Rivers

Geography

Hydraulic Action

When force of fast-flowing water hits the banks and river bed forcing water into the cracks which compresses air

Changes in air pressure weaken the channel

Vertically erodes upper course of the river

Laterally erodes lower course of the river especially when water hits a meander

Abrasion

Small boulders and stones scratch and scrape their way down a river during transport which wear down the river bed

Attrition

Affects rivers load. When stones first enter river they are angular and sharp but as they travel down they collide with each other and the river bed which makes the rocks smoother

Solution

Dissolving of rocks like chalk and limestone. Rivers travelling over these rocks will erode them this way

Suspension

Fine material is held up and carried with the rivers flow

Upper Course Characteristics

Steep V-Shaped Valley

Vertical erosion from hydraulic action

Narrow channel

Low velocity

Small discharge

Rough river bed due to large rocks from traction

Middle Course Characteristics

Less steep gradient

Lateral erosion more frequent via attrition and abrasion

Wide and deep channel

High velocity

High discharge

Less rough river bed and sediment is smaller and smoother

Lower Course Characteristics

Gentle gradient

Wide Valley

Lots of deposition

Wide and deep channel

High velocity & discharge

Smooth river bed

Only lateral erosion

Why does the river widen downstream

As gradient is less steep in middle course, river begins to meander and laterally erode into valley side

In lower course, deposition from floods build up flood plain and meanders migrate which builds up and widens the valley

Interlocking Spurs

Found in upper course in parts of high land that alternate from either side of valley

Steep gradient is separated from narrow valley floor

As river moves it erodes landscape through vertical erosion but it cant cut hard rock so winds around it

Land that sticks out between bends are called spurs

Because river weaves in and out around the spurs either side of valley, spurs interlock

Rapids

Fast flowing turbulent sections of river where bed has steep gradient

Upper course and found where there’s uneven river bed

Waterfalls

-Upper course

1). Found where there’s hard and soft rock but soft rock erodes

2). Splash back from water below causes hydraulic action to weaken rocks behind water

3). As soft rocks break off, a base is formed

4). Continued undercutting means hard rock loses support and overhangs the drop

5). As hard rock loses support and falls over, its caught in fast flowing water. Some rocks become trapped and drill into the bed creating a plunge pool

6). Undercutting continues creating new overhang and as more erosion happens, waterfall retreats upstream leaving gorge downstream

Gorge

Narrow steep sided valley with rocky walls

Found below waterfall

Boulders litter the river bed

As the waterfall retreats upstream gorge is formed and as overhanging cap breaks off, so does gorge

Meanders

On inside bank they are

-Curved, gentle convex slopes with sand, gravel and pebble sediment smoothened by attrition. Vegetation grows furthest from water

On outside bank they are

-Steep drop down into river, several meters high and composed of bare earth

Meander Formation

-Formed in middle-lower course of river in flat land where river has energy to move sideways

1). River begins to swing side to side following small differences in land or the obstacles creating bends

2). On outside of each bend, river flows faster with more energy causing lateral erosion by hydraulic action and abrasion cutting into riverbank forming river cliff

3). On inside of each bend, water is slower and shallower so there’s not enough energy to carry all it’s load so it deposits sand, silt and gravel forming slip-off slope

4). As erosion continues on outside, and deposition on inside, bends become more pronounced forming a meander

Oxbow Lake

Small, horseshow shaped lake which is found several meters from fairly straight stretch of river in its middle and lower courses

1). Overtime as meander gets more curved due to more erosion on outside bends, deposition builds up on inside bends

2). 2 outer bends of meander move closer forming narrow neck of land between them

3). During a flood, river has more energy and cuts straight through the neck carving a new channel. As this is the shortest path, the river takes it

4). Original meander is now cut off from main flow of river and becomes isolated U-Shaped body of water

5). Over time, sediment seals off ends of old meander loop and curved lake is left shaped like an oxbow

Levees

-Raised river banks made of gravel and stone

-Grade sediment with coarsest closest to river channel

-Steep-sided but steeper on channel side than land side

-Flat top covered by grass

1). When river bursts its bank, friction with land reduces velocity and causes deposition

2). Heavy sediment is deposited closest to river

3). Sediment size decreases with distance from river

4). With each flood, banks built higher. Overtime, river bed gains thick layer of sediment which raises river

Floodplain

-Large area of flat land either side of river prone to flooding

1). Width of flood plain is due to meander migration where outside bends erode sideways into valley

2). Position moves downstream cutting wider valley

3). When floods recede, floodplain is higher and more fertile due to deposits of silt and alluvium caused by flooding

Estauries

Tidal part of river where channel broadens and meets sea

High tidal range, wide, mudflats

-Tidal bores are huge waves that funnel up the river but travel so fast they damage river bank and nearby vegetation

1). Salinity increases towards sea as does sediment as it comes from sea and river

2). Sea level rises and falls with tide and when tide comes in, saltwater pushes up into river mouth

3). When tide comes out, freshwater flows into sea

4). As river reaches sea, it slows down and drops sediment.

5). As sediment forms mudflats and salt marshes on side of estuary areas are flooded at high tides and exposed at low tide

6). Constant actions of tides and deposition creates wide, funnel-shaped river mouth

Percolation

Water seeping deeper below the surface

Groundwater

Water stored in the rock

Throughflow

Water flowing through soil layer parallel to surface

Factors increasing flood risk

1). Depression- results in heavy rain which saturates soil so can no longer store water so surface runoff increases so rainwater enters river quicker so more discharge and floods

2). Sudden bursts of heavy rain resulting in infiltration rate being too slow to cope. Could occur after drought that has baked the soil. Surface runoff occurs so discharge increases causing flash floods

3). Too much light rainfall may cause floods if theres previous rainfall thats saturated soil

Human factors leading to flood risk

Urbanization- more infrastructure built with impermeable surfaces increasing flood risk

New Houses- Built on greenfield sites. Between 2001-2011, 72% increase in density of houses so multiple houses in spaces where there should be 1

People paving back gardens and concrete front areas to accommodate cars. 47% of UK households have 2 cars

How many tourists and revenue does Kielder Dam make

300,000 a year and $6M

How much electricity and water is Kielder Dam producing and how many trees planted around

6MW to serve town of 10,000

200M L of water

150M trees

Cost of Kielder Dam

$167M

How many trees and families removed

1.5M trees and 58 families

Effects of dam building

-Soil downstream becomes less fertile through lack of sediment from floods reducing crop yields

-Concrete dam affects path of migrating fish

-Algae collected behind dam which deoxygenates water

Channel straightening + Pros

-Meandering section of river is engineered to create wide, straight and deep course which improves flood risk

As its straightened, river moves water out of the area quicker as there’s less friction with bed and banks

Fast flowing water removes sediment that would be built up in height of river bed

Reduces length of river (Tees by 4km) which increased trade at (Stockton) port

Home owners will invest in properties as there’s less flood risk so less insurance

Channel straightening + Cons

-Doesn’t straighten entire river so still flood risk

-Dredging needs to be done to remove silt (costs $6M)

-Straightened river may have concrete lining which is unattractive and deprives burrowing river bank animals of habitat

-Causes pollution due to chemicals used

Embankments + Pros

Artificially raised river banks so more water stays in channel to reduce flood risk. Made by bulldozers which move mounds of impermeable soil into river banks to build up height

-Safer from flooding as channel has more capacity and less likely to burst

-Cheap, doesnt affect wildlife, provides walking paths

Embankments + Cons

-Harder for fishing + boating

-Not as reliable and could give false sense of security

-High maintenance costs due to constant repair and monitoring

-If embankment is breached, water lies on land for a while

Flood-relief Channel + Pros

Artificially made channel designed as a backup for river that frequently floods

Runs parallel to river and made by cutting out land using equipment then making a cut into river bank to join channels

-Removes risk of flooding from specific areas

-Exeter relief channels protected (3000) houses and built cycle tracks

-Calms the river

-More secure for houses and business

Flood-relief Channel + Cons

-People living in the path will have to be moved

-Settlements downstream will suffer from flooding as merging of water swells river

-Expensive

-Disturbs wildlife and looks bad

River Restoration + Pros

When river thats been hard engineered has been restored to natural channel

-Creates new habitats

-Increased water storage so less risk of flooding

-River Quaggy scheme stopped 600 houses from flooding

River Restoration + Cons

-Not always effective

-Loss of agricultural land and floods nearby crops

-Expensive (River Quaggy $11M)

Effects of Boscastle 2004

-3m wall of water flooded at 60km/h

-Cars swept away and destroyed buildings

-4 bridges washed away

-Economy ruined as 90% of income is from tourism

Why was flood so bad

-River was narrow at bridge

-Low bridges trapped boulders and trees

-Trees built next to river stopped water flow

How much did the scheme cost

$5M in 2008

What did the scheme include

-Widened bridge

-Widened river and deepened it to carry higher flows of water

-More drainage outfall

-Defence walls

-River level lowered by 75cm

-Replaced river channel next to RiverSide hotel

-Removed trees from river

-Land owners encouraged to maintain vegetation cover and remove dead trees

Effects of the scheme

-Reduced chance of flooding from 1:10 to 1:75

-Some felt it changed character of village replacing Elizabethan structures with metal poles