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water basins ((Presentation on theme: "DRAINAGE BASINS AND FLOOD…
water basins
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Any precipitation which falls into the basin is collected and drains into the main river or its tributaries by runoff, throughflow or groundwater flow.
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A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow.
Drainage basins are divided from each other by topographic barriers called a watersheds or water divides.
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Watershed: A ridge or other line of separation between two river basins or drainage systems. Drainage Basin: An area of land drained by a river system. River system: a system comprising of streams and a main river Tributaries: Streams or rivers that flow into larger rivers. They bring water into the main river increasing its discharge downstream.
Distributaries: Small channels that branch out from the main river and are mainly found in the lower course and delta. River Source: Point where the river starts River Mouth: Place where the river meets the sea / ocean (River ends)
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7 The following image shows the nature of drainage basins as determined from a topographic map sheet. The red lines delineate the watersheds for the drainage basins of first order streams. The yellow lines define the watersheds for two larger sub-drainage basins that form part of the larger basin
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They form part of a larger Global Hydrological Cycle and are commonly viewed by scientists as being open systems.
Inputs to these systems include precipitation, snow melt, and sediment.
Drainage basins lose water and sediment through evaporation, deposition, and streamflow. These are outputs.
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A number of factors influence input, output, and transport of sediment and water in a drainage basin.
Such factors include topography, soil type, bedrock type, climate, and vegetation cover.
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The drainage basin forms part of the hydrological cycle and can be described as an ‘open system’ involving a series of:
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Within the system, some of the water:
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Passes through a series of transfers or flow e.g. infiltration, percolation and throughflow
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Be held in storage in lakes, the soil or underground
Flow into a river to return, eventually to the sea as run-off.
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So, river runoff will be less than precipitation
If we have excess precipitation then, we get?
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16 As water cycles through a drainage basin via inputs, outputs, transfers and stores, it creates a basin hydrological cycle.
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Throughflow : water moving through the soil. Surface run-off : water that flows over the land surface
18 Inputs Main Input - precipitation. The types: rain or snow, hail, etc.
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19 Storage Water is stored - on the surface,
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Water can be temporarily stored via interception. – in the storage of water on leaf and plant stems.
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sheetwash - across the surface, or
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Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow (base flow).
Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.
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A hydrograph may be used to show how the water flow in a drainage basin (particularly river runoff) responds to a period of rain.
A hydrograph shows variations in a river’s discharge over a short period of time, usually during a rainstorm.
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25 This type of hydrograph is known as a storm or flood hydrograph and it is generally drawn with two vertical axes.
One is used to plot a line graph showing the discharge of a river in cumecs (cubic metres per second) at a given point over a period of time.
The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge.
The scale on the horizontal axis is usually in hours/days and this allows both the rain event to be recorded and the subsequent changes in river discharge to be plotted
26 The shape of the hydrograph varies according to a number of controlling factors in the drainage basin but it will generally include the following features:
The time difference between the peak of the rain event and the peak discharge is known as the lag time or basin lag time.
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The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow.
The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the river’s level falls. It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly throughflow which is making up the river water.
The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel.
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When a storm begins, discharge does not increase immediately as only a small amount of rain will fall directly into the channel.
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The gap between the time of peak (maximum) rainfall and peak discharge (highest river level) is called lag time.
A river with a short lag time and a high discharge is more likely to flood than a river with a lengthy lag time and a low discharge.
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30 After this point in time surface run-off will input water into the river. At Point X surface run-off and discharge are increasing rapidly.
At Point Y discharge is in decline. Surface run-off is still feeding water into the river but it has also passed its peak.
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At Point Z there are no longer surface run-off inputs and throughflow and ground flow are both in decline. The discharge of the river is almost back to its minimum base flow.
Point W on the graph marks the peak rainfall. At this moment, precipitation has not entered the river.
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A number of physical and human factors (known as drainage basin controls) influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.
These include the size and shape of the basin, the steepness of slopes, the type of geology within the basin, antecedent rainfall and land use within the basin.
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Hydrographs are graphs which show discharge (the amount of water passing a particular point in a river at a particular time).
Therefore, factors that affect discharge within the drainage basin will affect the shape of the hydrograph.
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The size, shape and relief of the basin are important controls.
Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one.
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Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins.
Smaller basins generally have shorter lag times because precipitation does not have as far to travel.
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Where gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb.
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If the drainage basin is already saturated (antecedent rainfall) then surface runoff increases due to the reduction in infiltration.
Rainwater enters the river quicker, reducing lag times, as surface runoff is faster than baseflow or through flow.
Thus, if it’s been raining heavily previously, the ground may be waterlogged so the lag time will be reduced because water will be unable to infiltrate and will instead travel via overland flow.
The amount of precipitation can have an affect on the storm hydrograph. Heavy storms result in more water entering the drainage basin which results in a higher discharge.
The type of precipitation can also have an impact. The lag time is likely to be greater if the precipitation is snow rather than rain. This is because snow takes time to melt before the water enters the river channel. When there is rapid melting of snow the peak discharge could be high.
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If a river is surrounded by non-porous and impermeable rocks (e.g., mudstone) it’s going to have a high peak discharge and a short lag time.
Impermeable rocks won’t let water percolate through them, forcing the water to travel via overland flow. This is much faster than groundflow, interflow and throughflow so the lag time is reduced.
Furthermore, non-porous rocks can’t store water so the peak discharge of a river is increased as more water enters the river rather than being stored in the drainage basin.
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The soil’s ability to let water infiltrate has a similar effect to the dominant rock type in a drainage basin.
Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin. In addition, water travels slowly through soil via throughflow. This reduces the peak discharge while increasing the lag time of a river.
On the other hand, extremely fine clay soils don’t allow water to infiltrate. As a result, water travels quickly as overland flow, reducing the lag time of a river.
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Drainage Basins with impermeable underlying rock are also associated with higher drainage density on the surface.
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If a drainage basin has a significant amount of vegetation this will have a significant affect on a storm hydrograph.
Vegetation intercepts precipitation and slows the movement of water into river channels. This increases lag time.
Water is also lost due to evaporation and transpiration from the vegetation. This reduces the peak discharge of a river.
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Drainage systems that have been created by humans lead to a short lag time and high peak discharge as water cannot evaporate or infiltrate into the soil.
Areas that have been urbanised result in an increase in the use of impermeable building materials. This means infiltration levels decrease and surface runoff increases. This leads to a short lag time and an increase in peak discharge.
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Vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere so reducing the amount of water available for overland flow
The large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.
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Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc
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47 We need to be careful not to exaggerate the importance of urbanisation in creating flash responses in rivers; after all the city or town only represents a small area of the drainage basin as a whole.
The construction of roads and parking lots does of course increase run-off as well as the building of houses and businesses on the floodplains.
In addition, the design of roads and drainage infrastructure to transport water quickly into the river further reduces lag time.
However, with the exemption of major metropolitan areas, these impacts need to be observed more generally at a local scale.
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As urban areas grow, the risk of flooding increases.