Topic 5- Hydrological Cycle Flashcards
(130 cards)
1
Q
What is the global hydrological cycle?
A
It is a global closed system that of the flows of water between different states around the Earth-atmosphere system.
2
Q
What are the three systems of operation for the global hydrological cycle?
A
Stores
Fluxes
Processes
3
Q
What are stores(stocks)?
A
Reservoirs where water is held, such as the oceans.
4
Q
What are fluxes?
A
The rate of flow between the stores.
5
Q
What are processes?
A
The physical mechanisms that drive the fluxes of water between the stores.
6
Q
What is the global hydrological cycle driven by?
A
It is driven by solar energy and gravitational potential energy.
7
Q
What does a closed system mean?
A
It does not have any external inputs or outputs.
8
Q
What does a closed system mean for the global hydrological cycle?
A
This means that there is a fixed amount of water in the Earth- atmosphere system. The amount of water is constant and finite.
9
Q
What is the systems approach to the global hydrological cycle?
A
Systems approaches study hydrological phenomena by looking at the balance of inputs and outputs, and how water is moved between stores by flows.
10
Q
What is the cryosphere?
A
The areas of the Earth where water is frozen into snow or ice.
11
Q
Give an example as to why sea levels would drop in reference to the cryosphere.
A
In the last ice. age more water was held within the cryosphere in a solid form as snow and ice; as less was held in the oceans. sea levels dropped considerably- over 140m lower than they are today.
12
Q
Give an example as to why sea levels would rise in reference to the cryosphere.
A
Recent global warming is beginning to lead to major losses of ice in Greenland and, more recently, Antartica, and significant rises in sea levels.
12
Q
Give an example as to why sea levels would rise in reference to the cryosphere.
A
Recent global warming is beginning to lead to major losses of ice in Greenland and, more recently, Antartica, and significant rises in sea levels.
13
Q
How have humans increased the security of their water supplies?
A
At a small scale, humans have built numerous water storage reservoirs to complement natural lakes in order to increase the security of their water supplies.
14
Q
What are the four major stores of water?
A
Coeans(96.5-97%)
Cyrosphere(1.9%)
Terrestrial surface groundwater
Atmosphere
15
Q
Describe the stores within the global hydrological cycle
A
- In the oceans, the vast majority of water is stored in liquid form, with only a minute fraction as icebergs.
- In the cryosphere water is largely found in a solid state, with some in liquid form as melt water and lakes.
- On the land the water is stored in rivers, streams, lakes, and groundwater in liquid form. It is often known as blue water. Water can also be stored in vegetation after an interception or beneath the surface in the soil. Water stored in the soil and vegetation is often known as green water.
- Water largely exists as vapour in the atmosphere, with the carrying capacity directly linked to temperature. Clouds can contain minute droplets of liquid water or, at a high altitude, ice crystals, both of which are a precursor to rain.
16
Q
What is blue water?
A
Blue water is the visible part of the hydrological cycle.
17
Q
What is green water?
A
Green water is the invisible part of the hydrological cycle.
18
Q
What are the two major fluxes in the hydrological system driven by?
A
Precipitation, evaporation, cryospheric exchange, and a run-off generation(both surface and groundwater).
19
Q
What is the most important process of the fluxes?
A
Evaporation from the oceans and precipitation onto land and the oceans.
20
Q
What is evaporation?
A
The change in state of water from a liquid to a gas.
21
Q
What is residence time?
A
Residence time is the average time a water molecule will spend in a reservoir or store.
22
Q
What is fossil water?
A
Fossil water is ancint, deep groundwater from former pluvial(wetter) periods.
23
Q
What is transpiration?
A
The diffusion of water from vegetation into the atmosphere, involving a change from a gas to a liquid.
24
What is groundwater flow?
The slow transfer of percolated water undergoround through pervious or porous rocks.
24
What is groundwater flow?
The slow transfer of percolated water undergoround through pervious or porous rocks.
25
What is precipitation?
The movement of water in any form from the atmosphere to the ground.
26
Why is water stored in the soil easily lost?
Water stored in the soil, for example, remains there very briefly as it is spread very thinly across the Earth. Because of its accessibility it is easily lost to other sites by evaporation, transpiration, groundwater flow or recharge.
27
Which store has the shortest residence time? For how long and why?
Atmospheric water, 10 days, as soon as it evaporates, condenses and falls to the Earth as precipitation. Atmospheric water is forced to fall as precipitaton due to gravitational potential energy and weight of the clouds.
28
Whta is the link between residence times and levels of water pollution?
There is a strong link between these two as the stores that are longer in situ tend to be more polluted.
29
Where is the vast majority of Earth's fresh water supply located?
Around 69% is locked up in snowflakes, ice sheets, ice caps and glaciers found in high altitude locations.
30
Why is the vast majority of our fresh water supply being locked up in glaciers and ice sheets an issue?
It is largely inaccesible for human use, although some streams in mountain areas are 'fed' from ice and snow as melt water. 30% of freshwater comes from groundwater which is very inaccesible and therefore only around 1% of freshwater is actually accesible, this means that agricultural and urban development maybe slower without a lot of water fuelling the development of sectors.
31
What are humans main source of surface water?
Rivers are humans main source of water, however, this only constitutes 0.007% of total water. This is why there are so many disputes about the usage of all this water.
32
How is technology being used to extend the availabilty of fresh water supplies?
The desalination of ocean water which which is used as fresh water due to the highly advanced filtration and advanced chemicals used to desalinate the water.
33
What is the drainage basin water cycle?
On a small scale(variable from regional to local, depending on the size of the drainage basin) the drainage basin is a subsystem within the global hydrological cycle. It is an open system as it has external inputs and outputs that cause the amount of water in the basin to vary over time.
34
How often do conditions vary in the drainage basin cycle?
The variations in the drainage basin cycle can occur at different temporal scales, from short-term horuly through to daily, seasonal and annual.
35
Define a drainage basin
The area of land drained by a river and its tributaries, and is frequently reffered to as a river catchment.
36
What is a watershed?
The highland which divides and separates water flowing to different rivers.
37
How big are drainage basins?
Drainage basins can be of any size, from a small stream possibly without tributaries up to a major international river flowing across borders of several countries.
38
What are the flows in the global hydrological cycle?
```
Precipitation
Cryospheric processes
Vapour transport
Surface run-off
Evaporation
Atmosphere
Transpiration
Evaporation
Precipitation
Groundwater
Soil
Percolation
Ocean
```
39
What are the stores in the drainage basin cycle?
```
Soil
Lake
Groundwater
Surface
River flow
Interception
```
40
What are the inputs into the drainage basin?
Precipitation
| Solar and gravitational energy
41
What are the outputs in the drainage basin cycle?
Evaporation from water surfaces
River discharge
Evaporation and transpiration from vegetation.
42
What are the flows in the drainage basin?
```
Soil
Rock
Infiltration
Overland flow
Throughflow
Base flow
Channel fall
Evaporation
```
43
What are the conditions necessary for precipitation to form?
Air cooled to saturation poijnt with a relative humidity of 100%.
Condensation nuclei, such as dust particles, to facilitate the growth of droplets in clouds.
A temperature below dew point
44
What are the main influencing factors for the drainage basin hydrological system? Describe each
1. The amount precipitation, which can have a direct impact on drainage discharge: as a general rule, the higher the amount the less variability in its pattern.
2. The type of precipitation(rain, snow or hail): the formation of snow, for example, can act as a temporary store and large fluxes(flows) of water can be released into the system after a period of rapid melting resulting from a thaw.
3. Seasonality. In some climates, such as monsoon, Mediterranean or continental climates, strong seasonal patterns of rainfall or snowfall will have a major impact on the physical processes operating in the drainage basin system.
4. Intensity of precipitation is also a key factoras it has a major impact on flows on or below the surface. It is difficult for rainfall to infiltrate if it is very intense, as the soil capacity is exceeded.
5. Variability can have a major impact: Secular variability happnes long term, for example as a result of climate change trends. Periodic variability happens in an annual, seasonal, montly or diurnal context. Stochastic variability results from random factors, for example in the localisation of a thunderstorm within a basin.
6. The distribution of precipitation within a basin. The impact is particularly noticeable in very large basins such as the Rhone or the Nile, where tributaries start in different climatic zones. At a local scale and shorter time scale the location of a thunderstrom wihtin a small rover basin can have a major impact temporarily as inputs will vary, with contrasting strom hydrographs for different stream tributaries.
45
What is interception?
Interception is the process by which water is stored in the vegetation. It has three main components: interception loss, throughfall and stem flow.
46
What is condensation?
Condensation is the change from a gas to a liwuid, such as when water vapour changes into water droplets.
47
What is dew point?
Dew point is the temperature at which dew forms, it is a measure of atmospheric moisture.
48
What is convectional rainfall?
When the land becomes hot, the air above it becomes warmer, expands and rises. As it rises, the air cools and its ability to holdwater vapour decreases. Condensation occurs an clouds develop. If the air continues to rise, rain will fall. (the clouds formed are cumulous clouds)§
49
What is cyclonic rainfall?
Cyclonic rainfall happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools, and its ability to hold water vapour decreases. Condensation occurs and clouds and rain form.
50
What is orographic rainfall?
When air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward (downward) slope receives relatively little rain, which is known as the rain shadow effect.
51
What is interception loss?
Interception loss is water that is retained by plant surfaces and later evaporated or absorbed by the vegetation and transpired. When the rain is light, for example, drizzle, or of short duration, much of the water will never reach the ground and will be recycled by this process.
52
What is throughfall?
This is when the rainfall persists or is relatively intense, and the water drops from the leaves, twigs, needles, etc.
53
What is stem flow?
This is when water trickles along twigs and branches and then down the trunk.
54
What does interception depend upon?
The cover of interception can vary with the type of tree, with the dense needles of coniferous forests allowing greater accumulation of water. There are also contrasts between deciduous forests in summer and in winter- interception losses are around 40% in summer for certain Chiltern beech forests, but under 20% in winter.
55
What is a rain shadow?
A rain shadow is a dry area on the leeward(downward) side of the mountain. It receives little rainfall as the mountains shelter it from rain-producing weather systems. As the moist air is forced to rise on the windward side of the mountain, rainfall occurs as a result of adiabatic cooling(when the volume of air increases but there is no addition of heat), and condensation to dew point. The air, without much water left in it, is then drawn over the mountains where it descends and is adiabatically warmed by compression. This leads to a very dry 'shadow' area, for example, the Owens Valley is in the rain shadow of the Siera Nevada range in California.
56
How does wind speed impact interception?
Wind speeds can decrease interception loss as intercepted rain is dislodged, and they can also increase evaporation rates too.
57
Define infiltration
The movement of water from the ground surface into the soil.
58
What is infiltration capacity?
The maximum rate at which rain can be absorbed by soil.
59
Define surface run-off.
The movement of water that is unconfined by a channel across the surface of the ground. Also known as overland flow.
60
Name some factors that affect the rate of infiltration.
Infiltration capacity decreases with the time through a period of rainfall until a more or less constant low value is reached.
The rate of infiltration also depends on the amount of water already in the soil as surface or overland flow will take place when the soil is saturated.
Soil texture- whether sand, silt, loam, or clay- also influences soil porosity, with sandy soils having an infiltration capacity of 3-12mm/hr less than permeable clays 0-4mm/hr.
The type, amount and seasonal changes in vegetation cover are a key factor, with infiltration far more significant in land covered by forests(50mm/hr) or moorland(42mm/hr), hence the recent drive to vegetate upland catchments that flow into areas liable to flooding. Permanent pasture has infiltration rates of 13-23mm/hr depending on grazing density and soil type.
The nature of the soil surface and structure is also important. Compacted surfaces inhibit infiltration(around 10mm/hr), especially when rain splash impact occurs.
Slope angle can also be significant: very steep slopes tend to encourage overland run-off, with shallower slopes promoting infiltration.
61
What is the relation between infiltration and surface run-off?
They are inversely related. As infiltration increases, surface run-off decreases.
62
What is throughflow?
The lateral transfer of water down slope through the soil via natural pipes and percolines.
63
What are percolines?
Lines of concentrated water flow between soil horizons to the river channel.
64
When does direct overland flow occur?
Capacity in puddles has been exceeded. Overland flow is also a feature of many urban areas, especially when the capacity of storm drains and sewers has been exceeded.
65
What is percolation?
The transfer of water from the surface or from the soil into the bedrock beneath.
66
What is saturated overland flow?
The upward movement of the water table into the evaporation zone.
67
Define albedo
A measure of the proportion of the incoming solar radiation that is reflected by the surface back into the atmosphere and space.
68
Define evapotranspiration
The combined effect of evaporation and transpiration.
69
What is channel flow?
The flow of water in streams or rivers.
70
What is channel storage?
The storage of water in streams or rivers.
71
What is potential evapotranspiration (PEVT):
the water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation.
72
Under what conditions is throughflow percolation increased?
With humid climates and vegetated slopes.
73
Why is groundwater flow important?
It is a vital regulatory component in maintaining a steady level of channel flow in varying weather conditions.
74
What are the three components of channel flow?
Overland flow, throughflow, or groundwater flow.
75
What are the climatic factors influencing evaporation rates?
```
Temperature
Hours of sunshine
Humidity
Wind speed
Although temperature is the most important factor.
```
76
List some factors that are not climactic that influence evaporation rates.
Size of the water body
Depth of water
Water quality
Type of vegetation cover and the colour of the surface(which determines the albedo or reflectively of the surface).
77
Define transpiration
The biological process by which water is lost from plants through minute pores(stomata) and transferred to the atmosphere.
78
Define transpiration
The biological process by which water is lost from plants through minute pores(stomata) and transferred to the atmosphere.
79
What factors affect transpiration rates?
Time of year
The type and amount of vegetation cover
The degree of availability of moisture in the atmosphere and the length of the growing season.
80
When is the difference between potential evapotranspiration and evapotranspiration greatest?
In arid areas than in humid areas.
81
What are the physical factors that affect the drainage basin?
Climate- it influences the type and amount of precipitation overall and the amount of evaporation, ie. the major inputs and outputs. Climate also has an impact on the vegetation type.
Soils- they determine the amount of infiltration and throughflow and, indirectly, the type of vegetation.
Geology- this can impact subsurface processes such as percolation and groundwater flow(and, therefore, on aquifer). Indirectly, geology alters soil formation.
Relief- Altitude can impact on precipitation totals. Slopes can affect the amount of run-off.
Vegetation- The presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurence of overland flow, as well as on transpiration rates.
82
What is cloud seeding?
It is the introduction of silver iodide pellets, or ammonium nitrate to act as a condensation nuclei to attract water droplets. The aim is to increase rainfall in drought-stricken areas. It has variable results.
83
Name some human impacts on evaporation and evapotranspiration.
Deforestation is a key influence. Also important is the increased evaporation potential resulting from the enormous artificial reservoirs behind mega-dams, for example, the Aswan Dam and Lake Nasser in southern Egypt. Conversely, the channelisation of rivers in urban areas inot conduits cuts down surface storage and, therefore, evaporation.
84
List some human impacts on interception.
It is largely determined by vegetation type and density, deforestation, and afforestation.
85
Define afforestation
The planting of trees in an area that has not been forested in recent time.
86
What affect does deforestation have on evapotranspiration?
Deforestation leads to a reduction in EVP and an increase in surface run-off. This increases flooding potential, leads to a decline of surface storage and a decrease in the lag time between peak rainfall and peak discharge. In other words, it speeds up the cycle.
86
What affect does deforestation have on evapotranspiration?
Deforestation leads to a reduction in EVP and an increase in surface run-off. This increases flooding potential, leads to a decline of surface storage and a decrease in the lag time between peak rainfall and peak discharge. In other words, it speeds up the cycle.
87
What research has shown afforestation to be ineffective for up to 30 years?
Research of the river severn in mid wales shows that there is a period of time just after planting young trees where there is an increase in surface run-off and sediment loss as a result of compaction of soil by tractors and planting equipment, which only stops after 30 years when the trees are more fully grown.
88
What are the possible impacts of deforestation in the Himalayan foothills of Nepal?
Rainwater moves quickly over the surface to rivers, causing flooding.
Rainwater does not soak into the ground, but stays at the surface causing run-off and erosion. Raindrop impact increases.
Rainwater strikes soil directly, or grass near surface causing soil compaction.
Little rainwater soaks into the ground, so the level of groundwater is depressed, and little reaches the streams. When the rain stops the rivers quickly run dry.
88
What are the possible impacts of deforestation in the Himalayan foothills of Nepal?
Rainwater moves quickly over the surface to rivers, causing flooding.
Rainwater does not soak into the ground, but stays at the surface causing run-off and erosion. Raindrop impact increases.
Rainwater strikes soil directly, or grass near surface causing soil compaction.
Little rainwater soaks into the ground, so the level of groundwater is depressed, and little reaches the streams. When the rain stops the rivers quickly run dry.
89
How do infiltration rates compare between land use of forests vs grassland?
Infiltration is 5x greater under forests compared to grassland.
90
What are the positives impacts of ploughing on infiltration?
The ploughing increases infiltration by loosening and aerating the soil.
91
What feature can be implemented to help mitigate waterlogging and salinisation?
Installing drainage.
92
In London, what has a reduction in water-using manufacturing activity have led to?
Less groundwater being abstracted. As a result groundwater levels have begun to rise, leading to a different set of problems, such as surface water flooding, flooding of cellars and basements in houses, and increased leakage into tunnels such as those used by the London Underground. The water supplies are also more likely to become polluted.
93
What is a water budget?
A water budget is the balance between precipitation, evaporation, and run-off, it can be useful at global, regional, and local scales.
94
How are water budgets useful at a regional scale?
They provide a more useful indication of available water supplies.
95
How are water budgets useful at a local scale?
They show the annual balance between inputs(precipitation) and outputs(EVT), and how this can impact soil water availability.
96
What is the soil moisture budget?
It is a subsystem of the catchment water balance and is of vital importance to agriculturalists.
97
What is another name for a drainage basin water budget?
Water balance
98
How are water balances usually expressed?
P=Q+E±S
```
P= precipitaton
Q= discharge(stream flow)
E= Evapotranspiration
S= changes in storage
```
98
How are water balances usually expressed?
P=Q+E±S
```
P= precipitaton
Q= discharge(stream flow)
E= Evapotranspiration
S= changes in storage
```
99
Will there be a positive or negative water balance if precipitation inputs exceed evaporation losses?
There will be a positive water balance.
100
Under what condition does water surplus occur?
Precipitation>potential evapotranspiration. Soil water store is full and there is a soil moisture surplus for plant use. Run-off and groundwater recharge.
101
Under what conditions does soil moisture utilisation occur?
Potential evapotranspiration>precipitation. Water store is being used up by plants or lost by evaporation(soil moisture utilisation).
102
What happens between the periods of soil moisture utilisation and soil moisture deficiency?
Soil moisture store is now used up. Any precipitation is likely to be absorbed by the soil rather than produce run-off. River levels fall or rivers dry up completely.
103
Under what conditions does soil moisture deficiency occur?
There is deficiency of soil water as the store is used up and potential evapotranspiration>precipitation. Plants must adapt to survive, crops must be irrigated.
104
Under what conditions does soil moisture recharge occur?
the precipitation is greater than the potential evapotranspiration. Soil water store starts to fill again.(Soil moisture recharge)
105
What happens after soil moisture recharge?
The soil water store is full, field capacity has been reached. Additional rainfall will percolate down to the water table and groundwater stores will be recharged.
106
Define a river regime
The annual variation in discharge or flow of a river at a particular point or gauging station, usually measured in cumecs.
107
Where does the majority of river flow come from in a river regime?
Much of the river flow comes from the groundwater between periods of rain, which feeds steadily into the river system from base water flow.
108
What are the factors that characterise the regime of a stream or river?
The size of the river and where measurements are taken in the basin: many large river have very complex regimes resulting from varied catchements.
The amount, pattern and intensity of the precipitation: regimes often reflect rainfall seasonal maxima or when the snow field or glacier melt(for snow the peak period is in spring, for glaciers it is early summer).
The temperatures experienced: evaporatio will be marked in summer as the temperatures are warmer.
The geology and overlying soils, especially their permeability and porosity: water is stored as groundwater in permeable rocks and is gradually released into the river as base flow, which tends to regulate the flow during dry periods.
The amount and type of vegetation cover: wetlands can hold the water and release it very slowly into the system.
Human activities, such as dam building, which can regulate the flow.
109
What is the most important factor that determines stream flow?
Climate
110
What is a storm hydrograph?
Storm hydrographs show the variation of discharge within a short period of time, normally an individual storm or a group of storms not more than a few days in length.
111
Before a storm begins what is the main supply of water to the river or stream and what is it once the storm develops?
Before a storm begins the main supply of water comes from groundwater base flow bu, as the storm develops, water comes to the stream by a number of routes. Some water infiltrates into the soil and becomes throughflow, while some flows over the surface as overland flow. This water reaches the river in a comparatively short time so is known as quick flow.
112
What is the rising limb on a storm hydrograph?
The part of a storm hydrograph in which the discharge starts to rise as a result of rainfall input.
113
What is the peak discharge on a storm hydrograph?
The time when the river reaches its highest flow. It is often reached after the peak rainfall because the water takes time to move through the system to the gauging station of the basin.
114
What is the lag time on a storm hydrograph?
The time interval between peak rainfall and peak discharge.
115
What is the falling limb?
Once the storm input has ceased the amount of water in the river starts to decrease.
116
What is the base flow in a storm hydrograph?
The normal discharge for a river.
117
What is the difference between a flashy hydrograph and a hydrograph?
A flashy hydrograph has very steep libs, especially rising limbs with a high peak discharge and a very short lag time; whereas a normal hydrograph has more stable characteristics, e.g. ordinary lag time.
118
List some factors that interact to determine a flashy river.
```
The weather/climate
Rock type
Soils
Relief
Basin size
Shape
Drainage density
Vegetation
Pre-existing conditions
Human activity
```
119
Explain the factors that determine the shape of a storm hydrograph.(LOTS OF DETAIL REQUIRED)
The weather/climate- Intense storm which exceeds the infiltration capacity of the soil. Rapid snow melt as temperatures suddenly rise above zero. Low evaporation rates due to low temperatures.
Rock type- Impermeable rocks, such as granite, which restrict percolation and encourage rapid surface run-off.
Soils- Low infiltration rate, such as clay soils(0-4mm/hr)
Relief- High, steep slopes that promote surface run-off.
Basin size- Small basins tend to have more flashy hydrographs.
Shape- Circular basins have shorter lag time.
Drainage density- High drainage density mean more streams and rivers per unit area, so water will move quickly to the measuring point.
Vegetation- Bare/low density, deciduous in winter, means low levels of interception and more rapid movement through the system.
Pre-existing conditions- Basin already wet from previous rain, water table high, soil saturated so low infiltration/percolation.
Human activity- Urbanisation producing impermeable concrete and tarmac surfaces. Deforestation reduces interception. Arable land, downslope ploughing.
120
Explain the factors that determine the shape of a flat hydrograph.(LOTS OF DETAIL REQUIRED)
The weather/climate- Steady rainfall which is less than the infiltration capacity of the soil. Slow snow melt as temperatures gradually rise above zero. High evaporation rates due to high temperatures.
Rock type- Permeable rocks, such as limestone, which allow percolation and so limit rapid surface run-off.
Soils- High infiltration rate, such as sandy soils(3-12mm/hr)
Relief- Low, gentle slopes that allow infiltration and percolation.
Basin size- Larger basins have more delayed hydrographs; it takes more time for water to reach gauging stations.
Shape- Elongated basins tend to have delayed or attenuated hydrographs.
Drainage density- Low drainage density means few streams and rivers per unit area, so water is more likey to enter the ground and move slowly through the basin.
Vegetation- Dense, deciduous in summer, means high levels of interception and a slower passage through the system; more water lost to evaporation from vegetation surfaces.
Pre-existing conditions- Basin dry, low water table, unsaturated soils, so high infiltration/percolation.
Human activity- Low population density, few artificial impermeable surfaces. Reforestation increases interception. Pastoral, moorland and forested land.
121
What is the most significant human factor that leads to increased flood risk>
Urbanisation.
122
Summarise the effects of urbanisation on the flood risk.
Building activity leads to clearing of vegetation, which exposes soil and increases overland flow. Piles of disturbed and dumped soil increase erodibility. Eventually the bare soil is replaced by a covering of concrete and tarmac, both of which are impermeable.
The high density of buildings means tat rain falls on to roofs and is then swiftly dispatched into drains by gutters and pipes.
Drains and sewers are built, which reduce the distance the storm water must travel before reaching a channel. The increase in the velocity occurs because sewers generate less friction than natural pathways: sewers are designed to drain water quickly.
Urban rivers tend to be channelised with embankments to guard against flooding. When floods occur they can be more devastating as the river overtops defences in a very confined space.
Bridges can restrain the free discharge of floodwaters and act as local dams for upstream floods.
In extreme weather events, urban areas such as Manchester, Leeds and York are highly vulnerable. They have to manage flood control problems with a higher , quicker peak discharge, well as pollution problems from the storm water which washes off the roads, containing toxic substances.
123
Name some methods that people who work in planning use to manage a catchment area as a whole.
Developing appropriate land use, such as forestry and moorlands, in the upper areas, and managing development in the lower part of catchments by land use zoning, and by limiting building on the floodplains so 'making space for water to flood'.
124
Name some low-cost strategies that can be used to lower flood risks.
Semi-permeable surfaces for car parks and high level wiring systems in houses, as well as the government developing affordable insurance. Additionally, building regulations can be tightened to ensure flood-proof property designs.
125
What are the main effects of urbanisation on flooding a flow chart?
Urbanisation= Population density increases & Building density increases.
1. Population density increases-> pollution control problems
