Classes taken this semester
Environmental Fluid Mechanics
- Turbulence
- Sediment Transport
- Turbulence
- Sediment Transport
Final Exam for the subject
The final exam covers the sediment transport problems. The chapters that are covered in final exam is from chapter 5 - chapter 8.
Create Spreadsheet for the final exam
Students are required to create the spreadsheet for the calculation of sediment transport.
Important Questions for Final exam
1) What factors are important to the transport of sediment ?
2 ) What are the types of sediment loads ?
3) What is sediment texture ?
4 ) What are modes of sediment transport ?
5 ) What are the important factors for the transport of a sediment grain, and what are the forces involved ?
6) What is saltation ?
Ans : Saltation is an important mode of transport for sandy and gravelly systems.
7) How does initiation of motion takes place ?
Ans: The initiation of movement of individual grains is dependent on a variety of factors, both deterministic and random.
Deterministic (forces on particles ) : Gravity , Fluid (Lift, Drag)
Randomness : Grain placement , Turbulence
8) What is the problem with Shields' curve ?
Ans: The problem with Shields curve is that it is an implicit relation. That is, it requires knowledge of the critical shear velocity u*c in order to calculate the critical Shields stress Tc*.
9) What is sediment flocculation ?
Ans: The colloidal wash load of streams mixes with saline sea water which changes the properties of the clay-electrolyte system. The suspended particles move about due to the Brownian motion, and relative to each other due to different settling velocities and velocity gradients. These movements cause collisions which lead to formation of aggregates (flocs).
10) What is Brownian motion ?
Ans: Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid (a liquid or a gas ).
11) What happens after the flocculation mechanism ?
Ans : Collisions arising from the Brownian motion and differential settling are gentle and the aggregates formed weak and fluffy. Rotational collisions due to vertical gradients usually lead to stronger aggregates. The aggregates are much larger than the individual particles. When a floc touches the bed it can bond to the bed surface. The bonding is controlled by the number and strength of inter-mineral contacts between the floc and the bed and the fluid stresses acting there.
12) What are the requirements for sediment motion ?
Ans: The requirement of sediment motion are one of the following :
13 ) What is viscous sublayer or laminar sublayer ?
Ans : Viscous sublayer or laminar sublayer is the region of a mainly turbulent flow that is near no-slip boundary and in which the flow is laminar.
14) What is average shear stress ?
Average shear stress is also called tractive force. T (written as tou) = gsDS (gs = gamma) where S is the channel's energy slope and D is the water depth.
15) Why is third auxiliary parameter introduced in Shield's diagram ?
Ans: There is a direct relationship between Tc and U* since U* = sqrt (Tc / rho). THus critical shear stress determination is convoluted. Because the shear stress determination is complicated by the dependency between Tc and U* an auxiliary third parameter was proposed.
16) What is stream bed armoring ?
Most alluvial sediments, with the exception of fine sands, have a broad grain size distribution. The geometric standard deviation of river gravels is normally about 3.5 to 4. The erosion of such beds under certain conditions can lead to the formation of an armour layer on the bed surface, also known as paving, which protects the bed.
17) What are different types of scour ?
The scour at a structure may be
a) General scour of the stream which would occur irrespective whether the structure was there or not
b) Constriction scour arising from the constriction of the water way, for example, when flows over flood plains have been intercepted by bridge causeways and channeled through the bridge waterway.
c) Local scour resulting from the effect of the structure on local flow pattern. The local scour may be superimposed on constriction and / or general scour.
Clear water scour: It refers to the condition when the bed material upstream of the scour area is at rest. The bed shear stresses in areas beyond the direct influence of the structure are at or below the critical or threshold stress for initiation of bed material movement.
Live bed scour : Occurs under conditions of general sediment transport and there is a sediment input into the scour zone from upstream.
18) Why is sediment transport important ?
Knowledge of sediment transport is most often used to know whether erosion or deposition will occur, the magnitude of this erosion or deposition, and the time and distance over which it will occur.
Aeolian : Sediment transport by wind.
Fluvial : Fluvial processes relate to flowing water in natural systems. This encompasses rivers, streams, periglacial flows, flash floods and glacial lake outburst floods. Results in formation of dunes and ripples.
Coastal : Due to motion of waves and currents.
2 ) What are the types of sediment loads ?
3) What is sediment texture ?
4 ) What are modes of sediment transport ?
5 ) What are the important factors for the transport of a sediment grain, and what are the forces involved ?
6) What is saltation ?
Ans : Saltation is an important mode of transport for sandy and gravelly systems.
7) How does initiation of motion takes place ?
Ans: The initiation of movement of individual grains is dependent on a variety of factors, both deterministic and random.
Deterministic (forces on particles ) : Gravity , Fluid (Lift, Drag)
Randomness : Grain placement , Turbulence
8) What is the problem with Shields' curve ?
Ans: The problem with Shields curve is that it is an implicit relation. That is, it requires knowledge of the critical shear velocity u*c in order to calculate the critical Shields stress Tc*.
9) What is sediment flocculation ?
Ans: The colloidal wash load of streams mixes with saline sea water which changes the properties of the clay-electrolyte system. The suspended particles move about due to the Brownian motion, and relative to each other due to different settling velocities and velocity gradients. These movements cause collisions which lead to formation of aggregates (flocs).
10) What is Brownian motion ?
Ans: Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid (a liquid or a gas ).
11) What happens after the flocculation mechanism ?
Ans : Collisions arising from the Brownian motion and differential settling are gentle and the aggregates formed weak and fluffy. Rotational collisions due to vertical gradients usually lead to stronger aggregates. The aggregates are much larger than the individual particles. When a floc touches the bed it can bond to the bed surface. The bonding is controlled by the number and strength of inter-mineral contacts between the floc and the bed and the fluid stresses acting there.
12) What are the requirements for sediment motion ?
Ans: The requirement of sediment motion are one of the following :
- FL = Ws
- Lift force = submerged weight
- FD = FR (drag force = resistance force )
- Mo = MR ( Overturning moment = resistive moment )
13 ) What is viscous sublayer or laminar sublayer ?
Ans : Viscous sublayer or laminar sublayer is the region of a mainly turbulent flow that is near no-slip boundary and in which the flow is laminar.
14) What is average shear stress ?
Average shear stress is also called tractive force. T (written as tou) = gsDS (gs = gamma) where S is the channel's energy slope and D is the water depth.
15) Why is third auxiliary parameter introduced in Shield's diagram ?
Ans: There is a direct relationship between Tc and U* since U* = sqrt (Tc / rho). THus critical shear stress determination is convoluted. Because the shear stress determination is complicated by the dependency between Tc and U* an auxiliary third parameter was proposed.
16) What is stream bed armoring ?
Most alluvial sediments, with the exception of fine sands, have a broad grain size distribution. The geometric standard deviation of river gravels is normally about 3.5 to 4. The erosion of such beds under certain conditions can lead to the formation of an armour layer on the bed surface, also known as paving, which protects the bed.
17) What are different types of scour ?
The scour at a structure may be
a) General scour of the stream which would occur irrespective whether the structure was there or not
b) Constriction scour arising from the constriction of the water way, for example, when flows over flood plains have been intercepted by bridge causeways and channeled through the bridge waterway.
c) Local scour resulting from the effect of the structure on local flow pattern. The local scour may be superimposed on constriction and / or general scour.
Clear water scour: It refers to the condition when the bed material upstream of the scour area is at rest. The bed shear stresses in areas beyond the direct influence of the structure are at or below the critical or threshold stress for initiation of bed material movement.
Live bed scour : Occurs under conditions of general sediment transport and there is a sediment input into the scour zone from upstream.
18) Why is sediment transport important ?
Knowledge of sediment transport is most often used to know whether erosion or deposition will occur, the magnitude of this erosion or deposition, and the time and distance over which it will occur.
Aeolian : Sediment transport by wind.
Fluvial : Fluvial processes relate to flowing water in natural systems. This encompasses rivers, streams, periglacial flows, flash floods and glacial lake outburst floods. Results in formation of dunes and ripples.
Coastal : Due to motion of waves and currents.
Application of Sediment Transport
- Movement of sediment is important in providing habitat for fish and other organisms in rivers. Therefore, managers of highly regulated rivers, which are often sediment-starved due to dams, are often advised to stage short floods to refresh the bed material.
- Sediment discharge into a reservoir formed by a dam forms a reservoir delta. Knowledge of sediment transport can be used to properly plan to extend the life of a dam.
- Flow in culverts, over dams, and around bridge piers can cause erosion of bed. This erosion can damage the environment and expose or unsettle the foundations of the structure. Therefore, good knowledge of mechanics of sediment transport in a built environment are important for civil and hydraulic engineers.
- Siltation: When suspended sediemtn transport is increased due to human activities, causing environmental problems including the filling of channels.
Bed Load
Bed load constitute 5-10% of the total sediment load in a stream.
Bed material load is the only part of sediment load that actively interacts with the bed .
Bed material load is the only part of sediment load that actively interacts with the bed .
Transport Stage
Bed load transport rates may be given by a ratio of bed shear stress to critical shear stress, which is equivalent in both the dimensional and nondimensional cses. This ratio is called the "transport stage" (Ts or phi) and is an important in that it shows bed shear stress as a multiple of the value for criteria for the initiation of motion.
Ts = Tb / Tc
Ts = Tb / Tc
Short coming of Engelund - Hansen
The bed material load formula of Engelund and Hansen is the only one to not include some kind of critical value for the initiation of sediment transport. The Engelund-Hansen formula is one of the few sediment transport formula in which a threshold "critical shear stress" is absent.
Prandtl's Mixing length turbulence model
Prandtl postulated that the mean fluctuating velocity for a flow would be expressed by a product of a mixing length and depth wise velocity gradient.
Meyer-Peter and Muller
It works well for wide channels, not in flood. It is also helpful if the bed material is well-sorted sand (i.e capacity- limited streams).
Capacity - limited
The amount of sediment delivered to the river mouth is limited by the water supply to the watershed. In other words, there is so much material available to be transported that sediment fluxes are limited by power supplied by the running water. Large foreland-basin rivers (e.g, the Mississippi) are an example.
Supply Limited
The amount of sediment available to be moved is limited. Bedrock mountain streams are an example.