Geography A/S Level - Rivers, floods and management

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The drainage basin, hydrocycle and water balance

  • A drainage basin is an area of land that is drained by a river or tributary. Its boundary is marked by a ridge of high land beyond which any precipitation drains into adjacent rivers.
  • The hydrocycle is the constant recycling of water as it consists of evaporation/evapotranspiration, the movement of water to the sea and precipitation into rainfall. This can occur in a drainage basin and is called a drainage basin system

Waterbalance shows the state of equilibrium in the drainage basin between inputs and outputs and can be expressed as:

P = Q + E +- Change in storage
P = Percipitation
Q = Run off
E = Evapotranspiration 

Seasonal variations can be found which is result of a surplus of moisture in the soil and the defict of moisture which is usually found in the summer months.

  • Soil 'Recharge' is when evapotranspiration levels begins to drop and precipitation levels to increase. This is usually found around the season of Autumn to Spring beforeevapotranspiration begins to rise again above the precipitation levels. 
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Factors affecting river discharge (Hydrograph) 1/3

There are 8 main factors that affect river discharge these are:

  • Basin size, shape and relief
    Size: If a basin is small then the rainfall will reach the main channel much quicker. Therefore lag time would be shorter. Longer/Bigger basin means much longer lag time.
    Shape: A circular basin has a much shorter lag time and a higher peak flow than a elongated basin. This is due to the water being equidistant from the river in a circular basin in comparison with an elongated basin where the water is at different distances.
    Relief: Slope of the basin and its valley sides. As a steeper valley/basin slopes cause water to reach the main channel thus reducing lag time. 
  • Types of Percipitation  
    Prolonged Rainfall :Ground becomes saturated; therefore high levels of surface run off.
    Intense Storm - Rainfall intensity is much higher than inflitration rates resulting in surface runoff.
    Snowfall - Water is held in storage and whent he temperature rapidly increases. The meltwater soon reaches the river. Frozen ground can impend infiltration rates. 

  

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Factors affecting river discharge (Hydrograph) 2/3

  • Temperatures can affect river discharge as extreme cold can freeze the ground restricting the infiltration rates thus resulting in surface run off while extreme heat will increase levels of evapotranspiration results in a deficit in soil moisture content. This means high levels of infiltration occurs due to 'soil recharge'. As a result the rivers discharge is lowered.
  • Land Use as planting vegetation can cause interception which results in less water reaching the main river.
    Fact: Rain forests intercept up to 80% of water in comparison to arable crops which intercept on 10%. 
  • Rock Type:
    Porous - Allows faster infiltration, less surface run off. E.g. Sandstone.
    Pervious - Allows water to flow along the joints. E.g. Granite with cracks (which becomes permeable)
    Impermeable - No infiltration occurs, resulting in surface run off E.g. (Granite)  
  • Soil Type - Depends on the soil moisture rate (Water balance table). 
    Pore spaces allow different volumes of water to be stored. Clays have small pore spaces and are less connected therefore reducing infiltration and through-flow but encourages surface runoff 
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Factors affecting river discharge (Hydrograph) 3/3

  • Drainage Density which is the number of surface streams in an area. Can result in flash floods if the drainage density is too high. Drainage density is high on impermeable rocks and clays. Furthermore the more surface streams you have, the more risk you are of flash flooding due to short lag times. It is low when rocks are permeable and sands.
  • Tides and storm surges as high water levels prevents water from escaping into the sea. Thus creating a reservoir further up stream flooding the area. (Near the lower profile of the river) 

  • Storm Hydrographs
    Key Terms:
    - Approch Segment
    - Rising Limb
    - Peak Discharge (Peak Flow)
    - Lag Time
    - Stormflow
    - Baseflow
    - Bankfull discharge 
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Long Profile, Erosion, Transportation, Deposition,

  • A river's long profile is the actual route of a river. It is when you slice a river in half and look at it from a horizontal view. This allows you to see the route from source to mouth. 
    Many processes occur from source to mouth but the initial overview is that erosional processes occur closer to the source while deposition processes occur towards the mouth. As a result different landforms can be found along the river. This is due to the river losing energy along the journey.
  •  Transportation
     Bed-load - Large particles which cannot be picked up are moved along the river. It is bounced across the river bed in a hopping motion. Saltation occurs when pebbles, sand and gravel are temporarily lifted and bounced along the bed.
    Traction occurs when the largest cobbles and boulders rolls or slide along the bed
    (River has lots of energy) 
     Suspended load - Very fine particiles of clay and silt are carried by turbulence in a fast flowing river. The greater the velocity the large quantity and size of particles can be carried. Material held in suspension usually is takes up to 80% of the total load thus giving the water its brown or black colours. 
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Long Profile, Erosion, Transportation, Deposition,

  • Transportation continued.....
    Dissolved or solution load is when material such as limestone is dissolved due to the water containing minute levels of acid within it. It is constantly dissolved and removed in solution. Material held in solution only holds a small portion of the total load. 
  • Competence is the maximum size of material which is a river is capable of transporting.
  • Capacity is the total load actually transported.

Hjulstrom Curve

  •  Shows the relationship between velocity and particle size. It shows the critical velocities needed to initiate movement (erosion) or deposition (sedimentation) 
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Long Profile, Erosion, Transportation, Deposition,

Erosion

  • Corrasion occurs when the river picks up material and rubs it along its beds and banks, this wears them away by abrasion, rather like sandpaper. It can cause potholes to form in the river bed common known as Turbulent Eddies. 
  • Attrition is when boulders collide with other material. The impact breaks the rock into smaller pieces. In time angular rocks become increasingly rounded in apperance. This is how pebbles are formed. 
  • Hydraulic action is when the sheer force of the water hits the banks. The air in cracks is compressed and causes the pressure to increase. Overtime this extends the cracks and cavitations is a form of hydraulic action. This is the slowest and least and effective erosion processes.  
  • Corrosion is the continuous dissolving of materials around the circumference of the river due to carbonic and humic acid.
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Long Profile, Erosion, Transportation, Deposition,

Deposition 

  • Factors that cause deposition include a reduced discharge rate due a period of low precipitation. 
  • Velocity is lessed on entering a sea/lake resulting in a delta. 
  • Shallower water occurs on the inside of a meander. 
  • The load is increased rapidly due to landslide. 
  • River overflows its banks therefore the velocity outside the channel is reduced. 
  • The heaviest bed-load is deposited first. This is the reasons channels of mountain streams are often filled with large boulders. Large boulders increase the wetted perimeter. 
  • Gravel, sand and silt transported either as bed load or in suspension will be carried further to be deposited over the floodplain or near its mouth. 
  • Finest particules of silt and clay which are carried in suspension may be deposited where the river meets the sea, either to infill an estuary or to form a delta. 
  • Dissolved load will not be deposited but will be carried out to sea where it will help to maintain the salinity of the oceans. 
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Valley Profiles, Changing cross profile, potential

  • Base level is the lowest part of the river in which erosion by running water can take place. 
  • Every river in the world attempts to reach its graded profile. But this is stopped due to the different composition of rocks causing irregularities along its graded profile such as waterfalls and lake. 
    Erosion would be greatest at the waterfall while deposition would be greatest at the lake.
  • Changes in base level.
    Climatic - Effects of glaciation and changes in rainfall
    Tectonic - Crustal uplift following plate movement or Isostatic uplift due to glaciers melting.
    Positive change - Occurs when sea level rises to the land. This causes a decrease in the gradient of the river with a corresponding increase in deposition.
    Negative change - Occurs when sea level falls in relation to the land for the land rises in relation of the sea and a knick point is formed. Increasing the rate of fluvial erosion.  
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Valley Profiles, Changing cross profile, potential

Graded Profile

  • Every in the world attempts to achievea profile of dynamic equalibrium. This is when the river progressively decreases its gradient without and sudden increase or decrease. This is where a balance has been achieved between erosional and depositional processes. 
    Such a balance between energy and work cannot occur at a particular moment in time but is suggested as a average postion over a long time. 

Potential and Kinetic energy

  • In relation to rivers potential or stored energy is fixed by the altiude of the source of the streams in relation to the base level.
    Kinetic energy due to movement is generated yb the flow of the river which converts potential energy into moving energy. The amount of kinetic energy is determined by the volume or amount of water flowing down (discharge), the slope (gradient) down which its travelling and its average velocity (speed). An increase in speed or discharge results in an increase in kinetic energy.  
  • All channel processes are dependant on the amount of energy avaliable. 
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Changing channel characteristics

Hydraulic Radius

  • It is the ratio between the area of the cross section of the river channel and the length of its wetted perimeter.
  • Cross section = Width * Mean depth
  • Length of wetted perimeter = total length of the bed and bank sides in contact with water.
  • Larger Hydraulic radius means less amount of water in its cross section is in contact with the wetted perimeter. This creates less friction which  in turn reduces the energy loss and allows for greater veloctiy.
  • Smaller Hydraulic means a large amount of water is in contact with the wetted perimeter. This results in greater friction, more energy loss and reduced velocity.
  • Every river has a point of maximum velocity, this is a region of water just below the surface of the water and depends on the shape of the river for its location.
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Changing channel characteristics

Roughness of channel bed and banks.

  • A river which runs through coarse materials with numerous protrusions and over large angular rocks meets more resistance than a river with cohesive clays and silts forming its beds and banks.
  • This is usually the case as the river's upper course is usually smaller and runs through coarse material and angular rocks in comparison to its lower course where it has a large wetted perimeter with small rounded bed load.
  • Upper part of the river has the most turbulence, roughness, bedload in comparison to discharge and friction.
  • Lower part of the river has the greatest discharge, velocity, highest average hydraulic radius and greatest cross sectional area.
  • The roughness can be calculated by using Manning's coefficient of friction.
    V = (R^0.67*S^0.5)/n
    V = Mean velocity | R = Hydraulic radius | S = Channel Slope | N = Boundary roughness
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Changing channel characteristics

Channel Slope

  • More tributaries, water from surface runoff, through flow and groundwater flow all entering the main river causes the discharge, channel cross section area and hydraulic radius all to increase.
  • Furthermore less energy will be lost through friction and erosive power of bedload material will decrease.
  • All these factors contribute to the river flowing over a gradually decreasing gradient. (Concave long profile)
  • Velocity of a river increases as it nears the sea unless, like the Colorado it flows through desert where water is lost through evaporation or by human extraction for water supply.
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Landforms of fluvial erosion and deposition

  • Potholes are round to oval shaped holes in the bedrock of a river bed.  They are created where sediment accumulates within naturally occurring small depressions on the rock surface on the river bed.  Turbulent flow swirls the stones around in the depressions, widening and deepening them through the prolonged process of abrasion.  As the holes gets bigger even bigger debris can become trapped in the pothole, and this material is again used as an abrasive tool.

  •  Rapids are areas along the rivers course where water becomes more turbulent (often creating white water which canoeists and rafters love so much!) .  It is caused by a localised increase in gradient along the rivers gradient or where the river flows over alternating bands of harder and softer rocks.  These are often linked in with pool and riffle sequences (the rapids form the riffles).  The pools are areas of deeper water whereas the riffles are areas of shallower water. The pool is an area of greater erosion  as the water is deeper and therefore flows faster, whereas the riffles encourage deposition because they are shallower.  It is thought that it is within these features that meanders start to develop from straight channels, as the water is forced to move from side to side around deposited obstacles.

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Landforms of fluvial erosion and deposition

  • Waterfalls are typical of the upper valley, but can be found in the rivers lower courses where the process of REJUVENATION creates enough potential energy for vertical erosion to recommence closer to the rivers mouth. Created by rivers and area created by vertical erosion over bands of varying rock resistances by the processes of hydraulic action, abrasion and acid found in the water.
  •  Meandering rivers result in widening of the river valley and the production of Ox-bow lakes.  They are typical of the middle and lower course of a river where vertical erosion is replaced by a sideways form of erosion called LATERAL erosion, plus deposition within the floodplain.  This Helicoidal flow of the river causes the river to move laterally across the flood plain.
  • Braiding are basically rivers that have multiple channels and islands of sediment in between those channels.  We find braided rivers in deltas, in areas where the river channel banks are made of easily erodible material or in areas of high sediment load where discharge varies (e.g. glacial melt water rivers).
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Landforms of fluvial erosion and deposition

  • Levees and Floodplains, these landforms are very closely linked to meanders and ox-bow lakes and are combined erosion and deposition landforms.  they are popular for farming and (unfortunately) building for towns and settlements as they are flat features that can have back swamps.
    The river in the middle reaches erodes laterally, from side to side, because of the fastest velocities being on the outside edge of meanders. This means that the river erodes the valley sides and widens the valley floor to create a floodplain.
    Where the river is deepest, next to the river channel, the largest size sediment is deposited (as seen on the Hjulstrom curve) and towards the edge of the floodplain where the river is shallowest the smallest sediment is deposited. The net result is a multilayered floodplain with large banks of sediment called levees next to the river channel.
  • Deltas are depositional landforms that are created from the loading of sediment onto the land as the rivers capacity to carry that sediment is reduced.  They are dynamic areas that change rapidly due to continual recreation of land or the erosion of unstable island and land during storm and flood events. A large percentage of the world's population live on these landforms, due to their high agricultural productivity and proximity to ocean resources.
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Process and impact of rejuvenation

  • A negative change in base level increases the potential energy of a river, enabling it to revive its erosive activity. In doing so, it upsets any possible graded long profile. This is basically what a rejuvenation means; when the river recovers its ability to be erosive further down stream.
  • In doing so knickpoints are found, these indicate the maximum extent of the newly graded profile.
  • River terraces are remnants of former floodplains which, following vertical erosion caused by rejuvenation, have been left high and dry above the maximum level of present day flooding. They offer excellent sites for the location of towns.
  • Incised Meanders; there are two types:  
    Entrenched meanders have a symmetrical cross section and a result from either a very rapid incision by the river or the valley sides being resistant from erosion.
    Ingrown meanders occur when the uplift of the land or incision by the river is less rapid allowing the river time to shift laterally and to prodouce an asymmetical cross valley shape like pinsors.
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Physical and human causes of flooding.

Physical Causes

  • Excessive levels of perciptation occuring over a prolonged period of time. This leads to the saturation of soil and eventually surface runoff.
  • Intensive percipitaion over a short period of time, particuarly when the ground surface is baked hard after a long period without rainfall.
  • Melting of snow, particually when the subsoil is still frozen so that infiltration capacity is reduced.
  • Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low pressure weather systemds in mid-latitudes between 23.5 degrees of the equator bring abnormally large amounts of perciptation.
  • The nature of the drainage basin has an influence of 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.
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Physical and human causes of flooding.

Human causes

  • Urbanistion; as humans have developed urban areas on flood plains. Due to this concrete and tarmac are used in urban areas for roads and pavement. Such surfaces are impermeable, so perciptation is unable to infiltrate slowly into the soil similar to that found in vegeated areas. In addtion less interception from trees as they are all cleared away.
    Due to his a higher proportion of the rainfall makes its way to the town or city.
  • Furthermore surface water is channeled directly into drains and swers in urban areas, so percipitation reaches the river quickly whch in turns reduces the lag time overall between peak rainfall and peak discharge.
  • Natural rivers may be constricted by bridges which can slow down discharge and reduce the carrying capacity of the river.
  • Deforestation is a big factor mainly is less developed countries. Rapid deforestation has taken place of recent decades. In region such as South America and Asia; once trees are removed there is a greater risk of soil erosion and sediments finds its way into rivers obstructing them.
    FACT: Rainforests intercept up 80% more water than Arable Crops with 10%. Deforestation usually occurs to build urban areas or to make way to farms.
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Flood Management stratergies

Hard Engineering

  • Multi-Purpose Dams - Built to store water and regulate the discharge of the river. Water is stored in a resvior upstream, this protects the catchment downstream from potential flooding. Large dam projects are used for clean hydro eletric power for large scale irrigation schemes that opens up the interior of countries for transport and trade. (E.g. 3 Gorges dam China)
    The downside side is that it is very expensive as the 3 Gorges dam cost over $25 billion. Furthermore dams can disrupt migratory fish and the resviour created could cause the displacement of millions of people (3 million due to 3 Gorges dam)
  • Channelisation - Is an manmade attempt to alter the natural geometry of the river. The river can be deepned and widened to increase the capacity, this in turn increases the hydraulic efficinecy and allows for greater discharge. This helps prevents flooding and the channel can be made straighter, through the use of artifical cut offs. They can be realligned to increase the long profile which in turn increases the velocity of the flow. 
    In general channelisation is seen as an efffective means of increasing the capacity, hydraulic efficiency and discharge rivers. It has been successfully used to prevent erosion and flooding for large river systems like the Mississippi, but there remains some significant questions in regard to cost, ecological impact and their effectiveness to cope with high magnitude low frequency flood events.
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Flood Management stratergies

Hard Engineering

  • Dredging the river is the process by which bedload is removed from the channel of the river. This is achieved through either heavy industrial pumps and diggers or through dislodging sediment that then encourages the natural flow of the river to transport it.The benefit of using dredging is that it maintains the natural aesthetics of the river channel. However, it is costly and time consuming process that is only suitable for small section of the river, for example close to or within urban environments. In addition, the prosess has high ecological impact on natural ecology.
  • Other forms hard engeering include:
    - Building up Levees
    - Diversion Spillways
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Flood Management stratergies

Soft Enginerring

  • Bank Protection; It is to help prevent bank erosion. This can be achieved through providing bank support. There are a number of different methods. Gabions, which are wire cages containing stones allow for a softer appearance. Groynes or spurs  achieve the same affect through deflecting the fastest flow away from the bank; deposition builds up on the headward side of the groyne.
  • An even softer approach  is the planting of bank vegetation or by allowing vegetation to develop. This is achieved by simply not cutting it back. Tree planting along the banks provides greater stabilty and may reduce erosion. However allowing vegetation to recover in a unregulated way may in fact encourage greater channel roughness and meander migration.The effect would be a rise in the watertable and an increase in the liklihood of floods. In contrast vegetation clearance reduces channel roughness, discourages bank deposition and increases the hydraulic efficiency of the river.
  • Other types include:
    - Forecasts and warnings
    - Wetland just as parks
    - River restoration.

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Case Studies to learn

Case Study 1
Location: Bostcastle, N. Cornwall; Rivers Valency & Jordan
Date: 16 August 2004.
www.thegeographer.co.uk/userfiles/boscastle-flood-summary.doc

Case Study 2
Location: Yichang, China; River Yangtze
Date: 2009+
http://www.arch.mcgill.ca/prof/sijpkes/arch374/winter2001/dbiggs/three.html

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