HYDROLOGICAL CONTEXT FOR WATER QUALITY AND ECOLOGY IMPACT ASSESSMENTS

Transcription

1 TECHNICAL APPENDIX B HYDROLOGICAL CONTEXT FOR WATER QUALITY AND ECOLOGY IMPACT ASSESSMENTS

2 Table of Contents B.1 Introduction... 1 B.1.1 Overview... 1 B.1.2 Importance of Hydrology to the Stage 3 LTOA Investigation... 1 B.1.3 Description of the LTOA... 2 B.1.4 Studies undertaken for the hydrological assessment... 8 B.2 Hydrological Context B.2.1 Thames Basin Overview B.2.2 Characterisation of the Flow Regime and its Management in the Study Area B.3 Hydrological Assessment B.3.1 Naturalised Flow Comparison...40 B.3.2 Tidal State Analysis for Water Quality Data Matching B.3.3 Flow Data Analysis in Support of Water Quality Assessment Models B.3.4 Temporal Flow Pattern Analysis B.3.5 Spatial Flow Pattern Analysis B.3.6 Tidal Level and Salinity Analysis B.3.7 Tidal Level and Water Quality Analysis B.3.8 Tidal Level and Navigation B.3.9 Review of Fish Pass at Molesey Weir B.3.10Velocity Review B.4 Summary of Issues for the Stage 3 Assessments of Water Quality, Aquatic Ecology and Navigation B.4.1 Hydrological Context B.4.2 Summary of Hydrological Assessment Findings... 68

3 B.1 INTRODUCTION B.1.1 OVERVIEW This appendix sets out the hydrological context to support the Stage 3 LTOA impact assessments of water quality, aquatic ecology and navigation. It updates the assessment presented in Stage 1 () using more recent data (collected during the Stage 2 monitoring period), and builds on that assessment following the 10 recommendations made in Stage 1. Stage 1 set out to identify and illustrate features of the flow regime of the River Thames and the influence of river flows on the Upper Tideway which potentially influence water quality and aquatic ecology and which may be impacted by Thames Water s abstractions. The importance of hydrology to the Stage 3 LTOA investigation is re-stated (from Stage 1) in Section B.1.2. An outline description of the LTOA is re-stated (from Stage 1) in Section B.1.3. In line with the Stage 1 recommendations, the 10 studies undertaken for the hydrological assessment are introduced in Section B.1.4. The hydrological context set out in Stage 1 has been updated in Section B.2. The 10 hydrological assessment studies are presented in Section B.3. A summary of issues for the Stage 3 assessments of water quality, aquatic ecology and navigation are presented in Section B.4. Data used in the study are listed in the accompanying Annex 1. B.1.2 IMPORTANCE OF HYDROLOGY TO THE STAGE 3 LTOA INVESTIGATION The hydrological functioning of rivers and estuaries plays a key role in determining the water quality and ecological dynamics of the supported ecosystems. The hydrology, viewed here as the flow of the river is a key habitat variable, together with its impact on tidal circulation of the estuary. The dynamic nature of the hydrology needs to be well understood, including the driving variables that dictate the spatial and temporal changes (e.g. response to meteorological conditions, land use etc.). In turn, the changes in hydrology can have a fundamental significance for both water quality and ecological response it is these relationships that this LTOA investigation is seeking to understand. It is through the assessment of the change in Lower River Thames hydrology resulting from LTOA abstractions that an understanding of the potential or actual impacts of the abstractions on water quality and/or ecology of the Lower River Thames and/or Upper Tideway will be identified. However, it is not only the flow that has hydrological relevance. The consequence of varying flow is that the water velocity, wetted area and wetted depth may also vary. Each of these aspects in isolation or cumulatively can have a significant effect on 1

4 habitat availability and function. It is therefore important that these variables are investigated to assess whether changes in the range of hydrological metrics can influence habitat availability and therefore ecological sustainability. Of note, hydrological function can and does vary diurnally, seasonally, annually and interannually, requiring consideration of flow variables at a range of scales to fully establish the influence on water quality and ecology. The variability in the abstraction regime under the LTOA also changes with time and spatially, and not necessarily at the same rate of change as the natural hydrology. The Lower River Thames ecosystem is therefore a complex one, with dependant and independent variability that requires careful consideration. Currently the available data will allow consideration of seasonal, annual and inter-annual variability, and the consequences for water quality and ecology it is these trends and relationships that are considered in this Stage 1 review. Finer resolution affects at the smaller reach scale or over shorter time-steps may become evident during the investigation that will be identified for consideration in Stage 2, should the potential effects be considered significant. The working boundary of the study area (see Figure 1) has been agreed between Thames Water and the Environment Agency. The upstream limit is the Windsor flow gauge on the River Thames upstream of Thames Water s Datchet intake the most upstream of the intakes included in the LTOA. The downstream limit is London Bridge - below which previous studies 1 have identified no significant influences of the river on the tidal cycle, salinity regime or water quality of the Upper Tideway. B.1.3 DESCRIPTION OF THE LTOA The LTOA is a Section 20 Water Resource Management Agreement under the Water Resources Act 1991 between the Environment Agency and Thames Water (originally under Section 125 of the Water Act 1989). The agreement, which came into force in 1989, regulates the licensed abstraction of surface water from the Lower River Thames between Windsor and Teddington (abstraction licence no. 28/39/M/2, known as the M2 licence) to Thames Water s Thames Valley reservoirs in west London and to the Lee Valley reservoirs in north London (via the Thames-Lee Tunnel). This provides for a balance between water required for public water supply and the environment. 1 e.g. Halcrow (1985) Teddington Weir tidal hydraulics study, report for Thames Water September 1985; Thames Water (2006) Lower Thames Drought Permit Environmental Report. 2

5 LTOA: Stage 2 - Completion of AMP5 Investigations Figure 1 LTOA Study Area and Key Features 3

6 The LTOA provides the management framework for day-to-day operational decisions on Thames Water s abstraction from the Lower River Thames. The agreement is based on ensuring that sufficient flow passes over Teddington weir (measured at Kingston gauging station), relative to the prevailing seasonal reservoir storage of the Thames Valley and Lee Valley reservoir groups. This requires very close monitoring of river flows and reservoir levels to set abstraction rates, with daily liaison between the Environment Agency and Thames Water. Daily abstraction rates are therefore controlled by: The licence conditions of the M2 abstraction licence: Overall licence: Annual maximum of 663,729Ml; daily maximum of 5,455Ml; daily maximum averaged over the year 1,818Ml. Individual intakes daily limits: Datchet 2,273Ml; Staines 682Ml; Laleham 1,364Ml; Walton 1,264Ml; Hampton (Thames-Lee Tunnel) 682Ml; Surbiton 109Ml. The control rules for deciding on the appropriate pass-forward flow (Teddington target flows, TTF) over Teddington Weir (the freshwater contribution of the River Thames to the Upper Tideway), as guided by the Lower Thames Control Diagram (LTCD, see Figure 2), which include: Date in calendar year Total London reservoir total storage Water use restrictions in place (relating to Thames Water s Level of Service). Four TTF values are included in the LTOA (see Figure 2). The LTOA conditions mean that Thames Water s abstractions cannot cause the pass-forward flow to drop below the TTF agreed for that day. There are significant other consumptive surface water abstraction licences in the study area. Affinity Water hold abstraction licences for public water supply at their River Thames intakes at Sunnymeads (daily limit 227.3Ml/d), Egham (daily limit 182Ml/d), Chertsey (daily limit 55Ml/d) and Walton (daily limit 55Ml/d). These licences are neither included in the LTOA, nor subject to river flow condition controls although they have an important role in determining the water available to Thames Water s abstractions managed by the LTOA and therefore the TTF in place. The importance of hydrology to the LTOA investigation of water quality and ecology, both directly and through hydraulic influence, such as velocity, is presented in Section B2. The management of Thames Water s abstractions through the TTFs in the LTOA/LTCD results in a number of key hydrological effects and patterns are reported in Section B3. 4

7 Figure 2 Lower Thames Control Diagram Ml/d 90 TARGET FLOW Total London Reservoir Storage (Ml) Ml/d 400 Ml/d 300 Ml/d Percent of usable capacity 800 Ml/d 600 Ml/d 400 Ml/d 300 Ml/d RESTRICTION Level 1 Level 2 Level 3 Level 4 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3 is an example of how the complex rules are operated and how river flows and abstractions are managed in association with reservoir storage. It is for illustrative purposes only. Normally there is a minimum residual flow of 800Ml/d (Note 1 on Figure 3). However, as the volume of available reservoir storage reduces, this triggers reductions in the TTF in a stepwise fashion (Notes 2 and 3 on Figure 3). Each step requires the implementation of greater demand control measures by the Environment Agency and Thames Water, for example hosepipe bans. This stepwise reduction progresses from 800Ml/d to 600Ml/d (Note 2 on Figure 3), followed by further reductions to 400Ml/d (Note 3 on Figure 3) and then 300Ml/d 2. Figure 3 shows that Thames Water s abstractions vary according to the river flow conditions but do not cause the pass-forward flow to reduce below the TTF in effect on any given day. Affinity Water s abstractions from the Lower River Thames (not included in the LTOA and not subject to river flow condition controls) remain fairly constant over the example timeline in Figure 3. As storage in the reservoirs recovers, the TTF is increased (Notes 4 and 5 on Figure 3). It is important to note from these real data that the pass-forward flow is not managed exactly as the TTF value. The pass-forward flow varies from day to day because of the practical difficulties of managing the abstraction of the available flow in combination with Environment Agency operations to manage water levels using weir settings 2 It is noted that the minimum residual flow requirement under the M2 abstraction licence is 200Ml/d - formal agreement from the Environment Agency is required in order to vary the Section 20 LTOA to this value. 5

8 throughout the study area. The responsibility for decisions on changes of TTF lies with the Environment Agency s Environment Manager, South East Area and Thames Water's Asset Management Director. Key features of the pass-forward flow regime are apparent from Figure 3 for further investigation in the initial hydrological assessment in Section B.3.3 including: Magnitude of the pass-forward flow to the Upper Tideway flow is managed to meet the TTF unless flow (without abstraction) is much greater than the normal TTF (800Ml/d). In Figure 2 flow reaches a minimum close to the 400Ml/d TTF (e.g. 401Ml/d on 2 August). High values are not shown (e.g. late October) as on these occasions the pass forward flow is relatively greater than compared to the abstracted volume. Proportion of the abstracted flow relative to the remaining flow passed forward to the Upper Tideway the proportion typically increases as the flow (without abstraction) decreases. In Figure 3, on 2 July the abstracted flow (2,095 Ml/d, including Thames Water abstractions of 1,767Ml/d) reaches almost 80% of the flow without abstraction (2,570Ml/d), with a TTF of 400Ml/d. Duration of pass-forward flows of a certain TTF - the 400Ml/d TTF has a duration of ~100 days between late June and early October, with an average flow of ~650Ml/d during this period. It should be noted that this is not a fixed flow but a minimum working flow. Seasonality of variation in the flow envelope of pass-forward flows. There is potentially seasonality in both high and low flows. For the low flows shown in Figure 3, abstraction impacts are seen in summer and autumn. The frequency of low pass-forward flows or the regularity of use of TTFs lower than the normal 800Ml/d. This is an inter-annual effect and is not shown on Figure 3. 6

9 Figure 3 Example of Use of Lower Thames Control Diagram in Guiding the Setting of Teddington Target Flows 5, ,500 4,000 3,500 3,000 2,500 2,000 1,500 1, T TF Flow Ml/d 1. Storage sufficient, TTF 800Ml/d 2. Reserv oir storage reduces to below monthly LTCD trigger, TTF reduced to 600Ml/d 3. Reserv oir storage reduces to below monthly LTCD trigger, TTF reduced to 400Ml/d 4. Reservoir storage recovers to above monthly LTCD trigger, TTF increased to 600Ml/d 5. Reservoir storage recovers to above monthly LTCD trigger, TTF increased to 800Ml/d Reservoir storage, % usable capacity 0 0 Jun Jul Aug Sep Oct 01/06/0 01/07/06 01/08/0 01/09/0 01/10/06 Total London reservoir storage (Thames Water reservoirs) Teddington Target Flow (TTF) in effect through agreement between Thames Water and EA, guided by LTCD Riv er Thames pass-forward flow at Teddington (5-day average) with water company abstractions from lower Thames without water company abstractions from lower Thames Lower Thames total water company abstractions (5-day av erage) Veolia Water component of total Thames Water component of total (bar stacked) 7

10 As the LTOA point of hydrological compliance is Teddington, the hydrological context will focus on the river flow regime at Teddington which is the pass-forward flow to the Upper Tideway. The potential impacts of Thames Water s abstractions spatially throughout the study area will be put into context. However, it is important to consider that the individual intakes do not have local hydrological compliance conditions in the river but are managed by the overall TTF at Teddington subject to the individual intake daily licence limits; the available storage in the reservoirs they each supply; and the annual abstraction maximum of the overall M2 licence. In effect, this means that although measured, historic abstraction records of individual intakes provide a guide to the scale and pattern of the intake s use, they reflect the management regime in place at that time to meet specific water resource supplydemand scenarios and do not necessarily reflect Thames Water s current water resource source management for the London Water Resource Zone or the full range of abstraction scenarios possible through the LTOA conditions. B.1.4 STUDIES UNDERTAKEN FOR THE HYDROLOGICAL ASSESSMENT A preliminary assessment of the hydrological context itself was presented in Stage 1 (Section B.3). This provided an overview of the Thames basin and characterisation of the flow regime and management in the study area. Emphasis was given to the flow regime 3 of the River Thames and the pass-forward flow to the Upper Tideway with, where appropriate, narrative on the hydrodynamic factors 4 of water depth, velocity, wetted width, flushing rate and estuarine physical processes which directly or indirectly influence water quality, ecology and navigation. This hydrological context has been updated in Section B.2. Further assessment was identified as required to address a range of emerging issues. Stage 1 identified that the assessment would investigate, in detail, the potential impacts of the LTOA on the flow regime and the potential consequent impacts on the hydrodynamics of the River Thames and Upper Tideway. The Stage 3 assessment in Section B.3 provides this assessment as follows: Section B.3.1 Naturalised flow comparison: Review naturalised flow inputs to WARMS against Environment Agency Technical CAMS (or Complex CAMS if available) naturalised flows. Section B.3.2 Tidal state analysis for water quality data matching: Analyse the measured tidal record data in order to characterise and assign stages in the tidal cycle (high water slack, upper ebb tide, lower ebb tide, low water slack, lower flood tide, upper flood tide) and tidal regime 3 The quantity of water over time. 4 The processes of physical forces on the flow and motion of water - particularly wetted width and depth, velocity, time of travel and flushing rate; estuarine physical processes affecting tidal height and salinity profile. 8

11 (spring/neap) across the ten year water quality assessment period. These data should then be used for impact assessment of the influence of river flow on tidal state, water quality (empirical analysis) and to identify boundary conditions in water quality and tidal regime modelling. Section B.3.3 Flow data analysis in support of water quality assessment models: Extant water quality models (Quest and Telemac 2-D models, see Technical Appendices C and F) should be reconfigured for the study area and calibrated for the range of flows relevant to the LTOA investigation. Flows should include TTFs of 300Ml/d, 400Ml/d, 600Ml/d and 800Ml/d together with the range of flows without abstraction that can be reduced to each of these TTFs. Modelling should be used to inform an impact assessment of how freshwater flows affect key water quality parameters in the River Thames (e.g. dissolved oxygen) and Upper Tideway (e.g. dissolved oxygen, salinity). Where impacts are identified at these lower flows, water quality modelling should also be undertaken across a wider flow envelope (moderate and high flows) to provide context to the scale of impacts of LTOA. Generate a time series of river flows without abstraction and with abstraction (recent actual demand together with licensed maximum where required by the Environment Agency) as boundary conditions for water quality modeling. This should be undertaken for the 86-year period through WARMS and adequately separate Thames Water s LTOA abstractions from Affinity Water s non-ltoa abstractions. Section B.3.4 Temporal flow pattern analysis: Undertake statistical analysis of temporal patterns in the freshwater reaches and pass-forward flow. Empirical and WARMS modelled data should be interpreted for consideration of seasonal, annual and interannual variability. Section B.3.5 Spatial flow pattern analysis: Undertake statistical analysis of spatial patterns of flow in the freshwater reaches of the study area, including the local significance of tributaries and abstractions on local flow changes, not just pass forward flow at Teddington. Section B.3.6 Tidal level and salinity analysis: Undertake statistical analysis of pass-forward flows and their interaction with the tidal regime to assess the significance of LTOA on tidal level and tidal prism (salinity). This should be undertaken across the range of relevant pass-forward flows (without and with abstraction) and full tidal regime using modelling (Telemac 2D). Where viable, the model should 9

12 also provide data for interpretation of mixing in the Teddington-Richmond pound across the tidal cycle. Where impacts on tidal regime are identified, investigation should also be also be undertaken of the potential impact of reduced velocities under reduced pass-forward flow conditions at low tide and the associated potential for sedimentation. Section B.3.7 Tidal level and water quality analysis : Interpret how flow and water quality affects ecological indicators (as identified in Stage 2), including model output data (Telemac 2D) on the impact of LTOA on tidal level and inter-tidal exposure. Section B.3.8 Tidal level and navigation: Assess how flows and levels affect navigation and recreation (to be scoped in Stage 2), including model output data (Telemac 2D) on the impact of LTOA on tidal level and inter-tidal exposure. Section B.3.9 Review of fish pass at Molesey Weir: Assess the effectiveness of the Molesey Weir fish pass, noting this may be the weir with the lowest flow in the study area. Section B.3.10 Velocity review: Review the need to include assessment of velocity in the upper Molesey- Teddington reach, the reach with the lowest flows in the study area. The scope would potentially include assessment of water quality and sedimentation pressures. 10

13 B.2 HYDROLOGICAL CONTEXT The review has been undertaken through desktop analysis using existing baseline data and site survey data, together with water resource modelling outputs from Thames Water. The characterisation seeks, through analysis of the measured record, to identify and illustrate features of the flow regime of the River Thames and the influence of river flows on the Upper Tideway which potentially influence water quality and aquatic ecology and may be impacted by Thames Water s abstractions. At a basic level these features are: a) The flow regime of the River Thames: How much flow is a typical flow that describes the general conditions of the river? How much flow is a low flow; or an extreme low flow that might occur during a drought and cause stress to the aquatic system? How much flow is a high flow; or an extreme high flow that might cause washout or habitat change in the system? The seasonality and frequency of these events. b) The spatial pattern of flows along the river resulting from: flow reductions from abstractions (e.g. Thames Water and Affinity Water s potable water supply abstractions) flow increases from tributaries (e.g. River Wey) and treated wastewater discharges (e.g. Thames Water s Windsor sewage treatment works (STW)) c) The management of River Thames and Upper Tideway water levels, principally for navigation and fish migration. d) The pass-forward flow from the River Thames to the Upper Tideway over Teddington weir and its influence on water level, water quality and habitat availability in the Upper Tideway, including the current extent of other freshwater sources such as tributaries to the Upper Tideway (e.g. River Wandle) and Thames Water s Mogden STW. e) The scale of Thames Water s potable water supply abstractions on the reduction of river flow and the pass-forward flow to the Upper Tideway. An overview of the Thames Basin - the river catchment and estuary system within which the Lower River and Upper Tideway of the study area sit, is provided in Section B.3.1. This outlines catchment characteristics and tributaries (B.2.1.1), general land use (B.2.1.2), geology (B.2.1.3), hydrogeology (B.2.1.4) and anthropogenic influences (B.2.1.5). This is useful in setting the general characteristics that govern the hydrological functioning of the Lower River Thames and Upper Tideway. 11

14 The existing flow regime characteristics of the Lower River Thames and Upper Tideway are established in Section B.3.2 from measured data for typical baseline conditions within the study area, together with a selection of high, moderate and low flows (in Section B.2.2.2). This illustrates the flow regime of the Lower River Thames and the pass-forward flow to the Upper Tideway commensurate with the likely scale and significance of any predicted changes in flow relative to the existing situation without LTOA. Together with the management practices it has also been used to develop an understanding of the hydrodynamic factors of water depth, velocity, wetted width, flushing rate and estuarine physical processes which directly or indirectly influence water quality and ecology. B.2.1 THAMES BASIN OVERVIEW This section provides an overview of the hydrological processes affecting the flow of water within the Thames Basin - the river catchment and estuary system. This outlines the key influences on the water quantity in the Lower River and Upper Tideway of the study area. B Catchment Characteristics and Tributaries The Thames catchment is one of the largest fluvial catchments in the UK. At the tidal limit of the River Thames at Teddington Weir the catchment area is almost 10,000km 2. At the Teddington Weir the River Thames is approximately 275km in length, a sinuous channel flowing from its source in the Cotswolds at Thames Head in Gloucestershire to the North Sea. The Thames passes through or adjacent to several large urban areas - Oxford, Abingdon, Wallingford, Reading, Henley-on-Thames, Marlow, Maidenhead and Windsor, finally passing through London as a constrained upper estuary (Tideway) before entering the Thames Estuary at the Dartford Crossing and ultimately the North Sea. The catchment ranges in altitude from around 350m to sea level and is characterised by three north-east to south-west trending ridges, the Cotswolds in the north-west, the Chilterns towards the centre of the catchment and the North Downs towards the south of the catchment. The Cotswolds and Chilterns are between m in altitude while the North Downs are lower, generally between altitudes if m. At Goring Gap the Thames cuts through the escarpment of the Chiltern Hills in a southerly direction. Between these ridges altitudes are much lower, between m with the lowest catchment altitudes are encountered around London, ranging from 10-20m, with Teddington Weir located at 4m. The catchment is situated in one of the driest regions in the UK and receives an average 690mm of rainfall per annum, much lower than the national average of 12

15 897mm 5. Of the annual average only 250mm is effective rainfall, the rest lost through evaporation and transpiration 6. The highest rainfalls are concentrated in the Cotswolds, Chilterns and the North Downs, with local annual rainfall totals up to 850mm at the highest altitudes. The lowest rainfall totals are experienced in the areas of lower altitude between the Cotswolds, Chilterns and North Downs with local annual totals measuring 600mm. Due to its spatial extent a very large number of tributaries flow into the River Thames along its path. In the upper reaches of the River Thames from its source to Dorchester the major tributaries include Ampney Brook and the rivers Churn, Coln, Cole, Leach, Windrush, Evenload, Cherwell, Ray and Ock. In the middle reaches of the River Thames from Dorchester to Windsor, the significant tributaries include the rivers Thame, Pang, Kennet, Loddon, Wye and The Cut. In the lower reaches of the River Thames from Windsor to Teddington Weir, the significant tributaries are the rivers Colne, Wey, Mole, Hogsmill and The Bourne (North and South). These tributaries of the Lower River Thames drain an area of 2,500km 2 directly into the study area. Several significant tributaries flow into the Thames Tideway between Teddington Weir and Dartford Crossing, namely the rivers Crane, Brent, Beverley Brook, Wandle, Lee, Ravensbourne, Ingrebourne, Rodding, Beam and Darent. Cumulatively, these watercourses drain a highly urbanised area of 2,800km 2. B General Land Use The basin is generally characterised by a rural landscape, although the eastern portion of the basin around London is heavily urbanised. Over 40% of the land is classed as an Area of Outstanding Natural Beauty, e.g. the Cotswolds. Land use in the western areas of the catchment are dominated by arable and horticulture, with interspersed grassland and woodland. The eastern half of the catchment is characterised by grassland and woodland, although urbanisation increases rapidly around London. Some heath and bog is present towards the southern most areas of the catchment. There are also extensive areas of surface water, notably Thames Water s west London storage reservoirs. B Geology The River Thames passes through a diverse range of lithologies from the Middle and Upper Jurassic, Lower and Upper Cretaceous, Palaeocene, Eocene and Quaternary systems. These strata dip gently southeast towards the London Basin syncline, becoming younger in age in an easterly direction. This diverse range of lithologies greatly influences the flow regime of the River Thames and the tributaries which supply it: from groundwater-fed (see Section B.3.1.4) ephemeral winterbournes to 5 Environment Agency (2004) Thames Corridor Catchment Abstraction Management Strategy. Technical Document. pp457 6 Environment Agency (2004) Thames Corridor Catchment Abstraction Management Strategy. Technical Document. pp457 13

16 clay-based rivers whose flow is dominated by surface runoff and have flashy responses to storm events. The River Thames flows through extensive alluvial deposits along its length, these increase towards the mouth of the Thames as the floodplain increases in size. Extensive Quaternary river terrace deposits characterise the superficial geology towards the middle and lower reaches of the River Thames, especially around Reading and upstream of London. These superficial deposits act as important aquifers within the catchment for public, industrial and agricultural water supply. B Hydrogeology Within the River Thames catchment there are several major aquifers which supply the River Thames and many of its associated tributaries with baseflow as well as provide significant water supply. The major aquifers consist of the Inferior Oolite Limestone and Great Oolite Limestone, the Chalk groups and alluvial gravels towards the Maidenhead area. The Chalk aquifers are the most important aquifers within the catchment. The Thames receives a significant flow contribution from tributaries fed by groundwater from the Chalk aquifer, especially the Kennet, Loddon and Wye catchments. At Teddington Weir approximately 50% of flow in the River Thames is baseflow from the Chalk aquifer. The Chalk aquifer is in continuity with the River Thames where the Thames flows over the Chalk, with several large public water supply abstractions located close to the river. Where public water supply abstractions are undertaken these can impact upon flow within the Thames. The Inferior and Great Oolite outcrop in the western area of the catchment in the Cotswold Hills around the source of the Thames and many of its major tributaries in the upper reaches. Superficial Quaternary deposits of alluvium and river terrace deposits composed of gravels and sands are distributed along the course of the River Thames forming important aquifers, especially in the stretch between Maidenhead and Staines B Anthropogenic Influences Due to the size of the Thames catchment and the large population which lives within its boundary, anthropogenic influences on the River Thames and its tributaries are large, ranging from direct influences on hydrology, the modification of channel morphology for the purposes of flood risk prevention and their use for leisure activities. Abstraction and discharges to the River Thames and its catchments form the main anthropogenic influence on the flow regimes. Within the entire Thames catchment 14

17 (to the Dartford Crossing) there are over 2,600 licensed abstractions with groundwater abstractions accounting for ~69% by number of all consented abstractions. Abstraction from groundwater and surface water for public water supply accounts for ~62% by number of the licensed abstractions in the whole catchment (and accounts for 90% by number of abstractions from the River Thames above Teddington Weir), with agriculture and aquaculture accounting for the bulk of the remaining abstractions. Industrial abstractions are generally more important in urbanised areas, although Didcot Power Station accounts for a large proportion of flow abstracted from the middle Thames. There are ~12,000 consented discharges located in the River Thames catchment which discharge into the River Thames and its tributaries. By far the largest number of consented discharges are located in the River Colne system, numbering ~2,000, with most being small and private discharges from houses. Throughout the catchment the greatest contributor to flow from these discharges are sewage treatment woks (STW), in most cases accounting for ~70-90% of all flow from the consented discharges. As such, the flows act to augment river flows, particularly in smaller tributaries. The River Thames and its tributaries act as high-value recreational resources. Most rivers have a path alongside with walking and fishing being common activities. The River Thames itself is followed by the Thames Path, a national trail which runs from the source of the River Thames in the Cotswolds to the North Sea. In addition the Thames provides amenity and aesthetic value through the urban areas it flows. Boating is a common activity on the River Thames, the Thames being navigable from immediately prior to Lechlade, Gloucestershire to the North Sea. This is aided by extensive lock systems (44 in total) and cut channels, e.g. Culham Cut. The extensive lock and weir system operated by the Environment Agency ensures that the River Thames is level controlled within a managed freeboard for navigation. In addition the Port of London Authority operate a lock and half-tide sluice at Richmond to maintain water level at low tide and provide navigable passage in the Upper Tideway. As the Thames flows through major urban areas modification of the channel is common, particularly for the purposes of managing flood risk and enhancing navigation 7. Weirs and locks are commonplace, as are reinforced and re-sectioned banks and flood levees etc. Specific flood conveyance channels exist within the basin, specifically the Jubilee River which bifurcates from the Thames near Maidenhead and rejoins prior to Windsor and the River Lee Flood Diversion channel in the River Lee sub-catchment. 7 Environment Agency (2009) Thames River Basin District: draft River Basin Management Plan 15

18 Thames Water Utilities Ltd B.2.2 CHARACTERISATION OF THE FLOW REGIME AND ITS MANAGEMENT IN THE STUDY AREA The historic measured record has been used to illustrate the flow regime of the passforward flow to the Upper Tideway and characterise flow variations in the River Thames in the study area. The flow variations illustrated include high, moderate and low flows; seasonal and inter-annual variations; spatial variations along the river reaches; and impacts on level variation in the Upper Tideway. The characterisation has been separated spatially into the pass-forward flow from the River Thames to the Upper Tideway at Teddington Weir (Section B.2.2.1), the Lower River Thames (Section B.2.2.2) and the Upper Tideway (Section B.2.2.3). B Characterisation of Pass Forward Flow and its Control Teddington Weir, first built in 1812, marks the normal tidal limit of the River Thames at a standard headwater level of 4.38m AoD. Environment Agency lock keepers are required to operate the weirs to manage the head water level between 0.075m and 0.400m above the standard headwater level for navigation purposes. While managing water level, the Environment Agency lock keepers have responsibility for ensuring the daily agreed pass-forward (Teddington Target Flow in the LTOA) is equalled or exceeded. The arrangements for controlling water level above Teddington Weir and the passage of water across the locks/ weir are illustrated on Plate 1. Plate 1 Management of Flow/Level at Teddington (looking upstream) Teddington Locks to north (left of island) Sharp-crested weir Radial gates 2 large roller sluices Fish pass Thames Tideway at low tide (level controlled by Richmond Sluice) The flow envelope of the pass-forward flow can be described through the Kingston flow gauge record, gauged since There are no significant flow inputs or abstractions between the gauge and Teddington Weir. The Environment Agency reports that hydrometric accuracy was not high pre-1951 due to leakage and lockages, resulting in underestimation of low flows prior to this time. A summary of the long term hydrological data for Kingston flow gauge is shown in Figure 4. It is important to reiterate that the measured record of the flow regime describes the pass-forward flows actually experienced as an environmental baseline and does not reflect the 16

19 potential scale of influence of the LTOA, as the current demand for water is significantly higher than historically. Figure 4 River Thames at Kingston, Complete Flow Gauging Record ( ) River Thames Flow (Kingston) Mean Daily Discharge Ml/d River Thames Flow (Kingston) River Thames Flow (Kingston) Mean Daily Discharge Ml/d Mean Daily Discharge Ml/d

20 Measured pass-forward flow variation is within a large range, varying from approximately 1Ml/d (on several dates in August October 1976) to 69,000Ml/d (November 1894). The frequency of occurrence of different flows is illustrated on Figure 5. Typically the average pass-forward flow at Teddington (Q50) is 3,516Ml/d, low flow (Q95) 669Ml/d, extreme low flow (Q99) 310Ml/d and high flow (Q10) 13,910Ml/d. A once every ten years flood event is in the order of 40,000Ml/d. Figure 5 River Thames Long Term Flow, Measured at Kingston ( ) River Thames at Kingston flow, Ml/d High river flow: River flow is above 17,971Ml/d (winter) 13,824Ml/d (all year) 7,050Ml/d (summer) for 10% of the time (Q10) Low river flow: River flow is below 807Ml/d (winter) 649Ml/d (all year) 562Ml/d (summer) for 5% of the time (Q95) River flow (full year) Summer flow (April - September) Winter flow (October - March) Percentage of time flow equalled or exceeded There are significant seasonal differences in the pass-forward flow regime. This is apparent from the typical cycle of high winter flows and low summer flows in Figure 4 and the differences in winter and summer flow statistics 8 shown on Figure 5. The monthly flow summary (Table 1 identifies that higher pass-forward flows are typically recorded during November to March, with low pass-forward flows can occur throughout the year but are typically lower in July October. Average flow (Q50) is typically highest in January and February. 8 Hydrological Winter is from October to March, Hydrological Summer is from April to September 18

21 Table 1 River Thames Long Term Daily Flow, Measured at Kingston ( ), Summarised by Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec High flow (Q10) 21,254 20,479 17,539 12,096 8,354 5,435 3,258 3,465 3,568 7,486 15,206 18,835 Average flow (Q50) 8,899 8,441 6,765 5,348 3,628 2,415 1,616 1,391 1,348 1,875 3,862 6,704 Low flow (Q95) 1,892 1,875 2,057 1, ,019 Extreme low flow (Q99) Footnote: Flow values are Ml/d recorded for all years 1/10/ /2/2012 Several extended dry periods and extended wet periods have been identified from Figure 4. These are useful to illustrate the flow conditions that can occur during extreme and normal conditions: 1933/1934 summer 1950/ /1976 summer 2007/ /1988 Dry summer followed by a dry winter and then a second dry Notably wet winter Dry summer followed by a dry winter and then a second dry Consecutive wet summer periods Average winter period and average summer period. Pass-forward in these periods has been plotted as time-series in Figure 6. These periods are examined in greater detail in the following text. High Flow High flows over the recorded period (Q10) at Kingston for winter are ~18,200Ml/d, while for summer they are ~6,808Ml/d, as indicated in Figure 5. The monthly flow summary (Table 1) indicates that within the winter period October high flows are generally lower than in other winter months, with summer high flows generally greater in the early summer months. This pattern is characteristic of a groundwater dependent river, where baseflow is topped up during the wet winter months and reduces during the drier summer period. Peak flows are generally higher than 20,000Ml/d. Flows of 20,000Ml/d occur at least once a year for 80% of the recorded years. Normally these peak flows occur during the winter months. The periods 1950/51 and 2007/08 have been identified as characteristically wet. Time series plots of both years are indicated in Figure 6. The period 1951/52 is wetter during the winter, particularly during the early months of It may appear that 1951/52 was not particularly wet compared to other years - for example peak flow is lower than in March 1933 (the dry year). However, on further inspection it can be seen that winter flows during the winter of October 1950 March

22 exceeded 20,000Ml/d over seven distinct periods. Following the wet winter of October 1950 March 1951, summer flows (in 1951) remain elevated while baseflow reserves diminish, regardless of the extent of the winter precipitation. In 2007/08 both summers were wet and thus the characteristic summer reduction in baseflow is not recorded. Peak flow in July 2007 was 23,328Ml/d which is over 8 times the long-term high flow (Q10) for July, and equivalent to high flows that often occur over winter. Moderate Flow Average flows during the winter months are generally between 5,000Ml/d and 10,000Ml/d. Average flows during the summer months are generally between 1,300Ml/d and 5,000Ml/d. October shows flows more characteristic of summer than winter. A representative average flow period from 1987/88 is presented in Figure 6. Flow is generally greater in winter and lower in the summer with the characteristic reduction in baseflow occurring as the summer proceeds. Flow in the winter of 1987/88 is reasonably high, with four peak flow periods when flows over 20,000Ml/d are recorded. Low Flow Lowest summer flows generally occur during August/September with low flows (Q95) during these months being around 400Ml/d. Extreme low flows during the same two summer months can below 200Ml/d. Using the Q95 statistic in isolation, when considering low summer flows, does not provide information on key characteristics of low period such as event duration, frequency or antecedent conditions. The MSM7 provides a measure of the lowest sustained low flow period 9. Since sustained low flow periods can represent one of the most significant ecological stressors, MSM7 is a useful index of the stress tolerance of aquatic species. The summer and winter mean seasonal MSM7 for each hydrological year of the baseline period is presented in Figure 7. Of the 127 years assessed, MSM7s of less than 800Ml/d occurred during 85 of these years (67% of the record). MSM7s of less than 600Ml/d occurred in 46 of the years (36% of the record) and of less than 400Ml/d occurred in 32 of the years (25% of the record). MSM7s less than 300Ml/d and less than 200Ml/d occurred in 22 (17%) and 13 (10%) of the 127 years respectively. 9 Mean seasonal minimum 7-day average flow (MSM7) provides a measure of the lowest sustained low flow period: the average of flow on seven consecutive days. The MSM7 is calculated as the average flow in the 7-day period and is not the minimum flow. The MSM7 is therefore the lowest 7-day average flow in each hydrological summer or hydrological winter season. After: Gustard A., Bullock A. and Dixon J. M. (1992) Low flow estimation in the United Kingdom. Wallingford: Institute of Hydrology, Report No

23 Figure 6 River Thames at Kingston, Selected Years River Flow at Kingston (Dry) River Flow at Kingston (Wet) River Flow at Kingston (Dry) River Flow at Kingston (Average) River Flow at Kingston (Wet) Mean Daily Flow in River Thames at Kingston, Ml/d Jan 1 Feb 1 Mar 1 Apr 1 May 1 June 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec 1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec 21

24 From Figure 7 it can be seen that lowest sustained low flows generally occurred during the summer period although there are some exceptions. A low flow period is observed in the River Thames at Kingston each summer. The average MSM7 low flow from the long-term record was 713Ml/d and this can be considered as typical summer minimum flow in the River Thames at Kingston. However, there was considerable variation in the magnitude of summer low flow and minimum flow in the long-term record, with summer MSM7 ranging from 1Ml/d with a Drought Order in place during an extremely dry summer (1976) to 2,971Ml/d in a wet summer (1931) (see Figure 7). Low flows during the winter months are generally less than 2,000Ml/d. Low flows in winter are more likely to occur during October/November before significant groundwater recharge by winter rainfall. Winter low flows have occurred regularly in the baseline period, but are not a hydrological feature of each winter. Winter MSM7 ranges from 231Ml/d ( ) to 6,637Ml/d ( ), with a long term average of 1,592Ml/d. Time series flow data for representative dry years are presented in Figure 6 (1933/1934 and 1975/76). Both periods recorded extended low flows resulting from two consecutive dry summers with a dry intervening winter period. The intervening winter period did not allow groundwater to recover significantly and thus the following summer flows were not supported by normal baseflow contributions and flows in both years dropped below 100Ml/d over several spells (see Figure 7). During 1933/34 flow dropped below 5,000Ml/d in May 2009 and only twice (and for just 6 days in total) returned to above 5,000Ml/d over the following 19 months (576 days). Of these 576 days, flow was less than 200Ml/d for 36 days, less than 300Ml/d for 78 days, less than 400Ml/d for 133 days, less than 600Ml/d for 176 days and less than 800Ml/d for 233 days. The period of low flow during the 1975/76 period was shorter although the reduction in flow levels was even more acute, in part due the presence of a Drought Order. The number of days where river flow at Kingston was below certain values was also investigated. The values investigated were 800Ml/d, 600Ml/d, 400Ml/d, 300Ml/d and 200Ml/d. In some years flows were maintained above all of these values throughout, most recent examples of these years include 1968 and The average number of days flow was recorded lower than each value, the maximum number of days and a count of the number of years when flow was not lower than the respective threshold is presented in Table 2. From this it can be seen that in the majority of measured record years flow has dropped below each of the thresholds indicated. Notably for around 75% of the total years examined flow has been less than 200Ml/d for at least one day within the year. The average number of days per year this has occurred is few, typically two days per year. The maximum number of days within a calendar year when flow dropped to less than 200Ml/d occurred in 22

25 1976 when it was below this threshold for almost 2 months of the year (58 days). Table 2 Statistics Relating to Flows Recorded for Each Year Below Selected Flow Values ( ) Maximum number of days per year flow is lower than selected value Average number of days per year flow is lower than selected value Minimum number of days per year flow is lower than selected value Number of years where selected flow value (or lower) is not recorded Selected flow value 800Ml/d 600Ml/d 400Ml/d 300Ml/d 200Ml/d The incidence of low flows below the same selected flow values are presented in Figure 8 to illustrate the duration and frequency of low flow periods. The extent of low flows varies between years - some years such as 1934 or 1976 have prolonged and protracted periods of low flow. Low flows do however appear to have become more frequent since Since 1975 only 5 years do not have at least one day of low flow (flow less than 800Ml/d) , 1979, 2001, 2007 and Between 1950 and 1970 the incidence of low flows was relatively rare, reasonably mild and of limited duration. This shift in low flow occurrence may be due to climatic variation, changes in river and catchment management, or reflect the increasing demand for potable water supply, or a combination of these. B Characterisation of Lower River Thames from Windsor to Teddington The Lower River Thames in the study area extends from the Windsor flow gauge on the River Thames upstream of Thames Water s Datchet intake to the normal tidal limit at Teddington Weir. This length of river includes eight navigable reaches each abutted by a lock and weir system (see Figure 9). Although flow may have been managed through weir systems previously, the eight locks in the study area were first added in the 11 year period The current weirs, level control arrangements and fish passes for each reach have been added or modified over time. Under high tides, predominantly around and at spring tides, Teddington Weir can be overtopped, with tidal level variations extending upstream to Molesey Weir. Overtopping is a regular occurrence (see Figure 10) occurring on average on 60 times a month between 1989 and 2011 with a maximum of 157 overtopping events in 1994 and no events in The average height of tidal overtopping of the weir is 0.10m, although in July 1994 this reached a maximum of 1.1m. Flow gauging on the Lower River Thames commenced at Kingston on the Molesey-Teddington reach in