sedimenation in reservoirs

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JAIPUR



SEDIMENTATION IN RESERVOIRS

by

VINAY CHANDWANI E.E., WRD



INTRODUCTION Sedimentation of reservoirs is a natural phenomenon

and is a matter of vital concern for storage projects inmeeting the various demands, like irrigation, hydro-electric power, flood control, etc, since it affects theuseful capacity of the reservoirs based on which theprojects are expected to be productive for a designperiod. Further the sediment deposition adds to theforces on structures in dams, spillways, etc.



INTRODUCTION The rate of sedimentation will depend largely on the

annual sediment load carried by the stream and theextent to which the same will be retained in the

reservoir. This in turn, depends upon a number offactors; such as the area and nature of the catchment,landuse pattern (cultivation practices, grazing,logging, construction activities and conservationpractices ), rainfall pattern, storage capacity, period of

storage in relation to the sediment load of the stream,particle size distribution in the suspended sediment,channel hydraulics, location and size of sluices, outlet works, configuration of the reservoir, and the methodand purpose of releases through the dam.



INTRODUCTION Sedimentation in dams reservoir is one of the main

problems facing too many dams worldwide. Accordingto the report published by International Committee ofLarge Dams (ICOLD), over 40,000 large dams exists inthe world with a total storage capacity of 7,000 billionm3, operated for different purposes such as watersupply, hydroelectric power generation and floodcontrol. On the other hand, an average rate of 0.5 to 1% of their storage capacity is lost each year due tosedimentation.



INTRODUCTION The growth of the dam development was intense during 1960 to 1980 as

shown in Figure. The new dam development since 2000 is significantlyless; however the loss of storage capacity is very high. It shows thatabout 1 000 km3 of volume will be lost by 2020, which is about 15% ofthe current gross available storage.



INTRODUCTION Reports produced by United Nations Environmental Programme

(UNEP) show that the total global sediment discharge is about 14 x109tonnes/year (UNEP, 2003). Figure shows the suspended sedimentdischarge per region, which demonstrates that Asia with PacificOceanic Islands contributes nearly 70% of the sediment delivery to the world’s oceans and seas.



INTRODUCTION



SEDIMENT FORMATION Weathering of rocks – chemical and physical

process by which rocks break down into smallerparticles.

Erosion – detachment and transport of weatheredmaterial from one location to another. Sediment Yield is the amount of eroded sediment

discharged by a stream at any given point; it is the

total amount of fluvial sediment exported by the watershed tributary to a measurement point and isthe parameter of primary concern in reservoirstudies.



SEDIMENT TRANSPORT Bed Load – Material that moves along the bottom

of the channel (by rolling) as a result of shear stresscreated by vertical velocity gradients in thestreamflow.

Suspended Bed Material Load bed material that becomessuspended by action of turbulence.

Wash Load –

fine material thatis carried by the flow insuspension, but is not representedin the bed material



STORAGE ZONES OF A RESERVOIR



STORAGE ZONES OF A RESERVOIR Full Reservoir Level (FRL): It is the level corresponding

to the storage which includes both inactive and activestorages and also the flood storage, if provided for. Infact, this is the highest reservoir level that can bemaintained without spillway discharge or withoutpassing water downstream through sluice ways.

Minimum Drawdown Level (MDDL): It is the levelbelow which the reservoir will not be drawn down soas to maintain a minimum head required in powerprojects.



STORAGE ZONES OF A RESERVOIR Dead Storage Level (DSL): Below the level, there are no

outlets to drain the water in the reservoir by gravity.

Live storage: This is the storage available for the intended

purpose between Full Supply Level and the Invert Level ofthe lowest discharge outlet. This may also be termed as the volume of water actually available at any time between theDead Storage Level and the lower of the actual water leveland Full Reservoir Level.

Zero Elevation Level : The level up to which all theavailable capacity of the reservoir was or is expected to belost due to progressive sedimentation of the reservoir up tothe specified time.



PROBLEMS ASSOCIATED WITH

RESERVOIR SEDIMENTATION Loss of Storage:

The reduction of the active storage capacity which may reducethe capability of the reservoir to deliver the benefits.

It may reduce availability of firm water in marginal years byincrease in both the number and quantum of failures.

Sedimentation at or near the dam face may tend to blockthe outlet causing difficulties in operation of the gates.

Sedimentation up to intake of the outlet may induce moresediment to be carried through the conservation outlets,thus causing problems of sedimentation of canals,machinery parts, etc.




RESERVOIR SEDIMENTATION Sediment accumulation at the dam-face may increase the

loading on the masonry/concrete dam structure beyond what has been provided for.

Sedimentation in upper portion of the reservoir maychange the back water profile from what it would havebeen put for sedimentation. The increase in flood levelsupstream of the reservoir may cause additionalsubmergence, formation of marshy lands, etc.

The river regime at the entry to the reservoir may getaffected due to sediment deposits. Delta formation andbraided river pattern may result and this may be unsightly.Tree growth in the delta lends increase evapo-transpiration.




RESERVOIR SEDIMENTATION The process of sedimentation in reservoirs may also

increase the turbidity of water resulting in theenvironment problems like deterioration of waterquality and reduction of visibility in the reservoir water for fish survival.



PLANNING OF RESERVOIRS :

EFFECT OF SEDIMENTATION Reservoirs are national assets created for economic

development with large investments. They havetraditionally been planned, designed, and operated on the

assumption that they have a certain life but still it is veryessential that such reservoirs continue to give economicbenefit as envisaged at the planning stage

Storage reservoirs built across rivers and streams loosetheir capacity on account of deposition of sediment. This

deposition which takes place progressively in time reducesthe active capacity of the reservoir. Accumulation ofsediment at or near the dam may interfere with the futurefunctioning of water intakes and hence affects decisionsregarding location and height of various outlets.



PLANNING OF RESERVOIRS :

EFFECT OF SEDIMENTATION Assessment of reservoir sedimentation problem, in a

particular case, may be made by comparing the expectedaverage annual volume of sediment deposition with the

gross capacity of the reservoir. If ratio is more than 0.5percent per year, the problem is usually said to beserious and special care is required in estimating thesediment yields from the catchment. If it is less than 0.1percent per year, the problem of siltation may be

insignificant and changes in reservoir capacity ran beneglected for studies of reservoir performance. Forcases falling between these two limits, the sedimentationproblem is considered significant and requires furtherstudies.



LIFE OF RESERVOIR Feasible Service Time: It is estimated as the time

after which the new zero elevation of the reservoir would equal the sill of the outlet relevant for thepurpose.

Full Service Time: For a specified purpose, the periodor notional period for which the reservoir provided isexpected to provide, a part of the full planned benefitinspite of sedimentation.



LIFE OF RESERVOIR The term ‘life of reservoir’ denotes the period during which

whole or a specified fraction of its total or active capacity islost.

Full service time shall not be less than 50 years after thestart of operation. Feasible service time shall not be lessthan 100 years after the start of operation.

For hydro-power projects expected to supply power to acommunity, in isolation the feasible and full service time

shall be the same as for the irrigation projects. For hydro-power projects supplying power to a grid, full

service time shall not be less than 25 years. Feasible servicetime shall not be less than 70 years.



REFER CODE IS:5477 (Part –II)

DATA REQUIRED FULL TANK LEVEL (F.T.L)

CATCHMENT AREA (sq.km or sq.mile)

SEDIMENTATION RATE (ha-m/sq.km/year orMcft/sq.mile/year)

CAPACITY AREA ELELEVATION CURVE AREA IN HECTARES (Ha) CAPACITY IN Ha-M

ELEVATION- Metres

LIFE OF RESERVOIR (years)

ASSESSING THE NEW ZERO

ELEVATION OF A RESERVOIR




ELEVATION OF A RESERVOIR REFER CODE IS:5477 (Part-II)

FROM SEDIMENATION STUDIES COMPUTE THERATE OF SEDIMENTATION

USUALLY RATE OF SEDIMENTATION ISEXPRESSED IN mm/year, Ha-m/Sq.Km/Year orMcft/Sq.Mile/Year.

Ha-m/Sq.Km/Year= 104/106 = 1/100 m/year or 0.01 x1000 mm/year = 10 mm/year or 1 mm/year = 0.1 Ha-m/Sq.Km/Year




ELEVATION OF A RESERVOIR CALCULATE THE TRAP EFFICIENCY BY BRUNE’S

CURVES (REFER IS: 12182)




ELEVATION OF A RESERVOIR TRAP EFFICIENCY : Trap efficiency of a reservoir, over

a period is the ratio of the total deposited sediment tothe total sediment inf low.

Calculate capacity at FRL (C)

Annual Inflow (I) or yield

Evaluate C/I ratio

Using Median Curve Relation 100*0.97 (0.19^LOG(C/I)), compute

the % trap efficiency of the reservoir.

Choose Life of Reservoir in years (Normally analysis isdone for 50 years, 70 years & 100 years)




ELEVATION OF A RESERVOIR Evaluate seriousness of sedimentation in the reservoir

Total Sedimentation (S) per year in Ha-m

Trap Efficiency (%) * Area of Reservoir (Ha-m) * Life ofReservoir (Years) * Sedimentation Rate (Ha-m/sq.km/year).

Evaluate Ratio S/C ; C is the capacity at FRL

If ratio S/C >= 0.5 then problem is serious

If ratio S/C <= 0.1 then problem is insignificant

If 0.1 < S/C < 0.5 then problem is significant




ELEVATION OF A RESERVOIR From Capacity-Area –Elevation Curve arrange Area

(Ha) and Capacity (Ha-m) in decreasing order startingfrom FRL.

Inflow I =59.74 McumCapacity at FRL (C) = 58.29 McumFor this case Trap efficiency = 96.9742%Sedimentation Rate =0.1 Ha-m/Sq.Km/yearor 1 mm/year

Area of Reservoir= 390 sq.KmLife of Reservoir = 70 yearsTotal Sedimentation= 3780.94 Ha-m




ELEVATION OF A RESERVOIR BY EMPIRICAL AREA

REDUCTION METHOD CHOOSE A NEW ZERO ELEVATION LEVEL

BETWEEN NSL AND FRL. LET US CHOOSENEW ZERO ELEVATION AS 375.15 metre

INTERPOLATE CAPACITY AND AREA FORTHIS NEW ZER ELV. AND TABULATE THEM AS SHOWN.

CALCULATE DEPTH OF RESERVOIR

SUCCESSIVELY FROM NSL TO FRL.

DRAW A LOG-LOG GRAPH FOR DEPTH OFRESERVOIR VERSUS CAPACITY

FIND THE SLOPE OF THIS LOG-LOG GRAPH.




ELEVATION OF A RESERVOIR Empirical Area Reduction Method

This method is based on the analysis of data of sedimentdistribution. In this method, reservoirs are classified into

four types, namely, (a) gorge, (b) hill, (c) flood plain-foot hill, and (d) lake, based on the ratio of the reservoir

capacity to the reservoir depth plotted on a log-log scale.

The equation for the design curve used is: A p = Cpm(1 -pn )

Where A p = a non-dimensional relative area at relative distance ‘p’ above the stream bed, and C, m and n= non-dimensional constantswhich have been fixed depending on the type of reservoir.




ELEVATION OF A RESERVOIR CLASSIFY THE RESERVOIR




ELEVATION OF A RESERVOIR AREA DESIGN CURVES




ELEVATION OF A RESERVOIR Calculate A p for assumed new zero elevation i.e for RL

375.15 m, where p = 0.76.

For this value of p evaluate A p by using curve for Type

II (Flood Plain) Reservoir, A p = 1.1795

compute K= Area at New Zero Elevation/A p;K = 472.723

Once K is evaluated, multiply it with A p at eachreservoir elevation to get the sediment area inhectares.





21

2 A Ah





For this case Trap efficiency = 96.9742%Sedimentation Rate =0.1 Ha-m/Sq.Km/year or 1 mm/year Area of Reservoir= 390 sq.Km

Life of Reservoir = 70 yearsTotal Sedimentation= 3780.94 Ha-m% Difference between expected sedimentation and computedsedimentation = (3780.94-3749.17)/3780.94*100 =0.8402% <1.0% HENCE CHOSEN NEW ZERO ELEVATION IS CORRECT





357

362

367

372

377

382

0 1000 2000 3000 4000 5000 6000

E L E V A

T I O N

m e t r e s

CAPACITY in Ha-metres

Capacity-Elevation Curve RAJGARH MIP

Life of Reservoir= 70 Years

Original

Revised



HOW TO CONTROL SEDIMENTATION ?

Use of check dams formed by building small barriersor dykes across stream channels.

They help arrest degradation of stream bed therebyarresting the slope failure;

They reduce the veIocity of stream flow, therebycausing the deposition of the sediment load.

Contour Bunding and Trenching

By these methods the hill side is split up into smallcompartments on which the rain is retained and surfacerun-off is modified with prevention of soil erosion.



HOW TO CONTROL SEDIMENTATION ? Gully Plugging

This is done by small rock fill dams. These dams will beeffective in filling up the gullies with sediment coming

from the upstream of the catchment and also preventfurther widening of the gully.

Scouring Sluices Scouring sluicing depends for its efficiency on either the

scouring action exerted by the sudden rush ofimpounded water under a high head through undersluices or on the scouring action of high flood dischargecoming into the reservoir.



HOW TO CONTROL SEDIMENTATION ? Scouring Sluices

Small power dams that depend to a great extent onpondage but not on storage;

Small irrigation reservoirs, where only a small fractionof the total annual flow can be stored;

Any reservoir in narrow channels, gorges, etc, where water wastage can be afforded; and

When the particular reservoir under treatment is a unitin an interconnected system so that the other reservoirscan supply the water needed.



REMOVAL OF DEPOSITED SEDIMENT Excavation

The method involves draining most of or all the water inthe basin and removing the sediment by hand or power

operated shovel, dragline scraper or other mechanicalmeans. The excavation of silt and clay which constitutemost of the material in larger reservoirs is more difficultthan the excavation of sand and gravel.

Dredging This involves the removal of deposits from the bottom of

a reservoir and their conveyance to some other point bymechanical or hydraulic means, while water storage isbeing maintained.



REMOVAL OF DEPOSITED SEDIMENT Draining and Flushing

The method involves relatively slow release of all stored water in a reservoir through gates or valves located near

bottom of the dam and the maintenance thereafter ofopen outlets for a shorter or longer period.

Sluicing with Controlled Water

controlled water supply permits choosing the time ofsluicing more advantageously and that the water may bedirected more effectively against the sediment deposits.



REMOVAL OF DEPOSITED SEDIMENT Sluicing with Hydraulics and Mechanical Agitation

Methods that stir up, break up or move deposits of asediment into a stream current moving through a

drained reservoir basin or into a full reservoir will tendto make the removal of sediment from the reservoirmore complete.

REFER IS: 6518-1992 “CODE OF PRACTICE FOR

CONTROL OF SEDIMENT IN RESERVOIRS” FORFURTHER DETAILS.



THANK YOU

sedimenation in reservoirs

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