The Flood Hydrograph

Question 1

What is discharge?

Answer 1

The volume of water passing a certain point in a river

Question 2

What is the equation for discharge?

Answer 2

Discharge = cross sectional area (m2) x velocity (meters per second)

Question 3

What are the characteristics of a flash hydrograph?

Answer 3

Short lag time and high peak

Question 4

Basin size of a flashy hydrograph?

Answer 4

Small drainage basins often lead to rapid water transfer

Question 5

Drainage density of a flashy hydrograph?

Answer 5

A high drainage density speeds up water transfer

Question 6

Rock type of a flashy hydrograph?

Answer 6

Impermeable rocks encourage rapid overland flow

Question 7

Land use of a flashy hydrograph?

Answer 7

Urbanisation encourages rapid water transfer

Question 8

Relief of flashy hydrograph?

Answer 8

Steep slopes lead to a rapid water transfer

Question 9

Soil water of a flashy hydrograph?

Answer 9

Saturated soils encourage rapid water transfer

Question 10

Rainfall intensity of a flashy hydrograph?

Answer 10

Heavy rainfall may exceed the infiltration capacity of vegetation, and lead to overland flow.

Question 11

Rock type of a low hydrograph?

Answer 11

Permeable rocks encourage a slow transfer by groundwater flow.

Question 12

Land use of a low hydrograph?

Answer 12

Thick vegetation (forest) intercept precipitation, holding the water on its leaves; slowing the movement of rainwater to the ground and so to river channels. 
Water is also lost to evapotranspiration from vegetation - reducing how much gets to the river.

Question 13

Soil water of a low hydrograph?

Answer 13

Dry soil soaks up water and slows down its transfer

Question 14

Define lag time.

Answer 14

The time between peak rainfall and peak discharge.

Question 15

Define peak discharge.

Answer 15

The point on a flood hydrograph when river discharge is at its greatest.

Question 16

Define storm flow.

Answer 16

Discharge resulting from storm precipitation involving hot tub overland flow, throughflow and groundwater flow.

Question 17

Define storm hydrograph.

Answer 17

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

Question 18

Why are storm hydrographs important?

Answer 18

Because they can predict how a river might respond to a rainstorm. This can help to manage the river.

Question 19

On a storm hydrograph, what happens when happens when all of the storm water has passed through?

Answer 19

The river returns to its base flow

Question 20

Explain drainage basins as a factor to make a hydrograph flashy.

Answer 20

Drainage basins with steel sides tend to have flashier hydrographs because water flows more quickly on sleep slopes, whether as throughflow or overland flow and so gets to the river more quickly.

Question 21

Explain drainage basins as a factor to make hydrographs subdued.

Answer 21

• drainage basins underlain by sandstones have a subdued hydrograph because they water soaks into the porous rock.
• THICK VEGETATION cover the drainage basin and intercepts precipitation, holding water on the leaves. This slows the movement of rainwater to the ground and so to the river channels. Water is also lost to due to evaporation and transpiration from the vegetation - this subdues the storm hydrograph, increasing lag time and reducing peak discharge.
• there’s more but see notes

Question 22

Typically discharge is greater in winter than winter due to decreased temperatures and subsequently less evapotranspiration. But why might peak discharge be in spring?

Answer 22

Spring maximum discharge caused by snow melting in the mountains.

Question 23

Why might discharge increase more so in October in America?

Answer 23

Increasing rainfall from extra-tropical cyclones increase discharge.

Question 24

In the U.K., why does discharge decrease throughout the spring?

Answer 24

As plants take up more water and leaves grow, evaporation increases.

Question 25

Precipitation of a flashy hydrograph?

Answer 25

Heavy storms with a lot of water entering the drainage basin over a short time = higher discharge.

Rain rather than snow

Question 26

Precipitation type of a subdued hydrograph.

Answer 26

Snow rather than rain because snow takes time to melt before the water enters the channel.
But when there’s rapid melting of snow the peak discharge could be high.

Question 27

Human factors leading to flashy hydrograph?

Answer 27

• deforestation reduces interception rates, allowing rainwater to hit the surface directly. The lack of vegetation roots reduces the infiltration rate into the soil. This leads to rapid overland flow. Deforestation also exposed the soil to greater rates of erosion, which leads to sedimentation of the channel. This reduces the bankfull capacity of the river and leads to a greater chance of flooding
• large number of animals on small areas compact soils leading to overland flow
• urbanisation

Question 28

Human factors leading to a subdued hydrograph?

Answer 28

• afforestation increases interception and infiltration which slows down the progress of water to the river channel and subdues any changes in discharge
• agriculture - ploughing breaks up the top soil and allows greater infiltration, subduing hydrographs.
• terracing on hillsides
• water abstraction reduces the base flow so more water must reach the channel before it reaches bankfull capacity