Coastal environments
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  • A useful animation looking at the global water balance can be found here: http://geography.uoregon.edu/envchange/clim_animations/flash/four_moist.html
  • Based on diagram produced by H D Mothersole, Wycombe High School
  • Good page on synoptic and radar charts for the 16th at http://www.metoffice.gov.uk/climate/uk/interesting/20040816.html

Transcript

  • 1. Rivers, Floods and Management AQA
  • 2. Contents Hydrological cycle and river discharge River processes Profiles and characteristics Landforms of fluvial erosion and deposition Causes and impacts of flooding Flood management strategies Exam questions
  • 3. The drainage basin Drainage basin or catchment area What is a drainage basin? An area of land (also called the catchment area) drained by a river and its tributaries What is the name for the boundary which divides one drainage basin from another? The watershed mouth
  • 4. The drainage basin as an open system The drainage basin forms part of the hydrological cycle, and can be described as an ‘open’ system involving a series of; Inputs Ways in which water enters the system Outputs Ways in which water leaves the system Can you define these terms? Stores Ways in which water is held in the system Transfers Ways in which water is moved through and within the system
  • 5. Elements of the system Name each store Name each transfer Name the Input Name each Output Transpiration Precipitation Throughfall + stemflow Interception by vegetation Infiltration Throughflow Soil water storage The water table (position varies) Groundwater storage Surface (overland) runoff Evaporation Percolation Groundwater (base) flow Saturated or impermeable rock Surface storage lakes, rivers, sea, depression storage
  • 6. The water balance This is the balance between inputs and outputs in the drainage basin. It is expressed as: P + Q +E (+/-), where; P = precipitation Q = run-off E = evapotranspiration (+/-) = change in storage The graph shows the main features of the water balance – average monthly precipitation and potential evapotranspiration levels
  • 7. Understanding the water balance graph In which month(s) is there a water surplus, and why is this? Jan, Feb, March, April, Oct, Nov, Dec, when precipitation exceeds potential evapotranspiration What is soil recharge and when does this occur? The replacement of soil moisture lost during drier periods; during autumn in the UK (Oct and Nov) Field capacity is attained after soil recharge. What exactly is field capacity? It’s the maximum amount of water soil can hold
  • 8. Understanding the water balance graph Why is a water deficit not shown on this graph? In the UK, precipitation (nearly) always exceeds evapotranspiration How might water balance graphs be a) useful?, and b) difficult to construct? They can be useful in predicting river regimes and managing drainage basins. It is however difficult to accurately measure evapotranspiration
  • 9. Discharge and the storm hydrograph • A hydrograph shows changes in a river’s discharge over time • A storm hydrograph shows this information following a rainfall event Peak ? discharge Discharge (cumecs) Rising ? limb Lag time ? rainfall Falling ? limb Storm ? flow Base ? flow • Name the main features of the hydrograph Time (hours)
  • 10. Factors affecting discharge Porosity of soils underlying basin Geology:per meability of underlying rock Drainage density Drainage basin relief Land use What factors affect river discharge? Precipitation: intensity and duration. Antecedent rainfall. Size and shape of drainage basin Vegetation: type and seasonal variations in cover. Deforestation
  • 11. How do these factors affect the storm hydrograph? Factor Effects on Hydrograph High intensity, long duration of rainfall, or antecedent rainfall Steep rising limb as infiltration capacity of soil exceeded Snow melt Greatly increased discharge, especially if ground frozen Porous soils and / or permeable rock Less steep or ‘flashy’ hydrographs Impermeable rock / frozen ground Reduced lag time and steeper rising limb Small drainage basin Faster response, so shorter lag time and steeper rising limb Elongated drainage basin shape Slower passage to river, so longer lag time Steep slopes within drainage basin Faster passage to river, so shorter lag time and steeper rising limb Summer vegetation Interception higher - slower response, peak discharge lower Deforestation Faster response and higher peak discharge
  • 12. Urbanisation and the storm hydrograph Steeper rising limb due to impermeable surfaces Higher peak flow as less water is ‘stored’; more water reaches the river discharge (cumecs) and rainfall (mm) Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc Rural time Urban
  • 13. River processes FLUVIAL EROSION The break-up of rocks by the action of the river TRANSPORTATION The movement of eroded material DEPOSITION The laying down of material which has been transported by the river Abrasion / corrasion Attrition What are the main processes of fluvial erosion? Corrosion / solution Hydrauli c action
  • 14. Definitions Can you define the following terms? What effect will each have on the river? EROSIONAL PROCESS DEFINITION EFFECT ON RIVER Corrasion / Abrasion Scraping, scouring and rubbing action of the load carried by the river Attrition Process where pebbles knock together Sediment load size and get smaller and more rounded decreases Hydraulic action Removal of loose Frictional drag and pressure created by material - river moving water as it is forced into cracks banks undercut and collapse Corrosion River banks wear away Where the naturally weak acidic water An increase in the dissolve some carboniferous rocks dissolved load such as limestone and chalk (solution)
  • 15. River transportation A river transports it’s load in four main ways. Can you describe them? Suspension: Very small particles of sand and silt (0.001 – 0.99 mm diam) carried in the flow Traction: Large stones and boulders (> 100 mm diam) rolled along river bed Saltation: Small stones (1.0 – 99.99 mm diam) bounced along river bed Solution: Dissolved minerals within water
  • 16. Deposition In flood conditions, when a river overflows onto its floodplain When river discharge is reduced due to a period of dry weather Under what conditions will a river deposit its load? There is shallow water, e.g. on the inside of a meander bend When there is a decrease in the gradient and / or velocity of the river e.g. at river mouth, or when entering a lake When there is an increase in the size of the sediment load caused by a landslide or tributary delivering larger particles
  • 17. The Hjulström curve The Hjulström curve shows the relationship between river velocity and the size of particles which can be eroded, transported and deposited. F. Hjulström collated data from 30 experimental studies into the competence of different flow velocities (competence is the maximum particle size which can be transported at specific velocities) 2 3 1 4 1. Silt/sand are picked up (entrained) at the lowest velocities. 2. Clays are as difficult to pick up as pebbles although they are small particles they are very cohesive and the claybed is very smooth. 3. Large boulders are dropped very easily. 4. Clay particles can be transported in suspension at very low velocities.
  • 18. The long profile • The long profile illustrates changes in altitude from the source of a river along its entire length to the mouth • The idealised model is smoothly concave, with a steeper gradient in the upper course becoming progressively gentler towards the mouth How would the processes of erosion, transportation and deposition change from A, through B, and to C? A Height (m) B C 60 Distance from sea (km) 0
  • 19. The long profile – changing processes Erosion A (Upper course) B (Middle course) C (Lower course) Hydraulic, attrition. Vertical erosion dominant Transportation Deposition Mostly large boulders. Some in suspension + solution Large bed-load only Mostly attrition, some hydraulic. Vertical erosion decreases + lateral begins Saltation and traction of smaller bed-load. Suspension increased. Some in solution Coarser material builds up in braiding, slip-off slopes and floodplain Some lateral erosion on outside of meanders Smaller sized bedload of sand and gravel, transported in suspension Mostly fine material deposited on levees, floodplain and slip-off slopes
  • 20. Bradshaw’s model Upstream Downstream discharge occupied channel width water depth water velocity load quality load particle size channel bed roughness gradient Bradshaw’s model illustrates the main changes expected down the long profile of a river
  • 21. The changing cross-profile A Height (m) B C 60 Distance from sea (km) A B Upper course Middle course 0 C Lower course
  • 22. Landforms of fluvial erosion What are the features of fluvial erosion? Erosional Features Upper Middle and course lower course Waterfalls Potholes & rapids River Meanders terraces Interlocking Ox-bow spurs lakes
  • 23. V-shaped valleys and interlocking spurs Predominantly vertical erosion results in a v-shaped valley. In areas of resistant rock (e.g. on Exmoor), meandering streams and rivers will be incised into the landscape, forming interlocking spurs. Interlocking spurs Rapids are formed by variations in rock resistance
  • 24. Potholes Potholes provide evidence of fluvial erosion What is the most dominant type of erosion here? Abrasion Where there are depressions / fissures, fine particles and larger boulders (‘grinders’) may become trapped and swirled around by the current Bourke’s Luck Potholes, South Africa In resistant rock, potholes require hundreds to thousands of years to form
  • 25. Waterfalls Where do waterfalls form? Usually where there is varying resistance in the types of rock or where there is a fault running across a river. Which processes operate? Soft rock Hard rock River Usually there is considerable hydraulic action due to the falling water. Abrasion is also likely to occur to create the plunge pool at the base of the waterfall Soft rock Broken pieces of hard rock Which feature is formed as the waterfall retreats upstream? A gorge or canyon
  • 26. High Force Waterfall, upper Teesdale, Yorkshire • Tallest waterfall in England at 22m high • 500m gorge downstream How was it formed? Resistant band of igneous rock (Whin Sill) overlying softer sandstone, limestone, shale and coal seam Water erodes softer rock more quickly, creating an overhang and plunge pool This eventually collapses and the waterfall retreats upstream
  • 27. Rejuvenation Significant breaks in slope (Knick points) along a river’s long profile may be due to rejuvenation. What is rejuvenation? It occurs when there is a fall in sea level (relative to land) or the land surface rises. Vertical erosion increases and, starting from the sea, the river adjusts to the new base level. The knick point (where the old profile joins the new) thus moves upstream knick point original long profile new long profile What feature will be found here? A waterfall original sea level relative fall in sea level (or rise of land surface) new sea level
  • 28. Rejuvenation Rejuvenation may also be caused by the sea eroding through and creating a breach in the coastal geology, e.g. River Lyn on the north Devon coast. Knick point waterfalls Original course of River Lyn Breach in coastal geology ‘Valley of the rocks’ Present – day village of Lynmouth
  • 29. Meanders • Sinuous bends in the river • Surface flow to outer bank and subsurface return to inner bank = helicoidal flow • Velocity across the meander varies and is related to depth • The fastest flowing water (the ‘thalweg’) is near the outside of the bend, where the water is deepest • Here, erosion occurs creating a river cliff • On the shallower inside slower moving water allows deposition to occur and a slip-off-slope or point bar forms 0.2 m/sec 0.1 m/sec Straight sections contain riffles or bars in the middle of the channel, where a ridge of bed load has been deposited in the middle of the river’s bed because the water velocity is slower here. Outside of meander Inside of meander
  • 30. Ingrown and entrenched meanders Renewed energy from rejuvenation results in increased vertical erosion and incised (deepened) meanders When incision is less rapid and lateral erosion is occurring, meanders become ingrown, e.g. the River Wye at Tintern. When incision is rapid and vertical erosion dominates, Steep river an entrenched meander is Inside of cliff formed, with steep sides meander slopes and a gorge-like gently appearance What are the differences in form between entrenched and ingrown meanders? FLOW Entrenched meanders have a symmetrical cross-profile (rapid uplift) whereas ingrown meanders are more asymmetrical (slower uplift)
  • 31. Meanders What are these features, and what do they represent? Ox-bow lakes. These show the former course of the river Can you remember how to describe the formation of ox-bow lakes? Erosion (E) and deposition (D) around a meander (a bend in a river) More erosion during flood conditions. The meander becomes bigger The river breaks through during a flood. More deposition causes the old meander to become an oxbow lake
  • 32. Floodplains and terraces River terraces result from which process? Are floodplains features of erosion or Both deposition? Rejuvenation forms river terraces How are they formed? • Lateral erosion What are they? Remnant of former floodplain Floodplain Terrace • Meander migration • Valley widened Bedrock How are they formed? • Deposition of sand and silt during floods • Following rejuvenation, river sinks into former valley • Old floodplain left at higher level • Several stages of rejuvenation can create several terraces, e.g. lower course of the River Thames
  • 33. Fluvial deposition – braided rivers What is braiding and under what conditions does it occur? It describes a section where the river has been forced to split into several channels separated by islands. It occurs in rivers supplied by large amounts of sediment load and / or rivers with variable / rapidly fluctuating discharges The river becomes very wide in relation to its depth The area where the sand or gravel is deposited is known as a point bar (also knows as an eyot) They are unstable and mobile features. When discharge and velocity increase they are easily eroded, and their position changes
  • 34. Levees Describe the process of levee formation shown in diagrams (a) – (d) (a) During times of high discharge, the river floods. The competence of the river decreases as velocity is reduced when the river breaks the banks. Heavier, coarse material is deposited first (b) Small banks of deposited material build up (c) Subsequent floods result in further deposition on these banks and the bed of the river (d) Raised banks, called levees are created and the river flows at a higher level than the floodplain
  • 35. Deltas • Feature of deposition at river mouth • Velocity (and therefore competence) decreases on entering a lake or the sea • They occur where river sediment load is very large, and where there are weak currents and / or small tidal range. The rate of deposition thus exceeds the rate of removal. • Flocculation occurs as sea water and fresh water mix • Clay coagulates and settles on the river bed • Sorting of deposits occurs into topset beds (larger heavier material), foreset beds (medium graded particles) and bottomset beds (finest particles) Curving shoreline and dendritic pattern of drainage, e.g. Nile Fingers of deposition build out into sea along distributaries, e.g Mississippi Pointed ‘toothlike’ delta; material spread evenly on either side of channel, e.g. Tiber
  • 36. Causes of river flooding Flooding occurs when a river exceeds its bankfull discharge Human factors Excessive, prolonged rainfall Physical factors Short, intense rainfall event Saturated soil Deforestation Local relative rise in sea level / storm surge What physical and human factors contribute to flooding? Urbanisation High drainage density Snow melt Impermeable rock Frozen soil River management Steep gradient
  • 37. High risk areas; Shrewsbury, UK Shrewsbury lies inside an incised meander loop of the River Severn. The town centre lies above the floodplain but some of the main transport routes lie vulnerably low, and recent development has taken place on low lying floodplain
  • 38. Boscastle, Cornwall Study the photograph which is looking upstream from Boscastle harbour towards the village of Boscastle. Suggest the ways in which the physical geography of the area may increase the speed of onset and severity of flooding
  • 39. Impact of flash flooding in Boscastle, - August 2004 • • • • • • • Intense low pressure weather system caused localised heavy thundery downpours 200mm rain fell in 24 hours (most between midday and 5pm on the 16th) on high ground to the east. Already saturated catchment – rapid runoff Boscastle lies in a deep valley just downstream of the confluence of the rivers Valency and Jordan 2m rise in river levels in one hour Debris caught under narrow bridge caused 3m high wave of water which burst down main street when bridge collapsed 70-80 cars swept away, significant structural damage, 100 people air lifted to safety but no loss of life
  • 40. Southern Britain, July 2007 Causes; • Abnormal track of jet stream • Rainfall totals for May-July highest since 1776 • Infiltration / percolation capacity minimal • Exceptional rainfall on 20th July – event only expected once in several hundred years Consequences; Normal Jet stream June – July 2007 • Flash floods across southern England; especially lower Severn and upper Thames catchments • Drainage systems overwhelmed and transport networks severely disrupted - £25 million damage to Gloucestershire’s road system – the year’s budget! • 45, 000 households lost power; 350,000 lost running water – £1billion cost to water industry • £3 billion damage covered by insurance. Equivalent amount uninsured loss • 50% crops lost in affected areas – shortages and price increases • 3 people died
  • 41. High risk areas - Bangladesh What are the physical causes of flooding? Himalaya ? Mountains; (monsoonal) rainfall and snow melt Major rivers converge ? What are the human influences? • Deforestation • Agricultural practices 80% of country ? occupies low-lying delta < 1m above sea level Storm surges, especially during cyclones / ? hurricanes. Also local sea level rises of 7mm/year • Densely populated • Urbanisation – Dhaka population over 1 million • Embankments built (road and river) – have prevented back-flow of flood water and increase siltation in drainage channels • Low GDP and lack of investment
  • 42. Flooding in Bangladesh, 2004 Exceptionally high rainfall totals in the monsoon of 2004 led to widespread flooding in July and August 36 million people made homeless (nearly 29% of total population) Spread of disease Flood waters mixed with raw sewage caused diarrhoea outbreak 38% land area flooded – worst floods for 6 years Consequences of flooding 800,000 ha agricultural land flooded – small scale farmers severely affected 800 dead by mid-September Capital city, Dhaka Flooded. Infrastructure severely Damaged – damage to roads, bridges, school and hospitals estimated at $7 billion $2.2 billion estimated cost of damage (4% of GDP for 2004)
  • 43. Flood management Hard engineering strategies involve the building of structures or alteration of the course / structure of the river The aim is to reduce the frequency and magnitude of flood events, and therefore reduce the damage that floods cause 1. Can you describe the measures shown in the diagram opposite? 2. What might be the advantages and disadvantages of hard engineering methods?
  • 44. Lynmouth – flood controls After the disastrous Lynmouth floods of 1952, the river was managed. What management can you see in the photograph? Cottages now set back from river bank Entry of East Lyn tributary is under a wide bridge to prevent damming Channel has been straightened Building of concrete revetment to increase speed of river flow in time of flood
  • 45. Hard engineering What are the arguments for and against hard engineering? FOR AGAINST Reduction in flooding and therefore Can lead to destruction of habitats protects property along river bank Takes water away from towns more quickly Increase in water supply e.g. on the Nile Improved navigation e.g. Mississippi Allows energy to be created e.g. hydroelectric power on the Colorado Can be visually intrusive It can dramatically increase peak discharge, duration and timing of floods downstream Where meanders have been straightened, the river will try to reestablish itself Straightening courses can lead to greater upstream erosion and downstream deposition
  • 46. Impact of the Three Gorges dam, China • • • Largest ever river hard engineering project Dam completed in 2006; 2.3 km long and over 100m high, with reservoir (660km by 1km) behind Project due to complete in 2009, at a cost of an estimated £25 billion Defence Criticism Produce HEP for 13 million people Cost Reduce the flood risk for 15 million people 1.2 million people relocated to newly built settlements following reservoir creation Improve navigation for faster, cheaper and safer shipping Cultural monuments lost due to creation of reservoir Boost local economic growth Downstream sediment supply decreased by 50% - increased erosion Improved water supply Disruption to habitats and environment
  • 47. The Three Gorges Dam
  • 48. Soft engineering What is soft engineering? Abatement strategies which aim to work with natural processes, and be more sustainable solutions to flooding • Afforestation • Contour ploughing and strip farming to reduce runoff • Floodplain zoning to allow (economically less valuable) areas to flood naturally • Conservation and restoration schemes; returning rivers to their original state and protecting, e.g. bales to improve water quality • Forecasting and early warning, e.g. Environment Agency flood watch and risk maps. Some small-scale community projects in Bangladesh have resulted in early warning systems and lives are being saved
  • 49. River restoration The River Cole near Swindon underwent a restoration project between 1994 and 1996. The aims were to change the water course back to a more natural state, improve water quality and manage bank side vegetation and habitats. The main strategies are shown below
  • 50. Prediction All flood protection methods are designed to cope with certain magnitudes and frequency of floods Hydrologists try to predict the likelihood of future flooding by examining historical discharge and flood records What is flood recurrence? The average number of years between floods of a certain size is the recurrence interval or return period Study the data shown to understand the recurrence intervals Feet above SL Magnitude Recurrence Interval (years) 796.8 1 57.00 792.3 2 28.50 791.4 3 19.00 791 4 14.25 789.9 5 11.40 789.8 6 9.50 789.7 7 8.14 789.6 8 7.13 789.5 9 6.33 789.4 10 5.70 785.1 20 2.85 782.8 30 1.90 779.5 40 1.43 774.1 50 1.14
  • 51. The Environment Agency What does The Environment Agency do? • Monitoring of water levels and flows • Building and maintenance of flood defences on coast and rivers • Use of radar rainfall data combined with discharge data to predict flood risk and issue flood warnings through; media, automated messages to risk groups, flood-line for the public • Implements and coordinates incident plans • Produce flood risk maps • Advises planners and developers on flood risk • Develops information, e.g. on the website about flood risk and what to do in the event of, and aftermath of a flood
  • 52. Exam question 1 How can planners help to restrict flood damage? (6) Typical answer Good use of Where? needs to be upstream appropriate term;They can stop the water getting Suspect choice of could just have to the settlement, by building a example; prior to said “water” reservoir and controlling the Aswan being built the farmers relied on Naïve; there are discharge. This was done when floods to provide very few ‘new’ the Aswan Dam was built on the fertile alluvium. How settlements River Nile. They can also make much damage was whose location sure that towns and cities are done? can be determined by not built too near a river which is Over-simplistic likely to flood.They can also planners. make sure that there are enough answer, with Impossible for some poorly sandbags to stop the floods every riverside chosen examples settlement have a affecting the nearby buildings. (2/6) stock - again, rather naive
  • 53. Exam question 1 How can planners help to restrict flood damage? (6) Specimen answer Useful additional detail Again, complex sentence allows point to be developed fully Two examples combined in the one sentence Planners can restrict the amount of building on the floodplain of river, or of buildings near to the river (as at Lynmouth). They can alter the channel of the river the river to straighten and speed up its flow. They can allow water meadows to flood, storing water, which can also be achieved by upstream retention e.g. reservoirs. Always a good idea to add a named example Although only three sentences, each is full, with examples. Would score as 3 x 2.
  • 54. Exam question 2 Examine the physical factors responsible for flooding. (25) Points to consider 1. Introduction could attempt to classify the causes, e.g. climatic, geomorphological, to indicate range of factors, and structure of essay. 2. Range of detailed examples needed, from MEDCs and LEDCs 3. Essay should demonstrate that many floods are multi-causal. 4. Maps and diagrams integrated into text discussion 5. Answer should be well structured and well written, with appropriate use of specific vocabulary. Conclusion could consider changing physical factors likely to make flooding more likely, e.g. global warming.