Flood Hydrographs

Question 1

What is a flood hydrograph used to represent?

Answer 1

Rainfall for the drainage basin of a river and the discharge of the same river on a graph.

Question 2

What is discharge?

Answer 2

The volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs).

Question 3

What is discharge made up of?

Answer 3

Baseflow and stormflow.

Question 4

What is the rising limb?

Answer 4

The line on the graph that represents the discharge increasing.

Question 5

What is the falling limb?

Answer 5

The line on the graph that represents the discharge decreasing.

Question 6

What is lag time?

Answer 6

The time between peak rainfall and peak discharge.

Question 7

What is baseflow?

Answer 7

The level of groundwater flow.

Question 8

What is stormflow?

Answer 8

Comprised of overland flow and throughflow.

Question 9

What is bankfull discharge?

Answer 9

The maximum capacity of the river.

Question 10

What happens if discharge exceeds bankfull discharge?

Answer 10

The river will burst its banks and be in flood.

Question 11

What is a flashy hydrograph?

Answer 11

Short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics.

Question 12

What is a subdued hydrograph?

Answer 12

Long lag time and low peak discharge.

Question 13

What is the lag time of a flashy hydrograph?

Answer 13

Short lag time.

Question 14

What are the rising and falling limbs like in a flashy hydrograph?

Answer 14

Steep rising and falling limb.

Question 15

What is the flood risk in a flashy hydrograph?

Answer 15

Higher flood risk.

Question 16

What is the peak discharge in a flashy hydrograph?

Answer 16

High peak discharge.

Question 17

What is the lag time of a subdued hydrograph?

Answer 17

Long lag time.

Question 18

What are the rising and falling limbs like in a subdued hydrograph?

Answer 18

Gradually rising and falling limb.

Question 19

What is the flood risk in a subdued hydrograph?

Answer 19

Lower flood risk.

Question 20

What is the peak discharge in a subdued hydrograph?

Answer 20

Low peak discharge.

Question 21

What are the nine natural factors which can affect a flood hydrograph?

Answer 21

High rainfall intensity, antedecent rainfall, impermeable underlying geology, high drainage density, small basin, circular basin, low temperatures, precipitation type and vegetation cover.

Question 22

What do some factors increase in a river to create a flashy hydrograph?

Answer 22

Increase surface runoff, decrease lag time and increase peak discharge.

Question 23

How does high rainfall intensity affect a river?

Answer 23

Higher discharge potential from the river and more likely for soil to reach its field capacity, thus increasing surface runoff and decreasing the lag time.

Question 24

What is antecedent rainfall?

Answer 24

Rainfall that occurs before the studied rainfall event.

Question 25

How does antecedent rainfall affect surface runoff?

Answer 25

Increased surface runoff as ground is saturated and soil has reached its field capacity.

Question 26

How does impermeable underlying geology affect percolation and throughflow?

Answer 26

Decreased percolation and therefore greater levels of throughflow.

Question 27

How does high drainage density affect lag time?

Answer 27

Many tributaries to main river, increasing speed of drainage and decreasing the lag time.

Question 28

How does a small basin affect lag time?

Answer 28

Rainfall reaches the central river more rapidly, decreasing the lag time.

Question 29

How does a circular basin affect lag time?

Answer 29

Rainfall reaches the central river more rapidly, decreasing the lag time.

Question 30

How do low temperatures affect peak discharge?

Answer 30

Less evapotranspiration so greater peak discharge.

Question 31

How does precipitation type affect flooding risk?

Answer 31

Snow or hail takes time to melt before moving towards the river, so rainfall increases the flooding risk.

Question 32

How does vegetation cover affect flood risk and lag time?

Answer 32

Forested areas intercept more rainfall, decreasing the flood risk, but exposed areas will transfer water to the river more rapidly, decreasing lag time.

Question 33

What are the three human processes that can affect a flood hydrograph?

Answer 33

Urbanisation, pastoral farming and deforestation.

Question 34

How does urbanisation affect runoff, storage and infiltration?

Answer 34

More impermeable surfaces, so runoff increased and surface storage and infiltration are reduced.

Question 35

How does pastoral farming affect interception and runoff?

Answer 35

Ground trampled so less interception and more surface runoff.

Question 36

How does deforestation affect water movement and flood risk?

Answer 36

Less interception by trees, so water reaches the ground and river more quickly. More surface runoff and greater flood risk.