3. Hydrographs

Question 1

What is a Hydrograph?

Answer 1

A hydrograph is a means of showing the discharge of a river at a given point over a short period of time (an individual rainfall event)

Question 2

Cumecs

Answer 2

Cubic Metres per Second

Question 3

Peak Flow / Peak Discharge

Answer 3

The highest level of water reached during a flood

Question 4

Bankfull Discharge

Answer 4

When the river is completely full - if it rises further there will be flooding!

Question 5

Rising Limb

Answer 5

Measures increasing discharge over time. Steeper limb = flashier flood!

Question 6

Falling Limb / Recession

Answer 6

Measures discharge after peak discharge / heavy flow.

• Decreasing in steepness
• Less steep than rising limb because of throughflow now reaching river (to increase water level).

Question 7

Runoff / Storm Flow

Answer 7

Stream discharge after a rainstorm, produced by a combination of overland flow and then throughflow (overland makes greatest contribution).

Question 8

Base Flow

Answer 8

(Normal flow ) - stream discharge produced by water seeping from bedrock

Question 9

Lag Time

Answer 9

The difference between peak rainfall and peak discharge.

Question 10

Rainfall Peak

Answer 10

The maximum volume of rainfall that occurrs in a storm event.

Question 11

Approach Segment

Answer 11

The discharge before rainfall

Question 12

Infiltration Capacity

Answer 12

The maximum rate at which the soil will absorb water

Question 13

Rainfall Intensity

Answer 13

Amount of rain in a given time

Question 14

If infiltration capacity is greater than rainfall intensity…

Answer 14

…the stores (capacity of the soil to store water) will fill up before overland flow occurs. This is typical of many locations in the UK.

Question 15

If infiltration capacity is lower than rainfall intensity…

Answer 15

…then rapid runoff occurs (known as Hortonian flow). Water cannot enter the ground so it runs straight off.

This often occurs in hot arid and seri-arid environments.

Question 16

What does a steeper / more peak-like rising limb represent?

Answer 16

A steeper / more peak-like rising limb represents a more rapid response to rainfall with a shorter time of rise / lag time.

Question 17

Which physical factors (catchment area) lead to steeper rising limbs?

Answer 17

Catchment area factors leading to steeper rising limbs:

• Impermeable rock in (most) of catchment area
• Extensive bare rock with little soil
• Cold climates with little vegetation
• Large, dendritic drainage basin (catchment area) with a high drainage density
• Basin with steep valley sides
• Thin soil with limited infiltration capacity

Question 18

Which physical factors (weather) lead to steeper rising limbs?

Answer 18

Weather factors leading to steeper rising limbs:

• Major intensive thunderstorm
• Saturated soil following prolonged rainfall (Aberfan)
• Fast snow melt

Question 19

Which human factors lead to steeper rising limbs?

Answer 19

Human factors leading to steeper rising limbs:

• Rivers without dams
• Urbanised river basins

Question 20

What does a flatter / gentler rising limb represent?

Answer 20

A flatter / gentler rising limb represents a slower response to rainfall and a longer lag time

Question 21

Which physical factors (catchment area) lead to gentler rising limbs?

Answer 21

Catchment area factors leading to gentler rising limbs:

• Catchment area contains permeable rock (porous or pervious)
• Large areas of vegetation / forest
• Humid climate and denser vegetation
• Small, linear drainage basin / drainage basin with a low drainage density
• Basin with gentle valley sides
• Thick / deep soil with a large infiltration capacity

Question 22

Which physical factors (weather) lead to gentler rising limbs?

Answer 22

Weather factors leading to gentler rising limbs:

• Long period of gentle rain
• Dry, thick soil (less prolonged rainfall so less saturated)
• Slow snow melt

Question 23

Which human factors lead to gentler rising limbs?

Answer 23

Human factors leading to gentler rising limbs:

• River with dams
• Rural river basins (not urbanised)

Question 24

What factors affect a river’s regime?

Very similar to factors affecting flashiness of flood

Answer 24

Factors affecting a river’s regime:

Climate - amount and distribution of rainfall e.g. seasonal
Snowmelt patterns
Geology - permeable / impermeable rock
Rates of evapotranspiration (vegetation)
Steepness of land
Human factors - abstraction for domestic water supply / dams / irrigation

Some factors not included here can be added from ‘factors affecting rising limb’

Question 25

Why is it useful to know about the river’s regime?

Answer 25

Why is it useful to know about the river’s regime?

• Irrigation
• Shipping
• Sewage disposal
• Recreation
• Water supply
• Seasons with increased flood risk
• Rice planting

Question 26

Why might humans build dams to regulate a river’s regime?

Answer 26

Reasons for dams:

• Hold back high flow to prevent flooding
• Hold back high flow as a water source e.g. for irrigation
• Hold back high flow to generate HEP evenly throughout the year