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Sediment Transportation in Estuary Ecology NARESH KUMAR MEHTA PHT-PA1-04

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Page 1: Sediments transportation

Sediment Transportation in Estuary Ecology

NARESH KUMAR MEHTA

PHT-PA1-04

Page 2: Sediments transportation

What sediment is ?

Sediment, a naturally occurring material that is

broken down by processes of weathering and erosion,

and is subsequently transported

by the action of wind, water, or ice, and/or by the

force of gravity acting on the particle itself.

Page 3: Sediments transportation

What are the sources of sediment ?

1. Land clearing

• There is a direct relationship between increased soil erosion and sedimentation.

• In areas where catchments have been cleared and river banks are consistently grazed, sediment loads are often high.

• Grazing along river banks can strip vegetation, disturb the soil and banks.

• Vegetation clearing increases = higher flows increase erosion of the stream channel, leading to the creation of deep gullies

Page 4: Sediments transportation

2. Road building-

• Unsealed roads can contribute significantsediment loads.

• The compacted road surfaces are subject toerosion which generates sediment.

• Points where roads cross streams areparticularly damaging.

• Roads can become long term sources ofsediment if they are not properly maintained.

Page 5: Sediments transportation

3. In-stream disturbance

• When dams and reservoirs are under construction, disturbance to the stream bank can generate large quantities of sediment.

• (Then PM Shri Atal Bihari Vajpayee declaration-2003)

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4. Other activities

• Sediment can also be increased by mining, (including off-stream quarries and in-stream sand extraction), dredging, some industrial processes and cleaning weirs that have filled with deposited sediment.

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Characteristics of sediments

• Sediments carried by estuarine waters typicallyencompass a range of sizes from less than 2mm (0.002 mm) to more than 4 mm, but thefiner sizes dominate most estuaries

• The bed and banks of most estuaries aredominated by clays and silts, with sand andlarger sizes depositing either at the head of theestuary (from upstream sources) or at the seaor ocean entrance (from downstream sources)

• Fine-grained sediments—clay sizes and some silts—include both inorganic and organic materials and are almost universally called mud.

Page 8: Sediments transportation

Characteristics of sediments

Size (μm) Wentworth scale classification cohesion

> 2000 gravels and cobbles cohesionless

63-2000 sand cohesionless

40-63 Medium silt to coarse silt Practically cohesionless

20-40 Fine silt to medium silt Cohesion increasingly important with decreasing size

2-20 Coarse clay to very fine silt Cohesion important

<2 Very fine clay to medium clay Cohesion very important

Page 9: Sediments transportation

• For transport purposes sediments are principallycharacterized by their size, by constituent composition,and by cohesion

• Estuarine sand is typically composed of quartz, althoughother minerals such as feldspar or various heavyminerals such as magnetite may be present or evenpredominate, depending on the sediment source.

• Fine sediments in estuaries are mixtures of inorganic minerals, organic materials, and biochemicals.

Conti..

Page 10: Sediments transportation

Cont…..

• Mineral grains consist of clays (e.g. montmorillonite, illite, and kaolinite) and non-clay minerals (e.g. quartz and carbonate).

• Organic materials include biogenic detritus and bacteria

• Organic fractions in suspended sediment ranging from 18% to 85% have been reported in various esturies

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• Cohesion describes the tendency of finesediment grains to bind together (aggregateor flocculate) under some circumstances,which significantly affects sediment behaviour.

• In general, smaller grains are more cohesive,with diameters greater than 40 mm essentiallycohesionless, and cohesion becomingprogressively more important as grain sizedecreases.

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Cohesionless Sediment Transport

• Transport Modes

• Cohesionless sediment (sand size and larger, pluscoarser silt are transported)

• At very low flow speeds, when the flow exertstractive forces on the bed that are lower than acritical value, no motion occurs.

• If the flow-induced forces slightly exceed thecritical value for initiation of motion, individualgrains begin to tumble or hop along the bed

Page 13: Sediments transportation

• At higher flow speeds the hops become longer jumps and the bed surface sediment is generally in motion, but with individual grains remaining on the bed between jumps.

• Finally, jumps of some grains take them high into the water column, where they can be transported significant distances before touching the bed again.

Page 14: Sediments transportation

• The simplest classification scheme divides the total sediment transport rate into

• bedload consists of grains rolling, sliding and jumping in frequent or continuous contact with the bed,

• suspended load consists of grains in suspension above the bed for extended periods of time.

• wash loadThat part of the suspended load that is not found in the bed is referred as wash load.

• Measurement of above loads is difficult so it is total bed or measured loads.

Page 15: Sediments transportation

Bed-load transport

Once the forces actingon particles are strongenough to intiate motion…

… particles slide, roll, and saltate down the river bed at a steady rate.

Figure from Chanson, p. 180

Figure from Chanson, p. 200

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Bed forms• The shape of the cohesionless bed surface varies

with flow and the rate of transport• An initially smooth, planar bed will remain smooth

for low transport rates,• then become covered with moving ripples at a

slightly higher transport rate.• At still higher rates the ripples coalesce into large

sand waves (or dunes)

Page 19: Sediments transportation

Dunes and ripples

Page 20: Sediments transportation

Conti…

• Sediment particles eroded from the upstreamdune face land on the steeper downstream faceand the waves march slowly downstream.

• Under the reversing flow of estuaries, sand wavesreverse their migration direction every few hours,but often exhibit a locally dominant direction thatcan be interpreted to establish dominant flowand transport directions

Page 21: Sediments transportation

• Effects of sedimentation

Page 22: Sediments transportation

How does sedimentation affectfluvial ecosystems ?

• Sediment suspended in water, can have significantimpacts on the flora and fauna living in the stream.

• Apart from the increased turbidity, water qualitycan also be affected by increased loads of nutrientsand toxic substances attached to sediment particles.

• Deposited sediment form of sediment has markedimpacts on stream flora and fauna, coating in-stream habitat and filling in pools.

Page 23: Sediments transportation

How does sedimentationaffect fish ?

• Increased turbidity has significantphysical, physiological and behaviouraleffects on fish.

• Sediment is harmful to the gills, clogginggill mucus and causing asphyxiation.

• Fish can also swallow large quantities ofsediment, causing illness, reduced growthand eventual death.

• Predatory fishes suffer from the food

• Clean, clear water is important for fishbreeding

• Recent studies on several species of

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What are the effects of sedimentation on invertebrates?

• Invertebrates (such as mayflies) use gills for respiration. In silt-laden water their gills become clogged and less effective.

• Some invertebrates filter food from the water using nets spun from silk, (such as caddisflylarvae) or other filtering devices,

• These devices become coated or clogged with silt, limiting their effectiveness

• Leeches, snails and some fly larvae are equipped with small suction devices

• Some species spend most of their life in these spaces within the stream bed, others use it as juveniles to escape predators, or during floods to avoid being washed away

Page 26: Sediments transportation

What are the effects of

sedimentation on aquatic plants ?• Increased turbidity reduces the penetration of

light for aquatic plants to photosynthesise andgrowth.

• Deposited sediment can smother plants livingon the bottom of the river or stream

• Increased amounts of nitrates and phosphatesin the water can create problems of excessiveplant growth and lead to blooms of algae andweeds.

Page 27: Sediments transportation

Actions to reduce sedimentation

• Prevent soil erosion through revegetation and stabilisation of catchments and riparian zones.

• Protect riparian vegetation: maintain continuous riparian vegetation along all waterways, ensuring dense groundcovers such as grasses.

• Construct and maintain road crossings or stock access routes to avoid erosion and runoff into waterways.

• Manage runoff from cleared or disturbed land so that it does not enter waterways.

• Use sediment traps or other means.

• Don’t flush sediment accumulated in farm dams or weirs downstream.

• Promote development and implementation of strategies or codes of practice to minimise

Page 28: Sediments transportation

reference

• Chapter 4, Sedimentation of rivers and streams. Department of Natural Resources and Environment (NRE) at www.nre.vic.gov.au

• COASTAL ZONES AND ESTUARIES – Sediment Transport in Estuaries - W. H. McNally, A. J. Mehta, USA.

• For further details

http://www.eolss.net/Eolss-sampleAllChapter.aspx

Page 29: Sediments transportation

Jai Hind Fish for food

Fish for health

Fish for environment