Siltation in ReservoirsPrevention and Cure

ContentsIntroduction to ReservoirsPreventing SiltationCureCost Benefit AnalysisConclusion

Introduction - Reservoirs Facts and FiguresDefinition: A lake or large pool, natural or man-made, used to store water

for public and industrial use, for irrigation, etc. Also more generally: a large expanse of water held back by a dam.1

Constructed for 3 main purposes

1. Irrigation

2. Storage/Flood Control

3. Hydro-power

75% of water consumption is used during irrigation 2

Irrigated land produces 33% of the worlds food

1% of reservoir storage is lost due to siltation every year3

Lake Kariba in Zimbabwe and Zamiba

Preventing SiltationPrevention through Design

Catchment CharacteristicsUpstream TrappingBypassing High Sediment LoadsMinimising Deposits

Sluicing Density Current Venting

Drainage Basin Area

Catchment CharacteristicsSlope

Gradient

Length

Runoff Velocity

Catchment OrientationSouthern

Facing Slopes in

the Northern Hemisphe

re

Drier Soil -> Higher

erosion

Large Catchmen

t

Lower Slope

Lower Sediment

Yield

Smaller % of erodible rock

More opportunity for sediment to be

deposited

Human Impact

Drainage Density

Catchment Characteristics

Deforestation

Urbanisation

Increases Erodibilit

y

Land UseCultivation

of Clay Soils

Decrease Erosion Rates

High Density

Higher erosion rates

Agriculture

Cultivation of Sandy

Soils

Increase Erosion Rates

Increasing Drainage Density

Catchment CharacteristicsVegetation

Root Systems

Reduces surface runoff and

velocity

Dissipates Rainfall Energy

Prevents Rain

reaching the ground

Reduces “Splash” Erosion

Holds soil in place

Maintains the soils

capacity to absorb water

Increase Infiltration

Upstream TrappingIntercept sediments as they travel downstream

Vegetation ScreenCheck Dam

Decrease speed of water flow Reduce erosion Allow sediment and

pollutants to settle

Bypassing High Sediment Loads High flow rates increases sediment concentrationThese waters should be bypassed by channels and/or

tunnels Water collected will have low/medium flow rates, where

sediment concentration is lower

Minimising SedimentationSluicing

Passing of sediments during flooding

Requires reduction in water level

Preserves the seasonal sedimentation loads downstream

Practical alternative to flushing

Density Current VentingNo reduction in water level

requiredOnly used in extreme

circumstances Density Current Venting (Morris and Fan, 1998)

Cure the Removal of Accumulated SedimentFlushingSiphoning Dredging and Digging

Not suitable for all reservoir shapesShape may prohibit suitable scouring channelsRiver discharge may be incapable of moving sediment

in large enough quantities

Flushing

Requires a significant change in the operation of the reservoirWater level needs to be

dropped dramatically

Water Level Drop

Inadequate Drawdown

Final Reservoir Condition due to Inadequate Drawdown

May cause environmental issues downstream: (1)Higher nutrient & metal contentAbrades fish gillsReduces light penetrationSwitzerland stipulates sediment concentration levels:

10 mg/l at the reservoirs in crystalline areas 70 mg/l at the reservoirs in non-crystalline areas

Flushing Cont.

Drawdown reduces the capacity to generate power

Reduction in annual water yield

A SuccessMangahao Reservoir

New ZealandConstructed 1924Lost 59% of storage

in 34 years1969 flushing

removed 75% of sediment

SiphoningInvolves special pumps to redeposit sediment downstreamThere are two approaches to siphoning sediment:

1. The pipeline goes over the dam crest into a downstream basin2. The pipeline travels through the dam’s body bottom outlet

Dredging and DiggingMost expensive of removal techniquesProcess requires:

ExtractionTransportation Disposal/Use

Mechanical Digging – carried out by specially equipped excavators in shallow waters

Dredging – fixed or mobile platforms

Cost Analysis300-400 new dams would need to be constructed a year

to keep current storage levels Costing between $9 and $13 billion1

Not accounting social, environmental or maintenance costsDredging costs approximately3 $8m-3

Flushing costs 10-30% of the cost to construct a new similar sized reservoir2

Performed once a 40-60% reduction of storage capacity is seen

Cost Cont.Process CostConstruction of new dam $44 MillionCost of Flushing average dam $13.2 Million

Cost to Dredge Mangahao at $8m-3 $70 Million

ConclusionTry to prevent through design100% prevention is unrealistic so implement sluicing and

density current venting if possible If removal is necessary flushing is the most cost effective

method

Prevention is better than cure!!