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Water ResourcesIntroduction
To implement an effective pollution control and river management system on the Tweed, it is necessary to have information on the rainfall and river flows within the catchment. This is achieved by a network of measuring sites and river gauging stations situated throughout the Tweed system, and this sheet describes how this information is gathered.
The Monitoring Network
Within the Tweed catchment there is a network of some 80 daily rainfall measuring sites, operated together with the recent addition of 7 rain gauges capable of measuring rainfall automatically every 15 minutes. The data from the automatic gauges are particularly valuable for providing more accurate assessments of the intensity and duration of rainfall likely to result in flooding.
The measurement of river flow is crucial in the development of a pollution control strategy. The greater the flow the greater the level of dilution available to enable the river to assimilate effluent discharges into it. A primary network of some 26 river gauging stations permit the continuous recording of the river level and measurement of a full range of river flows. Each gauging station houses cableway equipment for measuring flow and various for recording water level. Electronic logging equipment stores these measurements and via a connection with the telephone network the data can be transferred to a central point as well as generating alarms to a series of telephone numbers when predetermined river levels are reached. In addition to its obvious advantage at flood times the telephone link provides a very effective tool for day to day river management purposes. In addition to the primary network measurement, river flow is also measured on an ad hoc basis at several other locations. Comparisons of these measurements, with those from catchments with gauging stations, provide a reasonable means of deriving flow estimates on catchments without gauging stations.
The Importance of River Flow
The extremes of river flow are of great importance. For example, when designing for sewage works or trade effluent treatment plant a good knowledge of dry weather and drought flows is essential, since it is at these times that rivers are likely to be under most stress in their ability to absorb the impact of any discharge. At the other end of the spectrum, the occurrence of floods can cause damage and endanger life.
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Reservoirs and River Flows
Water supply reservoirs in the headwaters of several Tweed tributaries such as Megget and Whiteadder allow river flow to be enhanced, especially during the drier summer months. The prime purpose of these reservoirs is to collect and store water for water supply, but they also return water back into the river from the reservoir which is described by the term "compensation" water. Due to its involvement in pollution prevention, rainfall measurement and rainfall gauging, SEPA has developed a role as impartial negotiator in agreeing with the water supply interests, the amount of compensation water, rates of discharge etc. Modern policy on compensation water has moved away from the older system where a fixed amount of water was released daily from the reservoirs throughout the year. Under the new system higher rates of flow are released in the summer, with occasional releases of even larger flows known as freshets, which may last a day or more. Freshets are released when it is considered that it would be beneficial to the river, for example, after a prolonged period of low flow rates. The ability to provide adequate compensation flows throughout the year is generally beneficial, and in a dry summer, flows in sections of the Tweed may be nearly 50% higher that they would have been if no reservoirs existed.
Land Drainage and Flood Prevention
The legal position between Scotland and England with regard to land drainage and flood prevention is different. In Scotland, land drainage and flood prevention of agricultural land is the responsibility of the owner or occupier of the land. Within urban areas, the Local Authority has powers to carry out works to prevent or mitigate flooding.
In England and Wales, the Environment Agency has a general supervisory role over all matters relating to land drainage and flood defence. For those watercourses classified under the Water Resources Act as Main River, the EA has powers to maintain, improve or construct new works on them for drainage of flood defence purposes. It also has powers to prevent flooding from the sea. Additionally the EA has a comprehensive set of Byelaws to regulate the actions of owners of water courses and others, by preventing people acting in a manner which might obstruct the flow of the river or hinder the EA in the execution of its flood defence work. Details of flood defence works, etc already carried out or planned are available from the Northumbria and Yorkshire Region of EA for that part of the Tweed within English jurisdiction. For similar information on the rest of the river system contact should be made in the first instance with SEPA who would be able to direct any enquiries to the appropriate individual or authority.
Both the SEPA and the Northumbria and Yorkshire Regions of the EA operate flood monitoring schemes in their respective areas incorporating a series of warning states namely, Alert, Warning and Danger and various actions are taken under the overall supervision and authority of the Police. This information is readily exchanged where flooding occurs within the Tweed catchment with regular updating until the flood situation has passed.
Abstraction
The law relating to general river water abstraction in England and Wales is quite different from that in Scotland. The precise detail of this is not discussed here but can be obtained from the respective river authorities. In England and Wales the Environment Agency has a statutory responsibility to regulate water abstraction and impoundment. These statutory powers are contained within the Water Resources Act 1991 and mean that with limited exceptions abstraction of water requires a licence frrom the EA. However these water resources regulations powers are framed specifically to exclude their application to any inland waters that are part of the river Tweed. The existing legislative position therefore leaves the River Tweed with no regulatory body to control surface water abstraction or impoundment and legally water can be abstracted from the Tweed without any need for an abstraction licence. In this situation English Nature has a role in granting abstraction permissions and does so for example in the Till catchment.
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