Low Flows Investigations River Avon SAC Final Report for Steering Group Consultation March 2008 Report no: DM-#956729 River Avon SAC Low Flow Investigation Final Report for Steering Group Consultation Document Control This document has been prepared by Hyder and Wessex Water Services to meet the requirements of the Environment Agency. The information is provided in confidence and is intended solely for the use of the members of the Low Flow steering and project groups. No part of this document may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without prior reference and written consent from Wessex Water Services. Whilst not in use the document should be stored securely. Loss of the document should be immediately reported to Wessex Water Services. Requests for the release of the whole or parts of this document under the Freedom of Information Acts or the Environmental Information Regulations should be referred to Wessex Water Services. The document and its contents remain the property of Wessex Water Services, and is to be treated as Restricted. Map Licence Maps within this report are reproduced from Ordnance Survey Map with the permission of the Controller of Her Majesty’s Stationery Office. Crown Copyright reserved. Licence No. 100019539. Cover Picture: River Wylye (SAC reach) at Steeple Langford September 2006 River Avon SAC Low Flow Investigation Final Report for Steering Group Consultation Executive Summary This report summarises the work carried out in the Low Flow investigation of the impact of public water supply (PWS) sources on the River Avon Special Area of Conservation (SAC). The report presents the details of the work which comprises a desk review, a groundwater/river flow model and two years of ecological survey. The area of investigation comprises the Greensand and chalk aquifers which feed the River (Hampshire) Avon and its tributaries, the Wylye, Bourne, Nadder, Nine Mile River and Ebble. Within the catchment Wessex Water (WW) operates 20 groundwater sources and one river abstraction (downstream of Fordingbridge). Other PWS sources operated by neighbouring water companies are located in the eastern portion of the catchment and from the lower river, close to its discharge into Christchurch harbour. The River Avon SAC is designated for its chalk stream community (water courses of plain to montane levels with the Ranunculion fluitantis and Callitricho-Batrachion vegetation), the salmon, lamprey and bullhead populations. The overall objective of the investigation is to collate such information needed for the Environment Agency to determine whether the abstraction licences, alone or in combination, cause a loss of integrity of the SAC features, as required by the European Habitats Directive, 1994. The study has two main objectives: • To define the hydrological impact upon the River Avon SAC (and tributaries) due to Wessex Water groundwater abstractions • To determine whether these river flow changes, due to abstractions, adversely impact the integrity of the SAC. Hydrology A computer based numerical groundwater/river flow model of the Hampshire Avon catchment has been developed, jointly funded by the EA and WW. This model has been calibrated to historic river flows and is considered ‘fit for purpose’, though refinement work to improve the calibration/representation of the River Till is ongoing. The model allows natural (no abstraction, no discharges) river flows to be predicted. In addition the river flow under Full Licence abstraction conditions has been predicted. The model has also been used to assess the impact of individual sources. The key impact investigated is at summer low flows or Q95 but impacts at Q70 and Q50 have also been assessed. Natural England guidelines on ‘acceptable’ river flow reductions has been used to screen the model output to define whether flow reductions are ‘compliant’ or ‘non-compliant’. For the extent of the River Avon SAC impacted by WW operations, the allowable flow reduction is 10% of natural. At Full Licence the following impacts are predicted along the SAC reach of each tributary: Avon – small exceedance reach (~3km) around Durrington at Q95, which is mitigated in effect by leakage from the Kennet and Avon canal into the aquifer. Bourne – The largest impacts on the Bourne occur at times of low flow (> Q95). At Q95 exceedance occurs along the entire SAC reach, with a maximum reduction from natural of i River Avon SAC Low Flow Investigation Final Report for Steering Group Consultation 34%. An additional 5 Ml/d of water would be required in the river at Q95 to achieve compliance. Nadder – No exceedance of guideline values. Till – Awaiting model refinement. Wylye – Small reaches of exceedance occur at low flows (Q95), stream support and storage development affords a large degree of protection to flow at natural low flow times. Greater reductions in flow occur at higher river flows. Maximum reductions in river flow occur near the start of the SAC (25% at Q44) and between the confluences with the Chitterne Brook and River Till (29% at Q50). The reduction in flow at the start of the SAC is attributable to the Brixton Deverill PWS source. The reductions observed between the Chitterne and Till are in part a cumulative impact, but the large abstraction from the Codford PWS source has a major influence. The above impacts are theoretically possible but have never occurred to date. The contemporary impact of abstraction (i.e. the typical impact experienced in the river between 1995 and 2003) has been determined to establish the relationship between river flow (or drying period) and the present ecology: Bourne – contemporary abstraction has resulted in exceedance of guideline values, with up to a 25% reduction in flow (at Q95) compared to natural. Wylye – contemporary impacts along the Wylye are close to the guideline values at all Q values. The main exception is at the start of SAC, where reductions of up to 25% occur. The largest impacts due to abstraction occurs along the headwater triburtaries: Bourne, Wylye (upstream of Warminster) and Fonthill Brook (non SAC watercourse) Ecology Although the Avon has been intensively studied, as both a Site of Special Scientific Interest (SSSI) and due to public concerns over the decline of the fishery, the existing data failed to show a clear effect of abstraction and was unsuitable for doing so. Consequently a bespoke ecological survey programme for fish, plants and macroinvertebrates (aquatic insects) was undertaken in 2006 and 2007 in the headwaters where abstraction impacts occur. This aimed to separate the effects of historical abstraction from those of climatic flow change against the varying parameters of water quality, river shape and management. Independent technical experts to this study, advised that the macroinvertebrates were the organisms most likely to exhibit a ‘flow’ related stress. The survey years were climatically different. 2006 followed on from two low recharge winter and ‘dry’ summers. Whereas the 2006/07 winter was relatively wet and the 2007 summer was ‘wet’. Physical measurements were collected and the results were subject to statistical analysis against the modelled percentage of impact of abstraction on the flow. Areas with no abstraction pressure were included, such as the River Ebble. ii River Avon SAC Low Flow Investigation Final Report for Steering Group Consultation The data were subject to various statistical analyses to detect the effect of percentage impact of abstraction on permanently flowing stretches, and on drying period in the winterbourne stretches. This included identifying the key associations between the plant or macroinvertebrate community and the physical nature of the channel, water quality and flow, as well as with the degree of abstraction. The study was primarily undertaken on headwaters upto 30Ml/d of late summer flow (Q95), where tributaries with impacts of over 10% were located, along with non abstracted comparators. However, additional studies were done on the lower Wylye, which has more marginal impacts but is subject to considerable interest amongst fisheries associations, comparing it with the less abstracted Upper Avon tributary. The effect of abstraction on river temperature, water depth and plant life was also assessed. Where historical data was available this was also analysed. Relationships were found with the antecedent period of no flow, in the previous 12 or 24 months, along the winterbourne which allows the macroinvertebrate and macrophyte communities to be predicted. These community predictors, together with output from the model defining the period of extra drying due to abstraction, have been used to model the ecological change that would be produced by abstraction at Full Licence. For the perennial streams, no relationship was detected at present between the percentage reduction in flow and a change in ecological measure which would allow a relationship at that site to be predicted for full licensed abstraction rates. However, it is concluded that recent flows may dictate the riverine communities more closely than historical impacts and so further comparison with recent flows will be completed once the modelled values for 2007 are completed. The impact of current abstraction however is reported on the river ecology. Regarding the designated SAC interest the study has concluded that the River Avon headwaters and winterbournes contained diverse macroinvertebrate and plant communities of high conservation value, as befits it’s designated status. With regard to the effect of abstraction, the following effects were detected: Lamprey – No abstraction impact upon the juvenile Lamprey population was detected. Bullhead – An abstraction impact (at flow reduction >15%) has been detected by the Bullhead numbers, but the mechanism for impact has not been established. The 2006 survey found more Bullheads at umimpacted sites than impacted sites however, this difference was not detected in the 2007 data. Salmon – The survey area is outside of the majority of the known salmon habitat so few salmon were recorded except in the lower Wylye.