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Rivers, Floods and Management

QuestionAnswer
Bearing the cost a 'do-nothing' approach that deals with issues when they arise - often adapted in locations with a low level of risk that experience small, regular floods.
Hydrograph a graph showing, for a given point on a stream, the discharge, stage (depth), velocity, or property of water with respect to time, a graphical representation of stream discharge (volume/time) during a storm or flood event.
Laminar flow A horizontal movement of water that travels over the riverbed without disturbing it
Hydraulic radius A measurement of the efficiency of the stream for moving water. Area divided by wetted perimeter.
Levees A small, natural embankment alongside a channel. Can also be manmade to prevent flooding
Flood A temporary excess of water which spills over onto land
Sinuosity Actual channel length / straight line distance. The curving nature of the meander.
Soft engineering aims to work with the river's natural processes, and tends to have less impact on the environment
Precipitation all forms of moisture that reach the earth's surface, including rain, snow and dew
Floodplain zoning Allows certain areas of the floodplain to flood naturally - these areas have a limited land use of grazing and recreation, while other more economically valuable areas are located in areas with a longer flood return period
Drainage basin An area of land drained by a river and its tributaries
Floodplain An extensive, flat area of land on either side of a river, which periodically becomes flooded
Urbanisation an increase in the proportion of a country's population living in urban areas. It is sometimes used to mean the process of moving from rural to urban areas
Pools Areas of deeper water, formed by deposition of fine sediment
Deltas Areas of sediment deposited at the mouth of a river while it enters a slow moving body of water such as a sea / lake
Riffles Areas of shallow water, formed by deposition of coarse sediment
Eustatic change Change in relative heights of land and sea due to change in sea level
Negative feedback loops changes to the system are met with responses that redress the imbalance, and lead the system back to the original state
Cost benefit analysis Considers costs and benefits of a particular scheme to decide whether it should be implemented. Economic, envirmonmental and social factors must all be considered.
Turbulent flow Consists of a series of erratic eddies, in a downstream direction. May produce minor whirlpools on surface.
Potholes Cylindrical holes, drilled into the rocky bed of the river by turbulent, high velocity water, loaded with pebbles
Rapids Develop where the gradient of the river bed increases without a sudden break of slope, or where the strea, flows over a series of gently dipping bends of harder rock. This increases turbulence and erosive power
Contour ploughing/strip farming done in semi arid areas to reduce amount of surface runoff and therefore flood liability
Percolation Downward movement of water within rock under soil surface due to gravity. Cannot happen within impermeable rock.
Diversion spillways/bypass/relief channels excess water is redirected upstream of a settlement and follows an alternative route, bypassing the settlement until it is further downstream.
Secondary flow (+example) Flows which exist within the main flow, for example heliocoidal flow - a corkscrew movement and a series of converging and diverging lateral rotations.
River terraces Formed due to rejeuvenation, either from lateral migration of incised meanders, or by a slow drop in sea level and therefore the floodplain, causing 'steps'.
Incised meanders Formed during rejeuvenation when meanders have more potential for downcutting. Have a lower channel. Very deep = entrenched meanders
Transportation further carrying of material downstream
Isolated System Has no input or output of energy or matter
Frequency How often floods occur
Hjulström's curve Illustrates the relationship between velocity and competence: velocity at which a sediment will normally be eroded, transported or deposited.
Afforrestation Increased interception slows down lag time, evapotranspiration reduces discharge. Effective in long term but trees take gtime to mature and large areas need to be planted
(examples of) Transfers infiltration, percolation, overland flow, throughflow, groundflow
Hard engineering intervenes directly with the river's natural processes, through structural methods (offering protection through engineering)
Isostatic change Land rises relative to the sea as a result of crustal movements (eg glacier melt bounceback))
Knickpoints Mark current limit in regrading
Saltation Method of transport for bedload (bouncing)
Traction Method of transport for bedload (rolling)
Solution Method of transport for dissolved material
Suspension Method of transport for lighter material: held by turbulent eddies
Hydraulic action (cavitation) Movement of sediment by the frictional drag of the moving water
Solution (Corrosion) occurs when minerals in rocks dissolve into the water and are carried away, weakening the rock structure
Rejeuvenation Occurs when relative sea level drops, resulting in a lower base level, and formation of knickpoints, gorges, waterfalls, rapids, incised meanders, and river terraces
Positive feedback loops one change away frfom the original state triggers another change which leads even further away, and the system is unable to find redress
Channel Precipitation Precipitation that falls directly into a river
Levees and Embankments Raising height of riverbanks increases capacity, meaning river can hold more water before flooding occurs. Can be done fairly cheaply by dredging sediment. Concrete walls are more costly but effective, and are used in towns/cities.
Braiding Rapid deposition of coarse load blocks the main channel which will split into a series of smaller diverging and converging channels which find the easiest route around the sediment
Attrition Reduction in size of sediment particles as they collide with each other
Capacity refers to the total volume of sediment that a river can transport
River restoration schemes Return rivers to original state before management - improve water quality, allow functional floodplains upstream. Occasionnal flooding boosts soil fertility. This is a compromise between envorinmental gains and socio-economic considerations
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 vary, the river changes to move towards this equilibrium
Permeable rock Rock that contains pores and cracks large enough to allow water to flow through the rock
Porous rock Rock that contains pores and cracks which can be occupied by air. If large enough, these cavities will also be permeable.
Impermeable rock Rock that doesn’t contain pores or cracks large enough for water to pass through
Abrasion (Corrasion) Rubbing or scouring of the bed or banks by sedimentary material
River basin management Schemes that aim to reduce amount and speed of water flowing towards a river, and therefore flood risk, by managing land use.
Surface Storage Storage of water on the surface of the earth - eg in lakes, puddles, reservoirs.
Dams and flood storage reservoirs Store water in a reservoir during times of high flow and release it during low flow, allowing a steady release throughout the year. Are costly, flood large areas of land and trap sediment - but are multipurpose.
Waterfalls Sudden falls of water that occur where there are rapid changes of gradient in the river's course
Watershed The boundary between two drainage basins
Erosion The breaking away and removal of rock by the river
Infiltration the downward movement of water into the soil surface
Recurrence interval the interval at which particular levels of flooding will occur. (years of observation +1 divided by rank order
Thalweg The line of maximum velocity
Base level The lowest point that a river can erode to
Competence the maximum size (calibre) of load that a river is capable of transporting
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.
Wetted perimeter the portion of the perimeter of a stream that is in contact with the water
Interception The process by which raindrops are prevented from directly reaching the soil surface, eg by grass.
Transpiration the process by which water is lost by a plant through stomata in the leaves
Potential evaporation The rate of water loss from an area if there were no shortage of water (if soil and plants were saturated)
Magnitude The size of the flood
Regolith The soil and partially weathered rock close to the surface. This material is fragmental, so therefore permeable.
Velocity the spped and direction at which a body of water moves
Evapotranspiration the total amount of moisture removed by evaporation and transpiration from a vegetated land surface
Soil moisture the total amount of water, including water vapour, in an unsaturated soil
Cross sectional area the total area of the bed and the bank sides in contact with the water in the channel
Evaporation the transformation of water droplets into vapour, using energy provided by the sun/movement of air (wind)
Discharge the volume of water flowing in a river per second, measured in cumecs (m3/s). Cross sectional area x mean velocity
Open There are inputs and outputs of energy and matter
Closed System There is input, transfer and output of energy, but not of matter or mass
Outputs Things that come out of a system (evaporation, transpiration, runoff into sea)
Inputs Things that go into a system (precipitation, solar energy)
Lag time Time from peak rainfall to peak discharge
Channel Runoff Water carried via a channel (eg river) - brought into a and out of a system.
Saturated Zone Water collects above an impermeable rock layer, filling all pore spaces.
River straightening Water flows faster, so is transferred downstream more rapidly - but increases flood risk further downstream, creates and unnatural looking channel and damages habitats
Groundwater flow Water flows through bands of sedimentary rock. Can take thousands of years to reach river. Tries to balance out the saturated zone.
Groundwater storage Water stored underground in permeable rock strata
Storm flow Water than reaches the channel largely from runoff. This may be a combination of overland flow and rapid throughflow.
Throughfall (drip flow) Water thar drips off leaves duringa rainstorm. It occurs when more water falls onto the interception layer of the canopy than can remain on the leaves
Surface runnoff Water that flows away over surface, due to heavy rainfall / saturation of soil
Throughflow Water that moves downslope through the subsoil, pulled by gravity. Effective when underlying impermeable rock prevents percolation.
Baseflow Water that reaches the channel through slow throughflow and from permeable rock below the water table (groundwater flow).
Stemflow (Trickle flow) Water that runs down stems and branches of plants and trees during and after rain to reach the ground. Happens after interception.
Deposition when the transportation stops, and material is deposited as sediment
Pipeflow When water infiltrates into the soil it becomes waterlogged. Water flows through the top layer of the soil and creates a natural pipeline. Quicker than normal throughflow.
Zone of aeration (zone of percolation) Where water does not fully saturate the pores in the rock.
Created by: Rayrayy