Freshwater Flashcards
Drainage basin
The total area drained by a river and its tributaries
Why is the drainage basin an open system?
Because water can enter the system via precipitation and also leave, moving across the watersheds of multiple drainage basins. Matter can leave the drainage basin.
Inputs, outputs and flows of drainage basin
Input: precipitation
Flows/transfers = infiltration, throughflow, groundwater flow, percolation, base flow
Outputs: Evaporation, transpiration
Stores
Vegetation Aquifers Soil Cyrosphere groundwater
Definition of precipitation
Transfer of moisture from the Earth’s atmosphere to the earth’s surface in the form of rain, hail, sleet, snow.
Definition of interception
The capture of raindrops by plant cover that prevents direct contact with the soil. If rain is prolonged, retaining capacity of leaves will be exceeded and water will drop to the ground (through fall)
Evaporation is a function of…
- Vapour pressure
- Air temp
- wind
- rock surface - bare rocks have greater evap rates
Potentail evapotranspiration
The water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation.
Discharge definition + units?
The total volume of water flowing past a certain point per unit of time (cumecs)
Bradshaw model characteristics
Discharge increases Load size decreases Load angularity decreases Load amount increases Roughness decreases gradient decreases
5 river transport ways
Floatation - leaves and twigs carried on the surface of the river
Saltation - Heavier particles bounce or bump along the river bed
Attrition - particles collide with each other, becoming smaler and rounder
Solution - chemical ions are removed, dissolving particles in water
Traction - boulders are rolled along river bed
Factors affecting erosion
Rock type Vegetation Precipitation intensity Gradient Channel density Discharge Load angularity pH Urbanization
Explain the 4 types of erosion
Attrition - particles collide with each other, becoming smaller and rounder
Abrasion - particles scrape against river bed and banks and wear them away due to the friction
Hydraulic action - force of water directly weakens the rock or gets into cracks.
Solution - removal of chemical ions which dissolves rock
‘Capacity of a stream’ definition
The largest amount of debris a river can carry
‘Competence of a river’ definition
The diameter of the largest particle that can be carried.
Critical erosion velocity
The lowest velocity at which grains of a given size can move
Hjulstrom curve shows the relationship between…..
Debris diameter and velocity
3 main points about Hjulstrom curve
- Smallest and largest particles require high velocities to lift them (smallest is clay but requires big velocity because it is cohesive)
- Higher velocities required for entrainment than for transport
- When velocity falls below a certain level, deposition occurs
Factors affecting regime of a river (variation in the flow)
- Intensity of precipitation
- Porosity and permeability of rocks
- Amount of vegetation cover
Waterfall formation
- There are alternating layers of hard and soft rock
- Soft rock is more easily eroded than hard rock due to hydraulic impact
- This leaves the hard rock undercut
- It eventually collapses into a plunge pool due to lack of support by soft rock
- Abrasion occurs here, causing the waterfall to retreat.
- A gorge is formed
3 situations when deposition likely to occur?
- River floods
- River enters sea
- River enters behind a dam
Formation of a floodplain
They are flat areas found in the lower course, comprising of silt, clay and alluvium. Fertile lands.
Formation of levees
- When a river floods, it loses its velocity and thus energy and has to deposit some of its load
- It deposits the coarsest, heaviest material first
- This forms raised banks called levees at the edge of the river
- Build up over time as more deposition occurs
Factors affecting the formation of a delta?
- Salinity (salt makes particles more cohesive)
- size of load
- vegetation
- gradient of coastline
How is a delta formed?
When a stream flows into a standing body of water it may form a delta
For a delta to be formed, a river needs to carry a large volume of sediment
Deposition is increased if the water is salty as this causes salt particles to clump together (flocculation) and become heavier so they are deposited
Vegetation also increases rate of deposition by slowing down the water (bioconstruction)
Deposition occurs due to a rapid drop in stream velocity
There is a regular succession of deposits
Finest deposits are removed furthest (bottom-set beds) and coarsest material at the top
What does a hydrograph show?
How a river changes over a short period of time.
It shows how a river channel responds to the processes of the hydrological cycle
It measures the speed at which rainfall falling in a drainage basin reaches the river channel
Explain the rising limb, the falling limb, the lag time and the baseflow
Rising limb = indicates the amount of discharge and the speed at which it is increasing
Falling limb = How fast the river levels fall - is steeper than rising limb as overland flow has been carried away and it is mainly throughflow moving into channel
Lag time = The difference in time between when the rainfall peaks and when the river’s discharge peaks at.
Baseflow = movement of groundwater into the river channel
What is the recessional limb influenced by?
- Permeability of rocks
- Local aquifers
- Basin size
- Larger catchments
- Flatter gradients
Factors affecting flood hydrographs
- rock type
- climate
- urbanization
- vegetation
- gradient
- drainage density
- drainage size
- soils
How does a urban hydrograph differ from a rural one?
- shorter lag time
- steeper recessional limb
- steeper rising limb
- higher peak discharge
- discharge may peak earlier
4 urbanizing influences on flooding
- Deforestation: less interception, more saturated soils, less EVP
- Building of houses: more impermeable surfaces, decreased infiltration, more overland flow and surface run off, shorter lag time
- Channelization: straightening of channels may help carry water away quickly but may increase risk downstream
How can flood warnings be improved?
- improved rainfall and snow pack estimates; better and longer forecasts of rainfall
- better gauging of rivers, collection of meteorological information and mapping of channels
- Better and more current information about human populations and infrastructure
- More complete and timely sharing of info needed of meteorological and hydrological within international drainage basins
- Technology should be shared among all agencies involved in flood forecasting
Ways of reducing flood peaks:
- reforestation
- Treatment of slopes such as contour ploughing to decrease them, reducing run off coefficient
- Construction of natural water storage zones e.g: lakes
- vegetation protection
- clearance of sediment
2 examples of Flood Diversion
- levees
- reservoirs
Difference between economic and physical water scarcity
Physical - insufficient amounts of precipitation. Where water consumption exceeds 60% of supply.
Economic is having sufficient amount of water but lacking storage and transport facilities due to a lack of money to invest in projects
Drought definition
An extended period of dry weather leading to extreme dryness
Irrigation definition
The adding of water to areas where there is insufficient amounts of water for crop growth. Water is taken from aquifers, rivers, groundwater
Consequences of irrigation
- Less albedo: sandy surfaces replaced by dark green ones
- water tables lowered
- salinization
- changes in precipitation: storms, hurricanes
Eutrophication
- Fertilizers used in agriculture leach into lakes
- cause algal bloom
- plants grow tall, blocking light reaching smaller plants so smaller plants can’t photosynthesize. They die.
- microorganisms feed on decaying plant material, using oxygen in the water to respire aerobically
- no oxygen left for other species
Human pressures on lakes and aquifers
As population grows, demand for water increases.
As economic growth occurs, people’s lifestyles
may become more reliant upon water as their diets may change.
Also, economic growth leads to industrialization, more pollution and decreased water quality
Ramsar convention
An international treaty used to conserve the wetlands
Wetlands definition
“An area of marsh, fen, peatland or water that is static or flowing, permanent or temporary, natural or artificial”
Advantages of wetlands
- Flood control
- habitats for many species
- water purification
- tourism
- groundwater recharge
- storage of organic matter
- water transport
A wetland provides valuable storage for stormwater runoff reducing flooding as it holds the water up before it gets to streams
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What causes loss of wetlands?
- increased demand for agricultural land
- infrastructure development
- population growth
- river flow regulation
- drought, hurricanes
- invasion of non-native species
Kissimmee river
- Between 1962 and 1971 engineering was done to straighten, widen the K river
- transformed into a 90 km, 10 metre deep drainage canal. The river was channelized to provide flood protection for land adjacent to the river
Channelization had several unintended impacts:
- the loss of 2000 to 14000 hectares of wetlands
- a 90% reduction in wading bird and waterfowl usage
- continuing long term decline in game fish populations
Cost of restoration:
- project cost over $410 million
- Higher losses of water due to evapotranspiration during wet periods
Benefits of restoration:
- Higher water levels should ultimately support a natural ecosystem again
- Reestablishment of floodplain wetlands and associated nutrient filtration function should result in decreased nutrient loads to Lake Okeechobee
- Populations of key bird species such as wading birds and waterfowl have returned to the restored area.
- dissolved oxygen levels have doubled which is critical for the survival of fish and other aquatic species
- Potential revenue associated with increased recreational usage such as hunting and fishing and ecotourism on the restored river could significantly enhance local and regional economies.
Community participation
- retain water in reservoirs
- re-distribute water from wetter areas to drier areas
- water conservation
Water harvesting:
- extraction from rivers and lakes
- trapping behind dams and banks
- pumping from aquifers
GERD dam
- one of the largest in the world built near the Sudan border
- stores 70 bn litres of water in reservoir
- covers 1680km2 of forest area
- river Nile supplies water for over 11 Africa Nations
Costs:
- Egypt mostly disadvantaged - adds to pressures of water scarcity due to proliferation in number of those dependent upon it, primarily stemming from the rapid population growth
Sudan:
- transformation of the riverine environment into artificial lakes changes temp and pH - species can’t adapt as well, extinction possible
Ethiopia:
- costly, and may turn out to be counter-intuitive, contrary to the desirable betterment of relations between Egypt and Ethiopia if Egypt do not see benefits - war which would be a huge expensive for Ethiopia to bear.
Benefits:
- Ethiopia: 65% of them not connected to Grid. Provides electricity due to hydropower being generated
- Online learning
- Jobs during construction
- more income for investing money back into the economy
- source of national pride
Sudan:
- $1 billion revenue of hydropower
- flood control and reduced sedimentation
- lesser likelihood of floods means less people are displaced
- saves them $200 million which they can invest for other purposes
Possible solutions for Egypt:
- Unlike Ethiopia it is not landlocked and has a coastline where it can implement desalination plants using cheap technology.
- Also, treaty that gives Egypt 90% rights over the Nile is now outdated and it is a bigger concern that 11 LEDCs are dependent on the Nile so unjust for Egypt to claim exclusive rights over the Nile.
Aswan Dam
- Egypt
- The number of large dams being built (above 15m) are increasing globally
- Almost reaching a level of 2 completions per day
- The advantages of the dam are numerous:
- flood control and drought control - dams allow good crops in dry years
- Irrigation: 60% of water is used for irrigation
- 3.4 million hectares of desert are irrigated
- Hydroelectric power: 7000 million kilowatt hours each year
- Improved navigation
- Improved tourism
Costs:
- Water losses: less than 50% outputted
- Salinization: crop yields have been reduced on upto a 1/3 of the area irrigated by water from the dam due to salinization
- Groundwater seepage
- Displacement of population: 100,000 Nubian people removed from ancestral homes
- Seismic stress
Define economic water scarcity
Where water is available locally but not accessible for human, institutional or financial capital reasons
Define physical water scarcity
Lack of available water where water resource development is approaching or has exceeded unsustainable levels; it relates availability to demand and implies that arid areas are not necessarily water scarce
Define infiltration capacity
The maximum rate at which rain can be absorbed by a soil in a given condition
Decreases with time during a period of rainfall
Factors affecting infiltration
Antecedent moisture - decreases rate of infiltration
Duration of rainfall - decreases with duration
Vegetation cover - vegetation increases infiltration because it intercepts some rainfall and slows down the speed at which it arrives at the surface.
raindrop size - smaller sizes absorbed through pores in the soil more efficiently
Slope angle - larger gradient, more infiltration
2 ways in which overland flow occurs in
When precipitation exceeds infiltration rate
When soil is saturated (all pore spaces are filled with water)
Define field capacity
The amount of water held in soil after excess water drains away, that is, saturated
Describe the importance of aquifers
They are rocks which contain a significant amount of water and provide a greater reservoir of water
Water in aquifers moves very slowly and acts as a natural regulator in the hydrological cycle by absorbing rainfall that would otherwise reach streams rapidly.
In addition, aquifers maintain stream flow during long, dry periods.
Where water flow reaches the surface, springs may be found
Why does groundwater recharge occur?
As a result of infiltration of part of the total precipitation at the ground surface
Seepage through the banks and bed of surface water bodies
Groundwater leakage and inflow from adjacent rocks and aquifers
Artificial recharge from irrigation
Does a low width:depth ratio mean river is more efficient?
NO, a HIGH w:d ratio are more efficient since less energy is spent in overcoming FRICTION
Thus, the carrying capacity increases and a lower gradient is required to transport the load
Why does water flow?
Gravity
Frictional resistance
Conditions necessary for a turbulent flow
Complex channel shapes and meandering channels
High velocities
Cavitation in which pockets of air explode under high pressure
Conditions for laminar flow
Low velocities
Shallow channels
Smooth, straight channels
Hydraulic radius formula
Cross sectional area / wetted perimeter
The HIGHER the ratio, the more EFFICIENT the stream is and the lower the frictional loss is
What causes friction in rivers
Channel roughness
Irregularities in the river bed, boulders, trees and vegetation
Critical erosion curve?
Entrainment velocity
The critical erosion velocity is the lowest velocity at which grains of a given size can be moved
Causes of deposition
- gradient
- friction
- volume
Shallowing of gradient which decreases velocity and gradient
A decrease in the volume of water in the channel
An increase in the friction between water and channel
What does the rising limb indicate?
The amount of discharge and the speed at which it is increasing
Very steep in a flash flood or in small drainage basins
Peak flow or discharge (hydrographs)
Higher in larger areas
Steep catchments have lower infiltration rates
flat catchments have high infiltration rates so more throughflow and lower peaks
Define lag time
Time interval between peak rainfall and peak discharge
Influenced by basin shape, steepness and stream order
Define run-off curve
Reveals the relationship between overland flow and throughflow
Where infiltration is low, antecedent moisture high, surface impermeable and rainfall strong, overland flow will dominate
Define base flow
The seepage of groundwater into the channel - very important where rocks have high pore space
A slow movement, and the main long-term supplier of river’s discharge
Variation in river’s flow depends on:
Amount and nature of precipitation
Changes in vegetation cover
Impermeable rock
Seasonal variations in temperature
Urban hydrographs have
- Steeper rising limb
- shorter lag time
- higher peak flow
- steeper recessional limb
Explain 3 physical factors that affect flood risk
PRECIPITATION INTENSITY
Highly intensive precipitation is likely to produce overland flow and produce a greater flood
This is because the infiltration capacity of the soil gets exceeded and the soil becomes saturated
PRECIPITATION TYPE:
Precipitation that sits as snow on the ground eventually melts
Sudden, rapid melting can cause flooding and lead to high rates of overland flow and high peak flows
Explain 3 physical factors that affect flood risk
PRECIPITATION INTENSITY
Highly intensive precipitation is likely to produce overland flow and produce a greater flood
This is because the infiltration capacity of the soil gets exceeded and the soil becomes saturated
PRECIPITATION TYPE:
Precipitation that sits as snow on the ground eventually melts
Sudden, rapid melting can cause flooding and lead to high rates of overland flow and high peak flows
TEMPERATURE
High temps - more evap - less water getting into rivers
ANTECEDENT MOISTURE:
If it has been raining previously and the ground is saturated or near saturated, rainfall will quickly produce overland flow and a high peak flood
DRAINAGE BASIN AND SIZE
DRAINAGE DENSITY
SLOPES
VEGETATION TYPE
How does urbanization increase flood risk
Creation of impermeable surfaces - less infiltration, more surface runoff, more overland flow
Smooth surfaces served with a dense network of drains, gutters and underground sewers
Natural river channels are often constricted by bridge supports or riverside facilities, reducing their carrying capacity
Effects of urbanization on hydrological processes
More surface runoff, more water…
- increased erosion due to more water getting into rivers
- Increased speed of flow and transport of materials due to enlarged channels
- Less erosion due to riverbank protection schemes
Effects of deforestation
Less interception, more overland flow
Rivers carrying more sediment
Reduced evapotranspiration
How does channelization reduce flood risk
May create new channels
These are straight
They speed up water movement and so time lags are likely to be reduced
But may cause FLOODING DOWNSTREAM
Explain methods of flood mitigation
Dams:
hold back water during times of flood and release when flood risk is minimal, e.g: Aswan Dam protects lower Nile from flooding
Reservoirs:
- Store excess rainwater in upper course
- but only useful in small drainage networks
- e.g: in Bangladesh, around 66 milion cubic metres is needed to be stored to have an impact on major floods
Afforestation:
- interception, less overland flow, BLAH BLAH BLAH
Channel modification:
- straightening, enlarging
- e.g: Artifical levees divert and restrict water to low value land on the flood plain
- OVer 4500km of the Mississipi have these
Explain 4 mitigation strategies and their problems
Flood embankments with sluice gates
May increase flooding up and downstream
Channel enlargement to accomodate larger discharges
Enlarged channel becomes clogged with weed
Flood relief channel - expensive
Intercepting channels
Divert only a part of the flow away
How to deal with eutrophication?
Alter human activities that produce pollution e.g: using alternative types of fertilizer and detergent
Regulating and reducing pollutants at the point of emission e.g: in sewage treatment plants that remove nitrates and phosphates from waste
Restoring water quality by pumping mud from eutrophic lakes
Possible measures to reduce nitrate loss
Avoid use of nitrogen fertilisers between mid-september and mid-feb when soils are wet and more likely that fertiliser will wash through
Avoid ploughing up grass. This releases nitrogen
Using bales of barley straw to prevent the growth of green algae
It uses nitrogen as it decays and also locks up phosphorous
Effects of irrigation
Leads to changes in water table
In Indus Valley, Pakistan, irrigation has raised the water table by as much as 6 metres since 1922 and caused widespread salinization
Reduces Earth’s albedo by as much as 10%. This is because a reflective surface is replaced by a dark surface, DARK GREEN CROPS
Explain salinization
Increase in amount of salt in soil
Occurs when groundwater tables are close to the surface
Capillary forces bring water to the surface where it may be evaporated, leaving behind any soluble salts that it is carrying
Chemical changes: in Salinas, California, salinisation is characterised by an increase in dissolved salts and an increase in the ratio of chlorides to bicarbonates
Community level responses to water management
According to WaterAid, there are 748 million people without access to safe water, around 1 in 10 people of the world;s pop
Since 1981, waterAid has provided safe water to over 21 million people
WATER SAVING:
- Smaller-capacity baths hold less water
- Taking a shower rather than a bath saves water
- Water-efficient shower heads produce water flows that feel far greater than they actually are
- Taps with a low flow rate can be fitted to the bathroom and kitchen sinks
- Turn off taps when not in use: a dripping tap can waste more than 5500 litres of water a year so taps should be turned off properly
CUSTOMER PRICING:
- Water companies charge customers for the amount of water they use
NEW TECHNOLOGIES:
- Nanotechnology infiltration removes microbes and bacteria and other matter by using silver ions that destroy contaminants. Membrane chemistry enables water to be filtered and purified
- Pores of membranes used in ultrafiltration are 3000 times finer than a human hair
- recent breakthroughs have forced down the cost of desalinated water around the world
from $1 to $0.80 and $0.50 over 5 years
Seawater desalination:
- reverse osmosis consumes a large amount of energy making it very expensive
- Britain’s first ever mainland desalination plant was opened at the Thames Gateway Water Treatment Works in 2010.
- One solution being explored in Singapore is biomimicry, mickicking the biologival processes by which mangrove plants and fish extract seawater using minimal energy
- Smart monitoring technologies - help companies ensure the quality of their water supply networks
- pinpoint leaks can be detected much faster using wirelessly connected instruments to cloud-based monitoring systems
ZONING:
- aim: to allow aquifers to recharge
- Zoning may also maintain water quality and reduce the risk of subsidence due to over-abstraction
- nitrate vulnerable zones were identified and farmers were advised not to apply nitrogen fertilisers to these fields
WATER PURIFICATION:
- removal of impurities
What is rainwater harvesting
Making use of available water before it drains away or evaporates.
Efficient use or storage of water can be achieved in many ways:
- irrigation of individual plants rather than entire fiels
- covering expanses of water with plastic or chemicals to reduce evaporation
- storage of water underground in gravel-filled reservoirs
Evaluative point:
HICs can spend more on nanotechnology and have a large-scale and widespread impact, greater than LEDCs but LEDCs can build CHECK dams which cost only around $250, lasting for 5 years to recharge groundwater
