Secondly, a section about the interaction between humans and rivers. (Effects of flooding and strategies to prevent flooding etc)
The final column in the specification is scale. R/L means you need to talk on a regional and local scale, R/L/N means you need to talk on a regional, local and national scale.
Specification Guidance Scale The earth's crust is modified by fluvial processes which result in distinctive landforms . Long and cross profiles, processes of erosion (hydraulic power, corrosion, corrasion, attrition), transport (traction, saltation, suspension, solution) and deposition . The characteristics and formation of waterfalls, gorges, meanders, ox-bow lakes, levees, flood plains and deltas. Recognise and describe fluvial features on Ordnance Survey maps and photographs. How a river profile changes downstream and the relative importance of the processes at work such as the typical V shaped valley in the upper course where vertical corrasion dominates . Students will be expected to be able to name an example, describe each feature and to explain its formation making reference to the processes at work. There is an opportunity in this section to revise the OS skills as on page 37 as well as to introduce photographic interpretation etc. R/L R/L R/L
Specification Guidance Scale The interaction between people and fluvial environments. River basin management issues, the causes and effects of flooding in river basins in the context of both LEDCs and MEDCs. The short, medium and long term strategies used to attempt to manage the floods. Contemporary issues concerning use of hard vs soft strategies and attempts to achieve sustainability . The social, economic, environmental and political issues that have an impact upon strategies and the values and attitudes of interested groups. River basin management issues should be studied in the context of both LEDCs eg. Bangladesh and MEDCs eg. the European floods of the 1990s . One case study from the MEDC and LEDC should be chosen to study the physical and human causes of flooding and the impact of the floods. TV news is a good source of up to date information. Flood management should include efforts to prevent flooding and strategies used to ameliorate the impact. Candidates should be aware of how and why the methods used are different between the LEDC and MEDC and now currently there is a move away from hard engineering e.g. dams on the Colorado to softer approaches e.g. flood plain zoning, improved warning system . Opportunities to use ICT to research flood disasters. R/L N/R/L
Traction - Large boulders roll along the river bed.
Saltation - Smaller pebbles bounce along the river bed, picked up by the flow of the river.
Suspension - Fine sand and silt-sized particles are carried along in the flow, giving the river a brown appearance.
Solution - Minerals (eg limestone and chalk) are dissolved in the water and carried along with the flow
A river carries more material if: the volume of water is greater, the velocity of the water is greater, and the local rock types are more easily eroded.
When a river loses energy (due to lower velocity, higher friction with the land etc) it will drop some of its load, causing deposition . The dropped load is called alluvium . The heavier rocks are always dropped first.
On the outside of a meander the water is deeper and the current flows faster . On the inside of the bend the water is slack and the current is less strong .
This means erosion by corrasion etc occurs on the outside of the meander, creating steep river cliffs . On the inside of the meander there is deposition of sand and pebbles creating a gentle slip-off slope .
The lateral erosion on the outside bend widens the valley floor.
As the river flows downstream the meanders become larger and wider, and the erosion and deposition causes them to gradually migrate downstream.
This creates a line of river cliffs along the edge of the valley floor.
Ox-bow lakes form in the lower course of a river because the river meanders more vigourously.
Continued erosion (by corrasion etc) on the outside bend causes the neck of the meander to narrow. Eventually the neck is broken through to form a straight channel. This often happens during floods when the river is very powerful.
Continued deposition causes the old meander to be sealed off.
Gradually the ox-bow lake will dry out, forming a meander scar .
A flood hydrograph shows discharge over a period of time.
During rainfall, very little water directly falls into the river. The rising limb is where surface run-off and throughflow of water through the soil first reach the river, causing discharge to rise.
The falling limb is where discharge starts to fall due to the rainfall having finished, so surface run-off and throughflow are reduced.
The lag time is the time between the peak rainfall and the peak discharge - a river with a short lag time and high peak discharge is more likely to flood than one with a long lag time and low peak discharge.
Flood Hydrographs Page 45
Flood Hydrographs Page 45 This hydrograph shows the rising limb, falling limb and lag time.