Unit 1: Hydrology and Fluvial Geomorphology Flashcards
why are there so many
Hydrological cycle
The system of water movement around the earth
Lithosphere
The geological world (rocks that make up the earths crust)
Atmosphere
The layer of gases that surround the earth
Biosphere
All the living organisms found on earth
Drainage basin
Area of land that drains water to a single outlet into a lake or sea
Watershed
Boundary line of a basin usually a high ridge
Mouth
Where the river ends at a sea or lake
Source
Where the river begins usually in mountainous areas
Tributary
Smaller rivers or streams joining the main one
Confluence
Where a tributary meets the main river or where two rivers meet
Drainage patterns
The pattern or network of streams and rivers within a drainage basin can vary greatly. These patterns are often determined by the underlying geology
Dendtritic
A tree-like pattern where water may converge from a variety of directions before joining a min river channel
Rectangular
Where the streams and channels follow geological weaknesses and gaps in blocky bedrock
Radial
Where water drains away from a central high point, hill or mountain into separate channels
Trellised
Where streams follow slopes downhill and converge along areas of eroded rock
Endorheic drainage basins
Inland basins that do not drain into one of the worlds oceans
Instead they drain into a lake or small inland sea
System
Isolated system = no exchange with surroundings
Closed system = energy exchange with surroundings
Open system = energy and matter exchange with surroundings
Drainage basing inputs
Precipitation (all forms of rainfall, snow, frost, hail and dew)
This water is then stored or transferred in different parts of the drainage basing for different amounts of time
Drainage basin outputs
Evaporation
Transpiration
River discharge
Precipitation
Any transfer of water from the atmosphere to the land within a drainage basin. The nature of it can vary greatly so can have different impacts on the drainage basins and rivers within them. Variations:
Amount
Extent or distribution
Intensity
Type
Duration
Water storage
The parts of the system that hold or retain water for periods of time
Can be open stores on the surface, within vegetation or within rock structure. The amount of time it is stored for depends on the processes acting on it
Interception
Water that is caught and stored by vegetation. Affected by the size and coverage of plants.
Interception loss
Water retained by plants and later lost as evaporation
Through-fall and leaf-drip
Water slowed by running off and dropping from leaves/twigs
Stemflow
Water that runs down branches and trunks to the ground
Vegetation storage
When vegetation absorbs moisture directly through its root system it is stored within the organism or plant. The amount stored relates to the size and variety of plants and the local conditions at that time
Surface storage
Any parts of the system where water lies about the gorund. May accumulate in lakes, ponds and puddles or through human intervention structures like swimming pools. These stores have a high potential EVT rate
Channel storage
Water contained within a river channel or stream at one time
Groundwater storage
Water that has become stored in the pores and spaces of underlying rocks. May be stored here for 20000 years
Groundwater recharge
A result of percolation, infiltration from precipitation, leakage and seepage from the banks and beds or water bodies as well as artificial recharge from reservoirs and irrigation
Aquifers
Contain any large quantities of water. They are underground layers of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt) that can be found at depth. The ones near the surface are used for water supply and irrigation
Recharge rates
Areas that suffer from a large extraction of groundwater through wells and pumps need good recharge rates. Areas with little recharge consider groundwater non-renewable. Many groundwater reservoirs are being used at an unsustainable rate
Soil moisture storage
Water held in soil above the water table but below the surface. Water is held within the soil pores before being absorbed or moving upwards or downwards
Field capacity
The amount of water held in the soil after excess water drains away
Saturated soil
When the soil can no longer hold any more water
Soil moisture deficit
Water levels in the soil are falling as potential EVT exceeds PPT
Soil moisture recharge
Water level sin the soil are increasing as EVT is lower that PPT
Soil moisture surplus
Soil is saturated resulting in more overland flow
Soil moisture utilization
Soil moisture is draw to the surface and used as EVT is high
Springs
The point at which groundwater discharges onto the surface. A spring line is the intersection of the natural water level in the ground with the surface along which springs are commonly found. They are found along faults or areas or great topographic relief like cliffs or valleys
Groundwater depletion
Primarily caused by sustained groundwater pumping. Could also be caused by changes in climate
Throughfall
Water slowed down by leaves before reaching the surface
Stemflow
Water that runs down branches and trunks to the ground
Overland flow
The movement of water over the land, downslope to a body of water
How does overland flow occur
- When precipitation exceeds the infiltration rate
- When the soil is saturated
Infiltration
Water being absorbed or soaked into the soil
Differences in overland flow
When precipitation intensity is high but infiltration rates are low, overland flow is common
When precipitation intensity is low and infiltration rates are high, overland flow is rare
Channel flow/stream flow
The movement of water in a channel
How does water get in a channel
Direct precipitation
Overland flow
Groundwater flow
Throughflow
Tributaries
Porosity
The capacity of a rock or soil to hold water in its pores
Permeability
The ability to transfer water through a rock via joints and fissures
Infiltration capacity
The maximum rate at which rain can be absorbed by a soil in a given condition
Infiltration is inversely related to overland flow. When infiltration rates fall, overland flow increases
Vegetation can determine infiltration rates by slowing down water flows. Thick grassland can absorb almost 10x as much rainfall as bare ground
Percolation
When water moves slowly downwards from the soil into the bedrock
Often very slow but in rocks like limestone and chalk it can be very fast
Throughflow
Water flowing through the soil in natural pipes and percolines (lines of concentrated water flow between different types of soil)
Groundwater flow
Movement of water through rocks downslope
Baseflow
The proportion of the rivers discharge that is provided by groundwater flow. Tends to be very constant irrespective of the levels of precipitation
Transpiration
The process of evaporation of water from plants through pores in leaves. Broadleaved trees can hold more water so have greater potential for high transpiration. Some plants are specially adapted to retain moisture by reducing transpiration
Evaporation
Water converting to water vapour in the atmosphere. More significant where there are large bodies of water. Rates of evaporation depend on the climate like temperature, humidity, wind, the amount of water available, vegetation cover and albedo (reflectivity of the surface)
Evapotranspiration
The combined effect of evaporation and transpiration and is the major output of the drainage basin system
River discharge output
The volume of water moving in a river. Can also describe the output of river water from a drainage basin. At its lowest point a river will discharge into the sea
Other drainage basin outputs
When geology at lower levels causes leakage so water can seep from one drainage basin to another
Human water management may modify the system by creating reservoirs and dams affecting channel flow (taking water for irrigation, domestic or industrial use)
River discharge equation
Q=AxV
Q is the discharge, A is the cross-sectional area, V is the velocity
River regime
Changes to river discharge over the course of a year
Peak discharge
Maximum amount of water held in a channel
Peak rainfall
Maximum amount of rainfall
Lag time
The time taken between peak rainfall and peak discharge
Rising limb
Shows the increase in discharge on a hydrograph
Falling limb
Shows the return of discharge to normal/base flow on a hydrograph
Base flow
The normal discharge of the river
Catchment hydrology
The movement, distribution and quality of water in a drainage basin
Infiltration rate in hydrology
The flow of water through the soil and surface into a porous medium under gravity and pressure
Type of precipitation
Flooding often occurs after lots of rainfall when soil stores are full so less drainage is possible. The conditions before rainfall are antecedent conditions. During cold, water may temporarily be stored as snow or ice so less water is in the system but there may be a sudden release of water in a thaw. Intense storms are more likely to cause floods as the ground can’t absorb lots of water in limited time.
Relief
The size and shape of land affects the rate water can flow down it. Slopes with an angle less than 5 degrees have greater infiltration rates. The steeper the gradient, the greater surface run-off due to less infiltration opportunity. Higher up, rivers may cut steep valleys due to gravity but this is lessened downstream.
Parent material
The underlying geology of an area and the origins of the formed soil. This will determine permeability and how well the ground will drain
Sedimentary rocks
Formed through the deposition of sediment and the subsequent compression as additional layers are deposited about. Often porous so water can pass through them (permeable).
Metamorphic rocks
Sediments and rocks that have been transformed by heat and pressure. Their permeability will depend on the nature of the transformation
Igneous rocks
Formed by extreme heat and pressure in magmatic environments. Impermeable
Soil type, structure and density
Soil is composed of rock fragments, organic matter, water, air, organic material and organisms. The greater the clay content, the more water renetive it is since clay particles bond tightly restricting water flow. A sandy soil is free draining as the larger particles provide gaps for water to pass. Most soils contain a mix but become more easily saturated with more clay. Floodplains contain a lot of small particles deposited by flood (alluvial). Beaches are sand
Drainage density
The number of rivers and streams in an area. The greater the number of rivers, the easier the catchment will be able to drain. This may produce a quick rise in the hydrograph and a greater risk of flooding.
Antecedent conditions
The previous conditions that have affected an area. An area that has experienced a lot of precipitation may have partially or fully saturated soil increasing surface runoff. Dry conditions would allow for more water storage but too dry may mean there is a baked, impermeable crust which makes infiltration hard. Could mean a flashy hydrograph.
Land use
The land use of an area may be influential in determining catchment response
Urbanisation
Settlements are often heavily concentrated, very different to those on open moorland or arable farms. Urban populations are growing causing greater urbanisaiton and an increase in flood risk. Water can’t infiltrate tarmac and concrete combined with gutter and drains that channel and direct runoff, water can be carried quickly to the nearest waterway. Often runoff from roads and urban landscapes contain pollutants and waste unnatural for rivers causing damage to the ecosystem.
Vegetation
Vegetated areas have a greater capacity to intercept precipitation and absorb soil moisture. Rainforests intercept up to 80% of rainfall 30% of which may evaporate later but arable land may only intercept 10%. Broadleaved deciduous trees have a larger biomass and expansive canopy in summer causing greater interception than in winter where intake is reduced due to the loss of leaves in autumn. Deforestation is widely associated with flooding. The removal of vegetation for development or harvesting causes faster flows and implications on the regime. The stability of soil profiles can be compromised by logging trails and disturbed ground with areas vulnerable to erosion by the fast surface flows. The resultant runoff is heavily silted which makes rivers thick and dirty with sediment. Areas reliant on rivers for drinking suffer.
Tides and storm surges
The daily rise and fall of tides affects the base level of a river. High spring tides may prevent water discharging into the sea increasing flood risk. Low pressure system reduce the pressure acting on the sea level leading to a slight rise in water level. This along with strong winds creates pressure on low-lying coastal areas. Storm surges occur when strong winds affect a coastline, forcing waves landward and inward through estuaries.
Features of a river channel
River bank
River bed
River flow
The manner in which the water travels downstream through a river channel
What are the 3 types of river flow?
Laminar flow
Turbulent flow
Helicoidal flow
Laminar flow
The smooth horizontal movement of water. In reality this only happens in man-made straightened channels without steps, gradients and obstructions on the river banks
Turbulent flow
A series of erratic horizontal and vertical spiral flows. Also called eddies. They cause the surface of the water to be disturbed. Usually the dominant flow in rivers. Tubulance generally increases as river velocity increase and as friction with the river bed and banks increases
Helicoidal flow
A corkscrew-like flow that usually occurs when water travels around bends. In rivers, these are meanders and the formation of sediment bars and slip-off slopes
Thalwag
The name given to the path of least resistance through a river channel. This is where the influence of friction from the river beds, banks and surface are lowest so where flow is the fastest. On a straight river channel this is central but closer to the outside of a bank on a meander
What is river velocity determined by?
Gradient
Efficiency
Channel bed roughness
River efficiency
Measured by calculating the hydraulic radius
HR=cross sectional area/wetted perimeter
The larger the HR, the more efficient the channel
Erosion
The geological process in which earthen materials are worn away and transported by natural forces
Transportation
The movement of eroded material from one location to another
Deposition
The laying down of sediment carried by wind, water or ice
Hydraulic action
The force of water pushing into cracks and hitting against the rivers bank. It weakens the riverbank and air is compressed and pressure builds. Collapsing air bubbles creates cavitation
Corrasion
When sediment is thrown into or scraped along the banks and bed of a river. More effective during high flow. Potholes form when stones get trapped in hollows
Attrition
The process by which stones and sediment in a river become rounded. Material collides as it is transported causing sharp edges to break off
Solution
Water that has slight acidic properties will chemically dissolve and weaken certain rocks
Capacity
The total load of material actually transported
Competence
The maximum size of material a river is capable of transporting
Traction
When the largest boulders are rolled along the riverbed by turbulent flow
Saltation
Where smaller bedload are lifted and carried temporarily in a bouncing motion
Suspension
When fine particles are carried in suspension in fast-flowing water
Solution (transportation)
The process by which dissolved sediments are transported
When does deposition occur?
Following periods of low precipitation where river levels drop
Where river meets the sea
In areas of slow flow within a channel
When the load suddenly increases above capacity
When the water carried material outside the channel
Long profile of a river
The name given to the gradient of a river from the source to the mouth. They always work under the influence of gravity. The higher a rivers source, the greater GPE. As a result, the upper part of a river is often steep and has steep valleys. In the lower parts, as GPE decreases, rivers expel energy by eroding laterally. As water flows downhill, it seeks the path of least resistance. Channel are often rougher in upper parts but become deeper and more efficient downstream
The upper course
A high-energy environment that experiences high levels of erosion and turbulent flow
As water accumulates at the source it carves out shallow maths in the soil and vegetation before descending rapidly under gravity
At altitude the combination of weathering and fluvial erosion contribute to the high level of bed load and large angular material
Due to this, traction and saltation are common
The middle course
A longer section of river characterised by decreasing gradient and greater lateral erosion
Valley is less steep and river is more winding
River becomes more established and there are more tributaries
High proportion of suspended load and bed load is smaller and less angular
The lower course
The low-lying portion of the river that joins the sea
Has wide, flat, sweeping floodplains and large meanders
Is the depositional zone with small, rounded stones
High proportion of suspended material
Hjulstrom curve
Shows the relationship between particle size and velocity. The mean or critical erosion velocity curve shows the approximate velocity needed to pick up and transport particles of various sizes. The capacity of the river is responsible from most of the subsequent erosion. The mean fall or settling velocity curve shows the velocities at which particles of a given size become too heavy to be transported so fall out of suspension
Upper course characteristics
Often experiences large variations in weather and so the amount of erosion varies greatly
Large, angular boulders often dislodged from the valley side by freeze-thaw weathering are usually found in and around the river channel, increasing friction and creating even more turbulent flow
Vertical erosion will be especially high during peak discharge
V-shaped valleys
These are steep sided valleys in the upper course where the river channel occupies most of the valley floor. The exact shape may vary slightly depending on local factors such as geology
They are separated by interlocking spurs which are ridges of land form alternate sides of the valley that the river works its way around
Waterfalls
A cascade of water falling from a height, formed when a river or stream flows over a precipice or steep incline
How waterfalls are formed
- They first form when a river flows over a layer of harder rock followed by a layer of softer rock
- The soft rock erodes more quickly forming a step in the river bed
- The force of the water undercuts the hard rock and creates a plunge pool
- The hard rock is left overhanging and because it isn’t supported, it eventually collapses
- The fallen rocks crash into the plunge pool and they swirl around causing more erosion
- Over time, this progress is repeated and the waterfall recedes upstream forming a gorge
Potholes
They are round to oval shaped holes in the bedrock created by the swirling motion of the river and the abrasion of the river bed
Rapids
Areas where the river bed is very uneven causing very turbulent water flow. It is caused by alternating banks of hard and soft rock, or by sudden narrowing of the channel or change in gradient
Sinuosity
The natural path of a river is rarely straight
Meanders are a result of different rates of deposition and erosion
A straight river has an index of 1.0
Highly sinuous rivers have values above 4.0
Any river above 1.5 if meandering
Sinuosity index calculation
Actual channel length/straight line distance
Riffles
Shallower parts of the river with faster, oxygenated flow
Pools
Deeper parts of the river with slower flow
Meander formation
Meanders have an asymmetrical cross section. Flow is fastest on the outside of the bend so erosion deepens the channel. Flow is slower on the inside of bends so deposition occurs. Helicoidal flow occurs when surface water flows towards outer banks while bottom flow is toward the inner bank. Variations in flow create differences in cross-sections
River cliffs
Form on the outside of bends where erosion is greatest. The combined effect of hydraulic action and abrasion weaken the priverbank causing it to collapse. A steep bank will form with some of the collapsed material remaining on the riverbed
Slip-off slopes
On the inside of the bend where discharge is minimum and friction is greatest, deposition is greatest. Sediment accumulates to create a gentle sloping bar called a slip-off slope or point bar. The largest material is usually found on the upstream side of the bar
Meander characteristics
Meander wavelength is 10x the channel width
Riffles and pools are spaced 5-7x the channel width
The radius of the curvature of the bend is proportional to 2-3x that of the channel width
Meander amplitude is 5-7x the channel width
Ox-bow lake formation
- Erosion causes the outside of the bends to get closer
- Only a small piece of land is left between the bends
- The river breaks through the land, usually in a flood
- The river flows along the shortest course
- Deposition cuts off the meander
- Ox-bow lake is formed
Levees
Natural embankments which are formed when a river floods. When a river floods, friction with the floodplain leads to a rapid decrease in the velocity of the river so its capacity to transport material decreases. Larger material is deposited closer to river banks
Bluff
The edge of a floodplain is marked by a slightly raised line
River braiding
Braiding occurs when there is a high proportion of load in relation to discharge
May be a result of seasonal changes
At times of low flow, the river ma be forced to cut a series of paths that converge and diverge as they weave through deposited material
Braiding begins with a mid-channel bar that grows downstream as discharge decreases following a flood. Coarse bedload is deposited first
This form the basis of bars and as the flood is reduced, finer sediment is deposited
The upstream end stabilises and can become vegetated
The islands can alter subsequent flows, diverting the river and increasing friction
Deltas
Influenced by the river, tides and waves
Formed when large amounts of river load meet the sea and are deposited
Usually composed of fine sediments that are dropped during low energy conditions and are called deltas because they are traingular
As freshwater and saltwater mix, clay particles coagulate and settle to the seabed in a process called flocculation
The finest sediments are carried furthest and are deposited first as bottomset beds
Slightly coarser material is transported less far and is deposited as foreset beds
Coarsest material is deposited as topset beds
What are the 3 types of delta?
Arcuate delta
Cuspate delta
Bird’s foot delta
Arcuate delta
Having a rounded convex outer margin
Cuspate delta
Where material is evenly spread on either side of the channel
Bird’s foot delta
Where the sediment is distributed around many branches of the river (distributaries) in the shape of a bird’s foot
Base levels
Base level is defined as the limiting level below which a stream or river cannot erode its channel. For streams that empty into oceans, base level is sea level. On a more local level, the base level might be to a pond or lake
The river is constantly trying to produce the most efficient route to its base level while continually being influenced by the energy balance and outside factors
Changes in base level over time
The bse level of a river could change due to changes in sea-level. Sea levels rise during warm periods due to ice melt and the thermal expansion of water (eustatic rise)
Sea levels fall during colder periods due to water becoming locked up in ice and the thermal contraction of water (eustatic fall)
Isostatic rebound
This is when land that was previously compressed down by the weight of massive ice caps and glaciers during an ice age, slowly rises back upwards during warmer periods. This can exaggerate or negate global sea level changes
While some areas are uplifting since the ice age, this can cause other areas to sink
Graded profiles
The ultimate end point for the river when it is in full equilibrium and the base level is constant
Graded profiles may be altered by the presence of lakes or particularly resistant rock layers
Knickzones
Areas where the bed of the river is steeper than up or downstream - a cascade or area of fast water. Such oversteepened reaches can reflect faulting or the presence of strong rocks that are resistant to erosion
Knickpoints
Discrete jumps in elevation along a river’s bed or waterfalls. Such jumpts commonly retreat and grow less steep over time. Knickpoints can result from base level change, faulting, resistant rocks or the lingering effects of valley glaciation
Incised meanders
Asymmetrical as they are eroded more slowly. As the river channel erodes vertically as well as laterally, it will start to undercut on the outside of the bend creating an overhang in the river cliff. The inside of the bend, due to the continued deposition will form a gently sloping bar
Entrenched meanders
Formed, geologically, more rapidly. The meanders tend to be more symmetrical as they carve out a deep winding gorge across the landscape (grand canyon)
River terraces
Areas of higher ground surrounding a river. They are the former floodplains of a river that were carved out when it was higher up, now above the current level of flooding. Due to a change in base level, an increase in vertical erosion creates a newly cut river
Rias
A coastal inlet formed by the partial submergence of an unglaciated river. It is a drowned river valley that remains open to the sea. They typically have a dendritic outline although they can be straight without significant branches. They are funnel-shaped estuaries near the mouth
Why is water important to society and ecosystems?
Reliable freshwater is important for human health, agriculture, energy production, navigation, recreation and manufacturing. The stress that these put water resources under is likely to be intensified by climate change and population growth
Climate change and water
In many areas, climate change and population growth increases water demand while reducing water supplies. In some areas, water shortages will be less of a problem than increases in runoff, flooding or sea level rises
What is water quantity?
The amount of water available. The flows of the hydrological cycle can vary with location (latitude, altitude and continentality) and temporally through seasonal changes
How have human activities impacted water?
Human activities have increased global temperatures by 0.8 degrees Celsius over the last 30 years. The unpredictability of weather and climate means there is greater potential for extreme events such as droughts or flooding
What is water quality?
The cleanliness and usefulness of water to society and the environment. The way humans harness water is not always efficient, clean or sustainable
Human impact on precipitation
In heavily industrialised areas, precipitation rates are up to 10% higher due to an increased number of pollutants and particulate matter creating a greater extent and frequency of clouds.
For moisture to fall as rain, water vapour must attach to small particulate matter in the atmosphere (hydroscopic nuclei). As water vapour condenses and accumulates to form clouds, water droplets grow before falling under gravity
What is cloud seeding?
It is designed to encourage precipitation. It injects more particulate matter into the atmosphere to create rain. Silver iodine, carbon dioxide and ammonium nitrate are used and dispersed by aircraft or fired by rockets into the air
Urbanisation impact on the hydrological cycle
An increase in urbanisation creates large, impermeable surfaces which reduce interception and infiltration
This usually causes flashy hydrographs. As water run over impenetrable surfaces and into drains, it is carried quickly causing a quick response in the river, raising levels and increasing flood risk
How does the removal of vegetation affect hydrology?
Less than 1% of britain is covered by natural woodland due to human activities. The removal of vegetation for development or crop harvesting can affect the hydrological balance of an area. Where clearance is large in relation to vegetation coverage, effects will be heightened
How to interception rates affect the hydrological cycle?
Interception rates are determined by the type and extent of vegetation cover. Much of the land’s surface has experienced some clearance and modification causing widespread deforestation. This reduces EVT rates and increases surface runoff resulting in a flashier hydrograph and shorter lag time. Afforested areas have a greater capacity to absorb moisture and bind the soil. They are planted mainly for commercial reasons but it does help habitats and flood management
How do infiltration rates affect the hydrological cycle?
Infiltration is up to 5x higher under forest than pasture. Forested areas intercept precipitation before funnelling it ground-ward. Bioturbation (reworking the soil by animals) is high in fertile forest with macro-invertebrates constantly aerating the soil Pore spaces are larger and more plentiful than pastora land where the land is heavily compacted due to animals
The impact of dams and reservoirs on the hydrological cycle
Large stores of open water (reservoirs) increase the potential for evaporation. Where temperature is high, evaporation rates are high. This can be reduced by creating underground and covered storage with plastic or sand-filled dams which can be impractical for large applications. In warmer environments and drought-prone areas, underground storage containers and water tanks are used
Water abstraction impact on the hydrological cycle
The removal of water temporarily or permanently from lakes, rivers, canals or underground rock strata. The redirection of this water from natural flows in a drainage basin can be for commercial, industrial or domestic uses. This is closely regulates. Reasons for water abstraction include irrigation, groundwater withdrawal and inter-basin transfer/trans-basin diversion
Irrigation
Used to increase the productivity of an area through water redirection though the amount of water is managed to suit the crop
Problems with the reduction in agriculture and industrial water extraction leading to excess water at ground level
Increase in spring and river flows
Surface flooding and saturation of agricultural land
Flooding of basements and underground tunnels
Re-emergence of dry rivers and wells
Chemical weathering of building foundations
Groundwater effect on the hydrological cycle
Human activity has severely reduced the sustainable potential of groundwater in some areas. If groundwater usage exceeds recharge, the water table will drop. Many groundwater stores are in equilibrium where recharge and discharge are equal. A main problem of groundwater abstraction is in coastal areas where saltwater intrusion. This is the movement of saltwater into an aquifer that used to hold freshwater. Many coastal communities in the US have experienced this for decades
Overextraction
Can lead to subsidence. As water is removed from the rock, sediment particles fill pore spaces that used to be filled with water. This results in a compression of the land and a reduction in its height. Can be particularly problematic when under structures and buildings. Railway lines and pipes and be ruptured
Mining effect on groundwater
Mining can deplete surface and groundwater supplies. Groundwater withdrawals may damage or destroy streamside habitats many miles from the actual mine site. Mining can affect water quality. Heavy metal contamination like arsenic being leached out of the ground, sulphide-rich rocks reacting with water to create surface acid, chemical agents used to separate minerals that leak into nearby water bodies, erosion and sedimentation from ground disturbance that clogs waterways and smoother vegetation and organisms as well as silting up fresh drinking water
Energy generation
Hydropower uses water to turn turbines. This has a small impact on the quantity and quality of water as it is largely returned will little change in state. Less sustainable energy requires water for fossil fuel and nuclear energy production. Water is converted to steam that powers a turbine to generate electricity. The water is then returned to bodies of water with a lower oxygen content at different temperatures, damaging fish populations and habitats
Floods
An overflow of water onto an area of land that is usually dry
This occurs when the discharge of a river exceeds bankfull capacity
Climatological causes of floods
Rain
Ice melt
Snow melt
Part-climatological causes of floods
Estuarine interactions between streamflow and tidal conditions
Coastal storm surges
Other natural causes of floods
Earthquake
Landslide
Dam failure
Human causes of flood intensifying
More rapid discharge in urban areas due to impermeable surfaces and more drainage channels
Urbanisation and urban growth
Floodplain developments
Bridges, dams and obstructions
Changes in vegetation cover
River engineering works
Human-induced climate change
Importance of predicting floods
Considered the most serious natural disaster due to frequency
Floods causes about 10000 deaths per year globally
The risk is increasing as climate change causes more extreme flooding and world population grows
Flood forecasting
The ability to predict floods is improving. Technology and algorithms based on the characteristics of different drainage basins allow us to model scenarios and predict when life-threatening floods will occur. The complexity of nature means they won’t be completely accurate. The larger the drainage basin, the easier it is to predict floods. This is due to the longer lag time leading up to peak discharge. Flash floods in smaller drainage basins are harder to predict which makes them more dangerous
Monitoring rivers
Hydrologists collect lots of data to help predict floods. They can monitor:
-The amount of rainfall in real-time
-Water levels in the river at numerous points in real-time
-Duration, intensity and ariel extent of incoming storms
-Conditions of the drainage basin
Different agencies are responsible for monitoring flooding.
Recurrence intervals
Shows the likelihood of a flood of a particular strength happening in 1 location. The numbers are averages based on a minimum 10 years of data (not a guarantee)
A large flood could be a 1 in 100 year flood
A small flood could be a 1 in 5 year flood
Recurrence interval calculation
Recurrence interval = (N+1)/Rank
n= number of years of observation
rank = rank order (large floods are ranked each year according to magnitude in runoff volume)
This shows how many years within which a flood event can be expected
Does not take climate change into account
What does prevention mean?
Stopping something from happening
Amelioration
Making something better or improving it
Flood warning
Predict when a flood is going to happen and get the message to people as soon as possible
People can evacuate or put in place measures to protect their property
Flood resistance
Removable barriers on doors and windows
Temporary seals for doors and air bricks
One-way valves on toilets and drainage pipes to decrease the risk of sewage backing up into a building during a flood
Pump and sump systems which drain water from below floor level faster than it rises
Flood resiliance
Ceramic or stone tiles instead of laminate or wood flooring
Raising electric sockets to 1.5m
Stainless steel or solid wood kitchens instead of chipboard
UPVC window frames instead of wooden ones
How to reduce flood risk at home
Waterproof bags
Place items higher up
Water sensors
If flood is over 1m outside, let the water in because of the pressure on the walls
How can flood warnings be improved?
Better and longer forecasts of rain and snowfall
Better gauging of rivers and mapping of channels
Improved information on infrastructure and population in drainage basins
Better sharing of information between forecasters, agencies, relief and the public
Better international sharing of data for cross-border drainage basins
Better technology sharing between world agencies
MEDC and LEDC floods
Deaths from flooding have fallen in MEDC’s but risen in LEDC’s
Hard engineering definition
Creating or building something man-made to alter the normal pattern of river flooding such as dams, straightening, levees and diversion spillways
Dams
Built to control flooding, provide HEP and create reservoirs for recreation and supply of water
Allows water to be held upstream and slowly released during high rainfall or snow melt. This limits peak discharge downstream and reduces the flood risk
Stored water can be released during periods of low precipitation to maintain water levels in downstream rivers and free capcaity in the reservoir for future storage when required
Pros of dams
Production of HEP
Relatively green energy
Reservoirs for leisure
More recreation
Irrigation
Water supply guarantee
Controlled water flows
Protection against floods
Job creation
Waterways for transportation
Tourist attraction
Cleaner water
Increased food production
Cons of dams
Can break
People have to relocate
High construction costs
Long construction time
Maintenance required regularly
Experts needed for control
Sedimentation patterns changed
Problems for squatic life
Excessive algae
Deforestation
Methane production
Ecological imbalance
People can be cut off from water flows
Challenges in drought
Political tool
Channel straightening
Speeds up water so high volumes can pass through an area quickly, reducing flood risk
Channel straightening advantages
Improved navigation
Water doesn’t travel as far
Reduced insurance costs
Channel straightening disadvantages
Flooding downstream
More erosion downstream
Expensive and high-maintenance
Unattractivw
Levees
Embankments that run parallel to a river channel to prevent water spreading across what would normally be its floodplain
Levees advantages
More affordable
No resettlement
Levees disadvantages
Failure may result in more harm
Required maintenance
Diversion spillways steps
- Flood embankments with sluice gates
- Channel enlargement to accommodate larger discharges
- Flood relief channels
- Intercepting channels
- Flood storage reservoirs
- Removal of settlements
Diversion spillway definition
Additional channels built to aid the flow of water to reduce or redirect floods
Storm drains
Infrastructure designed to drain excess rain and groundwater from impermeable surfaces. Diverts water into natural water body
Culverts
A structure that channels water past an obstacle or subterranean waterway. Can move large volumes of water preventing backup of floodwater
Barrages
A low-head diversion dam consisting of lots of large gates control water. Control downstream river height
Dredging
The evacuation of material for a water environment. Enables land drainage in low-lying landscapes and pumps water away
Soft engineering
Follows a more sensitive approach to maintaining and controlling river flow. Approaches seek to utilise the natural environment where possible and use natural and local materials to modify the river while maintaining its character
Floodplain and drainage basin management
This is managing the area surrounding a river to reduce the impacts of flooding
It has 2 main aims: flood abatement and flood diversion
Flood abatement definition
Decreasing surface runoff to minimise peak discharge
Flood diversion definition
Deliberately allowing certain areas to flood to protect others
Flood abatement schemes
Reforestation
Reseeding of sparsely vegetated areas to increase EVT
Contour ploughing or terracing of slopes
Protecting vegetation from wildfires, overgrazing and clear cutting of forests
Clearing sediment and debris from streams
Constructing small water and sediment holding areas
Preserving and restoring natural water storage zones like lakes
Flood diversion schemes
Focused on encouraging flooding in natural areas to reduce the risk to neighbouring valuable land such as housing
These areas include floodplains and wetlands which are excellent wildlife habitats and are of recreational benefit
Diversion spillways may be used to direct floodwaters to these natural areas
Washlands
Areas of land that are periodically allowed to flood to reduce pressure elsewhere. Often these areas are farmland and farmers are compensated for any potential loss of income
Catchment management plans
Looks to coordinate all action within a drainage basin to prevent flooding and improve water quality. It would bring together all stakeholders into an agreed future action plan
Restoring and conserving river banks
During periods of flood, river banks can be eroded reducing the capacity of the river channel in the future. These banks can be rebuilt and stabilised with plants to reduce the potential for future floods
River restoration
This is returning a river that has previously been managed and changed by humans back to its natural course. This includes restoring river banks, creating space for meanders, digging out riffles and pools and reintroducing point bars and river cliffs. The purpose of this is to slow the river down and increase its capacity to hold water
