Flood Hydrographs

Question 1

What is peak rainfall?

Answer 1

The highest amount of rainfall per time unit.

Question 2

What does the rising limb mean?

Answer 2

Shows how quickly the discharge rises after a rain storm.

Question 3

What is peak discharge?

Answer 3

The highest recorded discharge following a rainfall event.

Question 4

What does the lag time show?

Answer 4

The time difference between peak rainfall and peak discharge.

Question 5

What does the falling limb show?

Answer 5

Shows the reduced discharge once the main effect of runoff has passed.

Question 6

What is the base flow?

Answer 6

The normal flow of a river when its water level is being sustained by groundwater flow.

Question 7

What is the size of the basin (flashy hydrograph)

Answer 7

Small- water will reach the channel rapidly, as it has a shorter distance to travel.

Question 8

What is the size of the basin? (Low, flat hydrograph)

Answer 8

Larger- water will take longer to reach the channel as it has greater distance to travel.

Question 9

What is the shape of the basin? (Flashy hydrograph)

Answer 9

In circular basins, the hydrograph will be flashy because water will take less time for the water to reach the channel, as all the extremities are equidistant from the channel.

Question 10

What is the shape of the basin? (Low, flat hydrograph)

Answer 10

In elongated basin, water will take a long time to reach the channel from the extremities of the draining E basin.

Question 11

What is the gradient of the drainage basin slope? (Flashy hydrograph)

Answer 11

Water flows rapidly downhill and reaches the Chanel quickly.

Question 12

What is the gradient of the drainage basin slope? (Low, flat hydrograph)

Answer 12

Water can infiltrate into the ground and travel slowly to the channel through the soil and rock.

Question 13

What is the soil type and porosity? (Flashy hydrograph)

Answer 13

Clay soils have a low porosity and the grains swell when they absorb water, so water infiltrates slowly. Thin soil becomes saturated quickly.

Question 14

What is the soil type and porosity? (Low, flat hydrograph)

Answer 14

Sandy soils have a high porosity, so the water can infiltrate. Deep soils allow more infiltration.

Question 15

What is the permeability of the rock? (Flashy hydrograph)

Answer 15

Water cannot percolate into the rock, increasing surface runoff to rivers.

Question 16

What is the permeability of the rock? (Low, flat hydrograph)

Answer 16

Water percolates through pore spaces and fissures into the groundwater store.

Question 17

Number of streams/ channels in basin? (Flashy hydrograph)

Answer 17

A small number of surface streams per km2 means the storm water will reach the main channel rapidly.

Question 18

Number of streams/ channels in basin? (Low, flat hydrograph)

Answer 18

A high number of surface streams per km2 means the water travels slowly through the soils and rocks back to the river.

Question 19

Effects of vegetation cover? (Flashy hydrograph)

Answer 19

-Thin grass intercepts little water
-Little loss from evapotranspiration, so more water reaches the channel rapidly.

Question 20

Effects of vegetation cover? (Low, flat hydrograph)

Answer 20

-Woodland/ forest intercepts water and has high rates of evapotranspiration, so less water reaches the channel and more slowly.

Question 21

What is the land-use like? (Flashy hydrograph)

Answer 21

Urban surfaces have more hard surfaces such as roads, and drains that carry the water reaches the channel rapidly.

Question 22

What is the land-use like? (Low, flat hydrograph)

Answer 22

Vegetated surfaces intercept water and allow infiltration so water travels slowly to the river channel.

Question 23

What is the rainfall intensity like? (Flashy hydrograph)

Answer 23

When rain falls faster the infiltration capacity, surface runoff occurs and transports the water rapidly to the river.

Question 24

What is the rainfall intensity like? (Low, flat hydrograph)

Answer 24

Low intensity rainfall means water can infiltrate into the soil and then travel slowly through the soil to the river channel.

Question 25

What is the rainfall duration like? (Flashy hydrograph)

Answer 25

During a short duration of rainfall the water table rises and the soil becomes saturated, causing surface runoff, which travels rapidly to the channel.

Question 26

What is the rainfall duration like? (Low, flat hydrograph)

Answer 26

During a prolonged period of rainfall most of the water infiltrates into the soil and travels slowly through the soil into the rocks before reaching the channel.

Question 27

What is the speed of the snow-melt like & what does it mean? (Flashy hydrograph)

Answer 27

Fast snowmelt means water cannot infiltrate into the frozen ground, so it flows rapidly to the channel.

Question 28

What is the speed of the snow-melt like & what does it mean? (Low, flat hydrograph)

Answer 28

Slow snowmelt: the ground thaws with the snow, so the melt water can infiltrate into the soils and rocks before reaching the channel.

Question 29

What are the drainage basin losses? (Flashy hydrograph)

Answer 29

Fewer losses from the drainage basin system will increase discharge into the river channel.

Question 30

What are the drainage basin losses? (Low, flat hydrograph)

Answer 30

High evapotranspiration losses will reduce discharge into the river channel.