Water On The Land

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The rivers long profile summary:

Upper course: Features include steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.

Middle Course: Features include wider, shallower valleys, meanders, and oxbow lakes.

Lower Course: Features include wide flat-bottomed valleys, floodplains and deltas.

The Drainage Basin:

The edge of the Drainage Basin is characterised by the highest point of land around the river, this is known as the watershed. The point at which a river starts is called it's source. As the river continues to flow down stream it may be joined by smaller rivers called tributaries. The point at which these smaller rivers join the main river is known as a confluence. As the river continues it's journey, eventually it reaches the sea - the point where the river flows into the sea is known as the river mouth.

Processes of Erosion:

Abrasion: This is when large pieces of Bed Load material wear away the river banks and the bed.

Attrition: This is when the Bed Load itself is eroded when sediment particles knock against the bed or each other and break, becoming more rounded and smaller.

Hydraulic Action: This is when the force of water erodes softer rock.

Solution: This is when acidic water erodes rock.

Types of Transportation:

Traction: Heaviest material is rolled along the river bed. This requires the most energy.

Saltation: Sand-sized particles may be bounced along the river bed on a 'leap frog' motion.

Suspension: Small particles are carried within the water by turbulent flow.

Solution: Some minerals will dissolve in water and be carried in solution.

Features of the upper course of the river: V-shaped Valley:

In the upper course of the river water flows quickly through a narrow channel with a steep gradient: as it does so it cuts downwards. The vertical erosion results in a number of distinctive land forms including the steep sloping V-shaped Valley through which the river flows in it's upper course.

So how does a V-shaped valley form?

1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley

2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley

3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.

4. This material is then gradually transported away by the river when there is enough energy to do so.

As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops. As their is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs.

Features of the upper course river: Waterfall:

Waterfalls are found in the upper course They usually occur where a band of hard rock lies next to soft rock. They may often start as rapids.

As the river passes over the hard rock, the soft rock below is eroded (worn away) more quickly than the hard rock leaving the hard rock elevated above the stream bed below.The 'step' in the river bed continues to develop as the river flows over the hard rock step (Cap Rock) as a vertical drop. The drop gets steeper as the river erodes the soft rock beneath by processes such as abrasion and hydraulic action. A plunge pool forms at the base of the waterfall. This erosion gradually undercuts the hard rock and the plunge pool gets bigger due to further hydraulic action and abrasion.Eventually the hard cap rock is unsupported and collapses. The rocks that fall into the plunge pool will continue to enlarge it by abrasion as they are swirled around. A steep sided valley known as a gorge is left behind and as the process continues the waterfall gradually retreats upstream.

Features of the middle course river: Meanders:

Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope.

Features of the lower course river: Ox-bow lakes:

As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and narrower. Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend. Deposition gradually seals off the old meander bend forming a new straighter river channel. Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake. Over time this feature may fill up with sediment and may gradually dry up. When the water dries up, the feature left behind is known as a meander scar.

Features of the lower course river: Levees:

Levees are basically caused by floods. In times of low flow any deposition takes place on the river bed and this raises the height of the river bed.

In times of flood the water leaves the channel. As it does so it loses energy and the courser and heavier material is deposited near to the river on the banks. Finer material is carried further onto the flood plain. After many floods the river builds a bank on either side of the river and this can lead to catastrophes. As the river has been raised above the flood plain and the banks are often higher still, when the river floods the water can no longer drain back into the river channel and often has to form a new one.

The Drainage Basin as a system:

The land based part of the hydrological cycle is called the Drainage Basin system. A drainage basin is the name given to the area of land which is drained by a river. When water reaches the surface there are a number of routes which it may take in it's journey to reach the river.

Hydrographs and River Discharge:

The amount of water in a river at any given point and time is known as the discharge which is measured in cumecs (cubic meters per second). This can be calculated by multiplying river velocity by channel volume at a given point and time. Hydrographs are graphs which show river discharge over a given period of time and show the response of a drainage basin and its river to a period of rainfall.A storm hydrograph shows how a river's discharge responds following a period of heavy rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of the river. Analysis of hydrographs can help hydrologists to predict the likelihood of flooding in a drainage basin. The response of a river to a rainfall event can be measured in terms of the lag time - the time between peak rainfall and peak discharge. Rivers with a short lag time respond rapidly to rainfall events and are therefore more prone to flooding than rivers with a longer lag time. River discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel. The river will start to respond initially through inputs from surface runoff (the fastest flow of water) and its discharge will later be supplemented through inputs from through flow and groundwater flow.

Variations in the shape of a Hydrograph:

The shape of a hydrograph is determined by the speed in which flood waters are able to reach the river. The nature of the drainage basin therefore has a great influence on the way a river responds to a river as it will determine the types and speeds of the flow of water to the river.The fastest route to the river is via overland flow. If most of the water in a drainage basin travels in this way, a river will respond quickly to heavy rainfall and the hydrograph shape will be 'peaky' (graph A) with steep rising and recessional limbs. The lag time will be short and there will be a greater risk of flooding. Where more water is able to pass into the soil and travel to the river via throughflow / groundwater flow, there will be a slower rise in discharge and the river will respond slower (graph B). The lag time will be longer and the risk of flooding will be much lower.

Factors affecting a flood hydrograph:

Characteristics of the Drainage Basin:

Impermeable rock and soil will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly - results in short lag time.

Permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow - results in a longer lag time.

Types and amount of precipitation:

Heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff (short lag time)

Slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow (longer lag time).

Land use and Human impact:

Impermeable man made surfaces: such as concrete and tarmac are impermeable therefore rivers in urban drainage basins tend to have a short lag time due to higher amounts of surface runoff and drainage systems taking water to rivers quickly.

Vegetated areas: help to reduce flood risk by increasing the time it takes for water to reach a river by encouraging infiltration, intercepting water by their leaves and taking up water in their roots.

Size of the drainage basin:

Large drainage basin: water will take longer to reach the river

Small drainage basin: water will enter the river quicker.

Causes of Flooding:

Human causes:

Deforestation - cutting down trees cause increased run-off. Rain water reaches rivers faster.

Urbanisation - man-made surfaces such as concrete result in greater run-off. Rain water reaches rivers faster and can cause flooding.

Natural Causes:

Heavy rainfall, Melting snow and Relief.

Hard Engineering Options:

Dam Construction - Dams are often built along the course of a river in order to control the amount of discharge. Water is held back by the dam and released in a controlled way. Water is usually stored in a reservoir behind the dam. This water can then be used to generate hydroelectric power or for recreation purposes. Building a dam can be very expensive.

River engineering - The river channel may be widened or deepened allowing it to carry more water. A river channel may be straightened so that water can travel faster along the course. The channel course of the river can also be altered, diverting flood waters away from settlements.

Soft Engineering:

Afforestation - Trees are planted near to the river. This means greater interception of rainwater and lower river discharge.

Managed flooding/zoning - The river is allowed to flood naturally in places, to prevent flooding in other areas.

Planning/Flood warning - Local authorities and the national government introduce policies to control urban development close to or on the floodplain.

Case Study Kielder Water Northumberland:

Facts and Figures: Largest man made reservoir, dam is 1.2km long and 50m wide and building work started in 1975 and finished in 1981.

It was built because it expected a rise in demand for water to support the increasing industrial economy.

The areas that benefit most from the scheme are: Tyneside and Cleveland. major cities include Newcastle, Sunderland, Durham, Darlington and Middlesborough.

The water is transferred to these areas by aqueduct - Airy Holm. Airy Holm transfers water 39km to Frosterly.

The site was a good choice because it was effective at regulating the flow of the Tyne, not too low or too high. It also encouraged income through tourism in the area. It was connected to the HEP station.

Yes the transfer scheme was successful, in 1995 droughts, reservoir at Kielder Water provided above major cities with a constant supply of water.

Benefits of the Reservoir, it created an income through tourism, it provides a reliable water source and it provides HEP

Problems of the Reservoir, the scheme involved conservation areas, the habitats were ruined and people had to be relocated to new homes.

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