T' Shrewsbury Flood Alleviatio
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w t' Shrewsbury flood alleviatio > > Your questions answered Environment WM A gency ENVIRONMENT Ag en c y Shrewsbury is one of the best preserved medieval towns remaining in the country, and its rich architectural and historical heritage is evident throughout the town. The town centre stands on a hill in the middle of a horseshoe loop on the River Severn in an enviable location near to the English/Welsh border. ...The River Severn has always played a central role in the development of the town, and it remains an important asset... SBURY Over many years development has spread down to the low-lying flood plain, particularly around the Welsh and English Bridge areas. This flood plain is the natural overflow area for the river in times of high flows, and buildings on this land are at constant risk of flooding. The increased frequency of recent floods has highlighted the need to look urgently at the possibilities for alleviating the effects of flooding in Shrewsbury. This booklet provides some background information and examines many of the potential solutions that have been suggested. It also outlines the process for designing and implementing a scheme that the Environment Agency will follow - this process ensures that any environmental impacts are addressed and opportunities to enhance the environment taken. Shrewsbury floods - November 2000 A. ENVIRONMENT AGENCY View of the River Severn • • • JL. Shrewsbury floods - November 2000 Floodin Shrewsbury has a history of flooding with the earliest reported flood occurring in 1338. Reliable records date from 1672. The highest recorded level was in 1795 when the water at Welsh Bridge rose to 5.70 metres above the riverbed. This is nearly half a metre deeper than the November 2000 floods that reached a level of 5.25 metres. in the The largest flood in living memory 400 properties can be affected by occurred in 1946, when a water a major flood, with water up to level of 5.43 metres was recorded, 2 metres deep in places. The with flood water entering the Abbey effect on the town is dramatic; Church. Over the last 350 years a this includes disruption to traffic sj» i f major flood has caused significant and to public transport (both bus damage, on average, every ten and rail), with a knock-on effect to years. However, the time between the emergency services. Amenities II floods can vary significantly and cannot be reached, trade and n i h: an unusually flood free period commerce in the town is lost I III during the 1970s, 1980s and early and affected home and business 1990s served to mask the ever owners suffer great upheaval and present risk. distress. This is made all the worse when clean up operations are When the river flows are high, wrecked by recurring flooding. roads and property in the low lying areas of the town, especially During the early 1990's a flood around the bridge crossings, are defence scheme was proposed vulnerable to flooding. As many as for Shrewsbury by the A helping hand in the floods of November 2000 ► Environment Agency's predecessor It is important to remember that the National Rivers Authority. Shrewsbury is being considered However, there were doubts about along with other towns and some aspects of the proposals, in villages that were affected by the particular the perceived visual autumn 2000 floods, but the impacts, and the scheme did not Agency has secured funds that proceed. Subsequently, Shrewsbury should allow a £6 million scheme was lucky to experience several to start during 2001. The years when no significant flooding Government has released extra occurred, however, this luck ran money for defence schemes that out and more frequent and can commence quickly, and the damaging floods returned. Agency is keen to ensure that Shrewsbury benefits. In the autumn of 2000, the worst flooding for over 50 years devastated the length of the River Severn, hitting Shrewsbury Shrewsbury Abbey ^ particularly badly. The town was extensively flooded three times in the space of six weeks. Public and political interest was raised, resulting in a renewed drive to .■* ' provide a flood alleviation scheme. The Agency had already begun to re-consider flood defences for Shrewsbury after floods in 1998, but following the recent events the programme has been accelerated. town The Agency has permissive powers to protect people and property from river and sea flooding. Money for this is raised through a levy on local authorities and from grants obtained from A Shrewsbury Abbey during the floods of November 2000 Central Government. To ensure that money is spent to the best effect, every scheme has to meet What about towns and villages upstream Treasury Rules implemented by the Ministry of Agriculture, and downstream of Shrewsbury? Fisheries and Food (MAFF). This process is carried out using a The scheme is being proposed in the context of a catchment wide benefit/cost analysis. This means River Severn Strategy, looking at the river as a whole. Although schemes that the limited funds available are being proposed for Shrewsbury and Bewdley these schemes are not are spent where most benefit can being developed in isolation, and careful calculations and extensive be gained. If a scheme does not computer modelling will be used to ensure that neither of these schemes meet the benefit/cost criteria, it impact negatively on other communities. When weighing up which are can not go ahead. the best options, neighbouring communities are a clear priority. • • o Shrewsbury floods - November 2000 What are the options Ifn i- . Shrewsbury? Shrewsbury floods - November 2000 There are many options that have ♦ been discussed. These include: • Creating upstream storage lakes • Dredging the river • Building more weirs and dams • Changing the course of the river into bypass channels or tunnels • Creating underground storage • Building defences in the town itself • A combination of solutions So what about upstream storage lakes? There are already three main areas of upstream storage which store water during a flood event - Llyn Clywedog reservoir, Lake Vyrnwy and the Severn/Vyrnwy confluence area. Firstly, at the top of the line has been in use since the late are only experienced close to catchment is the Llyn 1970s to optimise the modicum of the reservoir, and not as far away 1Clywedog reservoir. protection benefit. as Shrewsbury. This was built in the 1960's When full, Llyn Clywedog stores for the purpose of regulating flow up to 50,000 million litres of water. Thirdly, there is an existing along the River Severn, in order to The flood protection benefit can area of floodplain storage supplement low flows in the be significant immediately upstream of Shrewsbury summer during drought conditions. downstream at Llanidloes, but the 3 in the Severn/Vyrnwy The reservoir is owned and benefit diminishes rapidly as you confluence near Melverley. operated by Severn Trent Water go downstream. At Welshpool, Here a system of argaes (low flood and managed by the Agency. A some 55km downstream, the embankments) that date back statutory flood 'drawdown line' benefit is greatly reduced. In some 200 years, provide some requires that under non-flooding Shrewsbury, which is a further degree of flood protection. When conditions, the reservoir is 25km downstream, the benefit is the argaes are overtopped, a operated whenever possible, at a very small indeed. In fact, the series of sluice gates hold back level below the crest of the dam. Llyn Clywedog catchment approximately 20,000 million litres In fact, a much lower discretionary represents only 1.8% of the total of water which provides some catchment to Shrewsbury. additional flood protection to Shrewsbury. To protect Shrewsbury from a 1 in 100 year Lake Vyrnwy is located event* at least a further approximately 60km 40,000 million litres of storage upstream of Shrewsbury would have to be provided. This 2 beyond the confluence of would mean rebuilding and the Severn and Vyrnwy rivers. increasing the height of over The dam was built over one 40kms of floodbank. The cost of hundred years ago for the purpose providing this additional storage of supplying drinking water to would be substantial and certainly Liverpool. It is owned and at least more than five times the operated by Severn Trent Water cost of providing defences in but the water supply is licensed to Shrewsbury itself. If such a North West Water. The reservoir scheme were built, many homes also provides a small flow and other property would be component to the River Severn flooded. Large areas of agricultural regulation system and tops up low land would be inundated for summer flows. The catchment of much longer periods, thus this reservoir accounts for only 4% significantly reducing their of the total catchment area to value, and communities like Shrewsbury. Consequently, as with Melverley would suffer much Clywedog, any protection benefits bigger floods. A Quarry/River Severn * Floods are categorised by their sue and the frequency with which they can be expected to occur. A 1 in 5 year flood « one that has a 20% chance of happening in any year - this is a relatively minor Hood. A 1 in 100 year flood has only a 1% chance of happening In any year, but its effects can be enormous. Why not dredge the river? Dredging the river is often suggested as a simple solution. However, dredging would have big implications for the town and the wider riverside environment. During normal conditions, the To undertake such work, rock River Severn through Shrewsbury blasting of the bed would flows at just over 50 cubic metres be necessary. per second. During events that flood the town, flow is approximately If dredging were to be carried 450 cubic metres per second, out, bridges and other structures some 9 times greater than under adjacent to the river would need normal conditions.