Rivers, Floods And Management Flashcards
Watershed
Boundary around drainage basin
Hydrological cycle
Closed system
Inputs
Where water enters the drainage basin
Precipitation
Processes
The ways in which water travels
Stemflow, infiltration, groundwater flow, channel flow, throughflow, percolation, interception
Outputs
How water leaves the basin
Runoff, evaporation, transpiration,
Different stores
Soil storage, vegetation storage, channel storage, surface storage, groundwater storage
Inputs and outputs in winter and autumn
Less than summer due to less sunlight, less heat, less output
Ground water stores recharge in winter due to more rain and less output occurs therefore, groundwater stores fill up.
Spring and Summer inputs and outputs
Floods in March as groundwater stores full after winter. Soil moisture surplus, for plants and runoff into streams.
Rivers run dry in August and farmers may need to irrigate crops. Soil moisture deficiency due to hotter weather.
Factors that influence water flows within a drainage basin
Intense rainfall Shape of land Vegetation and land use Conditions in basin Soil type and depth Size and shape of river basin Bedrock
River discharge
Amount of water in a river passing a given point at any time
Storm hydrograph
Graph showing how river discharge changes as a result of heavy rain
Peak rainfall
Time of highest rainfall
Peak discharge
Time river reaches highest flow
Lag time
Time delay between peak rainfall amount and peak discharge
Rising limb
River rising on a graph
Falling limb
River levels falling on a graph
Bankful discharge
The maximum discharge of a river channel is capable of carrying before flooding
Regime
A river patterns flow
Discharge formula
Velocity x cross-sectional area
Q= V x A
Measured in m3/s
Features of a drainage basin that would produce hydrographs with a high peak discharge
Small lag time Steep valleys High river discharge Intense rainfall Bare ground Small bankful discharge
The long profile from source to mouth
Steep gradient, high elevation, vertical erosion
Lateral erosion, valley widens, lower elevation
Alluvium deposition, low gradient, little elevation, highest discharge and velocity here
Erosion processes
Hydraulic action
Attrition
Abrasion
Solution
Where material comes from that forms a rivers load?
Endogenic
Exogenic
Transport processes
Traction
Saltation
Suspension
Solution
The Hjulström curve
1 Fine particles suspended in near still water
2 Velocity needed to transport bigger particles is slightly less than needed to pick up particles
3 larger sand particles less erosion velocity than smaller bits as they’re stuck together and hard to pick up
4 smallest bits similar erosion velocity as boulders
5 large gap between erosion and fall velocity shows small bits need less energy to be transported
6 large particles carried for short time - small gap between ev and fv
Capacity of a river
Largest amount of material that can be transported
Competence
Size of largest particle that can be transported
When deposition occurs
Sudden reduction in gradient
Mountain streams where there is large boulders
Sudden inc in load as tributary brings more load
As river floods on floodplain
River entering sea
Reduction in velocity
Shallow water within channel
Factors that affect the amount, type and character of a river’s load
Size of drainage basin - wide gradient, smaller particles
Relief - low, more suspended load
Human activity - dams(more dissolved), deforestation(more load)
Precipitation - intense(more carbonic acid)
Underlying geology - resistant(less water, less load)
Erosion, transportation and deposition in the upper course
E- vertical, abrasion
T- boulders move in high discharge, traction
D- coarse material stays
Erosion, transportation and deposition in the middle course
E- mainly hydraulic, vertical/lateral
T- smaller size, traction or saltation
D- inside pf meanders, floodplain when floods
Erosion, transportation and deposition in the lower course
E- reduced but lateral, some hydraulic, outside of bends
T- sand by traction, fine in suspension
D- fine silts and clays on floodplains, coarser dropped first, levees
If base level changes…
Irregularities in long profile:
Waterfalls, rapids
Lengths where gradient reduced locally, lakes
Source to mouth
Increase: velocities, discharge, load amount, efficiency
Decrease: roughness, friction, turbulence, load size
Potential and kinetic energy
Source: high potential, little discharge so low velocity, very high above sea level
Mouth: high kinetic, down long profile river is bigger and moving quicker.
KE= 1/2mv2
Landforms of fluvial erosion and deposition
Potholes Rapids Waterfalls Meanders Oxbow lakes Floodplains Levees Braiding Deltas - bird foot and arcuate
Landforms of rejuvenation
River terraces - paired, unpaired
Incised meanders - entrenched, ingrown
Knickpoints
Drainage basin
Area of land drained by a river
Open system as have inputs and outputs
Potholes formation key terms
Hydraulic drilling - pebbles
Cylindrical depressions
In rocky beds with high velocity and turbulent flow
Pebbles rotate in hollow, deepening
Rapids formation key terms
Changes in geology
Water turbulent and erosive power increases
Soft rock erodes faster
Speeds up flow of water and increases turbulence
Waterfalls formation key terms
River Tees
Hard rock on top of soft rock - whinstone, limestone Hydraulic action, abrasion Soft rock erodes faster, steep gradient Plunge pool from power, splashback Overhang collapses Plunge pool deepens
Meanders formation key terms
River Till
River at base flow, thalweg zigzags between bars of sediment on opposing banks.
Pools and riffles form
Thalweg on banks undercutting, slower flow deposition, more bend
River not straight, helicoidal flow
River cliffs exaggerate bend from undercutting
More deposition, point bars
Helicoidal flow definition
When the river is no longer straight the thalweg downstream swings faster like a corkscrew as well as moving vertically.
Meander Migration
Occurs laterally downstream
Move as thalweg doesn’t match shape of bend
Zone of greatest erosion is midpoint downstream in meander bend
River bluffs
Outer part of bend reaches valley sides and erodes them
Eventually meanders migrate downstream and slowly widen the valley floor to form flat floodplain.
Ox-bow lakes formation key terms
Mississippi
Hydraulic action and abrasion Material deposited on inside as point bar deposit Meander shifts position, narrower neck Flood breaches neck, new straight path Meander scar
Flood plains formation key terms
River Till
Discharge increases, wider channels needed
Meanders migrate, widen valley floor
Floods, alluvium deposited
Floodwaters shallow and wide, extensive wetted perimeter, more frictional contact, lower velocities, deposition
Layers of sediment, inc fertility and height of land
Levees formation key terms
Competence reduced
Deposits heaviest load on sides
Finer sediments further away
Successive floods, builds up
Braiding formation key terms
Yellow River in China
Discharge variable - melt water in day, seasonal
When discharge falls, river spreads laterally, deposition
Sediment builds up, eyots
Can become colonised, vegetation
Often eroded as river changes shape
Deltas formation key terms
Velocity, competence, capacity falls - deposition
Hydraulic radius reduced, inc wetted perimeter as river spread laterally
Flocculation
Sediment can build up, split channel
Repeats, distributaries
3 types of sediment in deltas
Topset beds: made up of larger bedload
Foreset beds: middle-sized load, deposited further, form steep like wedges
Bottomset beds: finest sediments, travel furthest before low velocity and flocculation occurs
Bird foot deltas
Dominated by an extruding finger like branch of deposition. Fewer distributaries and fine sediments. Formed due to weaker ocean processes and high discharge.
Arcuate deltas
Nile
Gentle sloping shoreline, distinct pattern of branching distributaries, dominated by coarser material.
River terraces formation key terms
Formed when rejuvenated river cuts down through a floodplain, leaving old floodplain above present level of river.
Terraces cut back as new valley is widened by lateral erosion.
Paired terraces
On same level of each side of channel and so indicate rapid down cutting.
Unpaired terraces
Occur when fall in base level is slower. Terraces present on different sides of channel at different levels due to lateral erosion - through meander migration.
Incised meanders
If sea level continues to fall for an extended period of time, knickpoint extends upstream beyond the middle course.
Entrenched
Ingrown
Entrenched meanders
Caused by rapid vertical erosion or when valley sides are more resistant to erosion, creating a winding gorge.
Can happen when uplift is more rapid and results on a symmetrically shaped cross-profile.
River Wear, Durham
Ingrown meanders
Occur when vertical erosion is slower, over a longer period of time. Meander has time to erode laterally as well as vertically to form an asymmetrical river valley with steep cliffs on the outside bend.
River Wye, Tinton
Knickpoints
Sudden change in river’s gradient and mark the point at which the river is cutting down due to rejuvenation.
Form river terraces.
Begins at sea level and retreats up long profile. Knickpoint is where old profile joins new. Usually marked by waterfalls, rapids.
Dynamic equilibrium of a river valley
All factors in balance (kinetic energy for water transport with no excess for erosion or deposition).
If volume and load change both long profile and channel morphology will also change.
The Impact of rejuvenation on a river valley
If sea level falls relative to level of land rejuvenation will occur. New base level - potential energy inc.
Isostatic change
When land rises relative to sea (result of crustal movements).
Eustatic change
When sea level falls/rises (usually due to ice caps melting/growing).
Physical causes of flooding
Excessive rain over long time Intensive rain over short time Snow melt Climatic hazards (hurricanes) Nature of drainage basin Relief
Influence of human activity on flooding
Urbanisation
Deforestation
River management
Global warming
Magnitude frequency analysis
Used to predict probability of a flood of a particular magnitude happening. Based upon reccurance intervals and highest peak discharge recorded every year.
Soft engineering characteristics
Cheaper
Sustainable
Don’t affect environment
Hard engineering characteristics
Expensive
Unsustainable
Changes/damages environment
Soft engineering techniques
Do nothing
Warn people
Afforestation
Land use zoning
Hard engineering techniques
Build dams/reservoirs
Levees
Dredging
Straightening rivers
Management advantages and disadvantages of:
Do nothing
- discourage new floodplain dev
- flooding continues
- costly to recover damage
- doesn’t prevent flooding
+ cheap
+ river naturally floods
+ fertile silt/water supply for farmers
Management advantages and disadvantages of:
Land use zoning
- changes/damages environment
- difficult to move settlements and communications
+ cheap
+ promotes suitable land use
+ quick to implement
+ effective for a long time
Management advantages and disadvantages of:
Build dams/reservoirs
- high construction
- long time to build
- damages environment
- ugly
- if breaks disaster
+ reduces flooding and costs
+ promotes multi-purpose use
+ built in upper basin
Management advantages and disadvantages of:
Warning people
- useless if ignored
- doesn’t solve problem
+ allows evacuation
+ less flood damage
+ requires good communication network
Management advantages and disadvantages of:
Levees
- expensive
- serious flooding if breached
+ reduces flooding
+ increase channel capacity
Management advantages and disadvantages of:
Dredging
- affects local eco-systems
- costly
- temporary
+ inc channel capacity
+ reduces flooding
Management advantages and disadvantages of:
Afforestation
- trees may not be native - affect ecosystem
+ low cost
+ improve environment
+ wood can be used if sustainably done
Management advantages and disadvantages of:
Straightening rivers
- may inc flooding downstream
+ reduces flooding
+ remove excess quicker
+ faster transfer of water
Flood management scheme
Three Gorges Dam, China
Benefits
Socio-economic: afford protection from 1in100year flood, save lives and livelihoods
Economic: hydropower provide 10% China’s needs, create jobs, enable 5000 vessels to reach Chongqing at all times of year, water supply for towns
Environmental: reduced air pollution as hydropower replaces thermal power, reduced siltation in lakes
Flood management scheme
Three Gorges Dam, China
Costs
Socio-economic: some settlements completely submerged, some people have to be moved, some to higher altitudes-poorer soils-poorer quality of life, loss of social tradition(live in same place as born)
Economic: loss of businesses as submerged, $24bn, decreased tourism as 1200 sites of cultural heritage drowned
Environmental: increased pollution(sewage, drowned factories toxicate water), siltation if reservoir, ecosystems affected, large dams can cause earthquakes
York 2000 Flood
Causes
Physical causes: catchment area 3000km2, relief steep high runoff, vegetation heather moorland low interception, climate heavy rainfall
Human causes: farming-cattle eat grass low interception high runoff, urbanisation-impermeable surfaces-drains-water to river ouse quick
Specific to flood: wettest autumn on record, series pf deep depressions from atlantic, global warming wetter winters
York 200 flood
River management strategies
Effects
Clifton ings: natural floodplain: store 2.3m3: 1982 embankments raised £1.25m
Almery terrace: concrete floodwalls with rubber sealed gates protect houses
Effects: >300 homes flooded, residents evacuated, businesses flooded-Kings Head pub loss of revenue, high costs to repair, city centre car parks flooded, rail track maintenance paused due to flooding
Bangladesh Flooding 2004
Climatic processes
Summer:land warmer:low pressure:monsoon winds bring moist air from sea
Winter:land cold:high pressure:dry winds blow out from land to sea
Random variations of intensity, flood
One of wettest climates on record, 1525 mm rain/year hills:5080mm
Brahmaputra river swollen by rain, himalayan burst dam, flooded B dozens killed, millions to seek refuge.
4 days underwater, ave rain 300mm per day, himalayan snow melt every year
Bangladesh Flooding 200
Terrestrial processes
Relief rainfall in himalayan mtn. feeds rivers.
Most of B next to deltas of many rivers flowing from himalaya
1/3 country floods yearly
Approx 80% fertile land (fplain) prone to severe flooding which irrigates crops and adds fresh silt to padi fields keeping fertility.
Most of B is under 10m above sea level.
Bangladesh Flooding 2004
Human activity
Global warming
Extraction of groundwater for irrigation has lowered water table.
Use of water upstream for irrigation and reservoirs has reduced amount of silt deposited.
More urbanisation, higher peak discharge and shorter lag time.
Pop growth pressure on food so more land farmed so less interception-landslides/deposition-dec channel capacity.
