Theme 2- Rivers and River Management

Theme 2- Rivers and River Management

1. River processes

   • Rivers have gravitational energy so they transport load (boulders, gravel, sand, silt) downstream
     • When rivers have high energy (eg after heavy rainfall or if it flows quickly), they erode. When rivers have low energy, they deposit
   • Erosion
     • Abrasion- Scraping, scouring and smashing action of the load carried onto the river banks
       • Attrition- When pebbles and rocks carried smash against one another so they get smaller and more rounded
         • Hydraulic action- When friction and pressure cause water to be forced into cracks of the river bank and bed, causing the particles to break apart
           • Corrosion- When weak acidic water disscolve rocks such as limestone and chalk
   • Transportation
     • Solution- Dissolved minerals carried within water
       • Suspension- When small particles such as sand and silt are carried by the flow
         • Saltation- Small stones are bounced along the river bed
           • Traction- Large stones and boulders are rolled along the river bed
     • Factors affecting transportation
       • Size of particles
         • Type of rock as sofer rocks like limestone are more like to be dissolved as they are soluble
           • The velocity of the river
   • Deposition
     • Caused by a decrease in the gradient or velocity of the river
       • In floooding, rivers overflow onto the floodplain so they deposit material after floding
         • An increase in the size of sediment casues deposition
           • When there is shallow water (eg in the inside of a meander bend) rivers deposit its load

2. River landforms

   • V-shaped valleys
     • Formed in the upper course
       • The water erodes sharply downwards(vertical erosion) and cuts the bed like a knife, forming a steep valley
         • Rain washes stones ans soil from the sides so the valley becomes V-shaped
           • Steep gradient+high energy= lots of erosion
             • The river winds and bends and avoids areas of hard rock, creating interlocking spurs
   • Meanders and oxbow lakes
     • Water flows faster on the outer curve of its bend and slower on the inner curve
       • The outer bend is eroded whilst the lower bed has depsoited materials
         • As the outer bend gets eroded (forms a river cliff), the inner bedn grows (forms a point bar)
           • Over time, the process continues so the meander becomes more loopy
             • An oxbow lake is a cut-off meander
               • As erosion continues, the neck of the meander gets narrower
                 • During flooding the river takes a shortcut stright across the neck
                   • The loop of the meander gets sealed off
                     • In time, the lake gets covered by weeds and soil and disappears
                       • Happens in the middle course
   • Waterfalls and gorges
     • Water tumbles over a ledge of hard rock and erodes the sofer rock underneath it faster
       • This leaves a ledge of hard rockand a plunge pool
         • Overhang makes the ledge unstable so it falls into the plunge pool
           • This is repeated so the water reatrests upstream and carves out a gorge
             • Happens upstream
   • Floodplains
     • In the lower course, the river has a high voulme and a large discharge and deposition happens
       • A floodpalin is an area around a river that is covered in times of flood
         • Very fertile as it has alluvium deposits
           • More deposition= build-up of alluvium, which creates levees, which raise the river bank
   • Estuaries
     • it is where the river meets the sea.
       • The river here is tidal so during high tides, large areas of valey floor will be flooded.
         • As the tide falls, materila will be deposited, creating mudflats, which are important habitats fro wildlife

3. The Drainage Basin

   • Factors affecting the drainage basin
     • Geology- Impermeable rocks and clay soils do not allow precipiation to inflitarte, increasing surface runoff, which can increase the discharge and thus increase flooding risk
       • Deforestion+Afforestation- Trees reduce surface runoff due to interception as well as soaking up groundwater. More trees= less discharge and vice versa
         • Urbanisation- Buildings and roads are impermeable, increasing surface runoff sue to less inflitartion. Also, drains remove rainwater quickly from urban areas towrds rivers, increasing discharge
           • Amount and duration of rainfall- Haevy rain can cause floods as there is too mich water to infiltarte into the ground so more surface runoff. Long rain times causes the soil to get oversatuared so more surface runoff so higher discharge
   • Definitions
     • Drainage basin- an area of land drained by a river and its tribuateries
       • Watershed- The boundary between 2 drainage basins
         • River channel- Where the water flows
           • Confluence- Where two rivers meet
             • Source- the start of a river
               • Tribuateries- A small river joining the mian river
                 • Mouth- Where the river meets the sea
                   • Interception storage- Water stopped by vegetation
                     • Surface storage- Water stored on the surafce of the eartg
                       • Infiltartion- Water that soaks from the surface to soil
                         • Percolation- Water travelling from soil to rock
                           • Soil storage- Water stored in the soil
                             • Groundwater storage- Water stored in rocks
                               • Through flow- Water that travels through soil
                                 • Groundwater flow- Water that flows through the soil
   • Storm hydrographs
     • A hydrogarph is a way of displaying hwo the discharge of a river can chnage over a ranfall event
       • Discharge is the voulme of water flowing in a river and is measured in cumecs. Peak discharge is when a river is at its mots full
         • Lag time is the time taken for a channel to reach peak discharge after peak rainfall
           • Rising limb can be steeper due to less inflitartion
             • Peak discharge can be higher due to less infliltartion
               • Lag time also depends on the factors affecting a draingae basin eg longer lagtime due to more infiltration and groundwater flow

4. Case Study- The Cumbrian Floods- 2015

   • Cause
     • The warm air from the mid-atlantic caused relief rainfall over the Cumbrain mountains. The warmer the air, the more moisture it has
   • Event
     • In December 2015, Storm Desmond set a new record with 341.4mm of rain faling over 24 hours
       • Cockermouth was mainly affected despite its flood defences in response to previous floods
   • Previous flood defences
     • River dredging took place after 2009 to increase the capacity of the river
       • A self-closing flood barrier was installed in 2013 to increase the height of the river banks
         • The Environmental Agency moniters the area for potential flooding and issues flood warnings (RAG)
           • The government has peldged to spend £2.3 billion on flood defences between 2016 and 2022
   • Impacts
     • Social
       • Over 43,000 homes suffered from power cuts
         • 5200 homes were affected by flooding
           • One person died
     • Economic
       • Many businesses suffered from property damage
         • Estimated cost was £500 million across Cumbria. Government provided £50 million for flood victims and businesses
     • Environmental
       • Vegetation was flooded, causing habitat loss
         • Heavy rain washed nutrients fro soil
           • Rivers were largely altered sue to more erosion

5. Flood management strategies

   • Hard Engineering
     • Embankments- Raising the banks of a river so it can hold more water.
       • Pros- Allows for floodwater to be contained within a river. Cons- Water speeds up, increasing flood risk downstream+ looks unnatural
         • Levees- Embankments built along rivers so river can hold more water
           • Pros- Allows the floodplain to be built upon. Cons- If levees are breached, there is a risk of severe flooding+ expnsive
             • Dams- Huge walls built across rivers and resevoirs are built behind dams. Floodwater is caught by dams and is realesed by a steady flow throughout the year
               • Pros- Resevoirs provide a reliable water source and turbines in dams generate electricity. Also allows for irrigation. Cons- Wildlife affected as fish migrate upstream to breeding groungs+ very expensive
                 • Straightening- River seppeds up the water so high volumes of water can pass quickly
                   • Pros- Reduces flood risk in built-up areas. Cons- Increases flood risk downstream due to speeding of river
                     • Dredging- Makes the river deeper to hold more water
                       • Pros- More water held into the channel. Cons- Needs to be done frequently
                         • Flood relief channels- Water is diverted from areas being protected
                           • Pros- Removes excess water from the river channel to reduce flooding. Cons- Expensive+ relief channel can also flood if water levels continue to rise
   • Soft engineering
     • Flood plain zoning- Allowing only ceratin land uses on the flodplain to reduce flooding risk on buildings
       • Pros- Costlier buildings moved further from river so lower flood risk.+ less damage= less insurance claims Cons- Not always possible to change current land uses+ restricts development of an area
         • Wetalnd and river banck conservation- Wetland store water ad slow it down so planting vegetaion increases interception, increasing lagtime and reducing discharge
           • Pros- Creates wildlife habitats+ vegetation protects against soil erosion. Cons- Less alnd available for farming
             • Flood warnings and prepartion- Environmentqal agencies monitor rivers and issue warning via th emedia so people can prepare for floods
               • Pros- People have time to protect theor properties and evacuate+ reduces insurance claims. Cons- Some people can't access warnings and flash floods happen to fast for a warning to be effecive
                 • River restoration- Making the river more natural though man-made levees and allowing natural flooding
                   • Pros- Less maintainacne needed as river is left in its natural state+ river provides a better habitat to wildlife. Cons- Local flood risk increases

6. Views on flood defences

   • For
     • Planners advice that permeable materials such as gravel are used for more infiltartion
       • Soft engineering moves flooding away from urban areas
         • Expensive defences guarenatee home protection (home owner)
           • Peat bogs allow for more water to soak into marshland and protects habitats (envoronmnetalist)
   • Against
     • Motorists want impermeable materials such as tarmac on roads
       • Hard engineering increases discharge downstream and affects other towns (River scientist)
         • Floodplains must not be built upon as there is a higher flooding risk (housing minister)
           • Drains carry water downstream, increasing flooding risk (environmentalist)

7. The River Tees Case Study

   • Upper Course
     • Rises from the Penines and is located in the moorlands of Cumbria in soggy land
       • Annual rainfall is 1200mm and altitude is 800m
         • Bedload is large and angular and land is mainly made of impermeable rocks
           • There is a V shaped valley
             • A resevoir is built to store water for urban areas downstream
   • Middle Course
     • Bedload is medium and rounded and altitude is 350m
       • V-shaped valley becomes flatter and meanders and floodpalins forms
         • Highforce Waterfall is made of Whin sill (hard rock) and is 21m high and forms a gorge
           • Lateral erosion happens so more depsoition happens and river is wided and meanders erode the valley floor, creating a U shaped valley
             • More land is used for farming and pasture
               • Flood defences-Yarm
                 • Cost of defences was £2.1 million
                   • Embankmenst installed to increase the level of river banks
                     • Gabions reduce erosion along river banks
                       • Street lighting and flower bed placed to visually improve the area
   • Lower Course
     • Channel becomes wider and deeper
       • Bedload made of fine silt
         • Land use is mainly industrial (steel and iron factories) and there is a nuclear power plant
           • Levees formed sue to flooding
             • Estuary important for wildlife such as birds
               • River has been straightened for navigation+ to reduce flooding
                 • Flood defence- Tees Barrage
                   • Barrage stops tidal flows upstream and has opened 25km clean navigable waterway for leisure
                     • Regeneration of derelict land and attracted new jobs and businesses. Also led to increased tourism
                       • Barrage cost £54 million
                         • Around barrage there is a caranvan site, restaurants, woodlands, encouraging tourism