Physical Landscapes in the UK - Rivers

Meanders

Waterfalls

Ox-bow Lakes

River Profiles and Courses

Floodplains, Levees & Estuaries

Erosion, Transportation, Deposition

Rivers develop large bends called meanders in their middle and lower courses, in areas where the channel has both shallow and deep sections:


  • The current is faster on the outside of the bend because the river channel is deeper (there's less friction to slow the water down)


  • So more erosion takes place on the outside of the bend, forming river cliffs


  • The current is slower on the inside of the bend because the river channel is shallower (there's more friction)


  • So eroded material is deposited on the inside of the bend, forming slip-off slopes


Meanders get larger over time - they can eventually turn into an ox-bow lake:


  • Erosion causes the outside bend to get closer until there's only a small bit of land between the bends (called the neck)


  • The river breaks through this land, usually during a flood and the river flows along the shortest course


  • Deposition eventually cuts off the meander forming an ox-bow lake, e.g. the River Calder has formed several ox-bow lakes near Castleford, West Yorkshire

Four Processes of Erosion

Hydraulic action - The force of the river water colliding with the rocks breaks rock particles away from the river channel

Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock


  • The softer rock is eroded more (by hydraulic action and abrasion) than the harder rock creating a step
  • As water flows over the soft rock it is eroded more and more creating a steep drop
  • The hard rock is eventually undercut, is left unsupported and collapses
  • The collapsed rock is swirled around the foot of the fall eroding it (abrasion) creating a deep plunge pool
  • Over time more and more undercutting causes more collapses. The waterfall retreats leaving a steep sided gorge

Abrasion - Eroded rocks picked up by the river scrape and rub against the channel, wearing it away. Most erosion happens by abrasion

Attrition - Eroded rocks picked up by the river crash into each other and break into smaller fragments. Their edges also get rounded off as they rub together. The further the material travels, the more it is eroded - attrition causes particles size to decrease from a river's source to its mouth

Solution - River water dissolves some types of rock, e.g. chalk and limestone

Transportation

Traction - Large particles like boulders are pushed along the river bed by the force of the water

Saltation - Pebble-sized particles are bounced along the river bed by the force of the water

Suspension - Small particles like silt and clay are carried along by the water

Solution - Soluble materials (e.g. limestone) dissolve in the water and are carried along

Deposition

Deposition is when a river drops the material it's transporting. It occurs when a river loses velocity and energy. There are a few reasons why rivers slow down and deposit material:

  • The volume of water falls e.g. after a time of flood
  • The amount of eroded material increases
  • The water is shallower, e.g. on the inside of a bend
  • The river reaches its mouth

Vertical and Lateral Erosion

Vertical erosion - This deepens the river valley and channel, making it V-shaped. It's dominant in the upper course of the river. High turbulence causes the rough, angular particles to be scraped along the river bed, causing intense downwards erosion

Profiles

Long Profile

Lateral Erosion - This widens the river valley and channel during the formation of meanders. It's dominant in the middle and lower coursed of the river

Cross Profile

Floodplains

  • The flood plain is the wide valley floor on either side of a river which occasionally floods
  • When rivers flood, the water slows down, loses energy and deposits the material that it's transporting. This builds up the flood plain
  • Meanders widen as they migrate across floodplains laterally.
  • Overtime, meanders also migrate downstream
  • The deposition that happens on the slip-off sloped of meanders also builds up the flood plain

Hard & Soft Engineering Strategies

Levees

  • Levees are natural embankments along the edges of a river channel
  • During a flood, eroded material is deposited closest to the river channel, because it gets dropped first when the river slows down and loses energy
  • Over time, the deposited material builds up, creating levees along the edges of the channel

Estuaries

Estuaries are found at river mouths. The land is close to sea level and the river valley is at its widest

  • The water here is tidal - the river level rises and falls each day
  • When the water floods over the banks of the river, it carries silt and sand onto the valley floor
  • As the tide reaches its highest point, the water moves slowly and has little energy, so it deposits sediment
  • Over time, more mud builds up, creating large mudflats, e.g. the Severn Estuary, Gloucestershire
  • At low tide, the wide, muddy banks are exposed

Shows the gradient of the river

Courses

Upper - Steep, V-shaped valley, steep sides, narrow, shallow channel

Shows the valley and channel shape

Middle - Medium, gently sloping valley sides, wider, deeper channel

Lower - Gentle, very wide, almost flat valley, very wide, deep channel

River Discharge and Flooding

River Discharge

Peak discharge - The highest discharge in the period of time you're looking at

Lag time - The delay between peak rainfall and peak discharge

Rising limb - The increase in river discharge as rainwater flows into the river

Falling limb - The decrease in river discharge as the river returns to its normal level

Hard

Soft

Factors affecting flooding

Physical

Man-made structures built to control the flow of rivers and reduce flooding

Heavy rainfall - Heavy rainfall means that water arrives too quickly to infiltrate, so there's a lot of surface runoff, which increases discharge

Dams & Reservoirs

  • Barriers are built across the upper course of a river storing water behind the wall
  • Store water after heavy rain and prevent flooding downstream, can be used to generate HEP
  • Can flood existing settlements upstream, extremely expensive

Human

Prolonged Rainfall - Prolonged rainfall can saturate the soil. Any further rainfall can't infiltrate, increasing runoff into river channels

Geology - Clay soils and some rocks e.g. granite and shale, are impermeable so runoff is increased

Relief - If a river is in a steep-sided valley, water will reach the river channel quicker because it can flow faster down the steep slopes - this rapidly increases the discharge

Buildings are often made from impermeable materials e.g. concrete and surrounded by roads made from impermeable surfaces e.g. tarmac increasing surface runoff. Man-made drains then quickly transport runoff to rivers, increasing discharge

Trees intercept rainwater in their leaves, which then evaporates. Trees also take up and store water from the ground. This means that cutting down trees increases the volume of water that enters the river channel

Channel straightening

  • Meanders are removed by building straighter artificial channels
  • Water leaves the area quicker rather than building up
  • Flooding may happen downstream instead

Schemes set up using knowledge of a river and its processes to reduce the effects of flooding

Flood Plain Zoning

  • Restrictions prevent building on parts of a flood plain that are likely to be affected by a flood
  • Flood risk is reduced as fewer impermeable surfaces are created e.g. roads. The impact of flooding is also reduced - there are no buildings to damage
  • The expansion of an urban area is limited if there aren't any suitable building sites

Planting Trees

  • Planting trees in the river valley increases the interception of rain water therefore increasing lag time
  • Discharge and flood risk decreased, provides habitats for wildlife
  • Less land available for farming

Case Study: River Tees

Facts

  • 137km long, rising in Pennines
  • Upper course: V-shaped valleys interlocking spurs
  • High Force waterfall tallest in England (21m) - horizontal cap of Whinstone over Carboniferous limestone
  • Middle course: meanders, floodplain at Barnard Castle
  • Lower course: ox bows, levees, estuary, mudflats at Teesmouth (Middlesborough)
  • Petrochemical and nuclear plant at mouth
  • Seal Sands protected for wildlife

Management Scheme

  • 23,000 people at risk of flooding
  • Town of Yarn - £11 million invested in flood gates, gaboon's, flood warnings, flood plain zoning
  • Tees Barrage - reduces coastal flooding
  • Cow Green Reservoir - helps to reduce flooding downstream and provides 30 hectares of natural habitat for wildlife