4b.4 - Storm Hydrographs

Question 1

Once evaporated, whats the fastest route that water can take back to the ocean?

Answer 1

Precipitation straight into river channel or flows overland via surface run-off

Question 2

Once evaporated, whats the slowest route that water is likely to take back to the ocean?

Answer 2

Precipitation lands on permeable soil and is infiltrated into groundwater

Question 3

How is discharge defined?

Answer 3

The volume of water passing a particular point (Gauging Station) at a particular time.

Question 4

How is discharge calculated?

Answer 4

Cross sectional area of the river channel x river velocity

Question 5

What is a storm hydrograph?

Answer 5

Graph that shows the short term response of river discharge after a storm event

Question 6

What is lag time?

Answer 6

The time it takes between peak rainfall (water leaving cloud) and peak discharge (passing gauging station)

Question 7

How can a storm hydrograph vary?

Answer 7

It can be either….

(1) Subdued - Long Lag time and low discharge
(2) Flashy - Short lag time and high discharge

Question 8

Which type of storm hydrograph would be associated with a flooding event?

Answer 8

Flashy Hydrograph - water reaches the river channel very quickly (short lag time) to increase discharge rapidly

Question 9

How does GEOLOGY affect flooding (as shown on a storm hydrograph)?

Answer 9

Impermeable IGNEOUS rock = more surface run-off = short lag time = more discharge = flood (flashy hydrograph)

Permeable/porous SEDIMENTARY rock = more infiltration = long lag time = less discharge = no flood (subdued hydrograph)

Question 10

How does SOIL affect flooding (as shown on a storm hydrograph)?

Answer 10

Impermeable THIN, CLAY soil = more surface run-off = short lag time = more discharge = flood (flashy hydrograph)

Permeable THICK, SANDY soil = more infiltration = long lag time = less discharge = no flood (subdued hydrograph)

Question 11

How does SLOPE affect flooding (as shown on a storm hydrograph)?

Answer 11

Steep slope = quicker surface run-off = short lag time = more discharge = flood (flashy hydrograph)

Gentle/flat slope = more infiltration/slower surface run-off = long lag time = less discharge = no flood (subdued hydrograph)

Question 12

How do ANTECEDENT CONDITIONS affect flooding (as shown on a storm hydrograph)?

Answer 12

Lots of recent rainfall = SATURATED SOIL = more surface run-off = short lag time = more discharge = flood (flashy hydrograph)

Previous dry conditions = more infiltration into DRY SOIL = long lag time = less discharge = no flood (subdued hydrograph)

Question 13

What does ‘Antecedent Conditions’ mean?

Answer 13

“A preceding event “– what has come before?

Consider recent rainfall over the previous few days

Question 14

How does RIVER BASIN SIZE AND SHAPE affect flooding (as shown on a storm hydrograph)?

Answer 14

SMALL, ELONGATED BASIN = short lag time = more discharge = flood (flashy hydrograph)

LARGE, ROUND BASIN = long lag time = less discharge = no flood (subdued hydrograph)

Question 15

How does DEFORESTATION affect flooding (as shown on a storm hydrograph)?

Answer 15

Deforestation = less interception and uptake = more run-off = short lag time = more discharge = flood (flashy hydrograph)

Question 16

How does URBANISATION affect flooding (as shown on a storm hydrograph)?

Answer 16

Urbanisation = more impermeable surfaces (concrete) = more run-off = short lag time = more discharge = flood (flashy hydrograph)

Question 17

How does weathering affect river landscapes?

Answer 17

More weathering = more mass movement into river = gentle, wide v-shaped valley AND increased sediment load of river