

Cast iron 0.25-1.5 (4)

More detailed list can be obtained from different handbooks. As most practical problems in hydraulic

(civil) engineering occur in the region of quadratic resistance (full turbulence), even manning formula

could be applied with reasonable accuracy. Then better known values for n can be used and/or converted

to f. As given before for this case:

Minor or local losses are calculated based on experience and experiments. Some coefficients to

calculate local losses are given here:

Entrance: sharp – =0.5, rounded – =0.2, bell-mouth – =0.05, pipe sticking into reservoir –

=1

Sudden expansion: in regard to in-flowing velocity. If expansion is gradual

then this coefficient would be diminished (by multiplier k<1) depending on the angle of

expansion (for 5o – k=0.13, 15o – k=0.27, 25o – k=0.62…)

52

Sudden contraction: , (based on out-flowing velocity) for gradual contraction

coefficient would depend on angle and ratio of diameters.

Elbow: where L is arc length, R is bend

radius, D is pipe diameter, and is deflection angle of the curve.

Valves and gates: if open 0.05<<0.2 (0.10-0.12) depending on the type and condition. For

different closures values could be found in handbooks, but accurate ones only from

manufacturers.

Exit (into still water body): =1.0

For gravity flow conditions in pipeline conveyance systems, Manning’s interpretation of Chezy equation

gives good results. Maximum flow capacity is where hydraulic radius is max. i.e., for profile ca 94% full.

However, if there is any miscalculation or difference in roughness (or if pipeline is not fitting into project

line or if there was abrasion or deposition…) then flow capacity calculated for max. conveyance would

be overestimated. It is more reasonable to limit maximum filling of the pipe (up)to ca 81%. At this point

conveyance of the gravity flow equals that of full cross section (100% – without pressure)

7.2.2 Loads

Internal pressures

Vertical loads due to backfill (for buried pipes)

Horizontal backfill pressures (for buried pipes)

Surcharge and concentrations (for buried pipes)

Bedding and load distribution (for buried pipes)

Support and anchor loads (for pipes supported above ground)

Temperature loads (for open-air pipelines due to tendency of shrinkage and extension)

For pipes under pressure (buried or not), especially if subjected

to waterhammer, inner pressures (loads) might be most

important. The total shell tension due to internal pressure is

approximately: F = ½pD.

Tensile stress = ½pD/e for unit length of pipe. Here e is pipe

shell (wall) thickness (or equivalent thickness of tension taking

part). Thickness could be calculated from here if instead of tensile stress, allowable tensile stress (for

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given material) is used. Usual procedure would be to compute this thickness first, taking into account

pressure transients. This procedure requires iterations since the pipe thickness affects pressure wave

celerity a which is relevant for determination of pressure rise H. If thickness (obtained in this way) is

still less than certain structural minimum, than this later value should be adopted. Structural minimum

would depend on material used and pipe diameter (for steel pipes this should be 8 mm or more for larger

pipes, including up to 2 mm provision for corrosion and abrasion losses of the mass during operation).

Such dimensions should be checked if can withstand other loads, and adjusted if necessary.

Usually temperature induced loads should be alleviated using deformable coupling elements (expansion

joints) that can accommodate resulting deformations. Due to temperature changes pipe would tend to

expand (contract) between two fixed points (anchor blocks) depending on temperature difference between

particular moment and ambient temperature during pipe placement. Temperature linear expansion

coefficient is (m/moC). For steel it is about 12x10-6. Without anchors, extension of the pipe’s length

would be: L = Ltt. If expansion is disabled by anchor blocks reacting stress would develop:

= EL/L, E is modulus of elasticity of material (for steel 20x1010 Pa). These stresses and resulting

forces can be unacceptable, and to diminish them special pipeline construction arrangements can be

introduced – either expansion joints or harp-shaped pipeline deformable parts.

For free surface or low-pressure pipes loads caused by burying, backfill and surcharge are more

important.

The plate thickness required to resist buckling under uniform external pressure is approximately:

Own weight of the pipe and water in it must be taken into account for calculation of the forces acting on

supports and anchor blocks. Friction, inertial, deflection (centrifugal) and other effects must be accounted

for. Other loading conditions are going to be discussed more in the course Hydropower development.

7.2.3 Placement considerations

Pipes can be buried or open. Both arrangements have advantages and disadvantages. Decision is to be

made based on project needs, local conditions and the like.

Buried pipes:

Advantages – Keep water temperature pretty constant and protect from freezing; Ambient

temperatures do not impose extra loads (savings on expansion joints); Once placed they do not

consume extra space; Frequent anti-corrosion painting not needed…

Disadvantages – difficult accessibility for maintenance; Backfill pressures; Expensive trench

excavation, bedding material, careful back filling; Difficult placement – limited space for work;

Painful detection of leakage and other problems.

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Open pipes:

Advantages – simple accessibility for maintenance; No backfill pressures; No expensive trench

excavation, bedding material, careful back-filling; Simplified placement – plenty of space for

work; Easy detection of leakage and other problems

Disadvantages –water temperature affected by ambient and no protection from freezing; Ambient

temperatures impose severe extra loads (expensive expansion joints); They occupy a lot of

valuable space; Frequent anti-corrosion painting needed; Anchor blocks and numerous supports.

Economic considerations

Importance of optimized layout. Several alternatives should be compared. Saving in length and

diameter/wall thickness (as well as pumping facilities)

Selection of economic conduit size (Pumping stations, HPP, etc.). In case when plenty of head is

available (no pumping needed, or no HPP foreseen/feasible) then consideration of minimum diameter

– max. allowable velocity.

Comparison or combination with other types of conveyances if applicable.

In terms of materials for the pipes in hydraulic construction (larger scale) most common are steel and

concrete. Asbestos-cement introduced after the other two, seemed to be promising due to its favorable

properties (durability, ease of placement, etc.), but lately it is suspected to be responsible for potentially

causing cancer, and is no longer considered environment-friendly material.

Steel pipes are mostly welded nowadays, though other types of joints are still used. They are relatively

expensive and require protection (and maintenance) against corrosion. Otherwise, they are comparatively

easy to handle, they can stand extreme pressure and tension stresses, easy to make fittings, joints,

branches, expansions, contractions, bends, and whatever else needed.

Concrete pipes are relatively inexpensive in terms of material. However, they are very difficult to handle

(for large diameters they become very heavy – if prefabricated for howling reasons they have to come in

short rings – too many difficult joins – leakage possibilities – in addition chance to divert from desired

route in “zigzag” fashion is high, therefore affecting roughness and length of the pipe and thus its

conveyance capability. They require precise and well-done bedding, and gentle placement of the backfill

due to their brittle nature. Still, in certain range of diameters (say 0.5-2.5 m), and for low to medium

inner pressures, they can prove to be good competitors, and often far cheaper than steel alternatives.

Pipelines in general follow the ground morphology.

If free gravity flow is required then they basically (up to reasonable extent) “follow” contour lines. In this

case length of the pipelines connecting points A and B would tend to be much longer than strait distance

between these points. To reduce the length various structures are built such as:

Aqueducts

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Inverted siphons

Drop structures (if the accumulated head is unacceptable)

If pressure flow is more suitable, then length of the pipeline can be significantly reduced, but then the

pipe is usually going to be placed uphill downhill, as terrain requires. Since the pipes should be emptied

for various reasons (inspection, operation, etc.) additional structures should be provided for filling and

emptying:

Air vents (at summit points of vertical bends, to let the air in when emptying and let it out while

refilling the pipe with water again prior to next operation)

Silt outlets (at lowest points along the line in vertical bends to let the water and silt out when pipeline

is being emptied)

All these structures must be well maintained, and taken into account in design and cost estimate.

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8 Special Structures

Many hydraulic works can have adverse environmental impacts. They can be mitigated in different ways.

Some of them are possible to mitigate by use of special environmental hydraulic structures. A few of

those structures are briefly discussed in this chapter to give the emphasis on the importance and

sometimes necessity of such structures (in particular the structures that mitigate impacts on fish and water

quality).

Technologies of preserving natural fish reproduction in inland water bodies under intense hydraulic

construction (fish species conservation):

fish passageways, including single-lane fish locks with continuous attraction of fish, as well as a

mechanical fish hoist with a fish chute;

devices for diverting fish from water intakes, including fish-protection concentrating structures with

horizontal separation of fishes, and those with vertical separation of fishes, curtains;

the development of new types of artificial spawning grounds and channels, including use of

prefabricated spawning panels with an artificial substrate.

8.1 Fish Ladders and Passages

The design criteria to select the type of fish pass, layout and other facilities depends on the size of river,

magnitude of reservoir, the type of fish, and whether they are migrating upstream or downstream which

is best established by monitoring existing structures and modeling. There are two major groups:

For upstream migrating fish: fish ladders, fish locks (lifts), tramways, facilities for trapping and

trucking the fish

Downstream migrating fish (e.g., smolts young salmon): arrangements to collect fish in the forebay

at fine mash screens (fixed or movable) and directing them to safe by-pass systems.

Fish passes should be designed so that fish can find the entrance to the system and be able to swim safely

through.

Structures:

Fish ladder upstream fish passage for heads up to 20 m. (Fish entrance, ladder proper, and fish exit)

Auxiliary water supply can be provided to attract fish. It consists of drops of ca 30 cm between pools

with slope of 1:8 to 1:15. One rest pool (of double size) is provided after every 5-6 standard pools

12 m deep, 25 m long and 210 m wide) . Entrance should be downstream parallel to the river,

while exit must be away from the spillway. Water flow velocity in the ladder should be about 0.5

m/s.

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Fish lifts (locks) Operation similar to navigation lock, except that through-flow is maintained to

guide fish in and out of chamber. Cyclic operation. In comparison with the navigation locks and

overflow dam, the fish locks have the most complex and diverse mechanical equipment. (E.g., In

Tikhovsk Hydro Development Russia out of the total mass of 2560 tons of mechanical equipment the

fish locks accounts for 1130 tons! The upstream and downstream slotted gates form a working

chamber in which the pools are isolated and the fish are examined and counted. Directly behind the

upstream slotted gate is an ichthyological platform. The fish locks in addition to their main function forceful transport of fish from the lower to the upper pool, can serve as a natural laboratory for

developing ways of attracting various fish species).

Fish traps for high head structures, fish ladders are not sufficiently feasible. Tramways or cableways

are used. Hoppers into which fish swim are transported to reservoir by one of these ways.

Fish barrier dams low head weirs with electrical field that stop induce fish to swim into a ladder or

hopper downstream. From there they are transported by tank tracks with cooled and aerated water.

Downstream passing facilities screens to divert fish from intakes and spillways into safer by-passes.

They can be mechanical or electrical. Water velocity is around 0.5 m.

8.2 Spawning Channels

Most of the dams are equipped with fish ladders, elevators or the like but many passes are released with

varied success. Sometimes better long-term solution can be offered by implementation of artificial

spawning channels. By provision of stable favorable flow and bed conditions in the channel during

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spawning season for laying eggs, these facilities can even increase the survival rate (compared to natural

conditions) as much as nine-fold.

Design of spawning channels depends a great deal on type of original climate and biota specific for the

river. Natural conditions should be restored/imitated as much as possible. Fish behavior, number,

swimming ability and other factors should be studied and accounted for. Geometry of the structure is of

the vital importance. Usually rectangular or trapezoidal shapes are selected. The entrance is most crucial.

The flow of water attracts the fish into the channel. In order to adjust entrance to water level fluctuations

additional fish ladder could be provided between the channel and lower pondage. Entrance should be

close to hydraulic obstruction and easily perceivable by the fish. It is essential to establish correct

capacity of inflow, where ichthyologist should contribute in the design. Favorable bed condition (sand or

gravel), and optimal water depth and channel slope should be provided. Flooding of the channel in

spawning season must be avoided at any cost. Settling of suspended materials as well, since silt

deposition harms the eggs. Prevention of entrance of predatory fish is important, but care must be taken

to prevent humans as well. Density of fish (can reach over 60 per m2) may attract people to harm the fish.

To illustrate main problems and diversity in the area of spawning channels, a few examples taken from

the abstracts in the literature are listed below:

Selection of spawning sites by kokanees and evaluation of mitigative spawning channels in the

Green River, Wyoming.

Selection of spawning sites by kokanees Oncorhynchus nerka was assessed over a 1-km reach of the Green River in the tailwater of Fontenelle Dam in Wyoming, during 1990 and 1991. Within this reach three spawning channels were constructed in 1990 to mitigate losses of spawning habitat that were believed to have resulted from extensive deposition of gravel and rubble in 1986. The channels were built through an island of gravel and rubble in the river channel. Kokanees selected certain water depths, current velocities, and substrate sizes for spawning, but these three variables were not sufficient to fully account for spawning site selection. The distribution of recently deposited gravel and rubble, and the presence of shallow riffles that discouraged entry to a spawning channel also influenced spawning site selection. The spawning channels provided a substantial portion of the spawning habitat used by kokanees during 1990 and 1991, but high spring flows in 1991 substantially altered the morphology of the spawning channels and reduced their mitigatory value.

Installation of a Fish Migration Channel for Spawning at Itaipu

A fish migration channel for spawning at the Itaipu (Brazil and Paraguay) hydroelectric station was installed to improve fish recovery downstream from the dam. The complex had caused a significant reduction in the spawning area, with a deleterious effect on the reproductive cycle of the native species. An experimental model of a fish migration channel was installed, having a system of 'tank steps' at the foot of the dam, a sheet-metal ladder, a serpentine-style channel for the spawning of the fish, and two marginal lagoons for incubation of the eggs and growth of the larvae. The return migration of the hatchlings will be made through a trough linking the marginal lagoons directly to the Parana River. A 56-m-long ladder forms the first phase of the project. Preliminary data from the first phase has proved that fish (3000/day) of tropical climates can ascend ladders exceeding 8 m. The results of the first phase indicate the efficiency of the experimental project, with the entry and ascendancy of fish in a migration channel ladder. These data now provide the technical basis for implementation of the complementary spawning channel stage.

Rare Fishes of Himalayan Waters of Nepal

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The fishes of Himalayan waters are biologically diverse. More than 130 species occur in the rivers and mountain lakes of the Nepalese highlands. In some rivers, dams and barrages have been constructed for hydroelectric power stations and navigational purposes. The dams do not have fish ladders and they obstruct and prevent upstream and downstream movements of fish. Fisherman, poachers, and predatory animals, killing rare fishes exploit the shallow tailwater regions below dams where fish congregate. The scientific management of the Nepalese mountain streams has just begun. The local human population has increased dramatically. Sewage, detergents and herbicides entering the rivers are causing a critical deterioration of water quality. Spawning beds are being removed to provide sand and gravel for the building industry. Changes in land use and deforestation cause soil erosion and the rivers are loaded with silt in the rainy season. In order to conserve the rare and threatened species in Nepal, suitable water levels must be maintained in the tailwater regions of dams during the breeding season for stone carps, loaches, and catfishes. The provision of special spawning channels would help to maintain populations of masheer, snow trout, stone carp and torrent catfish. Consideration should also be given to the creation of one or more artificial river parks, containing channels and impoundments suitable for breeding rare fishes

Mitigation, Compensation, and Future Protection for Fish Populations Affected by

Hydropower Development in the Upper Columbia System, Montana, U.S.A.

The construction of Hungry Horse Dam resulted in estimated annual losses of 65,500 migratory juvenile westslope cutthroat and 1965 adult migratory bull trout from the Flathead Lake and River system. In addition, operations of Hungry Horse and Kerr dams caused annual losses conservatively estimated at 96,300 river-spawning and 131,000 lakeshore-spawning kokanee adults. Water level fluctuations caused by dam operations at Libby and Hungry Horse reservoirs result in: (1) altered thermal stratification; (2) indirect losses in phytoplankton and zooplankton production, (3) direct washout of phytoplankton and zooplankton through dam penstocks; (4) reductions in standing crop of benthic organisms and of insects on the water surface; and (5) reduced fish growth in the late summer and fall. Mitigative measures include: (1) 99.2 and 113.3 cu m/s minimum flows in the Flathead and Kootenai rivers respectively, to protect salmonid eggs and juveniles; (2) improvement of fish passage to restore migrations between the Flathead and Swan systems; and (3) biological rule curves for operations at Libby and Hungry Horse reservoirs. To compensate for fisheries losses, enhancement of spawning and rearing habitat, introductions of hatchery juveniles, and spawning channels are recommended. In addition, protection from further hydropower development for 100 stream reaches for fish species of special concern, and for outstanding sport fisheries is recommended. These and other measures will be considered by various agencies in developing an overall fisheries restoration plan which should be flexible, and employ principles of adaptive management.

Fish Passage, Control Devices and Spawning Channels in New Zealand

The construction of a hydroelectric power scheme on a river usually creates a barrier or impediment to the passage of fish travelling either upstream or downstream. This may adversely affect the fish population depending upon the river, the fish species involved and the location of their spawning grounds. Some of the fish species present in New Zealand, such as quinnat salmon, whitebait and some bullies, spawn in freshwater and the young in the form of eggs, larvae or juveniles move into the sea for a period before returning to freshwater to complete the life cycle. Some fish, such as eels, are catadromous, that is the adults migrate to the sea to spawn and the offspring move back into freshwater to complete the life cycle. Other species may reside in freshwater for their entire life but migrate from their residential areas to spawning areas. The need to maintain continuity between upstream and downstream fish populations, each of which can exist separately, is less clear-cut. The measures taken to maintain the fish population are usually either the provision of passage facilities or the provision of artificial spawning and rearing areas such as in spawning races and hatcheries. This chapter reviews information on the passage of fish past barriers, the problems associated with guiding or excluding fish and the construction and operation of spawning channels. It also discusses provisions, which have been made on New Zealand hydroelectric schemes.

Babine Lake Sockeye Salmon (Oncorhynchus nerka) Enhancement Program: Testing some

Major Assumptions

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The objective of the Babine Lake sockeye salmon (Oncorhynchus nerka) enhancement project was to increase fry outputs (and thus, smolt outputs and adult returns) by expanding and improving available spawning beds through the use of artificial spawning channels and related water flow control facilities. The project proceeded on four basic assumptions: (1) the artificial spawning channels would prove an effective means of producing sockeye fry, (2) the fry produced would be as viable as those produced from natural spawning beds, (3) the lake nursery area had the capacity to support larger juvenile populations, and (4) increased smolt outputs would result in increased adult returns. A before and after study has allowed these assumptions to be tested. Egg-to-fry survival in the channels was close to 40%, as expected. Comparisons of wild and channel-produced fry did not reveal any substantial difference in their distribution, growth, and survival in the lake. Increases in the abundance of fry were followed by corresponding increases in the abundance of under-yearlings in the lake and seaward migrating smolts. No significant change in the average size of the juveniles or their survival in the lake could be detected when population size increased. While the assumptions regarding juvenile production were found to be generally valid, adult returns did not meet expectations. This was due largely to the lack of response to increased smolt outputs from even-numbered brood years. Some options for future management are offered.

8.3 Selective Intakes

In reservoirs (especially larger ones) where stratification of water having different quality (oxygen

content, temperature, salinity, sediment, etc.) occurs (seasonally or all the time), there might be a

problem maintaining supplied water quality, if standard intake structures are used. For different users,

different parameters of quality really matter. For potable water most of them count, while for irrigation

(depending on crops) it’s usually salinity. For hydropower it may be favorable to avoid abrasive

sediments and air intrusion.

On the other hand quality of water in the reservoir may be of interest. Adverse impacts of long water

residence time, stratification and other processes can affect water quality for supply and life of aquatic

fauna as well. To a great extent the ecological safety depends on structure design, equipment, and control

systems. A pronounced transformation occurs in a river ecosystem when a dam is built and a reservoir is

created. A river ecosystem with a longitudinal gradient continues to exist in the free stretches of the river,

and a new ecosystem with vertical trophic distribution forms in the reservoir. The relationship between

these two systems depends on the design of the conveyance structure. For example, the use of a selective-

type intake makes it possible to regulate the water quality in the downstream pools and to transport

aquatic biota. In order to decrease the ecological impacts a greater emphasis is being placed on

monitoring systems and the use of highly skilled personnel to conduct biological surveys. The problems

of incorporating ecological concerns in the design and implementation of large reservoirs require the

development of new ideas in hydraulic construction. Again, there is not much that can be done with

conventional intakes.

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Both problems can be dealt with, up to certain extent, by implementation of several intakes at different

elevations or alternatively by single complex intake with inlets at different elevations – selective intake.

Selective intakes can have different designs, but basic idea is to allow for water withdrawal at different

elevations. In most cases three elevations satisfy demands for water quality control, though other number

could be more appropriate in certain cases.

The number will depend on water quality and stratification studies. Basic schemes are:

Separate intakes at different elevations leading to separate outlets

Separate intakes at different elevations leading to the same outlet

Former is usually more economically appropriate. Control structures (gates) and access are usually

dictating intake’s layout. Common layouts are:

Intakes and control structure in the dam body

Intakes and control structure in the intake tower

Intake at the abutment and control structure in the intake tower or underground shaft

Intake and control structure inclined on the abutment

Intake and (horizontal/sub-horizontal) outlet can be connected:

Each inlet via separate inclined conveyance pipe

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Water inlets connected via elbow joints with vertical/sub-vertical collection pipe

Water inlets connected via vertical collection shaft

In any case they are equipped with:

Trash racks (usually common one, but not necessarily)

Control/emergency gates (separate ones)

Aeration pipes

Operation: One (or two) water inlets are open at one time withdrawing water from most desirable

elevation where water quality is most appropriate. This depends on the usage of water, climate,

reservoir characteristics and the policy regarding the quality of the water that remains in the reservoir.

E.g., if the water is for drinking purpose and saving in water treatment process is important, water from

very top layer in the reservoir would probably be most convenient.

In principle different parameters of water quality vary in different ways along the depth of the reservoir,

for instance:

Temperature drops (or rises in the very cold winter) gradually in first ca 5 m then it drops rather

severely and then gradually reaching more or less constant temperature in great depths

Salinity (if any) can have different patterns in different seasons that would be dictated by inflow and

stage of the reservoir. In principle it tends to be lower at the surface.

Sediment content is highest near the bottom

Oxygen content is highest near the top, etc.

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In most cases decision is driven by demanded water quality for supply. However, in some cases it might

be important to maintain reasonable quality of remaining water in the reservoir and the river downstream

as well and then other criteria might be used.

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