Sediment Transport Flashcards
The process of eroding sediment from one place, transporting it in the flow and depositing it in another place.
Sediment Transport
Occurs when the shear stress applied on
the sediment bed exceeds the critical shear stress
Erosion
It takes place when the transport capacity of the flow is exceeded.
Deposition
(4) Basic sediment process
Resuspension
Transport
Settling
Consolidation & compaction
Sediment process is dependent on
1)Flow hydrodynamic conditions
2) Sediment Properties (size, shape, density, composition)
It is most fundamental property
governing the motion of the
sediment particles in water.
Particle Settling
Defined as the terminal velocity at which a single particle falls through quiescent water and depends on the particle’s size, shape, and density along with the water’s viscosity and density.
Settling Velocity
True/False
For Sediment Transport Modeling, the settling velocity of particles and their resistance to resuspension under shear
stress are the most significant.
True
True/False
PARTICLE SETTLING CHARACTERISTICS vary as they respond to hydrodynamic conditions in the water body.
True
True/False
Once near the bottom, DEPOSITION governs their removal from the water column.
True
2 major mechanisms for sediment transport
Currents & Wind waves
True/False
Sediment transport is significantly
influenced by the instability of the near-bed flows.
True
True/False
Sediment is moved either only suspended or bed load.
False (and/or)
Calculation of total sediment load
Suspended load + Bed load
It is the portion of the sediment load that is transported in suspension in the water column which includes:
sediments resuspended from the bed and wash load brought from upstream.
Suspended Load
It is used to analyze contaminant transport since contaminants are often attached to and transported by sediments in suspension.
Total Suspended Load
It has relatively fine material in near-permanent suspension
that is transported through the system without deposition.
Wash Load
It is comprised of particle sizes finer than those found in the
bed.
Wash Load
True/False
Cohesive sediments can be transported as suspend load and bed load.
False
True/False
Cohesive sediments can be transported as suspended and or bed load.
False (only as suspended load)
True/False
Cohesive sediments can be transported as suspended and or bed load.
False (only as suspended load)
True/False
Non-cohesive sediment can only be transported as bed load.
False (both suspended and bed load)
The 2 mechanism of transporting cohesive as suspend load:
Advection
Dispersion
It is a mechanism where the cohesive sediment is carried with the ambient water at the flow velocity.
Advection
It is a mechanism of transport where cohesive sediment is moved from an area of high to low sediment concentration.
Dispersion
It is a type of load where it is comprised of particles that move on/ near the bead by Saltation, rolling, or slid8ng in the bed layer.
Bed Load
True/False
The bed load movement occurs intermittently in a thick layer; on or in close contact with the bed.
False (thin)
It is a process by which individual
sediment grains make isolated or
serial jumps along the bed?
Saltation
t provides a transition from bed-load
transport that takes place
immediately above the bed to
suspended-load transport that takes
place in the overlying water column.
Saltation
It is defined as the frictional force per unit of bed area exerted on it by the flowing water.
Shear Stress
It is an important factor in the movement of bed material and is a useful parameter in describing sediment transport as well as erosion and deposition.
Shear Stress
True/False
Shear stress is exerted on the bed when water flows over it, as either steady flow or oscillatory flow under tides and waves.
True
True/False
Shear stress moves bed particles in a sliding motion downstream.
False (rolling)
True/False
If the shear stress is greater than the critical shear stress, only a fraction of the suspended material will be deposited.
True (higher shear stress-sediment go up)
True/False
If the shear stress is less than the critical shear
stress, suspended sediment will be deposited on bed gradually.
True
(3) bottom shear stress mechanism vary
Temperature
Topography
Salinity
3 Bottom shear stress mechanism vary
Temperature
Topography
Salinity
3 Bottom shear stress mechanism vary
Temperature
Topography
Salinity
4 Sources of sediment transport
Point discharges
Land surface runoff
Bank erosion
Bed scour
4 Predominant factors responsible for sediment transport
Currents
Winds
Inflows
Tides (in estuaries and coastal waters)
Major Mechanism of sediment transport
Currents & Waves
A major mechanism dominant in rivers.
Currents
A major mechanism that is dominant in lakes, especially during storm.
Wind wave
The two mechanism that is dominant estuaries.
Wind & tidal waves
True/False
In Steps 1 and 2, the predominant mode of transport is sediment grains moving along the bottom in the form of bed load. Particle movement begins as a sliding or rolling motion.
True
True/False
In Step 3, some particles also may move above the bed surface through saltation. These movements result in frequent contact of the moving particles with the bed and are transported as bed load.
True
In Step 4, particles begin to be suspended and are subject to _______ forces as the flow velocity and shear stress increase. Particles are
thrown up into suspension and are transported as suspended load.
turbulent
(2) Difficult to separately identify the phase of
resuspension and deposition in natural waterbodies
Resuspension &
Deposition
It determined the critical shear stress for erosion as a function of depth and the erosion rate as a function of both shear stress and depth. (Designed by McNeil)
SEDflume
It measures the erosion of sediments at high shear stresses and with sediment depth.
SEDflume
It is controlled primarily by BOTTOM SHEAR STRESS and also SITE-SPECIFIC BED PROPERTIES for erosion rates and scour depths.
Sediment Resuspension
True/False
Sediment on the bed will be eroded and transported when the bottom shear stress EXCEEDS A CRITICAL VALUE.
True
[Sediment RESUSPENSION]
The Immersed particle weight is the main stabilizing force.
Non-cohesive sediment
True/False
[Sediment RESUSPENSION]
For sediment with bulk density close to the water density, the main forces are interparticle adhesion and organic binding.
True
True/False
[Sediment RESUSPENSION]
Beds are layered with density and shea strength decreasing downward.
False (increasing)
True/False
Sediments move whenever the shear stress caused by water flow and wind waves REACHES A CRITICAL SHEAR STRESS FOR RESUSPENSION that is EQUAL TO THE SHEAR STRENGTH “HOLDING” the sediment to the bed.
True
A process by which suspended sediments LEAVE the water column, either temporarily or permanently, and become part of the bottom sediments.
Sediment Deposition
True/False
[Sediment DEPOSITION]
With non-cohesive sediment, the critical shear stress is only slightly less than that for erosion.
Particles settle to the bed almost as soon as the
shear stress is too small to erode it.
True
True/False
[Sediment DEPOSITION]
With cohesive sediment, critical shear can be much greater than that for erosion.
False (smaller)
It can be estimated as the
product of the settling velocity and the probability of deposition on contact with the bed, which is 0 for very turbulent systems and 1 for stagnant ponds.
Deposition Rate
True/False
For depostion, particles must overcome
RESISTANCES due to:
Turbulence in the Water Column
Thin Viscous Sublayer at the Interface
Chemical or Biological Activity at the Bottom
True
True/False
Probability of deposition depends on:
Bottom Shear Stress
Suspended Sediment Size
Cohesiveness of the Sediment
True
True/False
Once in SUSPENSION, sediments will settle out at a rate determined by the sediment concentration, the settling velocity, and the turbulence intensity.
True
