Candover Licence renewal Part 1 report. DRAFT. 11 August 2016
Renewal of the Candover Scheme Abstraction Licence Part 1 - Hydrological Impact Analysis
Final Draft to accompany licence renewal 11th August 2016
Candover Licence renewal Part 1 report. DRAFT. 11 August 2016
We are the Environment Agency. We protect and improve the environment and make it a better place for people and wildlife. We operate at the place where environmental change has its greatest impact on people’s lives. We reduce the risks to people and properties from flooding; make sure there is enough water for people and wildlife; protect and improve air, land and water quality and apply the environmental standards within which industry can operate. Acting to reduce climate change and helping people and wildlife adapt to its consequences are at the heart of all that we do. We cannot do this alone. We work closely with a wide range of partners including government, business, local authorities, other agencies, civil society groups and the communities we serve.
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© Environment Agency 11th August 2016 All rights reserved. This document may be reproduced with prior permission of the Environment Agency.
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Context of document This is the first in a series of three documents which provide the technical evidence to support our application to renew the Candover Scheme licence in the Summer 2016. Although the licence was varied in 2015 to account for changes necessary to meet the requirements of the River Itchen SAC Habitats Regulations Review of Consents Site Action Plan (2007), the renewal of the licence requires consideration of a greater range of habitats and species and an assessment of impacts on other rivers. Further analysis and changes within the catchment that have happened, or are likely to happen, have resulted in our recommendations to place further constraints on the licence. For that reason this is not simply a renewal of the licence on the same terms, it is a further tightening of conditions on the licence to improve environmental protection. As both the operator of the Scheme and also the environmental regulator determining the permit, this may be viewed as an unusual situation. This document is provided by the Solent & South Downs Area to support our application to renew the licence, which we intend to make in Summer 2016 to allow the licence to be renewed before it expires on 31st December 2016. In accordance with our standard procedures, the recommendation on renewal of the licence will be determined by our National Permitting Service (NPS) with the final decision offered to the Secretary of State for sign off. We are informing a range of local stakeholders about our proposals, giving them the opportunity to comment on this application. To help shape the revised licence, we have sought opinions from Natural England, Hampshire & Isle of Wight Wildlife Trust and our own technical specialists. Natural England will play a formal part in the process of renewing the licence as they are a statutory consultee for abstraction licences that have the potential to affect designated sites. This is the first of three documents that together make-up our assessment of the Candover licence renewal. This report will explain why the scheme is needed and what hydrological impacts it can have on the Candover Stream, River Itchen and surrounding watercourses. The second document follows-on from this hydrological assessment, and is an assessment of the ecological risks and consequences of operating the Candover licence, against the River Itchen SAC and SSSI interest and wider biodiversity. It presents the existing ecological data and considers risks against a number of ecological flow targets. The third, and final document, considers the outcomes of the first two assessments and presents our final conclusions and recommendations on the way forwards.
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Executive Summary
The Candover Scheme was licensed for use in 1980 following extensive test pumping in 1976. Since that time, the Scheme has been used several times by the Environment Agency during drought events to support flows in the River Itchen. At this time, we believe that the Candover Scheme still has a useful role for managing water resources in the Itchen catchment and so we must renew the abstraction licence before it expires on 31st December 2016. Before the licence can be renewed we need to carry out further assessment to clearly identify any associated environmental impacts when the scheme is used. This report contains the technical evidence to assess the hydrological impacts of the Scheme. We use this GW augmentation scheme to support downstream abstraction during extreme droughts. The current licence accounts for around 2% of all abstractions in the upper Itchen catchment which includes large abstractions for watercress, fish farms and public water supply. This report clearly shows the context in which these impacts are assessed. Abstractions for watercress and public water supply clearly have a far more significant impact on flows than the Candover Scheme as the impacts are larger and persist throughout the year. Use of the Candover Scheme can contribute to those impacts when it is used. The current version of the Candover Scheme abstraction licence, issued in 2015, contains detailed conditions which constrain when and how the Scheme can be used. This means that over the last 60 years, the scheme may have been needed up to 10 times. However, there is the possibility that the Scheme could be used several years in a row under the current terms of the licence: the licence currently allows repeated use. When the Scheme is used, the immediate impact is to increase flows in the Candover Stream and to cause drawdown in the groundwater of the upper Candover valley. The increase in flows in the Candover Stream is managed so that there is a gradual increase, but in the lower reaches of the Stream, flows can still be double what they would otherwise be. Licence conditions ensure that flows cannot rise above the highest flows seen naturally in the Candover Stream for the summer period. Further downstream on the River Itchen, flows typically remain within 10-15% of what would naturally be expected. The introduction of water from the Scheme leads to a lowering of daytime maximum water temperatures in the Candover Stream. There are not likely to be any other water quality impacts from use of the scheme as it introduces groundwater which would otherwise have been flowing naturally into the river system. Once the scheme has been turned off, the lowering of groundwater levels caused by its use results in a reduction in flow in both the Candover Stream and some of the surrounding watercourses - namely the Rivers Dever (in the River Test catchment), and River Wey (in the River Thames catchment), and Alre (upper Itchen catchment). The largest impacts on flows tend to be whilst flows are rising as a result of natural recharge but reductions in peak winter flows are also likely. Depending on how much the scheme has been used and how rapid the natural recharge has been, small scale impacts can persist during the Spring and Summer following use of the scheme and in exceptional conditions, very small impacts can persist for longer than a year. The scale of impact varies, ranging from a maximum reduction in flows of up to 50% in the upper Candover Stream; 10-20% in the middle reaches of the Candover Stream, 5-10% for the lower Candover Stream; and for the River Alre, River Dever and River Wey between 5 to 30% reduction in river flows, reducing after a few months to typically less than 2% for the duration of up to 12 months.
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The Candover Stream and surrounding watercourses often have long winterbourne reaches which naturally flow when groundwater levels are high. Use of the Scheme does not affect the pattern of flow in any of these winterbournes, although modelled predictions suggest that the rise in stream heads in the Autumn and subsequent fall in the Spring may be delayed by a few weeks with an typical impact of up to 250m (up to 1.75km maximum) on the location of the stream-head. Compliance with Water Framework Directive tests for both the surface water body of the Candover Stream and associated groundwater bodies has been assessed and use of the Scheme will not prevent achievement of good status by 2027 and will not lead to deterioration in ecological status.
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Contents
Renewal of the Candover Scheme Abstraction Licence ...... 1 Part 1 - Hydrological Impact Analysis ...... 1 Context of document ...... 3 Executive Summary ...... 4 1. Background ...... 7 1.1. The River Itchen ...... 7 1.2. Augmentation of the River Itchen ...... 7 2. Need for the Candover Scheme ...... 12 2.1. Environment Agency need for the Candover Scheme ...... 12 2.2. Southern Water need for the Candover Scheme ...... 14 2.3. Summary of potential scenarios of use ...... 14 3. Available Data and Tools ...... 17 3.1. Hydrometric Monitoring...... 17 3.2. Abstractions and Discharges ...... 19 3.3. Test Pumping ...... 19 3.4. Test & Itchen Groundwater Model ...... 19 4. Characterising the hydrology of the Candover Stream ...... 22 4.1. Upper River ...... 22 4.2. Lower River ...... 22 4.3. Summary of Hydrology of the Candover Stream ...... 25 5. Impacts of the Candover Scheme acting alone ...... 26 5.1. Candover Stream ...... 26 5.2. Impacts on flow in the River Itchen ...... 33 5.3. Impacts on flows in surrounding watercourses ...... 33 5.4. Impacts on groundwater levels ...... 36 5.5. Southern Water summary of impacts ...... 40 6. In combination impacts of the Candover Scheme ...... 41 6.1. Fobdown licence changes ...... 41 6.2. Upper Reaches ...... 42 6.3. Lower Reaches ...... 43 6.4. Summary of In Combination Abstraction Impacts ...... 50 7. Impact on Water Quality ...... 52 8. Summary of Abstraction Impacts ...... 53 9. Water Framework Directive ...... 57 9.1. Surface water body ...... 57 9.2. Groundwater Body ...... 57 9.3. Summary of WFD Compliance ...... 62 10. Conclusions of Part 1Report ...... 63 6 of 66
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1. Background 1.1. The River Itchen The River Itchen starts as three main tributaries rising from the Chalk in Hampshire around the Town of Alresford. The Cheriton Stream, River Alre and Candover Stream come together at Alresford to form the River Itchen, which then flows to Southampton Water via Winchester, Eastleigh and Southampton (see figure 1.1). Groundwater from the Chalk forms over 95% of the flow in the river providing a relatively stable hydrological regime where winter flows are typically no more than 4 to 5 times the flows in the summer. However, after heavy and prolonged winter rain, the tributaries may start flowing many kilometres above their usual sources, and increased flow in the river can cause flooding. Following lengthy dry periods, the river is remarkably resilient with flows being sustained from the Chalk aquifer. However, the sources of the tributaries naturally retreat down the valleys and flows in the rivers and levels in the adjacent wetlands fall, which can have an impact on the designated species and habitats. Much of the River Itchen is designated as a Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI). The designated features include those typically found in high quality Chalk Stream ecosystems, and are afforded the highest level of environmental protection under UK and European law. Located in South East England, the river is subject to many pressures from factors such as climate change, abstraction, waste water discharges and diffuse pollution. The river system has also been modified over a long period for uses such as navigation, water mills and water meadows. Recreational activities are also important with good access to the river along footpaths and a strong tradition of fishing. 1.2. Augmentation of the River Itchen The Environment Agency owned and operated two river augmentation Schemes which take groundwater from the Chalk and discharge it to the headwaters of the River Itchen. The Candover Scheme was developed in the 1970s by the Southern Water Authority to allow increased surface water abstraction lower down the river and to support water quality in the lower River Itchen under low flow conditions. The Alre Scheme was subsequently developed in the 1980s for the same purpose. The Schemes were transferred to the National Rivers Authority (NRA) in 1989, as their main purpose was defined at that time as environmental support, and so it was deemed more appropriate for the NRA to own and operate them at the time the Water Authorities were privatised. The Schemes subsequently passed to the Environment Agency in 1995. The Alre Scheme abstraction licence was revoked in 2015 and the Scheme is currently being decommissioned. 1.2.1. The Candover Scheme Initial proposals for the Candover Scheme were first made in 1971 with work starting in Autumn 1973. There are three pairs of boreholes in dry valleys upstream of Preston Candover at Axford, Bradley and Wield (See Figure 1.2). The boreholes are 73 - 120 metres deep and penetrate the Chalk aquifer. Each borehole can abstract around 4 - 6 Ml/d. The individual boreholes were initially tested in 1975 and a 13km overland pipeline was constructed to two discharge points on the Candover Stream prior to a full test of the Scheme in 1976. Over a six month period 5.06 million cubic metres were abstracted and discharged to the Candover Stream. Between 1972 and 1977 comprehensive monitoring of river flows, groundwater levels and ecological and water quality parameters were measured. Information about the scheme and its impacts was pulled together into a comprehensive report, The Candover Scheme Pilot Report, published by Southern Water Authority in 1979. 7 of 66
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Figure 1.1 Location Map
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Once the future of the scheme was secured, the pipeline was installed below ground and two discharge points were constructed - one at the major outfall; and one upstream of Northington, which was referred to as the Minor Outfall. Discharges from the Minor Outfall tend to soak away into the river bed and are not effective in supporting flows in the Candover Stream and so the outfall has not been used since 1997.
1.2.2. Candover Scheme abstraction licence history A copy of the current licence is included as Appendix A.
Initial issue of Candover Scheme licence The original licence for the Scheme was issued in 1980 following a substantial test of the Scheme in 1976. The Scheme was licensed for 36 Ml/d and 5,000 Ml/yr, with limited conditions.
Conversion of Candover Scheme licence to time limited status The licence was converted to add a time limit in 2007 The CAMS (Catchment Abstraction Management Strategy) Common End Date of 31st March 2013 was selected. No other changes were made.
Short term renewal of Time Limited Licence 2013 The licence expired on 31st March 2013 and was renewed with a very short time limit of 31st December 2016 to allow the work associated with the Review of Consents and work looking at the future of the Scheme, to be completed.
Review of Consents (RoC) licence changes - 2015 As the River Itchen is a Special Area of Conservation (SAC) the impact of all regulated permissions on site integrity had to be assessed. Following publication of the RoC Site Action Plan in 2007, it was determined that nine abstraction licences needed to be modified which included the Candover Scheme. The licence was subsequently modified in December 2015 when additional conditions were added to the licence to ensure that we could conclude that it could not have an adverse effect on the River Itchen SAC. A full explanation of the changes made and the reasons for them can be found in the Environment Agency document: Implementing the Review of Consents requirements for the Itchen Augmentation Schemes (2015). Whilst modifying the licence in 2015, we took the opportunity to work with Natural England and Hampshire & Isle of Wight Wildlife Trust to understand any concerns that they had over both the Alre and Candover Schemes. At that time, along with those other organisations, we joined Southern Water and WWF on Southern Water's Augmentation Technical Working Group where there was wider debate about the future of the augmentation Schemes. To assist in that debate, we shared a table summarising what possible licence changes may be needed in the future. That summary is attached as Appendix B. This acts as a useful starting point for issues that were raised in relation to factors that were not addressed in the licence changes in 2015
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Figure 1.2 Location of the Candover Scheme
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1.2.3. Time limited licence renewal changes - 2016 The Candover Scheme licence is time limited with an expiry date of 31st December 2016. When we apply to renew the licence, we must supply evidence to show how the abstraction meets the following three tests: Test one: Continued environmental sustainability Test two: Continued justification of need Test three: Demonstration of efficient use of water
Although we completed work in 2015 to ensure that the licence met the requirements of the Site Action Plan for the River Itchen SAC, to assess the environmental sustainability of the licence, we must consider wider issues such as: the impact of the licence on our Water Framework Directive (WFD) surface water and groundwater body objectives; our statutory obligations to protect and enhance SSSIs; and our duty to have regard to Biodiversity Action Plan (BAP) species and habitats - Priority Habitats and Species referred to under S41 of NERC Act. This requirement also extends to an assessment of possible impact on any surrounding watercourses such as the River Dever (River Test catchment), River Wey (River Thames catchment), and the River Alre (upper Itchen catchment). In addition, Natural England were not fully in support of the actions proposed in the RoC Site Action Plan (2007), and so any concerns they have about the River Itchen SSSI and SAC need to be addressed. Similarly, the Hampshire and Isle of Wight Wildlife Trust have residual concerns about chronic ecological effects of use of the Scheme, even after the 2015 licence changes. A key concern that was not resolved in the 2015 licence changes related to use of the scheme in successive years. We have clarified those remaining concerns in 2016 and will account for them in our assessment. In addition we must have regard to any changes that have occurred in the catchment that may affect our assessment of sustainability. This report provides the evidence needed to judge if the three tests have been met. It is accompanied by a separate report (Part 2) in which we set out our consideration of ecological risks and impacts and will address the issues raised by Natural England and others in 2015 and 2016. The final Part 3 report recommends any further licence changes that we need to make to ensure that the abstraction is sustainable. Together these three complimentary reports will be submitted in support of our application to renew the licence in Summer 2016.
Licence changes beyond 2016 There are several possible changes which could happen in the near future which would themselves trigger further modifications to the licences. These include:- • Change of ownership of the Candover Scheme; • Any movement of the main discharge point of the Candover Scheme; • Any changes to the use of the Scheme as a result of the above two points.
In 2016, if the licence is renewed, it will be issued with a new time limit at which point environmental sustainability will have to be assessed again. By the time the new time limit expires, there could potentially be new factors influencing that assessment including issues like: • new environmental flow targets for the River Itchen; • the possibility of Natural England designating the upper reaches of the Candover Stream as SSSI; • Changes in the operation of other licences in the catchment which could trigger a re- assessment of the WFD hydrological tests for the Candover Stream.
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2. Need for the Candover Scheme 2.1. Environment Agency need for the Candover Scheme When constructed, the purpose of the Candover Scheme was to support downstream Water Company abstractions by putting additional water into the River Itchen to provide better dilution of effluent and enable abstraction to continue. The Scheme also provided a benefit to other abstractors by allowing them to continue abstracting with no restrictions in dry periods. Abstractors benefitting from use of the Schemes paid additional supported source fees to the Environment Agency in recognition of this benefit. As we have worked with abstractors to modify their licences to protect the River Itchen the need for the Scheme has reduced as the environmental mitigation is now provided by a reduction in abstraction rather than abstraction support which we view as a move towards a more sustainable management of the river system. The number of sources defined as supported has reduced from seven in 2008 to four in 2016 and is likely to reduce further. However, Southern Water have clearly stated that they expect the Candover Scheme to continue to be used in the future to support their Otterbourne abstractions. Our Drought Plan explains how we would propose to use the Candover Scheme in a drought event and states that as flows approach 240 Ml/d at Allbrook & Highbridge, we would consider if we need to use the Scheme and if it is needed turn it on as flows reduce. Before assessing potential sustainability of the licence, it is important to understand how often the Scheme may be needed and at what rates abstraction is likely to occur.
2.1.1. Assessment of how much water is needed The way that the Scheme can be used under the terms of the current licence is heavily constrained by licence conditions imposed in 2015 which ensure that: • Annual abstraction is now only allowed up to a maximum of 3,750 Ml instead of the previous licensed rate of 5,000 Ml. • The Candover Scheme can only be used when flows in the main River Itchen as measured at Allbrook and Highbridge fall below 240 Ml/d. • Use of the Scheme has to be gradually increased and decreased which means that it takes several weeks before the full output of the Scheme is discharged. • Use of the Scheme cannot be allowed to cause flows in the Candover Stream, as measured at Borough Bridge, to rise above the maximum that could be expected for that time of year.
When considering historic flow data for the Gauging Stations at Allbrook and Highbridge, a scenario can be derived under the terms of the licence to simulate the maximum likely use during the period of available data of 1958 - 2015. Table 2.1 shows periods when flows have fallen below 240 Ml/d at Allbrook and Highbridge. In the fifty eight year period, flows have fallen below 240 Ml/d in fourteen years but have only fallen below 200 Ml/d in one year in 1976. There are several years when flows only marginally fall below 240 Ml/d or only fell below that flow for several days and in these circumstances, although the licence may allow use of the Scheme, it is very likely that we would choose not to use it. If operating under the terms of the current licence, the occasions when the licence could have been used and the extent to which it is likely to be used are shown in Figure 2.1. Realistically, we may have chosen to use the scheme in ten of the last fifty-eight years but for three or four of those years, use would have been minimal.
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Year Flows below Flows below Flows below 240 Ml/d 220 Ml/d 200 Ml/d 1959 Y Y - 1964 Y Y - 1965 Y - - 1973 Y Y - 1976 Y Y Y 1978 Y - - 1989 Y Y - 1990 Y Y - 1991 Y - - 1992 Y Y - 1996 Y - - 1997 Y Y - 2005 Y Y - 2006 Y Y -
Table 2.1 Years when flows fell below 240 Ml/d at Allbrook & Highbridge
Figure 2.1 shows this likely maximum use of the Scheme, as permitted under the terms of the 2015 licence conditions, compared with the actual use of the Scheme from its first testing in 1976. Clearly, maximum use of the Scheme will now be constrained by the new annual licensed amount of 3,750 Ml, and typically use of the Scheme is expected to be a lot lower than it has been historically with a modelled predicted maximum use of around 2,300 Ml in 1976.
Figure 2.1 Proposed maximum use of Candover Scheme
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The 1976 flow event is considered by many to represent an event with a probability of around 0.02 (2% or 1 in 50 years). If augmentation is required to support a more significant low flow event or if other abstractions in the catchment change, then the full annual abstraction of 3,750 Ml is likely to be required.
One limitation of this approach is that it relies on historical data and does not consider more extreme weather events than those observed. In addition, if consumptive abstraction by other licence holders increased beyond historic rates, it is possible that higher levels of abstraction would be needed and the scheme may be needed more frequently.. To address these limitations, in our technical work we have looked at a range of scenarios to ensure that we have considered the possibility of higher rates of abstraction. Time limiting the licence allows us to regularly review licences in the context of climate change and any other changing conditions.
2.2. Southern Water need for the Candover Scheme In their published Water Resources Management Plan (2014), Southern Water has stated that they want to acquire the Candover Scheme to offset the impact on their licences caused by imposition of the proposed hands-off-flow conditions to their Otterbourne licence. To form an effective part of their solution, they need to own the Scheme and have said they would wish to move the discharge point for the Scheme to the main River Itchen. To assess if the Scheme can provide the output they need in their Water Resource Management Plan, Southern Water were involved in working with the Environment Agency to test pump the Alre and Candover Schemes in 2011, at the extant discharge points. Their report "Pump testing and associated investigations of the Candover and Alre augmentation Schemes, summer 2011", written in August 2012, explains why they believed that the Alre Scheme was of no value to them but the Candover Scheme warranted further investigation. Southern Water proposed use of the Scheme would be potentially less frequent than the scenarios of use calculated in section 2.1.1 above, but when the Scheme is used it is proposed that it would be used more heavily. This is almost certainly due to a different analysis of likely maximum demands on public water supplies, whilst our own analysis has simply considered historic data. The type of use proposed by Southern Water would be permitted under the current terms of the extant licence. They have assessed the potential impact of this type of use on flows and groundwater levels in their report: Groundwater modelling assessment of the impact on river flows and groundwater level of using the Candover augmentation Scheme: Atkins Technical Assessment, 2014. This paper is solely concerned with renewing the abstraction licence whilst the Scheme is owned and operated by the Environment Agency. However, it is important to consider how the licence could be used in the future if ownership of the licence changed and if abstraction conditions in the catchment changed. Southern Water will need to assess the environmental risks of how they would operate the scheme, if they take ownership of it.
2.3. Summary of potential scenarios of use There are certain conditions currently contained in the abstraction licence for the Candover Scheme which dictate how frequently and how much the Scheme can be used which are described in the sections above. As there is uncertainty regarding how the scheme may be used in the future, we have considered several scenarios in our analysis.
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• Scenario1 - This scenario is simply based on operating the scheme under the terms of the current (2015) abstraction licence using recently experienced flows to predict the frequency and magnitude of use • Scenario 2 - We have already identified that use of the scheme in successive years is not likely to be desirable and so we have derived a scenario that restricts use in successive years. • Scenario 3 - As a sensitivity test the abstractions in scenario 2 were doubled whilst still remaining within the terms of the licence • Southern Water - This scenario is based on Southern Water's predictions of use which are based on a worse case assessment of flows in the River Itchen with enhanced levels of abstraction • Southern Water Stochastics - This scenario is based on modelled drought events which are more severe than have been experienced
Period of Environment Environment Environment Southern Actual Use use Agency Agency Agency Water (Ml) Scenario 1 Scenario 2 Scenario 3 Scenario (Ml) (Ml) (Ml) (Ml) 1973 427 427 818 3,308 Sep - Dec Sep - Dec Aug - Dec Sep 73 - Jan 74 1976 2,316 2,316 2,731 2,068 5,100 May - Oct May - Oct May - Sept Jul - Sep 1989 910 910 2,198 0 2,400 Jul - Dec 89 Jul - Dec 89 Jul - Dec 1990 668 0 0 0 1,654 Aug - Dec 1992 649 0 0 2,079 360 (in 1995) Jun - Nov Aug - Nov 1997 358 358 1,184 1,229 343 Aug - Dec Aug - Dec Jun - Dec Oct - Dec 2005 790 790 1,917 2,484 327 Jul - Nov Jul - Nov Jul - Oct Jul - Nov 2006 388 0 0 0 1,051 Aug - Oct Frequency 8 times in 47 5 times in 47 5 times in 47 5 times in 47 of use years years years years (1970 - 2016)
Table 2.3 - Potential Scenarios of use of the Candover Scheme
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In assessing the impacts of abstraction, it is important to consider the possibility of full uptake of the licence. This Scheme is not like many larger abstractions which are used constantly at a fairly stable rate as despite the licence being comparatively large, its use is infrequent. In addition, when the Scheme can be used, full use of the licence is further constrained by additional licence conditions. Scenario modelling, based on historic flow data, shows that under drought conditions, the full licence of up to 3,750 Ml may be needed but on most occasions, abstraction would be lower. To consider the possibilities of different patterns of use if there were more extreme droughts than observed in our historic flow records, Southern Water have carried out further analysis which is reported in Advice of the Technical Working Group (TWG) to the Steering Group, September 2015. They assessed four 36-month duration drought episodes constructed to be much worse than those in the historical record. The modelling showed that the scheme would need to be used in each of the four drought episodes tested and that the duration of use would be longer than found in relation to the droughts of the Standard Period (1970-2010) and would tend to reach the annual licence volume of 3,750 Ml/year. Impacts associated with that type of use are described in the following sections alongside a discussion of impacts associated with more normal patterns of use.
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3. Available Data and Tools 3.1. Hydrometric Monitoring When the Candover Scheme was first developed, a dense network of hydrometric monitoring sites was established, many of which are still in use. These sites give a way of characterising the conditions in the Candover catchment and also allow us to directly assess the impact of using the Scheme. The location of these sites is given in Figure 3.1, and form the basis for analysis of the frequency of extremes of low (and high) flows.
3.1.1. Gauging Stations Flow is recorded on a 15 minute basis at the following gauging stations. Site Name River Period of Record Borough Bridge Candover Stream 1970 to date Drove Lane River Alre 1970 to date Sewards Bridge Cheriton Stream 1970 to date Easton River Itchen 1982 to date Allbrook & River Itchen 1958 to date Highbridge Bransbury River Dever 1986 to date Alton Kings Pond River Wey 1991 to date
Table 3.1 Gauging Stations measuring river flow
3.1.2. Other flow measurements More intense periods of flow monitoring have occurred during tests of the Scheme and for academic studies. Notably, temporary weirs were installed on the Candover Stream to carry out detailed flow measurement during the 1976 tests and manual current metering have been carried out at several sites along the Candover valley to obtain information about how flows vary along the river length. Observations of the location of the springhead of the main chalk streams in Hampshire are recorded at least twice a year. Monitoring at these sites clearly identifies the perennially flowing sections of the Candover Stream and also helps define how often the winterbourne sections of the river flow and how often they are dry.
3.1.3. Groundwater levels Figure 3.1 shows the very dense network of groundwater monitoring observation boreholes which have been used to measure general groundwater fluctuations and the impacts on groundwater levels of using the Scheme.
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Figure 3.1 Location of Hydrometric Monitoring
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3.2. Abstractions and Discharges Figure 3.2 shows the location of abstractions in the area. A summary of abstractions in the area is given in a factsheet attached as Appendix C. 95% of abstractions in the Upper Itchen area are for fish farms and watercress beds but these abstractions return the majority of water abstracted back to the catchment and so are termed as non-consumptive. Of the remaining 5% of abstraction, 95% is used for public water supply where water is abstracted and removed from the local area which can result in local abstraction impacts. The sources at Totford and Lasham are located in the groundwater catchment for the Candover Stream. Of the consumptive abstraction from the Candover groundwater body, public water supply accounts for 92% of the total. Small private and agricultural supplies only account for 5% of consumptive abstractions in the Upper Itchen. Although there are a large number of them, their impact on the environment is considered to be minor as the cumulative volume of these abstractions is so small. Significant discharges are generally associated with the fish farms and watercress beds with larger waste water discharges predominantly to ground.
3.3. Test Pumping The best way to assess the impact of using the Scheme is to test it and carry out detailed monitoring to assess the impact that the Scheme is having. In the case of the Candover Scheme, two detailed test pumpings were carried out during dry periods in 1976 and 2011. These events were both well documented providing a wealth of information to assess impacts on both the Candover Stream itself and more distant watercourses. During the 1976 test, 5,060 Ml was abstracted from May to October and during the 2011 test, 969 Ml was pumped from September to November. The impacts observed as a result of the 1976 test will now be in excess of impacts expected from the reduced licence and in contrast those impacts observed in 2011 would not have been as large in extent, magnitude and duration as those potentially allowed within the conditions of the extant licence. However, both tests provide useful information to inform our assessment.
3.4. Test & Itchen Groundwater Model Although test pumpings provide valuable information, they only provide data for the hydrological conditions observed during the test. Use of a groundwater model enables various scenarios to be tested to explore different ways of operating the Scheme under differing hydrological conditions. In this way, the likely impact of using the Scheme at the new licensed rates and under the new conditions can be assessed within the uncertainties of the model. Information from test-pumpings has been used to inform development of the Environment Agency's Test and Itchen groundwater model which was last updated in 2013. The groundwater model can provide output from 1970 to 2011 and operates on a time-step of two periods a month. The model is based on a grid of 250m. There are four standard scenarios for the groundwater model: • Run 125 Naturalised - all abstractions and discharge turned off • Run 124 Historic - all abstractions and discharges set to historic actual values • Run 126 Recent Actual - a repeated annual sequence of abstraction based on recent abstraction patterns • Run 127 Fully Licensed - a repeated annual sequence of abstraction based on the maximum that could be abstracted under the terms of all the licences
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When updated in 2013, the model was accepted as fit for purpose although there are limitations in its use. There is low confidence in absolute values of flow for the winterbourne reaches and the model is not perfectly calibrated to match periods of winterbourne flow and drying out. For instance, the model predicts that naturally flows would dry out at Abbotstone but available data suggests that flows never dry up at that point. In addition, experience of local groundwater models shows that the impacts of abstractions appear to be extended both spatially and temporally beyond what tends to be observed during periods of test pumping which may have the effect of smoothing modelled impacts on groundwater levels and river flows. For those reasons, care must be used in analysing output and so this assessment considers observed as well as modelled data to assess potential impacts. This version of the groundwater model has been used by both Southern Water and Environment Agency to assess the impact of different scenarios of use of the Candover Scheme with output presented at a series of standard data points. Data from these points is used in sections 5 and 6 below and the location of the points are shown in Figure 4.2 as red dots.
Location NGR Candover Easting Northing Preston Candover 460400 141400 Chilton Candover 459200 140200 Brown Candover 457800 139400 Totford Bridge 457125 138125 Northington Bridge 457000 138000 Abbotstone 456400 134500 Borough Bridge 456750 132250 Gauging Stations Wey at Kings Pond 472250 139500 Dever at Bransbury 442000 142000 Alre at Drove Lane 457250 132500
Table 3.4 Standard locations for Groundwater Model output
Southern Water's work has been carried out by Atkins with the majority of their reporting in these documents: • Atkins (2014) for Southern Water Services, Groundwater modelling assessment of the impact on river flows and groundwater level of using the Candover augmentation Scheme: Atkins Technical Assessment • Rob Soley (2015) Technical note for Southern Water reference 29388tn568, Augmentation Technical Working Group, AMEC Foster Wheeler.
Where relevant, this work is referred to in this report as it often represents a scenario with more extreme impacts than the Environment Agency scenarios.
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Figure 3.2 Location of abstractions in the Study Area
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4. Characterising the hydrology of the Candover Stream 4.1. Upper River The upper reaches of the Candover Stream are characterised by being dry for variable amounts of time depending on the prevailing conditions. Flooding has occurred in Preston Candover in extremely wet winters but more typically the river rises in the winter around Chilton Candover. Flow gaugings and spring head survey data (shown in figure 4.1) show that in most summers the perennial head falls to a point below Totford - typically between Northington Bridge and the Swarraton/Waterwheel area.
Figure 4.1 Spring head data for the Candover Stream (1997 - 2016)
4.2. Lower River Flow data from Borough Bridge for years when the Candover Scheme was not in use shows that there is a large range in flows expected over the autumn, winter and spring months. The flood events of 2014 and 2000/01 are clearly evident.
Figure 4.2a Figure showing annual flows in the Candover Stream for periods when the Candover Scheme was not in use
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However, the natural characteristics of chalk streams lead to a relatively narrow range of flows in the summer months. The highest summer flows were observed in the very wet summer of 2014 whilst the lowest flows were observed in 1992 after a long period of low flows and use of the scheme in 1989 and 1990.. Notable low flow periods when flows were affected by use of the scheme, such as 1976, 1989, 1990,, 1997 and 2005 are not shown in this plot. In the summer months, flows typically range from 0.25 - 0.5 m3/s (cumecs) with flows in wetter summers as high as 0.5 - 0.7 m3/s. In winter, flows are typically within the range from 0.3 - 1.5 m3/s. Exceptionally wet winters can result in flows exceeding 4 cumecs but this is an infrequent occurrence. In contrast, during very dry winters such as 1992, flows in winter can remain as low as those observed in dry summers. Variability in flows between Borough Bridge and Abbotstone can be seen in flow data collected before the 1976 test pumping of the Candover Scheme. The locations of these sites are shown in Figure 4.2b and the data plotted in Figure 4.2c. The data from the 1976 test period (shown circled in red) shows how augmentation has an impact on the natural pattern of flows downstream of the discharge point. This is evident at Waterwheel and downstream of Grange Lakes at Abbotstone and Fobdown. During the test, discharges reached 36 Ml/d compared to the maximum of 27 Ml/d allowed by the current licence. Flows were sustained at a very low level upstream of the discharge point at Sheepdip but were dry at Totford.
Figure 4.1c Flow data at points of the Candover Stream (1972 - 1977)
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Figure 4.2b Hydrology of the Candover Stream
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Upstream of Grange Lakes, Figure 4.2c shows that flow was generally sustained for the full period at Waterwheel but the river progressively dries up as you move upstream and for a longer duration during the summer. This data also shows that despite very low flow conditions in 1973, flow was sustained at Abbotstone. In the lower reaches of the river, a significant increase in flows is evident at Fobdown where historically the watercress beds have been discharging significant quantities of water. Modelling of the river suggests that this discharge raises flows above what would naturally be expected, in effect providing an additional form of augmentation which has a particular hydrological benefit during low flow periods.
4.3. Summary of Hydrology of the Candover Stream
Figure 4.2b summarises the different hydrological areas of the Candover Stream.
4.3.1. Perennially Flowing section Downstream of Grange Lakes, available data for a range of flow conditions shows that the Candover Stream flows all year round. Flows are enhanced downstream of Fobdown cressbeds as a result of the discharge from that site.
4.3.2. Grange Lakes This lake is an artificially created (c. 1780) on-line lake set within parkland associated with The Grange. There is no record of it ever drying up and it acts as a distinctive change from the perennially flowing Candover Stream to the upstream reaches where flow is more variable.
4.3.3. Typical zone of fluctuation of summer head Available data shows that for the last few decades, the summer head of the Candover Stream is typically to be found within the 1.5km stretch of river between Grange Lakes and Totford. In most years, it is located in the field downstream of Northington road-bridge, with flows at Totford typically dry for several months in most summers.
4.3.4. Typical zone of fluctuation of winter head In most years, flows in winter will rise through Totford and Brown Candover to reach a location in a wet copse beside the road on the northern edge of Brown Candover. In wetter years the head extends into Chilton Candover, which is around 5km upstream of Grange Lakes.
4.3.5. Flow in very wet winters Only under exceptional conditions such as the winters of 2000/01 and 2013/14 does the Candover flow in Preston Candover which is about 7km upstream of Grange Lakes. For most of the time, the river bed between Chilton Candover and Preston Candover is dry.
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5. Impacts of the Candover Scheme acting alone
The measured impacts of using the Candover Scheme are well documented in the 1976 test pumping report and to a lesser degree in the 2011 test pumping report. The potential impact of future scenarios has also been considered using the groundwater model. A range of evidence from all of these sources is used to explain the impact the Scheme can have on the Candover Stream and surrounding watercourses. Further detail of the modelled impact of using the scheme on river flows and stream-head locations can be found in Southern Water's 2014 report. Impacts are first described in isolation from other abstractions and discharges and are then considered in combination with others in section 6. 5.1. Candover Stream 5.1.1. Winterbourne reaches The impact of the Scheme on the winterbourne reaches was not studied in any detail in the 1976 test pumping report, but has most recently been assessed in the Southern Water 2014 report which is based on the Southern Water scenarios of use. Output from that report is presented in this section to illustrate the possible impacts. For consistency the graphs are presented for the 1970s but graphs covering other periods are included in the original report. Run 126 (the black line) is the Recent Actual model scenario and run 131 (the dashed red line) is the Southern Water augmentation scenario.
Preston Candover Local evidence and output from the groundwater model shows that the stream hardly ever flows at Preston Candover. It is only exceptional rainfall that produces flow this high up the stream and it is winter to early spring that this occurs. There is an impact on stream flow following use of the augmentation Scheme during 1973-74 and 1976; however this impact is a small reduction of in the magnitude of winter high flows and does not impact on the length of time the river actually flows. Use of the Scheme at other times does not have any noticeable impact on flows at this point as naturally flows do not return to Preston Candover until several years after the Scheme has been used and any impacts on groundwater levels have dissipated.
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Figure 5.1a Groundwater Model Output for flows at Preston Candover based on Southern Water Augmentation Scenario (Taken from Southern Water Report, 2014). Run 126 (the black line) is the Recent Actual model scenario and run 131 (the dashed red line) is the Southern Water augmentation scenario.
Chilton and Brown Candover Flows tend to reach Chilton Candover during wetter winters but modelling shows that the impact of using the Scheme is to reduce winter peak flows over several weeks by up to a maximum of 15 litres per second (l/s). When there is flow at this point, the peak flow is typically in the range of 40 - 200 l/s. Modelling suggests that use of the Scheme does not change the period during which there is water in the channel and would not have caused flows to dry up any more than occurs currently. Flow occurs in most winters at Brown Candover but naturally the river also dries up in most summers. Use of the Candover Scheme can reduce winter peaks by up to 25 litres per second (l/s) in the winter following use of the Scheme. Figure 5.1b shows that typical winter peak flows range from 100 l/s to 450 l/s so reductions are of the order of 5 - 25%. The impact of reduced flows will be more pronounced in dry years like 1974 when winter peak flows are naturally lower. However, modelling suggests that the overall pattern of flows does not appear to be affected by use of the Scheme.
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Figure 5.1b Groundwater Model Output for flows at Brown Candover based on Southern Water Augmentation Scenario (Taken from Southern Water Report, 2014) Run 126 (the black line) is the Recent Actual model scenario and run 131 (the dashed red line) is the Southern Water augmentation scenario.
Totford and Northington Bridge Flow data and groundwater model output confirms that typically there is flow in most years at Totford and Northington, but the river dries up in most summers. Again the impact of using the Scheme is to reduce peak flows in the winter following use of the Scheme but there is no apparent change to the pattern of flows. Impacts on flows can reach 45 litres per second for a couple of weeks when flows are increasing but at this point in the river, winter peak flows typically range from 100 - 700 l/s so the scale of impact ranges from 5 - 45% of flows.
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Figure 5.1c Groundwater Model Output for flows at Northington Bridge based on Southern Water Augmentation Scenario (Taken from Southern Water Report, 2014) Run 126 (the black line) is the Recent Actual model scenario and run 131 (the dashed red line) is the Southern Water augmentation scenario. To illustrate the variation in impacts associated with different climatic conditions, the data for 2000 - 2010 for Northington Bridge is shown in Figure 5.1d below. This graph shows that following the dry winter of 2005, flows did not recover at Northington in 2006 and so the model predicts that the river would have been dry even without use of the scheme. Flows are not impacted at this point until they start to recover with the onset of aquifer recharge in 2007.
Figure 5.1d Groundwater Model Output for flows at Northington Bridge based on Southern Water Augmentation Scenario (Taken from Southern Water Report, 2014) Run 126 (the black line) is the Recent Actual model scenario and run 131 (the dashed red line) is the Southern Water augmentation scenario.
Use of the groundwater model suggests that the maximum impact on the location of the Candover streamhead is to move it 7 stream cells (equivalent to 1.75 km). This impact only occurred for 15 days and more typically the impact is to move the location of the stream-head 1-2 stream-cells (250 to 500m) downstream during a period of 6 to 12 months after the scheme has been used.
5.1.2. Perennially flowing reaches
Abbotstone Figure 5.1e shows groundwater model output for river flows at Abbotstone where all licences are being used at current rates both with and without augmentation (scenario 2). The large variation in flows is evident which is driven by the prevailing climatic conditions. The graph clearly shows the increase in flow as a result of the augmentation discharge but the reduction in winter flows are more difficult to discern. The maximum modelled reduction in flows is around 10 - 13% in October 1976 which had reduced to a 2-3% impact at the time of peak flows in February 1977.
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Figure 5.1e Impact of Candover Scheme on flows at Abbotstone (Scenario 2)
Borough Bridge The impact of use of the Candover Scheme on the Candover Stream is well documented in the 1976 test pumping report. Figure 5.1f below shows that the gain in flow during periods of augmentation is followed by a reduction in flow once the scheme is turned off. For the 1976 test, analysis suggested that flows had recovered to what would have naturally been expected by April 1977. However, the 1976 drought event was a particularly unique event as although flows became very low, there was significant recharge in autumn 1976 which rapidly restored flows and groundwater levels.
Figure 5.1f Augmented and estimated natural flows at Borough Bridge (1976 - 1977)
The groundwater model can be used to assess how flows are impacted both during and after periods of augmentation during a variety of hydrological events and augmentation scenarios. Figure 5.1g shows the modelled impact on flows in the Candover Stream at Borough Bridge under the Environment Agency Scenario 1.
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Figure 5.1g Modelled impacts of augmentation scenarios on flow in the Candover Stream at Borough Bridge (Environment Agency Scenario)
This shows the expected pattern of increasing flows whilst the scheme is in use followed by a period of reduced flows. The magnitude and duration of the reduction in flows following use depends on several factors including the extent to which the scheme was used and the prevailing hydrological conditions. Not surprisingly where the scenario shows that the scheme is used over successive years during the extended low flow period of 1989 - 1992, it is not until there is a particularly wet winter in 1994 that the impacts of using the Scheme are no longer apparent. Figure 5.1h shows the percentage reduction in flows at Borough Bridge for periods where the Scheme can be used on at least two occasions in a sequence of four years.
Figure 5.1h Reduction in flows at Borough Bridge after use of the Candover Scheme
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This shows that when the scheme is used several times in close succession, it is possible that flows will not have completely recovered before use of the scheme is permitted again. Our analysis suggests that the maximum reduction in flows is around 5% whilst analysis carried out by Southern Water for their scenarios suggested that the maximum impact on flows at Borough Bridge was a reduction in flows of 8% or 74 l/s in the February following the 1973 use of the Scheme. In all scenarios, the impacts on flows have reduced to below 1% after a year and are only likely to exceed 2-3% for a few months. The impact on flows from the discharge of water is significant with flows often more than doubled at Borough Bridge. Due to concerns raised after the 2011 test about the impact of creating a flow regime uncharacteristic of natural summer flows, licence conditions were applied in 2015 to reduce this risk. Notably for the period from 1st July to 31st October, use of the augmentation scheme cannot cause flows at Borough Bridge to rise above 0.5 m3/s. Also the phased increase in discharge which the licence requires ensures flows do not increase at a greater rate than could occur naturally.
Figure 5.1i Comparison of early switch on augmentation scenario with observed flows at Borough Bridge
Figure 5.1i shows a scenario where the augmentation scheme is turned on early in the year. Under these circumstances, only 20 Ml/d can be discharged to the river under the terms of the current licence until the end of August/start of September and augmented flows have to remain below 0.5 m3/s. The graph shows that the rate of rise in flows is comparable to that observed during the uncharacteristic increase in flows in the wet summer of 2012. Flows under the augmentation scenario rise from flows expected to be exceeded more than 95% of the time to the sort of flows only expected to be exceeded 5% of the time but remain below the highest flows observed in the river for the time of year.
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Figure 5.1j Comparison of late switch on augmentation scenario with observed flows at Borough Bridge Figure 5.1j shows a scenario of using the augmentation scheme later in the year where the current licence allows a more rapid increase in discharge and a higher discharge rate of 27 Ml/d. Until the end of October the discharge must still not allow flows of 0.5 m3/s to be exceeded. This graph shows that a more rapid rise in flows was observed naturally as a result of early recharge in 1993. Under the augmentation scenario, flows could rise from those which would be exceeded 75% of the time to flows only exceeded 5% of the time. In this circumstance though, flows are significantly below the maximum expected for the time of year. This analysis shows that clearly the increase in flow does change the natural flow regime but due to the conditions of the current licence, flows could not exceed those which may naturally be seen in the Candover Stream for the time of year and the rate of rise is not likely to be significantly different from rapid rises caused naturally by rainfall events.
5.2. Impacts on flow in the River Itchen As flows in the main River Itchen are significantly higher than those at Borough Bridge, the impacts of use of the Candover Scheme are correspondingly lower. The maximum reduction in flows measured at Easton as a result of using the scheme is 1 - 1.5%. Impacts are only likely to remain above 1% for a maximum period of 3 - 4 months. The increase in flows observed as a result of using the scheme can amount to up to 14% of natural flows. These maximum impacts were modelled to occur during and after the 1976 use of the scheme.
5.3. Impacts on flows in surrounding watercourses Analysis of the 1976 test pumping showed that of the 5.06 million m3 abstracted, 76% of that water was abstracted at the expense of flows in the Candover Stream. The remaining water was at the expense of flows in the lower Itchen (12%), Alre (6%) and Dever (6%). All flow impacts were reported to have recovered by April 1977, with no carry over impact into the following year, although the wet winter of 1976 make any conclusion uncertain.
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The groundwater model provides an alternative way of assessing how the impacts are distributed across the Candover Stream and surrounding watercourses. Using our scenario for the 1976 event suggests that around 40% of the impacts were modelled on the Candover Stream, 25% on the River Alre, 14% on the River Dever and 6% on the River Wey with the remaining impacts distributed over the whole of the model area. This analysis is in contrast to the test pumping data. To explore this apparent anomaly further, distribution of impacts for the scenario of use in 2005 were explored which showed a slightly higher proportion of impacts on the River Alre - up to a maximum of 30%. Analysis of the timing of the impacts using the groundwater model was also interesting as it showed that for the 1976 event, around 90% of the impacts were indeed modelled to occur in 1976 and 1977. However in 2005, the impacts on surrounding watercourses occurred over a longer time period with only 70 - 80% modelled in the first two years. This is because the rapid recovery in groundwater levels and river flows in late 1976 quickly replaced the water abstracted by the augmentation scheme, whilst in 2005 the slower rate of recharge meant that recovery of flows and groundwater levels took longer. What is clear is that different assessments tend to deliver variability in how the impacts of using the Candover Scheme are distributed to surrounding watercourses. The test pumping data from 1976, which relies on observations of impacts under specific conditions, suggests that the impacts remain closer to the Candover Scheme and can be short lived. However, the groundwater model tends to spread the impacts more in time and space, reducing apparent impacts closer to the scheme and perhaps over exaggerating more distant impacts. Despite this contrast, there are also clear indications about the timing and extent of the impacts of the scheme which are consistent. This is why it is important to not simply rely on one method of assessing impacts or only look at specific hydrological events, but consider different types of events using both modelled and real data.
5.3.1. River Dever Analysis of the 1976 Test Pumping suggested that 6% of the volume of water abstracted was at the expense of flows in the River Dever. However, output from the groundwater model suggest that as much as 15% of the water abstracted by the Candover Scheme could be at the expense of flows in the River Dever. Analysis by Southern Water (2014) of their scenarios of using the Scheme suggests that the maximum likely impact at the gauging station at Bransbury was a 1.9% reduction in flows recorded in summer 2006 following use of the Scheme in 2005.
Figure 5.3a Groundwater model output for the River Dever at Bransbury (Taken from Southern Water Report, 2014)
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In terms of the impact on the winterbourne reach of the Dever, the maximum impact is also modelled as occurring in 2006 due to lack of recovery through the subsequent winter months 2005-06. The report states that: "although the springhead location is further downstream than if the augmentation Scheme hadn’t been used in 2005, it is no further downstream than during the preceding summer months (2005) when the Scheme was brought online. There is a good recovery during the following winter (2006-07) and the remaining slight impacts occur during the early part of the recession spring 2007 and the following year spring 2008". Under the modelled scenarios, it is predicted that the effect of using the Candover Scheme is to move the spring head location by typically only one model stream-cell, which is equivalent to 250m. Under most scenarios, residual impacts on the stream-head of up to 250m can last for up to 6 - 12 months.
5.3.2. River Alre The analysis of the 1976 test pumping suggested that around 6% of the water abstracted by the Scheme is at the expense of theRiver Alre. In contrast, the groundwater model outputs suggest that this figure could be as high as 25 - 30%. Figure 5.3b shows that the groundwater modelling output suggests that the maximum impact on flows at Drove Lane are 2%, with impacts of more than 1% persisting for less than a year. The maximum impacts tend to be on peak flows and the subsequent early part of the recession.
Figure 5.3b Modelled impact of reductions in flow in the River Alre at Drove Lane (scenario 2)
Modelling by Southern Water suggests that although there is a possible slight shift in the winterbourne stream head, it is hard to detect an impact as the dominant impact of location of the stream head is strongly influenced by the operation of local watercress beds. The assessment suggests that the maximum extent of the movement of the stream-head as a result of using the scheme is 250m and any impacts have usually dissipated within 6 months of using the Candover Scheme.
5.3.3. River Wey Analysis of the 1976 test pumping data did not show any impact on the Wey, although simple observation of the cone of depression caused by use of the scheme suggests that it is quite possible that flows in the River Wey could be depleted by use of the Scheme. Analysis of our groundwater model output suggests that up to 5% of the abstraction for the scheme is at the expense of flows in the River Wey. 35 of 66
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Figure 5.3c taken from Southern Water's 2014 report suggests that it is hard to discern any impact on flows due to use of the augmentation Scheme, as the impact is so small. The figure also shows that in very dry years the modelled flow at the gauging station at Kings Pond can fall to zero during late autumn / winter months without the Candover Scheme being in use. Modelling does show a potential slight impact on the location of the stream-head potentially reducing the maximum location of the stream-head by 250m and on some occasions delaying the rise in stream-head by a few weeks when recharge starts.
Figure 5.3c Modelled impact of Candover Scheme on flows in the River Wey (Taken from Southern Water Report, 2014)
5.3.4. Cheriton Stream Analysis of the 1976 test pumping data did not show any impact on the Cheriton Stream and in fact it was used as a control catchment because at the time there was high confidence that it had not been affected by use of the Scheme. Recent analysis of groundwater modelling output confirms that this is highly likely to be the case.
5.4. Impacts on groundwater levels Figure 5.4a taken from the 1976 test pumping report shows how the cone of depression extended with time. This clearly shows the impact of the Scheme on groundwater levels propagating out towards Grange Lakes on the Candover Stream, but as the last contour shown in these plots is 1m, smaller more widespread impacts are not demonstrated. The impact of using the Scheme on groundwater levels was monitored during the 2011 test with the extent of the cone of depression around the Candover abstractions in 2011 very similar to that seen in 1976. The northern, eastern and western boundaries of the 2011 cone of depression lie very close to those of the 1976 cone. To the south of the Candover abstractions, the 2011 cone of depression is influenced by the Alre abstractions and the cone of depression extends further south than seen in 1976. The maximum drawdown seen after the shorter test in 2011 was 3.5 m, which is slightly greater than that presented in the 1976 test pumping report after 30 days of pumping.
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Figure 5.4a Contours of Drawdown taken from the 1976 test pumping report
The impact on groundwater levels can also be modelled with Figure 5.4b taken from the Atkins 2014 report to show the maximum extent of the cone of depression at the end of Southern Water's proposed 1973 use of the Scheme. This plot shows the cone of depression potentially extending out towards the River Dever, River Wey and River Alre. The curious shape in the south east of the cone of depression is as a result of a groundwater high point which is not susceptible to groundwater level changes. Presentation of data in Southern Water's 2014 report shows that for the 1976 event, groundwater level impacts reduced to around 0.1 - 0.2m in the upper Candover catchment by summer 1977 and virtually no impacts were detected a year after the scheme had been used. In contrast, the model suggests that for the scenario when the scheme was used in 2005, impacts of up to 0.4m persisted into the summer of 2006, with impacts of 0.1 - 0.2m still potentially evident a year after the scheme had been used. This is because recovery of groundwater levels as a result of recharge was far slower in 2006 than in 1977.
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Figure 5.4b Modelled extent of Cone of Depression after 1973 use of the scheme (taken from Southern Water 2014 Report)
To assess potential impacts on wetlands, a map has been produced to show the maximum likely extent of the cone of depression and where this has the potential to influence areas where groundwater levels may be high enough to relate to groundwater-dependent terrestrial (non- riverine) ecosystems. The maximum extent of the cone of depression is based on a consideration of both test pumping and modelled data and is shown in Figure 5.4c. Measurable impacts on groundwater levels in this area could typically persist for 6 - 18 months depending on how much the Scheme was used and what the prevailing conditions are. The risks to any sites within this area are then assessed in Part 2 of the Technical Report.
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Figure 5.4c Maximum likely extent of Cone of Depression from the Candover Scheme
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5.5. Southern Water summary of impacts In preceding sections some output from Southern Water's technical assessments has already been presented. The most recent Atkins Technical note 027-DG-1066_06 from September 2015 uses previous Atkins analysis alongside additional work carried out by Amec Foster Wheeler to summarise the impacts associated with operation of the Candover Scheme. That report states that: • after use of the Candover Scheme there is a period of recovery for the Candover Stream which is most marked in the first month or two of flow recovery, with small residual impacts (down to 5% of natural) during higher flow periods which may continue for up to a year. • Augmentation events might cause the same winterbourne reach to dry two weeks (one stress period) earlier during the spring recession period after the winter following augmentation than it would have done without operation of the augmentation scheme. • Typically there are no impacts on the “flowing” and “non-flowing” springheads during the following winter flow periods, which suggests that there is effectively no ‘carry-over’ impact on winterbourne behaviour; • There are some impacts during the higher flow periods of winters following the augmentation events, and to a lesser extent during the following winters, but the changes are small in the context of the much higher flows experienced at those times. Flow losses due to the augmentation scheme generally fall to below 10% of natural simulated flow roughly 2 months after switching the scheme off and this falls to less than 5% of natural within one year of operation. Not surprisingly, these conclusions are broadly similar to our own. However, it should be noted that in Southern Water's work they have assumed that the discharge is made to the River Itchen and not to the Candover Stream which is what is being assessed under the terms of the licence renewal. For that reason Southern Water's output for the lower reaches of the Candover Stream has not been presented in this report.
5.5.1. Possible influence of climate change To consider the possible use and impact of the Candover Scheme under drought conditions more severe than observed, Southern Water carried out further work which was presented in Technical Note 027-DG-1066_06, Sept15, impacts of augmentation events v1.2 and which was summarised in the Advice of the Candover Technical Working Group to the Steering Group (published in September 2015). An Aquator water resources simulation model was used to assess when the Candover scheme might need to be used under extreme events. The Test and Itchen groundwater model was then used to assess the impact of using the Candover Scheme for four modelled extreme drought periods of 36-month duration thought to equate to events that are only likely to occur less frequently than on average once in 200 years. During these periods of use, the duration of use was longer than previous scenarios and for some events, use of the scheme spanned two years. In addition, use of the Scheme tended to approach the full annual licence. It was reported that under these tests the impact on the recovery period extended over a longer period than found in the historical drought scenario tests but the magnitude of impacts was still typically of the order of -5% to -10% (within recovered flows) and the worst impacts were in the - 15% to -20% category. The additional hydrological impacts from operating the Candover scheme on flow and winterbourne signature recovery under such extreme conditions were reported as small (although associated ecological risks were not assessed). In addition, the Atkins Technical note 027-DG-1066_06 states that operation of the Candover Scheme for the four dry year events makes negligible difference (effectively none) to the simulated timing and location of springhead flow recovery between fully licensed model runs with and without augmentation (although there was no assessment of related ecological impacts).
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6. In combination impacts of the Candover Scheme
It is important to not only assess the impact of the Candover Scheme operating on its own but also the contribution it makes to having an impact on groundwater and river flows in the context of other abstractions and discharges. This area of Hampshire is relatively rural with only small towns and villages and no large urban areas. The location of major abstractors is shown in Figure 3.2. These abstractions are generally for the purpose of watercress growing and public water supply. Appendix C contains a summary of abstraction in the upper Itchen catchment. As there is no flow and groundwater data to show what conditions in the Candover Stream were like before abstraction started, the groundwater model can be used to explore what natural conditions may have looked like and what may happen if all licences were fully used. Testing these scenarios is problematic as it pushes the groundwater model beyond the limits of its calibration so model output must be treated with some caution, but it can indicate possible pressures and issues. As this report is concerned with the potential impacts of the Candover Scheme and is not a review of all other licences in the area, the in combination impacts are generally described in terms of the sectors that cause the impact. The main sectors of abstraction which act in combination with the Candover Scheme are: Public Water Supply and Watercress and Fish Farms.
6.1. Fobdown licence changes In June 2016, two major licences were revoked at Fobdown. These licences permitted around 14 Ml/d of water to be abstracted from the Chalk aquifer for the purpose of watercress growing. At least 95% of the abstracted water had to be returned to the Candover Stream. This significant change in conditions in the catchment is likely to affect our assessment of the in combination impact of the Candover Scheme licence and is described in the sections below. Abstraction at Fobdown is understood to have gradually reduced over the last few months to a point where all pumps were switched off. Examination of flow data at Borough Bridge does not show any overwhelming change to flow patterns in the lower Candover Stream but recovery of groundwater levels and river flows is likely to occur over a period of time and so impacts may be difficult to detect. Use of the groundwater model suggests that revocation of the Fobdown licence is likely to permanently increase flows in the perennial section of the stream upstream of Fobdown resulting in a possible improvement of the flow regime. However, cessation of the significant discharge from the site is likely to lead to a reduction in flows downstream of the site. Modelling suggests that this abstraction drew in water from the adjacent Alre and Itchen catchments resulting in it being a net contributor of flow to the Candover Stream. In the upper winterbourne reaches, there is likely to be minimal impact on flows as a result of revoking the Fobdown licence which on average only accounted for around 4% of abstraction impacts at Totford and an even smaller proportion of the impact further upstream. The analysis above shows that removal of the Fobdown licence is likely to lead to an apparent deterioration in flows in the lower Candover Stream whilst resulting in a net benefit to the River upstream of the Fobdown watercress beds. This change in flow status in the lower reaches of the Candover Stream is likely to lead to changes in various targets used to assess the impact of abstraction on the environment:
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• For WFD, flows are no longer expected to support good status and this change is likely to trigger an investigation into the wider impacts of all abstraction licences on the ecological status of the water body. • Under the Habitats Regulations, the change to the Fobdown licence changes the assessment of in-combination impacts of abstraction on the River Itchen SAC. The change in hydrological conditions, together with other changes in our understanding of the risks to the SAC since our 2005 RoC work will trigger the need to review the implications for our 2005 conclusions (see our Part 3 report). This is likely to lead to a need to work with Water Companies to carry out further work to address this issue. • Under the current Review of Consents target flow regime, it is possible that the flow targets will now be failed.
6.2. Upper Reaches
Winterbourne Reaches Although we have looked in detail at the impact of the Candover stream on flow in the winterbourne reaches, we haven't looked in detail at impacts in this area caused by other sectors of abstraction. It is clear however, that the impact of public water supply abstractors is likely to cause some impact on the location and magnitude of flows and the frequency and duration of dryness. Impacts from watercress abstractions are likely to be minimal.
Totford Figure 6.2a shows the scale of impacts from the Candover Scheme compared with impacts from all other abstractors. At Totford, the impact of watercress abstractions and other abstractions are minimal, with the dominant impact on flow from Public Water Supply abstractions. The maximum impact of these abstractions operating together is assessed to be 15 Ml/d in February 1990 of which the augmentation scheme contributed 2 Ml/d of impact. Use of the Candover Scheme can reduce natural flows by up to 20% but when acting in combination with other licences, flows can be reduced by 50 - 80% and in extreme conditions, when flows are naturally very low, abstraction can cause flows to dry up.
Figure 6.2a Abstraction impacts at Totford
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6.3. Lower Reaches
Abbotstone At Abbotstone, all impacts except the Candover Scheme have a negative impact on flows. At this point on the river, the impact from other licences is expected to be far larger than the impacts associated with the Candover Scheme. The green line in Figure 6.2b shows the modelled impact of using the Fobdown abstraction - an impact which has now ceased. This shows that the gain in flows expected from cessation of abstraction at Fobdown is likely to be comparable to any negative impacts on flow associated with use of the Candover Scheme.
Figure 6.2b Abstraction impacts at Abbotstone
Impacts from Public Water Supply abstractions are more significant than those from watercress beds. Impacts from all abstractions reduce to zero when the model estimates that flows would naturally be dry at this point. This is one weakness of the groundwater model, as our monitoring data shows that there is no record of flow drying up at this point even in the driest summers. There is a slight impact on modelled flows following use of the Scheme but the dominant influence is from other abstraction sectors.
Borough Bridge The impact of abstraction sectors acting in combination with the Candover Scheme on flows at Borough Bridge can be estimated using the groundwater model. Figure 6.2c shows the flows that would be expected with no abstractions or discharges and flows under a scenario of all licences being fully used. The graph clearly shows periods where flows are raised above what would naturally be expected by use of the Candover Scheme.
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Figure 6.2c Modelled flows at Borough Bridge for the natural and recent actual scenario (Environment Agency scenarios)
The graph also shows that in general the fully licensed flows are slightly below the flows expected if there was no abstraction. The data regarding abstraction impacts at Borough Bridge in figure 6.2d shows that the watercress beds at Fobdown were a net contributor and acted to augment river flow and largely offset the impacts of other watercress beds and public water supply abstractions. At times of low flow, this lead to actual flows apparently being higher than what might naturally be expected.
Figure 6.2d Abstraction impacts at Borough Bridge shown by abstraction sector
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There are seasonal fluctuations in the impact profiles which, for public water supply abstractions are related to groundwater level fluctuations and the influence of aquifer storage. When groundwater levels are low, large public water supply abstractions tend to have more of an influence on aquifer storage and the impact of that is to delay the recovery of winter flows and peak flows. This means that abstraction impacts on river flow are higher at times of high groundwater levels and high flow. It is clear that the residual reduction in flows after the scheme has been turned off is small when compared to the ongoing impacts on flow as a result of public water supply and watercress bed abstractions. The combination of abstractions and discharges operated at their maximum limits can be expressed in a time series graph of flow at Borough Bridge (Figure 6.2e). The graph shows how much flow is in the Candover Steam at Borough Bridge as a percentage of natural flow. This clearly shows that the combined abstraction impacts result in flows fluctuating generally around 75 - 95% of natural flows. When the Candover Scheme is used, flows at Borough Bridge can be increased to double the natural flow expected at that time.
Figure 6.2e Combined percentage impacts of flow in the Candover Stream at Borough Bridge
6.4. In combination groundwater impacts
The impact of using the Candover Scheme on groundwater levels has been shown in section 5.4. There are other significant groundwater abstractions in the area which can have an impact on groundwater levels. The most notable of these are the public water supply abstractions at Lasham and Totford and the local watercress abstractions. These impacts act in combination to reduce groundwater levels across a wide area of the Chalk aquifer. Figure 6.4a shows the cones of depression produced by the Test and Itchen groundwater model which are related to these major abstractions. These should be treated as indicative only but give the impression that groundwater levels could be reduced in the Candover catchment by all of these abstractions.
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Figure 6.4: Presenting the overlapping data superimposed all four contour sets where : Red = Totford, Black = Lasham, Green = Fobdown Cress and Purple = Alre & Cheriton watercress farms and fish farms licences (minimum contour level is 0.1 m)
Clearly, revocation of the licences at Fobdown would be expected to lead to a general rise in groundwater levels in the lower reaches of the Candover Stream extending from the confluence with the river Itchen to an area upstream of Grange Lakes.
Description of other abstraction licences considered in this in-combination with the Candover licence: Licence details:
Daily Annual Recent use of licence Totford 4.5 Ml/d 1,659 Ml/yr 62% 11/42/22.3/55 Lasham 27 Ml/d 5,455 Ml/yr 84% 28/39/30/0273
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Analysis of how those other licence have been used in recent years:
Southern Water – Totford Licence (11/42/22.3/55)
Southern Water Totford - yearly total abstraction 1300000
1200000
1100000
1000000
m3 900000
800000
700000
600000
Year
Southern Water Totford - monthly abstraction 140000
120000
100000
80000
m3 60000
40000
20000
0
Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul
Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Year and month
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South East Water – Lasham Licence (28/39/30/0273)
South East Water Lasham - yearly total abstraction 6000000
5500000
5000000
4500000
4000000 m3 3500000
3000000
2500000
2000000
Year
South East Water Lasham - monthly abstraction 700000
600000
500000
400000
m3 300000
200000
100000
0
Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul
Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan Jan 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Year and month
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The watercress and fish farm Licences (ref s???)
Pumped Watercress and Fish Farm licences in the Upper Itchen catchment – not including watercress beds fed by artesian overflowing boreholes. (Itchen Valley Trout Farm 11/42/22.1/136, Pinglestone 11/42/22.2/101, Franklyns Fish Farm 11/42/22.2/148, Drayton 32/068, Bighton Road 32/078)
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6.5. Summary of In Combination Abstraction Impacts In combination impacts of abstraction were assessed during the Review of Consents process using a previous version of the Test and Itchen groundwater model. Since that assessment, the in combination impacts have changed because the Alre Scheme licence has been revoked and the two large licences at Fobdown have also been revoked. Detailed mapping of the spatial impacts of abstraction carried out before the Fobdown licences were revoked confirms that there could be local issues related to abstraction in the upper reaches of the Candover Stream. These impacts exist regardless of use of the augmentation Scheme.
Although the current version of the Test and Itchen groundwater model provides a credible assessment of abstraction impacts, any detailed assessment of impacts from individual licences would need further investigation. For example, the groundwater model has not been calibrated to consider the detailed impact of the Lasham source on the River Itchen. South East Water have a different groundwater model to assess the impacts of this source on the River Wey and are currently carrying out an investigation in relation to the River Wey.
Augmentation The potential for in combination impacts caused by augmentation have significantly reduced since 2015 when the Alre Scheme licence was revoked. In general the negative impacts of the Candover Scheme are far smaller than the impacts on flows and groundwater levels caused by public water supply abstractions and are more comparable to impacts related to local watercress bed operations. With the cessation of operations at Fobdown, the augmentation scheme is now the only significant discharge to the Candover Stream.
Public Water Supply There is a strong seasonality to the impact caused by these significant abstractors which affect the entire length of the Candover Stream. Within the RoC Site Action Plan (2007), attention was focussed on the impact that this group of abstractors could have on the SAC designated river downstream of Grange Lakes. On that basis, the Lasham licence was affirmed, whilst it was decided that the impacts of the Totford licence could be mitigated during low flow periods by the appropriate use of the Candover Scheme. This action took no account of impacts on the winterbourne reaches as they are not designated as SAC. This recent analysis clearly shows that the dominant artificial influence on flows in the upper Candover is from Public Water Supply and that impacts from the Candover Scheme are small in comparison to those impacts. Impacts on groundwater levels are also predominantly caused by operation of public water supply sources. If further assessment of these impacts is deemed necessary, a detailed investigation would need to be carried out.
Watercress and Fish Farms There are significant watercress bed abstractions around Alresford and at Fobdown and there are also groundwater abstractions for fish farms. Before the licences were revoked Fobdown had the most significant impact on the Candover Stream by increasing flows downstream of the discharge and decreasing flows upstream of the beds. Modelling has shown that in the upper reaches of the Candover Stream the influence on flows from these licences is minimal due to increasing distance
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Other abstractions and discharges Other abstractors account for less than 5% of all abstraction in the Upper Itchen and the modelled impacts are considered to be trivial.
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7. Impact on Water Quality
Data to show the impact of the Candover Scheme on Water Quality was collected both during the 1976 and 2011 test. The majority of chemical variables tested during the 2011 test pump were within water quality standards set by Water Framework Directive (WFD) or River Itchen Special Area of Conservation (SAC) targets. The most significant impact on water quality is the impact on temperature with use of the Scheme having a measurable reduction in day time maximum temperatures as shown in Figure 7a. Statistical analysis showed that there was no correlation of overnight temperatures and flow rate.
Figure 7a Comparison of maximum daytime water (solid black line, open squares) and daily maximum ambient (dashed line, open diamonds) temperatures with daily mean flow rates (solid line, closed triangles) at Upper Fobdown survey site (a) and Lower Fobdown survey site (b) (taken from Crayfish report 2012)
Soluble Reactive Phosphate (Orthophosphate) levels were recorded during the 2011 test in the Candover Stream downstream of the discharge point at Abbotstone. Levels were between 0.02 and 0.036 mg/l with a mean value of 0.022mg/l. Levels measured in the discharge from the Candover Scheme were either below 0.02 mg/l or around 0.025mg/l. The standard required to meet high water quality is less than 0.04 mg/l. This evidence shows that the Candover Scheme does not introduce water with higher levels of phosphate than are already present in the river. Suspended solids concentrations in the Candover Steam remained below the limit of 10 mg/l - typically recorded as 3 - 4.5 mg/l during the test.
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8. Summary of Abstraction Impacts
The following tables attempt to summarise the impacts that the current Candover Scheme licence could have on riverflows and winterbourne reaches of streams both whilst the scheme is in operation and after it has been turned off. The preceding sections show how variable the impacts can be, particularly in relation to how much the scheme is used and what the prevailing conditions are. For that reason, these tables are simply a summary to give an impression of impacts.
Magnitude of impact Duration of Frequency of impact impact
Candover Reduction in autumn and winter Low level impacts upstream of peak flows depending on prevailing could prevail for 6
Grange conditions and actual use of – 18 months Lakes scheme depending on prevailing
conditions and Alone impact - As much as - 50% in actual use of upper reaches scheme. In-combination - not assessed in High impacts only Historically detail as flows are naturally very expected to last for scheme only likely variable. In rare events, acting with a few weeks. to be used once in other licences, impacts can be as 10 years. high as -50 to -100% with other abstractions thought to have a far more significant effect Current licence Candover Maximum impact: Impacts from allows scheme to downstream Scheme likely to be used in -5 to - 10% alone at Borough of Grange fall below 1% consecutive years Bridge Lakes within 12 months and several times - 10 to - 20% around Abbotstone and only exceed 2- in a sequence of years. 3% for several months -25% in combination during high flow periods at Borough Bridge -25% to - 65% at Abbotstone and can be higher during extreme events River Alre Maximum impact: Impacts from Scheme likely to -2% to -3% alone fall below 1% -10% In combination during high within 12 months flow periods Upper Itchen Maximum impact: Impacts from Scheme likely to -1.5% Alone fall below 1% -6% In combination within 3 - 4 months River Dever Maximum impact: Impacts from Scheme likely to -2% alone
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In combination not assessed in fall below 1% detail but no apparent abstraction within 3 - 4 months issues River Wey Typically less than -1 to -2% Low level impacts may persist for up In combination not assessed but to 12 months abstraction impacts on the Wey are already subject to an NEP investigation and impacts associated with this licence are minor in comparison to impacts from other sources Cheriton Not discernible N/A Stream
Table 8a Summary of Impact on River flows after Candover Scheme has been turned off
Magnitude of impact Duration of impact Frequency of impact
Candover Pattern of flow/no flow relatively Impacts up to 18 above unaffected. months maximum,
Grange typically lasting no Potential of a delay of several weeks Lakes more than 1 year in spring head reaching maximum winter position and spring head may be up to 1.75km lower than expected, although typically 250 - 500m.. River Alre Pattern of flow/no flow unaffected Impacts typically Historically lasting no more than scheme only likely Streamhead likely to move by up to 6 months to be used once in 250m upstream 10 years. River Pattern of flow/no flow relatively Impacts typically Dever unaffected lasting no more than 1 year Current licence Streamhead likely to move by up to allows scheme to 250m upstream be used in River Wey Pattern of flow/no flow relatively Impacts typically consecutive years unaffected lasting no more than and several times 1 year in a sequence of Streamhead likely to move by up to years. 250m
Table 8b Impact on stream-heads after scheme has been turned off
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Magnitude of impact Duration of impact Frequency of impact
Candover Groundwater levels may be Impacts greater than 1m will Stream reduced by up to 3m in the persist in the upper valley
upper Candover Valley with for 6 - 12 months. Smaller
reductions of the order of impacts of less than 0.5m Historically 0.2m at Northington and may last for 12 - 18 months. scheme only likely 0.1m further downstream at to be used once in Smaller impacts in the lower Abbotstone 10 years. valley may persist for up to 12 months from when use of the scheme stops Current licence River Alre Groundwater levels may be Impacts of up to 0.1m may allows scheme to reduced by up to 0.1m persist for up to 12 months be used in from when use of the consecutive years scheme stops and several times in a sequence of River Dever Groundwater levels may be Impacts of up to 0.2m may years. reduced by up to 0.4m persist for up to 12 months from when use of the scheme stops River Wey Groundwater levels may be Impacts of up to 0.1m may reduced by up to 0.2m persist for up to 12 months from when use of the scheme stops Table 8c Impact on areas of high groundwater level which may be linked to groundwater dependent terrestrial ecosystems
Magnitude of impact Duration of Frequency of impact impact
Upper Stream already likely to be dry N/A Candover upstream of discharge point so no
impact likely whilst scheme in use
Lower +110% alone Up to 6 months Candover Use of the scheme is likely to cause flows to rise from those expected to be Historically exceeded 95% of the time to flows only scheme only likely expected to be exceeded 5% of the to be used once in time. In other words from what might be 10 years. expected in a dry summer to flows expected in a wet summer. Flows beyond what would naturally be Current licence expected would not be permitted under allows scheme to the terms of the existing licence be used in consecutive years The rate of rise permitted by the licence and several times conditions matches the rate observed in in a sequence of the wet summer of 2012 and is not as years. steep as that observed during exceptional events when rapid recharge
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naturally starts in October as observed in 1993. Upper +10% alone Up to 6 months Itchen + 9 % in combination with all other abstractions and discharges from cressbeds. The underlying negative impacts of abstraction offset the positive influence on flows to make the increase in flows in combination with other licences smaller than the increase in flows if the scheme was acting on its own
Table 8d Impact of augmentation discharge on river flows
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9. Water Framework Directive
In order to meet our obligations under the Water Framework Directive (WFD), when determining applications for water resources licences, we must not: • deteriorate water body status, including deterioration of individual quality elements; • compromise the implementation of improvement measures. These are described in the River Basin Management Plans (RBMP) to ensure we aim to achieve good status, or good potential, in water bodies; • compromise the achievements of protected area objectives (SAC, SPA and Ramsar sites). These are described in the Annexes of the RBMPs.
The following sections explain how we have assessed the impact of the renewal of the licence in the context of our Water Framework Directive obligations.
9.1. Surface water body Figure 6.1a shows that the Candover Stream is included in one surface water body (GB 107042022620) which is also referred to as the Candover Brook. The 2015 South East River Basin Management Plan confirms that the ecological status of the Candover Brook water body is moderate status. Invertebrates and Fish are both recorded to be at good status but macrophytes are moderate status. The Candover Brook is predicted to achieve good status by 2027. River flow is a "supporting" element for achieving Good Ecological Status, but does not in itself determine status. To determine if flow is supporting good ecological status, calculations are carried out in Water Resources GIS (WRGIS) to compare flows with the Environmental Flow Indicator (EFI). This is carried out by analysing flow data which has been summarised into a flow duration curve. Compliance is reported at Q95 (low flows) but is also calculated for medium and high flows. It is important to note that WFD flow compliance is based on recent rates of abstraction and estimates of a future predicted scenario, rather than at full licensed abstraction. Flows in the Candover Stream are currently reported as supporting Good Ecological Status And so no investigation into abstraction pressures on the Candover Stream have been carried out for WFD purposes. However, removing the Fobdown abstraction and associated discharge from the WRGIS results in a position where Recent Actual flows would be below the EFI by 1.6 Ml/d at Q95 and fully licensed flows would be 2.9 Ml/d below the EFI at Q95. In this case, the hydrological status is likely to change to a position where flows would no longer be supporting Good Ecological Status.
9.2. Groundwater Body WFD groundwater body status can be either good or poor (there are no high, moderate or bad categories as for surface water). This overall classification is based on the groundwater body’s chemical status (in relation to a large range of pollution pressures) and its quantitative status (in relation to groundwater abstraction pressures). A series of four tests have been developed for groundwater body quantitative classification, based on WFD requirements and guidance provided at a UK level. The four tests consider the impacts of groundwater abstraction both on the groundwater body itself, and also on the ecological receptors which depend on it, and are described below.
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Figure 9.1a WFD Water bodies
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• Groundwater body resource balance: at its simplest this test compares the long term average recharge to the groundwater body with the long term average groundwater abstraction from it. The relatively few groundwater bodies where abstraction exceeds recharge are classified at poor quantitative status (with high confidence). The test also compares groundwater abstraction impacts with EFIs-based low flow limits aggregated for all of the surface water bodies supported by the groundwater body. If groundwater abstraction impacts exceed this aggregated low flow resource allowance, the groundwater body should also be classified at poor quantitative status (but with a lower level of confidence). • Dependent surface water body ecological status deterioration: this test considers the impact of groundwater abstractions on the ecological status of river, lake and estuary water bodies. Where groundwater abstraction has had, or is having, a significant impact on surface water ecological status, the groundwater body supporting both the abstraction and the surface water body should be classified at poor quantitative status. Additional tests are also incorporated considering the relative significance of groundwater abstractions in causing the failure, as distinct from surface water abstractions (groundwater abstraction impacts must account for more than half of the acceptable abstraction limits) • Saline & other intrusions: excessive rates of groundwater abstraction can result in the intrusion of saline or other poor quality waters. Groundwater level drawdown in coastal areas can result in the intrusion of sea water. Further inland older ‘connate’ groundwaters can also be drawn up from depth. • Dependent wetland significant damage: the WFD recognises that the ecological condition of wetlands may depend on protecting groundwater levels, flows and quality. ‘Groundwater Dependent Terrestrial Ecosystems’ (GWDTEs i.e. wetlands) are not classified with an ecological status but are assessed as to whether they have been ‘significantly damaged’ by groundwater abstraction - as part of the quantitative status classification of all groundwater bodies.
The final combined quantitative status result is based on the worst outcome from all four tests. Confidence in those status assessments is also given in terms of attributing high or low confidence to the result. In addition, the risk of deterioration in status is assessed by considering the risk to status from abstracting to the full extent of the licences. Figure 9.1a shows that the Candover Scheme is located within the River Itchen Chalk groundwater body (GB40701G505000) but previous sections show that groundwater levels may also be affected in the adjacent River Test Chalk groundwater body (GB40701G501200). The Candover Surface water body also extends onto the Basingstoke Chalk but previous analysis suggests that the scheme is unlikely to have a significant effect on that groundwater body. Impacts may also spread into the adjacent Alton Upper Greensand groundwater body but impacts are expected to be minor. The reason for the poor quantitative status of the River Itchen Chalk groundwater body is largely related to significant groundwater abstractions around Twyford and Otterbourne. The River Basin Management Plan states that good status will be achieved by 2027 principally by delivering changes to large groundwater abstraction licences which will improve the status of dependent surface water bodies. To assess the sustainability of this licence, it is important to assess if there is any risk that use of the licence could lead to a change in status of any of the elements of the four groundwater body tests or prevent achievement of good status. In a unit already failing the quantitative tests, opportunities to reduce licences should be actively pursued.
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River Itchen Chalk River Test Chalk (GB40701G505000) (GB40701G501200) GW Balance Test Poor (LC) Good (HC) Probably At Risk (LC) Probably Not At Risk (LC) Dependent Surface Water Test Poor (LC) Good (LC) At Risk (HC) Not at Risk (HC) Saline Test Good (HC) Good (HC) Not at Risk (HC) Not at Risk (HC) Groundwater Dependent Terrestrial Good (LC) Good (LC) Ecosystems Test Probably At Risk (LC) Probably At Risk (LC) Overall Quantitative Status Poor (LC) Good (LC) At Risk (HC) Probably At Risk (LC)
Table 9.2a Groundwater body status results for River Itchen and River Test Chalk ( LC = Low Confidence, HC = High Confidence)
We would not be able to renew the licence if that licence could be considered to be contributing to serious damage which within this context could be considered to occur if: • Groundwater body status moves from Good to Poor • Groundwater body status moves from Poor (Low Confidence) to Poor (High Confidence) There is also the provision to consider the “potential or risk of serious damage.” For groundwater the risk or potential of serious damage will include: Groundwater bodies where the overall WFD Groundwater Quantitative status could deteriorate from Good to Poor under the condition where licence uptake is equivalent to the full licence entitlement. The Candover Scheme will have no impact on salinity of either groundwater body as it will not induce sea water to flow into the aquifer and it will not lead to mining of connate water. Within the constraints of the standard WFD assessment carried out to define the overall status of the unit, no Groundwater Dependent Terrestrial Ecosystems were considered to be at risk. Therefore, using the standard assessment tests there is no potential for deterioration in status for those two tests. The tests for Groundwater balance and Dependent Surface Water Abstractions can be re- assessed to see if there is any risk in deterioration as a result of using the scheme.
9.2.1. Groundwater Balance Test
For both water bodies, the maximum impact of possible use of the Candover Scheme has been applied in both tests, despite this being a very unrealistic scenario. This analysis uses the most recent data set available but does not account for the fact that in the last year, there has been a reduction in the fully licensed scenario of 17 Ml/d due to revocation of the Alre Scheme licence and a reduction in the Candover Scheme licence. This reduction in risk more than offsets any potential increase in risk as a result of using the scheme. Even without accounting for that change, the River Itchen groundwater unit is in surplus for test 1 and is in deficit for test 2 which means that for the Overall Groundwater Balance test it is at Poor Status (Low Confidence). It would only deteriorate to Poor Status (High Confidence) if abstraction exceeded recharge which it will not. The use of the augmentation scheme would not result in any within class deterioration for this test for the River Itchen Chalk. 60 of 66
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The River Test Chalk Groundwater body is at Good Status and the minor change possible as a result of using the Candover Scheme will not affect that status as the unit will remain in surplus. There is therefore no risk of deterioration for either groundwater body.
River Itchen Groundwater Body River Test Groundwater Body Gw body 453 km2 970 km2 area (km2) Recharge 437 mm 395mm (mm) Recharge 543 Ml/d 1050 Ml/d (Ml/d) Natural 64 Ml/d 107 Ml/d Available Low Flow Resource
Recent Fully Recent Recent Actual Fully Recent Actual Licensed Actual with Licensed Actual with full use of full use of Candover Candover Scheme Scheme every year every year TEST 1 Long Term 168 Ml/d 270 Ml/d 178 Ml/d 137 Ml/d 193 Ml/d 147 Ml/d Average
Abstraction % of 31% 50% 33% 13% 18% 14% recharge Balance for 375 Ml/d 273 Ml/d 365 Ml/d 913 Ml/d 857 Ml/d 903 Ml/d Test 1 TEST 2 Low Flow 96 Ml/d 160 Ml/d 106 Ml/d 31 Ml/d 55 M/d 41 Ml/d abstraction impact Balance for -32 Ml/d -96 Ml/d -42 Ml/d 76 Ml/d 52 Ml/d 66 Ml/d Test 2
Table 9.2b Reassessment of Groundwater Tests if Candover Scheme is fully used
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9.2.2. Dependent Surface Water test For the River Itchen Chalk Groundwater body, Table 9.2c shows that more than 20% of Groundwater body is covered in units where surface water bodies are failing the EFI and the unit is therefore classified as Poor Status. As use of the scheme is only likely to improve the hydrological surface water body status for the Candover Brook or any other surface water bodies then if anything, use of the scheme will help achieve good status in relation to this test and there is no risk of deterioration.
Proportion of the River Proportion of the River Test Itchen Chalk Groundwater Chalk Groundwater body body covered by failing covered by failing surface surface water units water units Good (HC) 55% 66% Good (LC) 20% 24% Poor (LC) 25% 10% Poor (HC) 0% 0%
Table 6.2c Dependent Surface Water test results for River Itchen and River Test Groundwater bodies
For the River Test Chalk Groundwater body only 10% of the groundwater body is covered by failing surface water bodies which is well within the 20% threshold for this test. Use of the Candover Scheme is not expected to change compliance with the Environmental Flow Indicator for any surface water bodies associated with the River Test Chalk Groundwater Body and there is no risk of deterioration. 9.3. Summary of WFD Compliance Analysis of the tests used to check WFD compliance for both surface water and groundwater, shows that there is not likely to be any risk of deterioration as a result of using the Candover Scheme and use of the scheme will not compromise the stated aim of achieving good status by 2027. This analysis has considered the Candover Brook surface water body as well as the River Itchen and River Test Groundwater units. 9.4. Abstraction Licensing Strategy The Test and Itchen Abstraction Licensing Strategy was published in March 2013 and contains information relating to the renewal of time limited licences. The document states that for existing licences: Itchen SAC was assessed and it was decided to modify nine abstraction licences. Seven remain to be modified; three public water supply, two augmentation scheme licences and two fish farm licences. It is our intention to make those modifications by 2015 if that is possible within our funding constraints. local considerations. Renewals may be subject to minor changes including the addition of water efficiency conditions. Variations may also be subject to minor changes including a time limit. Within that document, to meet our WFD obligations, it is clearly stated that there is a presumption in favour of renewing existing abstraction licences.
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10. Conclusions of Part 1Report
In this report, evidence has been provided to: • Show how often and at what rates the Candover Scheme might be used under the conditions of the current licence • Demonstrate the impacts of the Candover Scheme on: o River flows in the Candover Stream o River flows in adjacent water courses o The movement of springheads o Groundwater levels o Water Quality • Determine if use of the scheme affects WFD compliance
This evidence will now be used in the Part 2 report to assess the environmental sustainability of the Candover Scheme which will focus on the ecological consequences of the hydrological impacts described above. It presents the existing ecological data and considers risks against a number of ecological flow targets for the River Itchen SAC and SSSI interest features and wider biodiversity. The third, and final document, considers the outcomes of the first two assessments and presents our final conclusions and recommendations on the way forwards.
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Appendices
Appendix A Candover Scheme Abstraction Licence
Appendix B Candover Scheme future licence changes
Appendix C Summary of abstraction in the Upper Itchen
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References
Augmentation Schemes: ecological and anomalous behaviour report Rushbrook et al., 2012 Hampshire & Isle of Wight Wildlife Trust / Environment Agency
M. J. Beard, D. M. Giles, H. G. Headworth, J. L. Hill, T. Keating, M. J. Packman and D Patterson (1979) The Candover Pilot Scheme Final Report Southern Water Authority: Botley Printers Limited
Environment Agency (2015). Implementing the Review of Consents requirements for the Itchen Augmentation Schemes
Pump testing and associated investigations of the Candover and Alre augmentation Schemes, summer 2011, Southern Water Services, August 2012 DRAFT
River Itchen Appropriate Assessment, Environment Agency, 2005
River Itchen Site Action Plan, Environment Agency, 2007Rob Soley (2015) Technical note for Southern Water reference 29388tn568, Augmentation Technical Working Group, AMEC Foster Wheeler. Rushbrook, B.J., Selby, T. & Evans, K. (2012). Investigating the potential ecological implications of the Upper Itchen flow augmentation Schemes: a specific focus on the resident white-clawed crayfish (Austropotamobius pallipes) population. A report prepared for the Environment Agency and Southern Water Services Limited. Hampshire and Isle of Wight Wildlife Trust. S.Watson (2014) Southern Water Services, Groundwater modelling assessment of the impact on river flows and groundwater level of using the Candover augmentation Scheme: Atkins Technical Assessment
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