Page 43 PCP Teachers Notes 1—About The Parrett Catchment Project The Parrett Catchment Project (PCP) is a partnership, established in 2000, whose long term goal is to reduce the frequent and severe flooding in the lower river Parrett basin. The partnership can do this by developing a way to manage the use of land and water which can easily be continued but with mini- mal long term effects on the environment (i.e. is sustainable). The partnership’s way of managing land and water will also need to try and benefit the economic, social and cultural life of the River Parrett catchment and protect and improve the environment for future generations. The Parrett catchment covers half of Somerset, incorporating 180 parish councils and approximately 138,000 households. The major towns in the catchment are Bridgwater, Taunton, and Yeovil. The catchment covers lots of different scenery from the arable uplands around Sherborne to the wet grass- lands of the Somerset Levels and Moors. Twenty seven stakeholder organisations now form the PCP partnership. The project is paid for by a group of nine organisations, including Somerset County Council and the Environment Agency. In 2002 the PCP also won lots of financial support and recognition as part of the European ‘Joint Approach for Managing Flooding’ (JAF) project. The JAF project also gets money from the Environment Agency. The money from the JAF project has enabled the PCP to deliver its innovative ‘Farming Water’ pro- gramme, which is made up of a number of demonstration and awareness raising projects: · Flood retention schemes · Flood storage schemes · Woodland development · Arable reversion to grassland · Soil management · Monitoring The JAF project also encourages co-operative working with partner organisations in Germany and The Netherlands, where there are lots of existing programmes already that provide ‘Farming Water’ schemes. To date the PCP has successfully delivered many of the components of the JAF project, and now that we are in the final year of JAF we are reviewing the future direction of the PCP and future funding sources. Page 44 PCP Teachers Notes 2 - What Causes Flooding? These notes may be useful to share with children during Task 3: Floods are not caused by any single thing and flooding in different places is usually the result of a different combination of issues. For example, snow can contribute to flooding in some places, but not others, and construction on the floodplain can make flooding worse sometimes, but not always. All floods have one thing in common however, and that is that there is a period when there is a lot of rainfall and the rivers cannot take the water from the land to the sea quickly enough, so they overtop their banks and inundate the surrounding area. High rainfall is usually the main cause of flooding, but other MAKE IT WORSEMAKE problems usually make it worse. They are: LOTS OF THINGS CAN FLOODING OR CAUSE Lower capacity of rivers—in other words, they can not 1 hold as much water. The capacity of a river is reduced when: The river is already at a high level, which reduces the amount of extra water it can take. A prolonged period of heavy rain or melting of snow can cause higher a) than normal water levels in rivers. The cross section of a river with a high water level and what happens when there is heavy rain. Page 45 PCP Teachers Notes 2 - What Causes Flooding? When a river is full of deposited silt, it reduces the size of the channel, making a river smaller and unable to cope with high rainfall levels. The effect is the same as if it had a high water level—the river cannot accommodate much extra water and may burst its b) banks. The tide can also make water levels higher than usual, especially in areas where the tide comes up the river. A high tide also has an impact further upstream as it prevents the river from discharging water to the sea. As a consequence, the river can back up like a c) clogged drain and gradually fill up to the point where it bursts its banks. Page 46 PCP Teachers Notes 2 - What Causes Flooding? 2 Increased runoff from land in the catchment Soil cannot absorb water from heavy rain because: The ground is frozen or covered in a deep layer of snow. The soil becomes less permeable, and a larger amount of rainfall than usual runs off the land and drains into rivers, a) which may be unable to cope with the increased flow. The soil has already absorbed as much rainfall as it can and is saturated. When this happens, the ground has no further capacity to absorb any more water and any rain b) that falls on it runs straight off and into rivers and streams. The soil has been compacted, meaning that it has been trampled or compressed, creating a hard top layer that is c) not permeable. Under the soil, the rock is impermeable. Some kinds of rock are more porous than others and can absorb water. Different types of rock also hold it for different lengths of d) time. The ground has been covered with an impermeable surface such as concrete or tarmac or e) buildings have been constructed. Roofs and other impermeable surfaces do not soak up water, so rain runs straight off them, down the drains and into rivers. Note: Flash flooding is a particular type of very short-term flood—it happens when there is ex- ceptionally heavy rain (during a thunderstorm, for example) and usually only lasts a very short time. They are, however, very dangerous as they are notoriously difficult to predict and people may not get very much warning about them. WHERE IS FLOODING MOST LIKELY TO HAPPEN? The floodplain is the flat, relatively low lying area of land around a river and is the first place to be affected when a river bursts its banks. It is also a place where a lot of development and construction has often taken place. Because rivers have, historically, been such important places for industry and transport, towns are usually built on their banks. More recently, people have built new homes near rivers because they are attractive places to live. These trends have put properties at greater risk of flooding. Page 47 PCP Teachers Notes 3 —What features of the Parrett Catchment make it prone to flooding? Earlier on we learnt a lot about what causes flooding or makes it worse throughout the world — rivers that have been straightened, woodland destroyed, land that has been made impermeable. Now it is time to look at some features specific to the Parrett catchment make flooding a problem in the area. 1) ‘EXTREME’ RAINFALL From the previous exercise you will know that the Parrett Catchment is not especially wet, so flooding isn’t due to a high average rainfall. Flooding only happens when there is excessive or extreme rainfall. In winter 2000 the rainfall experienced in Somerset was extreme. Two storms hit Somerset very close together allowing the rivers little time to evacuate the water from the first storm. With climate change extreme events are predicted to become more frequent. 2) DRAINING THE FLOODPLAIN The levels and moors have been gradually drained by man for agriculture over the centuries. The river system in this area does not act like a natural floodplain. It is man- aged by man through a system of sluices, locks, weirs and spillways. Large parts of the area are only a few metres above sea level. 3) POOR AGRICULTURAL LAND MANAGEMENT Changes in the way middle and upper catchment farmland is farmed can make a sig- nificant difference to flooding in the lower catchment. Poor farming practices can lead to compacted and capped soils which are impermeable to rainwater. Water runs quickly off fields often taking soil with it and depositing it in drains and on highways. During a rainstorm blocked drains and ditches can lead to serious localised flooding. 4) RUN-OFF FROM URBAN DEVELOPMENT Over the past 10 years, more and more homes have been built in the catchment. The hard surfaces of these developments, including tile, concrete, brick and Tarmac, do not soak up water. Rain runs straight off the roofs and roads and into the drains and rivers very quickly, placing stress on the drainage and river system. 5) LOSS OF WETLAND HABITATS Originally the Somerset Levels and Moors were wetlands, naturally flooded by a mix- ture of seawater and river water every winter. Wetlands, such as wet grasslands, reed- beds and wet woodland, act like giant sponges absorbing huge volumes of floodwater. By turning wetlands into farmland the amount of water that can be stored naturally in the catchment has been dramatically reduced. 6) REDUCED RIVER CAPACITY As indicated by the brown colour, the waters of the River Parrett are laden with silt or suspended solids. This silt is deposited along the sides of the river channels as mud and this reduces the size of the channel, making it smaller and less able to carry high volumes of water after rain storms. Page 48 PCP Teachers Notes 3 —What features of the Parrett Catchment make it prone to flooding? 6) RIVER BANKS FAILING In the Parrett catchment thankfully flooding has never happened due to river bank fail- ure. At the moment the banks are strengthened and upgraded regularly by the Envi- ronment Agency. But, with sea level rise in mind the tidal river banks cannot continue to be raised forever, because the river will get higher and higher thus increasing the danger.