QU: Where does sand do jail time? AIM: To explain the sediment cell concept as a system and what human and natural factors create/upset a dynamic equilibrium. ST: On your copy draw and label future depositional landforms you expect to develop. Will it look like the covered version below? 1
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The Coastal System A Coastal area can be seen as a system which produces, transfers and deposits sediment. It is an open system with inputs, sinks (stores) and transfers and outputs. 1 -Demonstrate this system resorting the statements as a group on the followin slide. (do this at the computer) Copy the completed table into your notes. 3
Longshore drift along coast Weathering and erosion of cliffs Sediment Outputs Sediment Transfer Beach/sand dunes Sediment inputs Sediment removal by human intervention Sediment lost to open sea Deposition by rivers Off shore bars Wind blown movement Sediment Sinks 4
Match the source with it's percentage Human replenishment schemes 0.2% 90% 4.8% 5% 0.2% Cliff and sub ariel erosion 4.8% Coastal sediment origins Sea bed combing* 90% River Bedloads 5% * Sea bed combing refers to material dragged up from the sea bed as sea levels rose after the last ice age. Copy into your notes before reading the next slide. 5
Generally inputs match outputs keeping the system in equilibrium. This equilibrium can be disrupted at times. eg. storms, but is restored over time. 6
On your copy of pg. 56 (OCR A/S) 1 On Holderness map highlight in different colours Imputs, Transfers, Sinks and Outputs. Add a key. 2 Activities 1 and 2 3 On sediment cells handout highlight the 11 main sediment cells on Uk as shown. Title your handout when you stick it in. 4 Use A3 worksheet (pgs.116 117 Edexcel book) Outline the characteristics of a sediment cell. 5 Define and example positive and negative feedbacks in sediment cells. Define DYNAMIC EQUILIBRIUM. Ext: Councils often operate in isolation when developing or protecting their coastlines. Why is it so important that this view is scrapped for a more wholistic view of sediment cells. 7
Sediment Cell definition revision slide. The processes of erosion, transportation and deposition within the coastal margin is largely contained within sediment cells or littoral cells. There is thought to be 11 large sediment cells in England and Wales as shown in the map. A sediment cell is generally thought to be a closed system, which suggests that no sediment is transferred from one cell to another. The boundaries of sediment cells are determined by the topography and shape of the coastline. Large features, like the peninsulas, such as the Llyn Peninsula in Wales act as huge natural barriers that prevent the transfer of sediment. In reality however, it is unlikely that sediment cells are fully closed. with variations in wind direction, and tidal currents. It is inevitable that some sediment is transferred between cells. There are also many sub cells of a smaller scale existing within the major cells. The sediment cell is usually in a state of equilibrium with inputs and outputs operating at similar levels. If sediment levels drop waves continue to transport material and new areas of coast, lacking a beach will be prone to erosion. This new erosion will re establish the equilibrium. This is known as a positive feedback. (one change has led to another) Conversely if too much sediment enters the system then the waves cannot carry it all so more deposition will take place. This is then removed later when sediment levels drop and the sea can handle the supply. Equilibrium returns. This is known as negative feedback. (The system returns to an original phase) The balance of this sediment is a changing process and is known as a dynamic equilibrium. Sediment cell clip if needed. https://www.youtube.com/watch?v=vqmh2kk1sy4 8
East Head Spit, Portsmouth The groynes placed up drift have starved the spit of sediment with severe consequences after a storm. This demonstrates the need to understand sediment cells and their characteristics along our coastline. The DYNAMIC EQUILIBRIUM of this system has been altered. 9
Sediment sinks are our friend and they help protect our coast. They can absorb wave energy. Consider what happens to waves when they hit a beach. Often depositional features are transitional and prone to disappearing or shrinking. From beaches to spits, they can change rapidly. We often seek to STABALISE the features and fix them in place. How can this be done? Think sandunes and saltmarshes. 10
Planting and fencing can hold sediment in place. This will lead to a succession sequence which will further stabalise the coast. Revise your notes from Lesson 5 succession) saltating sediment reaches back of the beach encounters debris, vegetation this creates turbulence slows the wind sediment is deposited builds up vegetation starts to colonise stabalising the coast succession occurs Spits, beaches and other depositional features can be held in place by plants. Spec Speak 11
We look at sediment cells closely to work out inputs, stores, transfers and ouputs. It helps us get an idea of what impacts we can have. Look how the Severn Estuary has been mapped. An event here in Dec 1981 really helped us understand how the dynamic equilibrium of sediment cells works. input input store 12
Over to you. Apply your knowledge. You will work in threes. You will have 5 pieces of information about an event in the Severn Estuary in December 1981. Your job is to answer the following questions in 10 minutes. What conditions created the coastal flooding? Be precise, use data. Explain why some areas of the coast did not flood. (knowledge of why waves break needed here) Explain how the sediment cell was changed at a local level and how this event is a good example of 'dynamic equilibrium'? What evidence is there that we should seek to plant and stabalise these environments for the future? Bonus Q: Why did human defences fail? We will go over this at the end of the lesson. 13
AIM: To explain the sediment cell concept as a system and what human and natural factors create/upset a dynamic equilibrium. If you were accurate with your answers to the group task then you should know this element of the specification. 14
Print next five slides for the group task 15
Air pressure The lower the pressure the higher the sea is able to rise as there is less air pushing down and 'levelling' the surface. Anything below 980 millibars often heralds large storms. 16
South west winds blew sea water and waves into the Bristol Channel. 17
Beaches or sand dune systems. Look closely at this map. What is significant about where the coastal flooding happened? 18
Sediment deposited (off shore bar) Sandunes eroded remember winter/summer profiles! 19
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Attachments Sediment cells