REJUVENATION- Causes of flooding

Causes of flooding

Flooding = occurs when a river's dicharge exceeds the capacity of its channel to carry that discharge = the river overflows its banks. 

Flooding may be caused by a number of natural causes or physical factors:

Excessive levels of precipitation occurring over a prolonged period of time - leads to saturation of the soil. 

When the water table reaches the ground surface = increased overland flow or runoff

Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall = the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case

The melting of snow particularly when the subsoil is still frozen = infiltration capacity is reduced

Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes = bring abnormally large amounts of precipitation

The nature of the drainage basin has an influence on the  likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.

---

REJUVENATION- Incised meanders

Incised meanders

If a rejuvenated river occupies a valley with well-developed meanders = renewed energy= they become incised + deepnd. 

Incised streams + rivers have cut deeply into the landscape in many parts of the British Isles.

 The nature of the landforms created is largely a result of the rate at which vertical ersion has taken place. 

When incision is slow + lateral erosion is occurring = an ingrown meander may be produced.

 The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip-off slopes on the inner bends. 

With rapid incision, where downcutting or vertical erosion dominates = the valley is more symmetrical, with steep sides and a gorge-like appearance, These are described as entrenched meanders.

FLOODING- Hard and soft engineering

Hard and Soft engineering: which is the better option?

Hard engineering strategies involve the use of technology - to control rivers
Soft engineering, adopts a less intrusive form of management, seeking to work alongside natural processes.

 Hard engineering = immediate results + are expensive. However, in the future, they may make problems worse or create unforeseen ones. 

Soft engineering = much cheaper + more sustainable = it does not interfere directly with the river’s flow.
 
What’s  more important?
In the aftermath of the 2009 Cumbrian floods, local people were angry that more hadn’t been done to prevent them. 

They accused the authorities of ‘putting salmon before people’ after their earlier request to lower the river bed by 3 metres in Cockermouth had been turned down because it might harm fish stocks.

The cost of protection
Professor Samuels advises the government on managing rivers. He said ‘It is technically possible to defend places like Cockermouth against extreme events, but only by building huge walls and embankments along the river, which would cost billions and alter the character of the town. 

For most people, that would be unacceptable as the floods.’

Hard Engineering

Hard engineering involves building structures to defend places from floodwater.

 Dams and reservoirs exert a huge degree of control over a river.

 The natural flow of water = prevented by a dam (often a concrete barrier across the valley), water fills the area behind it and is released or held depending on circumstances such as current and expected rainfall. 

Dams and reservoirs = normally constructed as part of a multi-purpose project rather than with just a single aim in mind.

Soft engineering

Soft engineering involves adapting to flood risks, and allowing natural processes to deal with the rainwater. 

It is a strategy that accepts the natural processes of the river + seeks to work with it to reduce the effects of folding rather than attempting to gain control of it. 

A conscious decision can be made to ‘do nothing’ but simply to allow natural events to happen, even if this involves the risk of flooding. In some poorer areas of the world, this is a necessary approach.

 In richer areas - money is set aside in years when flooding does not occur to provide relief after the event. 

However, there are many more positive approaches that can be adopted to reduce the risk of flooding without exerting a major force over the river and its processes.

FLOODING- Impacts

The impact of human activities on flooding

Urbanisation

More people are living in towns and cities

Population growth + urbanisation= demand for land to build on - floodplains are flat and are food for housing

Concrete and tarmac = impermeable = preciptation cannot infiltrate = into the river much more quickly = reaches bank full discharge = floods

Less interception - trees and plant matter is removed = precipitation gets into the river much more quickly = reaches bank full discharge = floods

Surface water = channelled directly into drains and sewers = precipitation reaches the river much more quickly.

Bridges over rivers can constrict rivers, slow discharge and reduce the carrying capacity of the river.

Deforestation

In poor countries = rapid deforetation has taken place.

Land = used for framing, settlement and mining etc.

No trees =  greater risk of soil erosion =  the preciptation is not intercepted.

Flood damage =  greatest near the mouth of a river - wide,flat floodplains = most susceptible to damage.

The volume of water is greatest here because many tributaires have joined the river.

River Management

River management = reduce the likelihood of flooding.

However, in some circumstances it can actually increase the risk:

Bangladesh: flood embankments have built along some river channels = increase river capacity but at times have prevented floodwater draining back into the rivers

Farakka Dam, India: Lots of rainfall = lake behind the dam could burst = floodgates of the dam were opened = stopped the dam from bursting = increased the discharge of the river in Bangladesh.

This coincided with the normal floods and made the severity much worse

FLOODING- Flood prediction

Flood prediction

Hydrologists = try to forecast the likelihood of futrue flood events using past records. 

Data = river discharge records - precipitation + flood recurrence interval graphs.

These graphs = calculate statistically the probability of flooding in the future based on past records.

The further back flood records go = more accurate the prediction.

Records of a river's discharge are ranked - longest period available,from highest peak discharge to the lowest recorded. 

The following formula is used to calculate the recurrence interval:

recurrence interval (years) = number of years on record +1 / ranking of flood being considered

When the recurrence interval = plotted against discharge as a scatter graph on semi-logarithmic graph paper,

= possible to use the line of best fit to predict when the next flood of a particular magnitude might occur

= flood return period. 

The Department for Environment Food and Rural affaris (DEFRA) and the Environment Agency 

=  main organisations responsible for flood management of major rivers

E.G Thames and the Severn in the British Isles 

Use flood recurrence interval graphs to plan flood defence strategies.

Densely populated urban areas = sufficiently protected against a 1 in a 100 year flood events 

Grassland and low productivity agricultural land should not be protected at all.

Hydrologists use past data records showing: 

The regime or yearly pattern of discharge in relation to annual precipitation patterns

= likelihood of seasonal flooding can be assessed. 

RIVERS- KEY TERMS

Key terms

Hazard: a natural event that threatens life and property. A disaster = realisation of the hazard. Flooding is an exmaple of a natural hazard

River management: River basins = subject to strategies desigined to prevent flooding and to ensure that there is an adequate supply of water

REJUVENATION- River terraces

River terraces

---

A river terrace - remnant of a former floodplain - has been left at a higher level after rejuvenation of the river. 

Where a river renews its downcutting- it sinks its new channel into the former flooodplianm leaving the old floodplain above the lovel of the present river. 

There terraces are cut back as the new valley is widened by lateral erosion.

 If renewed rejuvenation takes place-  the process is repeated + a new pair of terraces is formed beneath the original ones. 

The River Thames has created terraces in its lower course by several stages of rejuvenation. 

Terraces =  useful shelter from floods in a lower-course river valley + natural routeways for roads and railways. 

The built-up areas of Oxford + London are mainly located along the terraces of the River Thames.

REJUVENTATION- Knick points

Knick points

A knick point = a sudden break or irregualrity in the gradient along the long profile of a river. 

Some knick points = shaply defined- waterfalls, whereas others are barely noticeable. 

A number of factors can cause such features to occur- commonly attributed to rejuvenation.

When a river is rejuvenated= adjustment to the new base level starts at the sea + gradually works its way up the river's 

course= the river gains renewed cutting power (in the form of vertical erosion)=  encourages it to adjust its long profie.

The knick point is where the old long profile joins the new. 

The knick point - recedes upstream at a rate which is dependent on the resistance of the rocks- may linger at a relatively hard outcrop. 

Difficult to determine whether a waterfall occurs due to variability in rock type or rejuvenation.

REJUVENATION

Rejuvenation

Rejuvenation occurs - fall in sea level relative to the level of the land or a rise of the land relative to the sea. 

The river renews its capacity to erode as its potential energy = increased. 

The river adjusts to its new hase level, at first in its lower reaches and then progressively inland = a number of landforms may be created: knick points, waterfalls and rapids, river terraces and incised meanders.

LANDFORMS OF FLUVIAL EROSION- Deltas

Deltas

A delta= a feature of deposition- located at the mouth of a river as it enters a sea or lake. 

Deposition occurs as the velocity + sediment-carrying capacity of the river= decrease on entering the lake or sea= bed load + suspended material are dumped. 

Flocculation=  occurs as fresh water mixes with seawater + clay particles coagulate due to chemical reaction=  clay settles on the river bed.

Deltas form = rate of deposition excveeds the rate of sediment removal. 

In order for a delta to form the following conditions are likely to be met:· 
1) The sediment load of the river is very large, as in the Mississippi and Nile rivers

2)The coastal are into which the river empties its load = small tidal range + weak currents=  limited wave action and, therefore, little transportation of sediment after deposition has taken place. 

E.G Gulf of Mexico and the Mediterranean Sea

Deltas are usally composed of three types of deposit:
·        The larger + heavier particles = the first to be deposited as the river loses its energy=  form the top set beds
·        Medium graded particles=  travel a little further before they are deposited as steep-angled wedges of sediment, forming the foreset beds
·        The very finest particles= travel furthest into the lake before deposition and from the bottomset beds

Deltas = described according to their shape. 

`Arcuate delta (Nile delta) =curving shoreline + a dendritic pattern of drainage.
Many distributaries break away from the main channel as deposition within the channel itself occurs = the river braids. 
Longshore drift keeps the seaward edge of the delta relatively smooth in shape.

Bird's food delta he (Mississippi). Fingers of deposition build out into the sea along the distributaries' channels, giving the appearance from the air of a bird's claw. 

A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea cuurents or longshre 

LANDFORMS OF FLUVIAL EROSION- Floodplains

Floodplains

Floodplains = result of both erosion and deposition.

The accumulation of river deposits = suggests =predominately depositional features. 

Relatively flat areas of land either side of the river= form the valley floor in the middle+  lower courses of the river. 

Composed of alluvium - river deposited silts and clays. 

Floodplain= becomes wider and the depth of sediment accretions= increases. 

The width of the floodplain= determined by the amount of meander migration + lateral erosion that has taken place.

Lateral erosion = most powerful just downstream of the apex of the meadner bend. 

Results in the migration of meanders= leaving their scars clearly visisble on the floodplain. 

Interlocking spurs = removed by lateral erosion in the middle course- leaving behind a bluff line + widening the valley.

Alluvial deposits = depends partly on the amount of flooding in the past- floodplain creation is linked to extreme events. 

Point bars + old meanders scars become incorporated into the floodplain= adding to the alluvial deposits. 

These become stabilised by vegetation as the meanders migrate + abandon their former courses. 

LANDFORMS OF FLUVIAL EROSION- Levees

Levees

Middle + lower courses=  risk from flooding during times of high discharge= overflows the banks= deposition= the competence = reduced.

Coarsest material = deposited first= small raised banks (levees) along the sides of the channel. 

Subsequent floods= increase the size of these banks + further deposition occurs=  the river, with channel sediment build-up=  flows at a higher level than the floodplain= authorities sometimes strengthen levees and increase their height.

---

LANDFORMS OF FLUVIAL EROSION- Oxbow lake

Oxbow lakes

Outer banks - meander = eroded through processes - hydraulic action = neck = narrow= the two outer bends meet = river cuts through the neck of the meander (usually during a flood event = energy in the river is at its highest).

The water = takes its shortest route.
Deposition = seals off the old meander bend = a new straighter river channel is formed.
Deposition = old meander bend = left isolated from the main channel= an ox-bow lake.
The feature= fills up with sediment + may gradually dry up (except for periods of heavy rain)= a meander scar.

LANDFORMS OF FLUVIAL EROSION- Meanders

Meanders

Water flows fastest on the outer bend =  channel is deeper+ less friction= water being flung - towards the outer bend as it flows around the meander=  greater erosion = deepens the channel=  reduction in friction = increase in energy= greater erosion.

 This lateral erosion = undercutting of the river bank + formation of a steep sided river cliff. 

Inner bend water is slow flowing= low energy zone= deposition = shallower channel= increased friction = reduces the velocity =reducing energy=  further deposition=  a small beach of material builds up on the inner bend= slip-off slope or point bar. 

Helicoidal flow=  water in a meander flows in a corkscrew like movement - moves from the inside of the bend towards the outside of the bend. 

Meander = asymmetrical in cross-section = deeper on the outer bend -  greater erosion+  shallower on the inside bend - an area of deposition.

---

LANDFORMS OF FLUVIAL EROSION- Braided channels

Braided channels

Braiding = river is forced to split into several channels separated by islands. 
Rivers - supplied with large loads of sand + gravel. 
Occur - river has variable discharges. 
The banks = formed from sand + gravel = unstable +easily eroded= channel becomes very wide in relation to its depth. The river = choked- several sandbars + channels- constantly changing their locations.
Braiding also occurs in environments - rapidly fluctuating discharges:


1. Semi arid areas - low relief that receive rivers from mountainous area


2. Glacial streams - variable annual discharge. In spring, meltwater = river discharge + competence = increase=  transports more particles= temperature drops + the river level falls=  load is deposited as islands of deposition in the channel.  

LANDFORMS OF FLUVIAL EROSION- Potholes

Potholes

Potholes = cylindrical holes drilled into the rocky bed of a river - turbulent high-velocity water loaded with pebbles. 
The pebbles = trapped in slight hollows + vertical eddies in the water=  the sediment  grinds a hole into the rock by abrasion (corrasion). 
Attrition = rounds + smooths - pebbles caught in the hole = reduces the size of the bedload.
Potholes = vary in width from a few centimetres to several metres. 
Found in the upper or early-middle course of a river=  valley lies well above base level= more potential for downcutting +  river bed =  likely to be rocky.

LANDFORMS OF FLUVIAL EROSION AND DEPOSITION- Waterfalls

Waterfalls

Formation of a waterfall:
1.Waterfalls = upper course of a river. 
 Hard rock lies next to soft rock. 
Often start as rapids.

2. River passes over the hard rock, the soft rock below = eroded (hydraulic action, abrasion etc) =hard rock left elevated above the stream bed. 


3. The 'step' in the river bed = develops - the river flows over the hard rock step (Cap Rock) = vertical drop.


4. Drop = steeper - river erodes the soft rock beneath = a plunge pool forms at the base of the waterfall.


5. Gradually undercuts the hard rock =  the plunge pool gets bigger -further hydraulic action and abrasion= hard cap rock is unsupported + collapses= the rocks that fall into the plunge pool = enlarge it by abrasion - swirled around. 


6.A steep sided valley = gorge =  left behind and as the process continues the waterfall + gradually retreats upstream.

CHANGING CHANNEL CHARACTERISTICS- Other changes

Further changes

The shapes of the channel - influences the velocity of the river. 

 Upper course=  narrow + uneven channel - the presence of large boulders= large wetted perimeter. 

 Wetted perimeter = total length of the river bed + banks in cross section that are in contact with the water in the channel.

River levels = rise - heavy rain / snowmelt - upper course = relatively shallow. 

Large wetted perimeter=amount of water in the river= more friction.

Friction results = energy loss = slowed velocity.

Channels = larger + smoother- middle and lower course = more efficient. 

Wetted perimeter= proportionately smaller - volume of water flowing in the channel=less friction to reduce velocity. Mountain streams = slower velocity= energy is expanded overcoming friction on the uneven channel bed in the upper course- in the lower course = little to disrupt water flow

Hydraulic radius = cross-sectional area of the channel / wetted perimeter

A high hydraulic radius = river is efficient= moving water =loses proportionately less energy in overcoming friction - than when the ratio between cross-sectional area + the wetted perimeter = low. 
Larger channels = more efficient= area increases to a greater degree than wetted perimeter.

CHANGING CHANNEL CHARACTERISTICS- The Bradshaw Model

The Bradshaw model

Describes -changes we  would expect as a river travels from its source regions in its uppercourse with  increasing distance downstream towards its mouth. 

CHANGING CHANNEL CHARACTERISTICS- Channel cross profiles

Channel cross profiles

The channel cross profile = view of the river bed + banks - one side to the other - any one point on it course. 

As a river flows from its source to its mouth= a number of typical changes take place in the channel morhology.

Upper course:
Channel = narrow and uneven- presence of deposted boulders. 

Where both banks = eroded = channels = broadly rectangular in shape.

Middle course:
Meanders

Channel= asymmetrical- on the river bends

             = mainly smooth + symmetrical - on the straight stretches.

Lower course:
River widens + deepens further

Banks of deposition + eyots (islands of deposition) = can disrupt the shape of the channel cross-section=  a braided channel. 

Levees (embankments) =  either side of the channel (can also be man-made)

VALLEY PROFILE= A graded profile

A graded profile

A graded stream= a long profile in equilibrium with the general slope of the landscape. 

A graded profile = concave + smooth. 

Stream's maintain their grade= a balance between erosion + transportation + deposition.  

Erosion=  removes material - bumps in the profile+ deposition fills in dips.

Potential and kinetic energy

Precipitation - deposits it on an upland=  possesses potential energy/ energy of position. 

Water begins to flow down hill= potential energy =converted - kinetic energy/ energy of motion (turbulence+ friction within the moving water = kinetic energy.)

Kinetic energy=  water erodes away the surface of the  stream channel it is passing through + moves sediments along the stream bed= the river ends in a standing body of water (lake)= energy = converted- thermal energy/ heat energy. 

This heat energy= spread through the environment= most degraded form of energy + cannot accomplish anything. 

Potential Energy → Kinetic Energy → Heat Energy 

Kinetic energy to  heat energy=  performs work, i.e. the application of force over distance= physical work- moves the water + forms the river into what we see.' 

VALLEY PROFILES- Cross profile

Cross profile

The valley cross profile = view of the valley from one sie to another. 

Valley cross profile of a river in an upland area = a v-shape, with steep sides + a narrow bottom. 

Upper course: Dominant vertical erosion by the river= narrow steep-sided valley where the river occupies all of the valley 

Middle course: Lateral erosion= widens the valley floor = distinct valley bluffs+ a flat floodplain.

Lower course:  Lack of erosion+ reduced competence of the river= large-scale deposition + a very wide flat floodplain in which the valley sides = difficult to locate

VALLEY PROFILES- Long profile

Long profile

The long profile of a river= changes in altitude of the course of the river from its source, along the entire length of its channel, to the river mouth. 

 Long profile= smoothly concave

                    = gradient being steeper in the upper course+  becoming progressively gentler towards the mouth. 

Irregularities - gradient fequenetly occur = represented by rapids, waterfalls or lakes.

Rejuvenation= marked breaks + changes in slope=  knick points

Rejuvenation = sea level (in relation to the land) falls or when the land surface rises= allow the river to revive its erosion activity in a vertical direction= the river adjusts to the new base level- in its lowest reaches+ then progressively inland. 

RIVER PROCESSES- Deposition

A river deposits when there is a decrease in its level of energy= it is no longer competent to transport its load.

 Deposition usually occurs when:

• A reduction - gradient of the river (e.g when it enters a lake)
• The discharge = reduced (such as during and after a dry spell of weather)
•  Shallow water  (inside of a meander)
• Increase - calibre (size) of the load (due to a tributary bringing in larger particles, increased erosion along the river’s course, or a landslide into the river)
• The river floods + overtops its banks= reduced velocity on the floodplain outside the main channel

 Largest fragments = first to be deposited then - successively smaller particles
Finest particles = may never be deposited. 
Pattern of deposition = reflected in the sediments found along the course of a river. 
The channel of upland rivers = filled with large boulders. 
Gravels, sands and silts = carried further +  often depoisted further downstream. 
Sands and silts are deposited = flat floodplains either side of the river in its lower course.