1.2. Discharge Relationships within drainage basins

Question 1

Discharge

Answer 1

the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs).

Question 2

Cumecs

Answer 2

Cubic metres per second

Question 3

Peak Flow/Discharge

Answer 3

Maximum discharge in the river

Question 4

Rising Limb

Answer 4

The rising water in the river

Question 5

Falling Limb

Answer 5

Falling flood water in the river

Question 6

Lag Time

Answer 6

Time difference between the peak of the rain storm and the peak flow of the river

Question 7

Baseflow

Answer 7

Normal discharge of the river (via groundwater flow)

Question 8

Stormflow (run-off)

Answer 8

Overland flow + throughflow

Question 9

Bankfull discharge

Answer 9

When the discharge of the river is at the top of the banks - any further discharge will cause flooding

Question 10

Precipitation type and intensity (Factors affecting storm hydrographs)

Answer 10

Intense rainfall will create a steep rising limb and high peak flow. Low amount of rainfall will more likely infiltrate increasing the lag time and reducing peak flow. Snowmelt can cause flooding and lead to high rates of overland flow, and high peak flows.

Question 11

Temperature and evapotranspiration (Factors affecting storm hydrographs)

Answer 11

Higher temperature means more evaporation therefore decreasing the peak flow.

Question 12

Antecedent moisture (Factors affecting storm hydrographs)

Answer 12

If it has been raining previously and the ground is saturated or near saturated, rainfall will quickly produce overland flow and a high peak flow and short lag time.

Question 13

Drainage basin size and shape (Factors affecting storm hydrographs)

Answer 13

Circular drainage basins respond quicker than liner ones therefore the lag time is reduced in a circular drainage basin.

Question 14

Drainage density (Factors affecting storm hydrographs)

Answer 14

Urban areas with high drainage density reduce the lag time a lot; it is the opposite with areas with low drainage density.

Question 15

Porosity (Factors affecting storm hydrographs)

Answer 15

The more porous the surface the higher lag time and increases the peak flow there is as the water is more likely to be infiltrated. Areas that aren’t porous have a reduced lag time and reduced peak flows.

Question 16

Slopes (Factors affecting storm hydrographs)

Answer 16

Steeper slopes create more overland flow, shorter lag times and higher peak flows.

Question 17

Vegetation type (Factors affecting storm hydrographs)

Answer 17

Broad-leafed vegetation intercepts more rainfall, especially in summer, and so reduces the amount of overland flow and peak flow, and increases lag time. In winter, deciduous trees lose their leaves and so intercept less.

Question 18

Land use (Factors affecting storm hydrographs)

Answer 18

Land uses that create impermeable surfaces or reduce vegetation cover reduce interception and increase overland flow. If more drainage channels are built (sewers, ditches, drains) the water is carried to rivers very quickly. This means that peak flows are increased and lag times reduced.