Introduction
In order to effectively manage the water environment of the Tweed catchment, it is necessary to have information on rainfall and river flows throughout the catchment. This information is gained by analysing data from a network of measuring sites and river gauging stations situated throughout the Tweed system.
The monitoring network
Within the Tweed catchment there is a network of rain gauges providing valuable assessments of the intensity and duration of rainfall likely to result in flooding. SEPA operates 80 rain gauges and 26 river gauging stations within the Scottish Tweed and the Environment Agency has responsibility for water flow and water level monitoring in England.
The measurement of river flow is crucial for pollution control and flood warning. The greater the flow, the greater the level of dilution available to enable the river to assimilate effluent discharges into it. SEPA operates river monitoring stations that record river level and, from these recorded levels, produce 15 minute flow figures in m³/s. An important use for these stations is to provide data and warnings for the flood warning system covering the Tweed area.
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 plants, 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 to structures and land and endanger life.
Reservoirs and river flows
Water supply reservoirs in the headwaters of several Tweed tributaries, such as the 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, a process known as “compensation” water. Due to its involvement in pollution prevention, rainfall measurement and rainfall gauging, SEPA has developed a role as impartial negotiator in agreeing the amount of compensation water with water supply interests, as well as rates of discharge and so forth. Modern policy on compensation water has moved away from the old 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 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.
Flood prevention
While flooding is a natural process it can be exacerbated by activities such as canalisation; riverworks; modification and diversion of rivers; land drainage; poor agricultural and forestry practices; or floodplain development. Addressing those land management practices that increase flood risk will aid effective and sustainable flood management. However, there is also a need to avoid short-term localised solutions and instead use a more catchment-based approach which does not transfer problems from one part of the catchment to another. Indeed, a catchment-based approach was required by various EU legislation, such as the WFD and Floods Directive, as was the need for exploring land management practices that allow the natural processes of a river to function (Sustainable Flood Management), and it remains to be seen how much the UK will continue to support this post-Brexit.
The legal position between Scotland and England with regard to flood prevention is different, yet they are both subject to the over-arching Floods and Water (Amendment etc.) (EU Exit) Regulations 2019. For flood prevention in Scotland, SEPA and local authorities take the lead role, while, in England, this responsibility falls to the EA and Lead Local Flood Authorities. Development control and planning responsibilities, which are important components of flood management, are also split within the Tweed catchment between Scottish Borders Council on the Scottish side and Northumberland County Council on the English side. Both SEPA and the EA operate flood monitoring schemes in their respective areas, as well as flood warning services – Floodline in Scotland and a flood warning sign-up service in England. Information is readily exchanged where flooding occurs within the cross-border Tweed catchment, with regular updating until the flood situation has passed.
Abstraction
The law relating to river water abstraction in England and Wales is quite different from that in Scotland. In the Till/English Tweed, abstractions are authorised as per the Till and Tweed Abstraction Licensing Strategy. In Scotland, abstractions are regulated by the Environmental Authorisations (Scotland) Regulations 2018 (EASR).