11.9 Physical causes of flooding

Question 1

Precipitation def

Answer 1

Any source of moisture reaching the ground eg rain snow frost

Question 2

Interception def

Answer 2

Water being prevented from reaching the surface by trees or grass

Question 3

Surface storage def

Answer 3

Water held on the ground surface, eg puddles

Question 4

Infiltration def

Answer 4

Water sinking into soil/rock from the ground surface

Question 5

Soil moisture def

Answer 5

Water held in the soil layer

Question 6

Percolation def

Answer 6

Water seeping deeper below the surface

Question 7

Groundwater def

Answer 7

Water stored in the rock

Question 8

Transpiration def

Answer 8

Water lost though pores in vegetation

Question 9

Evaporation

Answer 9

Water lost from ground/vegetation surface

Question 10

Surface runoff def

Answer 10

Water flowing on top of the ground

Question 11

Throughflow def

Answer 11

Water flowing through the soil parallel to the surface

Question 12

Groundwater flow def

Answer 12

Water flowing through the rock layer parallel to the surface

Question 13

Water table def

Answer 13

Current upper level of saturated rock/soil where no more water can be absorbed

Question 14

Explain why flooding is more likely to happen in the city than in the countryside

Answer 14

• infiltration rates are faster in rural environments due to permeable rock and soil. The vegetation also intercepts the rainfall
• however, flash floods can occur when the soil is baked hard in the summer, the ground is frozen in winter or when a period of prolonged rainfall leads to the saturation of soil and the rise of the water table. In all these cases, the rain cannot infiltrate the soil and surface runoff increases, leading to flooding
• infiltration rates are slower in urban environments because concrete is impermeable. This means that there is more surface run-off, so water will return more quickly to the river and this will lead to flooding

Question 15

What type of rock is found in mountains, why does it not allow water to pass through and why is infiltration low in the mountains

Answer 15

• slate: it is impermeable
• the rock is often bare, with thin soils and little vegetation to intercept the rainfall

Question 16

Why is it difficult for infiltration to occur in low-lying areas

Answer 16

Often contain an impermeable clay soil. It is usually vegetated but the soil is so compacted that it is difficult for infiltration to occur. There is not enough gradient to remove the water

Question 17

Where is flooding less likely to occur

Answer 17

In areas with permeable rock (chalk + limestone) as water passes through these rocks

Question 18

Why is there increased surface runoff on mountainsides

Answer 18

Steep slopes mean that surface runoff happens on mountainsides before rain has time to infiltrate the soils

Question 19

Using the example of snowdonia, outline how precipitation, geology and relief are all connected

Answer 19

Precipitation: relief rainfall due to high relief
Geology: impermeable slate
Relief: steep gradient so there will be surface runoff
Therefore heavy rain falls on steep slopes and is unable to infiltrate. This leads to flooding