Unit 1 Exam Questions

Question 1

(a) (i) Define the fluvial terms traction and abrasion.

Answer 1

Traction:
The process where larger sediments, such as boulders and cobbles, are rolled or dragged along the riverbed by the force of flowing water.
(2 marks)

Abrasion:
The erosion caused by the rubbing or scraping of sediment particles against the riverbed and banks, similar to a sandpaper effect.
(2 marks)
(Total: 4 marks)

Question 2

(a) (ii) Describe the process of suspension within a river channel.

Answer 2

Suspension refers to the transport of fine-grained sediment, such as silt and clay, within the water column.

These particles remain suspended and are not in contact with the riverbed or banks due to the turbulence of the water.

The velocity and discharge of the river determine the amount of sediment that can be carried in suspension.
(3 marks)

Question 3

(b) Explain the formation of river bluffs and levées.

Answer 3

River bluffs:
These are steep edges formed at the boundaries of a floodplain.
Meandering rivers erode laterally over time, cutting into the valley sides and creating bluffs.
This process is often enhanced in areas with a steeper gradient or when rivers migrate across the floodplain.

Levées:
Levées are natural embankments formed along riverbanks during flooding.
When a river overflows its banks, the flow loses energy, depositing the heaviest (coarser) sediments closest to the channel.
Finer sediments are carried further and settle across the floodplain.
Repeated flood events lead to the buildup of these embankments over time.

Question 4

(a) (i) Describe the conditions which lead to overland flow on slopes.

Answer 4

Heavy or prolonged rainfall exceeds the infiltration capacity of the soil.

Saturated soils (antecedent moisture) prevent further infiltration.

Impermeable surfaces, such as urban areas or clay soils, limit infiltration.

Steep slopes encourage surface runoff due to gravity.

Question 5

(a) (ii) Explain how the shape of a drainage basin affects the shape of a storm hydrograph.

Answer 5

A circular drainage basin has evenly distributed tributaries, leading to a rapid concentration of flow at the main channel. This produces a short lag time and high peak discharge.

An elongated drainage basin has tributaries entering the main channel at different times, resulting in a longer lag time and a lower peak discharge.

The degree of symmetry in basin shape influences the rate and uniformity of flow concentration.

Question 6

(b) Explain how the Hjulström curve is used to explain erosion and deposition in a river channel.

Answer 6

Erosion Threshold:
Fine particles like clay require higher velocities to be eroded due to cohesion between particles, while medium-sized particles like sand require lower velocities because they are not cohesive. Larger particles such as gravel and boulders require significantly higher velocities due to their weight.

Deposition Threshold:
When the river’s velocity drops below a certain level, particles can no longer be transported and are deposited.
Finer particles such as silt and clay settle at very low velocities, whereas larger particles like gravel and boulders require higher velocities to stay in motion and are deposited first when the flow slows.

Question 7

(c) Explain two reasons for the variation of deposition along a river channel.

Answer 7

Change in velocity:
When a river enters a lake or sea, velocity decreases, leading to the deposition of heavier sediments first and finer materials further downstream.
Similarly, deposition occurs on the inside bends of meanders, where flow velocity is slower compared to the outer bend.

Channel gradient and friction:
In the upper course, high friction and turbulence due to a rough riverbed cause localized deposition in slower-moving areas like behind rocks or obstacles.
In the lower course, lower gradient and reduced friction allow finer sediments to settle.

Question 8

(a) (i) Briefly explain why precipitation may not always reach a river channel.

Answer 8

Interception: Vegetation traps rainwater, which may evaporate before reaching the ground.

Evaporation: Water can evaporate directly from surfaces or from intercepted water on vegetation.

Percolation and groundwater storage: Precipitation infiltrates the soil and becomes part of the groundwater, which may not contribute directly to river flow.

Storage in other reservoirs: Water can be stored temporarily in lakes, snow, or ice before reaching the channel.

Question 9

(a) (ii) Outline two factors which influence the formation of a braided channel.

Answer 9

Variable discharge:
Rivers with seasonal or daily variations in discharge (e.g., from glacial melt) experience frequent sediment deposition and reworking, forming multiple channels.

High sediment load:
Rivers with large amounts of sediment deposit material during low flow periods, forming islands (eyots) and shallow, wide channels.

Easily erodible banks:
Rivers with weak banks (e.g., sandy or silty soils) are prone to erosion, creating shallow channels that divide into multiple streams.

Question 10

(b) Describe and explain how soft engineering and hard engineering can be used to prevent river floods.

Answer 10

Soft Engineering:
Reforestation: Planting trees increases interception and reduces surface runoff, decreasing the likelihood of flooding.
Floodplain zoning: Restricting construction on floodplains minimizes property damage and allows natural storage of floodwaters.
Wetland restoration: Restoring wetlands increases the land’s ability to store excess water during floods.

Hard Engineering:
Dams and reservoirs: Dams store excess water upstream, releasing it gradually to prevent downstream flooding.
Levees and embankments: These structures raise riverbanks, increasing channel capacity to contain higher water levels.
Channel straightening: Reducing meanders increases the flow velocity, moving floodwaters out of an area more quickly.

Question 11

(a) (i) Describe the main features of a meander.

Answer 11

River cliff: The outer bend is eroded by faster flow, forming a steep bank.

Point bar/slip-off slope: Deposition occurs on the inner bend where flow is slower.

Asymmetrical cross-section: The channel is deeper on the outer bend and shallower on the inner bend.

Helicoidal flow: A corkscrew motion transfers material from the outer bend to the inner bend.

Pools and riffles: Alternating deeper and shallower sections develop within the meander.

Question 12

(a) (ii) Explain two factors which influence the level of a water table.

Answer 12

Rainfall: High rainfall increases recharge and raises the water table, while drought or reduced precipitation lowers it.

Vegetation: Dense vegetation promotes infiltration, raising the water table. Conversely, evapotranspiration removes water, potentially lowering the water table.

Rock type: Permeable rocks, like sandstone, allow water to infiltrate and raise the water table, while impermeable rocks, like clay, limit water infiltration and cause variability in water table levels.

Question 13

(b) Describe and explain the formation of deltas.

Answer 13

Deltas form where rivers flow into standing water (e.g., a sea or lake) and their velocity drops sharply.
Reduced velocity causes sediment deposition, forming distinct layers: topset, foreset, and bottomset beds.
Flocculation occurs when clay particles in the river combine in saline water and settle out.

Distributaries: As sediment builds up, the river splits into smaller channels.
Types of deltas:
Bird’s foot: Narrow projections into the sea, e.g., the Mississippi Delta.
Arcuate: Rounded shape with many distributaries, e.g., the Nile Delta.
Cuspate: A pointed shape formed by wave action, e.g., the Tiber Delta.

Question 14

(b) Describe the features of a braided river

Answer 14

The river channel is wide with multiple interconnected channels separated by bars or eyots.

Some bars are bare, while others are vegetated, indicating stability in certain areas.

The channel shows evidence of slight meandering and frequent branching.

The riverbed consists of sediment deposits, including gravel and sand, reflecting the high sediment load.

The channel appears to widen downstream, possibly due to decreasing gradient and reduced flow velocity.

Question 15

c) Explain the formation of the features of a braided river

Answer 15

High sediment load: The river carries a significant sediment load, which it deposits during periods of low flow.

Fluctuating discharge: During high discharge, the river erodes and transports sediment, while low discharge results in deposition, forming bars.

Unstable riverbanks: Erosion of weak or non-cohesive banks allows the channel to widen and split into multiple channels.

Bar formation and vegetation colonisation: Deposited sediments accumulate into bars.
Over time, some bars stabilise as vegetation grows, forming eyots.

Variable flow velocity: Faster-flowing sections erode material, while slower-moving sections deposit it, leading to the braided pattern.

Question 16

(a) (i) Define the terms throughflow and soil water.

Answer 16

Throughflow: The downslope movement of water through the soil under the influence of gravity, typically towards a river channel.
Example: Water moving through soil after infiltration during rainfall events.

Soil water: The water held in soil pores after infiltration, available for uptake by plants or for further movement through the soil.

Question 17

(a) (ii) Briefly explain how drainage density affects the shape of a storm hydrograph.

Answer 17

High drainage density:
Results in a flashy hydrograph with a steep rising limb and short lag time. This is because water reaches the channel more quickly due to the high number of streams and short flow paths.

Low drainage density:
Produces a gentle hydrograph with a longer lag time. Fewer tributaries and longer flow paths slow water movement to the river channel.

Question 18

(b) Explain the effects of land use change on catchment flows and catchment stores.

Answer 18

Urbanisation:
Increases surface runoff due to impermeable surfaces such as roads and buildings, leading to shorter lag times and higher peak flows.
Reduces infiltration and soil water storage, causing a decrease in groundwater recharge.

Deforestation:
Reduces interception and evapotranspiration, increasing the volume of water reaching the surface.
Soil erosion can compact the land, reducing infiltration and further increasing runoff.

Agricultural land use:
Irrigation can deplete groundwater stores, while over-cultivation may compact soil, reducing infiltration rates.
Drainage systems in farmland may alter flow paths, increasing channel flows.

Question 19

(c) Explain two reasons why some extreme rainfall events do not result in river flooding.

Answer 19

Drainage basin characteristics:
Basins with high infiltration rates due to permeable soils (e.g., sandy soils) or porous rocks (e.g., limestone) absorb more water, reducing surface runoff and the likelihood of flooding.

Vegetation cover:
Dense forests and vegetation increase interception and promote transpiration, reducing the volume of water reaching river channels.

Human interventions:
Effective flood management infrastructure, such as dams, reservoirs, and levees, can store excess water and prevent river overflow.

Question 20

(a) (i) Define the fluvial terms thalweg and bluff.

Answer 20

Thalweg: The line of fastest flow in a river channel, usually found in the deepest part.
Example: In a meander, the thalweg shifts to the outer bend, where erosion dominates.

Bluff: A steep, linear slope marking the outer edge of a river’s floodplain, formed by lateral erosion over time.
Example: The Mississippi River floodplain has distinct bluffs at its boundaries.

Question 21

(a) (ii) Briefly explain how turbulent flow occurs in rivers.

Answer 21

High velocity: Fast-moving water increases turbulence by generating chaotic flow patterns.

Channel roughness: Irregularities such as rocks, boulders, and vegetation increase friction, disrupting smooth flow and creating eddies.

Steep gradient: A steep slope accelerates water movement, amplifying chaotic interactions within the flow.

Question 22

(b) Explain how river erosion can lead to the formation of waterfalls.

Answer 22

Differential erosion:
Waterfalls form where a river flows over layers of hard, resistant rock underlain by soft, less resistant rock.
Erosion processes (e.g., hydraulic action, abrasion) erode the softer rock more quickly, creating a step in the riverbed.

Formation of a plunge pool:
The falling water scours the base of the waterfall through cavitation and abrasion, forming a deep plunge pool.

Undercutting and collapse:
Continued erosion undercuts the hard rock layer, leaving it unsupported. Eventually, it collapses, causing the waterfall to retreat upstream.

Knickpoint recession:
Over time, this process repeats, shifting the waterfall upstream, forming a gorge.
Example: Niagara Falls is retreating upstream at an average rate of about 1 meter per year.

Question 23

a) Label the main features of a delta.

Answer 23

Distributary channels: Clearly show how the river splits into multiple smaller channels.

Sediment deposits: Indicate areas where sediment has accumulated near the channels.

Vegetated areas: Highlight stabilised regions of the delta.

Eyots: Label any small islands formed by sediment deposition.

Question 24

(b) Briefly explain the formation of the features within a delta.

Answer 24

Distributary channels: These form as sediment builds up in the main channel, forcing the river to split into smaller branches.

Sediment deposits: When the river’s velocity decreases as it enters a standing body of water, sediment is deposited, forming these features.

Vegetated areas: Plant growth stabilises deposited sediments, reducing erosion and helping to anchor the delta.

Eyots: These small islands form from sediment deposits in areas of slow-moving water or where deposition exceeds erosion.

Question 25

Suggest how deltas may change shape over time.

Answer 25

Expansion: Continued sediment deposition could cause the delta to extend further into the sea.

Erosion: Marine processes like wave action and longshore drift may reduce the delta’s size by eroding its edges.

Channel changes: Avulsion or the creation of new distributary channels could alter the delta’s shape, redistributing sediment to different areas.

Question 26

(a) (i) Describe how drainage density is measured.

Answer 26

Drainage density is calculated as the total length of all streams and rivers in a drainage basin divided by the basin’s area.

Units are typically expressed as km/km².

Higher drainage density indicates a more dissected basin with a greater number of streams relative to the area.

Question 27

(a) (ii) Briefly explain how velocity affects erosion in a river.

Answer 27

High velocity: Increases the energy available for erosion, allowing the river to erode larger and more resistant particles through processes like hydraulic action and abrasion.

Hjulström curve: Demonstrates the relationship between particle size and velocity, showing that intermediate particles like sand require less velocity for erosion than finer (e.g., clay) or coarser (e.g., gravel) particles.

Low velocity: Reduces the river’s ability to erode and transport particles, causing deposition instead.

Question 28

(b) Explain how catchment flows and stores are affected by urbanisation.

Answer 28

Reduced infiltration: Impermeable surfaces like roads and buildings limit infiltration, increasing surface runoff.

Higher runoff rates: Water flows quickly to rivers, shortening lag time and increasing flood risk.

Reduced groundwater recharge: Limited infiltration reduces the volume of water stored in aquifers.

Stormwater systems: Urban drainage systems channel water directly into rivers, bypassing natural storage areas like wetlands.

Streamflow variability: Urbanisation often leads to flashier hydrographs, with higher peak flows and steeper rising limbs during storms.

Deforestation: Clearing vegetation for urban development reduces interception and evapotranspiration, increasing the volume of water reaching rivers.

Question 29

c) Explain two factors that affect the shape of a storm hydrograph.

Answer 29

Urbanisation:
Impermeable surfaces such as roads and buildings reduce infiltration and increase surface runoff.
This results in a steep rising limb, short lag time, and higher peak discharge due to rapid water flow into rivers.

Vegetation cover:
Forested or vegetated areas intercept rainfall, slowing its movement to the ground.
This increases lag time, reduces peak discharge, and creates a gentler rising limb.

Drainage basin shape:
A circular basin allows water to concentrate quickly, producing a flashy hydrograph with a steep rising limb and short lag time.
An elongated basin spreads water flow over time, leading to a flatter hydrograph.

Question 30

(a) (i) Describe how underground water recharge occurs.

Answer 30

Infiltration: Rainwater enters the soil through pores and cracks.

Percolation: Water moves downwards through permeable rock layers, driven by gravity.

Recharge of aquifers: Water enters the saturated zone or groundwater storage, increasing the water table.

Question 31

(a) (ii) Explain how channel straightening may help prevent river flooding.

Answer 31

Increases velocity: By removing meanders, water flows faster through the channel, reducing the time for water to pool.

Steepens gradient: A straighter channel creates a more direct path, enhancing flow efficiency.

Increases capacity: Faster flow removes water from vulnerable areas more quickly, reducing the risk of overflow.

Prevents backflow: In straightened sections, water is less likely to back up, further reducing flood risk.

Question 32

(b) Describe and explain different patterns of flow within a river channel.

Answer 32

Laminar flow:
Water flows in parallel layers with minimal mixing.
Occurs in smooth, shallow channels with low velocity.
Example: Occurs in straight sections with uniform beds.

Turbulent flow:
Water flows chaotically with eddies, whirlpools, and increased mixing.
Occurs in rough, steep channels with high velocity.
Example: Common in rapids where hydraulic action and abrasion are intense.

Helicoidal flow:
A corkscrew-like motion in meanders where water flows spirally downstream.
Leads to erosion on the outer bend (river cliff) and deposition on the inner bend (point bar).
Example: Found in mature meandering rivers like the Mississippi.