What factors affect the angle of a slope?

Transcription

1 Climate Rock type and Structure What factors affect the angle of a slope? Aspect

2 Fast mass movements Slides: Slides are movements along the SLIP PLANE, i.e. a line of weakness in the rock or soil structure. Usually material moves down the slope more or less intact then breaks up at the bottom of the slope. Leads to large slumped terraces

3 River Terrace

4 Rockfalls. Need steep slopes >70 Initial cause may be weathering e.g. freeze thaw Broken fragments of rock fall to the base of slope producing scree or talus slopes. Leads to slumped terraces In upland areas falls and slides are an important sediment source for rivers. Beachy Head 2001 illustrating the rockfall of 1999.

5 Slumps Occur on weaker rocks (e.g. clay) where the slide plane is CURVED e.g. Folkstone Warren, Kent where: Unstable gault clay lies below soft chalk. Heavy rain can saturate the chalk The base of the cliff is attacked by wave action.

6 Weathering processes Weathering is the decomposition disintegration of rocks in situ and (Not to be confused with erosion which is the removal of rock) There are three main types: Mechanical (physical) Chemical Biological

7 Freeze thaw/ Frost shattering - Water gets into cracks or joints in rocks - When the temperature falls below 0, it freezes - When it freezes it expands (by about 10%) exerting great pressure on the rocks. - Rocks are weakened during repeated freeze thaw cycles Most effective where there is; 1. Plenty of moisture 2. Frequent temp changes/ fluctuations between above and below 0 degrees. E.g. Mountain areas

8 Salt Crystal Growth -Salt/saline water enters cracks in the rock -Water evaporates leaving salt crystals -Salts can expand by 3 x their original size when wet -Expanding salt crystals break up the rock -Very common in semi arid areas (edge of sahara) which are hot and largely dry (salt solutions dry quickly)).

9 Disintegration -Rock heated by the sun expands -When temperature falls the rock contracts -Common in hot deserts where lack of cloud means high daytime temperatures and very low night time temperatures (i.e. 40 degrees C variation) -Rock is a poor conductor of heat, stresses take place in the outer layer and cause peeling (exfoliation).

10 Pressure Release -Where overlying rocks are removed by erosion then this pressure is released -Once cracks are formed then other processes will weather the rock -Common in areas that were once glaciated because of the removal of great weight of ice above when it melted

11 2. Chemical Weathering This is the breakdown of rocks caused by chemical changes in the minerals in the rocks The main agent of chemical weathering is CARBONIC ACID which is found in rain water. Carbonation solution: Occurs on rocks with calcium carbonate e.g. chalk and limestone. Rainfall and dissolved carbon dioxide forms a weak carbonic acid. Calcium carbonate reacts with the acid to form calcium bicarbonate (which is soluble and therefore carried away by water).

12 Hydrolosis. -Most common in type of chemical weathering and is the result of minerals disintegrating through reaction with carbonic acid -E.G. GRANITE contains the mineral FELDSPAR which decomposes when attacked by weak acid. -FELDSPAR decomposes into a clay substance called KAOLIN (china clay). Hydration -Certain minerals absorb water -Water causes a chemical change as well as causing mineral to swell -E.g. water reacts with anhydrate to form GYPSUM (building material plaster)

13 Oxidation: - The result of a mineral combining with oxygen (found in air and water) to produce a reddish brown coating

14 3. Biological weathering Breakdown of rocks by plants/animals -Plant roots enter rocks via cracks and as they grow and thicken they exert increasing pressure. -Burrowing animals can speed up the weathering process in rocks which have already been partly weathered.

15 What affects the rate of weathering? The rate of weathering (i.e. how quickly it happens) will depend on climate and rock type. CLIMATE Peltier s diagram (1950) shows a strong relationship between the rate of weathering and rainfall and temperature. E.g. Physical weathering is particularly active in cold climates where freeze-thaw predominates Chemical weathering tends to be most intense in wet and warm climates

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17 2. Geology Rock type influences the rate of weathering due to: Chemical Composition The nature of cements in sedimentary rock Joints and bedding plains. For example, limestone consists of calcium carbonate and is therefore sisceptible to carbonation solution, where as granite is prone to hydrolysis because of the orthoclase feldspa.

18 Factors affecting river flow and velocity Water is subject to two main forces, gravity (forces downstream) and frictional resistance with the bed and banks. This resistance opposes the flow downstream. What effects the amount of energy a river has? Volume of water Channel Shape The type of flow a river has will effect the landforms that are produced.

19 1. Turbulent Flow Not steady or uniform. It provides the upward motion in the flow that allows the uplifting of fine particles. This contributes to depositional landforms downstream. The conditions necessary for turbulent flow are; Complex channel shape e.g. meandering channels High velocities Cavitation the explosion of air in joints and cracks as a result of air at very high pressure.

20 2. Laminar Flow Movement of water in sheets. Common in groundwater or glaciers but not very common in rivers but can occur on the bed in lower course. Needs; 1. Shallow channels which are smooth and straight 2. Low velocities If laminar flow alone occurred in rivers, all the sediment would remain on the bed.

21 Factors affecting flow and velocity: Channel Shape The efficiency of a stream s shape is measured by its hydraulic radius. Hydraulic Radius = Cross-sectional area Wetted perimeter (Wetted perimeter: the total length of the bed and base of a river channel in contact with the water in a river) The higher the ratio, the more efficient the stream and the smaller the frictional loss. The ideal channel shape is semi-circular

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23 Factors affecting flow and velocity: Channel roughness Why? Roughness causes friction that slows down the velocity of water. Friction is caused by boulders, trees, vegetation, irregularities in riverbed, and contact between water, bed and bank.

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25 Downstream changes in discharge and channel variables Further downstream, the flow becomes less turbulent, the bed becomes smoother and consequently velocity increases despite appearances to the contrary.

26 Transportation Erosion provides the loose material (and minerals input by mass movement and weathering) which is carried by the river as its load

27 Homework Notes on Transport, erosion and deposition For Friday