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HYDROPOWER DEVELOPMENT

BOOK NO 8

HYDRAULIC DESIGN

D.K. LYSNE

B. GLOVER

H. STØLE

E. TESAKER

CHAPTER 9

SEDIMENT TRANSPORT

AND SEDIMENT HANDLING




9 SEDIMENT TRANSPORT AND SEDIMENT HANDLING

9.1 INTRODUCTION

Sediments are fragments of rock and minerals, loosened from the surface of the earth

due to weathering processes and the impact of rain and snow, blowing winds, flowingwater and moving glaciers. When the eroded material is carried by water in motion,

sediment transport occurs. The sources of sediments are the non-organic componentof soils, fluvial and other deposits and rock.

The forces acting on sediment particles are normally split in two categories, i.e. thestabilizing forces and the destabilising forces. Sediments have a higher density than

water, and if there is no movement of the water, the sediments will remain stable onthe bottom. Gravity, in addition to cohesive forces between fine sediment particles,

may in general represent the stabilizing forces, which resists movement. The flowvelocity or the turbulence level in the fluid may likewise represent the destabilizing

forces, which tends to pick up particles and carry them with the water flow insuspension or as bed- load.

Human activities in river basins have often caused an increase in the pressure on landresources due to road construction, mineral exploration, livestock grazing, agricultureand use of the forest resources. There has, however, been a tendency to blame man for

all land-erosion and the resulting sediment transport in the rivers. It is good to protectthe forest resources and it is good to plant threes, but there is no watershed

management programme able to remove the sediments from the rivers.

It is important to remember that sediment transport is a natural phenomenon. In a

geomorphologic context, land erosion and sediment transport processes are balancingthe geological processes creating mountains. An example is the tectonic uplift of the

Himalayas cause by the Indian plate, which collides with the Eurasian plate in theHimalayan area. The mountains of Norway are still rising after the heavy icecapmelted away at the end of the last ice age.

The names of many rivers are reflecting the fact that sediments coloured the water of

the river when man named these rivers, long before the natural vegetation cover was by any means threatened. The Yellow River in China, the Red River in Vietnam, theBlack River in Nepal, the Clay River in Norway and the Mud River in USA are a few

examples.

The general land-use practice in the hills has intensified land erosion and thusincreased the sediment transport rates in many mountain rivers where hydropower

plants may be developed. It is not likely that these man-made contributions to the

erosion and sedimentation processes will be reduced during the lifetime of these power plants. The general trend is increased sediment yield and not reduced yield

even though some efforts are made through erosion mitigation measures. Hydropower planners must therefore design hydropower plants able of handling the sedimentstransported by the water throughout the lifetime of the plants.

Some basic knowledge about sediment transport processes and theory is needed as a basis for design of sediment handling facilities at hydropower schemes. The hydraulic




designer must also be familiar with what, why and how sediment data is recorded,analysed and used during planning, design and operation of hydropower plants. This

chapter will therefore give a brief introduction to fluvial sediment transport as well assediment measurement and the use of sediment data in the context of hydropower

development. The reader is advised to go to the textbooks on fluvial sediment

transport for a more in-depth study of the very complex nature of sediment transport.

This Chapter is focusing on the hydraulic design issues related to the fact that thewater harvested for hydropower generation contains sediments and that facilities for

handling of these sediments are required for successful hydropower development insediment loaded rivers. Reference is also made to Section 4.3.1 in Book no 3“Environmental Effects” in this book-series where sediment transport is addressed

briefly in an environmental context. Some terms and units frequently used in theliterature of sediment transport are given in the following table.

Table 0.1: Some definition and units used in sediment transport

Erosion rate is the amount of surface material removed from the landscape by water, ice and wind

over a year. Erosion rate or the rate of soil loss is commonly used in connection with agriculture,forestry and land use studies. The most common unit for erosion rate is kg/ha/year.

Sediment yield is the amount of sediments transported by a river or a stream through a g iven cross-

section divided by the catchment area on an annual basis. Sediment yield is commonly used inconnection with hydrological, river hydraulics and geomorphological studies. The most common unit

for sediment yield is tonnes/km2 /year.

Denudation rate is equivalent to the sediment yield, but weight of sediments divided by the area isconverted to volume of sediments divided by the area, i.e. depth of an evenly distributed (virtual) soil-

layer, which is removed and transported out of the catchment by the river annually. The mostcommon unit is mm/year.

Sediment concentrat ion is the amount of sediments in the water flow. Concentration is a relativeterm by weight and the most common unit is ppm (parts per million), which is equivalent to one mg

sediments in one kg of water and sediment mixture. In practical terms ppm is considered to beequivalent to mg/l and g/m

3.

Bed-load covers all particles that mainly move close to the riverbed by sliding, rolling and jumping.

These particles are in frequent contact with stable particles at the riverbed and the velocity of thesediment movement is much less than the velocity of the water. The most common unit is kg/s or

tonnes/day.

Suspend ed load covers all particles that mainly are carried by the water flow in suspension and theytravel with more or less the same velocity as the water flow. These particles will hit the riverbed

occasionally but they will not tend to settle and rest on the riverbed. The most common unit is kg/s ortonnes/day.

Origin of mater ial versus mo de of transport : The terms bed-load and suspended load are used to

classify sediment transport according to mode. There is, however, no distinct physical divisionbetween these two modes of sediment transport. Sediment load are also classified according to

origin. Sediments that originates from the riverbed is labelled bed m ater ial load . The bed materialload will have the same characteristics as the riverbed material with respect to petrography and sizedistribution and it may be transported both as suspended load and as bed-load. Sediments that havebeen transported over a long distance and that originates far from the site is labelled wash load .




9.2 SEDIMENT TRANSPORT IN THEORY

The theory behind sediment transport is still not completely understood or described.Empirical relations and experimental data are therefore an integral part in every

practical use of the theory. It is useful, however, to start with some general principles

of sediment motion in order to understand the practical tools that has been developedfor its calculation.

It is common practice to distinguish between two main transport modes of bed

material load, i.e. bed-load and suspended load as defined in Table 9.1. The total loadis the sum of the bed-load and the suspended load. The two modes of transport mergewithout a distinct interface. By tradition, bed- load is often defined to occupy the

lower 5 per cent of the flow depth.

9.2.1 Classif ication and descri ption of sediments

Bed materials in a river system may be of mineral, organic or mixed origin. Organicmaterial is usually found in lakes and quiescent river reaches. The content of organicmatter in mixed samples is determined as weight loss after glowing of the sample.

Mixtures with more than 30 per cent of organic matter are termed organic.

Mineral particles are classified according to their size as shown in Table 9.2. For

particles coarser than silt, the grain size is usually determined by sieving in standardsieve sets. For the finer particles, various methods are used to determine the particle

fall velocity, which is then converted into equivalent diameter, assuming spherical particles.

Table 0.2: Classification of sediment particles according to grain size (mm)

Secondary classificationPrimary classification

Coarse Medium Fine

Boulders Larger than 250

Cobbles 250 - 60

Gravel 60 - 2 60 - 20 20 – 6 6 - 2

Sand 2 – 0.06 2 – 0.6 0.6 – 0.2 0.2 – 0.06

Silt 0.06 – 0.002 0.06 – 0.02 0.02 – 0.006 0.006 – 0.002

Clay Less than 0.002

River sediments are usually mixtures of grain or particle sizes. A common way todescribe a mixture is in terms of percent finer by weight of particles with increasinggrain sizes as shown in Figure 9.1. Mixtures with a wide spread of sizes are termed

"well graded", while mixtures with a narrow range of grain sizes are termed "poorlygraded" or "uniform".

The particle size is usually denoted by d. For the identification of a particular grainsize in the distribution, its "percent finer" value is used as sub index. The size d50 is

thus dividing between equal weights of finer and coarser particles in a sample. It issometimes erroneously termed "mean" diameter. The ratio d60/d10 is commonly used

as an indicator of grading. When d60/d10 is more than 5, the material is well graded,while the material has a uniform size distribution when the d60/d10 is less than 5.




Fig. 0.1: Grain size distribution curves, examples of various grading

9.2.2 Part icle fall veloci ty

The fall velocity of sediment particles is an important parameter for the understanding

of sediment motion. The turbulent motion of the flow tend to detach and lift the particles, but the falling motion as soon as the particles are free of the bed is

counteracting the motion. Particles in suspension will move forward by the water, and be lifted by the turbulent motion while at the same time sink under the action ofgravity.

Fig. 0.2: Fall velocity of quartz spheres in water and air after Rouse [26]




The fall velocity is affected by many parameters of which submerged weight andform are the most prominent in river flow. But also temperature and viscosity have

notable effects. Figure 9.2 shows the fall velocity of spherical quarts particles instagnant water.

9.2.3 Dr ag, li ft and gravity

It is sometimes useful to deal with the stability and motion of single sediment particle.

The water exerts forces on the particle often referred to as drag and lift . The dragworks in the main direction of the flow, and the lift transversally to the flow direction.Both are in principle proportional to the square of the ambient flow velocity, u, and

the area of exposure to the flow, A, but need empirical corrections, called drag and liftcoefficients, to include effects of form and orientation of the particle. The general

formulae for drag and lift forces are:

F D = C D⋅ A⋅ρ⋅u2 /2 and F L = C L⋅ A ⋅ρ⋅u2 /2 (9.1)

F stands for force and C for correction coefficient, while D and L stand for drag and

lift.

The third element in the stability analysis for single particles is the gravity force,which in suspended transport is balanced by the forces of the turbulent current, and in

bed load motion also causes resistance due to friction against the stationary bed.

9.2.4 Shear stress and turbulence

It is impossible to deal with each single particle when the transport is substantial and

includes a mixture of particle sizes. Sediment transport theory therefore operates with shear stress and turbulence as determining factors for the bed in general.

The shear stress is the average force per area exerted by the water on the bed. Theshear stress depends on the rate at which the velocity changes from zero at the bed

towards the free flow above the bed. In natural flows, the shear stress is the result ofturbulence, transferring momentum towards the bed. In viscous flow, the shear stress

is a direct effect of the viscous character of the fluid adhering to the bed particles.

True viscous flow is rarely found in natural watercourses.

Turbulence is defined as irregular flow motion resulting from eddies that are carried by the flow and swirling in an irregular manner. New eddies are continuously formed

by the shearing action inside the flow and against the bed, while other eddies dissipateinto heat due to viscous and boundary friction. Due to turbulence the flow velocity ata point is fluctuating around its average value, and the bed shear stress is fluctuating

in a similar manner. This is important for the stability of bed particles against motion,since it is the shear stress peaks of the fluctuating flow that determine whether a

particle may be moved or not. The more turbulent a flow is, the larger particles can bemoved by a given average shear stress.




Direct measurement of turbulence is impossible in practical cases. But turbulenceaffects the velocity distribution near the bed. If the average velocity in two points near

the bed is known, it is possible to assess the effect of turbulence and calculate the bedshear stress by use of the two formulas:

)log(/)(17.0 2121 z z uuu −−=∗ (9.2)

wu ρτ ⋅= ∗2

0 (9.3)

u∗ is a fictive parameter labelled "shear velocity", u1 and u2 are the two measured

velocities, z1 and z2 are the corresponding distances from the bed, τ0 is bed shear

stress, and ρw is the density of water.

In uniform flow, i.e. when bed and surface are parallel, the bed shear stress is found

directly by combining slope, S and hydraulic radius, R (R = A/P, the ratio betweenflow cross section A and wetted perimeter P):

S R g w ⋅⋅⋅= ρτ 0 (9.4)

9.2.5 Star t of motion

Shields combined expressions for the destabilising forces drag and lift against weight

or friction as the stabilising force into a general formula for the equilibrium of particles:

d g C w s

s ⋅⋅−= )(

0

ρρ

τ

(9.5)

The famous Shields' diagram, Figure 9.3, relates this parameter to a so-called particle

Reynolds' number Re = u∗ ⋅d/ ν, where u∗ is the shear velocity, d is the grain size, and ν

is the kinematic viscosity of water. The curve was found by experiments, using particles of different densities. It is therefore valid also for other materials than

ordinary rock and other fluids than water.




Fig. 0.3: Shields' diagram for start of motion, adopted from [26] and [27]

Values of Cs below the curve indicate stability against motion. Values on the curveindicate start of motion and thus labelled critical Shields number, Cc. The

corresponding shear stress, τc is labelled critical shear stress.

The diagram is made for uniform sediments, but may be used for mixed sedimentswith good accuracy by using the d60 (60 percent finer) for d. For practical use, Cc =

0.06 can be used for grain diameters larger than 1 mm.

9.2.6 Bed-forms in rivers with movable bed

When the limit for start of motion is exceeded at a granular riverbed, some particles

will start moving. The bed will gradually develop into undulating forms, which willadjust their size and shape dependent on the particle size and the shear stress. When

the limit of motion for fine sand is barely exceeded, small triangular ripples will firstform, moving slowly in the direction of the current. The ripples are the first in a seriesof bed forms that appear for increasing shear stress. Referring to Shields' diagram,

figure 9.3, the size of the bed-forms increases with increasing Cs. The ripples anddunes are mainly involved with bed load motion, but as the forms grow larger with

increasing flow turbulence, the forms contribute to the interchange of material between bed and suspended transport.

Bed forms do not form in gravel-bed rivers and rivers with coarser bed material. If theriverbed is flat, the flow resistance can be estimated based on Stricklers formula for

Manning number n = d901/6/26. M = 1/n is used in many countries.




9.2.7 Erosion and deposit ion

Erosion is what results if the current is able to carry more material away from an area

than is brought into it. On the other hand, if the sediment transport into an area is

larger than the current can carry onward, deposition will occur.

We may distinguish between general and local erosion. General erosion is the gradualdegradation of the ground over large areas due to precipitation, overland flow and

wind. This is the main source of the sediment transport in the rivers. The gradualdeepening of river valleys is also a type of general erosion. Typical places subject tolocal erosion are:

• Downstream of dams and sills obstructing the natural sediment transport

• In reaches with loose bed material downstream of reaches with naturally solidrock or protected bed

• Next to local lateral constrictions, e.g. due to groins, bridge pillars, roadembankments, or reclamation areas

Typical places were deposition is likely are:

• In reservoirs, pools, and lakes

• Downstream of expanded flow cross sections

9.2.8 Concentr ation of parti cles in suspension

The fall velocity of the particles is important for the particle concentration insuspended transport and for the pattern and rate of deposition e.g. in lakes orreservoirs. Particles in suspension tend to settle due to the gravity force, but are held

in suspension by the upward components of turbulence. The turbulent motion hasequal components of upward and downward motion, however. To compensate for thefalling motion due to gravity, the upward motion in turbulent flow must carry more

particles than the downward motion. In a stable suspended flow, therefore, theconcentration of particles must decrease upwards. The concentration gradient is

affected mainly by the fall velocity of the particles and by the turbulence intensity:

• The smaller the suspended particles are, the less is the fall velocity, and the moreuniform is the vertical distribution.

• The more intense the turbulence, the more uniform is the vertical distribution.

Hunter Rouse [29] developed a formula for the distribution of concentration c at any

level y above the bed in terms of the concentration ca at level a and water depth d:

z

a ad

a

y

yd

c

c

−

⋅−

= (9.6)

The exponent z = w/(β⋅k ⋅u∗) where w is the particle fall velocity and u∗ is the shearvelocity, while the constants β≈1.0 for fine particles and k ≈0.4 for clear fluids. Since




the formula requires a known value ca it cannot be solved in general. But Rouse presented a diagram showing the relative distribution for various values of z as shown

in Figure 9.4.

Fig. 0.4: Distribution of suspended sediment concentrations, adopted from [28]

9.2.9 Calcul ation of sediment transport

a) Bed-load transport formulae

Many research institutions have developed formulas for calculation of bed-load

transport. The first attempts combined empirical observations with simple theory andarrived at formulae suitable mainly fo r conditions similar to those that supplied theobserved values. A few have survived and may still be used for making simple

assessments of the required volume of bed-load traps and accumulation of coarsematerials in intake ponds. Here only two will be mentioned. Shields' formula is basedon Shields' diagram and is easy to use when the shear stress is known.

50

2

0

)/)((10

d S q g

ww s

c

s ⋅−−

⋅⋅⋅=ρρρ

ττ (9.7)

The Meyer-Peter and Müller formula was developed to fit data from steep flumes, and

is therefore useful in many hydropower cases with steep rivers.




( ) ( )

( )( )

3

3/23/1

50

'

/25.0

047.0/

−⋅⋅⋅−⋅−⋅⋅⋅⋅

= s s w s

w sw s

d g k k RS g g

ρρρρ

ρρρ (9.8)

gs is the bed load by weight per unit of time and width (e.g. N/(m⋅s))q is the unit discharge of water, i.e. flow per metre widthS is the slope of the energy line

k/k’ is a bed-form correction of the bed-frictionk/k’=1 for flat bed and k/k’=0.5 for a rough bed due to bed-forms etc.

ρs and ρw is the density of particles and water respectively

τ0 and τc is the bed shear stress and critical shear stress respectively

b) Calculation of suspended load

Suitable formulas for direct calculation of suspended load are not available. Formula(9.6) needs at least one reliable sediment concentration observation from sampling.

The formula includes the fall velocity of particles and some constants, which arenormally not available.

If sufficient sampling data are available, it is possible to apply formula (9.9) tocompute the suspended load Qs passing the area A at the time of sampling, where c is

the concentration of suspended sediments and v is the ve locity in the same point.

∫ ⋅⋅⋅= A

s dydz z yu z ycQ ),(),( (9.9)

If enough samples exist over the time period T, the total load QST is found by (9.10).

∫ ⋅=T

sST dt QQ (9.10)

Collection and analysis of suspended load samples is very time consuming, and

requires skilled personnel for good results.

c) Total load formulae

A simple and rather accurate way to estimate the sum of bed load and suspended loadis to use a method for total load calculation. Several methods exist. Engelund and

Hansen presented a reliable formula in 1967.

( )

3

50

0502

/)(05.0

⋅−⋅

⋅−⋅

⋅⋅⋅⋅=d g g

d U g g

w s

w sww s ρρ

τ

ρρρρ (9.11)

Where U is the average velocity in the cross section. The other parameters have beendefined above. This formula is recommended for riverbeds where d50 > 0.15 mm and

the standard deviation of grain size distribution is less than two. It is of course a

condition that sediments are available.




d) Relation between bed load and total load

The diagram in Figure 9.5, originally prepared by E.M. Laursen [30] shows therelative proportion of bed-load to total load. It is seen that bed-load is the dominating

transport mode as long as the fall velocity of particles is greater than the shear

velocity u∗ (defined above). This is the case when the bed particles are coarse inrelation to the flow velocity.

On the other hand, in rivers with fine bed material and strong currents, the bed-loadusually represents a small fraction of the total load. The arrows in Figure 9.5 show

that if u∗ /w = 6.6, the bed-load part of the total load is 16/400, i.e. 4% of the total

load.

Fig. 0.5: Relation between bed-load and total load, adopted from [28] and [30]

9.2.10 Morphology of natural r ivers

Transport, erosion and deposition vary along the river with the local conditions. Whilethe transport capacity of the river changes with slope and discharge, the actual

transport may be limited by the local supply of transportable sediments from thewatershed through tributaries and overland erosion. Figure 9.6 presents schematically

four typical processes that may characterise sections of a river course from source tooutlet:




1 Erosion: The river can transport more bed-material than the watershed cansupply. The bed and slope of the river will be controlled by bedrock, and

sediment deposits will only be found in local sheltered places.

2 Transport: The river transport capacity matches with the supply of sediments.

The river has a stable bed and it is then said to be in regime.

3 Deposition: The river cannot transport all the bed-material tha t is supplied.The surplus material will form shifting bars, channels and islands. The river is

unstable.

4 Delta development: The transport capability is low compared to the supply.

Sediment deposits constrict the river channels, and new distributor channelsdevelop as supplements or substitutes.

Fig. 0.6: Classification of river reaches after [31]

In normal topography all four conditions may in principle occur consecutively frommountain to outlet. In many cases, however, intermediate reaches with erosion and

deposition as well as inland delta formation may be found. Lakes usually represent breaks in and repetition of the normal sequence, initiating deltas at the upstream end

and eroding conditions below the outlet.

If a river runs through areas where the sediment supply or the original sediment bed

has a graded composition, the critical shear stress will vary from particle to particle.Due to the turbulent character of the flow, both fine and coarse particles will have

some probability of movement, but fine particles will have a much larger probabilityfor movement than coarser particles. This leads to a sorting of the particles at thesurface of the riverbed.

During time, the sorting process will remove a larger part of particles with critical

shear stress less than the average shear stress of the flow, while most of the coarser particles will remain in place or move infrequently. The original well-gradedcomposition of the bed will then change near the bed surface into a bed characterised

mainly by the coarser fractions of the original material. This bed will generally resista larger shear stress than a bed of the original composition. Small spatial

rearrangements of the armour stones by the flow may eventually result in a ratherorderly arrangement of stones in an interlocking pattern with more strength against

scour.




The armour layer will only have a thickness of a few diameters of the coarsest stones.Extreme floods may therefore now and then break up local parts of an established

armour layer, but it will usually recover quickly. Due to this natural armouring, astable riverbed may be established with a much steeper slope than corresponding to

the average grain size of the ground material.

9.2.11 Morphological changes due to hydropower

In the context of hydropower development, regulation of a river system often causesnotable changes in the sediment situation. Diversion of water from a river may causeshift from stable to unstable riverbed downstream of the diversion, because

proportionally more water than sediments is usually diverted. Downstream of a tail-water outlet, on the other hand, stable situations may change to eroding, particularly if

the outlet is in another river.

A dam will, like a natural lake, trap most of the sediments, and the water passingthrough spillways and bottom sluices may in many cases cause severe erosiondownstream of the dam in reaches where the original bed consisted of sediments in

equilibrium or unstable. An example of predicted erosion downstream of a proposeddam in Rufigi River in Tanzania is shown in Figure 9.7.

Fig. 0.7: Predicted erosion downstream of proposed dam in Stiegler's Gorge [32]

Surface sediments of riverbeds with nearly uniform grain composition, will move

whenever the critical shear stress for the representative grain size is exceeded, butwithout significantly changing the character of the bed material. The bed will erode if

a stable transport situation has not already been obtained. The natural reaction toerosion is a degrading of the bed, both in order to add to the transport towards thecapacity for the situation, and in order to reduce the slope until the actual sediment

transport becomes equal to the transport capacity corresponding to the new slope.




9.3 SEDI MENT DATA NEEDS FOR HYDROPOWER

The Norwegian based International Center for Hydropower and the Nepal basedHydro Lab Pvt. Ltd arranged a Sediment Workshop for South-Asia with the title:

Sediment Management for Successful Hydropower Development in September 2001.

Some sections from the opening address by Haakon Støle serves as an introduction tothis Section.




9.3.1 Sediment Data

Some information on sediment data is needed in connection with most riverengineering projects. In connection with planning and design of a hydropower plantthere are some basic questions related to sediment data, which must be answered.

• What type of sediment data is needed?

• How to get high quality sediment data?

• How to make use of the sediment records during planning and design?

It is important to be able to address these questions when:

• The terms of reference for a study is made

• A study team is put together and a time schedule is made

• The structure of a pre-feasibility, feasibility or final design report is made

• The hydraulic design of the project is done

• The tentative operation and maintenance plan is made

It is also important to collect sediment data during the operation phase in order to

optimise the production and document the actual sediment load, which has passedthrough the turbines as an example. Without any documentation, the owner has no

basis to address unexpected high rates of sediment- induced wear with the turbinesupplier at the end of the defects liability period. The supplier will always claim that

Hydro-metrological networks are built and maintained as a basis for assessing the water

resources potentials for food and energy production. We are gauging river flows toassess the availability of water for power generation. We collect flood data for design othe headworks and to obtain the necessary safety of the dams and the people living

downstream of them. Sediment gauging, however, seems to obtain much lesserattention than flow gauging. Why do we spend so much less money on obtaining goodand reliable sediment data records than flow data records?

We must agree that sediments have a flavour of problems and costs in the context ofhydropower development. Sedimentation of reservoirs and sediment-induced wear of

structures and turbines are well known problems from hydropower schemes inHimalayan River Basins. This may partly explain why the water resources authoritiesshow much less interest to systematic record sediment data than river flow data.

Sediment gauging is not easy and it is not free of costs. Analysis of sediment recordsand prediction of sediment yield are far from being an easy and precise science.

Sediment data must be collected systematically over time in order to facilitate planningand design of a storage scheme. I believe it will be almost impossible to obtaininternational finance for a seasonal water storage scheme in this region in the future if

we don’t have reliable and good sediment data records. Likewise it will be impossible toattract serious and sound private investors or developers to a project in lack of sedimentdata. Private developers will not collect sediment data for 10 to 15 years before they

actually know if they have an attractive project or not.

A reservoir scheme without a reliable estimate of the lifetime of the reservoir will

financially be too risky for private developers. The risk will not be less for projectsdeveloped by public funds partly through the World Bank, Asian Development Bank orvarious international donors. Long and reliable time-series of sediment data are

therefore needed to reduce the risk, to obtain loans and to attract private developers.




the sediment load has been higher than specified in the tender documents if reliabledata records do not exist. The main variables and parameters are:

• Concentration of suspended sediments in the water flow

• Particle size distribution of suspended sediments

• Bed-load transport rates• Particle size distribution of the bed-load

• Particle size distribution of the riverbed material / armoured layer

• Mineralogical and petrographical composition of the sediment load, i.e. in practical terms the content of hard minerals like quartz, garnet and feldspar.

• Content of organic matter in the suspended load

• Density of deposited sediments

Combining the primary sediment data listed above with hydrological and catchment

data, the following sediment load and sediment yield parameters may be computed.

• Daily, monthly and annual sediment load

• Sediment yield and denudation rate

The sediment transport patterns of mountain rivers are indeed complex, and ourunderstanding is limited. The rivers are often running through various climatologic

and topographic reaches from high up in the mountains were glaciers transform intomountain torrents flowing on the bedrock or as a “step-pool stream” where large

boulders and bedrock lines the waterway. Further downstream we tend to name themountain river a “boulder-bed river” to be followed by “gravel-bed river” before itflows gently on a bed of sand and silt. Only then it will behave according to the

classical science of river hydraulics and sediment transport. Hydropower plants areoften located upstream of the river reaches where the theories, the observation

techniques and the empiricism of river hydraulics are developed.

The size of material transported by the river will often vary from fine clay particles

originating from the glaciers to large boulders of tens of tons, but of more local origin.

The sediment load in a river varies largely from year to year. The fluctuation in theannual sediment load is much larger than the variations in water runoff. Reliable

prediction of sediment yield based on short time-series of data is therefore not

possible.

There are furthermore large seasonal variations in the sediment load. The major partof the sediment load is transported during the flood season. High sedimentconcentrations must, however, be expected during relatively small off-season floods

caused by local torrential rains.




Depth-integrating sediment sampler with additional

sinker-weight for cablewayuse at Yebesa in Mo Chhu

River, Bhutan. The sinker-

weight prevents samplingclose to the bottom. The

cableway is operated from therive bank and the sampler is

of the Swedish type. (Photo H.Støle)

Fig. 0.8: Depth-integrating suspended sediment sampler

The sediment supply to steep rivers is guiding the amount of sediments transported bythe river, and not the sediment transport capacity of the river. Mass wasting (mainly in

the tributaries) plays a more dominant role in the pattern of the sediment yield in theupper than the lower reaches of the rivers.

Sediment sampling in mountain rivers is difficult and the available data are oftenquestionable with respect to the data quality, partly due to shortcomings in the

available sediment measurement techniques. The correlation between water flow andobserved suspended sediment concentration is often poor. Water flow is therefore not

a reliable parameter to determine the sediment concentration in the river. As anexample, we do not know of any gauging station in any Himalayan River upstream ofthe plains, from Pakistan via India and Nepal to Bhutan, where a reliable and

consistent sediment rating equation has been observed when a reasonable samplingfrequency has been applied. The operation of the headworks of hydropower plants

with respect to sediment exclusion can therefore not be guided by the water flow onlyas shown in Figure 9.9.




Fig. 0.9: Concentration and discharge plot from Seti River, Nepal [31]

9.3.2 Needs for Sediment Data

There are two important issues the reader must look for when reviewing the content ofa feasibility report with respect to sediment data:

• Is the presented sediment data representative?

• How is the available sediment data used?

Like in all data collection programmes, the applied sediment data must berepresentative for the water, which will enter the intake at a run-of-river hydropower

plant or the reservoir at a storage scheme. The sampling method must produce data,which gives a true picture of the sediment transport pattern in the river over time.




The needs for data and the type of data required vary with the type of project studied.

In sediment- loaded rivers, we tend to distinguish between reservoir schemes and run-of-river schemes when it comes to sediment data and sediment handling. In principal,

a reservoir scheme must provide storage for all incoming sediments during the

lifetime of the plant while a run-of-river scheme must allow all sediments carried bythe river to pass the headworks structures to secure undisturbed operation of the

power plant.

a) Reservoir Schemes

A sediment study for a reservoir scheme shall focus on the following needs:

• Maintaining water storage capacity

• Obtaining acceptable and predictable operations and maintenance costs

The study shall first of all produce a reliable long-term average sediment yieldestimate so the predicted lifetime of the reservoir becomes as accurate as possible.Day to day variations in the sediment concentrations is of less interest. In order to

study the deposition pattern and the gradual loss of storage capacity that will take place, it is also important to collect information on the particle size distribution and

the density of deposits over time.

It is important to remember that the ratio between the 100-years flood, Q100 and the

two-years flood, Q2 may be in the range from 5 to 10, while the ratio between the100-years and the two-years annual sediment load may be 100 or more. A few years

of local sediment data records will therefore not be sufficient to plan a reservoirscheme where the lifetime of the reservoir is an important parameter with respect tothe profitability of the scheme.

When the

water levelin thereservoir

varies thedeposits

will be

eroded andre-

deposited several

times. It isa complex

study to

simulatethe deposition pattern in a reservoir. The density of the deposits will change over

time. It is also dependent on if the deposits will remain submerged or if they will beconsolidated through dewatering during drawdown or not. (Photo Haakon Støle)

Fig. 0.10: Deposition in Kulekani Reservoir, Nepal




Figure 9.11 shows that the actual observed sedimentation in some Indian reservoirs

were several times higher than what was predicted prior to construction of the dams.This is probably not only reflecting poor quality sediment sampling, it is also

reflecting the complex nature of sediment transport in these rivers.

Fig. 0.11: Predicted and observed sedimentation in some Indian reservoirs [33]

Documentation of actual deposition rates in existing reservoirs and lakes are a major

source of information in connection with planning of new water storage schemes.

b) Run-of-River Hydropower Plants

A sediment study for a run-of-river hydropower plant shall focus on the following

needs:

• Good performance of the headworks structures

• High regularity in the power production

• Acceptable and predictable operation and maintenance costs of the headworks,

the waterways and the hydraulic machinery

In order to optimise sediment exclusion at a run-of-river project it is necessary to have

sediment data, which includes the short-term variations (from day to day) as well asthe long-term variations (from year to year). A time-series of reliable sediment and

flow data with high sampling frequency is required. The study shall produce realtime-series of suspended sediment concentrations and river flows, with sufficient highfrequency to give a true picture of the natural variations we will experience during

normal operation of the hydropower plant.




Representative time-series of sediment and flow data facilitates simulation of normalwet-season situations, which will be crucial for assessing the performance, regularity

and operation & maintenance costs of the plant. A sediment-sampling programme in amountain river tailor-made for a run-of-river project should therefore have a high

sampling frequency of one sample or more per day. The data records will then

preferably contain information on short-term variations in the sediment load as well asthe long-term trends.

The content of hard minerals and particle size distributions of suspended sediments

are needed to compute resulting sediment exposure of the turbines and the amount ofsediments excluded from the withdrawn water in the settling basins.

At a run-of-river plant there is no income related to extreme floods, because theduration of these events are so short. The power plant should the refore not from an

economical point of view be designed to remain in full operation during floods orsediment concentrations with return periods more than 2 to 5 years. The costs (wear

and down-time during repair) will normally be many times the benefits earned fromcontinued operation during these short events with very high sediment concentrations.The plant shall, however, be designed so it survives these floods as well as the design

flood.

9.3.3 Use of Sediment Data

The main sources for sediment yield studies are time-series of sedimentconcentrations and river flows and observed sediment deposition in lakes andreservoirs in the region. If the time-series of daily suspended sediment concentrations

are in the range from five to ten years, it is possible to develop useful sediment ratingequations where a longer time-series of river flows are used to compute the resulting

influx of suspended sediments over the years as long as:

• The suspended load covers 75% or more of the total load

• Mass wasting plays a minor role in the sediment supply

• The size of the bed material particles and the transported particles are within

the same order of magnitude

The difference between bed-load and suspended load was addressed in section 9.1. It

does also make sense to distinguish between measured sediment load and unmeasuredsediment load. Large sediment loads may pass a sediment sampling station un-gaugedduring floods due to various limitations in the sampling technique or between regularsampling hours. When the flow velocity is high, it may be impossible to obtain water

samples, which are representative for the entire cross-section of the river. It isimportant to address these issues as a part of the sediment yield prediction process.

In addition to sediment yield, density of deposits and particle size distributions areneeded to carry out reservoir sedimentation studies. The process of a reservoir

sedimentation study is addressed in Section 9.5.

There are surplus of water available in the river during the wet season when the riverin periods carries high sediment loads. But there is no room available at a run-of-river




plant where partly cleaned water can be stored for peaking purposes. The waterabstracted from the river must therefore be cleaned simultaneously with the power

generation. The project's capability to handle high sediment concentrations in theabstracted water without interference with the power generation will determine the

regularity of the energy production at the project during the wet season. Several

unplanned close downs (full or partial) will give a low regularity even though theenergy losses may not be significant due to relatively short duration of each close

down.

The variations in sediment content, as well as particle size distribution andmineralogical distribution of the instantaneous sediment load, determine the amountof sediments which will be trapped by the sediment handling facilities and the amount

which will pass on to the hydraulic machinery in the power house. If the power plantshall operate with a high regularity, the headworks must be able to deal with most of

the short-term variations in the sediment concentrations. Hence, handling largevariations is equally important, as coping with the variations in the particle size

distribution of the suspended load.

The collected data will be used in a series of simulation of the operation of the

headworks of the plant under various settling basin arrangements. The main objectivewith the simulations of the headworks performance is to provide information and datafor the detailed optimisation of the settling basin design. The simulations link the

design process to expected performance during the operation of various alternatives,and thus cost figures during the operation phase, not only the construction costs.

Depending on the selected settling basin arrangement, it will be required to closedown the power plant partially or fully from time to time due to various reasons. The

performance study of the settling basin arrangement at Mangde Chhu hydropower plant in Bhutan is used as an example. The sediment data record applied in the

feasibility study had a sampling frequency of one sample per day. The performance ofthe basins was tested for each 24 hours period according to the four following criteria,which all will disrupt power generation more or less, and thus reduce the regularity

and the production:

• Too high concentration of suspended sediments in the water released fromthe settling basins for power generation. If the concentration exceeds theadopted upper limit of 3 000 ppm, the plant is assumed closed down.

• The amount of sediment load trapped in the basin exceeds the sedimentremoval capacity of the basins.

• Passage of bed load and flushing of the pond. The plant is assumed closeddown when the flow goes higher than the cut-off level of 300 m3/s, refer

section 9.6.2.

• Reduced energy production during the time of flushing for the alternatives,

which have to reduce the generation during flushing in order to maintainthe trap efficiency.

Based on these simulations, the effect of varying the size of the basins as well as

adoption of various flushing system were studied.




9.4 SETTL ING BASIN DESIGN

The objective with a settling basin is to reduce the turbulence level in the water flowto allow suspended sediment particles to settle out from the water body and deposit on

the bottom of the basin. The deposits are then removed from the basin by use of the

flushing system or through excavation if the amount of sediments is small.

Settling conditions are obtained by reducing the transit velocity of the water so theeffect of gravity increases relative to the effect of the turbulence. The suspended

particles will not follow the movement of the water because the fall velocity of the particles will create a flux of sediments downwards. The transit velocity in a settling basin will normally be in the range of 0.1 to 0.4 m/s, depending on the design criteria

and to some extent the size and shape of the area available for settling basins. At anearly stage of planning a transit velocity based on the net flow cross-section of 0.2 m/s

is normally adopted.

The performance of a settling basin is guided by its ability to trap suspendedsediments and its ability to remove the trapped deposits from the settling basins, i.e.the qualities of the adopted sediment flushing system.

The settling basins

at the 60 MW Khimti hydropower

plant in Nepal are furnished withSerpent Sediment

Sluicing System(S4) for removal of

trapped sedimentswhile the basinsremain in normal

operation.(Photo Haakon

Støle)

Fig. 0.12: Settling basins at Khimti hydropower plant

9.4.1 Design Cr iteri a

The coarser fractions of the suspended load shall be removed from the abstractedwater in the settling basins in order to fulfil the design criteria with respect to

sediment exclusion. It will never be possible to trap all suspended sediments in asettling basin, as the fall velocity of suspended silt and clay are to small compared

with the turbulence level in the settling basins. However, most of the sand fractions ofthe suspended sediments shall be excluded from the abstracted flow in order to:

• Maintain the hydraulic transport capacity of the waterways

• Reduce the sediment load to the turbines, valves etc.• Obtain the required power generation regularity




The parts of the hydraulic machinery of a run-of-river hydropower plant exposed to

the flowing water will always be subjected to sediment-induced wear. The higher thevelocity of the water is, the greater damage will occur. The material erosion rate of

steel subjected to water containing quartz is found to be proportional to the sediment

load and the velocity of the flow into the power of 3 to 4. This implies that thematerial erosion rate of the nozzle of a Pelton turbine (or most parts of a Francis

turbine) will be proportional to the head of the power plant into the power of 1.7 to 2.High head power plants are therefore extremely susceptive to sediment erosion.

Leakage water from a Francis turbine is passing through the gap between the runnerand the cover plate. This gap is normally about 0.5 mm wide and a function of

accuracy in the production, efficiency and the amount of leakages. Sediment issuesare normally not addressed when this gap is determined, except when it comes to the

selection of materials. Brass may be used for sediment free water, but stainless steelshould be used when there are sediments in the water. It is recommended to prevent

all angular quartz particles larger than 0.3 to 0.4 mm from entering into this gap asthey may cause severe damage. 0.4 mm may therefore be used as an upper limitedwith respect to size of particles, which may be released from the settling basins. This

will, however, in most cases not be the optimum criterion for sediment exclusion at ahigh head run-of-river hydropower plant.

There are many factors like various properties of the sediments, characteristics of theturbine and its operation, which are affecting the resulting sediment- induced wear of

the turbine components. It is, however, outside the scope of this book to discuss thesefactors in details. The hydraulic engineer involved in planning and design of a run-of-river hydropower plant should know that the main factors related to turbine wear are:

• The hardness of the sediments (i.e. practically the quantity of quartz)

• The total sediment load through the turbine

• The velocity of the water through the turbine

• The operation regime of the turbine

Out of these four factors, the design of the settling basins at the headworks is onlyaffecting one, i.e. the total sediment load through the turbine. The bigger the settling

basins are the smaller sediment particles are trapped and the majority of the totalsuspended sediment load is trapped in the basins.

The first rule for exclusion of suspended sediments is that most of the suspended sandshould be removed from the abstracted flow in settling basins. Most of the suspended

silt and clay will, however, pass through the waterways and the turbines of the power plant. Where there is hard sediments and a power head of more than 50 metres, therewill always be sediment-induced wear of the turbines.

The main parameter with respect to wear of turbines at high head run-of-river

hydropower plants is the total amount of sediment load (quartz) passing through theturbines and not the size of the individual particles in the sediment load. Standarddesign criteria for settling basins are, however, linked to the basins’ ability to trap

particles of a given size. It is relatively easy to prove that a design is satisfying a trapefficiency criterion, which is a more or less direct function of the size of the settling




basins. As the sediment exposure design approach is dependent on the complex natureof the sediment transport in the river, it is much more difficult to satisfy a set of

design criteria for the settling basins (tonnes of sediment load per overhaul andreplacement of various components). This requires sufficient reliable and

representative sediment data as well as information on the operation regime of the

power plant. Simulations of the operation of the plant are therefore needed in order todetermine the optimum size of the settling basins as well as the most cost-effective

operation regime for the plant.

The optimum sediment exclusion level at a run-of-river project is therefore dependenton the sediment load characteristics of the river as well as the features of the projectitself. An optimum sediment exclusion study shall minimise the total investment and

operation costs throughout the lifetime of the project of the four components listed below:

• Increased capacity to resist sediment-induced wear through improved turbine

technology, so called “silt-friendly” turbine design. This involves design,manufacture and installation of the turbines and replacement of runners,guide-vanes etc. Various coating techniques may increase the turbines initial

resistance to wear, but it may reduce the possibilities to carry out an efficientoverhaul programme later on.

• Increased capacity to resist sediment-induced wear through an efficientoverhaul and maintenance programme for the turbines. This aspect must

include generation losses due to necessary down time for maintenance andreplacement of components in addition to the investment and operation costs

of the required turbine maintenance facilities.

• Reduced sediment exposure through improved exclusion of sediments in the

abstracted water through the settling basins. Increased size of settling basinswill improve trapping and efficient sediment removal systems will reduce

generation losses associated with operation of the settling basins.

• Reduced sediment exposure through sediment guided operation regime basedon real time sediment monitoring. The sediment concentration in steep rivers

varies with time. The concentration goes from time to time very high sayabove 10 000 ppm. The sediment load in the water released from the settling

basins will then be high even if the settling facilities have high trap efficiency.There is most probably a level at each high head run-of-river plant where thecost of continued generation is higher than the benefits due to excessive

sediment induced wear and resulting maintenance needs and costs even ifrationing costs are introduced.

It is recommended to split the flow in two or more so the number of settling basins areat least two. This will enable dewatering of one basin during the dry season for

inspection and maintenance of the flushing system etc. without effecting the operationof the plant. The entire flow to the power plant will then be channelled through theremaining basin(s) as the concentration of suspended sediments in the water is low

and there are no needs for cleaning of the water in this situation.




9.4.2 Trap Ef fi ciency

The trap efficiency of a settling basin is mainly governed by the geometry of the basin, i.e. size and shape. A larger basin will facilitate exclusion of more suspended

load with respect to volume and particle sizes. The shape of the basin is important

with respect to the flow distribution in the basin. A good shape will produce an evenflow distribution in the basin and thus maintain optimum trap efficiency in the basin,

while a poor shape will produce unfavourable flow conditions and thus reduce thetrapping ability of the basin considerably. The main component of a typical settling

basin arrangement is shown in Figure 9.13. Note that the effective length and theuniform length is not the same. The effective length is the corresponding length to a

basin with the same cross-section area and complete uniform flow distribution over

the cross-section of the basins.

Fig. 0.13: Settling basins – definition sketch

The hydraulic design of a settling basin arrangement shall secure:

• An even flow distribution between the settling basins for various flows

• An even flow distribution internally inside each basin for various flows• Efficient removal of deposits during flushing of the basin

The flow distribution between the basins may be secured at the inlet or at the outlet ofthe basins. It is often convenient to have a small head- loss at the outlet of the basin tosecure even outflow from the basins as well as even outflow over the width of each

basin. A slotted outlet has proven to be effective in this respect. A small horizontalslot (about 0.5 meter high) over the width of each basin close to the surface will also

secure filling of the basin before water is diverted to the downstream waterway.Sediment trapping will then take place during filling of the tunnel, and prevent sandfrom passing through the basin during the time it takes to fill the tunnel. The slotted

outlet will also give the hydraulic engineer freedom to accelerate the flow fast and




turn it in any direction downstream of the basin without any effect on the flowdistribution inside the settling basins.

The main challenge for the hydraulic engineer is to design the geometry of the inlet

transition so the flow is evenly distributed over the width and the depth of the basin in

the settling part, often referred to as the main body of the basin with uniform width. Itis difficult to obtain an even flow distribution in a water body with low velocities and

low friction losses. An uneven flow distribution in the upstream end of a settling basintends to remain uneven throughout the basin. An uneven flow distribution is a settling

basin will reduce the trap efficiency drastically compared with a basin with even flowdistribution.

Fig. 0.14: Inlet transition with guide-walls

A symmetric layout is preferred. The approach canal should preferably be straight for

a length of ten times the width of the canal upstream of the start of the expansion inorder to avoid the effect of secondary currents (rotation flow) set up by a bend in theapproach canal. The expansion should be symmetric and smooth in order to prevent

that the flow separates from the sidewalls and the bottom of the inlet transition. It is possible to prevent separation if the opening angle of the inlet transition is less than

10 to 12 degrees if a generous curvature is applied. This narrow opening angle will,however, make the inlet transition very long. It is possible to shorten the inlettransition by carefully applying guide-walls in the transition so the opening angle

between two walls is small enough to prevent separation. The downstream end of theguide-walls must then be carefully shaped so the water velocity at the outlet of the

transition is not pointing towards the sidewalls, but parallel with the longitudinal axisof the settling basin.

A symmetric layout has the great advantage that it will secure optimum distribution ofthe water in the settling basin for all ranges of flows through the basin. It is also

possible to have a skewed inlet, which will compensate for a skewed approach flowcaused by a curved approach canal. The resulting flow in the downstream end of theinlet transition may then be evenly distributed over the cross-section for the design

flow, but most likely unevenly distributed for a different flow.

The topographical features of the headworks do not always favour a symmetric

design. In order to overrule the secondary currents generated through a bend, the flowmay be accelerated downstream of the bend in a pressurised canal and then carefully




retarded again to obtain even flow distribution. The best way to obtain the optimumhydraulic design may then be by use of a physical hydraulic model. The scale should

normally not be less than 1:25 due to the low transit velocities in the settling basin.

It is possible to replace the long and gentle inlet transition with a flow tranquilliser.

The tranquilliser is a sort of a filter where the flow is distributed over a cross-section by use of a head-loss as shown in Figure 9.15. The construction costs may be reduced

by use of a tranquilliser, but the associated head-loss in the range of 0.15 to 0.25 mmust be capitalized over the lifetime of the plant. It is also important to prevent trash,

floating debris and gravel from clogging part of the tranquilliser. The optimum flowdistribution effect of a tranquilliser is linked to the design flow. The distribution effectwill be less optimum for lower flows.

Fig. 0.15: Flow tranquilliser at the inlet to the settling basins at Modi hydropower

plant, Nepal (Photo Haakon Støle)

In order to dimension the main body of the settling basin we must be able to compute

the resulting trap efficiency of a basin with some basic geometric dimensions. The particle approach to trap efficiency computation is assessing the probability of one

particle being trapped or passed through the settling basin. The concentrationapproach is addressing the difference in average sediment concentration in the flowentering the basin and the flow leaving the basin.

The particle approach is based on a simple relation. If there is no turbulence inside the

basin, the ratio between the particles fall velocity, w and the horizontal transit velocityin the basin, vt must be the same as the ratio between the fall distance (i.e. the depth ofthe basin D) and the horizontal travel distance (i.e. the length of the basin L). In an

ideal basin, i.e. a basin without any turbulence, all particles with a fall velocity largeror equal to w will be trapped.




s

t

A

Q

L

v Dw =

⋅= (9.12)

As is the net surface area of the basin. As there is turbulence, some particles will notsettle as fast as the fall velocity indicates because turbulence will always move some

particles in the upwards direction. Camps diagram, shown in Figure 9.16 includes theeffect of turbulence on the trap efficiency [34].

Fig. 0.16: Camps diagram for trap efficiency including the effect of turbulence on the

fall velocity [34]

The trap efficiency η is found from the diagram, based on the two following parameters.

*U

w and

Q

Aw s⋅ (9.13)

U∗ is the shear velocity and Q is the discharge through the basin. Note that As/Q is the

same as L/D⋅vt. The term Q/As is labelled the surface loading of a settling basin. The

shear velocity can be found by use of Mannings formula for the energy gradient S e. Ris the hydraulic radius.

eS R g U ⋅⋅=∗ and 2

3/2)(

R A

QS e

⋅⋅= (9.14)




Vetters method, presented in [26] is based on the concentration approach. This is a

simplified version of Hazens method, assuming best performance with respect to flowdistribution in the settling basin.

)(

1 Q Asw

e⋅−

−=η (9.15)

For all simplified trap efficiency computations, it is important to make a reasonableassessment of the effective surface area for settling. The net surface area, As shall

only include the area of the basin where the flow distribution is close to uniform.

By applying computational fluid dynamics, it is possible to include the effect of the

inflow and the outflow conditions in the computation of the trap efficiency, refer toChapter 10. The trap efficiency computation based on a 3-D CFD simulation of the

performance of a settling basin will be based on comparison of the sediment flux into

the basin and the resulting sediment flux out of the basin after settling has occurred.

9.4.3 Removal of Deposits

It is always necessary to provide some dead storage in a settling basin where sedimentmay accumulate between the flushing processes. The size of the dead storage is

dependent on the sediment load as well as the adopted flushing method. The availableflushing technologies are classified in two main categories as shown in Table 9.3.Some flushing systems requires a closedown of the settling basin during the removal

process while other systems facilitates continuous operation of the settling basinduring flushing, i.e. incoming sediments will be trapped continuously also during the

removal process. Refer to [35], [36] and [37].

Table 0.3: Classification of flushing systems

SETTLING BASIN FLUSHING ARRANGEMENTS

Close down during flushing In operation during flushing

1

Conventionalgravity flow

flushing

2

Excavators andmanual unloading

3

Continuousflushing

4

Intermittentflushing

A swift flowing current must be generated inside the basin during conventionalgravity flow flushing in order to scour the deposits and transport them back to the

river downstream of the diversion weir. The flushing process is normally involvingoperation of flushing gates in addition to the gates in both ends of the basins. The inletgate shall adjust the flushing flow and the outlet gate must be closed to secure

continued operation of neighbouring basins during flushing. The flushing gates in thedownstream end of the basin must have sufficient capacity to secure an even

withdrawal of water from the entire width of the basin to prevent islands of deposits,which are not removed efficiently. The main weakness with this system is that theoperation of the settling basin is affecting the power generation directly. Generation

needs may overrule flushing needs. If the sediments are not removed when needed,




the basins will be overloaded and the trap efficiency will be drastically reduced.Increased sediment load to the turbines and increased wear will occur.

Continuous flushing systems in category 3 are designed to abstract water from the

bottom of a settling basin continuously during operation to prevent any deposition to

occur. The water consumption of these flushing systems is normally in the range of 20to 30 percent of the flow supplied to the waterways downstream of the basins. The

most common flushing system of this category has longitudinal hoppers with aflushing canal running along the bottom of the hopper. There are evenly spaced slits

connecting the basin with the flushing canal below. These slits are oriented normal tothe longitudinal axis of the basin. The flushing canal is increasing in size in the flowdirection in order to obtain a constant velocity in the flushing canal. This will give a

constant pressure difference between the basin and the canal and thus secure an evenabstraction of water from the basin along the bottom of the hopper. A flushing gate is

located in the downstream end of the flushing canal. It is important to keep theflushing gate open always when sediments may enter the basin, because it is

impossible to remove deposits from the basin if they have deposited while theflushing gate has been closed. In addition to the high continuous water consumption,the main weakness with the continuous flushing system is that it cannot be reactivated

by normal operation of the flushing gate after deposition has occurred.

Sediments are removed from the basins while these basins remain in normal operation

for both category 3 and category 4 flushing systems. In category 4, the flushing process is intermittent, and there is therefore no loss of water during the time between

flushing operations. Several intermittent flushing systems are developed [35]. Thehopper system is the most common of these The bottom of the basin is covered withhoppers furnished with a sediment ejection pipe in the bottom of each hopper. The

slope of each of the four sides of the hopper is normally 1:1. A gallery of valves islocated below or next to the basin for flushing. As the head often is a limiting factor,

there will be many hoppers with a costly concrete structure as well as many pipes andvalves to be operated.

Some other category 4 systems are the Bieri system [8], the Serpent SedimentSluicing System (S4) [35] and the Slotted Pipe Sediment Excluder [37]. Bieri and S4

are patented systems. Removal systems making use of dredgers and scrapers do alsofall into this category. As there are not room for presentation of all these systems, the

basic principles of the S4 system is selected as a representative of category 4 her.

The “serpent” (a heavy-duty rubber tube) seals a longitudinal slit between the settling

basin and a flushing canal along the bottom of the basin when it is filled with water.There is a flushing gate in the downstream end of the flushing canal and an operationvalve facilitating filling the serpent with water or dewatering the serpent so it

becomes buoyant.

The S4 system is removing deposits from the settling basin in two modes. In openingmode the serpent is gradually lifted from the slit along the bottom of the basin to thesurface. In closing mode as shown in Figure 9.17, the serpent is gradually closing the

slit over the flushing canal in the bottom of the basin as it is filled with water and

subjected to the suction from the flushing canal.




Fig. 0.17: Serpent Sediment Sluicing System (S4) [37]

The serpent is gradually sinking or rising in the basin and thus closing or opening theslit between the basin and the flushing canal as a zip- fastener. The sluicing-area where

water and sediments are sluiced out from the basin is gradually moved from one endof the basin to the other end and then back again. The 90 metre long serpents are seenfloating in one basin and resting over the slit in the other basin of Khimti hydropower

plant in Figure 9.12. The flushing-water consumption is 10% during flushing only.




9.5 RESERVOIR SEDIMENTATI ON

The CIR is a convenient term to use for classification of reservoirs in relation toreservoir sedimentation. CIR is the volume of the reservoir divided by the average

annual inflow to the reservoir in volume as illustrated in Figure 9.18.

Fig. 0.18: Classification of hydropower plants according to CIR

Rooseboom has given the following recommendations after studying sedimentmanagement at reservoirs in the Southern Africa over the years [38].

• Plants with CIR 3-30% tend to be particularly vulnerable to sedimentation

problems.

• Continuous and long-term sediment records are needed prior to detailed designof storage projects. A record between 6 and 10 years are required as a

minimum.

• Reservoirs should be located as far up in the catchment as possible.

• Regional study of sedimentation in existing reservoirs shall be included in allreservoir studies.

Mahmood estimated for the World Bank in 1987 in [39] that about 1% of the gross

water storage capacity worldwide is lost annually due to reservoir sedimentation. TheWorld Commission on Dams refers in [40] to a loss-rate of 0.5% to 1.0%.

Sediment yield is far from equally distributed over the earth as shown in Figure 9.19.It is therefore important to base any study on local data and experience. Semi-arid and

arid areas are most vulnerable as the average sediment concentration in the water flowhere is high even if the sediment yield may be moderate. This is in areas where longterm storage of water is important due to long periods without rain.




Fig. 0.19: Annual suspended sediment yield after Walling and Webb and average sediment concentrations after Jacobsen, [41]

It is important to remember that in most cases a considerable part of the incoming

sediments will deposit in the live storage of the reservoir. Loss of storage volume willtherefore occur long before the dead storage is filled up. This is complicating the termreservoir lifetime, which has been defined by many as the time it takes to fill up the

dead storage.

A reservoir sedimentation study will normally involve a three-dimensional simulationof the flow pattern in a reservoir over a long period of time. Numerical hydraulic




models are developed and they are now able to give reasonable good predictions ifthere is a basis for calibration of the model. When the flow pattern in the reservoir is

known, it is possible to simulate the transportation and deposition pattern of thesuspended sediments, which enters the reservoir together with the water flows. The

uncertainties are today more linked to the lack of correct and detailed input data than

to the modelling technique itself.

A long time-series of data is required in order to enable a realistic simulation of the behaviour of a planned reservoir. The most important data requirements are

topography and long corresponding time-series of inflow to the reservoir, suspendedsediment data (concentration and particle size distribution) and operation record forthe reservoir, i.e. outflow of the reservoir to the power plant and through the spillways

in addition to evaporation data.

Most reservoirs will reach equilibrium over time where the sediment inflow and thesediment outflow balances each other. The remaining storage capacity may then be in

the range of five to ten percent of the initial size of the reservoir.




9.6 HANDL ING OF SEDIMENTS IN RESERVOIRS

As it for many reasons becomes more and more difficult to build new reservoirs, itwill be more and more important to take care of the surface water storage facilities we

do have in lakes and man-made reservoirs. The demand for storage capacity, fuelled

by needs for water, food and electricity as well as flood mitigation, is growing.

9.6.1 Measures

Measures to prevent or reduce reservoir sedimentation are under development. Itmust, however, be observed that most sediment handling techniques for reservoirs are

limited to small reservoirs. The various concepts in use and under development arecategorised as follows:

1. Reducing sediment inflow through watershed management and erosion

control.

2. Removal of deposits from the reservoir through:

• flushing techniques

• hydraulic sluicing

• excavation and dredging

3. Reducing deposition of sediments in the reservoir by providing passage ofsediments through the reservoir by reservoir-management measures. Thisincludes mainly water level control and possible flushing.

4. Bypassing the reservoir through a sediment bypass arrangement.

The sediment bypass concept shown in Figure 9.20 may be combined with all theother methods listed above. The sediments are trapped in the trapping reservoir or

sluiced through this during floods and thus reducing the sediment loads entering themain reservoir.

Fig. 0.20: The sediment bypass concept




Reservoirs with sediment bypass should preferably be located in river loops where the

distance between the trapping reservoir upstream and the downstream balancingreservoir are as short as possible. The downstream balancing reservoir shall secure

that the sediment load is carried further downstream during floods so the sediment

transport pattern is as close to the natural sediment transport pattern as possible.

The alternative to sediment bypass schemes is normally much larger reservoirs wherea considerable part of the reservoir volume serves as sediment storage instead of

water storage.

Hydraulic sluicing is a method of removing deposits in a reservoir without emptying

the reservoir as needed for conventional flushing. The sluicing capacity generated bythe level difference between the reservoir and the outlet downstream of the dam is

moved to the deposits by means of a pipeline. The main challenge has been to designan efficient but safe mouthpiece. The sediments shall be picked up efficiently and fed

into the pipeline without causing the entire mouthpiece to be sucked into thesediments causing a complete blockage of the inlet and deflation of the pipeline due tothe vacuum, which occurs when the water in the pipeline continues to move.

The Saxophone Sediment Sluicer (SSS) is invented by Tom Jacobsen [41]. Thisdesign, shown in Figure 9.21 has solved the problem of balancing the feeding rate

with the transport capacity of the pipeline between the mouthpiece and the outlet. Themouthpiece can be placed directly on the deposits and left in position until the crater

is as deep as desired before it is shifted to another location.

A: Normaloperation

B: Bottom slotsare coveredC: Bottom slots

are reopened D: General

arrangement

1: Outlet pipe

2: Suction head3: Bottom slots

4: Balancing flow5: Opening forextra water

6: Sedimentdeposits

Fig. 0.21: The Saxophone Sediment Sluicer [41]




9.6.2 Dai ly Peaking Reservoirs

Reference is made to Section 4.1 where the value of small daily peaking reservoirs orhead-ponds is addressed. It is normally possible to schedule a flushing process when

the deposits consist of silt, sand and gravel and the reservoir has a favourable shape. If

stones and boulders are deposited in a reservoir during a high flood, it is more or lessimpossible to remove these deposits during a scheduled flushing as the available flow

will be limited to normal wet season floods.

The size of the pond is small compared to the annual water flow passing through it(CIR < 3 %). The reservoir volume may in many cases be in the same range as thevolume of the annual sediment load of the river. Protective measures must therefore

be introduced in order to prevent sediments from filling up the pond after short time.The headworks of this reservoir plant must therefore be designed as a run-of-river

plant in many ways as the plant in the long run must bypass all the incomingsediments.

The sustainability of the pond is the most critical aspect with respect to the adoptedheadworks concept and sediment transport. It is important to prevent accumulation of

coarse sediments (boulders) in the pond which cannot be removed through normaloperation of the pond and thus reduce the active part of the daily peaking reservoirover time.

The adopted operation strategy, aiming at preventing permanent deposition of

sediments in the pond is shown in Figure 9.22 through Figure 9.27. This strategy ismaking use of reservoir management measures for maintaining the pond capacitythroughout the lifetime of the project. This concept is adopted for the Mangde Chhu

project in Bhutan, and it has also been used in both the feasibility design and thetender design for Kohala and Neelum Jhelum hydroelectric projects in Kashmir,

Pakistan. The adopted concept is presented below [42].




Fig. 0.22: Dry season operation when Qriv > Q pow

Figure 9.22 shows how the pond is being operated during the dry season. Thesediment load carried by the river during this season is moderate and the river is not

able to transport any large particles, say gravel, boulders and blocks. As long as theflow of the river, Qriv, is higher than the sum of the installed capacity, Q pow, and the

minimum flow of the river just downstream of the dam, Qmin, the upstream water levelwill be maintained at HRWL.

Fig. 0.23: Dry season operation when Qriv < Q pow

When Qriv is lower than the sum of Q pow and Qmin, the upstream water level will vary

between HRWL and LRWL during the day due to peaking needs, as shown in Figure9.23.




Figure 9.24 shows normal rainy season operations when the flow is less than theaverage annual flood, Q(2). It is necessary to maintain the upstream water level at

LRWL throughout the rain period whenever the power plant is in operation in order tominimise the deposition of sediment load in the active part of the head-pond between

HRWL and LRWL.

Most of the coarser part of the sediment load will deposit in the dead storage below

LRWL. This volume is rather small and the sediment load is expected to pass throughthe pond after some time as shown on the lower part of Figure 9.24. If there is a side

intake upstream of the dam, under-sluices are required to facilitate bed control at theintake and thus preventing inflow of bed load to the intake.

Fig. 0.24: Normal rainy season operatio

Figure 9.25 shows the situation during moderate floods, i.e. when the flow is betweenan average flood Q(2) and a flood with a return period of 20 years, Q(20). The

upstream water level shall be lowered down to the "as before situation" during thesefloods in order to facilitate removal of deposits from the pond and passage of the




sediment load carried into the pond during these flood situations. It is assumed thatthe river will be able to transport boulders of any size, which can be supplied to it in

some quantity during a 20 years flood. It is necessary to prevent that large bouldersare deposited in the pond during these floods because it will be impossible to remove

them by flushing after the flood when the available flow is less than Q(2).

The lower radial gates in the dam must have capacity to pass any flow up to Q(20) as

a free surface flow. It is therefore assumed that the river will flow in an "as beforesituation" during floods between Q(2) and Q(20), and that bed load transported by the

river into the pond area will be carried on and pass through the lower gates.

Fig. 0.25: Reservoir flushing and sediment passage during moderate floods

The peak flow during large floods is in the range from Q(20) to Q design. When the flowexceeds a 20 years flood, the capacity of the bottom radial gates is exceeded withrespect to passage of the flood as a free surface flow.




Fig. 0.26: Deposition and passage during the peak of a large flood

The upstream water level will then gradually rise, causing the pressure on the gatesand their capacity to increase until water starts spilling over the crest. This situation,when the dam is throttling the river flow, is shown in Figure 9.26. Sediments will then

deposit in the pond. The gates shall remain open during the falling limb of the floodand the deposits in the pond are expected to be removed when the flow is

corresponding to Q(20) and the water level has dropped as shown in Figure 9.25.

The peak flow of an extreme flood is in the range from Qdesign to QPMF. Water will be

discharged over the crest as from a free overflow weir as shown in the upper part ofFigure 9.27. Sediments will deposit in the pond. The gates shall remain open during

the falling limb of the flood and the deposits in the active part of the pond areexpected to gradually be removed when the flow is decreasing from Qdesign to Q(20)as shown in the lower part of Figure 9.27. The remaining deposits are expected to be

removed when the flow is corresponding to Q(20) and the water level has dropped asshown in Figure 9.25. If the lower radial gates are not fully open during the falling

limb of a large or an extreme flood, it will be impossible to remove the deposits in the pond by a scheduled flushing operation later on when the available water flow will bemuch less than Q(2).




Fig. 0.27: Deposition and passage during the peak of an extreme flood

It is of prime importance that the recommended operation strategy for the pond is

followed throughout the rainy season in order to safeguard the pond and maintainingthe daily peaking capacity for the coming dry season.

The power generation losses caused by passage of bed load at a low upstream waterlevel during floods should be considered the minimum cost (insurance premium) that

the owner has to pay to maintain and re-establish the full peaking capacity for eachdry season.




9.7 SCOUR PROTECTI ON WORKS

9.7.1 Pri nciples and common practice

Scour is another word for erosion, indicating removal of surface material by thecurrent. Scour protection works are designed to avoid damage to structures due to

such removal. Common sites where scour protection is being applied include outsidecurves of riverbanks, artificial channels and constrictions, downstream of sills, etc.

In connection with hydropower development, many rivers are altered and theirdischarges regulated and transferred, with the result that new local areas are being

threatened by erosion. Tailrace channels may need protection as well. Other importantareas for protection lie downstream of spillways and at the foot of dams where

uncontrolled erosion may cause great damage and even risk of dam failure. Inregulated lakes, protection against wave action and degradation of creek outlets are

frequently needed.

Traditional scour protection consists of a two- layer covering of the threatened area.

The upper layer, "the cover layer", usually consists of irregularly placed rocks orartificial blocks, large and heavy enough to withstand the current under the mostextreme discharge conditions. The second layer, called "filter layer or sub-layer", may

serve two purposes. Its primary purpose is to avoid movement of the natural materialsunder the protection layer, causing them to be washed away through the openings

between the cover layers. A second purpose is to act as a pillow for the coarse coverlayer elements, in order to avoid penetration into and mixing with the original ground,

both during placing of the blocks, and during the later action of the current.

Modern development of scour protection has introduced some quite economic

solutions. In many cases artificial woven fibre materials, "geotextiles", now substitutethe original granular filter layers. The fibre geotextiles may be placed in single layersor as mats or bags filled with sand, depending on the local conditions.

Cover layers of blasted rock fragments still prevail, provided large rocks are locally

available. But artificial blocks of concrete, sometimes interlocking or bound together by stainless steel bars, are frequently used in order to reduce the thickness of the coverlayer without loosing effect.

Composite solutions are sometimes found where the function of cover and filter is

combined in one layer. In its most complex form, protection consists of multi-layeredmats of fibre material, with sand in-fill, asphalt etc. It may consist of thick layers ofwell-graded quarry material, placed directly on the riverbed.

9.7.2 Dimensioning of cover layers

Cover layers of rock material may in principle be chosen according to the theory of

critical shear stress described above. As soon as the dimensioning shear stress has

been determined from hydraulic analysis and statistical flow data, the minimum sizeof the cover blocks may be found from e.g. Shields' curve, Figure 9.3. This value




needs a safety factor to be added before selecting block size. Other curves have been prepared for direct selection of the safe dimension.

In many cases, water velocities are more readily available than the shear stress value.

Many curves for practical use therefore use the local velocity as parameter, but then a

correction for water depth is needed as well. Figure 9.28 shows a diagram presentinga combination of codes from USA and the former USSR, with a separate diagram for

depth correction.

For dimensioning of artificial cover blocks of concrete etc., advice from the supplieror consultant is needed. Special formulae for protection against waves are available in

coastal protection literature, see ref. [43].

Fig. 0.28: Practical dimensioning of cover layers made of common quarry stones withdensity 2650 kg/m3 , adopted from [28]




9.7.3 Traditional fi lter layers

Traditional filter layers are composed of granular materials with grain sizes in

between the base material and the cover layer. Special rules have been developed in

order to avoid both penetration of base material through the filter, and transfer of filtermaterial through the cover layer. These rules are defined in terms of grain size

fractions of the underlying layer compared to the overlying layer, and are thereforevalid for any interface between two layers.

If grain sizes are denoted by d for the underlying layer and by D for the overlyinglayer, the traditional filter rules are as shown below. Both criteria must be fulfilled for

all interfaces.

5⋅ d 15 < D15 < 5⋅ d 85 and D50 < 25⋅ d 50 (9.16)

Layers of granular materials should be at least twice as thick as the largest stone in thelayer, and minimum 0.30 m thick because accurate placing of thinner layers isdifficult.

The procedure for dimensioning of a traditional scour protection is therefore as

follows:

1. Determine the safe size of the cover layer stones, e.g. from the diagram above

2. Chose an available material for filter, and check dimensions against the filterrules

3. Check the chosen filter material against the base material, also using the filterrules

4. If one or both checks are not satisfactory, try another filter if available.

5. In some cases a third layer between the filter and the base may be necessary inorder to fulfil the filter rules from base to top.

9.7.4 Use of geotexti les

Geotextiles are now in many cases replacing or substituting the granular filter layer.

The fibre filters need to comply with similar requirements as the granular filter, i.e.

preventing escape of base material through the cover layer. In addition it is necessaryto prevent clogging of the filter by base material, since this may instigate local

pressures and cause unwanted local migration and accumulation of base materialunderneath the filter. Geotextiles may be woven or non-woven. The latter include a

variety of products, mostly mat-like fabrics of non-regular, intermingled long fibrefilaments. Rules for dimensioning of geo-textile filters have been proposed in ref.

[44]. If O90 represent the filter opening where minimum 90 per cent of openings are

less, the criterion for holding back the base material is as follows:For stationary loads:

• O90/d90 < 1.0 for woven geotextiles

• O90/d90 < 1.8 for non-woven geotextiles



For dynamic loads:

• O98/d85 < 1.0 is usually acceptable

9.7.5 Ar tif icial armour layers

Both granular and fibre filters are difficult to apply according to the strict rules forsatisfactory armouring. Therefore in many cases protection by one single layer of

graded material has been shown to be sufficient. The graded material of the armourlayer forms it own filter, but it is necessary to check against the filter rules for the

base material.

The upper part of the graded layer will contain a surplus of fines that will be washed

out after some time. The remaining coarser material has to comply with the generalrules for stability against scour. If the original graded material contains sufficient

coarse particles, the result will be an artificial armour layer of similar nature to manynatural riverbeds. It is possible to calculate the process of washing out of fines, byapplying statistical functions for the probability of a surface particle to move or rest.

A result of such calculations is shown in Figure 9.29, where the grain size curve of theoriginal material is presented against the resulting stable armour layer for an imposed

shear stress of 50 N/m2.

An inherent consequence of using graded material for development of an armour layer

is that some extra material is required. The more fines that have to be washed out inorder to obtain a stable result, the more extra material has to be dumped, but the cost

of the extra volume is often well justified due to the simpler placement.

Fig. 0.29: Computed stable artificial armour layer for shear stress of 50 N/m2

chapter 9 book8 hs2 with figures

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