Hydraulic Structures LN9

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B. Petry Lecture Notes N. Lukovac

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Hydraulic Structures

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

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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)

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• 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

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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

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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

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• 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 • Storm patterns, hydro-meteorological condition • Routing through channel system.

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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

A

v

x

+ vB

y

x

+ B

y

t

= q

∂

♦ Methods

• Full hydraulic method – complete equations • Diffusion method f 0

S = S -

y

x

∂

• Kinematic Wave Sf = So

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• 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 1

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

1 2 1 2 1

C =

2( t - t )

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

1

2

1 C =

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

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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: 32

1

2

3 /

2 C

g

b

H

Q

=

⋅

(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:

2 3 3 2 3 2

2 g

b

H

C

b

H

C

Q

=

⋅

=

⋅

♦ 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:

85 . 0 85 . 1

5 .

0

−

=

x

H

y

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 JUMP

3 FLAP GATE SHAFT SPILLWAY SPILLWAY CHUTE WATER CUSHION

4 FUSE PLUG SIPHON FREE SURFACE

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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

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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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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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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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σ

=

γ

α

p / - p / + h

+

hv

gr

v / 2g

a v 2 2

cos

Where: pa = atmospheric pressure pv = vapor pressure

γ = specific weight of water g = acceleration of gravity r = radius of curvature Cavitation criterion: σ > σcr

σcr = critical value for incipient cavitation For fairly smooth surfaces σcr = 0.25

♦ aeration

If σ≤ σcr cavitation may occur. A good control of cavitation is aeration of flow

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

Eu = v/√∆p/ρ - Ëuler number – Inertia/Pressure difference We = v/√ τ/ρL - Weber number – Inertia/Surface tension force τ = surface tension

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

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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:

(

1.35

)

) ( 9 . 1 ) (

25 )

(

kg

L

_m

H

_m

W

=

⋅

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)=300⋅ Capacity (tons), where capacity may vary from 75% to 150% of the gate leaf weight. For traveling type hoists W (kg)=167⋅ Capacity (t) 1.33.

Vertical-lift (sliding) gates:

(

1.75

)

) ( 5 . 1 ) (

9 .

25 )

(

kg

L

_m

H

_m

W

=

⋅

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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)=68⋅ Capacity (t).

Rolling gates and Hoists:

(

1.67

)

) ( 5 . 1 ) (

9 .

55 )

(

kg

L

_m

H

_m

W

=

⋅

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:

(

)

1.33 ) ( ) (

332 )

(

kg

L

_m

H

_m

W

=

⋅

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 = 59.5⋅ f (W) 0.74 _{(in tons)} Open utility type W = 28.9⋅ f (W) 0.456 _{(in tons)}

Where:













₊

+

⋅

=

S

B

A

S

C

W

f

2

1 1000

)

(

;

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:

y

ydy

c

A

g

a

H

T

a a H H y H H y a

−

=

_∫

= =

1

2

2 2 1 1

A = reservoir area A(H) a = Control section outlet area

Ha = steady reservoir level for given inflow

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Here

∑

+

=

ξ

D

L

f

c

1

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

_f

₌

₁₂₄

_.

₆

_n

2 3

_D

_{). For details on this consult lecture-notes “Basic}

Hydraulics or chapter 7 of these lecture-notes.

Σξ 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 1 Intake or Intake Structure 2 Conduit Waterway Tunnel 3 Gate 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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♦ 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 • Radial gates: ⋅ Tainter ⋅ Top-seal radial

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• 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:

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• 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 • Uplift

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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 = ξαv’2_/2g _{where v’ is velocity at the end of the spillway,}_α_is the Coriolis coefficient, and ξ is the head loss coefficient related to the velocity coefficient ϕ (ratio of actual to theoretical velocity) as:

ξ

ϕ

=

1 +

1

2 Relative head loss:

=

1 −

ϕ

2

E

e

After Novak and Čabelka (1981) for S/H<30 and smooth spillways: ϕ1≈ 1 − 0.0155S/H

The value of ξ can be increased (ϕ 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 1−3 (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%.

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• 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:













+

⋅

=

β

ϕ

β

ϕ

cos(

)

sin

'

0 .

5 sin

2

0 2 2 0 2

H

y

S

H

L

Where

H0 = S + H − S’− y/2 energy to the middle of the off-taking jet. β is take-off angle

And y = depth at the off-take of the ski-jump ϕ 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 β (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}_β

• 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

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Hydraulic jump stilling basin:

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

2 1 2 2 1

2 g

y

q

y

E

ϕ

α ⋅

+

=

and then second conjugate depth for rectangular basin:













+

−

=

₃ 1 2 1 2

1

8

2 gy

q

y

Depth of the stilling basin respecting need for certain “submergence” as safety measure: D = σ⋅y2 − y0 →

y0 is normal depth in the river downstream of the stilling basin usually obtained for computation from tail water flow-rating curve.

σ 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

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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:



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H* is difference between upstream and downstream levels 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: 3 3 4

435 .

1 







=

d o

y

v

H

L

, 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:

0 8

_y

d

H

Cq

y

_z w y x s

=

−

β

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:

0 1 . 0 * 6 . 0

5 .

1 q

H

y

_s

=

−

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.

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• After flip bucket

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

1

tan

6

_cr

β

s

y

=

β1 = upstream angle of the scour hole as a function of flip bucket exit angle β For 100_<_β_<400 _⇒ ₁₄0_<_β1_<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). This figure shows comparison of the model and actual situation

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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

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:

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• 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 1

The 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 2

Now 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 3

When 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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Adh

dt

gh

ca

Qdt

=

2 =

−

For instantaneous complete opening:

g

ac

H

A

h

dh

g

ac

A

dt

T

H T

2

0 0

=

−

=

∫

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)

∫

1

=

−

1 0

2

hT H t T

h

a

dh

g

c

A

dt

→

(

1

)

1 0 1

2

T T

h

H

g

ca

AT

tdt

=

−

∫

(

)

g

ca

h

H

A

T

2

4

1 1

−

=

And total filling time:

g

ca

H

A

T

g

ca

h

A

T

2

1 1 1

+

=

+

=

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:

(

A

)

ca

g

H

A

T

2

2 1 2 1

+

=

And for A1=A2

g

ca

H

A

T

2 =

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

(

h

i

h

i

)

g

ca

A

t

=

−

∆

₋1

2

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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 deAc-celeration 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.

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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

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

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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.”

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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

C en tr if ug al Rotodynamic pump

Radial-flow type Low >30mHigh 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 Medium

23−30m

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.

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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:

4 5

H

Q

N

_s

=

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 η. H = H / η

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For clean water and Hm (m), Q(m3/s), and η=ηp⋅ηm⋅ηt<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

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/ρg = pa/ρg − (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):









+

∆

−

≤

g

v

H

g

p

h

s s v a s

2 )

(

2

ρ

Net positive suction head (NPSH):









+

∆

+

−

=

g

v

H

h

g

p

NPSH

s s s v a

2 )

(

2

ρ

Thoma’s cavitation number σ is: σ = NPSH / Hm

NPSH values are supplied by pump manufacturers. Nss (suction specific speed) is: Nss = NQ1/2/(NPSH)3/4; σ = (Ns/Nss)4/3; 4700<Nss<6700 for most centrifugal pumps. Critical cavitation number (from model tests):

σc = 0.103 (Ns / 1000)4/3

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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.

Pump - Pipeline Characteristic

Q

H

pump characteristic

system

characteristics

efficiency

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Belmouth intake provides good inflow conditions and minimizes the entrance losses. Minimum submergence of the intake should be:

gd

v

b

a

d

h

₌

₊

Where a = 0.5−1.5; and b = 2.0−2.5 (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 60−90o_{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 (surges−waterhammer) 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:

g

av

H

₌

_

0

∆

Where a is celerity of the pressure wave:

e

D

k

eE

D

K

a

+

=











 +

=

50

10

1

4

ρ

For water ρ=1000 kg/m3_{, bulk modulus K}_≈₂₀_⋅ ₁₀8_N/m2_{, k=10}11_/E

For steel E≈20⋅1010_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 (0≤T≤τ, τ=2L/a).

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

gT

Lv

H

₌

₂

0

∆

. If along the

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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:













⋅

±

=

t

LV

gAH

gAV

LH

Q

H

0 0 0 0 0 0

sin

0 0 0 0 min

gAV

LH

Q

H

=

−

From here Vmax can be computed: Vmax1.2⋅ Hmin=V01.2⋅H0

Period of oscillation is:

0 0

2 LV

gAH

T

=

π

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

Hydraulic Structures LN9