Physical Geography - Hydrology and Geomorphology

Drainage Basin

The area within the watershed in a catchment system

Watershed

The highest point around the drainage basin containing the water system

Source

Where the river starts

Mouth

Where the river ends

Confluence

The meeting points between tributaries and the main channel

Tributary

Other streams of water that lead into the main channel

Channel

The main path that the river will take

Precipitation

Water Falling from the sky. Rain is the most important form of precipitation

Evaporation

The Surface level of water is absorbed by the sun and evaporated into the sky

Evapotranspiration

Water evaporates from puddles stream etc, and plant absorb water through transpiration. Together this total loss of water is called evapotranspiration

River Discharge

The volume of water flowing in the river

Interception

When precipitation lands on vegetation

Soil Water

Water being stored in the soil

Surface Water

Any store of water above ground in the drainage basin

Throughflow

Water flowing downhill through the soil

Baseflow

Water that reaches the channel through slow throughflow

Groundwater

Water stored underground

Permeability

How easily water passes through material

Channel Storage

How much water the channel can store

Throughfall

The downward movement of water from the leaves onto the ground

Stemflow

Water running off a tree or a plant

Overland Flow

Water running across the soil into the Channel

Infiltration

The downward movement of water into the soil

Percolation

The movement of water through the bedrock

Soil Moisture

The amount of moisture stored in the soil

Soil Moisture Surplus

If precipitation exceeds evapotranspiration and the excess is not being used by plants

Soil Moisture Deficit

Evapotranspiration exceeds precipitation

Soil Moisture Recharge

Replacement of water lost during drier periods

Field Capacity

The maximum amount of water soil can hold

How Does Distance Downstream Affect River Discharge?

The further downstream the river you are, the greater the discharge is

How do climate characteristics affect discharge

In the summer, Alpine areas experience glacial melt which increases discharge

How does land use affect discharge

Deforestation decreases interception, increasing discharge

Afforestation increases interception, decreasing discharge

How does water abstraction affect discharge

Wells and boreholes remove groundwater stones decreasing discharge as base flow cannot occur

How do channel modifications affect discharge

Channel straightening increases discharge and river bank planting decreases discharge

What are river regimes

River regimes are annual hydrographs which show the levels of discharge of different rivers over a year

How does a river gradient change over its course

It is steeper at the top of the river and decreases as it goes down the river

How does the balance between erosion and deposition change?

Erosion is more dominant up river but as you go down the river deposition become more dominant

How does the size of material change along the profile?

As you go down the river material become smaller and more rounded

CASE STUDY: River Severn - Description

Upper Course - Soggy moorland, hills in mid Wales - plyulimon hills

Rain dumped on moorland as It come from Atlantic

 Trickles become rivulets that form a stream

 V-shaped valley

 Erosion is dominant - ACASH

Difference in geology -> waterfalls/gorges

Middle Course - Sinuosity index increases

 River begins to meander

 Erosion is lateral and occurs on the outside bend. Deposition will occur on the inside

 Oxbow lakes will eventually form

Lower Course - Floodplains prevalent

 Human impact on the hydrological cycle… interception?

 Drainage of urban areas can cause rivers to flow

 Right at the mouth risk of flooding decreases due to width of channel

 15m difference between HWM and LWM

 Attraction of industry due to space around the estuary

What is ACASH

Attrition - Rocks have become smoother/more rounded as they knock into each other when being transported in the flow

Cavitation - Bubbles that formed in the water may implode under high pressure. This generates tiny jets of water which will, over time, erode the rock

Abrasion - Sediment cuts as ammunition for the river. As this material moves it will scrape away at the bed/banks

Solution - Rocks that contain soluble material (e.g. gypsum) can be dissolved as river water passes through the channel

Hydraulic Action - Joints/cracks from bedding planes expand as water passes through the channel, water is forced into cracks opening them up over time

What are the 3 C’s

Capacity - the volume of material that can be held in the channel

Competence - the ability that the river has to carry the largest sediment

Calibre - the size of each individual pebble or particle

When does deposition occur

Deposition occurs when:

• there is a sudden reduction in gradient

• the river enters a lake or sea

• discharge has been reduced following a period of little rainfall

• Where there is shallower water

• There is a sudden increase in the volume of sediment available, such as at a confluence/ where a land slide has occurred 

• River overflows its bank so velocity outside the channel is reduced. (resulting in floodplain)

Explain why sediment size in a river may vary at different locations

Sediment size changes at different locations in a river channel for a number of reasons. In  the upper reaches of a river sediment if often new so would be bigger, but as it travels down the river it would get more and more eroded, meaning that the sediment size would get smaller. In the upper reaches there is much more vertical erosion meaning that the sediment is much large and more sharp but as you move into the middle and lower reaches,  the sediment size changes into more rounded as the erosion changes as the velocity and discharge of the river also increases.

What is a thalweg

Line of maximum velocity in the river, occurs the maximum distance it can from the bed and banks as that is the path of least resistance, travels the outside bank to the outside bank of meanders - river cliffs may form on the outer bank

Laminar Flow

Water flowing downwards over a smooth surface in a simple sheet, with no eddies or meanders

Conditions needed - Smooth,Straight Channel

Turbulent Flow

Water flows in a series of erratic eddies, both vertical and horizontal, in a downstream direction with no regular pattern

Conditions needed - Channel bed roughness and friction influenced, Swirling vortices and eddied throughout the water, both horizontal and vertical

Helicoidal Flow

Spiralling cross channel motion

Conditions needed - Channels carrying lots of material may create pools/rifts

What are the 3 main channel types

Straight Channel - SI <1.0, Rare because helicoidal flow dominates most rivers, resulting in meandering

Braided Channel - SI 1.0-1.5, These are rivers that contain a large number of islands and bars made of sediment. Need to be in an area where discharge varies a lot during the year

Meandering Channel - SI >1.5

Human Activity Modifications - PACTS CUT

P - Precipitation - directly alter levels of precipitation through cloud seeding. Chemicals such as silver iodide + salt released into atmosphere by plane. The aim is for these chemicals to act as a condensation nuclei

A - Abstraction of water - removal of water for human use (Domestic, Agricultural, Industrial)

C - Changing channel characteristics, Canalisation - improving the water network for accessibility/efficiency of transport, Straighter and deeper channels, Moves water efficiently, decreases lag times, Influence of concrete channels 

T - Transferring water between drainage basins - 3 major industrial sites in the north of England - Headwaters further up profile (upper course) - reservoir on the Tyne

S - Storing water behind dams or groundwater recharge

• Irrigation within agriculture

• HEP (sustainable alternative energy source)

• Control of flooding

  Dams/reservoirs control the release of water downstream

C - Changing agricultural land use

U - Urbanisation

T - Tree cover (deforestation vs afforestation)

What are the 3 main parts to flood management

Forecast and Warning (FIRE)

Hard Engineering Solutions (SPREAD)

Soft Engineering Solutions (SAFER)

What is FIRE

F - Flood forecasting and warning service

I - Insurance

R - Reliant Housing

E - Emergency Services

What is SPREAD

S - Straightening of channel

P - Platforms for raising buildings

R - Reservoirs and dams

E - Embankments and levees

A - Artificial diversion spillways

D - Dredge the river

What is SAFER

S - Sacrificing 'washlands' downstream

A - Afforestation

F - Floodplain zoning and land use management

E - Establishing wetlands and riverbank conservation

R - River restoration schemes

CASE STUDY: River Quaggy

Where is it Located?

It passes through the south-east of London boroughs of Bromley, Greenwich and Lewisham. It rises from two sources near Farnborough hospital near Locksbottom 

How was the River Quaggy managed previously and what did they change

Since the 1960s it was heavily managed with artificial channels and culverts but they restored the river and let it run through the park

CASE STUDY: 3 Gorges Dam

Where is it located?

It is a dam downstream of the city of Chongqing, the Chang Jiang flows through a deep, narrow sections of its valley known as the 'Three Gorges'

CASE STUDY: Bangladesh 1998 Statistics

30 million people affected

780 - 1500 deaths

550 000-900 000 houses damaged