Measurements of suspended load become increasingly difficult andinaccurate near the bed, and bedload can be satisfactorily measured only in specialcircumstances such as laboratory or small scale field experiments.
Sediment Transportation in Estuary Ecology
NARESH KUMAR MEHTA
What sediment is ?
Sediment, a naturally
occurring material that is
broken down by processes
of weathering and erosion,
by the action of wind,
water, or ice, and/or by the
force of gravity acting on
the particle itself.
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
2. Road building• Unsealed roads can contribute significant
• The compacted road surfaces are subject to
erosion which generates sediment.
• Points where roads cross streams are
• Roads can become long term sources of
sediment if they are not properly maintained.
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 declaration2003)
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.
Characteristics of sediments
• Sediments carried by estuarine waters typically
encompass a range of sizes from less than 2
mm (0.002 mm) to more than 4 mm, but the
finer sizes dominate most estuaries
• The bed and banks of most estuaries are
dominated by clays and silts, with sand and
larger sizes depositing either at the head of the
estuary (from upstream sources) or at the sea
or ocean entrance (from downstream sources)
• Fine-grained sediments—clay sizes and some
silts—include both inorganic and organic
materials and are almost universally called
Characteristics of sediments
Wentworth scale classification
gravels and cobbles
Medium silt to coarse silt
Fine silt to medium silt
Coarse clay to very fine silt
Very fine clay to medium clay
Cohesion very important
• For transport purposes sediments are principally
characterized by their size, by constituent composition,
and by cohesion
• Estuarine sand is typically composed of quartz, although
other minerals such as feldspar or various heavy
minerals such as magnetite may be present or even
predominate, depending on the sediment source.
• Fine sediments in estuaries are mixtures of inorganic
minerals, organic materials, and biochemicals.
• Mineral grains consist of clays (e.g.
montmorillonite, illite, and kaolinite) and nonclay minerals (e.g. quartz and carbonate).
• Organic materials include biogenic detritus
• Organic fractions in suspended sediment
ranging from 18% to 85% have been reported
in various esturies
• Cohesion describes the tendency of fine
sediment grains to bind together (aggregate
or flocculate) under some circumstances,
which significantly affects sediment behaviour.
• In general, smaller grains are more cohesive,
with diameters greater than 40 mm essentially
cohesionless, and cohesion becoming
progressively more important as grain size
Cohesionless Sediment Transport
• Transport Modes
• Cohesionless sediment (sand size and larger, plus
coarser silt are transported)
• At very low flow speeds, when the flow exerts
tractive forces on the bed that are lower than a
critical value, no motion occurs.
• If the flow-induced forces slightly exceed the
critical value for initiation of motion, individual
grains begin to tumble or hop along the bed
• 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.
• 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.
Once the forces acting
on particles are strong
enough to intiate motion…
Figure from Chanson, p. 200
… particles slide, roll, and
saltate down the river bed at a
Figure from Chanson, p. 180
• 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)
• Sediment particles eroded from the upstream
dune face land on the steeper downstream face
and the waves march slowly downstream.
• Under the reversing flow of estuaries, sand waves
reverse their migration direction every few hours,
but often exhibit a locally dominant direction that
can be interpreted to establish dominant flow
and transport directions
How does sedimentation affect
fluvial ecosystems ?
• Sediment suspended in water, can have significant
impacts on the flora and fauna living in the stream.
• Apart from the increased turbidity, water quality
can also be affected by increased loads of nutrients
and toxic substances attached to sediment particles.
• Deposited sediment form of sediment has marked
impacts on stream flora and fauna, coating instream habitat and filling in pools.
How does sedimentation
affect fish ?
physical, physiological and behavioural
effects on fish.
• Sediment is harmful to the gills, clogging
gill mucus and causing asphyxiation.
• Fish can also swallow large quantities of
sediment, causing illness, reduced growth
and eventual death.
• Predatory fishes suffer from the food
• Clean, clear water is important for fish
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 caddisfly
larvae) 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
What are the effects of
sedimentation on aquatic plants ?
• Increased turbidity reduces the penetration of
light for aquatic plants to photosynthesise and
• Deposited sediment can smother plants living
on the bottom of the river or stream
• Increased amounts of nitrates and phosphates
in the water can create problems of excessive
plant growth and lead to blooms of algae and
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
• Promote development and implementation of strategies or
codes of practice to minimise
• 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
Fish for food
Fish for health
Fish for environment