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
The slowest water transfer?
Ground water flow
26
A fast water transfer?
Infiltration
27
The fastest water transfer?
Surface run off
28
What does a storm hydrograph show?
- how a river responds to a rainfall event
- annual hydrographs are called a river regime
29
What is lag time?
Time in hours from peak precipitation to peak discharge
30
What is base flow?
Discharge of the river before and after rainfall
31
What is storm flow?
Discharge above base flow - mostly from overland flow
32
What conditions can lead to overland flow?
Freezing - reduces infiltration capacity + rate
Precipitation - affects rate
Saturation- infiltration capacity full
33
What is infiltration capacity
How much water the soil can store
34
What is infiltration rate?
The velocity at which water enter the soil
35
How does precipitation affect storm hydrographs?
- 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
How does sandy soils affect storm hydrographs?
- large pore spaces = high infiltration capacity + allows rapid infiltration (greater throughflow)
- reduces peak discharge and increases lag times
37
How does clay soils affect storm hydrographs?
- much smaller pore spaces = low infiltration capacity + allows little infiltration (less throughflow)
- increases peak discharge + reduces lag time
38
How does thin soils affect storm hydrographs?
- 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
How do deep soils affect storm hydrographs?
- 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
How does forest vegetation affect storm hydrographs?
- 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
How do grasslands and moorlands affect storm hydrographs?
- 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
How do lakes and reservoirs affect storm hydrographs?
- 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
How does urbanisation affect storm hydrographs?
- 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
How does agriculture affect hydrographs?
- 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
How does shape of a drainage basin affect a storm hydrograph?
- 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
How does size affect the hydrograph?
- 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
How does drainage basin density affect the hydrograph?
- 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
How does relief affect storm hydrographs?
- 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
How does rock type affect a storm hydrograph?
- 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
What is discharge?
The volume of water flowing through a river channel in a given time
51
What is a water budget?
Inputs (precipitation/flows into basin) - outputs (flows out, evapotranspiration, loss to deep flows)
52
What is laminar flow?
- flowing in layers -> happens on efficient rivers flowing in smooth channels
- top + bottom layers slower due to friction, middle flowing fastest
53
What is turbulent flow?
- 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
What is helicoidal flow?
- 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
What is sinuosity?
- 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
What is Thalweg?
Line of fastest flow
57
How does a meander + oxbow lake form?
- **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
How do gorges form?
- retreat of the river
- antecedent drainage
- glacial overflow
- collapse of underground caverns
59
How do levees + floodplains form?
- 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
How is flood risk measured?
- measure using recurrence interval
- e.g. 10 year flood is the largest flood you can reasonably expect in the next 10 years
61
Causes of flooding
- 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
Forecasting/ warnings for floods
- use of weather satellites - geostationary
- river gauging + monitoring
- uk - 50% of dwellings have 6+ hrs notice of flooding
63
Loss sharing of floods
- insurance -> spreads cost of flood damage
- disaster relief
64
Hard engineering for flood prevention
- 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
Land use zoning for flood prevention
- using flood maps to create where houses can’t be built etc.
66
Soft engineering : flood reduction
- reforestation
- reseeding sparsely vegetated areas
- contour ploughing/ terracing -> slows overland flow + therefore infiltration increases
- protection of vegetation e.g. national park
67
Soft engineering: flood diversion
- allow certain areas to flood + increase infiltration in these areas
68
How do delta’s form?
- 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
What are the types of delta’s?
- 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
Outputs in the drainage basin
- 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
Stores in drainage basin
- 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
Flows in drainage basin
- above ground flows: throughfall, stemflow, overland flow, channel flow
- below ground flows: infiltration, percolation, throughflow, groundwater flow , base flow
73
What is the water table?
- 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
How can the water table change?
- 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
What is groundwater recharge?
- 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
How is groundwater recharged?
- 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
How do waterfalls + gorge form?
- 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
How is a bluff formed?
- 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
How does water storage cause flooding?
- 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
