ALL RIVERS CARRY SEDIMENT ◦ BED LOAD

◦ SUSPENDED LOAD

ABOUT 20 B tons OF SEDIMENT CARRIED ANNUALLY TO THE SEA WORLDWIDE

INDIAN SUBCONTINENT CONTRIBUTION ESTIMATED AT 6 B tons

IF THE INFLOW AND OUTFLOW OF SEDIMENT IN A RIVER REACH IS EQUAL, THE REACH WILL BE IN EQUILIBRIUM

ANY DISTURBANCE IN THE REACH CAN AFFECT SEDIMENT REGIME

THE DISTURBANCE MAY BE IN THE FORM OF ANY OBSTRUCTION OR SOME NATURAL PHENOMENON LIKE LANDSLIDES ETC.

RESERVOIR BASED ◦ CONSTRUCTION OF DAM AND WATER STORAGE IN

THE RESERVOIR CREATED

RUN OF RIVER ◦ WATER DIVERTED INTO A POWER CHANNEL

◦ NO APPRECIABLE STORAGE

BOTH PRESENT PROBLEMS OF SEDIMENT MANAGEMENT OF DIFFERENT TYPES

PROBLEMS OF SEDIMENT DEPOSITION IN THE RESERVOIR

DEGRADATION DOWNSTREAM ◦ CHANGES IN RIVER MORPHOLOGY

◦ ECOLOCICAL IMPACTS

REDUCED D.O.

TEMPERATURE CHANGES

LOSS OF CAPACITY DUE TO SEDIMENT DEPOSIT

ESTIMATED LOSS OF CAPACITY 1% ANNUALLY WORLDWIDE

INDIA HAS ABOUT 0.5% ANNUAL LOSS OF CAPACITY BY SEDIMENTATION IN RESERVOIRS

lOCATION Percent sedimentation rate (annual)

India 0.5%

China 2.3%

U.S.A. 0.22%

Turkey 1.2%

Morocco 0.7%

Tunisia 2.3%

World 1%

ESTIMATING LOSS OF CAPACITY ◦ PRELIMINARY ESTIMATE POSSIBLE IF SEDIMENT

YIELD FROM CATCHMENT AND TRAP EFFICIENCY KNOWN

◦ SEDIMENT YIELD UNIVERSAL SOIL LOSS EQUATION

MODIFIED UNIVERSAL SOIL LOSS EQUATION

GARDE AND KOTHIYARI’S RELATIONSHIP FOR INDIAN CATCHMENTS

ARTIFICIAL NEURAL NETWORKS

◦ TRAP EFFICIENCY BY BRUNE’S CURVE

UNIVERSAL SOIL LOSS EQUATION (USLE)

A = R*K*L*S*C*P A = Average annual soil loss tons per ha per

year R = rainfall erosivity factor K = soil erodibility factor LS = slope length and slope steepness factor C = cover management factor P = support practice factor.

GARDE AND KOTHIYARI’S RELATIONSHIP ◦ Y = 0.2 Fe

1.7 S-0.25 Dd0.10 (Pmax/P)0.9 Pa

m

Y is annual sediment yield in cm, Dd is the drainage density, S is the land slope, Pa is annual rainfall in cm, Pmax is average maximum monthly rainfall in cm and Fe is erosion factor defined as

◦ Fe = (1/∑Ai) (0.8AA+0.6AG+ 0.3AF + 0.1AW)

AA, AG, AF and AW are the arable, grass and scrub, protected forest and waste areas respectively in km2

and Ai is an arbitrary coefficient.

ANNs HAVE ALSO BEEN USED IN RECENT YEARS TO ESTIMATE SEDIMENT YIELD FROM CATCHMENTS

MULTILAYER CONFIGURATIONS AND DIFFERENT TRAINING ALGORITHMS HAVE BEEN TRIED

BRUNE’S CURVE – TRAP EFFICIENCY vs. RATIO OF STORAGE CAPACITY TO ANNUAL INFLOW VOLUME

COARSEST MATERIAL ENTERING THE RESERVOIR DEPOSITS FIRST – DELTA DEPOSITION

FINER MATERIAL DEPOSITS CLOSER TO THE DAM

CHARACTERISTICS OF DELTA DEPOSITS ◦ ABRUPT CHANGE OF SLOPE BETWEEN TOPSET AND

FORESET DEPOSITS

◦ SEDIMENT COARSER IN TOP SET BEDS

◦ ELEVATION OF TRANSITION FROM TOP SET TO FORE SET DEPENDS ON RESERVOIR OPERATING RULE AND POOL ELEVATION

◦ PROFILE CHANGES WITH PASSAGE OF TIME

REDUCING SEDIMENT YIELD ◦ CATCHMENT TREATMENT BY VEGETATIVE AND

STRUCTURAL MEASURES TO REDUCE EROSION

SEDIMENT ROUTING ◦ SEDIMENT PASS THROUGH BY PARTIAL DRAWDOWN

OR VENTING DENSITY CURRENTS

◦ SEDIMENT BYE PASS

RESERVOIR FLUSHING – INVOLVES OPENING THE DAM’S BOTTOM OUTLETS, ALLOWING THE DEPOSITED SEDIMENTS TO RE SUSPEND AND FLUSHED OUT ◦ FULL DRAWDOWN FLUSHING MORE EFFECTIVE BUT

CONSUMES LOT OF WATER BESIDES OTHER PROBLEMS

◦ PARTIAL DRAWDOWN FLUSHING

PARTIAL DRAWDOWN FULL DRAWDOWN

LIKELY ENVIRONMENTAL IMPACTS ON THE DOWNSTREAM WHICH MAY OCCUR SOMETIMES AND NEED TO BE CONSIDERED DURING FLUSHING ◦ CHANGES IN TEMPERATURE

◦ REDUCED D.O. LEVELS

◦ CHANGES IN RIVERINE HABITATS

◦ FISH GILL CLOGGING

WATER DIVERTED BY CONSTRUCTION OF A BARRAGE OR LOW DAM ◦ NOT MUCH STORAGE AND HENCE SEDIMENT

DEPOSITED UPSTREAM OF DIVERSION WORKS CAN EASILY BE FLUSHED OUT

LARGE PART OF SEDIMENT – BOTH BED AND SUSPENDED LOAD – ENTERS POWER CHANNEL ◦ POWER CHANNEL TO BE DESIGNED TO CARRY THIS

LOAD

◦ EXTRACT SEDIMENT ABOVE A PARTICULAR SIZE TO PREVENT DAMAGE TO TURBINE COMPONENTS

SEDIMENT EXTRACTION

DEVICES GENERALLY USED ◦ VORTEX CHAMBER SEDIMENT EXTRACTORS

◦ SETTLING BASINS

CYLINDRICAL CHAMBER WITH AN ORIFICE AT THE BOTTOM

WATER LED TANGENTIALLY AT HIGH VELOCITY IN THE CHAMBER

RANKINE COMBINED VORTEX FLOW DEVELOPS – FORCED VORTEX AT PERIPHERY AND FREE VORTEX NEAR THE ORIFICE

SECONDARY FLOW MAKES SEDIMENT SETTLE AT BOTTOM AND MOVE TOWARDS THE ORIFICE

SEDIMENT FLUSHED OUT THROUGH THE ORIFICE INTO A CHANNEL OR PIPE

USES LESS WATER FOR FLUSHING

OPERATES CONTINUOUSLY

USEFUL FOR SMALL SCHEMES ◦ CHAMBER DIAMETER REQUIRED FOR EFFICIENT FLUSHING

IS ABOUT FIVE TIMES THE CHANNEL WIDTH

PRINCIPLE ◦ ENSURE SETTLING OF SEDIMENT ABOVE DESIRED

SIZE BY REDUCING FLOW VELOCITY

◦ INCREASE CROSS SECTIONAL AREA BY INCREASING WIDTH AND DEPTH TO REDUCE VELOCITY

◦ PROVIDE LENGTH REQUIRED TO ACHIEVE DESIRED EFFICIENCY OF REMOVAL OF THE SMALLEST SIZE OF SEDIMENT

SEDIMENT SETTLED AT THE FLOOR CAN BE REMOVED BY ◦ CONTINUOUS FLUSHING

◦ INTERMITTENT FLUSHING

◦ MECHANICAL MEANS

CONTINUOUS FLUSHING REQUIRES ABOUT 15 TO 20% OF THE CHANNEL DISCHARGE FOR FLUSHING – CHANNEL HAS TO BE DESIGNED FOR THIS EXTRA DISCHARGE

DESIGN CONSIDERATIONS ◦ DETERMINE WIDTH, DEPTH AND LENGTH FOR

DESIRED EFFICIENCY OF REMOVAL

◦ MANY EMPIRICAL AND SEMI EMPIRICAL PROCEDURES AVAILABLE SUCH AS OF CAMP, DOBBINS, SUMER ETC.

◦ ANALYSIS OF THE AVAILABLE DATA BY DONGRE

DONGRE PROPOSED THE EQUATION FOR A

BASIN WITHOUT FLUSHING ɳ = 102.5 {1-exp (-0.3(Ab/Aa))}{1-exp (-0.1(L/D))}{1-exp (-

0.42(ω/u*))}

where ɳ is the efficiency, Ab and Aa the cross sectional areas of the basin and approach channel respectively, L is the basin length, D the basin depth ω the fall velocity of the given size of particle and u* is the shear velocity.

RANGARAJU et.al. GAVE THE EXPRESSION FOR BASINS WITH FLUSHING AS

ɳf/ɳ = 1- 0.12Qf-0.105{ω/u*}

0.312

where ɳf is the efficiency with flushing and Qf is the flushing discharge expressed as a percentage of the discharge entering the basin.

SOME INNOVATIVE DESIGNS ◦ MULTI HOPPER CONFIGURATION

◦ LONGITUDINALLY PARTITIONED BASIN

◦ SLOTTED PIPE SEDIMENT SLUICERS (SPSS)

◦ SERPENT SEDIMENT SLUICING SYSTEM (S4)

FLUSHED SEDIMENT GOES TO THE RIVER

RIVER DISCHARGE LIKELY TO BE LOW AND THE SEDIMENT MAY GET DEPOSITED AT OUTFALL

TEMPORARY EFFECT ON RIVER MORPHOLOGY

EFFECT TEMPORARY AS THE SEDIMENT WILL GET WASHED AWAY DURING FLOODS

EROSION DEPENDS ON ◦ SIZE, SHAPE AND COMPOSITION OF SEDIMENT

PARTICLES

◦ ANGLE OF IMPINGEMENT OF SEDIMENT PARTICLES

◦ CHEMICAL COMPOSITION OF WATER

◦ MATERIAL HARDNESS AND ELASTICITY OF TURBINE COMPONENTS