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River processes, landforms and flooding
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Rivers shape the land as they flow from hills to the sea, carving valleys and waterfalls and building floodplains.
Processes
Erosion: hydraulic action (force of water), abrasion (stones scraping the bed and banks), attrition (stones knocking together and getting smaller and rounder), solution (rock dissolving).
Transportation: traction (rolling boulders), saltation (bouncing pebbles), suspension (fine material carried in the water), solution (dissolved material).
Deposition: dropping material when the river slows down.
Transportation: traction (rolling boulders), saltation (bouncing pebbles), suspension (fine material carried in the water), solution (dissolved material).
Deposition: dropping material when the river slows down.
Landforms
Upper course: steep V-shaped valleys, interlocking spurs and waterfalls. A waterfall forms where soft rock below hard rock erodes faster, making an overhang that collapses. Example: High Force on the River Tees.
Middle and lower course: meanders (bends): fast water erodes the outside bend, slow water deposits on the inside. An ox-bow lake forms when a meander is cut off.
Floodplains and levees build up from deposits when the river floods.
Middle and lower course: meanders (bends): fast water erodes the outside bend, slow water deposits on the inside. An ox-bow lake forms when a meander is cut off.
Floodplains and levees build up from deposits when the river floods.
Floods and hydrographs
A flood hydrograph shows rainfall and river discharge after a storm.
Lag time is the gap between peak rainfall and peak discharge. A short lag time means a flash flood is more likely.
Flood risk rises with heavy rain, steep slopes, impermeable rock, saturated soil and towns.
Hard engineering (dams, flood walls) or soft engineering (planting trees, letting floodplains flood) can reduce risk.
Lag time is the gap between peak rainfall and peak discharge. A short lag time means a flash flood is more likely.
Flood risk rises with heavy rain, steep slopes, impermeable rock, saturated soil and towns.
Hard engineering (dams, flood walls) or soft engineering (planting trees, letting floodplains flood) can reduce risk.
Peak rainfall was at 09:00 and peak discharge at 15:00. What was the lag time, and what does it suggest?
- Lag time = time of peak discharge minus time of peak rainfall.
- 15:00 minus 09:00 = 6 hours.
- A longer lag time means water reaches the river more slowly, so the flood risk is lower than with a short lag time.
Answer: 6 hours
Erosion: hydraulic action, abrasion, attrition, solution. Transport: traction, saltation, suspension, solution. Upper course: V-shaped valleys and waterfalls. Lower course: meanders, ox-bow lakes, floodplains and levees. Lag time is the gap between peak rainfall and peak discharge.
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