Holford combe (landscape) case study


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  • Erosion on the north bank at the ford
  • The path leading to the ford
  • Holford combe (landscape) case study

    1. 1. Holford Combe The Quantock Hills Somerset <ul><li>A study of a river valley from source to mouth in an </li></ul><ul><li>Area of Outstanding Natural Beauty </li></ul><ul><li>(AONB) </li></ul>
    2. 2. The Quantock Hills <ul><li>These hills are found in North Somerset. They are a range of high, rounded hills with narrow, wooded river valleys. </li></ul><ul><li>The area has been used for farming and related industries in the past and is now a rural area which attracts tourists </li></ul>
    3. 3. Area of Outstanding Natural Beauty <ul><li>An area under special planning rules which protects the natural landscape and prevents development will change the nature of the environment. </li></ul><ul><li>Managed by the local authorities </li></ul><ul><li>Wardens patrol the area </li></ul><ul><li>Visitors are allowed but not encouraged </li></ul><ul><li>Few facilities are provided </li></ul>
    4. 4. Small, discrete signs inform visitors of the rules that apply to the AONB.
    5. 5. The Ordnance Survey map at 1:25,000 You can see how the river rises in an upland area of steep slopes with wooded valleys. The land in the lower course or lowland section of the river is flatter. Water here can easily flood the surface & drainage channels have been built. You can recognise these by their straight lines and the way they have been diverted around the edges of fields.
    6. 6. The Quantock Hills lie to the south of the Bristol Channel. The hilltops are rounded,and covered in grasses, bracken and shrubs such as bilberry. The valleys are steep and wooded
    7. 7. A view looking north from Wilmot’s Pool area Rough grazing on the high land Arable and pastoral farming on the fertile and flatter lowland near the sea
    8. 8. Some of the well rounded hills have been used as ancient forts in the past The surface is uneven with a few footpaths for walkers
    9. 9. The drainage area of the Holford Combe river valley. The river valley drops steeply to the north from its sources in the highland area of the Quantock Hills
    10. 10. The Basin of the Holford Combe The area drained by Holford Combe and its tributaries is shown within the red boundary. This area is called the drainage basin . The area draining into a river system is also known as the catchment area source mouth The watershed The catchment area or The drainage basin
    11. 11. Occasional hawthorn trees can survive the windy hills The River Parrett flows to the Bristol Channel on the flat plain, west of The Quantocks
    12. 12. Water moves downhill and collects in hollows where reeds and mosses grow Sheep graze freely on the hills Footpaths follow the ridges and valleys This is the area just above the stream
    13. 13. The rock is Old Red Sandstone. This was laid down about 360 million years ago and it has been compressed into an impermeable rock Impermeable rock will only allow water to flow over the surface or through the cracks
    14. 14. The source of the stream Overland flow Headward erosion groundwater flow
    15. 15. How water enters the stream Stem flow Leaf drip Through flow infiltration surface runoff Groundwater flow
    16. 16. Features found in the Upper Course <ul><li>Waterfalls form where bands of harder & softer rocks are worn away, or where the river is cutting down to form a new long profile as a result of sea level change </li></ul><ul><li>Gorges form where vertical erosion dominates and a river cuts through hard rock, usually because of a change in base level </li></ul>
    17. 17. A steep gorge forms below the source as the river cuts down through the resistant rock. The gorge is a steep, straight section of the river’s course. The stream here is still very small. Could this small stream have formed this deep valley? The most popular explanation is that this formed at a time when there was a lot more water flowing on the land.
    18. 18. Waterfalls and plunge pools are common features in the steep upper course
    19. 19. The steep slopes in the upper course help water to move downhill quickly. Water washes away some of the soil at the surface where footpaths have worn away the vegetation and topsoil
    20. 20. The Upper Course A narrow channel with many obstructions. A steep V-shaped valley Turbulent flow
    21. 21. The bed load is made up of angular and sub-angular rocks. Vegetation and leaves clog up the channel. Rocks forming the bedload Vegetation in the channel
    22. 22. Incised meanders suggest dominant vertical erosion – vertical corrasion
    23. 23. The confluence of Frog Combe & Lady Combe to form Holford Combe The channel gets wider but is still shallow with lots of rocks slowing flow A wider flood plain forms – the river has more volume & more energy to erode laterally
    24. 24. The middle course
    25. 25. The Middle Course The channel is wider and well developed meanders wind across the flood plain The valley is wider and the footslopes less steep
    26. 26. Deposition occurs on the inside bend of the meander where there is slower flow A river cliff forms on the outside bend where faster flow undercuts the bank – lateral erosion
    27. 27. A point bar or slip-off slope A river cliff Flood plain Valley slopes
    28. 28. Features found in the middle course <ul><li>Meanders </li></ul><ul><li>Formed as the river winds across the flood plain. </li></ul><ul><li>River cliffs are formed by erosion on the outside of the meander bend where water flow is faster. </li></ul><ul><li>Point bars are formed by deposition on the inside meander bend, where water flow is slower. </li></ul><ul><li>Flood Plain </li></ul><ul><li>Formed either side of the river channel as it meanders across the valley bottom, carving a wider, flatter base to the valley. </li></ul><ul><li>Ox-bow lakes are sometimes found in the flood plain. These form when the meander loop is cut off in a flood, the river runs on a straighter course and cut-off or ox-bow lake is left in the old river channel. </li></ul>
    29. 29. The old mill pond is surrounded by a bank. People confuse the pathways and wear away the bank The ford and the main pathway Erosion of the north bank as people avoid the wet, muddy ford
    30. 30. Tree roots are exposed and soil washed away as visitors take an alternative path at the ford.
    31. 31. Point bar River cliff The river flows out of the Hodder’s Combe section of the valley, into a more open valley.
    32. 32. View looking up the valley from the grassy meadow below the ford.
    33. 33. The Holford Combe flows through a deep ravine as it leaves the middle course and enters the lower course of the river at the village of Holford.
    34. 34. The river flows across the flat coastal plain. This area is low lying and prone to flooding, so the river is channelled and straightened by people.
    35. 35. Management and flood control in Kilve
    36. 36. Combining management with landscape enhancement
    37. 37. The river enters a pool just above the beach.
    38. 39. At it’s mouth the river spills out of the pool and across the shingle beach at Kilve to enter the sea.
    39. 40. The river enters the Bristol Channel
    40. 41. Features found in the Lower Course <ul><li>Wide flood plain </li></ul><ul><li>Where the river has emerged from the highland area it can flow across the low plain toward the sea. </li></ul><ul><li>Deposition </li></ul><ul><li>This is the dominant process and can lead to the build up of material forming a delta. This can cause braiding and a changing river course. </li></ul><ul><li>Flooding </li></ul><ul><li>Can be common where there is a wide,flat plain. </li></ul><ul><li>Levees may build up naturally where the river carries a fine clay load. </li></ul><ul><li>Flood or Dainage channels are common man-made features to manage the river flow. </li></ul>
    41. 42. Revising Holford Combe <ul><li>This is an example of a landscape that you have studied in detail </li></ul><ul><li>You should know where it is and what it is like </li></ul><ul><li>What landforms are found there? What processes are acting on the land? </li></ul><ul><li>How do people use the area? </li></ul><ul><li>How is the area managed? </li></ul><ul><li>What conflicts are there in this area? How could they be resolved? </li></ul>