Rivers

Question 1

Explain why the velocity needed to pick up particles varies with their 5 size

Answer 1

-Larger particle sizes require higher velocities to be picked up
because they are heavier.
• Sand (0.1–1.0 mm) requires the lowest velocity for entrainment
because of its small size and non-cohesive nature (granular nature).
• Clays have a more cohesive nature, which requires higher velocities
for entrainment.

TALK ABOUT DIFFERENT SEDIMENT TYPES

Question 2

Explain why the size of sediment deposited varies along a river.

Answer 2

• Velocity is reduced on entering sea or lake
• Shallow water on inside meander bend
• Change of river gradient
• River overflows its banks with a reduction in velocity
• River bed roughness (upper / lower course of river)

Question 3

Explain why, during a rain event, there could be more overland flow 5 than infiltration.

Answer 3

The amount and intensity of rain overcoming the infiltration capacity of
the soil
• the infiltration capacity of the soil which is governed by factors such as
the porosity and permeability of the soil
• antecedent moisture conditions
• different land uses (e.g. forestry, arable, pastoral) and surfaces (e.g.
impermeable surfaces in urban areas)
• steep gradient

Question 4

Explain how soils and rock type influence the amount and rate of 5 infiltration in a drainage basin

Answer 4

porosity
• permeability
• infiltration capacity influenced by different soil characteristics
• antecedent moisture in the soil might also be considered, as some
types of soil will retain water from previous rainfall more than others

Question 5

Explain why catchment flows change after urbanisation.

Answer 5

Decrease in vegetation means less evapotranspiration, thus more water reaches the ground surface.
• Less interception leads to more water reaching the surface and possibly more overland flow.
• Reduction in roots, as a result of less vegetation, leads to decrease in infiltration as fewer pathways for water to go.
• Impermeable surfaces, a result of urbanisation, reduces infiltration, increasing overland flow and directing flows of water through artificial drainage, percolation, throughflow and base flow reduced.
• Human impacts on flows – abstraction of groundwater reducing groundwater flow, dam building/drainage systems, associated with urbanisation.

Question 6

Suggest the factors that led to the flooding

Answer 6

High rainfall amounts/high intensity rainfall/high river discharge
• High antecedent moisture in the drainage basin
• High runoff from impermeable surfaces
• Changes to catchment characteristics
• Low lying land
• Levée insufficiently high
• Lack of specific flood prevention work

Question 7

Briefly explain why abrasion/corrasion varies along a river channel.

Answer 7

Velocity
• Amount of sediment load
• Size of sediment load
• Discharge
• Erodibility of river bed