River Environments Skimmed

4 Transfers of the Hydrological Cycle

Evaporation & transpiration, condensation, precipitation, runoff

4 Areas of a Drainage Basin

Channel network, mouth, source, watershed

7 Factors that make a Basin more prone to flooding

1. Large quantities of rain in a short period of time
2. Smaller basins are more easily overwhelmed by storm systems
3. Increased temperatures causing rapid snowmelt
4. Deforestation etc. so limited vegetation interception storge
5. Impermeable rocks or rocks saturated or sealed with impermeable material (e.g. tarmac) so limited infiltration
6. Steeper slopes encourage rapid runoff to channels, instead of infiltration or evaporation
7. The absence of lakes, reservoirs and tributaries as buffers encourages water to travel rapidly to the main channel

Hydraulic Action

Flowing water **entraining** sediment from river beds and banks

* Rate increased by cavitation (imploding air bubbles send out shock waves)


* If channel is soft sediment/alluvium can have bank undercutting

Abrasion

Sediment transported downstream grinds away river bed and banks

* Sometimes produces **abrasion potholes**

Solution (Erosion)

(In channels with limestone) dilute rainwater acids dissolve rock and enlarge the channel

* Makes water “hard”

Attrition

Transport sediment is fragmented by collisions with:

* The river bed and banks
* Other particles of transported load

Suspension

Flowing water is turbulent, and supports small particles of alluvium/sediment, moving them downstream

Saltation

Coarser alluvium cannot be entrained and supported by flowing water, so it is bounced along the river bed

Traction

Very coarse alluvium is only rolled and slid along the channel bed

Solution (Transport)

Dissolved load is transported downstream

Deposition

Occurs when flowing water velocity is lower than criticalt fall velocity for a sediment size

Climate effect on river, weathering and mass movement processes

River erosion and transport processes:

* Highest in flashy river regimes with high peak discharge in areas of high precipitation

Frost action:

* Wet climates, cold temperatures

Chemical weathering:

* Humid, tropical climates

Mass movement processes:

* Heavy rainfall

Slope effect on river, weathering and mass movement processes

River erosion and transport processes:

* If river long profile is steep, GPE converts to KE faster, so more energy for erosion

Mass movement processes:

* Steep valleys are more vulnerable

Geology effect on river, weathering and mass movement processes

River erosion and transport processes:

* Resistant rocks harder to erode

Weathering:

* Hard rocks (except limestone) more resistant

Mass movement processes:

* Resistant rocks form more steep sided gorges due to higher internal cohesion

High altitude effect on river, weathering and mass movement processes

River erosion processes:

* Since close to source of river system
* River regimes are flashier, so high flow velocity cause more erosion processes

Weathering:

* Intensified

Mass movements:

* More triggered by steeper slopes

Low altitude effect on river, weathering and mass movement processes

Weathering:

* Lower gradients so low flow velocities meaning → more deposition → more weathering

Aspect effect on river, weathering and mass movement processes

Direction of slope determines exposure to sunshine, and temperature

Valley profile and gradient of the Hilden Brook

Upper course:

* Steep gradients, around 5-7, up to 16
* Small V-shaped valleys e.g. at Ducks Grove north of Underriver

Middle course:

* 1-3 gradient for 3 km
* Joint by many tributaries, now a ‘3rd order stream)

Lower course:

* 3 km below 1 degree
* 1/2 km cross section, enters Medway floodplain, some small levees

Channel shape, discharge and velocity changes of the Hilden Brook

Upper course:

* 40-60 cm wide,

Sediment size and shape in the Hilden Brook

Upper course:

* 1st km no bedload
* V-shaped valleys: angular sediment (400 Cailleux index), 7 cm long index

Middle and lower course:

* \~4 cm long axis, 600 Cailleux index score

Human causes of flooding

1. Deforestation
2. Urbanization
3. Agriculture
4. Burning fossil fuels

Physical causes of flooding

1. Weather
* Prolonged precipitation exceeds infiltration capacity or exceeds water table as soil is saturated
2. Rock permeability (Limit percolation)
3. Soil
4. Slope
5. Drainage density
* Higher drainage density means more tributary streams carry rainwater to main river
6. Vegetation

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