1.2- Flood Hydrographs

Question 1

River flow is studied by measuring

Answer 1

The discharge of a river

Question 2

Define discharge

Answer 2

Volume of water passing a measuring point in a given time

Question 3

What is the equation to work out discharge?

Answer 3

Discharge ( m^3 per seconds) = cross-sectional area (m^2) X velocity (metres per second)

Question 4

What is a storm hydrograph?

Answer 4

A graph that shows how river discharge changes as a result of a period of heavy rain

Question 5

When it rains, only a very small proportion of the precipitation falls directly into the river channel- most of the rain falls on the valley sides and takes time to reach the river = the water makes its way downslope and the river rises as shown by

Answer 5

The rising limb of the hydrograph

Question 6

Flow increases to the point of peak discharge; from that time the amount of water reaching the river as a result of the rainstorm will begin to

Answer 6

Decrease (receding or falling limb)

Question 7

The starting and finishing level show

Answer 7

The base flow fed by groundwater

Question 8

The time delay between peak rainfall and peak discharge is known as

Answer 8

Lag time

Question 9

Rivers that have a rapid rise in discharge over a short period of time are said to have

Answer 9

Flash responses and produce a ‘peaky’ graph

Question 10

Why do some rivers produce a flash response and thus peaky graph?

Answer 10

Water is mainly entering the river via surface run-off

Question 11

Other rivers will have a more benign response to the same rainfall event- their discharge will not rise so high and the lag will be extended over a longer period of time = graph much flatter. This is because

Answer 11

More water is entering the river via throughflow and groundwater flow and less via surface-run-off- this has the effect of spreading spreading out the flow over an extended period of time; hence the graph is flatter (the reasons for this can be explained by the characteristics of the drainage basin itself)

Question 12

The way a river responds to a rainfall event has significant implications for its management- flash responses lead to a very abrupt rise in discharge over a very short period of time which can lead to

Answer 12

More frequent flood events- the characteristics of the drainage basin will explain how a river responds to a rainfall event

Question 13

Water enters the river through 3 main routes:

Answer 13

Surface run-off, throughflow and groundwater flow (very little precipitation can fall directly into the channel so this can be discounted)

Question 14

_______ ___-___ is by far the fastest route to the river, therefore drainage basins that encourage this and limit infiltration are likely to lead to a more ‘peaky’ hydrograph

Answer 14

Surface run-off

Question 15

How does vegetation restrict surface run-off?

Answer 15

• deciduous forest in summer increases rates of transpiration by as much as 40%
• this is achieved through interception
• water doesn’t even reach the surface
• vegetation therefore restricts surface run-off by reducing the quantity of water reaching the surface
• speed at which precipitation reaches the surface is slowed down by the forest canopy

Question 16

Rainfall reaches the surface via the canopy in a process known as

Answer 16

Throughfall

Question 17

What is throughfall likely to lead to?

Answer 17

• this slow transfer of water to the surface is more likely to lead to infiltration and percolation rather than surface run-off

Question 18

Any form of human intervention or development that removes dense forest vegetation is likely to increase

Answer 18

Surface run-off

Question 19

What is a river regime?

Answer 19

The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics

Question 20

There are almost ____ river systems in the UK and they vary greatly, being extremely sensitive to variations in climate, land-use changes and water abstraction

Answer 20

1500

Question 21

River systems are also influenced by

Answer 21

The landscape through which they flow- mountain rivers fed by high rainfall will differ from less flashy groundwater fed streams in drier areas of southeast England

Question 22

UK rainfall is fairly evenly distributed throughout the year with a slight autumn/winter maximum. By contrast, seasonal variations in temperature and sunshine intensity ensure that

Answer 22

• evaporation loses are heavily concentrated in the summer half-year (April- September)
• this results in a marked seasonality on river flows with maximum flows normally in the winter and minimum flows normally occurring in the summer or autumn
• however urban catchments may not conform to this pattern

Question 23

In addition to climatic conditions, explain how geological characteristics of individual catchments, their permeability especially, has a major influence on river flow patterns

Answer 23

• take example of neighbouring catchments in Thames basin
• although they experience almost identical climatic conditions, the flow regime for the Lambourn, which is fed by outflows of from the underlying chalk, is markedly more stable than that for the Ock which drains a largely impermeable clay catchment, allowing a more immediate response to rainfall events