1.2 Rainfall - discharge relationships within drainage basins

Question 1

Define discharge

Answer 1

• The total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs)
• Discharge = cross sectional area x velocity

Question 2

What is a flood hydrograph?

Answer 2

shows how the discharge of a river varies over a short period of time

Question 3

What does the rising limb show?

Answer 3

When the storm water enters the drainage basin the discharge rate increases

Question 4

What is the peak discharge?

Answer 4

The highest flow in the channel

Question 5

What is the base level?

Answer 5

The lowest point to which it can flow

Question 6

What does the falling limb show?

Answer 6

The fall in discharge back to base level

Question 7

What is the lag time?

Answer 7

The delay between the maximum rainfall amount and the peak discharge.

Question 8

Climatic factors affecting the flood hydrograph

Answer 8

• Precipitation type and intensity
• Temperature and evapotranspiration
• Antecedent moisture: moisture in the soil that is retained after a period of rainfall

Question 9

Drainage basins characteristics affecting the flood hydrograph

Answer 9

• Size
• Shape
• Drainage density
• Slopes

Question 10

Geology, soils and vegetation affecting the flood hydrography

Answer 10

• Porosity and impermeability of rocks and soils
• Vegetation type
• Land use

Question 11

Precipitation type and intensity

Answer 11

• Intense rainfall => overland flow => steep rising limb and high peak discharge
• Low-intensity rainfall => water likely to infiltrate and percolate => increased lag time and low peak discharge

Question 12

Temperature and evapotranspiration

Answer 12

• High temperature => evaporation => less water getting into the river
• Affects type of precipitation
• Warm air can hold more water => potential high peak discharge

Question 13

Antecedent moisture

Answer 13

If it has been raining previously and the ground is saturated => more overland flow, high peak discharge and shorter time lag

Question 14

Drainage basin shape and size

Answer 14

Smaller drainage basins respond more quickly to rainfall => smaller peak discharge but less distance travelled => shorter lag time

Question 15

Drainage density

Answer 15

Basins with high drainage density respond more quickly (more areas to take water in) => shorter lag time

Question 16

Slopes

Answer 16

Steeper slopes create more overland flow => shorter lag time and higher peak discharge

Question 17

Porosity and impermeability of rocks and soils

Answer 17

• Impermeable surfaces create more overland flow
• Rocks such as chalk and gravel are permeable and allow water to infiltrate and percolate => reduces peak discharge and increases time lag
• Rocks such as clay is more impermeable

Question 18

Vegetation type

Answer 18

• Broad leafed intercepts more rainfall => reduces peak discharge and increases time lag
• Contrast to deciduous trees

Question 19

Land use

Answer 19

• Usually creates more impermeable surfaces, reduce vegetation => increase overland flow => shorter time lags and increase peak discharge
• Drainage channels (sewers, drains) carries water into the river very quickly