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Rivers and Flooding
N/A
49
Geography
11th Grade
01/11/2014

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Cards

Term
What is a drainage basin?
Definition
The catchment area from which a river system obtains it's water
Term
What is watershed?
Definition
Area of high land which separates to drainage basin
Term
What is the water balance and it's equation?
Definition
The balance between the river's inputs and outputs.
Precipitation = Streamflow + Evapotranspiration +/- changes in storage
Term
What is river discharge?
Definition
Volume passing a measuring point in a river at any given time
Term
Equation for drainage basin discharge
Definition
Precipitation - Evapotranspiration +/- Changes in storage
Term
What is lagtime?
Definition
The difference between peak rainfall and peak discharge
Term
Name some factors that affect discharge
Definition
- Intensity and duration of a storm
- Precipitation
- Rain before storm
- Soil type
- Impermeable rock types
- Size and shape of drainage basin
- Slope angle
- Temperature
- Vegetation
- Land Use
- Urbanisation
Term
Factors resulting in a flashy hydrograph (sharp points and jumps)
Definition
- Short lived but intense rainfall
- Impermeable underlying rock e.g. granite
- Urban area
- Small round drainage basin
Term
Factors resulting in a long hydrograph
Definition
- Permeable rock e.g. limestone
- Dense woodland vegetation
- Elongated drainage basin
- High infiltration capacity of the soil
Term
Explain Abrasion
Definition
The process by which the bed and banks are worn down by the rivers load.
The river throws these particles against the bed and banks (sometimes at a high velocity)
Term
Explain Attrition
Definition
Material carried by the river bumps into each other and are smoothed/broken down into smaller partices
Term
Explain Corrosion
Definition
The chemical action of river water, the acids slowly dissolve the beds and banks
Term
Explain Hydraulic Action
Definition
The process where air becomes trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.
Term
Explain Vertical and Lateral Erosion
Definition
V = downwards, usually in upper reaches of land (high above sea level)
L = sideways, in middle/lower reaches due to lots of energy near sea level
Term
Why does vertical erosion dominate?
Definition
The river attempts to cut down to it's base level. High river level/velocity makes the river cut into it's valley by abrasion and hydraulic action
Term
Why does lateral erosion occur more frequently than vertical?
Definition
The river possesses a lot of energy which is used to widen the valley. The strongest current occurs on the outside of the bend and hydraulic action causes the bank to collapse.
Term
Explain Traction
Definition
Boulders and pebbles are rolled along the river bed at times of high discharge
Term
Explain Saltation
Definition
Sand particles are bounced along the river bed by the flow of water
Term
Explain Suspension
Definition
Fine clay + sand particles are carried along within the water even at low discharges
Term
Explain Solution
Definition
Some minerals dissolve in water such as calcium carbonate (this requires very little energy)
Term
Factors that affect erosion
Definition
- Weight of water (more mass more energy)
- Height of river (higher = more energy)
- Steepness (controls speed and KE energy)
- Friction
Term
Factors that effect deposition
Definition
- When gradient is reduced
- When discharge is reduced
- When in shallow water
- When size of load increases
- When there is reduced velocity on floodplain
Term
What is sediment yield?
Definition
The quantity of sediment transported through a stream cross section at any given time
Term
Factors affecting river load
Definition
- Extreme events (prolonged rainfall)
- Course of river
- Erosion
- Discharge + Velocity
- Size of drainage basin
- Rock type
- Precipitation
- Human Activity (deforestation, urbanisation)
Term
Define wetted perimeter
Definition
Total length of the river bed and bank sides that are in contact with the river
Term
Define Cross Sectional Area
Definition
Width x Mean Depth
Term
Define Hydraulic Radius
Definition
Cross sectional area / Wetted Perimeter
Term
A higher wetted perimeter means..
Definition
more friction, more energy lost and a slower velocity
Term
A higher hydraulic radius means..
Definition
less contact with banks for area of water, less energy lost, more efficient
Term
Velocity is influenced by:
Definition
- Channel shape in cross section
- Roughness of bed/bank
- Channel Slope
Term
A long profile...
Definition
illustrates the changes in altitude of course of the river from source to mouth
Term
What is the idealised long profile?
Definition
- Smoothly concave
- Graded profile, river is balanced between erosion and deposition
Term
A river changes in these ways..
Definition
- Erosion types and amounts
- Transportation types and amounts
- Deposition types and amounts
- Material sizes found in the river
- Landforms found on the river course
- Cross profile of the valley
Term
Characteristics of the river's upper region:
Definition
(In order of travelling downwards)
- Source: soggy, messy land
- Releases water through drips
- High altitude
- V Shaped Valley
- Reservoirs
- High Energy
- Most powerful, a lot of erosion
- Steep Gradient
- Broad Curves
- Waterfalls
Term
Characteristics of the river's middle region:
Definition
- Broadens out
- Lower altitude
- Energy declines
- Rocks are smaller/smoother
- Meanders are formed
Term
Characteristics of the river's lower region:
Definition
- Marshland
- Meanders are bigger and wider
- Floodplain
- Estuary mud, everything deposited
Term
How are potholes formed?
Definition
Abrasion occurs as turbulence swirls a rivers bedload in a circular motion causing it to rub and scrape out holes. These are then enlarged
Term
How are rapids formed?
Definition
A sudden increase in channel slope. They are steep sections of river with turbulent flow where there are several sections of hard rock
Term
How are waterfalls formed?
Definition
- Band of soft rock meets hard rock. Soft erodes more quickly making a step in bed.
- Water flowing over step speeds up (lack of friction) water has greater erosive power causing more s. rock erosion and undercutting of h. rock.
- H. rock can collapse. Deep plunge pool is created by abrasion at foot of waterfall from turbulence.
- Over time: more undercutting = more collapse, waterfall retreats leaving a steep sided gorge
Term
Waterfall case study
Definition
River Tees:
- NE England, in the Pennines, flows east to North Sea
- H. rock = Whinstone. S. rock = Limestone
- 22 metres High
- 500m Gorge
Term
How are meanders/ox bow lakes formed?
Definition
- Alternating pools and riffles
- River channel is deeper, more efficient and more erosive power (although energy is lost over riffles due to friction)
- Side to side flow due to spacing of pools and riffles
- Water speeds up and twists/coils
- Helicoildal flow causes more erosion and therefore more deposition on inside bend
- Combinatation exaggerates bends creating meander
- Neck of meander is broken through during flooding, deposition closes loop leaving lake
Term
Explain rejuvination
Definition
- Increase in river energy
- Caused by a change relative to sea level giving it more PE
- Making it higher above sea level so has to erode downwards
- River adjusts to new base level
Term
Rejuvination: knick points
Definition
- Sudden break in gradient
- It marks the limit of how far inland the river has adjusted to new base level
- knick point moves upstream
- sharply defined creating waterfalls/rapids
Term
Rejuvintation: River terraces
Definition
- Former floodplains that are left at a higher level after rejuvination
- Terraces are cut back as the new valley is widened by lateral erosion
Term
Rejuvination: Incised Meanders
Definition
- Renewed energy means meanders are incised/deepened
- happens slowly: deep (ingrown meanders)
- happens quickly: gorge (entrenched meanders)
Term
Causes of flooding
Definition
- Urbanisation (less vegetation)
- Decreased lag time
- More precipitation
- Impermeable rock types (more surface runoff)
- Small drainage basin
- Low lying ground
Term
How are deltas formed?
Definition
- River reaches sea, energy is absorbed
- River deposits load, builds up on sea bed
- Rises above sea level, partially blocks river mouth
- River braids into several distributaries to reach sea
Term
How are levees formed?
Definition
- During a flood material deposits across whole flood plain (as river loses velocity + energy due to friction)
- Heaviest material is dropped first closest to channel
- Material builds up and formes levee (natural raised embankment)
Term
What is braiding?
Definition
- River carries vast amount of eroded sediment
- If velocity drops/load becomes too much it's deposited
- River divides into man small winding channels which will eventually open up to form one
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