The Flood Storm Hydrograph

Question 1

Storm hydrograph

Answer 1

A graph of discharge of a river over the time period when the normal flow of the river is affected by a storm event

Question 2

How are they important?

Answer 2

They predict how a river might response to a rainstorm which can help manage the river.

Question 3

What does the storm hydrography start with ?

Answer 3

Base flow.

River is fed by throughflow of soil water and ground water.

This slow movement of water means that the changes in discharge are small

Question 4

What happens to this slow change in discharge when a storm event occurs?

Answer 4

As storm water enters the drainage basin the river begins to be fed by more fast flowing water.

Discharge rises - shown by rising limb.
Eventually reached peak discharge.

Question 5

Lag time

Answer 5

Time taken from peak rainfall to peak discharge

Shorter lag time more water entering river.

Question 6

What shows the fall in discharge ?

Answer 6

Receding limb.

Question 7

What happens when all the storm water has passed through ?

Answer 7

River returns to base flow.

Question 8

Bankfull

Answer 8

Point at which river will begin to over flow

Question 9

What does bar chart represent ?

Answer 9

Rainfall events .

Question 10

What does line on graph represent ?

Answer 10

Discharge of river

Question 11

Flashy hydrograph

Answer 11

Short lag time
High peak discharge
Steep rising limb
Steep falling limb

Question 12

Subdued hydrograph

Answer 12

Gentle rising limb
Gentle falling limb
Long lag time
Low peak discharge

Question 13

Name 8 physical factors affected the drainage basin shape

Answer 13

Shape of basin 
Sides of basin 
Drainage density 
Saturation 
Soil / rock type.
Vegetation 
Precipitation intensity 
Size of basin

Question 14

Shape of drainage basin

Answer 14

Circular
More flashy hydrograph than those of long and thin shape.
We each point is roughly equidistant from measuring point

Question 15

Sides of drainage basin

Answer 15

Steep sides
More flashier than gently sloping sides.

Because water flows more quickly on steep slopes as either overland flow or throughflow so gets to river more quickly.

Question 16

Basin drainage density

Answer 16

Higher drainage basin ( lots of surface streams acting as tributaries to main river ) have flashier hydrograph a.

Question 17

Basin saturation

Answer 17

Is already saturated by antecedent rainfall then overland flow increases because infiltration capacity has been reached.

Since overland flow is fastest of the transfers , the lag time is reduced.

Peak discharge higher - flashy hydrograph

Question 18

Soil / rock type

Answer 18

Impermeable ( Clay ) overland flow will be higher.

Throughflow and infiltration will be reduces meaning flashy hydrograph.

Surfaces baked hard by the sun during long period of dry weather of frozen surfaces leads to same thing.

Sandstone basins have more subdued graph as water speaks into porous rocks

Question 19

Vegetation cover

Answer 19

Vegetation intercepts precipitation, holding the water on leaves, this slows movement of rainwater to ground and so river channels - thicker vegetation more subdued graph is.

Water lost due to E and ET from vegetation - reduces how much water gets to storm hydrograph - increasing lag time / reducing peak discharge.

Question 20

Intensity / amount of precipitation

Answer 20

Heavy storms with lots of water entering drainage basin over short time results in higher discharge - flashy.

Lag Time higher if it’s snow not rain. As snow takes time to melt before entering river channel

Question 21

Size of drainage basin

Answer 21

Large basins catch more precipitation and so have higher discharge compared to smaller basins.

Smaller basins have shorter lag time because precipitation as it doesn’t have far to travel.

Question 22

Name 5 humans impacts on hydrograph

Answer 22

Deforestation 
Afforestation 
Agriculture 
Growth of urban areas 
Engineering 
Water abstraction

Question 23

Deforestation

Answer 23

Reduces rates of interception.
Rain water hits surface directly.
Lack of vegetation roots reduces infiltration rate into the soil - rapid overland flow and flashy hydrographs

Question 24

Name another negative of deforestation

Answer 24

Exposes soil to greater rates of erosion - leaving to greater sedimentation of channel.
This reduce bankfull capacity of a river and leads to greater chance of flooding

Question 25

Afforestation

Answer 25

Opposite effect to deforestation
Allows for interception and infiltration.
Flood prevention measure

Question 26

Agriculture

Answer 26

1. Plough breaks up topsoil meaning greater infiltration - subdued.
2. Terracing on hillsides stops movement of water downhill - subdued
3. Grass crops increasing infiltration - subdued
4. Large numbers of animals on small areas impact soils leaving to more overland flow

Question 27

Urban areas

Answer 27

Impermeable surfaces like roads lead to flashy graphs.
Settlements built on flood plains.
Transfers water quickly away from human settlement to nearest river

Question 28

Engineering

Answer 28

Soft engineering flood ma management schemes attempt to reduce flashiness.
Afforestation increases interception and infiltration

Question 29

Water abstraction

Answer 29

Reduces base flow and so more water much reach channel before it reaches bankfull capacity