geos 206 - sediment transport

what factors are used to classify fluvial sediment

measurement principle, transport mechanism, sediment size, and morphological role

what makes up the washload, and why is washload important

silt and clay

• all sediments except washload will be found on the bed

what materials are in suspension

clay and silt

what materials are in saltation

silt, sand, and gravel

what materials are in traction

gravel, cobble, and boulders

what does bed material make up

it makes up the banks and forms the cross sectional shape

do fine or coarse sediments require a higher shear stress for motion

fine sediment

• silt and clay

• due to cohesion of fine particles so they do not behave as individual particles but larger clumps

what is number for fully turbulent flow and why?

𝜽𝒄 = 0.06 ≈ constant

• for particles larger than 5mm

• 𝜽𝒄 (dimensionless shear stress)

surface armory

the surface of the bed has coarser material and finer material beneath it

• finer sediment is carried faster therefore deposited first and then larger sediment is deposited later

what is the primary factor to initiate motion of material

the shear stress needed to initiate motion depends on grain roughness

how does morphology affect material movement

surface structures redistribute shear stress, increase hydraulic roughness and turbulence, and modify critical shear stress

what is a factor when considering the entrainment of mixed size sediment

large grains protrude into the flow more and small grains tend to hide in between them

• this makes large grains easier to move than smaller grains

• grain packing and relative grain size complicate things

what affects suspended load transport

Q

• bed material can be in suspension part of the time and partially on the bed

• washload (related to sediment supply) is always in suspension and is not related to Q → a river will support as much washload as is supplied

what are some measurement methods for sediment

helly-smith (fin + bag contraption)

sediment traps

belts

hydrophones

check dams

morphological budgeting

what is suspended load

sediment that is transported in suspension within the water column

why is suspended load significant

• floodplain formation

• many contaminants to water quality travel on the surface of suspended particles

• if it is too high it is a poor habitat for many aquatic species

how is concentration maintained in flow

• upward turbulent dispersion of sediment must balance downward settling

how is rouse number and grain size related

if grain size gets larger then the rouse number will also increase and the grain is more likely to settle

why does suspended load increase with Q

when Q increases so does velocity

• higher U means more erosion and more turbulence thus more sediment is carried