Q: What is a lag time and runoff?
Suggested Answer
Lag times and run-off are part of a flood (or storm) hydrograph which is a graph that shows how a particular river basin is likely to respond to a flood or period of sustained heavy rainfall. The lag time is the time lapse between the peak rainfall and the peak discharge (when the water discharge at a given measuring station on a river is at its maximum). Run-off is the water that travels over the ground to reach the rivers, rather than percolating through the soil and rocks. Storm run-off is the extra volume of water that reaches a river quickly as the ground cannot absorb it. This is what can cause a river to flood. There is more information about flood hydrographs page ,
Q: Why in a storm hydrograph there is a lag time between the peak rainfall and the peak discharge.
Suggested Answer
A storm hydrograph shows the discharge of a particular river at a given point in the main channel over a short period of time after an individual event, such as a rainstorm. It reveals how the drainage basin responds to this extra input of water in the form of precipitation. The discharge is the water in the river channel that has reached there by surface runoff (overland flow), through the soil profile (throughflow) and groundwater (baseflow). The period of time between the maximum peak of precipitation and the peak or maximum discharge from the river is known as the lag time. The steeper the rise in discharge to peak levels, the shorter the lag time. The reason that there is a delay between the input of precipitation and peak flow is that although some precipitation will fall directly into the channel, most falls elsewhere in the drainage basin system and takes time to reach the channel itself.
There are several factors that affect the lag time of a drainage basin. Firstly, the size of the river basin itself is important - a larger basin will mean that water has further to travel to reach the main channel. Secondly, the density and pattern of the drainage network will affect the speed and efficiency of the movement of water to the point of measurement. Thirdly, a circular shaped basin will be likely to have a shorter lag time than a longer, elongated one, where it takes longer for the water to travel from some parts of the drainage basin into the main channel; the shape of the drainage basin will affect the lag time.
The relief (such as the steepness of the slopes in the upland valleys of the basin) also affects the speed at which water flows downslope into the channel. The soil and rock types in the basin also affect the lag time. These affect the capacity of water to infiltrate into the soil and percolate into the groundwater store. Water from these stores takes much longer to get into the river channel than water passing directly into the channel by overland flow. The amount of water already present in soil and groundwater stores also affects the lag time - if these stores are already saturated, then more water will be transferred into the channel quickly by overland flow. The type and amount of vegetation present in the drainage basin also contributes to the lag time, as vegetation intercepts a certain amount of falling precipitation, storing moisture and delaying transfers into the river channel. Human activities such as deforestation and urbanisation decrease the lag time and contribute to higher peak discharge, increasing the likelihood of flooding, because less interception takes place and impermeable surfaces such as tarmac and concrete increase overland flow. Artificial drains, sewers and gutters associated with urban areas increase the drainage network density, once again reducing the lag time.
Q: How overland flow, throughflow, and groundwater flow contributes to quick flow or storm flow to the drainage basin.
Suggested Answer
Overland flow is when water flows over the earth’s surface, joining rivers and eventually, the sea. This is the fastest form of water transfer. Throughflow is the movement of water through the ground. It is eventually likely to form a spring as it re-emerges on the earth’s surface after meeting an impermeable layer of rock. Groundwater is the water that is stored in the rocks below the surface. This is the slowest form of water transfer.
The rate at which the rainwater reaches the river depends on a number of factors. Impermeable rock (i.e. that which doesn’t allow rainwater to sink through) such as granite, will mean there is a greater amount of water flowing over the surface (surface flow or overland flow). This will reach the river quicker than water, which percolates slowly through the rocks. Therefore, permeable or porous rocks (such as limestone and chalk) will mean that water will be slower reaching the river. Impermeable rocks will contribute to the inability of a drainage basin to cope with a storm or prolonged period of heavy rainfall.
The relief of the land, whether there are steep or gentle slopes, the type of vegetation, the amount of urbanization, the current level of saturation and the land use will all affect the flow of the water in a drainage basin. As a general rule, the greater the proportion of water that is held as groundwater and that travels as throughflow and groundwater flow, the less chance there is of a sudden surge or flood. Where there is a greater amount of water flowing over land the drainage basin is prone to floods, which are often damaging in nature.
Q: How does human affect the drainage basin system?
Suggested Answer
Humans affect the drainage basin system by altering the amount of water in the system, or by pollution or changing the system in some way. Usually the reasons behind this are to lessen the risks of flooding, to build upon the flood plain or to use the water in some way.
Further research is needed how humans alter various aspects of the drainage basin and how this then affects the river and the drainage basin system.
For example, water can be taken from rivers to use for irrigation, to use as drinking water and for cooling in industries. This would then affect the system as there is less water in the river and when the water is returned, it may be polluted. This pollution could be caused by leaching of pesticides and other chemicals into the river or it could be that the water from industrial cooling is still heated, which would then kill the vegetation and plant life in the river.
Urbanisation continues unabated and more and more houses are being built on flood plains, despite the recent years of well-publicised flooding. Urbanisation increases the flood risk as water is taken, via drains, into the river more quickly leaving a shorter run-off time and higher water levels.
People can also alter the river itself, either by building dams, straightening meanders, building levees (for example, the Mississippi) or clearing sediment from the channel bed. And their effects in changing the drainage environment. For example, how would building a dam affect the flow of the river?
As this is AS or A2 level named examples are essential to refer to. You could start by researching the Mississippi, the Nile and the Rhine, which will cover the building of dams, the use of rivers for irrigation, industrial uses and pollution of river systems and flood management strategies.
Q:
Saturated overland flow percentage would be influenced by conditions in the basin prior to the start of the rainstorm (its part of a data response question about the components of the rainfall-runoff process during a rainfall)
Suggested Answer
Drainage basin systems are open which means that they have inputs and outputs (of water). The input of precipitation is stored and transferred between different places in the drainage basin system. The stores are like sponges that can hold a set amount of water. Once they are full, they release the water in them, which then flows into other stores, eventually leaving the drainage basin as discharge (the output of water into the sea or another river basin). Think of the system as a cascading system — like a series of champagne glasses piled up in a pyramid. As champagne is poured into the first glass it fills up and overflows into the next, and so on, down the pyramid until all the glasses are full.
Precipitation falling directly onto the land surface (input) moves downslope by overland flow, or surface runoff, and into river tributaries via which the water is transferred to surface stores such as lakes, reservoirs and puddles. The amount of overland flow will depend on the threshold level of the capacity of each precipitation store. Water on the surface is transferred vertically into the soil by infiltration and from there into the groundwater by percolation into underlying rock. The rate and amount of these transfers will affect the rate of overland flow. These depend on the soil and rock type in the area.
Water is taken up by the capillary spaces between individual soil particles (infiltration) and is only released from the soil when these capillaries are filled. The amount of pore space available will depend on the type of soil — for example, sandy or clay-rich — its texture and also its structure. These factors will all affect the field capacity of the soil and so the amount of surface water it can absorb. The quantity of water already present in the pore spaces before a rainstorm will directly affect the ability of the soil to absorb more from the surface. This antecedent moisture (water which is already present) will therefore influence overland flow rates. If the soil is already saturated with water, overland flow will be rapid and greater in volume than if there is plenty of capacity available in the soil store. The timing of the last rainfall to take place, its intensity and amount will be important factors in determining the capacity for storing new inputs. The percolation rate will depend on the nature of the soil store and also directly on the permeability of underlying rock which is present.
Therefore, when these soil and groundwater stores are full, or saturated, then water is transferred into the river channel, partly by overland flow. Some of the water input into the system is also transferred horizontally along the soil profile by throughflow into river channels. Finally, some water flows slowly at depth underground by groundwater flow (sometimes called baseflow). Smaller soil and groundwater stores will lead to increased overland flow rates overall because the ground will become saturated more quickly.
Q: How does the amount and location of water change in a hydrological cycle?
Suggested Answer
The hydrological, or water cycle is a model used to explain the movement of water on the planet. The hydrological cycle is extremely complicated and the relationships between some parts of it are still poorly understood, although it is becoming clear that there are serious implications to humans’ interference with its workings.
The hydrological cycle is a closed system, which means that the total amount of water in the global system stays the same, or is conserved, overall. The total amount of water in the Earth’s hydrological cycle can only be estimated: the US Geological Survey suggested the total water volume to be around 1,384,000km3. The approximate distribution of this water is shared between the oceans (97%), ice sheets and glaciers (2.1%), soil moisture and groundwater (0.6%), fresh water in lakes and rivers (0.1%) and water vapour in the atmosphere (0.001%). These totals in the cycle are not constant in time and space, as there is a constant recycling of water between these stores in the atmosphere, land and ocean, as water in different forms is transferred between them.
Heat from the sun causes evaporation of sea water and surface water in lakes and streams, a vertical transfer as water vapour in the atmosphere, which is then transferred horizontally by wind. Condensation of that water vapour as clouds leads to precipitation (rain, snow, hail, dew etc), a vertical transfer down to the surface of oceans and land . The terrestrial, or land-based, part of the system is what interests us most for thinking about water resource issues. The drainage basin system - rivers - forms an important part of the hydrological cycle. Drainage basin systems are ‘open’ which means that they have inputs and outputs (of water). The input of precipitation is stored and transferred between different places in the drainage basin system. The stores are like sponges which can hold a set amount of water. Once they are full, they release the water in them, which then flows into other stores, eventually leaving the drainage basin as ‘discharge’ (the output of water into the sea or another river basin). Precipitation falling directly onto the land surface (input) moves downslope by overland flow, or surface runoff, by which the water is transferred to surface stores such as lakes, reservoirs, or puddles or flows into the river channels. Some of the precipitation is intercepted by vegetation (interception) and stored there until it drips off leaves and down trunks and stems to reach the surface store. Some evaporates from the leaf surfaces and is returned to the atmosphere by transpiration. Water on the surface is also transferred vertically into the soil by infiltration and from there into the groundwater by percolation. The rate and amount of these transfers will depend on the soil and rock type in the area. When these soil and groundwater stores are full, or saturated, then water is transferred into the river channel. Some of the water is also transferred horizontally along the soil profile by throughflow into river channels (caused by the presence of less permeable layers in the soil). Some flows slowly at depth underground by groundwater flow (sometimes called baseflow).
Human activity can affect the hydrological cycle in several ways. For example, water can be pumped from groundwater for irrigating crops or drinking water for cities, reducing the amount in these stores. Water can be transferred from one river basin to another. Artificial surface stores such as reservoirs can alter the natural flow of river channels. Removing natural vegetation or building on parts of a drainage basin cause more and faster overland or surface flow of water. Finally, the amount of water stored as ice and snow depends on global temperatures. There is growing evidence that global warming caused by human activities is now causing these surface stores of water to melt, returning their water to a liquid form. This increases the transfer of water into the ocean store, causing sea level to rise worldwide. When the world was colder, during the Ice Age, large volumes of water were stored on the land as ice sheets and glaciers, possibly up to three times the current amount. It is thought that this caused global sea levels to drop by 100 metres or more, as the frozen water was stored on the land surface instead of being liquid in the oceans.
Q: Why the global hydrological cycle is an example of a closed system?
Suggested Answer
The global hydrological system, or cycle, is the constant recycling of water that takes place on a global scale between the oceans, atmosphere and land. It is described as a ‘closed’ system because the overall amount of water in the system remains constant and is conserved. There are no inputs and outputs from the system as a whole - it is self-contained. About 2% of global water is stored as polar ice, snow and glaciers; 0.7% is fresh water in the form of rivers and lakes (surface storage, 0.1%), soil moisture and groundwater in aquifers (0.6%), or atmospheric moisture (water vapour) - only 0.001%. The remainder - the vast majority - is stored as salt water in the world’s seas and oceans. The origins of the world’s water lie in the distant geological past, when volcanic gases from the interior of the young Earth and ice from comets colliding with the planet put water into the atmosphere and oceans. Since that time, thousands of millions of years ago, the overall amount of water on the planet has remained fixed.
Water is now constantly recycled from one store to another - the hydrological, or water, cycle. Very little water is stored in the atmosphere itself; instead, it is recycled between the stores of the ocean, land and atmosphere. Heat from the sun causes the evaporation of water from the ocean, rivers and other surface stores, which is transferred around the atmosphere as water vapour by winds. Water is also returned to the atmosphere by plants, through the process of transpiration from leaf surfaces. Condensation of this water vapour as clouds leads to precipitation in a variety of forms - rain, snow, dew, hail, etc. - transferring the water back to the surface into lakes, ponds, rivers or as snow and ice storage. This water then moves through river basins by surface runoff and through the soil (throughflow) and by slow groundwater flow underground, eventually returning to the ocean. In this way, the global water supply is recycled and conserved.
By contrast, a river or drainage basin is an ‘open’ system - it forms part of this global hydrological cycle and has inputs and outputs - it is linked to other parts of the overall hydrological system. Water enters the drainage basin as precipitation from the atmosphere (input) and leaves it as discharge from the river channel into the sea or another drainage basin, or is lost back to the atmosphere by evaporation and transpiration (outputs).
RIVER CHANNEL PROCESSES AND LANDFORMS
Q: Explain, with diagrams, how and why the position of a meander changes over time?
Suggested Answer
A meander (or a bend in a river) experiences both erosion and deposition.
On the outside of the bend where the faster current is, erosion occurs. On the inside of the bend, deposition occurs as the water is slower and shallower (as a meander has an asymmetrical cross section).
It is thought that the material eroded from one bend is moved downstream using a corkscrew movement known as helicoidal flow. Much of this material is deposited on the inside of the next bend with the remainder being carried further downstream.
As these processes of erosion and deposition continue to occur, the outside of the meander is eroded further, more so in the direction of the river flow. i.e. if the river is flowing to its mouth southwards, the southern end of the meander is eroded more. This then means that the meander will migrate (move) southwards very slowly, over time. It is this meander migration that moves the position of the meander over time and also contributes towards widening the valley and the flood plain.
Q: Why do rivers meander?
Suggested Answer
The reasons why rivers start to meander are still uncertain, but meander formation seems to be closely linked to the occurrence of pools and riffles. Pools are fairly deep sections that have an efficient shape (high hydraulic radius) whereas riffles are shallower sections with a less efficient channel shape (low hydraulic radius). This means that more energy is required to overcome the friction of the riffles than that of the pools, and in a straight river this means there is an energy imbalance between pools and riffles.
As less energy is required to overcome friction in pools there is more energy available for erosion. This tends to take place on the outside of the bend because the main line of flow (thalweg) flows towards the outside of the meander, undercutting the bank and increasing the depth of the cross section at this point. The water on the inside of the bend tends to be slow moving and deposition therefore occurs on the inside of the bend, resulting in an asymmetric cross section. When the river flows through a pool on a meander there is more friction to overcome than when the river flows through a pool on a straight river. This is because there is friction where the water hits the bank on the outside of the bend. Therefore in a meandering river the energy requirements of pools and rivers are much more in balance than in a straight river.
Q: Explanation formation on the following river features: ox-bow lakes, meanders, slip off slopes, rejuvenation and knick points.
Suggested Answer
Meanders are bends in a river. When water flows round a meander the fastest current is on the outside of the bend. As the fastest flow is on the outside this is where erosion takes place and where the river is deeper.
On the inside of a bend therefore, the water is slower flowing and the river lacks the energy needed to carry the material (silt, soil etc). This material is then deposited on the inside of the meander, where it gradually builds up, making the inside of the bend shallower. A slip-off slope is usually formed here which is a gentle slope from the top of the bank into the river, composed of the deposited material.
An ox-bow lake is a meander bend that has been cut off from the main river by erosion and deposition. As the meander erodes on the outside of the bend, the neck of the meander will eventually get narrower. It only takes a period of heavy rainfall for there to be more water than usual in the river. Once the neck is very narrow, the extra water will force the river to break through the narrow neck and flow straight on, leaving the loop of the meander. As the water is now flowing straight on (not round the loop, as the water will always take the quickest route), deposition occurs at the side of the channel and effectively blocks off the loop, leaving an ox-bow lake. This may retain water, particularly after a storm but it may also be dry after a period of time .
For diagrams to show the formation of meanders look at the ROSS text book or any book.
Rejuvenation is the term given to the renewal of the erosive power of a river, perhaps because of a change in the river’s gradient or because the river contains extra water. The extra water can be due to glacial meltwater or because of river capture where the river has eroded headward and has intercepted water from another river. The change in gradient could be because of a fall in sea level or an increase in the height of the land, for example, because of land movement due to tectonic uplift. The knick point is the point at which he increased erosion occurs. It is a break in the river usually marked by a small waterfall or turbulent water. The knick point is gradually eroded and will work its way upstream.
Q: Why does a river's efficiency increase down stream?
Suggested Answer
A river is more efficient if it has a greater velocity and less friction.
In a highland area, the channel tends to be rough in shape with rock protrusions. The load is angular and often consists of larger boulders and rocks. These all mean there is an increased wetted perimeter (the surface of the banks and the bed that the river is in contact with). This leads to greater friction.
A lowland river, however, will have a wider channel and a much smaller load, meaning a reduction in friction, leading to an increase in the river’s efficiency.
Q: By using Hjullstrom curve, explain how river processes are related to discharge.
Suggested Answer
The Hjulstrom curve (careful with the spelling) shows the relationship between the speed of the river flow (velocity) and particle size. It shows the critical velocities needed for erosion and deposition.
Discharge is the velocity of the river multiplied by its volume. It is the amount of water passing a point in a river at a given time and it is measured in cubic metres per second (cumecs).
It is very difficult to describe the Hjulstrom curve without a copy of the diagram!
Across the x-axis the size of the particles is shown (measured in mm), ranging from the smallest (clay at about 0.001mm), through silt, sand, gravel, pebbles and boulders (at 100mm and over). The other axis shows the velocity of the river (usually measured in cumecs). The two lines or curves marked on the diagram show the speed at which different sized particles will be eroded, transported and deposited. The mean or critical erosion velocity gives the velocity at which material will be picked up by the river and transported. The mean fall velocity curve (the lower of the two on the diagram) shows the velocities at which various sizes of particles become too heavy to be transported and are therefore deposited.
To explain how river processes are related to discharge. The processes you need to discuss are erosion, transportation and deposition. You will need to refer to the Hjulstrom curve to explain how different discharges mean different sized particles are either, eroded, transported or deposited. For example, at a velocity of 100 cm per second, sand would have enough force to erode the banks of a river. At a velocity of 10 cm/second, sand would be transported but would not erode, whereas if the river had a velocity of 0.5 cm/second, the sand particles would be deposited.
Q: What factors determine the efficiency of a river?
Suggested Answer
The efficiency of a river is determined by: velocity, friction, the wetted perimeter and the channel shape.
The velocity of a river is the speed. It is measured in metres per second. The wetted perimeter is the surface of the banks and the bed that the river is in contact with.
A river is more efficient if it has a greater velocity and less friction. In a highland area, the channel tends to be rough in shape with rocks sticking out of the river bed. The load is angular and often consists of larger boulders and rocks. These all mean there in an increased wetted perimeter. This leads to greater friction.
A lowland river, however, will have a wider channel and a much smaller load, meaning a reduction in friction, leading to an increase in the river’s efficiency.
Q: What are factors can be used to assess the flood risk in the global hydrological cycle?
Suggested Answer
The following are some of the factors, which you can expand on and give examples of:
Rainfall and other precipitation
Obviously the more rain the higher the flood risk, but the extent of the risk depends on what happens to the rainfall as below.
Rate of runoff
This is affected by angle of slope, soil and vegetation cover.
Infiltration rate
If most of the rainfall infiltrates into soil and permeable rock, this slows runoff into rivers. If infiltration is reduced by covering land with buildings, concrete and tarmac, water is rushed off into drains and reaches rivers quickly, causing flooding.
Interception
Vegetation, particularly trees, trap some rainfall as leaves get wet . If vegetation is removed, there is less interception, more runoff and more flooding.
Evaporation
During warmer seasons, evaporation (& transpiration) removes water before it can cause flooding. In cold seasons, with less evaporation (& transpiration), more water reaches rivers.
Transpiration
Vegetation, particularly trees, take up large amounts of rainfall through roots and give it off as transpiration. If vegetation is removed, there is less transpiration, more runoff and more flooding.
Snow melting causes flooding if it happens quickly with a sudden rise in temperature.
Silting of river channels makes drainage less efficient and can cause them to flood.
Global warming causes a rise in sea level leading to flooding by sea and stopping the drainage of water from rainfall. It can increase rainfall and flooding in some areas of the world.
The biggest risk of flooding is caused by combinations of the above factors.