Hydrology + Fluvio-geomomorphology Flashcards
1
Q
What does the Hjulström curve show?
A
- it shows the relationship between material size and the energy (velocity) needed to transport and erode it
- the larger the material, the higher the velocity needed to keep the particles transporting
- the settling velocity curve is the point at which the material no longer has the energy required to be transported, meaning it is deposited
2
Q
Erosion
A
- The process of the wearing away of soil and rock
- the force of water and fragments of rock cause the river bank to wear away in certain areas over time
3
Q
Abrasion/Corrasion
A
- the wearing away of the bed + bank by the load carried by a river - usually causing the river bed to deepen and widen
- a high river velocity increases abrasion due to the river having enough energy to carry larger rocks
4
Q
Attrition
A
- wearing away of load carried by river, creating smaller and rounder particles
5
Q
Hydraulic action
A
The force of air + water on the sides of rivers + in cracks
6
Q
Cavitation
A
- water forces its way into small cracks
- air in these spaces get compressed and put under pressure - fragmenting and damaging the rock
7
Q
Corrosion/ solution
A
- process of water dissolving parts of rock/soil that makes up the river channel
- factors affecting rate: bed rock, solute concentration of stream, discharge, velocity
8
Q
Traction
A
- the movement of large rocks and pebbles through water by rolling them along the river bed
- rocks are heavier and therefore can not be carried by water
9
Q
Saltation
A
Pebbles are bounced along river bed
10
Q
Suspension
A
Small pebbles and material are carried (suspended) within the water
11
Q
Solution
A
- soluble materials are carried within the water
12
Q
Factors affecting the rate of erosion
A
- load —> heavier + sharper = greater potential for erosion
- velocity —> increase in velocity = increase in erosion
- gradient
- geology —> soft, unconsolidated rocks such as sand are easily eroded
- pH —> acidic water increases erosion
- human impact —> deforestation, dams + bridges infers with natural flow
13
Q
Why does velocity needed to pick up particles vary with size?
A
- larger particles require higher velocities to be picked up because they are heavier
- sand (0.1-1.0mm) requires the lowest velocity to be picked up because of its small size
- clays are more cohesive so require higher velocities to be picked up
14
Q
Evapotranspiration
A
- water loss from the ground surface to the atmosphere (evaporation) combined with water given off by plants (transpiration)
15
Q
Interception
A
- vegetation, particularly trees, intercepts some precipitation on its way to the ground
- water is then lost back into the atmosphere by Evapotranspiration
- the intercepting plants also use some water for growth
- ## vegetation reduces and slows down water transfer
16
Q
Depression storage
A
When water is stored temporarily on the ground surface in the form of puddles
17
Q
Soil moisture
A
- the existing moisture in the soil determines whether precipitation will be absorbed or be forced to flow as overland flow
- clay soils can be very wet and boggy - leading to overland flow
- where as sandy soils tend to absorb more precipitation
18
Q
Baseflow/ground water flow
A
- very slow transfer of water through rocks
- only in limestone areas where there are extensive underground channels, can the flow be faster
19
Q
River channel
A
- the river is an important store of water
- forms the ‘exit’ for water transferred through the drainage basin
20
Q
Percolation
A
- the deeper transfer of water into permeable rocks - those with joints (pervious)or those that are porous
21
Q
Throughflow
A
- downhill transfer of water through the soil layer to the river
- this shallow transfer can be quite rapid in very porous sandy soils
22
Q
Infiltration
A
- involves water moving from the ground surface into the soil
- the rate of infiltration (infiltration capacity) depends upon the moisture content of the soil and it porosity (the number of airspace’s contained within)
23
Q
Overland flow
A
- this is the rapid form of water transfer over the surface of the ground
- most likely to occur during heavy periods of rainfall, or when the soil has completely saturated
24
Q
What is a drainage basin?
A
An area of land divided by a river and its tributaries
25
Q
The slowest water transfer?
A
Ground water flow
26
Q
A fast water transfer?
A
Infiltration
27
Q
The fastest water transfer?
A
Surface run off
28
Q
What does a storm hydrograph show?
A
- how a river responds to a rainfall event
- annual hydrographs are called a river regime
29
Q
What is lag time?
A
Time in hours from peak precipitation to peak discharge
30
Q
What is base flow?
A
Discharge of the river before and after rainfall
31
Q
What is storm flow?
A
Discharge above base flow - mostly from overland flow
32
Q
What conditions can lead to overland flow?
A
Freezing - reduces infiltration capacity + rate
Precipitation - affects rate
Saturation- infiltration capacity full
33
Q
What is infiltration capacity
A
How much water the soil can store
34
Q
What is infiltration rate?
A
The velocity at which water enter the soil
35
Q
How does precipitation affect storm hydrographs?
A
- prolonged rainfall -> flooding most frequently occurs following a long period of heavy rainfall -> the ground has become saturated-> infiltration replaced by overland flow
- intense rainfall -> intensity may be greater than the soil infiltration capacity -> resulting in high levels of over land flow
- snowfall -> heavy snowfall means water is held in storage -> temperature rises, snow melts -> infiltration capacity is exceeded or ground may still be frozen -> overland flow
36
Q
How does sandy soils affect storm hydrographs?
A
- large pore spaces = high infiltration capacity + allows rapid infiltration (greater throughflow)
- reduces peak discharge and increases lag times
37
Q
How does clay soils affect storm hydrographs?
A
- much smaller pore spaces = low infiltration capacity + allows little infiltration (less throughflow)
- increases peak discharge + reduces lag time
38
Q
How does thin soils affect storm hydrographs?
A
- thin soils can only store small amounts of water (soil moisture storage)
- soil becomes saturated quickly -> increasing the amount of throughflow and/or overland flow
- high peak discharge + short lag time
39
Q
How do deep soils affect storm hydrographs?
A
- can store a large amount of water (soil moisture storage)
- field capacity/full saturation is rarely reached -> increasing the amount of infiltration
- low peak discharge + long lag time
40
Q
How does forest vegetation affect storm hydrographs?
A
- forest vegetation have high levels of interception -> increasing the amount of water stored in the drainage basin + slows down flow of water to river
- as more water is intercepted, interception loss increases + less water enters the river
- soils underneath forested areas have high infiltration capacities -> allows increased amounts of water to enter the river by throughflow
- low peak discharge + long lag time
41
Q
How do grasslands and moorlands affect storm hydrographs?
A
- lower vegetation density = less interception loss + more water will travel through the drainage basin to the river
- soil infiltration capacity is lower -> intense rainfall will exceed infiltration capacity -> overland flow
- soil moisture capacity low -> less water need for soil to reach 100% saturation -> overland flow
42
Q
How do lakes and reservoirs affect storm hydrographs?
A
- reservoirs and lakes may control the flow of a river
- water entering the drainage basin after a rainfall event will be stored in the reservoir
- this will have an affect of ‘flatting’ the profile of a storm hydrograph
43
Q
How does urbanisation affect storm hydrographs?
A
- less permeable surfaces -> less surface storage and interception, as well as less infiltration by soil and rocks
- reduced surface storage and interception = less evaporation outputs
- water cannot infiltrate through tarmac and concrete -> increased overland flow
- increasing peak discharge, reducing lag time and lowering base flow
44
Q
How does agriculture affect hydrographs?
A
- farming activities may speed up or slow down delivery of water to rivers
- soil conservation measures are designed to reduce run off and soil erosion = reduced overland flow
- hill slope terracing or planting of permanent grassland on vulnerable slopes = reduces overland flow
- BUT land drainage systems is to accelerate the transfer of water to the stream
- intensive farming modify natural processes -> changes in vegetation coverage will affect interception, surface run off, infiltration and percolation
- ploughing of fields, application of fertilisers, and impact of machinery alter soil texture -> damaging storage and infiltration capacities
45
Q
How does shape of a drainage basin affect a storm hydrograph?
A
- a more circular basin = higher peak discharge and shorter lag time than an elongated basin
- all points on the watershed of a circular basin are approximately an equal distance from the gauging point
- in a elongated basin it takes longer for the rainfall from the extremities of the basin to reach the gauging point
46
Q
How does size affect the hydrograph?
A
- as size increase, peak discharge becomes higher and lag time increases
- bigger drainage basin collects water from a greater area - therefore more water enters the river
- the distance the water has to travel to reach the river is however larger - increasing the lag time
47
Q
How does drainage basin density affect the hydrograph?
A
- density refers to the number of surface streams in a given area
- the higher the density the shorter the lag time and greater the peak discharge
- short lag time due to the shorter distances that the rainfall has to travel to reach the river
48
Q
How does relief affect storm hydrographs?
A
- in a steep sided upland valley, water is more likely to reach the river quickly than in a gently sloping valley
- shorter lag time
49
Q
How does rock type affect a storm hydrograph?
A
- permeable rocks are either porous or pervious
- porous (e.g. sandstone and chalk) - lots of pores able to store water
- pervious (e.g. limestone) - allows water to flow down joints within the rock
- peak discharge lower + greater lag time
- impermeable rocks (e.g. granite) - don’t allow percolation -> more throughflow and overland flow
- peak discharge higher + shorter lag time
50
Q
What is discharge?
A
The volume of water flowing through a river channel in a given time
51
Q
What is a water budget?
A
Inputs (precipitation/flows into basin) - outputs (flows out, evapotranspiration, loss to deep flows)
52
Q
What is laminar flow?
A
- flowing in layers -> happens on efficient rivers flowing in smooth channels
- top + bottom layers slower due to friction, middle flowing fastest
53
Q
What is turbulent flow?
A
- vertical + horizontal eddies caused by obstruction in the river + pressure imbalance
- leads to the creation of fast + slow areas of the river
- erosion + deposition happens in the river, creating deeper pools and shallow riffles
54
Q
What is helicoidal flow?
A
- corkscrew motion generated by pressure differences on a meander
- higher pressure on the outside of the bend - undercutting banks to form a river cliff
- lower pressure on the inside - deposits material to form a point bar
55
Q
What is sinuosity?
A
- the measurement of how meandering a river is
- if channel length is less than 1.5 x it’s valley length it is ‘straight’
- over 1.5 x = sinuous or meandering
56
Q
What is Thalweg?
A
Line of fastest flow
57
Q
How does a meander + oxbow lake form?
A
- turbulent flow leads to the formation of pools and riffles, with pools occurring at points of greater erosion and riffles forming due to deposition
- the thalweg flows from side to side and a cork screw like motion called helicoidal flow moves material from the outside of one meander bend and deposits it on the inside of the next bend
- the outside of the bend has a higher pressure - undercutting the bank to form a river cliff
- the inside of the bend has a lower pressure - river deposits load to form a point bar
- continuous erosion on the outer bank + deposition on the inner bank forms a meander
- over time the shape of the meander will change + the neck will be cut off to form an oxbow lake
58
Q
How do gorges form?
A
- retreat of the river
- antecedent drainage
- glacial overflow
- collapse of underground caverns
59
Q
How do levees + floodplains form?
A
- caused by rivers bursting banks (flooding)
- hydraulic radius increases
- efficiency decreases
- velocity decreases
- deposition increases
- so, material deposited by river due to the drop in energy, with repeating floods this will continue to build up the levee over time
60
Q
How is flood risk measured?
A
- measure using recurrence interval
- e.g. 10 year flood is the largest flood you can reasonably expect in the next 10 years
61
Q
Causes of flooding
A
- high precipitation - e.g. in uk caused by Atlantic depression, season rainfall (the monsoon) or extreme weather such as hurricanes
- river management - e.g. dams/defences push flooding down stream
- impermeable surfaces - urbanisation
- deforestation
- climate change
62
Q
Forecasting/ warnings for floods
A
- use of weather satellites - geostationary
- river gauging + monitoring
- uk - 50% of dwellings have 6+ hrs notice of flooding
63
Q
Loss sharing of floods
A
- insurance -> spreads cost of flood damage
- disaster relief
64
Q
Hard engineering for flood prevention
A
- embankment + sluice gates -> controls flow into + out of urban areas
- enlarged channels -> can silt up in low areas
- flood bypass channel
- dams/flood storage -> can be temporary
- intercepting channel to take water away
- removal of urban area
65
Q
Land use zoning for flood prevention
A
- using flood maps to create where houses can’t be built etc.
66
Q
Soft engineering : flood reduction
A
- reforestation
- reseeding sparsely vegetated areas
- contour ploughing/ terracing -> slows overland flow + therefore infiltration increases
- protection of vegetation e.g. national park
67
Q
Soft engineering: flood diversion
A
- allow certain areas to flood + increase infiltration in these areas
68
Q
How do delta’s form?
A
- form where a river flows into the sea(estuary) - where the rate of deposition is greater than the rate of erosion
- they grow outwards + benefit from flocculations, which is when opposite charges of fresh water + salt water cause clay particles to clump together + be deposited
- e.g. the river Nile into MED, Mississippi into Gulf of Mexico
69
Q
What are the types of delta’s?
A
- arcuate —> longshore drift affects shape - stays smooth, fan shape - e.g. river Nile
- cuspate —> pointed, foot shaped - caused by opposing currents - e.g. Ebro
- Birds foot delta - huge amounts of deposition, delta twos along pattern of rivers
70
Q
Outputs in the drainage basin
A
- evaporation (liquid to gas) - occurs when water in the system is heated by solar energy causing it to evaporate into gas + rise into atmosphere
- evapotranspiration - occurs in plants when they respire through their leaves, releasing water they absorb through their roots, which then evaporates due to heating by sun
- river discharge - volume of water passing through a cross-sectional point of the river at any point in time (cumecs ) - water leaves through streams which drains the basin, may flow as tributaries into other rivers or directly into lakes + oceans
71
Q
Stores in drainage basin
A
- interception - water intercepted by plant branches + leaves before reaching the ground - water is stored short term
- soil water - water stored in upper levels of soil which is utilised by plants
- surface water - water stored in puddles, ponds, lakes etc. - depending on size can last from hours to years
- ground water - stored in pore spaces of rock or lower soil
- channel storage - stored in the rivers channel
72
Q
Flows in drainage basin
A
- above ground flows: throughfall, stemflow, overland flow, channel flow
- below ground flows: infiltration, percolation, throughflow, groundwater flow , base flow
73
Q
What is the water table?
A
- the level at which the pore spaces + fractures in the ground become saturated
- meaning above the water table is unsaturated soil + below is saturated soil
74
Q
How can the water table change?
A
- if the surface dips below the water table, groundwater will fill the surface space to become surface water
- if the surface water dries up, groundwater will continue to replenish the area that is underneath the water table
- if water table rises/falls = surface water rises/falls
75
Q
What is groundwater recharge?
A
- when groundwater levels deplete (by human extraction or by groundwater replenishing surface water levels) groundwater can be recharged
- ground water levels fall when its being used more - e.g. hot temps = less rainfall, more surface water dries up so groundwater will try to replenish these stores
- causing water table to fall as no water is replenishing the used groundwater
76
Q
How is groundwater recharged?
A
- precipitation infiltrating the ground until it reaches ground water - when precipitous is high, evaporation is low (e.g. winter) - groundwater levels will be recharged
- surface water bodies (e.g. lakes + rivers) seeping into groundwater stores
77
Q
How do waterfalls + gorge form?
A
- river flows over soft rock + hard rock - soft rock erodes away more quickly creating a step
- overtime soft rock erodes away further - undercutting the hard rock
- hard rock suspended in the air as an overhand
- rotational movement of water quickens erosion = deep plunge pool
- gravity causes overhang to collapse - broken hard rock accelerates erosion + deepens plunge pool
- erosion continues to undercut hard rock causing it to collapse
- the waterfall retreats upstream over time - gorge
78
Q
How is a bluff formed?
A
- formed on the outside bend on a meander where erosion is dominant
- water erodes the lower section of the river bank
- upper section is unsupported + collapses
- creating a steep bluff
79
Q
How does water storage cause flooding?
A
- reservoirs can raise the water table + can affect soil saturation if not careful
- usually very large = more water enters atmosphere through evaporation + increases cloud formation - affecting precipitation
