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G£🌍graph¥ > Hydrology and Fluvial Geomorphology content > Flashcards

Hydrology and Fluvial Geomorphology content Flashcards

1
Q

What are the inputs into the drainage basin system

A

Rain

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2
Q

What are the outputs in the drainage basin system

A

evaporation, evapotranspiration and river discharge

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3
Q

What are the stores in the drainage basin system

A

interception
soil water
surface water
ground water
channel storage

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4
Q

What are the above ground flows?

A

throughfall
stemflow
overland flow
channel flow

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5
Q

What are the below ground flows?

A

infiltration
percolation
throughflow
groundwater
baseflow

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6
Q

what are the underground water stores?

A

water tables
ground water
recharge
springs

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7
Q

What are River Regimes?

A

Annual hydrographs, also known as ’river regime’ is the difference in the discharge of the river throughout the year

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8
Q

What are Long term variations in river discharge relative to?

A

Mainly:
Climate change
A Change in Land Use

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9
Q

What are short term variations in river discharge relative to?

A
  • weather (monsoon)
  • Snowmelt and glacial melting
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10
Q

What is the antecedent flow rate (STORM HYDROGRPAH)

A
  • It is the rate of discharge before the storm event.

It is the initial discharge (then precipitation occurs), which leads to the discharge spiking as rainwater reaches the channel.

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11
Q

What causes a storm hydrograph’s rising limb?

A

Surface runoff begins and later throughflow causing the rivers discharge to rise.

The steeper the rising limb the more likely a flood is to occur.

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12
Q

What is the highest point of discharge called

A

“peak flow”

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13
Q

What does a river’s falling limb show?

A

Shows the decreasing discharge in the river channel. Usually less steep than the rising limb because although overland flow may have stopped the throughflow continues more slowly.

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14
Q

What is a basin’s “lag time”

A

The length of time between peak precipitation and peak flow/river discharge.

short lag time combined with a high peak flow suggests flooding.

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15
Q

What is baseflow?

A

The volume of water being added to the river from underground (i.e. not surface flows).

Does not depend on precipitation.

It is sustained between episodes of precipitation.

Look up a hydrograph and see the base flow line, as it is sometimes represented on the hydrograph

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16
Q

What is the impact of antecedent moisture on a storm hydrograph

A

Antecedent moisture can lead to a shorter lag time, due to saturated soils.

(anticedent moisture = wet conditions pre storm).

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17
Q

What is the impact of vegetation on the storm h¥drograph

A

Vegetation can increase the rate of interception, leading to a larger lag time in a hydrograph.

It will also exponentially increase evapotranspiration, further increasing the lag time.

Roots also increase infiltration by creating gaps in the soil

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18
Q

What is the impact of temperature on a storm hydrograph?

A

High temperatures increase the rate of evapotranspiration thereby reducing discharge and increasing lag time.

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19
Q

How does increased relief impact a storm hydrograph (or annual hydrograph)

A
  • high relief can lead to increased rainfall (orographic)
  • Increases surface runoff and less infiltration, leading to a flashy hydrograph.
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20
Q

how does precipitation type impact on a storm hydrograph?

A

The lag time is likely to be greater if the precipitation is snow rather than rain, as snow takes time to melt.

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21
Q

Why is geology important in determining the shape of a storm hydrograph?

A

porosity
- porous rocks hold more water therefore meaning more groundwater
- Permeable rocks allow for more infiltration
It also influences percolation, groundwater, throughflow & soil moisture
infiltration affected indirectly as the more permeable the rock the greater the infiltration capacity.

Geology might also influence vegetation types

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22
Q

What is drainage density?

A

the length of stream channel per unit area of a drainage basin.

The amount of streams in a drainage basin.

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23
Q

What is the “watershed”

A

The watershed is the boundary of a drainage basin.

Its edges.

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24
Q

What are the 4 types of fluvial erosion?

A
  • abrasion/corrasion
  • solution
  • cavitation and hydraulic action
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25
Q

What is cavitation?

A
  1. as water accelerates pressure drops.
  2. This drop in pressure can turn some of the water into vapor (bubbles)
    3.These bubbles implode/collapse causing tiny jets of water to be released (often very fast)
  3. This chips away at even the hardest rocks and is a form of hydraulic action
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26
Q

What is abrasion (and how does it impact a river channel)

A

when rocks grind along a river bank, causing a sand paper like effect.

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27
Q

What is solution (and how does it impact a river channel)

A

Solution involves the dissolving of soluble particles and rock (such as limestone and chalk) by slightly acidic river water, breaking down the bed and banks chemically.

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28
Q

What are the 4 types of fluvial transportation?

A

traction
saltation
suspension
solution

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29
Q

What is traction?

A

Large stones/boulders are rolled along the bottom of a river (as they are too big to suspend)

30
Q

What is saltation?

A

Small rocks or pebbles which are too big to be carried within the water are transported and bounce along the bottom of the river bed.

31
Q

What is suspension?

A

Sediment (probably eroded from rocks), is carried in the water. When the sediment is deposited from the water it is known as silt.

32
Q

What is solution?

A

The transport of dissolved rocks. This varies along the river depending on the presence of soluble rocks

Dependant on erosion such as attrition (where rocks bump into each other) and abrasion.

33
Q

why does the hjulstrom curve’s erosion line slope downwards and then upwards?

A

At smaller sediment sizes, more velocity is required to erode due to increased electrostatic cohesion between particles.

34
Q

Problems with the Hjulstrom curve

very important as it has not been asked yet

A

particles can be made of different densities and materials. The Hjulstrom curve only considers length whether length and not mass.

particle shape: A concave rock (facing the direction of the river) is more likely to be transported than a round rock which is smaller than it.

If confused about second point think of an aircraft landing with a parachute behind it, (the curve of the parachute catches more air).

does not consider previous rainfall events (anticedent flow may NOT be baseflow)

35
Q

What is the Thalweg?

A

the line or zone of fastest flow

Most erosion occurs here due to speed.
Transportation may be higher here but it depends on the size of particles.

36
Q

Explain Laminar flow and what causes it to occur.

A

A smooth flow with the streamlines aligned in ‘sheets’. Water flows parallel to the channel bed and banks, with no eddies (swirls).

What does it require to form?:

A very smooth, straight channel with a low velocity is required.

37
Q

Explain turbulent flow and what causes it to occur.

A

the movement of water in all directions in a river channel created by rocks and protuberance, often creating eddies.

What causes turbulent flow?

uneven bed and banks; the channels contain obstacles like boulders; and/ or they flow too fast.

Turbulence can involve some degree of ‘backwards’ flow- which means that velocity levels are low, although the forces applied to particles is relatively high. This wouldn’t be reflected in the Hjulstrom curve (another criticism)

38
Q

What is helicoidal flow, and what causes it?

A

Helicoidal flow is the movement of water in a “corkscrew” motion.

It is a secondary flow, often stemming from turbulent flow.

It is thought to move sediment from one meander to the next.

“possibly contributes to the formation of river features such as river cliffs and slip-off slopes in meanders.”

What causes helicoidal flow?

Meandering rivers have a sinuous, curved shape as they flow across the landscape. As water moves through these bends, centrifugal force causes the water to move faster on the outside of the bend and slower on the inside. This velocity gradient can induce helical flow patterns.

39
Q

Characteristics, formation, and where do straight river channels occur?

A
  • Often found in the upper course of the river
  • Single, straight channel
  • Lots of vertical erosion, water moves fastest in the upper course
  • Thalweg moves from side to side
40
Q

Characteristics, formation, and where do meandering river channels occur?

A
  • Characterised by a series of bends
  • Normally found in the middle and lower course
  • Lots of lateral erosion on the outside of the bends, deposition on the inside
41
Q

Characteristics, formation, and where do braided river channels occur?

A
  • Large deposits of sediment mean the river breaks into multiple channels seperated by islands of sediment
  • Usually occurs where discharge fluctuates
  • River deposits when it loses energy
  • Not found in the upper course usually
42
Q

How does a waterfall form?

A
  • Where there is both hard and soft rock, the soft rock erodes more quickly, leaving a step
  • The further erosion of the soft rock leaves an overhang of hard rock, the rotational movement of the water creates a plunge pool
  • The unsupported overhang collapses and the rock falls into the plunge pool, where it leads to further erosion, creating a deep plunge pool
  • The process repeats, with more steps and overhangs being created, collapsing and deepening the plunge pool
43
Q

How is a gorge formed?

A

When a waterfall continually retreats and forms a deep channel with steep sided walls

44
Q

How does an oxbow lake form?

A
  • When a river is in flood and breaks its banks, the river will flow along the shortest available course
  • In some meanders this means across the neck, cutting it off
  • Eventually due to deposition from this new channel, the meander becomes cut off and an oxbow lake if formed
45
Q

What is a river cliff and how does it form?

A

A cliff created on the outer bend of a meander when the bank if undercut by erosion

46
Q

What is a point bar and how is it formed?

A

A deposit of fine sediment such as sand on the river bank inside of a meander bend, usually changing position as the meander changes shape and leaving a mark where it once was (left behind as the meander moves further outwards)

47
Q

What is a riffle-pool sequence and how does it form?

A

Alternating patterns of shallow and deeper water in the channel, common in rivers with gentle slopes.
They are formed because a river tends to adjust the course of its flow to efficiently transport its load downstream, and higher energy on the outer bend leads to deeper pools being formed
A riffle is the higher, shallower area with material deposits
A pool is the lower, deeper area of the sequence

48
Q

What is a bluff and how does it form?

A
  • A large rounded cliff on the side of a river
  • Formed as the water erodes the lower section of the river bank, reducing the stability of the top section and making it unsupported, leading to it collapsing and leaving a steep bluff
49
Q

What is a floodplain and how does it form?

A
  • A large flat area of land near a river formed due to erosion and deposition
  • When a river is in flood the water spreads over the banks, loses energy and deposits sediment
  • The deposition builds up over time and makes it higher. The soil is usually very fertile
50
Q

What are levees and how do they form?

A
  • Natural embankments along a river
  • Formed due to repeat flooding and creating raised edges on the bank, making it less likely to break its banks again
  • Large sediment that takes most energy to transport is deposited first, then finer sediment, leading to a “tapered” shape
  • Act as natural flood defences
51
Q

What is a delta and how does it form?

A
  • Form at the mouth end of a river where sediment load is high
  • When a river meets a body of water like a lake or an ocean, the difference in velocity causes mass deposition of sediment
52
Q

Reminder to google image “different types of delta” - simple but can only be explained well by a photo

A
53
Q

How does deforestation affect a river catchment?

A
  • Decreases evapotranspiration
  • Decreases infiltration (roots provide gaps in the soil to allow water to enter and percolate to groundwater strores)
  • Decreases interception
  • Increases channel flow
54
Q

How does urbanisation affect a river?

A
  • Impermeable surfaces reduce infiltration and increase overland flow
  • Deforestation and a lack of trees/greenery decrease transpiration - leaving more water in the catchment area
55
Q

What is water abstraction and what are its effects?

A
  • The removal of water from a water body such as a lake, river or grounwater store
  • Can deplete water levels on the surface and groundwater
  • Depleted groundwater may lead to less seepage, decreasing levels of above-ground water also
56
Q

Name types of above ground water storage and what they do

A
  • Dams directly stop and control channel flow and can lead to the formation of a reservoir behind them
  • Reservoirs are usually very large, promote evaporation and can increase cloud formations and precipitation
  • If reservoirs are not properly managed they can lead to a lowering of the water table in other areas, as water that would be flowing in other areas is instead stores in a reservoir
57
Q

Causes of flooding

A
  • Heavy rainfall
  • Prolonged rainfall periods
  • Impermeable surfaces
  • Melting snow or glaciers contributing to channel flow
57
Q

Name a method to predict flood risk

A

Floor recurrence intervals

58
Q

What are flood recurrance intervals?

A
  • Probability of a flood of a given size occuring in a time interval of years
  • A 100 year flood DOES NOT mean it will happen every 100 years, but that there is a 1% chance of it occuring every year
  • This allows for the flood risk of certain areas to be assessed
59
Q

How are forecasts and warnings used?

A
  • Precipitation forecasts, streamflow data and streamflow route models are used to issue warnings days before so people can take action and evacuate, sandbagging etc.
  • The met office issues alerts (amber), severe flood warnings (red), amber means “be prepared”, red indicates a danger to life
60
Q

Hard engineering: what do Dams do and what are associated issues?

A
  • Physically obstruct a river’s natural course, leading to a reservoir building up, some built for flood control some for water storage
  • Effective at preventing flooding by restricting channel flow
  • They are expensive and the displacement of settlements and wildlife can occur

e.g Fish, such as salmon, migrating upstream

61
Q

Bonus: issues caused by the 3 gorges dam

A
  • Recently 300,000 people were evacuated upstream in Chongqing due to flooding concerns
  • Flooding was predicted to last 5 days
  • Drought concerns about the middle course of the Yangtze
  • The baiji dolphin and ancient river sturgeon rely on the Yangtze for their survival as a species, and other species have been damaged extensively by disconection of floodplains (from rivers) as well as dredging
62
Q

Hard engineering: what does channel straightening do and what are associated issues?

A
  • Bends and irregularities in the channel are removed using machinery, reducing likelihood of flooding and overflow, making water flow faster through the channel
  • However, water is just sent further downstream where it can enhance erosion and lead to further flooding (especially if channel downstream has not been straightened/meanders)
63
Q

Hard engineering: what do levees do?

A
  • Levees can occur naturally or be reinforced or constructed
  • Artificial levees provide a raised embankment so water has further to rise before exceeding bankful capacity
64
Q

Hard engineering: what do diversion spillways do and what are associated issues?

A
  • Allows excess water to flow into it when the channel is overflowing
  • Might direct water further downstream or to a different river
  • On many spillways there are floodgates that are opened and closed
  • However, they may direct water to areas which do not normally flood, potentially damaging both the environement and settlements if it floods
65
Q

Soft engineering: Floodplain and drainage basin management - what is it and how does it work?

A
  • Afforestation and tree management to decrease overall surface runoff
  • However, if improperly managed fallen over trees can block rivers and lead to more flooding
  • Managing urbanisation on floodplains is used
  • Minimising agricultural land use on floodplains as this is one of the most damaging land uses (particularly livestock trampling soil and monocultures)
  • Floodplain mapping uses river flooding records to map areas at most risk, and ensure land use is appropriate for the level of risk. An example sequence may be pasture, sports fields, car parks and roads, industry then residential property
66
Q

Soft engineering: Wetland and river bank conservation - what is it?

A
  • Wetlands support aquatic life in oxygen-limited, submerged environments.
  • They serve as natural flood buffers, filling with excess water to mitigate floods in surrounding areas.
  • Threats like climate change and agricultural use endanger wetlands.
  • Protecting wetlands involves restrictions on drainage, vigilant monitoring, and restoration efforts such as reintroducing species and recreating wetland conditions.
  • River banks can also be conserved by protecting and reintroducing vegetation to limit erosional processes and strengthen the river bank
67
Q

Soft engineering: River restoration

A
  • Process of restoring a river bank to its original state before it was altered
  • Often used after failed hard engineering attempts that have increased flooding or caused damage
  • River restoration may even increase flooding in one area to limit discharge downstream
  • Often occurs on land not seen as valuable, where flooding would not be an issue
68
Q

What is the difference between a porous and a permeable rock?

A

Porous rocks are able to hold water in empty spaces, permeable rocks allow water to pass through them

69
Q

What are different types of delta?

A
  • Arcuate delta (Nile delta example), fan shaped and filled with tributaries formed where the waves hit the delta head on
  • Cuspate delta, often fewer tribuaries, formed where currents are strong but hit from opposing directions, tooth-like shape
  • Bird’s foot delta, formed when waves are weak and river current is stronger, weak irregular tides hit from all angles and give it an irregular appearance (Mississippi delta example)
70
Q

How is a spring formed?

A
  • A spring is the outflowing of water from a slope/ the ground
  • Occurs due to throughflow being impeded forcing water to the surface
  • Or due to impermeable zones of the rock/soil
  • Or due to an overall rising of the water table

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