Unformatted text preview: HYDRAULIC
STRUCTURES Introduction
►A hydropower development includes a
number of structures, the design of
which will be dependant upon the type
of scheme, local conditions, access to
construction material and also local
building traditions in the country or
region. Cont’d
The following structures are common in a hydro
scheme:
► Diversion structure ► Dam
Spillway
Energy dissipation arrangement
Fish pass
Residual flow arrangements Water conveyance system Intake
Canals
Tunnels
Penstocks
Power house Design aspects
Dams
Dams and weirs are primarily intended to divert the river
flow into the water conveyance system leading to the
powerhouse.
It produces additional head and provide storage capacity.
Its choice depends largely on local topographical and
geotechnical conditions. For instance if sound rock is
not available within reasonable excavation depth,
rigid structures such, as concrete dams are difficult.
Conversely, for narrow valleys, it can be difficult to
find space for separate spillways, and concrete dams
can be the natural choice with their inherent
possibilities to integrate spillways etc in the dam
body.
1. Cont’d
World wide, embankment dams are the more common
partly due to the following characteristics:
► Can be adapted to a wide range of foundation
conditions.
► Construction uses natural materials, which can often
be found locally, limiting needs for long
transportation.
► The construction process can be continuous and
highly mechanized.
► The design is extremely flexible in accommodating
different fill materials.
Disadvantages with embankment dams are that they are sensitive to
overtopping and leakage, and erosion in the dam body and its foundation.
There is a higher mortality rate among embankment dams as compared to
concrete dams. Cont’d
Concrete dams on the other hand have
drawbacks that correspond to the pros of
the embankment dams:
► Require certain conditions with respect to
the foundations.
► Require processing of natural materials
for aggregate at the site, hauling of large
quantities of cement and has a labor
intensive and discontinuous construction
process, leading to large unit costs. Cont’d
Advantages of Concrete Dams
► They are suitable for most ranges of topography
that is for wide and narrow valleys, provided that
foundation conditions are right.
► They are not very sensitive to overtopping.
► A spillway can be placed at the crest, and if
required over the entire length of the dam.
► Chambers or galleries for drainage, tubing and
ancillary works can readily be housed within the
dam body.
► Powerhouses can be placed right at the toe of the
dam. Cont’d
► The development of the Concrete Faced
Rock fill Dam (CFRD) neutralizes many of the
drawbacks with core-type embankments. In
particular, sensitivity to leakage and erosion
is reduced, and dependence of good core
material is removed.
► The development of the Roller Compacted
Concrete Dams (RCC-dams) introduces a
continuous, highly mechanized construction
process and low unit costs.
► New large dams are almost always CFRD
and RCC designs. Embankment Dams
1. Homogeneous dams: These dams are used for low
embankments (H<4m) and often as
secondary dams. For dam safety reasons,
some type of drainage is almost always
provided. 2. Zoned embankment dams: These are used for dam heights for 4m and
above. Constructions are extremely sensitive
to the engineering design and construction,
and it is therefore vital to engage highly
skilled consultants/Engineers and contractors. It requires experienced site-supervision
engineers. Critical components of these dams
are the core, the transition zones (filters)
surrounding the core and drainage capacity of
the dam toe. Cont’d
3. Embankments dams with membrane:
The membranes can be of different
types and be located either at the
upstream front of the embankment or
vertically in the centre of the
embankment. Membranes can be made
from concrete (as in the CFRD), asphalt
(Norwegian type) or in the form of a
geomembrane on the upstream slope. Concrete Dams
Generally, concrete dams are
categorized according to how they
function statically Cont’d
1. Gravity dams: These are dependent on their own mass for
stability. Their cross-section is basically
triangular in order to provide adequate stability
and stress distribution across the foundation
plane and their upper part is normally
rectangular in order to provide adequate crest
width for installation and transportation.
Design issues include stability analysis (sliding
and overturning), stress control, temperature
control during construction to avoid cracking,
control of uplift pressures under the dam, etc. Cont’d.
2. Buttress dams: These dams consist of a continuous
upstream face that is supported by
buttresses at regular intervals. The
upstream face is normally divided into
vertical sections by dilatation joints, each
section being supported by a buttress. Cross-sections are similar to those of
gravitation dams. Cont’d
3. Arch and Cupola dams: These dams function structurally as horizontally laid
out arches that transfer the water pressure on the
upstream face into the abutments rather than into the
foundation.
Arch dams can be designed with a constant radius over
the dam height, or with varying radii (Cupola dams).
Arch dams with a constant radius have a vertical and
“straight” cross-section. These dams will be subject to
considerable vertical strain forces since the
deformation of the dam will tend to be greatest in the
vertical centre of the dam. This requires that the dam
be heavily reinforced to avoid cracking with
accompanying leakage. Cont’d
► The Cupola dam is designed to have only
compression forces for all directions and at
all sections. This requires the radius of the
curvature to vary over the dam height,
which produces a curved vertical crosssection.
► The arch and cupola dams are structurally
efficient and greatly reduce the required
volume of concrete. They require, however,
a narrow valley topography and strong
foundation rock in the abutments. Weirs and spillways
► The large majority of small hydro schemes are of
the run-of-river type, where electricity is
generated from discharges larger than the
minimum required to operate the turbine.
► In these schemes a low diversion structure is
built on the streambed to divert the required
flow whilst the rest of the water continues to
flow over it. Such a structure is commonly
known as a weir, whose role is not to store the
water but to increase the level of the water
surface so the flow can enter into the intake. Cont’d
► Weirs and spillways can be subdivided
into fixed and mobile structures. Smaller
fixed structures are generally referred to
as weirs, whereas larger structures are
often referred to as spillways. Spillways
are often divided into un-gated and
gated spillways, corresponding to fixed
and mobile structures, the un-gated
spillway in fact being a large-scale weir. Cont’d
► Fixed storage structures, such as weirs
and ungated spillways have the
advantage of security, simplicity, easy
maintenance, and are cost effective.
However, they cannot regulate the
water level and thus both the water
level and energy production fluctuates
as a function of discharge. Cont’d
► Mobile storage structures such as gated
spillways can regulate the water level such
that it stays more or less constant for most
incoming flow conditions. Depending on gate
configuration and discharge capacity they may
also be able to flush accumulated sediment
downstream. These structures are generally
more expensive than fixed structures, for both
construction and maintenance, and their
functioning is more complicated Weir Types
The sharp-crested weir: 1. • It is easy to construct and relatively costeffective. Special attention has to be paid
to the shape of the downstream face of
the upper part of the weir in order to
obtain sufficient aeration between the
lower nappe of the jet and the structure.
If the lower nappe of the jet sticks to the
structure, vibrations may be transferred
from the flow to the structure. Cont’d.
2. The broad-crested weir:
• It is often applied for temporary
structures or for structures of secondary
importance, such as in case of temporary
flow diversion. Its design is simple and
cost-effective. The hydraulic conditions
are far from optimal, expressed by a low
discharge coefficient and the presence of
under-pressures along the weir crest and
downstream face. The discharge depends
on the form of the structure. Cont’d
3. Ogee weir:
• It is hydraulically the most ideal solution giving
the highest discharge coefficient. Its curved shape
is defined by the jet trajectory that would appear
for the design discharge HD. For lower or higher
discharges, over-or under-pressures will appear
along the downstream face. For discharges much
higher than the design discharge, these underpressures may lead to cavitation and damage to
the downstream concrete face. Recent work
suggests fortunately that separation will not occur
until H > 3HD. Weir configurations Gated Spillways
The installation of mobile elements on dams or
weirs allows control of the flow conditions
without changing the water level. This is
performed by means of gates, which are
designed such that, when the gate is fully open
(and the structure functions as if it where
fixed) the discharge has to pass the structure
without noticeable water level increase
upstream. Gate operation needs permanent
maintenance and an external energy source.
As a result, there is a risk that the gate
remains blocked during floods. Other spillway types
► Flashboards Cont’d
► Inflatable weirs Cont’d
► Fuse gates Cont’d
► Siphon spillways Cont’d.
► Shaft (or Morning glory) spillways Cont’d
► Labyrinth weir Energy dissipating
structures
► Stilling basin
► Baffled apron drop
► Plunge pool
► Chute cascades Intake structures
►A water intake must be able to; Divert the required amount of water into a
power canal or into a penstock without
producing a negative impact on the local
environment and with the minimum
possible head losses. Handle a major challenge of debris and
sediment transport. Points to be considered in the
design of intake structure
► Hydraulic and structural criteria common to
all kind of intakes.
► Operational criteria (e.g. percentage of
diverted flow, trash handling, sediment
exclusion, etc.) that vary from intake to
intake.
► Environmental criteria characteristics of each
project (eg requiring fish diversion systems,
fish passes, etc). Intake types
The first thing for the designer to do is
to decide what kind of intake the
scheme needs. These can be classified
according to the following criteria. Power intake: The intake supplies water
directly to the turbine via a penstock.
These intakes are often encountered in
lakes and reservoirs and transfer the
water as pressurized flow. Intake types Conveyance intake: The intake supplies
water to other waterways (power canal,
flume, tunnel, etc.) that usually end in a
power intake. These are most frequently
encountered along rivers and waterways
and generally transfer the water as free
surface flow. Conveyance intakes along rivers can be
classified into lateral, frontal and drop
intakes. Intake characteristics Cont’d
► The lateral intake functions by using a river
bend or by using a gravel deposition channel
► This intake favorably applies the presence of a
strong secondary current along the outer bend
of the curved river. This secondary current
prevents bed load from entering the intake.
The installed discharge Qep has to be smaller
than 50 % of the critical river discharge Qcr,
where the latter is defined as the discharge for
which the bedload transport starts. Typical layout of lateral
intake Frontal Intakes The frontal intake is always equipped with a
gravel deposition tunnel and is well adapted for
rectilinear river reaches. The deposition tunnel
has to be flushed in a continuous manner and
the maximum river width is 50 m.
A major advantage of this type of intake is its
ability to handle large quantities of both bed and
suspended load. However, this needs continuous
flushing and thus large losses of water. The
frontal intake is largely applied in regions with
very large bed and suspended loads. Cont’d.
The drop intake is generally used in steep sloped rivers,
such as torrents, and for rectilinear reaches Drop Intake Tyrolean intake
Longitudinal view Cont’d.
Lateral view Head losses
For small hydro plants, head losses can be
of huge importance to the feasibility of the
project and should thus be minimized as
much as possible. Approach walls to the trashrack designed to
minimize flow separation and head losses Piers to support mechanical equipment including
trashracks, and service gates Guide vanes to distribute flow uniformly. Vortex suppression devices. Appropriate trashrack design. Cont’d The velocity profile decisively influences the
trashrack efficiency. The velocity along the
intake may vary from 0.8 - 1 m/s through the
trashrack to 3 - 5 m/s in the penstock. A good
profile will achieve a uniform acceleration of
the flow, minimizing head losses. A sudden acceleration or deceleration of the
flow generates additional turbulence with flow
separation and increases the head losses. Cont’d A constant acceleration with low head
losses requires a complex and lengthy
intake, which is expensive. A trade-off between cost and efficiency
should be achieved. The maximum acceptable velocity
dictates the penstock diameter; the need
for a reasonable velocity of the flow
approaching the trashrack dictates the
dimensions of the rectangular section. Cont’d The research department of "Energy, Mines and
Resources" of Canada commissioned a study of
entrance loss coefficients for small, low-head
intake structures to establish guidelines for
selecting optimum intake geometry. The results showed that economic benefits
increase with progressively smoother intake
geometry having multiplane roof transition
planes prepared from flat form work. In addition,
it was found that cost savings from shorter and
more compact intakes were significantly higher
than the corresponding disadvantages from
increased head losses Cont’d Analyses of cost/benefits suggests that the
best design is that of a compact intake
with a sloping roof and converging walls,
whilst the length of the intake is unlikely to
be the major factor contributing to the
overall loss coefficient. The K coefficient of
this transition profile was 0.19. The head
loss (m) in the intake is given by: ΔH = 0.19 V2/2g Cont’d Where V is the velocity in the penstock
(m/s). Head losses due to the trashrack
depend on spacing and shape of the bars,
orientation of the trashrack compared to
the flow and eventual obstruction due to
debris and are discussed in more detail
below. Trashracks One of the major functions of the intake is to
minimize the amount of debris and sediment carried
by the incoming water, so trashracks are placed at
the entrance to the intake to prevent the ingress of
floating debris and large stones. A trashrack is
made up of one or more panels, fabricated from a
series of evenly spaced parallel metal bars. If the watercourse, in the flood season, entrains
large debris, it is convenient to install, in front of the
ordinary grill, a special one, with removable and
widely spaced bars (from 100 mm to 300 mm
between bars) to reduce the work of the automatic
trashrack cleaning equipment. Cont’d Trashracks are fabricated with stainless steel or
plastic bars. Since the plastic bars can be made
in airfoil sections, less turbulence and lower head
losses result The bar spacing varies from a clear width of 12
mm for small high head Pelton turbines to a
maximum of 150 mm for large propeller turbines. The trashrack should have a net area (the total
area less the bars frontal area) so that the water
velocity does not exceed 0.75 m/s on small
intakes, or 1.5 m/s on larger intakes to avoid
attracting floating debris to the trashrack. Cont’d Trashracks can be either be bolted to the
support frame with stainless steel bolts or
slid into vertical slots, to be removed and
replaced by stoplogs when closure for
maintenance or repair is needed. In large trashracks it must be assumed that
the grill may be clogged and the supporting
structure must be designed to resist the
total water pressure exerted over the whole
area without excessive deformation. Cont’d
Prefabricated
Booms Cont’d The trashrack is designed in such a way
that the approach velocity (V0) remains
between 0.60 m/s and 1.50 m/s. The maximum possible spacing between
the bars is generally specified by the
turbine manufacturers. Typical values are 20-30 mm for Pelton
turbines, 40-50 mm for Francis turbines
and 80-100 mm for Kaplan turbines. trash rack head losses Cont’d Cont’d Vorticity A well-designed intake should not only
minimize head losses but also preclude
vorticity. Vorticity can appear for low-head
pressurized intakes (power intakes) and
should be avoided because it interferes
with the good performance of turbines especially bulb and pit turbines. Cont’d.
Vortices may effectively
Produce non-uniform flow conditions.
Introduce air into the flow, with
unfavorable results on the turbines:
vibration, cavitation, unbalanced loads,
etc. Increase head losses and decrease
efficiency. Draw trash into the intake. Cont’d The criteria to avoid vorticity are not well
defined, and there is not a single formula
that adequately takes into consideration
the possible factors affecting it. According to the ASCE Committee on
Hydropower Intakes, disturbances, which
introduce non-uniform velocity, can
initiate vortices. Cont’d
Vortices can be initiated by: Asymmetrical approach conditions;
Inadequate submergence;
Flow separation and eddy formation;
Approach velocities greater than 0.65 m/sec;
Abrupt changes in flow direction; Lack of sufficient submergence and asymmetrical
approach seem to be the most common causes of
vortex formation.
An asymmetric approach is more prone to vortex
formation than a symmetrical one. When the inlet
to the penstock is deep enough and the flow is
undisturbed, vortex formation is unlikely. Cont’d Empirical formulas exist that express the
minimum degree of submergence of the
intake in order to avoid severe vortex
formation. Nevertheless, no theory
actually exists that fully accounts for all
relevant parameters. Cont’d
► The submersion is defined as ht Cont’d
► The following formulas express the minimum values for
Submergence (ht) Cont’d
It is important to highlight that V is the
velocity inside the downstream
conduit in m/s and D is the hydraulic
diameter of the downstream conduit in
m. Sediment traps Conveyance intakes are designed on rivers in order
to eliminate possible floating debris and bedload
transport. However, they cannot prevent the entrance of
suspended sediment transport. For this, a
sediment trap is projected downstream of an
intake. The main objective of such a trap is to avoid
sedimentation on downstream structures (canals,
shafts, etc.) as well as to limit the possible damage
of sediments on the hydro mechanical equipment. Cont’d A sediment trap is based on the principle
of diminishing the flow velocities and
turbulence. This results in a decantation of
suspended sediments in the trap This diminishing is obtained by an
enlargement of the canal, controlled by a
downstream weir Cont’d Cont’d
Efficiency of a sediment trap It is a measure of the grain diameter that
deposits in the trap. The choice of efficiency depends on the
type of hydro mechanical equipment and
on the gross head difference of the power
plant Cont’d
Reparation intervals of Francis turbines Around 6-7 years for a sediment trap
efficiency of 0.2 mm, 3-4 years for an efficiency of 0.3 mm and 1-2 years for an efficiency of only 0.5 mm.
Experience has shown that the most
economical solution is around 0.2 mm
efficiency for severe conditions
(significant gross head, quartz particles)
and around 0.3 mm for normal conditions. Cont’d
Design The necessary length of a sediment...
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