CHANNEL PROCESSES- Transport processes

A river transports its load in four main ways:

• Traction– large stones + boulders are rolled along the river bed by water moving downstream. This mainly happens during periods of high discharge + consequently high energy levels.

• Saltation– small stones bounce or leap-frog along the channel bed. This process is associated with relatively high energy conditions. Small particles = thrust up from the bed of the river only to fall back to the bottom again further downstream. As these particles land = dislodge other particles upwards= more such bouncing movement to take place

• Suspension– very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried but it is also picked up, mainly through the turbulence that exists within the river. Suspension = largest proportion of sediment to the load of the rover. The suspended load= main cause of the brown appearance of many rivers and streams

• Solution– dissolved minerals are transported with the mass of moving water

CHANNEL PROCESSES-Transport theory

• River energy-not used for erosion = not lost through friction =  transport a river's load. 

A river obtains its load from two main sources:

• Material- washed, or has fallen, into the river from the valley sides
• Material-  eroded by the river itself from the bed or banks

Capacity and competence.

• Influenced by the velocity = therefore the discharge of the river
• Capacity of the river = a measure of the amount of material it can carry =the total volume of the load. 
• Competence of a river = the diameter of the largest particles that it can carry for a given velocity= the faster flowing a river = greater the turbulence = river is better able to lift particles from the river bed.

CHANNEL PROCESSES-Erosion processes

Abrasion

• Scraping, scouring and rubbing action - materials carried along by a river (load). 
• Rivers carry rock fragments in the flow of the water + drag them along the bed= wear away the banks and bed of the river channel.

Hydraulic action

• Sheer power of moving water. 
•  Movement of loose unconsolidated material - frictional drag of the moving water on sediment lying on the channel bed.

Corrosion

• Rocks that contain carbonates, such as limestone and chalk. 
• Minerals = dissolved by weak acids in the river water + carried away in solution.

Attrition

• Reduction in the size of fragments and particles within a river due to the processes describe above. 
• Fragments strike one another as well as the river bed. 
• Become smoother, smaller + more rounded as they move along the river channel. 

CHANNEL PROCESSES - Erosion theory

• Three main processes= erosion, transportation and deposition. 
• The dominant process operating = depends on the amount of energy available. 
• Governed - velocity of the flow + the amount of water flowing within the channel (discharge).

Erosion theory.

Rivers erode because they process energy.

Their total energy depends on:

• weight of the water = the greater the mass of water= more energy it will possess - the influence of gravity on its movement
•  height of the river above its base level (usually sea level) = source of potential energy + the higher the source of the river =more energy
• steepness of the channel =  controls the speed of the river= determines how much kinetic energy it has

Kinetic energy

Lost through :

• through friction
•  internally through turbulence within the flow of the river, 
• externally through contact with the bed and banks of the river channel. 

Energy loss through friction = impacts- all parts of a river.

 Upland river channel. 

Channel = rocky bed + many large boulders. 

Rough shape = means that the wetted perimeter (the overall length of the bed and banks that the river is in contact with) = large. 

Energy = lost through friction + the river’s velocity = energy level is reduced. 

Normal conditions= unable to perform much erosion. 

River contains large quantities of water following heavy rain or snowmelt=  energy to perform great amounts of erosion.

FACTORS AFFECTING RIVER DISCHARGE.

River discharge

River discharge= volume of water passing a measuring point/ gauging station in a river in a given time. 

Measured = cubic metres per second (cumecs). 

 Overall discharge - drainage basin = the relationship between preciptiation + storage factors.

Formula for drainage basic discharge:
Drainage basin discharge= precipitation - evapo-transpiration + or - changes in storage

Long term variation=  hydrographs =  annual patterns of flow ( the river regime) in response to climate. 

Short-term variation= flood of storm hyrdograph.

Storm Hydrograph.

• Shows variations in a river's discharge over a short period of time, usually during a rainstorm. 
• Base flow= starting and finishing level of the river.
• Rising limb= rising discharge as storm water enters the drainage basin.
• Peak discharge= highest flow in the channel.
• Receding limb= fall in discharge back to base level.
• Lag time= time delay between maximum rainfall amount and peak discharge.

Factors affecting a river's discharge

Rock and soil type

•  Permeable rocks + soils (e.g sandy soils) = absorb water easily= so surface run-off is rare
•  Impermeable rock + soils (e.g clay soils) =  more closely packed= rainwater can’t infiltrate=  water reaches the river        more quickly
• Pervious rocks (e.g limestone) = allows water to pass through joints + porous rocks (e.g chalk) =  spaces between the rock particles

Land use

• Urban areas=  roads = impermeable =  water can’t infiltrate/percolate = runs into drains= gathers speed + joins rainwater from other drains = quicker spillage into the river.
• In rural areas= ploughing up and down (instead of across) hillsides =  channels which allow rainwater to reach rivers faster = increased discharge= meet its bankful discharge.
• Deforestation = less interception= so rain reaches the ground faster= ground  saturated =surface run-off will increase.

Rainfall

•  Amount + type= will affect a river’s discharge
• Antecedent rainfall = is rain that has already happened= ground already saturated= further rain= surface run-off towards the river
• Heavy continual rain or melting snow= more water flowing into the river

Relief

• Steep slopes = rainwater = runs straight over the surface= quicker= before infiltration can occur.
• Gentle slopes= infiltration is more likely.

Weather conditions

• Hot dry weather = baked soil=  water can’t soak in= surface run off= straight into the river.
• High temperatures= increase evaporation rates from water surfaces + transpiration from plants = reduced discharge 
• Long periods of extreme cold weather= frozen ground= water can’t soak in

RIVERS- KEY TERMS

KEY TERMS:

Evaporation= Water turning into gas

Evapotranspiration= Water coming out of vegetation pores and evaporating into the sky

Groundwater flow= The slowest transfer of water within the drainage basin

Infiltration= The passage of water into the soil

Interception= The process by which raindrops are prevented from directly reaching the soil surface

Percolation= The downward movement of water within the soil surface

Waterfall= Formed when soft rock is eroded more easily than hard rock

Ox-bow lake= A horseshoe-shaped lake separated from an adjacent river

Potholes= Cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water

Traction= The rolling of the largest material on the river bed

Load= Any material carried by a river

Meanders= Bends in the river

Delta= Found at the mouth of the river formed by deposition

Floodplain= Created as a result of both erosion and deposition

Levees= Small raised banks found along the sides of the channel

Saltation= Small stones bounce or leapfrog along the channel bed

Abrasion= The scraping and rubbing action of materials carried along in the load

Hydraulic Action= the sheer power of moving water

Corrosion= Most active on rocks that contain carbonates

Attrition= The reduction in the size of fragments and particles within a river

WATER BALANCE OR BUDGET.

WATER BALANCE OR BUDGET.

Water balance/budget= the balance between inputs (precriptation)+ outputs (runoff, evapo-traspiration, soil and groundwater storage)- within a drainage basin.

Rivers= present on the surface = stores = capable of releasing water + there is direct surface runoff. 
= Dynamic relationship=River levels rise + fall - short term following heavy rainfall + often show an annual pattern (called the river's regime) = discharge.

The water balance formula =
Precipitation (P) = streamflow (Q) + evapo-transpiration (E) + or - changes in storage (S)
P = Q + E + or - S

 Precipitation >  evapo-transpiration =  pores of the soil = filled with water. 
 Soil = saturated= excess water =difficulty infiltrating into the ground= surface run off.

The water balance of a particular location= studied using a water budget graph. 
Shows - relationship between temperature, precipitation and evaporation- rates over the year. 
During the months - precipitation > evapo-transpiration= soil  recharged = water surplus= available to supply rivers and streams.
Months- precipitation< evapo-transpiration= water deficit= unable to supply rivers and streams.

RIVERS,FLOODS AND MANAGEMENT.

RIVERS,FLOODS AND MANAGEMENT.

Drainage basin hydrological cycle.

• Drainage basin = catchment area- river system obtains its water.
•  Watershed = delimits one drainage basin from another- follows a ridge of high land- any rain falling on the other side of the ridge = flow into another river - adjacent drainage basin.

The drainage basin hydrologicals system = open system = inputs and outputs.

Inputs 

•  Energy from the sun for  evaporation
• Precipitation - rain  & snow

Outputs 

• Move moisture out of the drainage basin
• Evaporation and  transpiration from plants (collectively called evapo-transpiration)
• Runoff into the sea
• Water percolating deep into underground stores = effectively lost from the system

Stores of  water

•  On the surface - glaciers, lakes, rivers, puddles
•   Vegetation stores water by interception and plants
•    The soil can hold water
• Groundwater is stored in permeable rocks

Transfers and flows

• Moves water through the system 
• Enable inputs of water to be processed - one store to another
•  Throughfall, stemflow, infiltration, throughflow and groundwater flow.