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Rivers, floods and management
Transcript of Rivers, floods and management
The storm hydrograph
The storm hydrograph shows variation in a river's discharge over a short period of time, usually during a rainstorm.
Rivers, floods and management
Interception - The prevention of rain from reaching the Earth's surface by trees and plants
Soil moisture - The total amount of water, including the water vapour, in an unsaturated soil
Factors affecting the shape of the hydrograph
Water budget P = Q + E +/- Change in storage
P = precipitation
Q = runoff
E = evaporation
Water balance graph
Condensation - The process by which water vapour is converted into water
Channel flow - The movement of water within the river 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
Throughfall - The water that drips off leaves during a rainstorm. It occurs when more water falls onto the interception layer of the tree canopy than can remain on the leaves.
Stemflow - The water that runs down the stems and branches of plants and trees during and after rain to reach the ground. It takes place after interception has occured
Discharge - The water flowing in a river channel and represents the total volume of water in a river channel passing any given point every second. Usually measured in cumecs (cubic metres per second)
Flood - A temporary excess of water which spills over onto land
Baseflow - Water that reaches the channel largely through slow throughflow and from permeable rock below the water table.
Stormflow - Water that reaches the channel largely through runoff. This may be a combination of overland flow and rapid throughflow.
The intensity and duration of the storm
The antecedent rainfall
Porous soil type
Size of drainage basin
Shape of drainage basin
Scraping, rubbing, scouring action of materials carried by the river
Reduction in size of fragments due to other erosional processes and the rocks bashing together
Sheer power of the moving water
Most active on rocks that contain carbonates, such as limestone and chalk. Minerals in rock dissolved by weak acids
This is a measure of the amount of material it can carry, that is, the total volume of the load. For example, if a river's velocity doubles then its capacity increase by eight times.
This is the diameter of the largest particle that it can carry for a given velocity. For example, if a river's velocity doubles, then its competence increase 64 times.
A river deposits when there is a decrease of energy within the river and it is no longer competent to transport its load.
There is a reduction in the gradient of the river or it enters a slower body of water, for example a lake
The discharge is reduced, for example after dry weather
There is shllow water, for example on the inside of a meander
There is an increase in the size of the load, could be due to a tributary, increased erosion or a landslide
The river floods and overtops its banks, resulting in reduced velocity on the flodplain
Sand particles are moved by lower critical erosion velocities than smaller silts and clays or larger gravels. The small clay and silt particles are difficult to pick up (entrain) because they tend to stick together. They lie on the river bed and offer less resistance to water flow than larger particles. Much more powerful flows of water are required to lift them into the water
Once entrained, particles can be carried at lower velocities than those required to pick them up. However, for larger particles there is only a small difference between critical erosion velocity and the settling velocity. Such particles will be deposited soon after they have been entrained
The smallest paricles (clays and silts) are only deposited at very low velocities. Indeed, some clay particles may never be deposited on the river bed and can be carried almost indefinitely. This explains why such deposits occur in river estuaries. Here the fresh water of the river meets the salt water of the sea, causing chemical settling of the clays and silts to occur and creating extensive areas of mudflats. This process is called flocculation. This coagulation of the clay and silt particles cause them to sink more rapidly.
Over a long period of time a river may display an even and progressive decrease in gradient down the valley, creating the typical smooth concave shape which has adjusted to the discharge and load of the river.
Some geographers define a graded river as one which uses up all its energy in the movement of water and sediment so that no free energy is left to undertake further erosion.
Such a balance between energy and work cannot occur at any particular moment in time but is suggested as an average position over a long time. It can never reach equilibrium because there are constant alterations to the profile so that it is constantly changing.
Potential or stored energy is fixed by the altitude of the source of the stream in relation to base level. Kinetic energy, or energy due to movement, is generated by the flow of the river which converts potential energy into moving energy.
The amount of kinetic energy is determined by the volume of flowing water (discharge), the slope of channel gradient down which it is flowing and its average velocity. An increase in velocity or discharge results in an increase in kinetic energy.
Potential and kinetic energy
The shape of the channel influences the velocity of the river. In the upper course, where the channel is narrow and uneven due to the presence of large boulders, there is a large wetted perimeter. As the channel becomes larger and smoother they tend to become more efficient. The wetted perimeter is proportionally smaller than volume of water flowing in the channel and so more efficient at transporting sediment.
Channel shape ias described as hydraulic radius. A high hydraulic radius means that the river is efficient.
The wetted perimeter is the total length of the river bed and banks in cross section that are in contact with the water in the channel. When there is a large wetted perimeter in relation to the amount of water in the river, there is more friction resulting in a lower velocity.
Channel cross profiles
Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high velocity water loaded with pebbles.
The pebbles become trapped in slight hollows and vertical eddies in the water are stong enough to allow the sediment to grind a hole into the rock.
They are generally found in the upper or early middle course as this is where the valley lies well above base level, giving more potential for downcutting, and where the river bed is more likely to be rocky.
Braiding occurs when the river is forced to split into several channels separated by islands. It is a feature of rivers that are supplied with large loads of sediment. The banks formed from the sediment are generally unstable and easily eroded.
A result of this is that the channel becomes very wide in relation to its depth.
It is most likely to occur when a river has variable discharges. For instance semi-arid areas of low relief that receive rivers from mountainous areas, or a glacial stream with varying annual discharge. In spring, meltwater cause the competence and discharge to increase, as temperature drops the river level falls.
Waterfalls and rapids occur when there is a sudden change in the gradient of the river as it flows downstream.
They may be caused by resistant band of rock occuring acroos the course of the river, at the edge of a plateau, or when the area is rejuvinating giving the river renewed erosional power.
Waterfalls and rapids
Plateau = An area of highland, usually consisting of relatively flat terrain.
Meanders are formed when in low flow conditions straight channels are seen to have alternting bars of sediment on their beds anad the moving awter is forced to weave around these bars. This creatse alternating shallow sections (riffles) and deeper sections (pools). The swing of the flow that has been induced by the riffles directs the maximum velocity towards one of the banks, and results in erosion by undercuttng the side. An outer concave bank is therefore created. Depostion takes place on the inside of the bend, the convex bank.
The outer bank forms a river cliff or bluff with a deep pool close to the bank. This bank is undercut by erosion. The inner bank is a gently sloping deposit of sand and gravel, called a point bar.
Helicoidal flow = the corkscrew-like movement of water from the concave outer bank to the convex inner bank.
They occur generally in the middle to lower course
Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake separated from an adjacent river. The water is stagnant (has no current or flow), and in time the lake gradually silts up, becoming a crescent-shaped stretch of marsh called a meander scar.
In its middle and lower courses, a river is at risk from flooding during times of high discharge. If it flods, the velocity of the water falls as it overflows the banks. This results in deposition, because the competence of the river is suddenly reduced. It is usual for the coarsest material to be deposited first, forming small raised banks (levées) along the sides of the channel. Subsequent floods increase the size of these banks and further deposition on the bed of the river also occurs. This means that the river, with channel sediment build-up, now flows at a higher level than the floodplain.
Floodplains are relatively flat areas of land either side of the river, which form the valley floow in the middle and lower courses of the river. They are composed of alluvium - river deposited silts and clays. Over time, a floodplain becomes wider and the depth of sediment accretions increases. The width of the floodplain is determined by the amount of meander migration and lateral erosion that has taken place. The depth of the alluvial deposits depend partly on the amount of flooding in the past, so floodplain creation is linked to extreme events.
Accretion - growth or increase by the gradual accumulation of additional layers of matter
A delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake. Deposition occurs as the velocityy and sediment-carrying capacity of the river decreases on entering the lake or sea, and bedload and suspended material are dumped. Flocculation occurs as fresh water mixes with seawater and clay particles coagulate due to chemical reaction. The clay settles on the river bed. Deltass form only when the rate of deposition exceeds the rate of sediment removal.
In order for a delta to form the following conditions are likely to be met:
The sediment load of the river is very large, as in the Mississippi and Nile rivers.
The coastal area into which the river empties its load has a small tidal rangeand weak currnets. This means there is limited wave action and, therefore, little transportation of sediment after dposition has taken place. This is the feature of the Gulf of Mexico and the Mediterranean sea.
Deltas are usually composed of three types of deposit:
The larger and heavier particles are the first to be depositied as the river loses its energy. these form the topset beds.
Medium graded particles travle a little further before they are deposited as steep-angled wedges of sediment, forming the foreset beds.
The very finest particles travel the furthest ito the lake before deposition and form the bottomset beds.
Types of Deltas
Bird's foot delta
Cuspate delta (Ebro delta)
Has a curving shoreline and a dendritic pattern of drainage. Many distibutaries break away from the main channel as deposition within the channel itself occurs, causing the river to braid. Longshore drift keeps the seaward edge of the delta relatively smooth in shape
Fingers of deposition build out into the sea along the distibutaries' channels, giving the appearance, from the air, of a bird's claw
Is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea currents or longshore drift
Dendritic = Having a branched form resembling a tree
In upper Teesdale an outcrop of igneous rock called the Whin Sill causes teh formation of the High Force waterfall. The Whin Sill is resistant cap rock which overlies softer sandstone, limestone, shales and coal seams. These are eroded more quickly, leaving the overhang of High force. The water fall created is 22m high - the tallest in England. Ahead of it lies a gorge stretching over 500m downstream.
Rejuvination occurs when there is either a fall in sea level relative to the land or a rise of the land relative to the sea. This enables the river to renew its capacity to erode as its potential energy is increased. The river adjusts to its new base level, at first in its lower course and then progressively inland. In doing so, a number of landforms may be created: knick points, waterfalls and rapids, river terraces and incised meanders.
A knick point is a sudden break or irregularity in the gradient along the long profile of a river. Some knick points are sharply defined, for example waterfalls, whereas others are barely noticeable. It is most commonly created from rejuvenation and is in a sense where the old long profile meetts the new, the knick point recedes upstream at a rate which is dependent on the resistance of the rocks, and may linger at a relatively hard outcrop.
A river terrace is a remnant of a former floodplain, which has been left at a higher level after rejuvenation of the river. When the river rejuvenates it sinks its new channel into the former floodplain, leaving the old flooplain above the level of the present river. The terraces are cut back as the new valley is widened by lateral erosion. The River Thames has created terraces in its lower course by several stages of rejuvenation, they provide natural dfense from floods, provide natural routeways for roads and railways. The build-up areas of Oxford and London are mainly located along the terraces of the Thames.
If a rejuvenated river occupies a valley with well-developed meanders, renewed energy results in them becoming incised or deepened. Incised streams and rivers have cut deeply into the landscpae in many parts of the British Isles. When incision is slow and lateral erosion is occuring, an ingrown meander may be produced. The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip-off slopes on the inner bends. With rapid incision, where downcutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance. These are described as entrenched meanders.
Physical causes of flooding
Flooding occurs when a river's discharge exceeds the capacity of its channel to carry that discharge. The river overflows its banks. Flooding mat be cause by a number of natural causes or physical factors.
Excessive levels of precipitation occuring over a prolongued period of time. This eventually leads to saturation of the soil. When the water table reaches the groud surface, there is increased overland flow or runoff.
Intensive precipitation over a short period of time, particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the groun cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case.
The melting of snow, particularly 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 systems in mid-latitud bring abnormally large amounts of precipitation.
The nature of the drainage basin has an influence on 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. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during times of heavy rainfall.
Impacts of Human activities on flooding
Urbanisation has been an important feature in many less developed countries, coupled with natural population growth, urbanisation has led to an increasing demand for space to build housing for other urban land uses. Floodplains were an obvious hoice - their flat land is suitable to build on and good communications are relatively easy to establish. However floodplains are very susceptible to flooding. Concrete and tarmac are used in urban areas for roads and pavement. Such surfaces are impermeable, so precipitation is unable to infiltrate slowly into the soil, as it would in a vegetated area. In addition to this, there is less interception from trees and uptake from plants is reduced. Overall, a higher proportion of the original rainfall makes its way into a river in a town or city. Surface water is channeled straight into drains and sewers, so precipitation reaches the river quickly, this leads to reduced lag time between peak rainfall and peak discharge.Natural river channels may become constricted by bridges, Which can slow down discharge and reduce carrying cpacity of the river. In times of spate, debris can be deposited directly behind the supports holding up the bridge and exaggerate the effects of a flood. During the Boscastlke floods in August 2004, huge amounts of debris blocked culverts upstream of the town, which had been constructed to allow water to drain quickly through the town.
In some (mainly less developed) coutries rapid deforestation has take place over recent decades. The rainforests of South America, Africa and Asia have been at particular risk as new land has been opened up for farming, settlement and other uses. Other countres, for example Nepal in the Himalayas, have also suffered from deforestation - timber is a valuable resource, used for building and firewood. Once trees have been removed there is a greater risk of doil erosion and sediment finds its way into rivers, obstructing them and adding to the flood risk. Trees intercept water and take it up through their roots, so in deforested areas more water reaches the channel as runoff. Flood damage is greatest near the mouth of a river because wide, flat floodplains are most susceptible to damage. Here, the volume of water is at its greatest because many tributaries have joined the river. Bangladesh lies downstream from Nepal and most fo the land is low-lying floodplain that is less than 1m above sea level, forming the delta of the rivers Brahmaputra, Meghna and Ganges. During the spring snowmelt occurs and once the heavy monsoon rains starts in early summer there is a natural rise in the volume of water in the rivers. In recent years, it has been claimed that flooding in Bangladesh has been more severe, partly as a consequence of deforestation in Nepal.
In Bangladesh, embankments have been built along the river channels in some places. These are designed to increase river capacity, but at times have prevented flood water draining back into rivers.
The Farakka dam lies on the upper reaches of the river Ganges in northern India. In 1988, the Indian government allowed the floodgates of the dam to be opened during the rainy season, because the reservoir behind the dam was at risk of flooding. This saved the land surrounding the dam but downsteram in Bangladesh it was a different matter. The extra discharge in the river coincided with the normal floods expected at that time of the year and greatly increased their severity.
The Mississippi river in the southern states of USA ise of the most managed rivers in the world. Artificial embankments (levées) have been built along te lower reaches of the channel to protect the heavily settled floodplain. The city of New Orleans lies below sea level on the banks of the Mississippi and is at particular risk of flooding but is protected by levées and diversion channels, built by the government. In August 2005, devastating floods occured, submerging the city as the levées were breached. A storm surge, brought about by Hurricane Katrina, gushed up the river from the coast. This, coupled with the heavy rainfall brought by t he storm, cause the river to rise dramatically. Major damage to the embankments resulted as they were breached in several places.
Some rivers in urban areas have been channelised. This involves lining the river channel with concrete and straightening it. Channelisation enables water to be directed through the Urban area more rapidly. It may protect the immediate surrounding area, but there is greater flood risk downstream. This is because water is delivered to downstream areas more rapidly than usual and the unmanaged river channel in these stretches is unable to cope wi the the rapid increase in discharge.
Hydrologists try to forecast the likelihood of future flood events using past records. The data they use include river discharge records in relation to precipitation, and flood recurrence interval graphs. These graphs calculate statistically the probability of flooding in the future based on past records. The further back flood records go, the more accurate the prediction. Records of a river's discharge are ranked over the longest period available, from highest peak discharge to the lowest recorded. The following formula is used to calculate the recurrence interval.
When the recurrence interval is plotted against discharge as a scatter graph on semi-logarithmic graph paper, it is possible to use the line of best fit to predict when the next flood of a particular magnitude might occur. This is called the flood return period. In addition to studying the likelihood of flooding on an annual basis, hydrologists use past data records showing the regime or yearly pattern of discharge in relation to annual precipitation patterns. In this way the likelihood of seasonal flooding can be assessed.
By Daniel White