DESIGN OF HYDRAULIC WORKS.docx

8/10/2019 DESIGN OF HYDRAULIC WORKS.docx

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DESIGN OF HYDRAULIC STRUCTURES

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PEDRO RUIZ GALLO NATIONAL UNIVERSITY

FACULTY OF CIVIL, SYSTEMS ENGINEERING AND

ARCHITECTURE

COLLEGE OF CIVIL ENGINEERING


RESERVOIR OUTLET WORKS

INSTRUCTOR: Msc. Ing. Jos Arbul Ramos

STUDENTS:

Santa Mara Carlos, MarianoPea Chaquila, Danner

December 2nd, 2014


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RESERVOIR OUTLET WORKS

I. INTRODUCTION

Water from the reservoir of a dam is released through two principal types of structures:

1. Spillways, which are provided for storage and detention dams to release surplus water or

floodwater that cannot be contained in the allotted storage space.

Spillways

2. Outlet works, which regulate or release water impounded by a dam. It can release incoming

flows at a retarded rate, as does a detention dam; it can divert incoming flows into canals or

pipelines; or it can release stored waters at rates dictated by downstream needs, by

evacuation considerations, or by a combination of multiple-purpose requirements.

Outlet works


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II. STORAGE DAM INTAKE WORKS

Outlet works are so named because they release out water from the reservoir. Some of these

are equipped with an Intake Structure if the water is fed into a canal or a conduit for serving

some specific purpose like meeting irrigation water requirement or hydropower generation,

etc.Occasionally, the outlet works may be placed at a level high enough to deliver water to a

canal, while a bypass is extended to the river to furnish necessary flows below the dam. Such

bypass flows may be required to satisfy prior-right uses downstream or to maintain a live

stream for preservation of aquatic life, or other purposes. Dams constructed to provide

reservoirs principally and also for recreation or for raising fish and wildlife conservation

require a fairly constant reservoir level. For such dams an outlet works may be needed only to

release the minimum flows necessary to maintain a live stream below the dam. In certain

cases, the outlet works of a dam may be used in lieu of a service spillway combined with an

auxiliary or secondary spillway. In such a case, the usual outlet works installation might be

modified to include a bypass overflow so that the structure can serve as both an outlet worksand a spillway.

An outlet work may act as a flood control regulator to release waters temporarily stored in

flood control storage space or to deplete the storage of a reservoir in anticipation of flood

inflows. Furthermore, the outlets may be used to empty the reservoir to permit inspection, to

allow needed repairs, or to maintain the upstream face of the dam or other structures

normally inundated. The outlets may also aid in lowering the reservoir storage when

controlling objectionable aquatic life in the reservoir is desired.


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Outlet works under embankment dam

III. GOALS

The outlet works their main goal is to making controlled release of reservoir water impounded

behind the dam. The outlet works consists of an intake structure that has valves openings at

various heights that allows reservoir water to flow into the structure at a selected flow rate.Water then flows down the intake tower down to an outlet conduit and through the bottom

of the dam back into a river or channel.

Intake Works


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Outlet works

IV. FUNCTIONS OF OUTLET WORKS

The type and design of the outlet works facility will be greatly influenced by the project, itspurposes, and structure-specific functions. An evaluation of each function is absolutely

necessary in any design of the structure selected.

a. Flood control. Outlet works for flood-control projects generally require designs having large

flow capacities and less regulation capabilities. Typically, the outlets are gated for flow

regulation. However, the conduits may be uncontrolled (no gates) for reservoirs that are low

or empty during non-flood periods.

b. Irrigation. Gates and valves for irrigation require close regulation and lower discharge

ranges than flood-control outlet controls. Releases may be discharged into a channel or

conduit rather than into the original riverbed.

c. Water supply. Municipal water supply intakes are generally a secondary project function.

Reliability and water quality are of prime importance in the design. Water intakes are located

and controlled to ensure that the water is free of silt and algae, to obtain desired

temperatures, and to allow intake cleaning.

d. Power. Power penstocks within intake structures should be located so as not to cause any

undesirable entrance flow conditions such as eddies that might jeopardize turbine operation.

Power intakes may require smaller trashrack openings to limit the size of debris that enters

the penstock.

e. Sediment. Projects designed for sediment retention should be designed to pass flows asthe sediment level rises in the reservoir and to prevent sediment from passing through and

damaging or blocking both the entrance and the outlet works itself. Smaller releases are

controlled by multilevel intakes that are closed by gating or stoplogs as the sediment level

rises in the reservoir. For projects with flood flows that are released through the outlet works,

a high-level intake that provides protection against large sediment buildup may be necessary.

V. COMPONENTS OF OUTLET WORKS

For an open-channel outlet works or for a conduit-type outlet where partial full flow prevails, the

control gates or valves should determine the outlet works capacity. Where an outlet works

operates as a pressure pipe, the size of the waterway and that of the control device shoulddetermine the capacity. The overall size of an outlet works is determined by its hydraulic head and

the required discharge. The selection of the size of some of the component parts of the structure,

such as the tunnel, is dictated by practical considerations or by interrelated requirements such as

diversion, reservoir evacuation, and initial filling.

When the type of waterway has been chosen and the method of control established, the

associated structures to complete the layout can be selected. The type of intake structure depends


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on its location and function and on the various appurtenances, such as fish-screens, trash racks,

stop log arrangements, or operating platforms that must be furnished. A means for dissipating the

energy of flow before returning the discharge to the river should normally be provided. This can

be accomplished by a flip bucket, a stilling basin, a baffled apron drop, a stilling well, or a similar

dissipation device. Gate chambers, control platforms, or enclosures may be required to provide

operating space and protective housing for the control devices. An outlet works may also requirean outlet channel to return releases to the river and an entrance channel to lead diversion flows or

low-reservoir flows to the intake structure.

Tunnels

Because of its inherent advantages, a tunnel outlet works is preferred where abutment and

foundation conditions permit its use and it is more economical than the other types of outlet

works. A tunnel is not in direct contact with the dam embankment and, therefore, provides a

much safer and more durable layout than can be achieved with a cut-and-cover conduit. Little

foundation settlement, differential movement, and structural displacement is experienced with a

tunnel that has been bored through competent abutment material, and seepage along the outer

surfaces of the tunnel lining or leakage into the material surrounding the tunnel is less serious.Furthermore, it is less likely that failure of some portion of a tunnel would cause failure of the dam

than the failure of a cut-and-cover conduit that passes under or through the dam.

Cut-and-Cover Conduits

If a closed conduit is to be provided and foundation conditions are not suitable for a tunnel, or if

the required size of the waterway is too small to justify the minimum sized tunnel, a cut-and-cover

conduit should be used. Because this type of conduit passes through or under the dam,

conservative and safe designs must be used. Numerous failures of earthfill dams caused by

improperly designed or constructed cut-and-cover outlet conduits have demonstrated the need

for conservative procedures.

Control Devices

Selection of the outlet works arrangement should be based on the use of commercially available

gates and valves or relatively simple gate designs where possible. The use of special devices that

involve expensive design and fabrication costs should be avoided. Cast iron slide gates, which may

be used for control and guard gates, are available for both rectangular and circular openings and

for design heads up to about 15 metre. However, higher head installations require special gate

designs. Simple radial gates are available for ordinary surface installations, and top-seal radial

gates can be secured from manufacturers on the basis of simple designs and specifications. For

low heads up to about 15 metre, commercial gate and butterfly valves are suitable for control at

the downstream end of pressure pipes if they are designed to operate under free dischargeconditions with the jet well aerated all around. Gate and butterfly valves are also suitable for use

as inline guard valves and can be adapted for inline control valves if air venting and adequate

aeration of the discharge jet are provided immediately downstream from the valve.

The control gate for an outlet works may be placed at the upstream end of the conduit, at an

intermediate point along its length, or at the lower end of the structure. Where flow from a

control gate is released directly into the open as free discharge, only that portion of the conduit


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upstream from the gate is under pressure. Where a control gate or valve is placed at the lower end

of the structure, full internal pressure should be considered in the design of the conduit tunnel or

pipe. However, when a control discharges into a free-flow conduit, the location of the control gate

becomes important in the design of the outlet. Upstream gate controls for conduits are generally

placed in a tower structure with the gate hoists mounted on the operating deck (Figure 2). With

this arrangement, the tower must extend above the maximum water surface. If controls are to belocated at some intermediate point along the conduit, high-pressure gates, slide gates, and top-

seal radial gates may be used. These controls may be located in a wet-well shaft that extends

vertically from the conduit level to the crest of the dam. Typical arrangements of these

installations are shown in Figures 1 to 4.


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Intake Structures

In addition to forming the entrance to the outlet works, an intake structure may accommodatecontrol devices. It also supports necessary auxiliary appurtenances (such as trashracks,

fishscreens, and bypass devices), and it may include temporary diversion openings and provisions

for installation of bulkhead or stoplog closure devices. Intake structures may appear in many

forms. The type of intake structure selected should be based on several factors: the functions it

must serve, the range in reservoir head under which it must operate, the discharge it must handle,

the frequency of reservoir drawdown, the trash conditions in the reservoir (which will determine

the need for or the frequency of cleaning of the trashracks), reservoir wave action that could

affect the stability, and other similar considerations. Depending on its function, an intake structure

may be either submerged or extended in the form of a tower above the maximum reservoir water

surface. A tower must be provided if the controls are placed at the intake, or if an operating

platform is needed for trash removal, maintaining and cleaning fish-screens. Where the structureserves only as an entrance to the outlet conduit and where trash cleaning is ordinarily not

required, a submerged structure may be adopted.

The necessity for trashracks on an outlet works depends on the size of the sluice or conduit, the

type of control device used, the nature of the trash burden in the reservoir, the use of the water,

the need for excluding small trash from the outflow, and other factors. These factors determine

the type of trashracks and the size of the openings. Where an outlet consists of a small conduit

with valve controls, closely spaced trash bars are needed to exclude small trash. Where an outlet

involves a large conduit with large slide-gate controls, the racks can be more widely spaced. If

there is no danger of clogging or damage from small trash, a trashrack may consist simply of struts

and beams placed to exclude only larger trees and similarly sized floating debris. The rackarrangement should also be based on the accessibility for removing accumulated trash. Thus, a

submerged rack that seldom will be dewatered must be more substantial than one at or near the

surface. Similarly, an outlet with controls at the entrance, where the gates can be jammed by trash

protruding through the rack bars, must have a more substantial rack arrangement than one whose

controls are not at the entrance.


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Energy Dissipating Arrangements

The discharge from an outlet, whether of a gate valve, or free flow conduit, will emerge at a high

velocity, usually in a nearly horizontal direction. If erosion-resistant bedrock exists at shallow

depths, the flow may be discharged directly into the river. Otherwise, it should be directed away

from the toe of the dam by a deflector. Where erosion is to be minimized, a plunge basin may be

excavated and lined with riprap or concrete. When more energy dissipation is required for freeflow conduits, the terminal structures described for spillways may be used. The hydraulic-jump

basin is most often used for energy dissipation of outlet works discharges. However, flow that

emerges from the outlet in the form of a free jet, as is the case for valve-controlled outlets of

pressure conduits, must be directed onto the transition floor approaching the basin so it will

become uniformly distributed before entering the basin. Otherwise, proper energy dissipation will

not be obtained.

Entrance and Outlet Channels

An entrance channel and an outlet channel are often required for a tunnel or cut-and-cover

conduit layout. An entrance channel may be required to convey diversion flows to a conduit in an

abutment or to deliver water to the outlet works intake during low reservoir stage. And an outlet

channel may be required to convey discharges from the end of the outlet works to the river

downstream or to a canal.


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VI. INLET STRUCTURES

The inlet structures intake works to consist mainly of trashracks a combination of trashracks

and control gates.

Depending on the particular design at each dam, the intake works must correspond to the

foundation conditions, required discharge operating loads, variations in reservoir water levelsand amount of floating solids that can catch the outlet.

VII. INTAKE IN CONCRETE OR MASONRY DAMS

In the case of concrete dams, irrigation intake structure can be located either at the toe when

operating head is low or in the body of the dam itself when operating head is medium or high.

Typical section of such an intake is shown in Figure 11.

VIII. INTAKE IN EARTHEN DAMS

When the reservoir is formed by an earthen dam, the irrigation tunnel is laid below it or in the

abutment. The intake structure for such situations will be a sloping intake or tower type of

intake. Typical layouts for sloping and tower type intakes are shown in Figure 12 and 13

respectively. As far as possible, reinforced cement concrete pressurized system should be

avoided in the body of the earth dam. Measures like provision of steel liners and suitable


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drainage downstream of core, provisions of joints for differential settlements when not

founded on rock should be considered in case pressure conduits are provided under earth

dams.


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Intake In Earthen Dams

IX. TRASH RACKS FOR INTAKES

Trashracks are needed to prevent clogging and debris damage to outlet control gates and turbines.

The trashrack type and size of openings depend on the pool elevation, intake elevation, the size of

the outlet conduit, the reservoir trash conditions, type of control device used, use of the water,

and the need to exclude the trash.

a. Intakes for flow regulation. A simple trash structure, usually of reinforced concrete, with clear

horizontal and vertical openings not more than two-thirds the gate width should be provided at

the upstream end of the outlet works to catch trees and other large trash which may reach the

entrance and be capable of blocking the gated passages. Large trash at the tunnel entrance occurs

more often when the permanent pool is only slightly above the entrance than when thepermanent pool is high above the entrance. Only in special cases in which trees and floating

debris are absent from the reservoir and watershed should the trash structure be omitted.

(1) Metal trashracks fabricated of closely spaced bars are generally used for small conduits with

control valves and water supply intakes that require screening of small debris.

(2) To facilitate trash removal immediately after a flood, the working platform at the top for

raking and removal of trash usually should not be lower than the top elevation of the conservation

or maximum power pool. The usual trash structure consists of upright beams, inclined slightly

downstream from the vertical to facilitate raking, and horizontal beams.

(3) If the gate structure is located at the upstream end of the outlet tunnel, the trash structure iscombined with it for economy.

(4) The area of the openings in the trash structure should limit the local net-area velocity to less

than 10 to 15 ft/sec.

b. Floating trash and debris control facilities. Floating trash and debris control can be provided by

a basic trash boom constructed of logs or floating pontoons. Trashracks would still be necessary


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to protect intakes from occasional water-logged trees or large logs that pass under the trash

boom.


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X. GATES

1. Vertical gate

Similar to that used for crest type gates (Figure 1), but usually for deep-seated purposes likecontrolling flow to hydropower intake either the ones with roller wheels (Figure 13), or the sliding

- type without any wheels (Figure 14), are used.

2. Sliding gates

Slide gates, as the name implies, are the gates in which the operating member (that is, gate leaf)

slides on the sealing surfaces provided on the frame. In most cases, the sealing surfaces are also

the load-bearing surface. Slide gates may be with or without top seal depending whether these

are used in a close conduit or as crest gate. A typical installation of a slide gate is shown in Figure


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14. These consist of a gate leaf and embedded parts. These embedded parts serve the following

purposes:

a) Transmit water load on the gate leaf to the supporting concrete (structure),

b) Guide the gate leaf during operation, and

c) Provide sealing surface.

According to the Bureau of Indian Standards code IS: 5620 Recommendations for structural

design criteria for low head slide gates, slide gates may be classified into the fol lowing three

types depending upon their service conditions.

(i). Bulk head or stop-logs

These are usually located at the upstream end of river outlet conduits or penstocks where in

addition some other equipment is used to cut off flow and are subjected to relatively high heads.

(ii). Emergency or guard gates

These are designed to be operated under unbalanced head, that is, with water flowing through

the conduit or sluice but are not meant for regulation. These are kept either fully opened or fully

closed and are not operated at part gate opening.

(iii). Regulating gates

These are used for regulating flow of water. These are also operated under unbalanced head

condition and are designed to be operated at any gate opening.


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3. Deep-seated radial gates

These are low level radial outlet gates. These gates have sealing on top apart from on all sides.

They are located at sluices in the bottom portion of dam (Figure 15). The hoisting arrangement is

usually at the top but could also be provided near the elevation of top seal to reduce hoist stroke.


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XI. HEAD LOSSES

Pressure Flow in Outlet Conduits

If a control gate is placed downstream from the conduit entrance, that portion above the control

gate will flow under pressure. An ungated conduit can also flow full depending on the inlet

geometry. The phenomena and the hydraulic equations for flow through an ungated conduit

under pressure. The hydraulic design of a gated pressure conduit should be similar to that for an

ungated pressure conduit.

For flow in a closed pipe system, as shown on figure 10-11, Bernoullis equation can be written as

follows: Where:

= Total head needed to overcome the various head losses to produce discharge,= Cumulative losses of the system, and= Velocity head at the valve.Equation (3) can be expanded to list each loss as follows:


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Where:= Trashrack losses,= Entrance losses,= Bend losses,

= Friction losses,

= Expansion losses,= Contraction losses,= Gate or valve losses, and= Velocity head exit loss at the outlet.In equation (4), the number subscripts refer to the various components, transitions, and reaches

to which head losses apply.

For a free-discharging outlet, is measured from the reservoir water surface to the center of theoutlet gate or the outlet opening. If the outflowing jet is supported on a downstream floor, the

head is measured to the top of the emerging jet at the point of greatest contraction; if the outlet

portal is submerged, the head is measured to the tailwater level.Where the various losses are related to the individual components, equation (4) may be written:

Where:

= trashrack loss coefficient,= entrance loss coefficient,= bend loss coefficient,= friction factor in the Darcy-Weisbach equation for pipe flow.= expansion loss coefficient,= contraction loss coefficient,= gate loss coefficient, and= exit velocity head coefficient at the outlet.Equation (5) can be simplified by expressing the individual losses in terms of an arbitrarily chosen

velocity head. The velocity head chosen is usually that in a significant section of the system. If the

various velocity heads for the system shown on figure 10-11 are related to that in the downstreamconduit, area (1), the conversion for xarea is found as follows:

Since:


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Then:

Equation (5) can then be written:

*

( )

( )+If the bracketed part of the expression is represented by , the equation can be written:

Then:


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XII. POSITION (ELEVATION) OF OUTLET WORKS IN RELATION TO RESERVOIR STORAGE

LEVELS.

The establishment of the intake level and the elevations of the outlet controls and the conveyance

passageway, as they relate to the reservoir storage levels, are influenced by many factors.

Primarily, to attain the required discharge capacity, the outlet must be placed sufficiently below

the minimum reservoir operating level to provide the head required for outlet works flows.

Outlet works for small detention dams are generally constructed near riverbed level because

permanent storage space, except for silt retention, is ordinarily not provided. (These outlet works

may be ungated to retard the outflow while the reservoir temporarily stores the bulk of the flood

runoff, or they may be gated to regulate the releases of the temporarily stored waters.) If thepurpose of the dam is only to raise the reservoir and divert incoming flows at low heads, the main

outlet works generally should be a headworks or regulating structure at a high level. A sluiceway

or small bypass outlet should also be provided to furnish water to the river downstream or to

drain the water from behind the dam during off-season periods. Dams that impound water for

irrigation, for domestic use, or for other conservation purposes, must have outlet works low

enough to draw the reservoir down to the bottom of the allocated storage space; however, the

outlet works may be placed above the riverbed, depending on the established minimum reservoir


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storage level. It is common practice to make an allowance in a storage reservoir for inactive

storage to accommodate sediment deposition, for fish and wildlife conservation, and for

recreation. The positioning of the intake sill then becomes an important consideration; it must be

high enough to prevent interference from the sediment deposits, but at the same time, low

enough to permit either a partial or a complete drawdown below the top of the inactive storage.

The size of an outlet conduit for a required discharge varies according to an inverse relationship

with the available head for producing the discharge. This relationship may be expressed by the

following equation:

Where:

The above relationship for a particular design is shown on figure 10-8(A). This example shows that

if the head available for the required outlet Works discharge is increased from 1.6 to 4.6 feet, the

corresponding conduit diameter can be decreased from 6 to 4.75 feet. This shows that the conduit

size can be reduced significantly if the inactive storage level can be increased. The reduction in

active storage capacity resulting from a 3-foot increase in the inactive storage level must be

compensated for by the addition of an equivalent capacity to the top of the pool.

The reservoir capacity curve on figure 10-8(B) shows that for equivalent storages (represented by

de and gh), the 3 feet of head (represented by cd) added to obtain a reduced outlet works size

would require a much smaller increase (represented by fg) in the height of the dam. Thus,


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economic studies can be used to determine the proper outlet size in relation to the minimum

reservoir storage level.

Where an outlet is placed at riverbed level to accommodate the construction diversion plan or to

drain the reservoir, the operating sill may be placed at a higher level to provide a sediment and

debris basin and other desired inactive storage space, or the intake may be designed to permit

raising the sill as sediment accumulates. During construction, a temporary diversion opening may

be formed in the base of the intake to handle diversin flows. Later, this opening may be plugged.

For emptying the reservoir, a bypass around the intake may be installed at riverbed level. This

bypass may either empty into the lower portion of the conduit or pass under it. Water can be

delivered to a canal at a higher level by a pressure riser pipe connecting the conduit to the canal.

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