Hydrology exam questions
. Explain why the minimum velocity needed for sediment erosion varies.
– Larger particles such as boulders, need a higher velocity for them to be picked up because
of their large size
– However, the exception to this rule is clay and silt, as even though the particles are very
small , the particles tend to stick together, making them hard to pick up.
– Higher velocities are needed for picking up (entrainment) than just for transporting.
4 . Explain two reasons for the variation of deposition along a river channel. (4)
– Drop in velocity on entering a lake
– Amount of friction, higher in upper course of river, greater turbulence thus very variable
local velocity, step-pool sequences
– Hydraulic radius increases downstream therefore less friction and less deposition
– Deposition behind obstacles
– Entering riffles
– Effect of meander shape, usually in middle/lower zone of rivers e.g. lower velocity on
inside bends e.g. point bars
– Change in gradient downstream with effect on velocity
3 – Change in sediment supply such as from tributaries
– Decreased sediment size downstream as a result of attrition means less likely to be
deposited
. Briefly explain why precipitation may not always reach a river channel. (3)
Precipitation may:
– Be intercepted by vegetation and then uptake by plant roots
– Be lost by evapotranspiration from vegetation
– Be used by humans
– Evaporate from impermeable surfaces
– Permeate through bedrock and enter ground water and eventually an aquifer
– Be stored in a lake
– Fall as snow and become consolidated as ice
2 . Outline two factors which influence the formation of a braided channel (4)
– Variable discharge e.g. seasonal/daily variations in discharge by glacial/snow melt or
seasonal discharge in semi-arid areas
– High sediment load enabling deposition as eyots in times of low discharge
– Multiple channels created in times of low flow
– Easily erodible riverbanks leading to wide, shallow channels
– Stabilisation of eyots with vegetation in times of low discharge
1There should be some discussion of the nature of braided channels (eyots, vegetated or
, 5
unvegetated, multiple channels) and therefore the factors that influence their formation
. Describe the variations in velocity of flow for transport and deposition (4)
– The velocity required to transport sediment is always higher than that required to deposit
sediment.
– For clay, the velocity to transport sediment can be less than 0.1 cm/s, as very little velocity
is needed once the sediment is entrained.
– At higher velocities, larger sediments are deposited whilst the smaller sediments are
transported.
– Variations in velocity of flow could be taken to mean variations horizontally across the
graph.
. Describe and explain the formation of river deltas:
– Distributaries/Braided Streams: As a river approaches its mouth, it often splits into multiple
channels called distributaries or forms a network of interconnected braided streams. This
occurs when the river carries more sediment than it can transport, causing it to divide into
smaller channels.
– Shape: The shape of a river delta can vary depending on various factors such as the river's
sediment supply, wave and current patterns, and tides. There are different types of delta
shapes, including bird's foot, cuspate, and arcuate deltas.
– Deposition: As the river water enters the body of water, it slows down, losing its ability to
transport sediment effectively. Consequently, the sediment carried by the river is
deposited. The deposition of sediment results in the accumulation of material, gradually
building up the delta over time.
– Eyots: Eyots, also known as river islands or riverine islands, are small landforms that can
be found within river deltas. They are formed by the deposition of sediment around
obstacles such as vegetation or rocks, creating elevated areas within the delta.
– Topset/Foreset/Bottomset Beds: Within a river delta, sediment deposition occurs in
distinct layers. The topset beds consist of coarse sediment, such as sand and gravel,
which are deposited at or near the water's surface. The foreset beds consist of finer
sediment, such as silt and clay, which slope down from the delta front into the body of
water. The bottomset beds are composed of fine-grained sediment that settles in deeper
water beyond the delta front.
– Sorting of Sediment: Sediment within a river delta is often sorted based on its size and
weight. Coarser sediment tends to settle closer to the river mouth, while finer sediment is
carried further into the delta or offshore. This sorting process leads to distinct sediment
layers within the delta.
– Flocculation of Clay: Flocculation refers to the process by which clay particles in the river
water bind together to form larger clumps or aggregates. Flocculation occurs due to the
presence of certain chemicals or changes in water conditions, such as a decrease in
velocity or an increase in salinity. Flocculated clay particles are less likely to remain in
suspension and are more easily deposited in the delta.
. Explain why the minimum velocity needed for sediment erosion varies.
– Larger particles such as boulders, need a higher velocity for them to be picked up because
of their large size
– However, the exception to this rule is clay and silt, as even though the particles are very
small , the particles tend to stick together, making them hard to pick up.
– Higher velocities are needed for picking up (entrainment) than just for transporting.
4 . Explain two reasons for the variation of deposition along a river channel. (4)
– Drop in velocity on entering a lake
– Amount of friction, higher in upper course of river, greater turbulence thus very variable
local velocity, step-pool sequences
– Hydraulic radius increases downstream therefore less friction and less deposition
– Deposition behind obstacles
– Entering riffles
– Effect of meander shape, usually in middle/lower zone of rivers e.g. lower velocity on
inside bends e.g. point bars
– Change in gradient downstream with effect on velocity
3 – Change in sediment supply such as from tributaries
– Decreased sediment size downstream as a result of attrition means less likely to be
deposited
. Briefly explain why precipitation may not always reach a river channel. (3)
Precipitation may:
– Be intercepted by vegetation and then uptake by plant roots
– Be lost by evapotranspiration from vegetation
– Be used by humans
– Evaporate from impermeable surfaces
– Permeate through bedrock and enter ground water and eventually an aquifer
– Be stored in a lake
– Fall as snow and become consolidated as ice
2 . Outline two factors which influence the formation of a braided channel (4)
– Variable discharge e.g. seasonal/daily variations in discharge by glacial/snow melt or
seasonal discharge in semi-arid areas
– High sediment load enabling deposition as eyots in times of low discharge
– Multiple channels created in times of low flow
– Easily erodible riverbanks leading to wide, shallow channels
– Stabilisation of eyots with vegetation in times of low discharge
1There should be some discussion of the nature of braided channels (eyots, vegetated or
, 5
unvegetated, multiple channels) and therefore the factors that influence their formation
. Describe the variations in velocity of flow for transport and deposition (4)
– The velocity required to transport sediment is always higher than that required to deposit
sediment.
– For clay, the velocity to transport sediment can be less than 0.1 cm/s, as very little velocity
is needed once the sediment is entrained.
– At higher velocities, larger sediments are deposited whilst the smaller sediments are
transported.
– Variations in velocity of flow could be taken to mean variations horizontally across the
graph.
. Describe and explain the formation of river deltas:
– Distributaries/Braided Streams: As a river approaches its mouth, it often splits into multiple
channels called distributaries or forms a network of interconnected braided streams. This
occurs when the river carries more sediment than it can transport, causing it to divide into
smaller channels.
– Shape: The shape of a river delta can vary depending on various factors such as the river's
sediment supply, wave and current patterns, and tides. There are different types of delta
shapes, including bird's foot, cuspate, and arcuate deltas.
– Deposition: As the river water enters the body of water, it slows down, losing its ability to
transport sediment effectively. Consequently, the sediment carried by the river is
deposited. The deposition of sediment results in the accumulation of material, gradually
building up the delta over time.
– Eyots: Eyots, also known as river islands or riverine islands, are small landforms that can
be found within river deltas. They are formed by the deposition of sediment around
obstacles such as vegetation or rocks, creating elevated areas within the delta.
– Topset/Foreset/Bottomset Beds: Within a river delta, sediment deposition occurs in
distinct layers. The topset beds consist of coarse sediment, such as sand and gravel,
which are deposited at or near the water's surface. The foreset beds consist of finer
sediment, such as silt and clay, which slope down from the delta front into the body of
water. The bottomset beds are composed of fine-grained sediment that settles in deeper
water beyond the delta front.
– Sorting of Sediment: Sediment within a river delta is often sorted based on its size and
weight. Coarser sediment tends to settle closer to the river mouth, while finer sediment is
carried further into the delta or offshore. This sorting process leads to distinct sediment
layers within the delta.
– Flocculation of Clay: Flocculation refers to the process by which clay particles in the river
water bind together to form larger clumps or aggregates. Flocculation occurs due to the
presence of certain chemicals or changes in water conditions, such as a decrease in
velocity or an increase in salinity. Flocculated clay particles are less likely to remain in
suspension and are more easily deposited in the delta.
