flood risk and hydrographs

Question 1

what is river discharge?

Answer 1

the volume of water flowing in a river per second. (measured in cumecs)

Question 2

what is the difference between water infiltration and percolation?

Answer 2

infiltration: water seeping from the surface into the soil.
percolation: water seeping from the soil into permeable rock.

Question 3

what is the difference between streamflow, throughflow and groundwater flow?

Answer 3

streamflow: water flowing in the river channel.
throughflow: water flowing downhill through soil.
groundwater flow: water flowing through permeable rock.

Question 4

how can surface run-off affect floodrisk?

Answer 4

surface run-off is rapid. if this is increased, the flood risk increases, as the river is likely to reach its bankfull capacity faster.

Question 5

how can precipitation increase flood-risk?
(physical)

Answer 5

PROLONGED RAINFALL:
- saturates the soil
- reduces infiltration
- increases surface run-off
- increases river discharge

INTENSE, SUDDEN RAINFALL:
- infiltration doesn’t occur fast enough
- increases surface run-off
- increases river discharge

Question 6

define ‘transpiration’:

Answer 6

evaporation of water from plants.

Question 7

how can geology within the drainage basin increase flood risk?
(physical)

Answer 7

• impermeable soil and rock doesn’t allow infiltration or percolation
• increases surface run-off
• increases river discharge

Question 8

how can the relief of the drainage basin increase flood risk?
(physical)

Answer 8

• steep valley sides means that infiltration will not occur fast enough
• increases surface run-off due to gravity
• increases river discharge
• however, low-lying areas are also at risk. the gradient is not steep enough to remove flood water.

Question 9

how can urbanisation affect flood risk?
(human)

Answer 9

• buildings and roads are made from impermeable material (e.g. concrete, tarmac)
• reduces infiltration
• increases surface run-off
• increases river discharge

Question 10

how can deforestation within the drainage basin increase flood risk?
(human)

Answer 10

• removing trees reduces interception, vegetation storage and transpiration
• increases the amount of precipitation that reaches the surface
• increases surface run-off
• increases river discharge

Question 11

what is a hydrograph?

Answer 11

it shows how a river discharge (at a certain point within the river) changes overtime, in relation to precipitation.

Question 12

what is lag time on a hydrograph?

Answer 12

the time delay between the peak rainfall and the peak discharge. there’s a delay because not all of the rain falls directly into the river channel. it will either flow over the surface (rapid) or flow underground (slower).

Question 13

what is the rising limb and falling limb on a hydrograph?

Answer 13

RISING LIMB:
- increase in river discharge as the rainwater enters the channel
- the steeper the line, the faster rainwater is entering

FALLING LIMB:
- the decrease in river discharge as the river returns back to normal (base flow)

Question 14

how can the relief of the drainage basin affect the hydrograph results?

Answer 14

• steeper valleys will reduce infiltration and increase surface run-off
• rainwater will reach the river channel faster, reducing the lag time
• this will create a steeper rising limb and higher peak discharge

Question 15

how can the geology of the drainage basin affect the hydrograph results?

Answer 15

• impermeable soils and rock will reduce infiltration/percolation and increase surface run-off
• rainwater will reach the channel faster, reducing lag time
• this will create a steeper rising limb and higher peak discharge

Question 16

how can urbanisation of the drainage basin affect the hydrograph results?

Answer 16

• increasing buildings and roads increases impermeable surfaces
• this will reduce infiltration and increase surface run-off
• rainwater will reach the channel faster, reducing lag time
• this will create a steeper rising limb and higher peak discharge

Question 17

how can deforestation of the drainage basin affect the hydrograph results?

Answer 17

• removing trees reduces interception and transpiration
• this increases surface run-off, as more rainwater reaches the surface
• rainwater will reach the channel faster, reducing lag time
• this will create a steeper rising limb and higher peak discharge