Riover

River Long Profile

• Path of river downhill is called its course

• Steep upper course / gently sloping middle course / flat lower course

• Long Profile helps to see how gradient changes

• River form channels and valleys as they flow downhill

• The shape of river valleys change as river flow downstream

• Cross profile (cross section)

Upper Course

• Gradient : Steep

• V shaped valley

• Narrow shallow Channel

Middle Course

• Gradient : Medium

• Gently sloping valley sides

• Wider Deeper Channel

Lower Course

• Gradient: Gentle

• Very wide / Almost Flat Valley

• Very wide deep channel

Vertical Erosion

• deepens river valley and channel

• V - Shaped

• Dominant in upper course

• High turbulence - cause rough angular particles to be scraped along river bed

• Intense downward erosion

Lateral Erosion

• Widens river valley

• Channel during formation of meanders

• Dominant in the middle and lower courses of the river

Fluvial Processes - Erosion

• Hydraulic Action

• Abrasion

• Attrition

• Solution

Fluvial Processes - Transporation

• Traction - Large Particles like Boulders are pushed along the river bed by force of water

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

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

• Solution - Soluble materials dissolve in water and are carried along

Deposition

• River drops sediment

• River loses velocity and energy

  • vol of water falls

  • amount of eroded material increases

  • water is shallower

  • river reaches its mouth

• Particle Size - Affects how far a material is transported → smaller particles transported further and deposited closer to rivers mouth

Waterfalls

• River flows over an area of hard rock followed by an area of softer rock

• Softer rock is eroded by (HA + AB)

• Creates a step in the river

• Water flows over step it erodes more of the softer rock

• Steep drop is created (waterfall)

Gorges

• hard rock is eventually undercut by erosion

• becomes unsupported and collapses

• collapsed rocks are swirled around at foot of the waterfall

• where they erode the softer rock (AB)

• deep plunge pool

• more undercutting cause more collapses

• water fall retreats → leaving steep side gorge

Interlocking Spurs

• Upper course → most erosion veritcally downwards

• Steep sides V shaped valley

• Rivers lack power to erode latteraly so have to wind around the high hillsides

• Hillsides that interlock with each other as river winds are called interlocking spurs

Meanders

• Winding curve or bend in the river

• Water current flows fastest outside of the river where channel is deeper and there is less friction

• lateral erosion (ha +ab) results in undercutting of river bank and formation of steep cliff

• inside of bend where river flow is slower material is deposited on a slip off slope due to more friction (as river channel shallower)

• eroded material deposited on inside of bend forming slip off slopes

• over time meander gradually change shape and move across the floodplain and migrate downstream

Ox-Bow-Lakes

.Over time the horseshoe becomes tighter (erosion) , until the ends become very close together.

As the river breaks through the neck, eg during a flood when the river

as river flows along the shortest course

has more energy, and the ends join, the loop is cut-off from the main

channel.

The cut-off loop is called an oxbow lake.

Flood Plains

• Wide valley floor on either side of river that occasionally floods

• Rivers flood the water slows down - loses energy and deposits material that it is transporting

• builds up the flood plain

• Meanders widen as they migrate across flood springs laterally

• Meanders also migrate downstream

• Deposition that happens on slip off slopes of meanders also builds up flood plain

Levees

• Levées form in the lower course of a river

• They are created during flooding, when the river overflows its banks

• As the river spreads out, it loses energy

• Coarse material (sand and gravel) is deposited first near the river banks

• Finer sediment (silt and mud) is deposited further away on the floodplain

• Repeated flooding causes sediment to build up along the banks

• This creates raised river banks (levées) that are higher than the surrounding floodplain

What are levees

• natural embankment along the edges of a river channel

Estuatries

• tidal areas where the river meets the sea

• estuaries are found in river mouths → land is close to sea level → river valley at its widest

• water here is tidal → river level rises and falls each day

• water floods over the banks of the river it carries silt and sand onto valley floor

• tide reaches highest point water moves slowly and deposits sediment

• over time more mud builds up → creating large mudflats

• low tide the wide muddy banks are exposed

Discharge

• Volume of river water flowing per second measured in cumecs

Hydrographs Features

• Peak Discharge → Highest discharge in period of time your looking at

• Lag Time → Delay between peak rainfall and discharge

• Rising Limb → Increase in river discharge as rainwater flows through river

• Falling Limb → Decrease in river discharge as the river returns to its normal level

Why does lag time occur?

• Most rainwater doesn’t land directly in the river channel

• Flows quickly overland (surface runoff)

• or soaks into ground (infiltration) and then flows slowly underground to the channel

Physics and that affect Flood Risk

• Heavy Rainfall → Water arrives too quickly to infiltrate so there is a lot of surface runoff increases discharge

• Geology (Rock Type) → Clay soils and some rocks eg. shale are impermeable (dont allow infiltration) so runoff is increased

• Prolonged Rainfall → Saturate soil → Further rainfall cannot infiltrate → Increasing runoff into river channels

• Relief → Steep sided valley water will reach river channel quicker because it can flow faster down steep slopes → rapidly increases discharge

Human Factors - Flood Risk

• Land Use

  • Buildings are often made from impermeable materials → surrounded by roads made from impermeable surfaces → increased surface runoff → man made drains then quickly transport runoff to rivers increasing discharge

  • Trees intercept rainwater on their leavs → evaporates → take up and store water from the ground → cutting down trees increases the vol of water that enters river channel

Why does flooding occur?

• River level rises so much that it spills over its banks

• Several factors shorten lag time

• So peak discharge and flooding is more likely to occur

Hard Engineering

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

Dams and Reservoirs (HE)

• Dams are barriers built across rivers and resevoir (artifical lake) formed behind dam

• Benefits:

  • Reservoirs store water - control water flow - prevent flood downstream

  • Generates HEP

• Disadv

  • Expensive to build

  • Creates a reservoir that can flood existing settlements

  • Material is deposited in reservoir along the river course

  • farmland downstream can become less fertile

Channel Straightening

• Meanders are removed by building straighter artifical channels

• Water leaves the area more quickly rather than building up so flood risk is lower

• Flooding may happen downstream instead

• Faster moving water may cause more erosion downstream

Embankments

• Raised walls are built along river banks

• River can hold more water → Floods are less frequent

• Quite expensive and there is a risk of severe flooding → if water rises above the level of embankment or if they break

Flood Relief and Channels

• Channels are built to divert water around built up areas or to divert excess water is level too high

• Gates on channels mean that release of water can be controlled reducing flood risk

• Increased discharge where the releif channel rejoins the river → flooding

Soft Engineering

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

Flood Warning and Prepariton

• Environment Agnecy issues flood warnings

• Buildings are modified to minimse flood damage

• Residents can prepare sandbags and floods board prior to floods

• ADV - Warning give people time to move possessions upstairs and put sandbags in place or evacuate

• Reduces the impact of flooding

• DISADV - Dont prevent floods

• People may not have access to warnings