Lesson 5- What are the factors affecting river discharge and hydrohraphs?

Question 1

Labels on storm hydrograph

Answer 1

Question 2

What is river discharge?

Answer 2

• This is the volume of water mocing past a point in a river per given time is called the dishcharge- metres cubed/ seconds or cumecs
• is calculated as Q= A x V
• A= the cross sectional area in metres squared
• Q= is discharge is cumecs
• V= velocity in m/s

Question 3

What is the river dishcrage effected by?

Answer 3

• River discharge is affected by rate of precipitation and the speed water transfers from where it lands to the river (speed across drainage basin).
• We need to know the amount of precipitation (inputs) and the river discharge (outputs) to work out how much water is stored in the drainage basin.

Question 4

Why is it important to understand river discharge?

Answer 4

• To manage water resources (e.g. irrigation)
• The design of water related structures (e.g. bridges, reservoirs)
• Flood warning and defences
• HEP (Hydro Electric Power).
• With climate change expected to impact very unevenly on river discharge patterns, keeping records of river flows is the key to quantifying and interpreting hydrological trends.
• Developing more effective ways of dealing with future flood and drought events.

Question 5

3 ways water from drainage basin can be transferred to the river

Answer 5

• Directly into the channel – not much
• Surface flow – most often
• Infiltration - through and baseflow

Question 6

What are the two types of hydrographs?

Answer 6

• Annual hydrographs
• Storm hydrographs

Question 7

Type of hydrographs-Annual hydrographs

Answer 7

• Annualhydrographs, also known as ariver’s regime, show the pattern ofseasonal variation that takes place through a drainage basin to river discharge over a year
• Different conditions in different locations produce different levels of discharge over the course of a year

Question 8

Type of hydrographs- Storm hydrograph

Answer 8

• Show changes in a river’s discharge during and after a storm
• Usually, they are drawn to show how a river reacts to an individual storm
• They compare two variables - rainfall received during an event in mm and river discharge m3/sec

Question 9

Bankfull discharge

Answer 9

• The maximum amount that can fit in a river without flooding

Question 10

Baseflow

Answer 10

• The normal day to day amount of water flowing in a river

Question 11

Stormflow

Answer 11

• The amount of water above the normal amount flowing in a river during a storm event

Question 12

Discharge

Answer 12

• The amount of water passing a given point in a river per second

Question 13

What are the types of storm hydrographs?

Answer 13

• Flashy hydrograph
• Subdued hydrograph

Question 14

Types of Storm hydrograph- Flashy hydrograph

Answer 14

• Flashy hydrographs have a steep rising limb and a very steep falling limb.
• This means water is reaching the river very quickly but also being transported away from the area quickly.
• They also have a higher peak discharge which means more water is in the river at the same time.

Question 15

Types of storm hydrograph- Subdued hydro graph

Answer 15

• Subdued / flat hydrographs have a much more gentle rising limb this means water is taking a much longer time to reach the river.
• They also have a much lower peak discharge which means much less water is in the river at one time.

Question 16

Factors affecting hydrographs

Answer 16

• The Drainage Basin
• Relief
• Soil and Rock type
• Vegetation
• Precipitation
• Precipitation
• Deforestation
• Agriculture
• Urbanisation
• SUDS
• Other points

Question 17

Factors affecting hydrographs- The drainage basin

Answer 17

• Large basins will have high peak discharges because they catch more precipitation.
• Big basins also have longer lag times than small basins because the water takes longer to reach the rivers as it has to travel further.

Question 18

Factors affecting hydrographs- Releif

Answer 18

• Water flows more quickly on steep slopes, (overland flow or throughflow).
• This means it reaches the river quickly and more of it at the same time creating a higher peak discharge and steeper rising limb.

Question 19

Factors affecting hydrographs- Soil and Rock type

Answer 19

• If water can’t soak in throughflow and infiltration will also be reduced. This will result in a flashier hydrograph (shorter lag time).
• If the soil is already saturated by antecedent rainfall then overland flow increases because the soils’ infiltration capacity has been reached. This will also create a more flashy hydrograph.

Question 20

Factors affecting hydrograph- Vegetation

Answer 20

• Thick vegetation cover means more precipitation will be intercepted and stored.
• The more vegetation the more subdued the hydrograph as it will take water longer to get to the river.
• Water may also be lost from the local system due to evapotranspiration. This will result in a more subdued hydrograph, characterised by a longer lag-time and lower peak discharge.

Question 21

Factors affecting hydrograph- Precipitation

Answer 21

• For intensity, the quicker it rains the more likely to have a flashy hydrograph as lots of water is reaching a river very quickly, this is often when we get flash flooding as infiltration capacity of the soil is reached and it just runs off.
• For amount of rainfall this can effect how high our peak discharge is, more rainfall ultimately should result in a higher peak discharge however if this occurs slowly you may have a lower peak but for a longer period of time.

Question 22

Factors affecting hydrographs- Deforestation

Answer 22

• Deforestation reduces interception rates allowing the water to hit the surface directly.
• This leads to rapid overland flow and flashy hydrographs.
• Deforestation can also lead to greater rates of soil erosion. This can result in sedimentation of nearby channels. This subsequently reduces the bankfull capacity of the river causing flooding.

Question 23

Factors affecting hydrographs-Agriculture

Answer 23

• Animals compact the soil surface. If this happens over a long period of time it can make the top layer of soil impermeable.
• This means we get increased surface run off and therefore water reaches the river quicker creating a more flashy hydrograph.

Question 24

Factors affecting hydrographs-Urbanisation

Answer 24

• Urban areas mean big impermeable surfaces which means we get flashy hydrographs.
• Lots of houses are built on floodplains.
• Most drainage systems are designed to transfer water as quickly as possible away from humans to the nearest river.
• This is achieved through road camber (slope), building design and drainage systems (e.g. sewers and drains).

Question 25

Factors affecting hydrographs- SUDS

Answer 25

• SUDS can have a big positive impact on urban flood hydrographs and discharge. They are designed to slow the flow of water down and therefore can help to reduce the peak discharge and help make the hydrograph less flashy.
• CASE STUDY OF SUDS:
  Manor Fields, Sheffield.
• At Manor Fields ) Swales/Retention Basin/Pond–during prolonged rainfall this willfill and store water, slowing downhow quickly it reaches the river.

Question 26

Factors affecting hydrograph-Other factors

Answer 26

• Water abstraction- reduces the base flow and so more water must reach the channel before it reaches bankfull capacity.
• The construction of bridges may restrict the channel, becoming blocked at periods of high discharge. This can create temporary dams.
• The construction of reservoirs and dams, which hold back water, will delay its arrival to the lower channel.