Geography: UK's Evoloving Physical Environment

River processes & changes down its course, storm hydrographs and the Bostcastle floods

What is the gradient, discharge, depth, channel shape, velocity and valley shape in the upper course of a river?

Gradient: Steep

Discharge: Small

Depth: Shallow

Channel shape: Narrow, steep sides

Velocity: Slowest (of the 3)

Valley shape: V-Shaped

What are the features in the upper(2), middle(2) and lower courses(4)?

Upper: Waterfalls, interlocking spurs

Middle: meanders, floodplains

Lower: meanders, floodplains, levees, ox-bow lakes

Name the 4 main processes of erosion.

Hydraulic Action: The force of the water on the bed and banks of the river

Attrition: River load bumping together wearing down into smaller, smoother pieces

Abrasion: River load rubs against the bed and bank - wears away at them like sandpaper

Solution: Some rock minerals dissolve in river water

Name the 4 main types of transportation

Traction: Large boulders roll along the river bed

Saltation: Smaller pebbles bounced along the river bed - picked up then dropped from flow

Suspension: Finer particles carried along in the flow - can give a brown appearance

Solution: Minerals from rocks (e.g. limestone & chalk) dissolved in the water and carried in the flow - cannot be seen

What is deposition in rivers?

When the river loses energy it may drop some of its load

How is a waterfall formed? (5 steps)

1. More resistant rock lies over less resistant rock

2. Less resistant rock is erroded quicker

3. More resistant rock is undercut forming and overhang

4. Overhang cannot be supported and falls into plunge pool - so waterfall retreats upstream

5. Powerful waterfall errodes plunge pool with falling rocks

How do interlocking spurs form?

Rivers at their source are small with low energy

They flow side-to-side around ridges called “spurs”

Spurs ‘interlock’ with those on the other side of the valley

Explain the formation of ox-bow lakes.

• Meanders cause lateral erosion on the outside of bends (faster current) and deposition on the insides (slower current)

• The neck of the meander narrows due to the lateral erosion

• During a flood or period of high water flow the river takes the easier path straight across the meander

• Eventually deposition seals off the meander creating an ox-bow lake

Explain the formation of floodplains.

• Lateral erosion erodes away the valley sides, making the valley flatter

• The meanders migrate downstream

• Deposition occurs, depositing fine silt particles when the river floods - forming the floodplane over 1000’s of years

What are 3 of the key features of storm hydrographs? (& what the 2 graphs mean)

Bar graph shows rainfall in milimeters (mm)

Line graph shows discharge in meters cubed per second (m³/s)

Features:

• Rising & falling limbs

• baseflow discharge - represented by a line crossing the discharge often in a different colour

• Lag time - time between peak rainfall and peak discharge

What are 6 physical factors that affect storm hydrographs and how?

• Soil Type: more impermeable soils absorb less water - more surface runoff

• Antecedent Conditions: pre-saturated ground causes more surface runoff

• Vegetation: since plants absorb water less vegetation causes more surface runoff

• Area of Drainage Basin: wide basins with tributaries close together causes faster surface runoff - steeper rising limb & shorter lag time

• Geology: resistant rocks absorb less water - more surface runoff

• Elevation (elevation): steeper slopes cause faster surface runoff

What are 4 human factors that affect storm hydrographs?

• Urbanisation & Building on floodplains: less permeable surfaces - shorter lag time & steeper rising limb

• Deforestation & Land-use changes: reduce vegetation - steeper rising & falling limbs and shorter lag time

What are 4 hard engineering methods to manage flood risk? INC pros and cons

• Embankments & Levées:

✅Natural resources that blend in with environment

❌May burst - causes widespread flooding

• Flood Walls

✅Minimal maintenance

❌Block view of the river

• Dams and Reservoirs

✅Can regulate & control water flow

❌Very expensive to build

• Flood Barriers

✅Can be moved to where needed quickly

❌Doesn’t provide long-lasting protection

What are 3 soft engineering methods to manage flood risk? INC pros and cons

• River Restoration

✅Can reduce downstream flooding

❌Citizens may not want land use to change

• Floodplain Retention / Washlands

✅Somewhere for floodwater to go

✅Attractive, space for leisure & recreation

❌Restricts economic development

• Afforestation (plant trees)

✅Increased infiltration

❌Not suitable for all locations

What, where & when was the Bostcastle flooding?

Devastating flood, ~2 billion litres of water - 0 deaths

Cornwall Northern coast

2004

What were the 6 main physical causes of the Bostcastle Flood?

• Drainage basin shape (wide) - less time for water to infiltrate soil

• Antecedent conditions (saturated ground) - water can’t infiltrate soil: more surface runoff

• Rainfall

• Tide (high) - floodwater cannot as easily reach the sea

• Gradient (steep) - less time for water to infiltrate soil: more surface runoff

• Geology (impermeable) - mainly clay & shale which water can’t percolate: more surface runoff

What were the 4 main human causes of the Bostcastle Flood?

• Lack of flood defence

• Building on the floodplain

• Bridges & Channel narrowing - low bridges acted as dams: caused water to back up then burst through

• Land use & Deforestation - clearing of upstream woodland: less tree canopy interception: more surface runoff

What were the Social, Economic & Environmental Impacts of the Bostcastle Flood?

Social Impacts:

• Displacement

• Mental health issues

• No fatalities

Economic Impacts

• Tourism losses

• Property & infastructure damage

• Insurance costs rose

Environmental Impacts

• Coastal & marine pollution

• Habitat destruction

• Debris & silt