Unit 1 Dynamic Planet by hedongchenchen


									                                             Specification Check List

                       Unit 1 Topic 6: River       Processes and Pressures
              SPECIFICATION CONTENT                                      ACTION

I know what erosion is

I can describe how a rivers shapes its channel using 4
types of erosion

I know what transportation is

I can describe how a river transports its load (4 types)

I can explain the formation of a waterfall

I know what weathering is and can describe biological,
physical and chemical weathering

I know what mass movement is

I know what a meander is and how it is formed

I know what a flood plain is, where it is located and
some pressures placed on the flood plain

I can describe and explain the processes and features in
the upper, middle and lower course of a river

I can describe and explain a rivers long profile and cross

I can interpret storm hydrographs

I can explain, using the hydrological cycle, factors that
might cause flooding

I know the causes and effects of one named flood event

I can evaluate hard engineering strategies to prevent
flooding and give examples

I can evaluate soft engineering strategies to prevent
flooding and give examples

I can describe sustainable river management
                                                                                     Unit 1: Dynamic Planet

                    Topic 6 River Processes and Pressures
Don’t forget the role of rivers in the hydrological cycle; Hydrology is the study of water. The
hydrological cycle is the movement of water between the atmosphere, land and oceans.
    Precipitation - all forms of rainfall, snow, frost, hail and dew.
    Interception - precipitation that is collected and stored by
    Infiltration - water that seeps into the ground.
    Transpiration - water loss from vegetation to the atmosphere.
    Evapotranspiration - the combined losses of transpiration and
    Water storage - the temporary storage of water in lakes, dams,
        reservoirs and rivers.

The main processes of a river at work

Erosion is the wearing away of land, there are 4 main types of erosion:

 Type of Erosion                     Diagram                                  Explanation
                                                                 This is when the river throws
                                                                 material against the channel

(or corasion)

                                                                 This is caused when rocks and pebbles
Attrition                                                        bump into each other and break into
                                                                 smaller pieces

                                                                 This is where salts and other acids in
                                                                 the salt water slowly dissolve away the
(or solution)
                                                                 This is when air gets trapped in a crack
                                                                 in the river bank. The air gets
Pressure (or
                                                                 compressed and expands, opening the
hydraulic action)
                                                                                       Unit 1: Dynamic Planet

Rivers can erode:

 Vertically – erodes material from the river bed

 Laterally – erodes material from the river banks

                                             More acidic water
                                             increases rates of


         Soft rocks erode                                              Human      Deforestation, dams
        faster than harder                                             Activity   and bridges interfere
                rock                                                              with the natural flow
          The steeper the                                                         The heavier and sharper
        gradient the greater   Gradient                                 Load      the load the greater the
       the erosion potential                                                         erosion potential


                                                The faster the river
                                               flow, the greater the
                                                 erosion potential

The material that the river carries is called its load. The load can be large boulders, rock, pebbles or
tiny grains of silt. How much load a river can carry depends on the energy or power the river has.

Transportation is how the load is carried within the river

     Traction – where large rocks and boulders are rolled
      along the river bed.

     Saltation – where smaller stones are bounced along the
      river bed in a leap frogging motion

     Suspension – where very small grains of sand or silt
      are carried along with the water

     Solution – where some material is dissolved
      (like sugar in a cup of tea) and is carried
                                                                                     Unit 1: Dynamic Planet

Deposition occurs when the river’s load is too big to carry and it can create some interesting and
unique features.

The most significant process within a river valley or drainage basin is erosion by the river itself.
Although the sub-aerial process of weathering which operates on the valley sides is important too!

Weathering is the breakdown of rocks in situ. This means it happens where the rock is. Rocks are
weakened by being chemically attacked and mechanically broken down. The main ways that rock is
weathered are:

Freeze-Thaw Weathering

Chemical Weathering
Most rain contains a small amount of acid. When it falls on rock it slowly eats into it. It makes
the rock rot and crumble away
Onion-Skin Weathering
The sun heats the rock and its outer layer expands. When it cools down at night it shrinks again.
After this happens many times, the outside of the rock peels off like an onion.
Biological Weathering
A seed falls into a crack in a rock and it may start to grow. As the roots grow they force the crack to
get wider. The rock breaks into pieces.

Mass movements can occur once the rock on the valley sides is broken up. The fragments of rock
can move fairly rapidly in the form of a landslide, or slowly, most common is soil creep.

 Landslides (and mudflows) are relatively rapid
movement of material. They are less common in the
UK, but can occur along the coast and railway

 Soil creep is a slow process where slope
material moves at about 2cm per year. Over
decades it can have many effects such as, bent
trees, leaning telegraph poles and/or walls.

The shape of a river valley is affected by 3 things:

 The speed of weathering

 The speed of mass movements

 How quickly the river can move material brought by mass movements
                                                                                      Unit 1: Dynamic Planet

The Long Profile of a River

                                                                                      A river has a very
                                                                                      distinct profile that
                                                                                      changes from
                                                                                      source to mouth.
                                                                                      There are distinct
                                                                                      sections of a rivers’
                                                                                      profile and each
                                                                                      section has features
                                                                                      and processes
                                                                                      associated with it.
                                                                                      The long profile is a
                                                                                      section along a river
                                                                                      from its source to
                                                                                      its mouth. The
                                                                                      upper section is
                                                                                      normally steep,
                                                                                      whereas the lower
section is usually flatter. The river is able to erode through its flood plain down to the new lower sea

                               The UPPER course of a river is V-shaped. It has a steep gradient and
                               the valley is relatively narrow. The river is often quite shallow and
                               relatively fast flowing. There is lots of friction with the large boulders
                               found on its bed and the river uses most of its energy to overcome
                               friction. One of the most impressive features in this section is a

Waterfall Formation

Waterfalls frequently occur on horizontally
bedded rocks. The soft rock is undercut by
hydraulic action and abrasion. The weight of the
water and the lack of support cause the waterfall
to collapse and retreat. Over thousands of years
the waterfall may retreat enough to form a gorge
of recession. Gorges form through the retreat of
waterfalls over thousands of years.
                                                                  Unit 1: Dynamic Planet

                    In the MIDDLE Course the slopes are gentle. Lateral (sideways)
                    erosion is beginning to form a flood plain. There is also a larger
                    volume of water in the river due to tributaries. One of the most
                    impressive features in this section of the river is a meander.
                    Meanders are caused by helical flow; this means water on the
                    outside of the bend moves faster and therefore erodes, while
                    the slower moving water is on the inside of the bend. This means
                    material is deposited to form a point bar (a little beach in the
                    inside bend).

Meander Formation

                                                      Continued erosion at the neck
                                                      of the meander can create an
                                                      Ox-bow Lake.
                                                                                       Unit 1: Dynamic Planet

In the LOWER Course, the Valley sides are very gentle, almost flat. Typical processes include erosion
(on the outer banks, especially during times of flood), transport, and deposition (especially on the
inner bends and on the floodplain). Typical features include levees, ox-bow lakes, floodplains and
terraces. Intense use is made of lower courses of rivers on account of their gentle gradient, large
supply of water, fertile valleys, and terraces which are free from flooding.

A floodplain is an area of land at the side of a river that fills up when the river floods. Floodplains are
often flat and fertile and may be used for farming. However, floodplains normally flood each year, so
they may be used for animals rather than crops. Floodplains often contain levees and sometimes
they have ox-bow lakes.

                              A levee is a raised bank on the edge of a river, formed by deposition of
                              material each year when the river floods. When the river floods its
                              speed is reduced. As its speed is reduced it has to deposit some of its
                              load. It drops the coarser, heavier material first and the finer, lighter
                              material last. Levees are built up of coarse material, while the flood
                              plain consists of fine silt and clay.


For deltas to be formed a river needs to:

 Carry a large volume of sediment

 Enter a still body of water

The coarser material is deposited first, and the finest material
last, and furthest away.
                                                                                              Unit 1: Dynamic Planet

A river regime is the annual variation in the flow of a river. In Britain, river flows are higher in winter

             higher rainfall

             lower temperatures and hence lower evapotranspiration

             less interception by deciduous vegetation

                                                      A storm hydrograph (or flood hydrograph) shows how
                                                      a river changes in response to a storm or period of
                                                      heavy rainfall. The rising limb shows how quickly the
                                                      flood waters begin to rise. The peak flow is the
                                                      maximum discharge of the river. The time lag is the
                                                      time between the height of the storm and the
                                                      maximum flow in the river. The recessional limb is the
                                                      speed with which the water level in the river declines
                                                      after the peak. Baseflow is the normal level of the
                                                      river, which is fed by groundwater.

                                                                                      Urban hydrographs are different
                                                                                      to rural ones.

                            Human                                                     They have:
                            Activity                                                   A shorter time lag
                                                                                       A steeper rising limb
                                                                                       A higher peak flow
                                                                                       A steeper recessional limb
                     Drainage             Factors
                                         Affecting                  Vegetation
                      Density           Hydrographs                                   This is because there are more
                                                                                      impermeable surfaces in urban
                                                                                      areas as well as more drainage
                                Slope                    Infiltration
                                Angle                     Capacity

                                        Rock Type

                                                                                 Link your knowledge of the
                                                                                 hydrological cycle to
                                                                                 storm/flood hydrographs.

                                                                                 Use the language of the
                                                                                 hydrological cycle.

Where (location): Morpeth, market town in Northumberland, UK. On the banks of the River Wansbeck.

When: 6th September 2008, the River Wansbeck breached its banks. The narrow steep topography of the
Wansbeck Valley and increased urbanization caused exaggerated surface run-off and a lag time of only 8

~ River Wansbeck has tributaries in the uplands of rural Northumberland and flows through many small
villages and towns along its course.
~ Environment Agency monitor river levels every 15 minutes
~ 150mm (6in) fell between Friday 5th and Saturday 6th October 2008
~ 24 hours of prolonged heavy rainfall filled river channels and saturated the catchment area
~ Highest ever peak discharge recorded by the EA
~ Narrow steep drainage basin which encourages overland flow
~ 8 hour lag time
~ Confluence just north of Morpeth

~ 2000 homes flooded
~ People were being cared for at the Town Hall and local school. Many of them, especially the elderly, were
distressed at having to leave their homes
~ 1000 people evacuated
~ RAF helicopter from nearby Boulmer airlifted the most vulnerable
~ “Staff from Castle Morpeth are handing out sandbags on a priority basis.” As the Environment Agency issued
a flood alert for the whole of Morpeth
~ Major roadways in Durham and Northumberland were closed; with rural villages such as Powburn and
Rothbury completely cut off and isolated.
~ The wet weather in the area at this time also affected Newcastle City Centre and sporting events such as the
cricket at Durham.
~ Damage estimated to cost £10 million
~ Many homes and business are still unoccupied today

~ Fire crews using inflatable dingies ferried families to safety
~ Morpeth Lions coordinate a Flood Victims Appeal and collect donations from Newcastle to Alnwick.
~ Morpeth Rotary Club shop has received over 300 donations of pieces of furniture.
~ Immediate needs were addressed by the Red Cross
~ Red Cross volunteers are systematically calling at houses, street by street. In this way, they can quickly
identify those who have been adversely affected by the recent weather and ascertain what help they might
~ Red Cross fire and emergency support vehicles are stations in streets as a focal point for information.
~ Local Authorities help with the clean up and re-house residents in social housing
~ RNLI lifeboat crews from Newbiggin-by-the-sea, Amble and Blyth also assisted with evacuations
                                       Flood Management Methods

Type of flood management                     Benefits of flood management                                Costs of flood management

                                       Cheap; easy to distribute                                Not very effective in large floods; not easy for elderly
                                                                                                or people with disabilities to put in place

                                       Store very large volumes of water; can be used for       Very expensive; can lead to unwanted impacts such
                                       multi-purpose schemes                                    as increased tectonic activity, displacement of
                                                                                                people, silting of dam

                                       Useful and flexible way of diverting flood waters away   Transfer the flood elsewhere; densely populated
                                       from centres of population                               areas may not have total flexibility in location of relief
                  Relief channels                                                               channels

                                       Relatively cheap and easy way of protecting one or       Do not protect against very large floods
                                       both sides of a river
               Raised banks (levees)

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