W11 Sedimentation in rivers - VU Collaborate PDF

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NEC 2203 – Hydraulics River Hydraulics and Sedimentation Week 11 – 9th October 2018 Dr. Nitin Muttil Room D416d Email: [email protected]

Topics covered River Hydraulics  Importance of rivers  Drainage basins  River morphology Sedimentation in rivers  Basic sediment processes  Sediment production  Sediment transportation  Sediment deposition Reservoir sedimentation and loss of capacity  Bed load estimation

River Hydraulics

Importance of Rivers

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Rivers and their drainage basins constitute a basic building block of our planet 

Societies deeply influenced by their presence

Examples of importance of rivers:      

Water they carry is essential for life itself Rivers as resources Rivers as recreational assets Means of transportation Habitat for huge range of animals and plants Rivers as legal boundaries

• Drainage basins as fundamental basic units

River as a legal boundary

Drainage basins Drainage divide or ridge

Outlet

Outlet: Point where surface water leaves the catchment

Drainage basins

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Most of the earth’s surface consists of drainage basins that supply rivers with water and sediment Why is sediments in rivers significant?

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Sedimentation has 2 types of negative impacts:

1) River morphology issue – Can fill in water control structures and adversely affect fish habitat

2) Water quality issue due to suspended sediments

River Morphology

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River morphology is the science dealing with the changes in shapes of river channels over time River channel processes reflected in river morphology are: Erosion (sediment production)  Transportation (sediment transport)  Sedimentation (sediment deposition) 

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These processes have shaped the present landscape of the world They can cause severe engineering and environmental problems

Sedimentation in Rivers

Basic sediment processes

• The three basic and important sediment processes are: 1) Sediment production (Erosion) 2) Sediment transport 3) Sediment deposition

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As flowing water enters the lake or ocean, its velocity decreases. The water's ability to carry sediments also decreases. Sediments carried by the stream are deposited where the slowing water can no longer move them.

Sediment production Sediment transportation Sediment deposition

Sediment Production • Sediment production refers to the addition of new sediment to the channel system  Landslides  Surface erosion  River bank erosion

• Human activity also lead to changes in natural sedimentation processes

Sediment Production • Riverine sediment production is related to the supply. In some watersheds the supply of sediment is the factor that limits the total sediment transported

Sediment Production • One of the earliest processes in sediment • •

production is from raindrop impact Raindrops or hailstones striking the ground can dislodge sediments making them available for transport. Bare ground that lies between sparse vegetation experiences more of this soil erosion.

Factors Controlling Sediment Production

• Factors are:  Increased

runoff  Size of storm  Vegetated areas  Bushfires  Deforestation  Basin and river slope  Basin area  Basin shape  Stream density  Meandering

Vegetated / Forested Areas

• If the area is heavily vegetated, many of the

• •

raindrops are intercepted by the plant canopy which can lower the fall velocity of the water that eventually reaches the ground surface Forested areas are also more likely to have a layer of forest litter that reduces the ability of the raindrop to directly disrupt the soil surface. Grassy surfaces also have the ability to mute the impact of rain or hail on the soil surface

Bushfires

• Bushfire removes the tree canopy and much of the • •

forest litter Raindrops and runoff can have much greater impact on the open surfaces of burned areas In many areas, fire is a major contributor to sediment production

Bushfires and Deforestation

• Disruption of the natural vegetation through bushfire and deforestation can greatly increase the potential of the landscape to produce sediments

Basin Slope

• The slope of a basin

affects the amount and the timing of runoff • With the land sloping, gravity no longer pulls the water directly into the ground, so more water is likely to become surface runoff. • Also less infiltration to ground`

Basin Slope

• Amount of sediment

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carried by flowing water (Erosion- water removes sediment from the ground surface) increases with increasing slope In general, the steeper the basin slope and the steeper the drainage channels, the quicker the flow response and the higher the peak discharges

Basin Area

• Basin Area has direct influence on the total volume of runoff that drains from that basin.

• If rain falls in a uniform manner over a larger basin and a smaller basin, the larger basin produces more runoff volume

• All things being equal, a drainage area that twice as large can generate twice as much runoff volume as its smaller counterpart

Basin Shape

• Basin shape also has an influence on magnitude and timing of the peak flow at the basin outlet

• Runoff in the more round basin will arrive more quickly at the basin outlet

• • •

Stream Density Stream density is the length of all channels within the basin divided by the area of the basin Higher stream density allows the landscape to drain more efficiently following a storm event More efficient drainage means that water moves into streams and creeks faster, causing peak storm flows to be larger and to occur sooner

Meanders

• A bend in a sinuous watercourse or river • Forms when moving water in a stream erodes the outer banks and widens its valley, and the inner part of the river has less energy and deposits silt

• Meanders in the stream channel add to the distance that water must travel from upstream to downstream

• Meanders increase the travel time of runoff through the basin, and may reduce the overall runoff volume

Meanders

Sediment production Sediment transportation Sediment deposition

Sediment Transport

• Once sediment is produced, it can be transported to • •

and within the river channel Sediment that makes it to the river channel may be transported, or routed downstream, or deposited within the river channel or floodplain There are three primary modes of sediment transport: 1) Dissolved load 2) Suspended load 3) Bed load

Sediment Transport

Sediment Transport

• Dissolved load refers to the process where the • • •

material is completely dissolved in the water and will precipitate out at a downstream location Suspended load is the sediment that travels for relatively long distances within the water Large suspended loads lead to a muddy appearance to the water The suspended load is related to the transport capacity which is a function of velocity, volume and turbulence, as well as the size of the sediment particle

Sediment Transport

• In general, suspended load involves smaller particles • •

such as clay and silt, and is a major contributor to sediment transport Bed load is the sediment that is pushed along the river bed and usually the larger particles such as sand and gravel, and in the case of high transport capacity it may include cobbles and boulders Some of the smaller bed load particles, such as sand, may bounce along the river bed in a process called saltation

Sediment production Sediment transportation Sediment deposition

Sediment Deposition

• The capacity of a river is a term used to describe how much sediment it can carry. When the capacity drops due to changes in the energy of the flow, sediment may be deposited

• Deposition occurs anywhere within a watershed or channel. Enhanced deposition typically occurs where the river slows down, such as where a river enters a lake, reservoir, or ocean

Sediment Deposition

Sediment Deposition • River Mouth: As a river flows, it picks up sediment from the river bed, eroding banks, and debris on the water. The river mouth is where much of sediments (gravel, sand, silt, and clay, called alluvium) carried by river is deposited

Sediment Deposition • River Delta: A landform that forms at the mouth of •

a river, where the river flows into an ocean, sea, estuary, lake, or reservoir. Deltas form from deposition of sediment carried by a river as the flow leaves its mouth.

Sediment Deposition

• Meandering rivers experience enhanced deposition in point bars on the inside bend where the momentum decreases. On the outside bend, higher momentum of sediment may result in enhanced erosion and cut banks

Sediment Deposition

• Human structures such as reservoirs enhance deposition, especially where the river enters the more stagnant water of the reservoir. This process is cited as a primary reason for reduced sediment volume reaching the mouth of managed rivers worldwide

Problems of Sediment Deposition

• Channels, waterways and harbors  Decrease carrying capacity and thus increase flooding  Requires extensive dredging to maintain navigation

• Reservoirs  Reduces the storage capacity

Reservoir sedimentation and loss of capacity

Reservoir Sedimentation

• Rivers carry suspended sediments and bed load • When a river enters a reservoir, velocity and turbulence are reduced. Bed load and larger suspended material settles down near the head of the reservoir and finer material is deposited further along the reservoir or passed through outlet works

• If sediment load is large compared to reservoir capacity, useful life of reservoir may be very short

Reservoir Sedimentation

• To estimate useful life of reservoir, we need to know original capacity and estimate volumetric loss rate, as below:  Estimate suspended load carried by river (in tonnes/year)  Estimate bed load carried by river (in tonnes/year)  Find % of inflowing sediments retained in the reservoir

(i.e. trap efficiency of the reservoir)  Use specific weight (W/V) of settled sediment to allow weight to volume conversion

Suspended Load Estimation

• One method involved sampling the stream water at appropriate locations/depths, filtering to remove sediment, drying and weighing filtered material.  Load is obtained by multiplying concentration with

discharge in that section

Bed Load Estimation

• Much research has been undertaken on this topic and many formulae have been developed. The simplest formulae are as follows:

Where Gb = Bed load N/s Q = water flow rate (m3/s) S0 = bed slope τ0 = bed shear stress and τc is its critical value (N/m2)

Ss = specific gravity of sediment particles d = mean particle diameter (m) B = river surface width; D = avg. depth γ = specific weight of water (N/m3) ψ = a sediment parameter (N/m3/s), can find approx. from (105 * d-0.75)...