Geog 1 - Rivers and flood management - Key terms Flashcards
Abrasion
The wearing away of the shoreline by sediment carried by waves. Also erosion by friction scraping, scouring and rubbing of load in contact with banks and bed. (corrosion)
Afforestation
Planting a substantial area of the catchment with trees to increase interception storage and evapotranspiration.
Aquifer
Rocks, porous and permeable which can store water.
Antecedent conditions
Moisture that was in the soil prior to more rain falling. (also called antecedent moisture)
Attrition
The reduction and rounding of particles of sediment carried in water by repeated collision with each other and the shore.
Bank full
The state of flow of a river when it completely fills its channel.
Baseflow
Water that reaches the channel largely through slow throughflow and from permeable rock below the water table.
Bedload
Larger material, cobbles , pebbles and sand transported by saltation.
Braided stream
Made up of many interconnected channels separated by small islands of deposited materials.
Capacity
Is the total volume of sediment a river can carry.
Calibre
Is the measurement of the long axis of sediment in a river.
Catchment area
The land which drains water into a river system.
Channelisation
Attempted to alter the natural geometry of the watercourse. Historically the chosen hard engineering or structural solution “against nature”.
Char
An island formed from silt deposited in a delta. The land is about at seas level. It is very fertile and attracts settlers desperate for land. However, it can easily be washed away by monsoon floods and cyclones. Even if the cyclones do not destroy the chars, they flood them with salt water which reduces their fertility.
Competence
Is the maximum size (calibre) of load a river is capable of transporting.
Condensation
The process by which water vapour is converted into water.
Channel enlargement
Deepening / widening the channel to accommodate larger discharge and get it out of the area quicker.
Channel flow
The movement of water within the river channel.
Contour ploughing
Farmers work around hills not up and down - to reduce runoff, soil erosion and silting of river channels.
Corrasion
Erosion by friction of load in contact with banks and bed (abrasion)
Corrosion
Includes the dissolving of carbonate rocks (e.g. limestone) in sea water and the evaporation of salt crystals which expand on formation and help the rock to disintegrate.
Cross sectional area
The total length of the bed and the bank sides in contact with the water in the channel.
Culverts
Rivers in cities may be covered over or in concrete pipes to allow development and remove the increased amount of runoff created by impermeable surfaces.
Dams
Barriers engineered to hold back water, may be multipurpose; storage, flood management, recreation.
Deficit
A shortage e.g. soil moisture
Deltas
Form when the amount of sediment delivered at the mouth of a river exceeds the amount removed by waves and tidal currents.
Deposition
Decrease in energy makes a river no longer competent to carry the load so it deposits. E.g. Enters lake, sea, floods onto wide floodplain, shallow inside of meander or drought.
Discharge
The volume of water flowing in a river per second measured in cumecs (Cubic meters per second)
Dissolved load
(Or solution load) most common in chalk or limestone areas where weak acids (e.g. carbonic acid from rainwater) may remove material in solution.
Distributary
Small channel which leaves the main river on a delta
Diversion spilways
Overflow channels which can take surplus water during times of flood. The Bonnet Carre Floodway begins 50 KM north of New Orleans. In times of flood it diverts excess water from the Mississippi along a 9 Km spillway, through 350 small bays (reservoirs), into Lake Pontchartrain, and eventually into the Gulf of Mexico.
Do minimum
Maintain existing flood measures but no more.
Do nothing
An approach that only deals with issues when they arise.
Drainage basin
The catchment area of a river and its tributaries.
Dredging
To remove sediment from the river bed to increase the depth of the channel.
Dynamic equilibrium
Rivers are constantly changing over time to reach a state of balance with the processes that determine their form. As the flows of energy and materials passing through a river system vary, the river changes to move towards this equilibrium.
Eddies
Fast - flowing circular currents of water in the river flow.
Erosion
The wearing away of the surface of the land. It includes the breakdown of rock and its removal by wind, water or ice.
Eustatic
Changes in sea level caused by variations in the amount of water in the oceans.
Evaporation
The transformation of water droplets into water vapour by heating.
Evacuation
In the worst situations people are alerted to vacate their properties.
Field capacity
The normal amount of water that can be held in the soil.
Evapotranspiration
The loss of water from a drainage basin into the atmosphere from the leaves of plants.
Flocculation
River load particles join together on contact with the salt in the sea water, increasing their weight and causing them to drop / be deposited.
Flood
A temporary excess of water that spills over onto land.
Flood Abatement
Reducing the possibility of flooding by managing land use upstream e.g. afforestation.
Flood embankments
Often made up of earth with rubble fill - more common in rural areas - simply the building up of levees
Flood forecasts
The meteorological office inform the environment agency of any flood hazards from precipitation.
Flood interception schemes
Intercepting channels, divert only part of the flow away, allowing flow for town and agricultural use and flood retention areas, polders and washland.
Flood proofing
May be temporary; sandbags to raise the height of flood walls and protect household doors OR permanent; new buildings can be constructed with flood-proof ground floor walls, or have flood gates that can be moved into place, or on stilts.
Flood Relief Channel
Constructed to redirect excess water upstream of a settlement via an alternative route.
Flood prediction
Records of river discharge and flooding are kept in order to predict future events.
Flood strategy reservoirs
Aim to store water in the upper reaches of the river catchment. Water is held back y the dam and released in a controlled way. (aka Flood storage reservoirs).
Flood walls
Increase height of channel, preventing water spilling out over the floodplain - common in cities.
Flood plain
The valley floor is broad and flat created by successive flooding events depositing material usually called alluvium.
Floodplain zoning
Planning controls based on maps of relative risk.
Flood warnings
The Environment Agency warns residents when floods are likely to occur, by loudhailer, telephone and with emergency officers on the ground.
Frequency
How often floods occur.
Gorge
The narrow, rocky, steep-sided valley, created by recession of a waterfall.
Graded profile
Long profile of a river where erosion, transport and deposition are in equilibrium,
Groundwater storage
The storage of water underground in permeable rock strata.
Hard engineering
Flood management strategies that are structural measures offering protection through engineering. Expensive, need maintenance and relatively short term solutions.
Helicoidal flow
Water flow pattern where the fastest current spirals across the channel and downstream in a corkscrew motion.
Hjulstrom’s curve
Graph showing the relationship between velocity, erosion and deposition. Size of particles are clay, silt, sand, gravel, pebble and boulders (in order)
Hydraulic action
Force exerted by moving water on the bed and banks of a river.
Hydraulic radius
The ratio of the cross sectional area of the channel and the length of its wetted perimeter.
Hydrograph
A graph showing for a given point on a stream the discharge, stage (depth), velocity or other properties of water with respect to time; a graphical representation of stream discharge (volume / time) during a storm or flood event.
Groundwater flow
The deeper movement of water through the underlying rock strata.
Infiltration
The downward movement of water into soil surface.
Infiltration rate
The speed (mm/sec) at which water passes through the ground surface into the soil (faster in sandy soils)
Insurance schemes
Modifying the burden of loss
Intercepting Channels
Diverting only part of the flow, allowing water for urban and agricultural use e.g. Great Ouse Protection Scheme.
Interception
The prevention of precipitation from reaching the Earth’s surface by tress and vegetation.
Interception storage
The total volume of water held on the surface of vegetation.
Isostatic
Changes in sea level resulting from the rise and fall of land masses.
Kinetic energy
Erosion caused by the mass of water in motion.
Knick point
A break of slope in the long profile of a steam. Often the upper limit along which down cutting triggered by rejuvenation has reached - marked by rapids and waterfalls.
Land use management on floodplain
Records of historic flood events can be used to predict the extent of future flooding. Maps of relative risk are the basis of planning controls.
Lateral erosion
Middle and lower sections where river has high energy especially if close to bank full. Widens the valley especially strong on outside meanders where hydraulic action undercuts river cliffs.
Levees
Natural parallel ridges formed by deposition of coarser material closer to the river channel during flood events, alongside rivers. May be reinforced by engineers to form flood embankments.
Lining the channel
With concrete mattresses (25m by 8m long on the Mississippi) making it smoother; reduced friction increases velocity away from the town.
Load
The material carried by a river.
Magnitude
The size of the flood.
Meanders
Bends in a river formed by Helicordial flow, with erosion on the outside and deposition on the inside. Asymmetrical cross section. realisation
Naturalisation
Restoring rivers to a state closer to their original course by removing hard engineering and other restrictive structures. A more holistic and “with nature” approach, sustainable whole basing management.
Overland flow
The movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense for infiltration to occur.
Peak rainfall
The time when the maximum amount of rain was falling.
Percolation
The gravity flow of water within soil.
Point bar
Sediments laid down on the inside of a meander.
Potholes
Are formed by corrosion (abrasion). Pebbles carried by the river are swirled around on the river bed.
Potential energy
The erosive power that is related to the height the water has to fill downhill to reach sea level. (gravity)
Precipitation
All forms of moisture that reaches the Earth’s surface, including rain, snow and dew.
Pre conditions
Moisture that was in the soil prior to more rain falling. (also called antecedent moisture)
Profile
Changing height of the river from source to mouth, also called long profile.
Rapids
Rapids are stretches of fast-flowing water tumbling over a rocky and shallow riverbed.
Realignment
(Straightening) shortening the river course by removing meanders, which increases gradient therefore removing water more quickly.
Recurrence interval
The interval at which particular levels of flooding will occur.
Regime
The annual pattern of river discharge.
Rejuvenation
A renewal of energy which permits accelerated erosion and transport. E.g. SE England - sea level relative to land is falling, so rivers adjust to new base levels.
Removal of settlement
E.g. Valmeyer village flooded by Mississippi in 1993 relocated 2 miles and 400 feet higher.
Resectioning
Widening and deepening of a channel to improve its hydraulic efficiency -> increases capacity.
Revetments
Made of concrete, steel piling or gabions are used to strengthen banks.
Riffles and pools
Shallow (riffles) alternate with deeper (pools) sections along the meandering sections of a river.
Risk categories
For floods low; 1 in 200 years or less; moderate 1 in 75 years to 1 in 200 years significant 1 in 75.
River cliff
Outside of a meander - steep undercut bank.
River restoration
Returning uplands into peat bog increasing absorption to historic levels and delaying water entering streams that threaten towns like Sheffield. Controversial (land owners) and costly.
Roundness
The shape of the sediment in a river which changes downstream as a result of attrition. Highly angular –> smooth / rounded.
Runoff
Water flowing over the land surface as channel flow and overland flow. (aka surface flow and overland flow).
Saltation
Small stones bounce and leap-frog along the channel bed.
Sinuosity
The curving nature of a meander described as; actual channel length divided by straight line distance.
Sluice gates
Barriers that hold back water, may even pump water in the opposite direction to flow with a pumping station.
Soft engineering
Flood management strategies that are non-structural measures more “naturalistic”.
Soil moisture
The total amount of water, including water vapour, in an unsaturated soil.
Solution load
Dissolved minerals transported within the mass of the moving water.
Stemflow
Flow down plant trunks and stems following interception.
Stormflow
Water that reaches the channel largely through runoff. This mat be a combination of overland flow and rapid throughflow.
Straightening
To increase the velocity of removal of water near to an urban area - may cause flooding downstream. May make navigation quicker.
Strata
Layers of rock.
Surface storage
The total volume of water held on the Earth’s surface in lakes ponds and puddles.
Surplus
More than is needed e.g. soil moisture
Suspended load
This is the bulk of sediment transported by a river and consists of muds, clay and sand. It is the reason why rivers appear muddy when bank full or approaching the river mouth.
Suspension
Sand and silt carried along by the flow of a river.
Temporal variations
Changes over time.
Throughflow
The movement of water downslope within the soil layer.
Traction
Large stones rolled along the river bed.
Urbanisation
an increase in the proportion of a country’s population living in urban areas.
Velocity
The speed and the direction at which a body of water moves (Metres per second)
Vertical erosion
Dominates upper reaches of river cutting into the bed by abrasion and hydraulic action.
Water budget
Relationship between inputs and outputs in a drainage basin. May be shown as a graph.
Washland restoration
Wet lands that are deliberately allowed to flood at times of high discharge. Allowing water flood over agricultural land in the floodplain and have that as part of the management plan of farm.
Waterfalls
Is a steep or vertical part of the river. Waterfalls occur when a band of hard rock lies across the river with softer rock downstream which is more rapidly eroded.
Water meadows
Return farmland to marshes, was land and allow river meander.
Watershed
Boundary of drainage basin, usually ridges of higher land.
Water table
The surface of the saturated later of soil or rock.
