Appendix B Hydrology and Physical Environment Assessment
January 2018 Version: Final Draft for EA/NE Review
Contents
B.1. Introduction ...... 4 B.1.1. Drought orders on the Test and Itchen ...... 4 B.1.2. Gaters Mill and Lower Itchen drought orders ...... 5 B.1.2.1. Portsmouth Water’s existing operations ...... 5 B.1.2.2. Southern Water’s existing operations ...... 6 B.1.2.3. Southern Water’s proposed drought order operations ...... 6 B.1.3. Structure of appendix ...... 8 B.2. Catchment setting ...... 9 B.2.1. Catchment overview ...... 9 B.2.2. Baseline data availability ...... 11 B.2.3. Hydrology...... 12 B.2.4. Geology ...... 14 B.2.5. Hydrogeology ...... 16 B.3. Hydrological and hydrogeological impact assessment ...... 19 B.3.1. Approach ...... 19 B.3.2. Reference conditions ...... 23 B.3.3. Environmental impact pathways ...... 23 B.3.4. Impact on flow ...... 24 B.3.4.1. Historical context ...... 25 B.3.4.2. Relationship of flow impact and drought severity ...... 25 B.3.4.3. Analysis of example drought events ...... 28 B.3.4.4. Common Standards Monitoring Guidance ...... 31 B.3.5. Impact on river hydraulics ...... 31 B.3.5.1. Implications of drought orders on habitat variables ...... 31 B.3.6. Impact on groundwater heads ...... 35 B.3.6.1. Impact on Chalk aquifer ...... 35 B.3.6.2. Impact on the hydrological functioning of wetlands ...... 35 B.3.7. Hydrological and hydrogeological impact summary ...... 38 B.4. Physical environment assessment ...... 41 B.4.1. Geomorphology ...... 41 B.4.1.1. Baseline ...... 41 B.4.1.2. Assessment ...... 42 B.4.2. Water quality ...... 45 B.4.2.1. Chalk aquifer ...... 48
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B.4.2.2. Reach A-B – River Itchen upstream of Allbrook and Highbridge gauging station ...... 48 B.4.2.3. Reach B-C – River Itchen downstream of Allbrook and Highbridge gauging station to Riverside Park gauging station...... 52 B.4.2.4. Reach C-D – Itchen Estuary (part of Southampton Water WFD waterbody) ...... 57 B.4.2.1. Water quality summary ...... 60 B.4.3. Environmental pressures ...... 61 B.4.3.1. Abstraction pressures ...... 61 B.4.3.2. Water quality pressures ...... 62 B.5. Cumulative impacts ...... 62
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B.1. Introduction As part of its Draft Drought Plan 2018, Southern Water is required to undertake environmental assessments of each of the drought permit and drought order options contained in the plan. This Appendix forms the hydrological / hydrogeological impact and physical environment assessment for the proposed drought order to temporarily modify the abstraction licence conditions for Southern Water’s Lower Itchen sources (Otterbourne groundwater (including Twyford Moors) and surface water abstraction and Twyford groundwater abstraction), as well as temporary modifications to the Portsmouth Water Gaters Mill surface water abstraction licence on the Lower Itchen.
The document firstly explains the proposed drought order, then the conceptual understanding of the sources and their hydrological connections to the River Itchen before estimating the hydrological and hydrogeological impacts of the drought order. The assessment of the physical environment considers the impacts on river flows, water levels, water quality and geomorphology. Consideration is also given to other water users. B.1.1. Drought orders on the Test and Itchen Southern Water’s resources in its Western Area are dominated by the abstractions on the Rivers Test and Itchen at Testwood and Otterbourne, respectively. A schematic of these two river systems is presented in Error! Not a valid bookmark self-reference.. Other key water sources, including the Environment Agency’s Candover Augmentation Scheme boreholes, are also shown for reference.
The Testwood and Lower Itchen sources are the subject of a forthcoming public inquiry into proposed abstraction licence changes that would constrain abstraction at times of low river flows such that drought orders would be required to help maintain water supplies to customers. Southern Water has therefore developed four drought orders as part of its Draft Drought Plan 2018 to help maintain water supplies to Western Area on the assumption that these licence changes are enforced. Due to the connectivity of the water resources in the Hampshire area, it is proposed that these drought orders are operated in the following order to limit environmental impact:
Testwood drought order; Candover drought order (to permit use of the Environment Agency’s Candover Augmentation Scheme boreholes and discharge water to the River Itchen) Gaters Mill drought order (to vary Portsmouth Water’s abstraction licence); then Lower Itchen drought order. This EAR is concerned with the impacts of the Gaters Mill and Lower Itchen drought orders. The drought order to vary the abstraction licence conditions for the Portsmouth Water Gaters Mill source will be used in preference to the drought order to vary the abstraction licence conditions for Southern Water’s Lower Itchen sources with the view to limiting potential impacts on the River Itchen. In practice, it is likely that these two drought orders would be applied for simultaneously (potentially as one single drought order application), and as such they have been assessed here in a cumulative fashion as a combined drought order.
4 Appendix B Hydrology and Physical Environment Assessment
Figure 1 Schematic of River Test and River Itchen
Candover scheme
Lasham Totford
MU3 MU4 MU2 MU1 Easton Winchester
Twyford
Otterbourne
Romsey MU5 Allbrook & Legend Highbridge
Eastleigh Management Unit (MU) Surface water abstraction Groundwater abstraction MU6 Gaters Mill Groundwater scheme for river Testwood Southampton Riverside Park augmentation To tide Surface water gauging station To tide HOF location
B.1.2. Gaters Mill and Lower Itchen drought orders B.1.2.1. Portsmouth Water’s existing operations Portsmouth Water abstracts water from the River Itchen at Gaters Mill less than 1 km upstream from the Riverside gauging station, which itself is located approximately 600 m upstream of the tidal limit at Woodmill. Abstraction is restricted by a Hands-off Flow (HOF) condition at the Riverside Park flow gauging station. The Portsmouth Water abstraction licence summary details are provided in Table 1.
Table 1 Portsmouth Water’s Gaters Mill abstraction licence details Daily Annual Source Licence number Conditions (Ml/d) (Ml/d) Riverside Park gauging station HOF Gaters Mill 11/42/22.10/134 45.5 15916.0 of 194 Ml/d Data source: EA data request in January 2017
5 Appendix B Hydrology and Physical Environment Assessment
B.1.2.2. Southern Water’s existing operations Southern Water operates a number of water sources on the Lower Itchen which are collectively referred to in this assessment as the ‘Southern Water Lower Itchen sources’. These consist of:
Otterbourne, which comprises of: - Otterbourne surface water (SW) abstraction - Otterbourne groundwater (GW) abstraction, which includes abstraction from Twyford Moors Twyford GW abstraction. Revised abstraction licence details for the Lower Itchen abstraction licences were set out in the Site Action Plan of The Habitats Directive Review of Consents for the River Itchen Special Area of Conservation (SAC). Time was allowed for Southern Water to bring in replacement sources of water to maintain water supply resilience, but on 17th November 2016, the Environment Agency gave notice that it would vary the terms of the Lower Itchen licences under Section 52 (s52) of the Water Resources Act 1991 (as amended by Section 22 of the Water Act 2003) and Regulation 31 of the Water Resources (Abstraction and Impounding) Regulations 2006 before these alternative supplies could be developed. The current licence details (as of September 2017) and the proposed Environment Agency Section 52 (s52) revisions to the Lower Itchen sources licence conditions are summarised in
6 Appendix B Hydrology and Physical Environment Assessment
Table 2.
For the purpose of this assessment, the HOF proposed under the s52 abstraction licence conditions will simply be referred to as the HOF.
B.1.2.3. Southern Water’s proposed drought order operations Southern Water’s Draft Drought Plan 2018 covers the period 2018-2023 and assumes that the s52 abstraction licence variation has been implemented in full. Water resources modelling identifies that the s52 abstraction licence conditions will constrain abstraction under low flow and drought conditions. Therefore, drought orders may be required as set out in
7 Appendix B Hydrology and Physical Environment Assessment
Table 3 to maintain water supplies to customers in the Western Area.
The drought order for the Portsmouth Water Gaters Mill source will be used in preference to the drought order for Southern Water’s Lower Itchen sources with a view to limiting potential impact on the River Itchen. In practice, however, it is likely that these drought orders would be applied for simultaneously, and as such have been assessed here in a cumulative fashion. The Portsmouth Water drought order will enable Portsmouth Water to continue to provide a bulk treated water supply to Southern Water in drought conditions of up to 15 Ml/d.
8 Appendix B Hydrology and Physical Environment Assessment
Table 2 Southern Water’s Lower Itchen abstraction licence details1,2,3,4 Licence Daily Annual Source Conditions number (Ml/d) (Ml/d) Current licence Otterbourne GW 71.6 21,230 3.4 Ml/d compensation discharge to 11/42/22.7/94 Twyford (68.19*) (20,002*) the Nightingale Stream Moors GW Twyford 11/42/22.6/92 36.37 13,319 GW Otterbourne 11/42/22.6/93 45.46 16,638 SW s52 proposed abstraction licence changes (and used for this assessment) Maximum monthly abstractions: • June – 4,110 Ml per month (137.0 Ml/d) • July – 3,940 Ml per month Lower (127.1 Ml/d) 11/42/22.7/94 42,000 Ml Itchen • August – 3,445 Ml per month 11/42/22.6/92 per year sources 11/42/22.6/93 (115.1 Ml/d) (111.1 Ml/d) aggregate • September – 2,280 Ml per month (76.0 Ml/d) and Allbrook & Highbridge gauging station hands-off flow (HOF) of 198 Ml/d. * Amount for PWS abstraction
1 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Otterbourne (surface water). 2 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Otterbourne (groundwater) 100641 3 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Twyford Moors 4 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Twyford 100055
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Table 3 Summary details of the proposed Lower Itchen Drought Orders Portsmouth Water’s Gaters Mill source (Gaters Mill drought order) HOF control Riverside Park Watercourse River Itchen Abstraction source Gaters Mill Surface Water Normal HOF / licence Licence details as per Table 1 details Proposed drought Reduction in the HOF at Riverside Park from 194 to 150 Ml/d permit / order Permit or order Drought order Yield (Ml/d) 44* Option 2 – Southern Water’s Lower Itchen sources (Lower Itchen sources drought order) HOF control Allbrook & Highbridge Watercourse River Itchen Otterbourne SW Abstraction sources Otterbourne GW (including Twyford Moors) Twyford GW Normal HOF / licence Licence details as per Table 2 details Proposed Drought Reduction in the HOF at Allbrook & Highbridge from 198 to 160 Ml/d Permit / Order Permit or Order Drought order Yield (Ml/d) 38* *Assessed as the difference between the s52 HOF and the proposed drought order HOF B.1.3. Structure of appendix This appendix is set out as follows:
Section B.2 Hydrological and hydrogeological impact assessment; Section B.3 Physical environment assessment; and Section B.4 Cumulative impacts.
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B.2. Catchment setting This section details the understanding of the River Itchen catchment, enabling an assessment of the impact on the drought orders on hydrology and the physical environment to be undertaken in later sections. B.2.1. Catchment overview The River Itchen in Hampshire supports a range of diverse plant and wildlife species5. As such, the river and many of its tributaries are designated as Sites of Special Scientific Interest (SSSIs) under Section 28 of the Wildlife and Countryside Act 1981 (as amended and inserted by section 75 and Schedule 9 of the Countryside and Rights of Way Act 2000), Section 17 of the Water Resources Act, 1991 and Section 4 of the Water Industry Act, 1991. In addition to this, the River Itchen is also internationally important for its wildlife and habitat and is designated as a Special Area of Conservation (SAC) under the European Commission Habitats Directive (River Itchen SAC).
The River Itchen supports game fishing, largely provided by brown trout, and to a lesser extent salmon and sea trout5. Almost the entire river is managed to maintain and facilitate fishing for brown trout, with fishing for sea trout and Atlantic salmon (Salmo salar) also taking place along the lower reaches. In the uppermost reaches of the River Itchen, native populations of brown trout (Salmo trutta) are believed to persist, and bullhead (Cottius gobbio) and brook lamprey (Lampetra planeri) are notable elements of the natural fish fauna and are of European importance.
The Itchen valley is also important in terms of its landscape, having extensive water meadows and associated historic landscape features, structures and mills. The water meadows are still used extensively for agriculture, although they are not generally ‘floated’ in the same way as they would have been traditionally. In the headwaters, upstream of Itchen Abbas, multiple watercress beds still operate commercially and there are also a number of fish farms in operation.
As a result of its historic, commercial and amenity value, the river has been progressively and extensively managed and modified over time. It now comprises a complex array of multiple channels with mill races and the historic Itchen Navigation, a canal between Southampton and Winchester. The banks and in-channel vegetation are, in large sections, heavily maintained for ease of access for fishing activities, although the views and practices on this are progressively changing with approaches to in-stream and riparian vegetation management gradually becoming more sympathetic to the riverine habitat.
The Itchen is augmented by a number of spring and pumped water sources along its length.
The study area is shown on Figure 2.
5 Atkins, 2013. Test and Itchen River Restoration Strategy Technical Report
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Figure 2 Study area
Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426
12 Appendix B Hydrology and Physical Environment Assessment
B.2.2. Baseline data availability The surface water gauging stations along the River Itchen and its tributaries are listed in Table 4 along with the data availability from the National River Flow Archive6.
Table 4 Surface water flow monitoring along the River Itchen6 Gauge location in relation Data Gauge River Data range to HOF frequency Upstream Allbrook & Sewards Bridge Cheriton Stream 1956 - 2015 Daily Highbridge Upstream Allbrook & Drove Lane River Alre 1970 - 2015 Daily Highbridge Candover Upstream Allbrook & Borough Bridge 1970 - 2015 Daily Stream Highbridge Upstream Allbrook & Easton River Itchen 1975 - 2015 Daily Highbridge HOF location for Southern Allbrook & River Itchen Water’s Lower Itchen 1958 - 2015 Daily Highbridge sources HOF location for Portsmouth Water’s Gaters Mill source. Riverside Park River Itchen 1981 - 2015 Daily Downstream of Allbrook & Highbridge Data source: National River Flow Archive, accessed January 2017
The hydrogeological monitoring points within a 5 km radius of Otterbourne are listed in Table 5. All observation boreholes listed in Table 5 are assumed to be monitoring groundwater levels in the Chalk unless stated in the monitoring name (as per the table).
The Portsmouth Water Gaters Mill source on the River Itchen is located on low permeability Tertiary deposits and is unconnected to the underlying Chalk aquifer (see Section B.2.5). As such, groundwater monitoring near this source is not a relevant parameter for this assessment.
Table 5 Groundwater monitoring sites within 5 km of Otterbourne7 Approximate distance Name Grid reference from Otterbourne (km) Otterbourne8D SU47032323 0.0 Otterbourne8C SU47032323 0.0 Twyford Moors 1 SU47422320 0.0 Oakwood Copse SU46482371 0.8 Twyford Moors SU47952361 1.0 Highways Road SU46362431 1.3 Four Dell Farm, Shaw SU45572467 2.1 Twyford Reservoir SU49292405 2.3 Martins Fields, Comp SU46432606 3.0 New Barn Farm SU48612592 3.1
6 National River Flow Archive (http://nrfa.ceh.ac.uk/ accessed January 2017) 7 Environment Agency GeoData store (https://data.gov.uk/data/search accessed January 2017)
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Approximate distance Name Grid reference from Otterbourne (km) Hazely Down Farm SU50252528 3.8 Hatchers Lane, Henst SU50992408 4.0 5 Bridges Rd Ob7 SU47412732 4.1 Chalk Dale Owslebury SU51152265 4.1 Old Kennels SU45122693 4.2 Upper Sharland, Hur SU43602581 4.3 IBM Ob 14 A31/A3090j SU42832484 4.5 Morestead Hill SU50852575 4.6 Data source: Environment Agency GeoData store accessed January 2017
B.2.3. Hydrology River Itchen
The main channel of the River Itchen is approximately 45 km in length with a surface catchment of around 470 km2[5]. The headwater tributaries consist of the Cheriton Stream from the south, River Arle from the east and Candover Stream from the north. These three tributaries converge between New Alresford and Itchen Stoke. The river then takes its course in multiple channels (including the Itchen Navigation) through the city of Winchester and flowing broadly in a south-westerly direction and discharging into Southampton Water. The river becomes tidal downstream of Woodmill.
The Itchen Navigation is an 18th century canal system linking Winchester to the sea at Southampton and would have transported coal and other goods. It divides the flow of the River Itchen in Winchester and flows alongside the river through Shawford and Twyford, then disappears in the Bishopstoke area and reappears as a dry watercourse beside Eastleigh (Southampton) airport before re-joining the river at Mansbridge. The stretch is internationally important to wildlife as part of the larger River Itchen SAC.
There are three gauging stations on the River Itchen and three gauging stations on the headwater tributaries. The flow statistics for these six gauges are shown in Table 6 and highlight the increase in flow with distance downstream in the catchment. Of the three headwater tributaries, the River Alre provides most water to the Upper River Itchen due to its very large groundwater catchment, whilst the flows in the Candover Stream and Cheriton Stream are generally similar. The River Itchen continues to gain flow from the Chalk between Alresford and Easton, but between Easton and the Tertiary boundary at Allbrook & Highbridge, this gain in flow is markedly lower. Southern Water’s Lower Itchen sources are located just upstream of Allbrook & Highbridge. Downstream of Allbrook & Highbridge, the river flows over low permeability bedrock and connectivity with the Chalk aquifer is negligible - river flow is influenced by surface water management activities, such as the discharge from the large Southern Water Chickenhall Wastewater Treatment Works (WwTW) at Eastleigh and Portsmouth Water’s surface water abstraction at Gaters Mill just upstream of Riverside Park gauging station. The Monks Brook joins the River Itchen downstream of Riverside Park gauging station, just above the tidal limit.
The River Itchen gains its water from the Chalk aquifer, which supplies most of the streams and rivers in the area, as well as most of the water abstracted in the area8. Chalk rivers are characterised by a baseflow dominant flow regime as shown by the Base Flow Index (BFI): the slow release of
8 Environment Agency, 2012, Catchment Abstraction Management Strategy (CAMS) Conceptualisation
14 Appendix B Hydrology and Physical Environment Assessment
water from the aquifer attenuates rapid recession during periods of low and/or no rainfall events and recharge to the Chalk can attenuate rapid surface water runoff from high rainfall events.
Typically, Chalk rivers tend to have relatively few tributaries on areas of Chalk outcrop. As a result, the drainage density is low due to minimal surface runoff. As evident from Table 6, the River Itchen is subject to a relatively stable flow regime due to a high BFI (which describes the ratio of annual baseflow in a river to the total annual run-off)5. There are few other permanent surface water features on the Chalk outcrop because the nature of the soils and depths to the water table are such that all rainfall either evaporates or infiltrates. There is one unnamed tributary in the vicinity of Otterbourne that follows the line of the Chalk outcrop and joins the River Itchen to the south of the Otterbourne abstraction. Based on the geology, it is assumed that this stream is baseflow fed, capturing the groundwater that overflows as the Chalk dips beneath the Tertiary sediments. Downstream of Otterbourne, the river network density increases and the BFI reduces, reflecting the presence of these lower permeability deposits.
There have been two groundwater schemes for river augmentation of the River Itchen, owned, licensed to and operated by the Environment Agency and its predecessor bodies. The original Candover Augmentation Scheme was developed in 1976 and was followed by the Further Itchen River Augmentation Scheme on the River Alre in 1984. The Environment Agency has recently surrendered its abstraction licence for the Alre scheme and it is understood that the boreholes are being decommissioned. Further downstream, Southern Water provide up to 3.4 Ml/d compensation discharge from the Otterbourne GW source to the River Itchen via the Nightingale Stream. Nightingale Stream is located at Otterbourne, on the east side of the River Itchen.
Table 6 Summary of flow statistics from flow gauges in the River Itchen catchment6 Site Name River Period BFI Mean Q95 Comment of annual (Ml/d) record flow (Ml/d) Sewards Cheriton 1956 - 0.96 58.0 24.0 Cheriton Stream is ephemeral Bridge Stream 2015 in upper reaches. Drove Lane River Alre 1970 - 0.98 141.6 89.4 Baseflow dominated regime 2015 with narrow flow range Borough Candover 1970 - 0.96 50.8 24.6 Runoff influenced by Bridge Stream 2015 groundwater abstraction. Impact of the Itchen groundwater scheme for river augmentation is notable during droughts. Easton River 1975 - 0.97 375.9 226.6 Largely natural baseflow Itchen 2015 dominated regime Allbrook & River 1958 - 0.96 476.4 259.3 Combination of flows Highbridge Itchen 2015 measured at two gauging stations: Allbrook & Highbridge. Riverside River 1981 - 0.91 510.4 246.2 Flows artificially influenced by Park Itchen 2015 abstractions at Gaters Mill and Otterbourne, and a large sewage treatment works discharge at Chickenhall, Eastleigh. Data source: National River Flow Archive, accessed January 2017
15 Appendix B Hydrology and Physical Environment Assessment
River Itchen tributaries – Nun’s Walk Stream, Bow Lake Stream, Monks Brook and Allington Lane Stream
As stated above, the River Itchen has few tributaries over the Chalk outcrop. Aside from the three headwater tributaries, three tributaries that join the River Itchen further downstream have been identified as distinct water bodies. Nuns Walk Stream joins the River Itchen immediately downstream of Easton, Bow Lake Stream joins downstream of Allbrook & Highbridge gauging station, and Monks Brook joins downstream of Riverside Park gauging station. The fourth main tributary, Allington Lane Stream, is part of the Itchen waterbody.
There are no gauging stations on Nun’s Walk Stream, Bow Lake Stream or Allington Lane Stream. Nun’s Walk Stream is anticipated to exhibit similar behaviour to the River Itchen, namely high baseflow-driven responses from connectivity to the Chalk aquifer. The degree of baseflow dominance in Bow Lake Stream is less evident - due to the geology (Section B.2.1.4), the upper reaches are believed to capture groundwater. Monks Brook and Allington Lane Stream traverse over low permeability deposits. Unlike the River Itchen, Monks Brook reacts quickly to rainfall and has naturally lower flows. The BFI for Monks Brook is 0.426. B.2.4. Geology The geology of the study area is shown in Figure 3 and the bedrock and superficial deposits are listed in
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Table 7 and
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Table 7 Bedrock geology
Group (Epoch) Formation Lithological description Hydrogeology Marsh Formation Bracklesham Earnley Sand Group Sand, silt and clays Formation (Eocene) Wittering Formation London Clay Whitecliff Sand Essentially no Thames Group, Pebble Beds groundwater London Clay Silty clays and sands Durley Sand (Eocene) Portsmouth Sand Nursling Sand Lambeth Group Reading Beds Mottled clays, locally sandy (Palaeocene) Soft white chalk with relatively Tarrant Chalk widely spaced but large flint seams Soft to medium hard smooth white Newhaven Chalk chalks with numerous marl seams and flint bands1 Soft white chalk with seams of White Chalk Seaford Chalk Subgroup large nodular and semi tabular flint1 (Upper Interbedded hard to very hard Lewes Nodular Highly Cretaceous) nodular chalks with soft to medium Chalk productive hard grainy chalks and marls1 Chalk aquifer Pure massively bedded chalks with New Pit Chalk conspicuous marl seams10 Medium hard to very hard nodular Holywell Chalk chalks10 Grey Chalk Medium hard greyish, becoming Subgroup ZigZag Chalk white blocky chalk, with some thin (Upper limestones1 Cretaceous)
Table 8 respectively.
The Chalk is at outcrop for the majority of the River Itchen catchment, which is estimated to be 80% Chalk5. In the lower catchment, the Chalk is overlain by Tertiary sediments deposited unconformably on an eroded surface. Gentle folding and erosion brings the older Chalk deposits to the surface, for example in the core of the Winchester anticline, and can also reduce the Chalk thickness.
As stated earlier, the Chalk is overlain by Tertiary deposits to the south of the catchment, south of Otterbourne. The deposits in this area comprise the Readings Beds that directly overlie the Chalk and the London Clay and Bracklesham Beds9. These low permeability formations are essentially devoid of groundwater and generate rapid runoff into the River Itchen and its tributaries in these areas5.
9 BGS, 1987 1:50,000 Solid and Drift Geology of Southampton, Sheet number 315
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There is a reasonable coverage of superficial deposits, with Alluvial and River Terrace deposits lining the River Itchen valley. There is a swath of Clay-with-Flints crossing the study area from the north- west to the south-east. These have been reported to be up to 8 m thick10.
10 Entec, 2003 River Itchen Catchment Groundwater Modelling Study
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Table 7 Bedrock geology Group (Epoch) Formation Lithological description11 Hydrogeology Marsh Formation Bracklesham Earnley Sand Group Sand, silt and clays Formation (Eocene) Wittering Formation London Clay Whitecliff Sand Essentially no Thames Group, Pebble Beds groundwater London Clay Silty clays and sands Durley Sand (Eocene) Portsmouth Sand Nursling Sand Lambeth Group Reading Beds Mottled clays, locally sandy (Palaeocene) Soft white chalk with relatively Tarrant Chalk widely spaced but large flint seams Soft to medium hard smooth white Newhaven Chalk chalks with numerous marl seams and flint bands1 Soft white chalk with seams of White Chalk Seaford Chalk Subgroup large nodular and semi tabular flint1 (Upper Interbedded hard to very hard Lewes Nodular Highly Cretaceous) nodular chalks with soft to medium Chalk productive hard grainy chalks and marls1 Chalk aquifer Pure massively bedded chalks with New Pit Chalk conspicuous marl seams10 Medium hard to very hard nodular Holywell Chalk chalks10 Grey Chalk Medium hard greyish, becoming Subgroup ZigZag Chalk white blocky chalk, with some thin (Upper limestones1 Cretaceous)
Table 8 Superficial geology Deposit Formation Lithological description11 Occurrence in the (Epoch) study area Fluvial Deposits Alluvium Clay, silt, sand and gravel – soft to River Itchen valley (Quaternary) firm consolidated Fluvial Deposits River Terrace Sand and gravel, locally with River Itchen valley (Quaternary) Deposits lenses of silt, clay or peat Residual Clay-with- Clay, silt, sand and gravel – Patches north west to Deposits Group Flints unbedded and heterogeneous south east across (Quaternary) Formation study area Mass Head Clay, silt, sand and gravel – poorly Movement sorted and stratified Dry valleys Deposits (Quaternary)
11 BGS GeoIndex Onshore website (http://mapapps2.bgs.ac.uk/geoindex/home.html)
20 Appendix B Hydrology and Physical Environment Assessment
Deposit Formation Lithological description11 Occurrence in the (Epoch) study area Chemical Tufa Inorganic or organic calcium River Itchen valley Deposits carbonate or silica deposited at or around Otterbourne (Quaternary) near springs and seepages Intertidal Tidal Flat Consolidated soft silty clay, with Deposits Deposits layers of sand, gravel and peat Estuary (Holocene)
B.2.5. Hydrogeology The hydrogeology of the River Itchen has been comprehensively reviewed as part of the development of the Test and Itchen groundwater model (T&I GW model)10.
The main aquifer is the Chalk aquifer. Whilst the regional groundwater flow is towards the south 12, the flow direction and groundwater catchments vary seasonally10 and are locally influenced by abstractions and the River Itchen.
The Chalk aquifer towards the north of Otterbourne is unconfined (except for superficial coverage). Confining conditions are present towards the south, as the Chalk dips beneath the lower permeability younger formations of the Reading Beds. These formations act as an aquiclude. As expected, the transmissivity and storage of the confined aquifer are lower than the unconfined aquifer10.
The Chalk is a ‘dual porosity’ aquifer, consisting of fractures and fissures as well as porous fine grained matrix, and typically has complex spatial variations in hydraulic conductivity and storage. Entec10 noted that summer water levels in observation boreholes do not vary much even in a drought year, and hypothesised that, whilst in some instances this may be due to borehole construction, this response may reflect reduced permeability with depth.
Chalk permeability is also affected by preferential flow paths which lead to increased dissolution. Indeed there is geophysical evidence within the catchment of increased fissuring and enhanced permeability at certain elevations which has been hypothesised to be related to the past or present groundwater table. There is marked spatial permeability contrasts between the valleys and interfluves, with higher permeability along the valleys where groundwater flow is concentrated. Entec10 concluded that the River Itchen is locally karstic and consequently the hydraulic parameters of the Chalk aquifer are highly variable.
There is believed to be high connectivity between the Chalk aquifer, superficial deposits and the River Itchen. Baseflow, particularly to the upper reaches, is an important contribution to surface water flow. This is particularly true in the Alre catchment where groundwater levels are close to the surface and boreholes can become artesian10. Chalk groundwater levels near the watercourse typically show relatively flat hydrographs indicating good connectivity with the surface water (or high storativity from increased fissuring). Moreover, a similar response is observed in the drift deposits along the river valley. The exception to this is in the vicinity of abstractions and adits (part of the Otterbourne GW source) where groundwater levels are depressed.
This surface water–groundwater connectivity is highlighted by the conclusions from the Easton pumping test10 from which it has been estimated that the groundwater source obtains 80% of its water from the river. Similarly, the diurnal abstraction pattern from the Otterbourne GW source can be identified in a subdued form in the flow record at Easton gauging station10. It may therefore be assumed that Otterbourne GW also has a high contribution from surface water. It is worth noting
12 BGS, 1979. Sheet 9 : Hydrogeological Map of Hampshire and the Isle of Wight (1:100,000)
21 Appendix B Hydrology and Physical Environment Assessment
that Entec10 also identified that significant changes in Otterbourne SW, and probably the groundwater component, resulted in fairly rapid recovery responses in flow at Allbrook & Highbridge.
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Figure 3 Conceptualisation of the Lower Itchen
Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426 and British Geological Survey data 2008/00
23 Appendix B Hydrology and Physical Environment Assessment
B.3. Hydrological and hydrogeological impact assessment The Gaters Mill and Lower Itchen drought orders would reduce the HOF conditions at Riverside Park and Allbrook & Highbridge, respectively, so as to enable continued abstraction from Portsmouth Water’s Gaters Mill source and Southern Water’s Lower Itchen sources during severe drought conditions. The purpose of this section is to assess the potential hydrological impact caused by these drought orders.
Although the Gaters Mill drought order will be utilised in advance of the Lower Itchen drought order, modelling has shown that the two orders will be required in quick succession. Therefore, this assessment considers the cumulative impact from the two drought orders as a precautionary approach. B.3.1. Approach For the Test and Itchen drought order environmental assessments, hydrological impacts have been assessed using a combination of Southern Water’s Western Area Aquator water resources model and the T&I GW model.
Southern Water’s Aquator model was developed for the Water Resource Management Plan (WRMP) 2014 and it has been refined during 2017 for use in Southern Water’s draft WRMP19 and draft Drought Plan 18. Aquator is an industry standard tool for modelling water demand, abstractions, river flow and supply deficits.
The T&I GW model has been applied to a range of water resources investigations by both the Environment Agency and Southern Water over recent years. Although there are some differences between the modelled behaviour and observations, for example around Abbotstone, calibration at the Borough Bridge gauges is good and the model is accepted as the best available tool for assessing the complex relationships between climate, abstractions, groundwater levels and flows.
A schematic summarising the key inputs, outputs and relationships between the two models is show in Figure 4.
Figure 4 Inputs and outputs from the T&I GW model and the Aquator model
Additional details of the modelling tools and approach are set out in a separate method statement, but other key points to be aware of are:
The T&I GW model operates on bi-monthly stress periods. The naturalised river flow inputs to the Aquator model are in daily timesteps which have been interpolated between these bi-monthly outputs. The impacts of a drought order scenario are compared with a ‘reference condition’ – the situation that would occur during drought but without the drought order in place (described further in Section B.3.2) The main steps in the hydrological impact assessment are summarised in Figure 5.
24 Appendix B Hydrology and Physical Environment Assessment
Figure 5 Main steps in hydrological impact assessment
Step 1: Model reference conditions for historical and stochastic climate sequences
Step 2: Impacts of drought orders on low flows
Step 3: Impacts on habitat variables
Step 4: Impacts on groundwater heads (if relevant)
In-line with the approach taken for the draft WRMP19, a stochastically-generated climate sequence has been used to help assess potential demand and supply balances and environmental impacts under more severe and extreme droughts. To generate naturalised flows for the Aquator model (as shown in Figure 4), two climate sequences were simulated in the T&I GW model:
An 80-year historical period from 1918 to 1997; and A 2000-year stochastic sequence. The T&I GW model is the best available tool to assess the impacts of groundwater abstraction and augmentation from the Candover boreholes because it includes explicit, three-dimensional representation of groundwater and surface water processes, whereas the Aquator model is based on simplified assumptions for these interactions. However, because the run times for a 2000-year sequence in the groundwater model are very long, a selection of droughts from the 80-year and 2000-year sequences were compiled for simulation in the groundwater model. The compiled sequence includes a period of ‘run-in’ to the target droughts followed by a period of recovery (Table 9). This provides a manageable approach to assessing the differences in impacts on flow and groundwater heads between the reference condition and drought order scenarios.
Table 9 Compiled climate sequence used in the T&I GW model Simulation year Climatic year Run 163 Year Purpose 1 Average warm up 2 Average warm up 3 Average warm up 4 1919 2719 historical run in 5 1920 2720 historical drought 6 1921 2721 historical drought 7 1922 2722 historical drought 8 Average recover 9 Average recover 10 Average recover 11 1972 2772 historical run in 12 1973 2773 historical drought 13 1974 2774 historical run in 14 1975 2775 historical run in 15 1976 2776 historical drought 16 Average recover 17 Average recover 18 Average recover 19 1990 2790 historical run in 20 1991 2791 historical drought 21 1992 2792 historical drought 22 Average recover 23 Average recover 24 Average recover
25 Appendix B Hydrology and Physical Environment Assessment
Simulation year Climatic year Run 163 Year Purpose 25 3704 SWS run in 26 3705 SWS WRMP19 selected 1 in 50 27 Stochastic 3706 SWS WRMP19 selected 1 in 50 28 3707 SWS WRMP19 selected 1 in 50 29 3708 SWS WRMP19 selected 1 in 50 30 Average recover 31 Average recover 32 Average recover 33 4080 SWS run in 34 4081 SWS WRMP19 selected 1 in 50 35 Stochastic 4082 SWS WRMP19 selected 1 in 50 36 4083 SWS WRMP19 selected 1 in 50 37 4084 SWS WRMP19 selected 1 in 50 38 Average recover 39 Average recover 40 Average recover 41 4132 SWS run in 42 4133 SWS WRMP19 selected 1 in 100 43 Stochastic 4134 SWS WRMP19 selected 1 in 100 44 4135 SWS WRMP19 selected 1 in 100 45 4136 SWS WRMP19 selected 1 in 100 46 Average recover 47 Average recover 48 Average recover 49 4313 SWS run in 50 4314 SWS WRMP19 selected 1 in 200 51 Stochastic 4315 SWS WRMP19 selected 1 in 200 52 4316 SWS WRMP19 selected 1 in 200 53 4317 SWS WRMP19 selected 1 in 200 54 Average recover 55 Average recover 56 Average recover 57 3286 SWS run in 58 3287 SWS WRMP19 selected 1 in 500 59 3288 SWS WRMP19 selected 1 in 500 60 Stochastic 3289 SWS WRMP19 selected 1 in 500 61 3290 SWS WRMP19 selected 1 in 500 62 3291 SWS WRMP19 selected 1 in 500 63 3292 SWS WRMP19 selected 1 in 500 92 recover 93 Average recover 94 recover
This environmental assessment utilises a combination of output from both the Aquator and T&I GW models. It is therefore important to understand how these impacts predicted from both models are correlated and whether comparison of impacts between models is a valid activity.
26 Appendix B Hydrology and Physical Environment Assessment
Figure 6 presents the predicted surface water impact during the extreme 1:500-year drought (stochastic year 3290) for both of these models at Allbrook & Highbridge. Figure 6 demonstrates that the two models generate comparable predictions of the surface water impact. Both models are based on naturalised flows in bi-monthly time steps derived from the T&I GW model. The influence of demand, restrictions and abstraction is then simulated in daily time steps in the Aquator model whereas they are simulated in bi-monthly time steps in the T&I GW model. The other main difference is that the T&I GW model includes an explicit 3-dimensional representation of groundwater-surface water interactions, whereas in the Aquator model these are based on more simplified assumptions – as informed by the GW model. Bearing these differences in mind, the comparison shown in
27 Appendix B Hydrology and Physical Environment Assessment
Figure 6 and equivalent interpretation for alternative drought years, provides confidence that the two models indicate similar surface water impacts.
28 Appendix B Hydrology and Physical Environment Assessment
Figure 6 Relationship of predicted surface water impacts between the Aquator and T&I GW model1314
A&H (T&I173 (DP0004) -172 (DP0003)) A&H (Aquator DP0004 - DP0003) 50 50 40 30 20 10 0 -10 -20 -30 Flow impact (Ml/d) impact Flow -40 -50 -50 01/01/3288 31/12/3288 31/12/3289 31/12/3290 01/01/3292 31/12/3292
Aquator model runs DP0003_a (without drought orders) and DP0004_a (with drought orders). Test and Itchen model runs T&I 172(DP0003) (without drought orders) and T&I 173(DP0004) (with drought orders)
B.3.2. Reference conditions During any drought, a number of factors determine the ‘reference conditions’ for river flows. The principal factors are:
Climate Water demand Pre-agreed demand restrictions The deployable output of sources (taking account of licence constraints); and Southern Water’s water imports and internal transfers. The reference conditions for the Test and Itchen drought orders are based on water demand and restrictions and the resultant abstractions and transfers in the Hampshire area, assuming that the River Itchen s52 licence changes and the s52 abstraction licence changes for the Testwood abstraction (River Test) are in place. These conditions are simulated for an 80 year historical climate (1918-1997) and a 2000 year stochastic climate sequence.
With regards to the Candover Augmentation Scheme drought order, the reference condition assumes that no abstraction takes place from the Environment Agency’s Candover Augmentation Scheme boreholes. B.3.3. Environmental impact pathways The two Lower Itchen drought orders have the potential to affect the environment in subtly different ways. Whilst this impact assessment considers the cumulative impact of the two drought orders, these differences, set out below, should nevertheless still be noted.
13 Aquator run version ‘a’ outputs are presented in Figure 6 since output from this run was used as inputs to the T&I model (see Figure 4). The Aquator model has since been re-run with minor changes to the assumptions around the triggering of the Gaters Mill drought order. Therefore, output from Aquator versions ‘g and ‘f’ without and with drought orders, respectively) is presented later. However, the implications of this on groundwater impact (for which the T&I model is used) is negligible, therefore the T&I model has not been re-run. 14 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx
29 Appendix B Hydrology and Physical Environment Assessment
Gaters Mill drought order
This drought order has the potential to impact surface freshwater flows in the 1.6 km reach of the River Itchen between Gaters Mill and Woodmill. Downstream of Woodmill, the river is tidal and the small changes in flow are considered to be negligible in comparison to the influence of the tidal system. Potential environmental impacts in the tidal system are considered further in Appendix D of the EAR.
Over this 1.6 km extent to Woodmill, the river traverses over low permeability Tertiary deposits. It is therefore hydraulically unconnected from the underlying Chalk aquifer, which is over 100 m below surface (the Chalk is recorded as being 185 m at Bunkers Hill borehole, approximately 12 km west of the Itchen15). Therefore no groundwater impacts are anticipated as a result of this drought order.
Southern Water Lower Itchen sources drought order
The Southern Water Lower Itchen sources drought order has the potential to generate both surface water and groundwater impacts.
The drought order will result in a flow reduction at Allbrook & Highbridge. This flow impact could be translated downstream to the tidal limit at Woodmill. It is assumed in this assessment that Southern Water’s Chickenhall WwTW at Eastleigh, located between Allbrook & Highbridge gauging station and the Gaters Mill abstraction, will discharge a minimum of 20 Ml/d at low flow conditions and that other minor inflows will be unchanged.
The spatial extent of any impacts would influence a much greater length of the freshwater River Itchen compared to the Gaters Mill drought order so, for this reason, it is proposed that the Lower Itchen sources drought order is used second to the Gaters Mill drought order. More broadly, the draft Drought Plan 2018 also indicates that these two drought orders should only be implemented after the Testwood and Candover Augmentation Scheme drought orders to support the Western Area.
Southern Water’s Lower Itchen sources include groundwater abstractions from Otterbourne GW and Twyford. Therefore, abstraction at these sources beyond that anticipated under reference conditions will result in additional groundwater drawdown. This impact on the Chalk aquifer has the potential consequence of reducing groundwater-surface water interaction over the extent where the Chalk is unconfined (i.e. north of Allbrook & Highbridge gauging station), with a resulting impact on surface water flows in this reach.
The nature of the drought order impact will be dependent on the operational split of the groundwater and surface water sources; increasing the component of groundwater abstraction will increase the groundwater impact. However, the overall impact on surface water flow may decrease as more water is obtained at the expense of aquifer storage. B.3.4. Impact on flow The potential impact that the Gaters Mill and Lower Itchen sources drought orders would have on flow in the River Itchen has been assessed by comparing the reference condition flows (drought flows with no drought order in place) to those predicted to arise in a drought with the drought order in place. To do this, the assessment has considered both the historical and stochastic flow timeseries generated from the Southern Water Aquator model runs DP0003b (without drought orders) and DP0004b (with drought orders).
B.3.4.1. Historical context Figure 7 provides an initial understanding as to the likely scale and frequency of flow impacts associated with the drought order under historical climate conditions (1918 – 1997). Figure 7 plots the modelled daily mean historical flows at Allbrook & Highbridge gauging station (the HOF location for the Southern Water Lower Itchen sources) and Riverside Park (the HOF location for the Gaters Mill abstraction) with and without the four Test and Itchen drought orders in place (Testwood, Candover, Gaters Mill and Lower Itchen). The Testwood and Candover Augmentation Scheme drought orders are assumed to be in place in advance of the Gaters Mill and Lower Itchen sources drought orders in the model.
Figure 7 demonstrates that both the Lower Itchen and Gaters Mill drought orders would not have been required under the historical climatic conditions examined; neither the Allbrook & Highbridge HOF nor the Riverside Park HOF are breached. The modelling therefore indicates that these two drought orders are only required for droughts of greater severity. It is important to note, however, that this assumes the Testwood and Candover Augmentation Schemes drought orders have been implemented first. The Candover Augmentation Scheme drought order would discharge water to the River Itchen upstream of Easton gauging station, increasing river flow at Otterbourne SW source; this drought order is used on two occasions in the model over the historical period: 1921/22 and 1992.
15 BGS borehole log ID 406528 http://scans.bgs.ac.uk/sobi_scans/boreholes/406528/images/10737902.html, accessed September 2017
30 Appendix B Hydrology and Physical Environment Assessment
Figure 7 Historical flow at Allbrook & Highbridge and Riverside Park gauging stations with and without drought orders16
600 600 550 Allbrook & Highbridge 550 500 500 450 450 400 400 350 350
300 300
Flow (Ml/d) Flow Flow (Ml/d) Flow 250 250 200 200 150 150 100 100 50 50 0 0 01/01/1918 01/01/1926 01/01/1934 01/01/1942 01/01/1950 01/01/1958 01/01/1966 01/01/1974 01/01/1982 01/01/1990 01/01/1998 01/01/1918 01/01/1926 01/01/1934 01/01/1942 01/01/1950 01/01/1958 01/01/1966 01/01/1974 01/01/1982 01/01/1990 01/01/1998 A&H s52 HOF Drought A&H HOF A&H flow (with drought orders) A&H flow (without drought orders) Riverside Park HoF Drought Riverside Park HOF Riverside Park flow (with DO) Riverside Park flow (without DO)
Model run output – DP0003g and DP0004f Riverside Park B.3.4.2. Relationship of flow impact and drought severity The Gaters Mill and Lower Itchen sources drought orders are only expected to be required for drought severities greater than those experienced in the historical record. Therefore, the assessment of drought order impacts requires the use of stochastic time series.
Figure 8 plots the annual minimum flows at the Allbrook & Highbridge and Riverside Park gauging stations, as calculated from the daily mean Aquator flow output, for the two model scenarios (i.e. with and without drought orders), from the stochastic time sequence. The Y axis has been translated to return periods (as calculated from the Aquator output), plotted in red along the top.
The Allbrook & Highbridge plot shows that the Lower Itchen sources drought order is anticipated to be required during drought severities of approximately 1:150 year or higher. For lower return periods, a series of sequential demand management measures, bulk supplies, transfers and the implementation of other drought orders cumulatively act to keep river flows above the HOF. These interventions are:
Demand management measures:
Level 1: Drought awareness campaigns; Level 2: Temporary use bans; and Level 3: Temporary use bans and non-essential use bans. Bulk supplies and transfers:
Portsmouth Water bulk supply to a maximum rate of 15 Ml/d (utilised once the ‘Level 1’ demand management intervention has been triggered); then Utilisation of internal treated water transfers from Southampton West to Southampton East water resource zones to a maximum of 24 Ml/d. Drought orders:
Testwood drought order; then Candover Augmentation Scheme drought order.
Beyond approximately a 1:150 year drought event, under reference conditions, the HOF constrains abstraction from Southern Water’s Lower Itchen sources, although this results in large public water supply deficits. For the most extreme droughts, even with the s52 licence constraints in place and no drought orders implemented, river flows would fall below the HOF. The drought order impact on annual minimum flow is the difference between the two lines.
The equivalent plot for Riverside Park gauging stations (the HOF location for the Gaters Mill drought order) shows similar features; the flows only fall noticeably below the HOF beyond approximately 1:150 year drought events.
16 Atkins Spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx
31 Appendix B Hydrology and Physical Environment Assessment
Figure 8 Annual minimum flows (daily mean) at Allbrook & Highbridge and Riverside Park gauging stations under the stochastic climate17 Model output from DP0003g and DP0004f. The timeseries for circled drought years are plotted in Figure 9. Analysis has been conducted on annual minimum flow. Thus drought 3290/3291 appears twice in this plot.
Return period when drought order scenario falls below A&H HoF of 198 Ml/d 300 2.5 10 20 50 100 200 500 1000 2000 280
260
240
220 4315 200
2911 180 2911 3291 160 3290 140
Annual minimum of the meam daily flow (Ml/d) flow daily meam of the minimum Annual 120
100 0 1 2 3 4 5 6 7 8 9 Logistic reduced variate
Return period (years) A&H HOF 198 Ml/d A&H DrO HOF 160 Ml/d
All Drought Orders (DP0004f) No Drought Orders (DP0003g)
Return period when drought order scenario falls below Riverside Park HoF of 194 Ml/d 300 2.5 10 20 50 100 200 500 1000 2000 280
260
240
220
200
180
160
140
Annual minimum of the meam daily flow (Ml/d)flow daily meam of theminimum Annual 120
100 0 1 2 3 4 5 6 7 8 9 Logistic reduced variate
Return period (years) Riverside Park HOF 194Ml/d Riverside Park DrO HOF 150 Ml/d
All Drought Orders (DP0004f) No Drought Orders (DP0003g)
17 Atkins Spreadsheet: DP0003g_DP0004f_DP0005b_INQ005_INQ0006_INQ0007_INQ0008f_INQ009g_INQ010g_FFC_v0.1_ITCHEN.xlsx
32 Appendix B Hydrology and Physical Environment Assessment
B.3.4.3. Analysis of example drought events The potential impact on flow at the Allbrook & Highbridge and Riverside Park gauging stations during example drought events from the stochastic Aquator modelling sequence has been considered.
Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts
33 Appendix B Hydrology and Physical Environment Assessment
Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8
34 Appendix B Hydrology and Physical Environment Assessment
presents the modelled flow at Allbrook & Highbridge and Riverside Park gauging stations with and without drought orders in place for droughts of increasing severity. The drought years can be cross-referenced with Figure 8, which provides context as to drought severity and maximum impact. The flow impact is summarised in Table 10, which also shows the public water supply deficits that are predicted to have occurred if the drought orders were not in place. The flow data in brackets in Table 10 indicate the equivalent river flow without the drought orders in place.
Three of the stochastic drought years presented have been selected from Southern Water’s draft WRMP19 for 1:200 and 1:500 events. The years used in the draft WRMP19 are:
1 in 200 Events: Stochastic years 3594, 4503, 4564 and 4315 1 in 500 Events: Stochastic years 2995, 3686 and 3290 Of the four 1:200 year drought events, the stochastic year 4315 has been selected as it is the drought year with the lowest flows based on Southern Water’s Aquator modelling. Of the three 1:500 year drought events, year 3290, which actually extends into 3291, is the most severe.
The other two drought years presented in Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts
35 Appendix B Hydrology and Physical Environment Assessment
Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8
36 Appendix B Hydrology and Physical Environment Assessment
(4782 and 2911) represent intermediate return period drought events. Figure 8 indicates a step-increase in flow impact around drought events with return periods of approximately 1:300 year. Years 4782 and 2911 are representative of events either side of this step-increase.
The magnitude and duration of flow impact depends on drought severity, and varies over the course of the drought. This is in part due to the climatic conditions and the supply-demand balance during the drought, but also due to the operation of the Candover Augmentation Scheme drought order which discharges water to the River Itchen upstream of the Otterbourne SW abstraction.
Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts
37 Appendix B Hydrology and Physical Environment Assessment
Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8
38 Appendix B Hydrology and Physical Environment Assessment
highlights that for the approximate 1:200 year drought events considered, the magnitude by which river flows fall below the HOFs is very small (up to a maximum of 7 Ml/d and 12 Ml/d at Allbrook & Highbridge and Riverside Park, respectively), and that the durations are very short. Flows are maintained at or very close to the two HOFs by operation of the Candover Augmentation Scheme drought order which augments flow in the River Itchen upstream of the Lower Itchen sources. This augmentation provides a net benefit to flows in the Upper Itchen relative to the situation without this drought order in place.
Under more severe drought events, the Candover Augmentation Scheme drought order becomes constrained as the drought progresses by either the annual groundwater abstraction limit (3,750 Ml/d) or the seasonal groundwater abstraction restriction (to 20 Ml/d during May-August rather than 27 Ml/d at other times of the year). As a result, the Candover Augmentation Scheme ceases to discharge water to River Itchen and there is an increased requirement for the Gaters Mill and Lower Itchen sources drought orders. Thus, flow at the Allbrook & Highbridge and Riverside Park falls below the respective HOFs. Under the extreme 1 in 500 year drought event (stochastic year 3290), a maximum flow impact of 36 Ml/d and 43 Ml/d at Allbrook & Highbridge and Riverside Park, respectively, is predicted. However, this degree of impact is not sustained throughout the entire drought event.
39 Appendix B Hydrology and Physical Environment Assessment
Table 10 Balance of low flows at Allbrook & Highbridge and Riverside Park gauging stations with public water supply deficits18 River Itchen low flows with (without) drought orders Public Water Supply deficits without drought orders in place Allbrook & Highbridge Riverside Park Maximum deficit (Ml/d) Duration of deficit (days) Minimum Duration below HoF (days) Minimum flow (Ml/d) Duration below HOF (days) flow (Ml/d) Stochastic 198 0 193 2 ~1:200 (yr 4315) (WRMP year) 101 139 (198) (0) (194) (0) 191 12 182 12 ~1:200 (yr 4782) (Aquator) 116 224 (198) (0) (194) (0) 175 129 165 116 ~1:300 (yr 2911) (Aquator) 123 271 (198) (0) (194) (0) 160 306 151 296 ~1:500 (yr 3290) (WRMP year) 147 426 (193) (32) (191) (17) Statistics from Aquator model runs DP0003g and DP0004f. Years plotted in Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts
18 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx
40 Appendix B Hydrology and Physical Environment Assessment
Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8
41 Appendix B Hydrology and Physical Environment Assessment
.
42 Appendix B Hydrology and Physical Environment Assessment
Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts19 Stochastic period (4313-4317) 1 in 200 (WRMP) Stochastic period (4780-4784) 1 in 200 A&H s52 HOF Drought A&H HOF A&H flow (with drought orders) A&H flow (without drought orders) A&H s52 HOF Drought A&H HOF A&H flow (with drought orders) A&H flow (without drought orders) 400 400 350 350 300 300 250 250
200 200
Flow (Ml/d) Flow Flow (Ml/d) Flow 150 150 100 100 18/01/4312 17/01/4313 17/01/4314 17/01/4315 18/01/4316 17/01/4317 17/01/4318 17/01/4319 20/01/4779 21/01/4780 20/01/4781 20/01/4782 20/01/4783 21/01/4784 20/01/4785 20/01/4786 Riverside Park HOF Drought Riverside Park HOF Riverside Park flow (with drought orders) Riverside Park flow (without DO) Riverside Park HOF Drought Riverside Park HOF Riverside Park flow (with drought orders) Riverside Park flow (without DO) 400 400 350 350
300 300
250 250
200 200
Flow (Ml/d) Flow Flow (Ml/d) Flow 150 150 100 100 18/01/4312 17/01/4313 17/01/4314 17/01/4315 18/01/4316 17/01/4317 17/01/4318 17/01/4319 20/01/4779 21/01/4780 20/01/4781 20/01/4782 20/01/4783 21/01/4784 20/01/4785 20/01/4786 Candover discharge (due to drought orders) Lower Itchen abstraction (due to drought orders) Gaters Mill abstraction (due to drought orders) Candover discharge (due to drought orders) Lower Itchen abstraction (due to drought orders) Gaters Mill abstraction (due to drought orders) 50 50
(Ml/d)
(Ml/d)
Discharge Discharge Discharge Discharge Abstraction / 0 Abstraction / 0
Stochastic period (2909-2913) 1 in 300 Stochastic period (3288-3293) 1 in 500 (WRMP) A&H s52 HOF Drought A&H HOF A&H flow (with drought orders) A&H flow (without drought orders) A&H s52 HOF Drought A&H HOF A&H flow (with drought orders) A&H flow (without drought orders) 400 400 350 350
300 300
250 250
200 200
Flow (Ml/d) Flow Flow (Ml/d) Flow 150 150 100 100
07/01/2908 06/01/2909 06/01/2910 06/01/2911 07/01/2912 06/01/2913 06/01/2914 06/01/2915 08/01/3287 09/01/3288 08/01/3289 08/01/3290 08/01/3291 09/01/3292 08/01/3293 08/01/3294 08/01/3295 Riverside Park HOF Drought Riverside Park HOF Riverside Park flow (with drought orders) Riverside Park flow (without DO) Riverside Park HOF Drought Riverside Park HOF Riverside Park flow (with drought orders) Riverside Park flow (without DO) 400 400 350 350
300 300 250 250
200 200 Flow (Ml/d) Flow (Ml/d) Flow 150 150
100 100 07/01/2908 06/01/2909 06/01/2910 06/01/2911 07/01/2912 06/01/2913 06/01/2914 06/01/2915 08/01/3287 09/01/3288 08/01/3289 08/01/3290 08/01/3291 09/01/3292 08/01/3293 08/01/3294 08/01/3295 Candover discharge (due to drought orders) Lower Itchen abstraction (due to drought orders) Gaters Mill abstraction (due to drought orders) Candover discharge (due to drought orders) Lower Itchen abstraction (due to drought orders) Gaters Mill abstraction (due to drought orders)
50 50 (Ml/d)
(Ml/d)
Discharge Discharge Discharge Discharge Abstraction / 0 Abstraction / 0 Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8
19 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx
43 Appendix B Hydrology and Physical Environment Assessment
B.3.4.4. Common Standards Monitoring Guidance The Joint Nature Conservation Committee (JNCC) Common Standards Monitoring Guidance (CSMG) methodology defines the minimum set of common standards required to consistently monitor the condition of features of interest in designated conservation sites (SACs and SSSIs) to help in their conservation and preservation. Flow targets are one component of a CSMG assessment20. CSMG targets have not been adopted for the River Itchen, but the Environment Agency and Natural England are working towards agreeing long term flow targets by 202121.
CSMG flow targets are thresholds for a maximum deviation from daily naturalised flow, expressed for different components of the flow regime ( As discussed above, use of the Lower Itchen drought orders is very infrequent and they are not predicted to be required during historical climatic conditions. Because CSMG compliance is based on differences from naturalised flows over recent decades rather than extreme (stochastic) droughts, implementation of the Lower Itchen drought orders will have no impact on CSMG compliance. B.3.5. Impact on river hydraulics B.3.5.1. Implications of Lower Itchen drought orders on habitat variables To assess the potential impact of reductions in river flow in the Lower Itchen on key habitat variables, hydraulic calculations have been carried out for a series of cross-sections between Otterbourne and Woodmill at the tidal limit of the River Itchen. Whilst a hydraulic model (ISIS model) is available for this reach of the River Itchen, it was developed for the purpose of flood risk assessment and thus calibrated to high flows. There are very few spot flow gauging records from this reach that could be used to assess calibration and therefore confidence in the hydraulic model calibration under low flows is limited. Consequently, the model has not been run as a predictive tool but instead has been used as the source of cross-section profiles and information on longitudinal gradients. These data have then subsequently been used to support hydraulic calculations to assess the sensitivity to changes in flow of key habitat variables, specifically velocity and water depth. As discussed in Section B.2, the lower reaches of the River Itchen comprise multiple channels including the historic Itchen Navigation, a canal between Southampton and Winchester. Downstream of Otterbourne, the Allbrook gauging station is located on the Itchen Navigation and the Highbridge gauging station on the River Itchen. Based on spot flow data from periods of relatively low flow in 1997 and 1998, the average flow split between these two flow gauging stations was ~75% of flow to the River Itchen and ~25% of flow to the Itchen Navigation. However, the split of flow between the River Itchen and Itchen Navigation channel, and indeed between the other channels and ditches in this complex braided system, could vary significantly depending on the operation of river control structures. The precise split of flows between the River Itchen and other channels is not known in any detail and this is one reason why the velocity and water depth calculations presented here should be taken as indicative only. In addition to the effects of the flow splits, flows in the River Itchen are also influenced by abstractions, discharges, inflows from tributaries and any baseflow contributions/losses from groundwater. However, analysis of data from the T&I GW model indicates changes in low flows 20 JNCC (2016) Common Standards Monitoring Guidance for Rivers. September 2016. 21 193 Rivers Itchen & Test: Proposed targets for SAC and SSSI conservation objectives (based on revised Common Standards Guidance) and interim progress goals for uRBMP (RIVPDF193 Additional) (RIV193ADD). http://publications.naturalengland.org.uk/publication/5953871591505920 22 JNCC (2014) Common Standards Monitoring Guidance for Rivers. http://jncc.defra.gov.uk/pdf/CSM_rivers_jan_14.pdf 44 Appendix B Hydrology and Physical Environment Assessment downstream of Otterbourne are dominated by the effects of the discharge from the Chickenhall WwTW at Eastleigh and the surface water abstraction at Gaters Mill. Based on an assumed River Itchen–Itchen Navigation flow split of 75:25 and a low flow discharge rate of 20 Ml/d at Chickenhall WwTW (as assumed in the Aquator modelling), reach-specific flows have been estimated for inflow conditions of the Allbrook & Highbridge s52 licence HOF of 198 Ml/d and equivalent flows for a drought order HOF reduced to 160 Ml/d. Downstream of Gaters Mill, the assumed inflow conditions are the current abstraction licence HOF of 194 Ml/d and a drought order HOF reduced to 150 Ml/d (Table 11). Table 11 Reach-specific flow conditions Estimated flow (Ml/d) Otterbourne to Chickenhall WwTW Chickenhall Downstream of WwTW to Gaters Gaters Mill Mill** Single Split: 75%- Split: 25%- Single channel Single channel channel River Itchen* Itchen Navigation* 198 149 50 218 194 160 120 40 180 150 *Assumes 75:25 split in total River Itchen flow based on spot flow monitoring in 1997-1998. Allbrook gauging station is on the Itchen Navigation channel, and Highbridge gauging station is on the main River Itchen. **Assumes 20 Ml/d discharge from Chickenhall WwTW. Thirteen cross-sections between Otterbourne and Woodmill were selected from the hydraulic (ISIS) model (Figure 10) and, for each of these, based on the cross-section profile and estimated slope gradient, ‘ratings’ tables based on Mannings calculations were exported from the ISIS model. Depth and velocity for the reach-specific flows were then calculated from the ratings curves and the results are summarised in 45 Appendix B Hydrology and Physical Environment Assessment Table 12. The main points to take from the results in 46 Appendix B Hydrology and Physical Environment Assessment Table 12 are: Velocities at all but three cross-sections are estimated to be above 3 m/s both for the abstraction licence HOF and the proposed drought order HOF. At the three cross-sections where velocities are estimated to be below 3 m/s, the change in velocity between the licence HOF and the reduced drought order HOF is very small (0.01 to 0.02 m/s). Water depths at all but one cross-section are estimated to be above 0.4 m both for the licence HOF and the proposed drought order HOF. At the cross-section where the depth is estimated to be below 0.4 m, the change in depth between the licence HOF and the proposed drought order HOF is very small (0.04 m). There are a number of uncertainties that need to be taken into account. Firstly, there are insufficient spot flow data between Otterbourne and Woodmill with which to assess the accuracy of estimates of low flow hydraulic characteristics. Secondly, this assessment has been carried out without any field visits. Therefore, the suitability of cross-sections has been based on map and model data only and the influence of local controls is unknown. Finally, the Lower Itchen is a highly braided system with a large number of flow and level control structures. The operation of these control structures and their influence on flows, velocities and depths in the Lower Itchen is not known. Finally, as noted in Section 0, even under extreme drought events with 1:200 and 1:300-year return periods, flows are only predicted to fall below the Allbrook and Highbridge HOF of 198 Ml/d by a relatively small amount and for only a short duration. Nonetheless, the calculations give a reasonable indication of the sensitivity of velocities and depths to changes in low flow, and specifically the reduction in flow due to the drought order changes to the HOFs for the Lower Itchen. The ecological significance of this is considered in Appendix D of the EAR. 47 Appendix B Hydrology and Physical Environment Assessment Figure 10 Location of cross sections for hydraulic calculations 48 Appendix B Hydrology and Physical Environment Assessment Table 12 Depth and velocity calculations for the abstraction licence HOF and proposed drought order HOF at selected cross-sections ISIS model cross-section Inflow / Flow at Velocity Velocity Depth Depth node and reach HOF Section (m/s) change (m) change description (Ml/d) (Ml/d) (m/s) (m) Otterbourne to Chickenhall (River Itchen) Upstream of Highbridge gauging station 28.008 198 149 0.41 -0.02 0.64 -0.04 Otterbourne to Highbridge 160 120 0.39 0.60 28.022 198 149 0.24 -0.02 0.98 -0.09 Otterbourne to Highbridge 160 120 0.22 0.89 28.034 198 149 0.35 -0.02 0.47 -0.04 Otterbourne to Highbridge 160 120 0.33 0.43 28.040 198 149 0.37 -0.03 0.40 -0.03 Otterbourne to Highbridge 160 120 0.34 0.37 Downstream of Highbridge gauging station 28.058 198 149 0.20 -0.01 0.85 -0.07 Highbridge to Chickenhall 160 120 0.19 0.78 02.226 198 198 0.22 -0.01 0.97 -0.07 Highbridge to Chickenhall 160 165 0.21 0.90 02.247 198 149 0.51 -0.03 0.83 -0.07 Highbridge to Chickenhall 160 120 0.48 0.76 Chickenhall to Gaters Mill (River Itchen) 02.261 198 218 0.42 -0.02 0.60 -0.05 Chickenhall to Gaters Mill 160 180 0.40 0.55 01.046 198 218 0.55 -0.03 0.77 -0.06 Chickenhall to Gaters Mill 160 180 0.52 0.71 01.031 198 218 0.51 -0.02 0.92 -0.06 Chickenhall to Gaters Mill 160 180 0.49 0.86 Downstream of Gaters Mill (River Itchen) 01.020 194 0.47 -0.03 0.69 -0.05 Gaters Mill to Riverside Park 150 0.44 0.64 01.009 194 0.54 -0.03 0.91 -0.06 Gaters Mill to Riverside Park 150 0.51 0.85 01.003 194 0.55 -0.04 0.69 -0.07 Riverside Park to Woodmill 150 0.51 0.62 49 Appendix B Hydrology and Physical Environment Assessment B.3.6. Impact on groundwater heads To assess the potential impact of the Lower Itchen sources drought order on the Chalk aquifer, output from the T&I GW model has been analysed. As noted previously, the Gaters Mill drought order will not impact the Chalk aquifer and therefore this assessment of groundwater heads is purely related to the Southern Water Lower Itchen sources drought order (see Section B.3.3). B.3.6.1. Impact on Chalk aquifer Maintaing groundwater abstraction during drought conditions from the Otterbourne GW and Twyford GW sources has the potential to impact the Chalk aquifer. However, the degree of impact will depend on the operational abstraction volume split between Otterbourne SW and groundwater sources. 50 Appendix B Hydrology and Physical Environment Assessment Figure 11 presents the potential groundwater drawdown five months into the 3290-3291 stochastic drought event (the worst drought event on the 2000-year stochastic record). During this first five months, groundwater abstraction is modelled as being approximately 8 Ml/d greater than it would have been without the drought orders in place. The drawdown has been calculated by the difference in groundwater heads under the drought order and the reference conditions for 31/05/3290. It is noted that Candover Augmentation Scheme drought order is also operational at this point and there is no change in the modelled Twyford GW abstraction rate from the reference conditions. Figure 13 indicates a maximum groundwater drawdown impact close to the Otterbourne GW boreholes of up to 1 m. Decreasing impacts are predicted up to a radius of 6 km where a 0.2 m drawdown impact is modelled. Whilst the groundwater model results are sufficient to indicate the approximate extent and magnitude of impacts during an extreme drought, the exact way in which abstraction from the Lower Itchen surface water and groundwater sources would be managed is uncertain. Also, the period selected is not at the end of this most severe drought period. As a result, the predicted impacts on groundwater heads in the Chalk aquifer should not be regarded as a precise delineation of impact. B.3.6.2. Impact on the hydrological functioning of wetlands The main purpose of the T&I GW model is to predict groundwater heads in the Chalk aquifer and flows in rivers as a result of different abstraction scenarios. The model has not been configured to represent the local groundwater-surface water interactions that will control wetland water levels. It is therefore not appropriate to rely on the groundwater model results to assess potential impacts on the wetland hydrology in any detail. To provide a high level assessment of the potential hydrological impacts of the drought orders, modelled gradients between groundwater heads and the ‘wetland surface’ have been considered. The modelled elevation of the stream cell top has been used as a proxy for the wetland surface as this will be a key control on groundwater-surface interactions in the model. The modelling shows that with and without the drought orders operating, baseflow / groundwater emergence is predicted to continue throughout drought conditions in SSSI Units 84 and 85. The slight reduction in head due to the drought order could theoretically slightly reduce the upward gradient for groundwater seepage, but this is considered to be a negligible impact. At SSSI Unit 83, closer to more of the Otterbourne boreholes, the T&I GW model predicts a more noticeable response in groundwater head, which fluctuates above and below the modelled stream cell top. Because the groundwater model is not configured to represent the wetland water table, these results need to be treated with caution. As part of the River Itchen Sustainability Study, a more detailed study of abstraction impacts on wet grasslands was carried out23. This study included the installation of a series of drift piezometers at three sites in the River Itchen valley, one of which was within the SSSI Unit 83 at Otterbourne. The study presents a conceptual model for surface water–groundwater interactions in which low permiability alluvial deposits overly high permability peats / gravels, which in turn overly low permeability Chalk. The River Itchen, Drift and Chalk are envisaged as being in reasonably good hydraulic continuiity, with hydraulic heads in the river been transmitted through the high permeability gravels to the underside of the alluvial deposit layer. In the Halcrow River Itchen Sustainability Study (2004), a local hydrological model was developed, focusing on three wet grassland sites (Winnal Moors, Easton and Otterbourne), based on the conceptual model described above. The local model used inputs from ISIS modelled river levels, 4R 23 Halcrow, 2004. River Itchen Sustainability Study, Wet Grassland Modelling, Technical Ref H(PR)09 51 Appendix B Hydrology and Physical Environment Assessment modelled rainfall and potential evapotranspiration (from the T&I GW model) and MODFLOW leakage transfers (also from the T&I GW model). For the Otterbourne wetland site, results from the local model indicated the east of the site was well- drained and that abstraction scenarios ranging from naturalised to fully licensed would have ‘no effect on the soil water regime at the site at all’. On the western half of the site, closer to the River Itchen, the report noted that despite the close proximity to the Otterbourne boreholes, there were areas that were surface wet for much of the year. The report concluded that these areas would be insensitive to climate change or naturalised abstraction regimes but could become drier under maximum abstraction scenarios. In summary, the possibility of an impact on water levels in wetlands very close to the abstraction boreholes cannot be ruled out from available models and previous assessments. However, wetland water levels at locations close to the River Itchen are likely to be primarily controlled by water levels in the River Itchen, which have been shown in Section B.3.5 to have a low sensitiivity to changes in low flows. At locations near to the Otterbourne abstractions but more distant from the River Itchen, the wetlands are likely to be more free-draining and insensitive to changes in abstraction. Finally, at the wetland sites further away from the main Otterbourne groundwater abstractions, the groundwater model suggests that upward head gradients are likely to be maintained. Overall, the hydrological impacts from the Lower Itchen sources drought order are considered to be very small. 52 Appendix B Hydrology and Physical Environment Assessment Figure 11 Modelled impacts on Chalk groundwater heads with/without drought orders five months into a 1:500 year drought 53 Appendix B Hydrology and Physical Environment Assessment B.3.7. Hydrological and hydrogeological impact summary This environmental assessment has considered the cumulative impacts of two drought orders in the Lower Itchen: the Gaters Mill drought order, which relates to a reduction in the HOF at Riverside Park gauging station to enable continued abstraction from Portsmouth Water’s Gaters Mill source and the Lower Itchen sources drought order, which pertains to a reduction in the HOF at Allbrook & Highbridge gauging station to enable continued abstraction at the Lower Itchen sources (Otterbourne SW, Otterbourne GW and Twyford); These two drought orders are part of a collection of four Southern Water drought orders on the rivers Test and Itchen. The other two, the Testwood drought order and the Candover Augmentation Scheme drought order, would be applied for in preference to the Lower Itchen and Gaters Mill drought orders, and for the purpose of this hydrological and hydrogeological assessment, are assumed to be in place. Although the Gaters Mill drought order will be utilised in advance of the Lower Itchen sources drought order, modelling has shown that the two orders will be required in quick succession. Therefore, this assessment considers the cumulative impact from the two drought orders as a precautionary approach. It is anticipated that these drought orders would only be required during drought events of a severity greater than approximately 1:150 year. The assessment has considered the potential impacts on various parameters in turn. These are summarised here in a spatial context. These reaches are marked on Figure 12 (presented further below in Section B4.1). Impact on the Chalk aquifer (GB40701G505000) Only the Lower Itchen sources drought order will impact the Chalk aquifer. The Gaters Mill drought order relates only to surface water abstraction towards the River Itchen tidal limit in a locality where there is no hydraulic connectivity to the aquifer. The maximum groundwater impact will depend on the operational abstraction split between surface water and groundwater abstraction from the Lower Itchen sources. For drought events with a return period of up to 1:200 years, the magnitude of additional abstraction as a result of the drought order is negligible relative to the background reference condition abstraction rates. Impacts on the Chalk aquifer will therefore also be negligible. For drought events of higher return periods, the duration of additional abstraction will increase. Taking the worst drought event in the 2000-year stochastic period as an extreme example, within the first five months of the drought order being implemented, under the modelled abstraction profile for the drought, additional groundwater drawdown is predicted to be 1 m, with a 0.2 m impact extending over a 6 km radius. Impact upstream of Allbrook & Highbridge gauging station (Reach A to B) Only the Lower Itchen sources drought order will impact this reach. The impact on river flow is related to the drought characteristics, including severity and duration, and the operation of the Candover Augmentation scheme drought order. The maximum impact on flow at Allbrook & Highbridge in a 1:500-year drought event (stochastic drought year 3290) is predicted to be 36 Ml/d. The minimum flow during this type of extreme event may reach close to the proposed drought order HOF of 160 Ml/d. However, this degree of impact is not sustained throughout the drought event. Surface water flow impacts upstream of the Otterbourne surface water intake are attributable to baseflow impacts arising from the groundwater component of the Lower Itchen sources. 54 Appendix B Hydrology and Physical Environment Assessment Baseflow / groundwater emergence is likely to still occur during drought conditions (assessed at 5 months only) near the edge of the Chalk outcrop, upstream of Allbrook & Highbridge gauging station. Further up-hydraulic gradient, the reduction in groundwater head may result in leakage as the vertical hydraulic gradient reverses. Due to the uncertainties in the groundwater assessment, the upstream extent of impact (boundary A in Figure 14) is indicative only. With the drought orders in place, river flow velocity is typically anticipated to be above 3 m/s and water depth above 0.4 m. Where this is not the case, the change in velocity and water depth is very small (0.02 m/s and 0.04 m respectively). The ecological significance of this is considered in Appendix D of the EAR. Impact between Allbrook & Highbridge and Riverside Park gauging stations (Reach B to C) The Lower Itchen sources drought order drought order will indirectly impact the surface water flow throughout this reach. The reduction in flow at Allbrook & Highbridge gauging station will propagate downstream. It is assumed that the Chickenhall WwTW at Eastleigh continues to discharge a minimum of 20 Ml/d throughout drought periods, which supports flows in the reach between the discharge and Gaters Mill. The discharge is also assumed to continue at the same minimum discharge without the drought orders, and therefore does not change the absolute flow impact induced by the Lower Itchen sources drought order. The Gaters Mill drought order will only impact the lower 1 km of this reach of the River Itchen, downstream of the abstraction intake. The impact on river flow is related to the drought characteristics, including severity and duration, and the operation of the Candover Augmentation Scheme drought order. The maximum impact on flow at the Riverside Park gauging station in a 1:500-year drought event (stochastic drought year 3290) is predicted to be 43 Ml/d. The minimum flow during this type of extreme event may reach close to the proposed drought order HOF of 150 Ml/d. However, this degree of impact is not sustained throughout the drought event. The flow impact at Riverside Park is due to the cumulative impact of the Gaters Mill and Lower Itchen sources drought orders. With the drought orders in place, river flow velocity is typically anticipated to be above 3 m/s. Where this is not the case, the change in flow velocity is very small (0.01 m/s). Water depth is predicted to remain above 0.4 m. The ecological significance of this is considered in Appendix D of the EAR. Impact downstream of Riverside Park gauging station (Reach C - D) Both drought orders will impact this reach. However, the impact downstream of Woodmill (the tidal limit) is considered to be negligible due to the comparative size of the tidal influence. In the 600 m freshwater reach upstream of Woodmill, impacts will be equivalent to those indicated above for Reach B to C. Flow velocity is anticipated to remain above 3 m/s and water depth above 0.4 m under both drought orders. Impact on the hydrological functioning of wetlands Whilst the possibility of an impact on water levels in wetlands very close to the Lower Itchen sources abstraction boreholes cannot be ruled out, the hydrological impacts are considered to be small. 55 Appendix B Hydrology and Physical Environment Assessment - wetland water levels at locations close to the River Itchen are likely to be primarily controlled by water levels in the River Itchen, which have a low sensitivity to changes in low flows. - locations near to the Otterbourne abstractions but more distant from the River Itchen, are likely to be more free-draining and insensitive to changes in abstraction - upward head gradients are likely to be maintained at wetland sites further away from the main Otterbourne groundwater abstraction sites. 56 Appendix B Hydrology and Physical Environment Assessment B.4. Physical environment assessment B.4.1. Geomorphology B.4.1.1. Baseline The River Itchen is a significant sized river flowing through an approximately 400 km2 catchment. Within the river length, 89 km has designated as a SSSI due to its classic Chalk stream and river, fen meadow, flood pasture and swamp habitats. In-channel vegetation is particularly important within this site, being dominated by Ranunculus spp. Habitats adjacent to the river channels are also important features supporting designations, such as the extensive water meadows, ditches and side channels as well as areas of wet woodland. The River Itchen is also designated as a SAC because it supports Habitats Directive Annex I habitats and Annex II species, primarily Ranunculion fluitantis and Callitricho-Batrachion vegetation, as well as the populations of southern damselfly (Coenagrion mercuriale) and bullhead (Cottus gobio). There is significant overlap between the reasons for SSSI and SAC designation on the River Itchen. Notable tributaries of the River Itchen include the River Arle, Candover Stream and Bow Lake Stream. Bedrock geology is dominated by Chalk outcrops (80%) but in the downstream, lower (southern) section of the river, clays and sands of Tertiary deposits overlie the Chalk. The assessment of the baseline geomorphology for the study area has been informed from a number of sources, namely: survey work completed as part of the Test and Itchen River Restoration Strategy and by numerous River Habitat Surveys (RHS). The Test and Itchen River Restoration Strategy focused specifically on the SSSI reaches within the catchment and included the hydrological reaches A-B and B-C. The main hydrological reaches in the study area have good RHS coverage but not full coverage (see Figure 12). River Itchen upstream of Allbrook & Highbridge gauging station (Reach A - B) As with most of the River Itchen, this is a multi-threaded channel over its 7.5 km length. Channel dimensions from the 17 RHS surveys within the overall reach indicate a medium bank height of channel, with left and right bank heights varying between 0.2 m to 1.2 m. Predominantly, bank heights equal bankfull height and this is the same for bankfull width when compared to channel water width, which generally ranged from 7 m to 19 m. The wide range in channel widths observed reflects the large number of channel threads on the River Itchen which vary in size significantly. The channel is extensively impounded and overly deepened reflecting both the presence of weirs and sluices throughout the reach. The channel has a number of riffles, pools and glides which is reflected in most of the RHS reaches surveyed. The reach has historically suffered from dredging, over- widening and uniform banks. Siltation is also prevalent and there is a general lack of woody debris. The shading of the channel is variable overall. It ranges from occasional clumps of trees all the way to continuous tree coverage along the banks in places. The reach also suffers from inappropriate control of vegetation and scrub. The channel is modified in places but not in others: the Habitat Modification Score varies between 2 (predominantly unmodified) and 5 (severely modified). It is modified due to the presence of bridges, reinforced banks, weirs, sluices, embankments and channel re-sectioning. River Itchen between Allbrook & Highbridge and Riverside Park gauging stations (Reach B- C) As with most of the River Itchen this overall reach is a multi-threaded channel over its 8 km length. Channel dimensions from the 19 RHS surveys within the overall reach indicate a medium height of 57 Appendix B Hydrology and Physical Environment Assessment channel, with left and right bank heights varying between 0.4 m to 1.2 m. Predominantly bank heights equal bankfull height and this is the same for bankfull width when compared to channel water width, which generally ranged from 9 m to 24 m. The wide range in channel widths observed reflects the large number of channel threads which vary in size significantly. The overall reach has a good proportion of the length being fairly natural but with localised issues such as poaching and modification. The channel is impounded and over-deepened in places reflecting both the presence of weirs and sluices through the reach. This is more localised than the upstream reach. The channel has a number of riffles, pools and glides, which is reflected in most of the RHS reaches surveyed. The reach has locally suffered from over-widening and modified, uniform banks. The shading of the channel is variable overall. It ranges from occasional clumps all the way to continuous tree coverage along the banks in places. The channel is modified in places but not in others: the Habitat Modification Score varies between 2 (predominantly unmodified) and 5 (severely modified) due to the presence of bridges, reinforced banks, weirs, sluices, embankments and re-sectioning. River Itchen downstream of Riverside Park gauging station (Reach C - D) Downstream of the Riverside Park flow gauging station to Woodmill Lane, the channel has been modified. Woodmill Lane marks the downstream limit of the freshwater River Itchen as it becomes tidal downstream of this location. This also marks the point at which the SSSI ends. This section has probably been modified to channel water down to the original mill at this location. There is evidence of significant bank protection along the left bank, which is also embanked. Downstream of Woodmill Lane, the channel widens significantly and is evidently estuarine. B.4.1.2. Assessment All the reaches examined above on the River Itchen have been modified to some degree. This has either been through channel widening, vegetation cutting, dredging, embankments, sluices or weirs. As a result, the current river system is less resilient to respond to drought conditions than it would be if it was more of a natural form. In a naturalised form, the cross-section would be more varied compared to the uniform trapezoidal channel that is commonplace along the River Itchen thus meaning that, as flow reduces in the river, there will always be deeper sections which remain cooler. The modifications to the channel is further exacerbated by the fact that numerous channels exist that are all connected to the main channel thread. In the event of a drought, the fact that there are multiple channels clearly impacts the overall wetted width as there is only so much more water that can be distributed across the system. None of the reaches downstream of Otterbourne have been subject to restoration measures detailed in the Test and Itchen River Restoration Strategy as these reaches have not been highlighted in updates on the strategy in 2014, 2015 or 2016. Upstream of Allbrook & Highbridge gauging station a number of reaches have been enhanced as a result of the undertaking of schemes in the Test and Itchen River Restoration Strategy. These improvements will help make theses reaches more ecologically resilient to any drought conditions. Hydrological reach A-B has a greater number of sluices and structures than reach B-C in particular, and thus the impact on wetted width (and the resultant effects) of the drought order could be dampened by the fact that there would be backwater effects in many locations leading to higher volumes of water being maintained in modified reaches compared to the less modified reaches in reach B-C. Therefore, there is a low risk of change to the wetted width in reach A-B and a medium risk to B-C. Hydrological reach A-B would also benefit from the upstream discharges of water provided under the Candover Augmentation Scheme drought order. Modelling has shown that of the maximum 25 Ml/d drought order augmentation discharge made directly to the River Itchen upstream of Easton gauging station, 24.5 Ml/d of this flow would reach the Allbrook & Highbridge gauging stations. Thus, the augmentation scheme drought order would enhance water levels within the 58 Appendix B Hydrology and Physical Environment Assessment system and therefore impacts on water levels during low flow levels would be mitigated against with no detrimental impact on the geomorphology. Any effect is amplified in reach B-C as, by the start of this reach, surface water abstracted under the Lower Itchen sources drought order will have been removed and thus this reach will have less water within it than the one upstream. Hydraulic calculations have been produced to show how water depths and flow velocities would be affected along 13 cross-sections between Otterbourne and Woodmill due to the reduction to the HOF from 198 Ml/d to 160 Ml/d at Allbrook & Highbridge. This analysis demonstrates that there would be water depth reductions ranging from 6% to 10% and flow velocity reductions ranging from 4% to 8% reduction across the cross-sections. These calculations show that this drought order would only lead to small changes in average water depth and flow velocity. With a reduction in wetted width, there could be an increased risk in fine sedimentation due to lower flow velocities. This risk is classified, overall, as being of low impact in all the reaches examined as a consequence of the analysis presented above. The issue of fine sediment entering the River Itchen is a wider catchment issue due to its modified state. Any reduction in flows caused by the drought order could lead to increased sedimentation across the catchment. The presence of trees shading the bank will help to inhibit the potential increases in water temperature as the water levels become shallower in the summer. Where trees are absent, the impact could be greater due to increased water temperatures. Potential decreases in wetted widths and water depths will also impact habitat availability. Emergent in-channel vegetation is a significant feature of the River Itchen, particularly in the less shaded sections. The risk of habitat decline is of low to medium risk in all the reaches examined. However, management of the vegetation in all reaches could be improved to increase resilience. Reductions in wetted widths and water depths can present potential risks to bank collapse due to the drying of steep banks. In hydrological reaches A-B and B-C, the risk and significance of bank collapse is related to the hydrological impact and is considered low. In hydrological reach C-D, the impact to fluvial geomorphological characteristics from any reductions in flow are considered to be very low. Firstly, the only non-tidal section is located upstream of Woodmill Lane and much of this section is ponded and thus not impacted significantly by fluctuating flows. This is further dampened downstream of Woodmill Lane as the reach becomes tidal and thus is subject to normal tidal cycles which masks changes to the flow levels. 59 Appendix B Hydrology and Physical Environment Assessment Figure 12 Location of RHS and water quality monitoring sites Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426 60 Appendix B Hydrology and Physical Environment Assessment B.4.2. Water quality This section sets out the baseline water quality of the impacted reaches identified above and examines changes over time and with respect to river flows. Environmental pressures on river water quality (such as discharges from wastewater treatment works), which may cause increased deterioration in water quality with the drought order in place, are discussed separately in Section B.4.3. To support the assessment of potentially sensitive environmental features, an understanding has been developed of the water quality of the river reaches within the hydrological zone of influence of the drought order. For Water Framework Directive (WFD) classification, the Environment Agency has set out (according to UKTAG evidence) what pressures, including water quality pressures, each biological quality element is capable of responding to. For the purposes of this drought order assessment, the relevant supporting water quality parameters are as follows: for fish and macroinvertebrates (where identified as sensitive features), the key parameters are dissolved oxygen saturation and total ammonia concentration; and for macrophytes and algae (phytobenthos / diatoms) (where identified as sensitive features), the key parameters are soluble reactive phosphorus. Potential impacts on water temperature have also been considered. Environment Agency routine monitoring data were reviewed to provide an overview of water quality in the hydrological zone of influence. In the River Itchen catchment, within the extent of influence of the Lower Itchen sources drought orders, there are seven water quality sampling sites, as detailed in Table 13 and indicated in Figures 14 and 15. Values at the limit of detection were halved in line with standard Environment Agency practice. Table 13 Summary of Environment Agency water quality monitoring sites Fish EA site ID Site name NGR Reach designation G0003795 River Itchen St Cross SU4715220591 Bridge A-B Salmonid G0003796 Otterbourne Pumping SU4704423248 Station G0003793 River Itchen Bishopstoke SU4646119411 Salmonid B-C G0003786 River Itchen Gaters Mill SU4537215634 River Itchen at Cobden G0003787 Bridge SU4376114079 N/A River Itchen at Kemps C-D G0003781 Boatyard SU4383912877 N/A Table 15 provides a comparison of key water quality data for freshwater sites in Reaches A to C against WFD Environmental Quality Standards (EQS) set out in Table 14. It should be noted that this information is provided for interpretive purposes only based on the available dataset from 2005-2016, and does not provide a formal WFD water quality assessment, which is based on annual datasets. Water quality assessment for the Itchen estuary (Reach C to D) is based on relevant WFD parameters for transitional water bodies. 61 Appendix B Hydrology and Physical Environment Assessment Table 14 Relevant WFD Environmental Quality Standards (EQS) for freshwater EQS EQS EQS EQS Determinand High Good Mod Poor Notes Total ammonia (mg/l) (EQS is a 90thpercentile) 0.3 0.6 1.1 2.5 River Itchen St Cross 0.051 0.092 0.216 1.1 Bridge Otterbourne pumping 0.052 0.094 0.219 1.107 station Soluble Reactive Phosphorus Bow Lake Stoke (mg/l) 0.048 0.088 0.209 1.084 Common (EQS is an annual average) River Itchen 0.053 0.095 0.222 1.113 Bishopstoke River Itchen Gaters 0.055 0.098 0.227 1.123 Mill Dissolved Oxygen (% saturation) 80 75 64 50 Salmonid waters (EQS is a 10th percentile) 70 60 54 45 Cyprinid waters pH (EQS is 5th and 95th percentiles for High and Good; 10th percentile for Moderate and Poor) 6 to 9 6 to 9 4.7 4.2 Temperature (°C) 20 23 - - Salmonid waters (EQS is a maximum temperature) 25 28 - - Cyprinid waters Table 15 Summary statistics for freshwater sites against EQS River Itchen Otterbourne Bow Lake River 2005-2016 River Itchen Site: St Cross Pumping Stoke Itchen dataset Bishopstoke Bridge Station Common Gaters Mill Min 0.015 0.015 0.015 0.015 0.015 Total Mean 0.032 0.023 0.229 0.020 0.071 Ammonia Max 0.166 0.074 2.880 0.089 0.384 90% 0.064 0.043 0.551 0.036 0.191 Soluble Min 0.010 0.020 0.005 0.010 0.014 Reactive Mean 0.064 0.056 0.071 0.051 0.104 Phosphorus Max 0.770 0.141 0.432 0.126 0.557 Min 83.5 59.3 31.6 71.1 71.4 Dissolved Mean 101.8 98.9 83.0 100.1 93.4 Oxygen Max 134.9 130.3 140.6 130.7 132.2 10% 89.5 89.9 54.7 89.7 83.5 Min 7.32 7.39 5.33 7.40 6.94 Mean 8.00 8.11 7.74 8.08 8.05 pH Max 8.97 8.72 8.96 8.60 8.59 95% 8.25 8.33 8.25 8.36 8.34 5% 7.71 7.84 7.23 7.73 7.64 Min 5.2 4.6 0.5 4.6 4.5 Temperature Mean 11.3 11.4 10.9 11.2 11.3 Max 17.9 17.7 21.6 19.0 18.5 62 Appendix B Hydrology and Physical Environment Assessment Figure 15 Location of estuarine water quality monitoring sites 63 Appendix B Hydrology and Physical Environment Assessment B.4.2.1. Chalk aquifer There is little groundwater quality data available for this water body. However, there are no known pollution incidents in the vicinity and the drought order covering the Southern Water Lower Itchen groundwater sources will not lead to any saline intrusion. It is possible that there may be a change in composition in the surface water reaches as a result of reduction in baseflow, but no impact on groundwater quality is predicted. B.4.2.2. Reach A-B – River Itchen upstream of Allbrook and Highbridge gauging station Water quality analysis for this reach has been undertaken based on the data available at the River Itchen at St. Cross Bridge (G0003795) and at the Otterbourne Pumping Station (G0003796) water quality monitoring sites. pH and Temperature The average pH values recorded were 8.0 (St. Cross Bridge) and 8.11 (Otterbourne), with the 5 and 95 percentile values in line with WFD High status. Maximum temperatures recorded were 17.9 and 17.7°C, respectively, in line with WFD High status for Salmonid waters. Figure 16 Measured pH at River Itchen at St Cross Figure 17 Measured pH at River Itchen at Otterbourne Pumping Station 64 Appendix B Hydrology and Physical Environment Assessment Figure 18 Measured temperature at River Itchen at St Cross Figure 19 Measured temperature at River Itchen at Otterbourne Pumping Station Total ammonia concentration Total ammonia concentration data for the River Itchen at St. Cross Bridge and Otterbourne Pumping Station were reviewed and are presented in Figures 20 and 21 against the relevant WFD standards for a lowland high alkalinity river. Total ammonia concentration measurements were consistently compliant with the WFD standard to support high status (0.3 mg/l) for fish and invertebrates for a lowland high alkalinity river at both sites. Peaks in concentrations were not linked to low flows. 65 Appendix B Hydrology and Physical Environment Assessment Figure 20 Total ammonia concentration at River Itchen St. Cross Bridge against WFD status bands 2.6 Bad 2.4 2.2 2.0 1.8 Poor 1.6 1.4 1.2 1.0 Total Ammonia mgN/l 0.8 Mod 0.6 0.4 Good 0.2 High 0.0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 21 Total ammonia concentration at River Itchen Otterbourne Pumping Station against WFD status bands 2.6 Bad 2.4 2.2 2.0 1.8 Poor 1.6 1.4 1.2 1.0 Total Ammonia mgN/l 0.8 Mod 0.6 0.4 Good 0.2 High 0.0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Dissolved oxygen saturation Dissolved oxygen saturation data for the St. Cross Bridge and Otterbourne monitoring sites were reviewed and presented in Figures 22 and 23 against the relevant WFD standards for a lowland high alkalinity river with salmonid designation. Dissolved oxygen saturation measurements were consistently compliant with the WFD standard to support high status (80% saturation; salmonid designation) for fish and invertebrates for a lowland high alkalinity river at both sites. There was one instance in the data record at Otterbourne where this standard was not met with the dissolved oxygen 66 Appendix B Hydrology and Physical Environment Assessment saturation value on this occasion indicative of WFD poor status. Dissolved oxygen saturation displays some moderate seasonality at both sites however this is not linked to low flow conditions. Figure 22 Dissolved oxygen saturation at River Itchen St. Cross Bridge against WFD status band 140 135 130 125 120 115 110 105 100 High 95 90 85 80 75 Good 70 65 Mod Dissolved Dissolved Oxygen Saturation(%) 60 55 Poor 50 45 Bad 40 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 23 Dissolved oxygen saturation at River Itchen Otterbourne Pumping Station against WFD status band 140 135 130 125 120 115 110 High 105 100 95 90 85 80 75 Good 70 65 Mod Dissolved Dissolved Oxygen Saturation(%) 60 55 Poor 50 45 40 Bad 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Soluble reactive phosphorus concentration Soluble reactive phosphorus concentration data at St. Cross Bridge and Otterbourne were reviewed and data are presented in Figure 24 and 25 against the relevant WFD site specific standards provided by the Environment Agency. Soluble reactive phosphorus concentrations were not completely consistent with the WFD standard to support good status (0.092 and 0.094 mg/l for the two sites respectively) for fish and invertebrates for a lowland high alkalinity river: a small number of elevated soluble reactive phosphorus concentrations were observed throughout the two site records, indicative of WFD moderate status (0.216 and 0.219 mg/l) and one observation indicative of WFD 67 Appendix B Hydrology and Physical Environment Assessment poor status (0.78mg/l at St. Cross). Peak soluble reactive phosphorus concentration values were rarely linked to low flow conditions at either of the two sites. Figure 24 Soluble reactive phosphorus concentration at River Itchen St. Cross Bridge against WFD status bands 1.2 Bad 1.1 1.0 0.9 0.8 0.7 Poor 0.6 0.5 0.4 0.3 Soluble Reactive Phosphorus mgP/l 0.2 Mod 0.1 Good 0.0 High 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 25 Soluble reactive phosphorus concentration at River Itchen Otterbourne Pumping Station against WFD status bands 1.2 Bad 1.1 1.0 0.9 0.8 0.7 Poor 0.6 0.5 0.4 0.3 Soluble Reactive SolubleReactive PhosphorusmgP/l 0.2 Mod 0.1 Good 0.0 High 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow B.4.2.3. Reach B-C – River Itchen downstream of Allbrook and Highbridge gauging station to Riverside Park gauging station Water quality analysis for this reach was undertaken based on the data available at the River Itchen at Bishopstoke (G0003793) and at Gaters Mill (G0003786) water quality monitoring sites. 68 Appendix B Hydrology and Physical Environment Assessment pH and temperature The average pH values recorded were 8.08 (Bishopstoke) and 8.05 (Gaters Mill), with the 5 and 95percentile values in line with WFD High status. Maximum temperatures recorded were 19.0 and 18.5°C, respectively, in line with WFD High status for Salmonid and Cyprinid waters. Figure 26 Measured pH at River Itchen at Bishopstoke 9 8.5 8 7.5 pH 7 6.5 6 Figure 27 Measured pH at River Itchen at Gaters Mill 69 Appendix B Hydrology and Physical Environment Assessment Figure 28 Measured temperature at River Itchen at Bishopstoke Figure 29 Measured temperature at River Itchen at Gaters Mill Total ammonia concentration Total ammonia concentration data for the River Itchen at Bishopstoke and Gaters Mill were reviewed and are presented in Figures 30 and 31 against the relevant WFD standards for a lowland high alkalinity river. Total ammonia concentration measurements were consistently compliant with the WFD standard to support high or good status (0.3-0.6 mg/l) at both monitoring sites. 70 Appendix B Hydrology and Physical Environment Assessment Figure 30 Total ammonia concentration at River Itchen Bishopstoke against WFD status bands 2.6 Bad 2.4 2.2 2.0 1.8 1.6 Poor 1.4 1.2 1.0 Total Ammonia mgN/l 0.8 Mod 0.6 0.4 Good 0.2 High 0.0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 31 Total ammonia concentration at River Itchen Gaters Mill against WFD status bands 2.6 Bad 2.4 2.2 2.0 1.8 1.6 Poor 1.4 1.2 1.0 Total Ammonia mgN/l 0.8 Mod 0.6 0.4 Good 0.2 High 0.0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Dissolved oxygen saturation Dissolved oxygen saturation data for the Bishopstoke and Gaters Mill monitoring sites were reviewed and presented in Figure and 33 against the relevant WFD standards for a lowland high alkalinity river (with salmonid designation for Bishopstoke). Dissolved oxygen saturation measurements were consistently compliant with the WFD standard to support high status (80% saturation, salmonid designation) for fish and invertebrates for a lowland high alkalinity river at both sites. There was one instance in both data records where the high status DO standard was not met, with DO saturations being indicative of WFD moderate status. The dissolved oxygen saturation data displays some moderate seasonality although this is not linked to low flow conditions. 71 Appendix B Hydrology and Physical Environment Assessment Figure 32 Dissolved oxygen saturation at River Itchen Bishopstoke against WFD status bands 140 135 130 125 120 115 110 High 105 100 95 90 85 80 75 Good 70 65 Mod 60 Dissolved Dissolved Oxygen Saturation(%) 55 Poor 50 45 Bad 40 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 33 Dissolved oxygen saturation at River Itchen Gaters Mill against WFD status bands Soluble reactive phosphorus concentration Soluble reactive phosphorus concentration values at Bishopstoke and Gaters Mill were reviewed and data are presented in Figures 34 and 35 against the relevant WFD site specific standards provided by the Environment Agency. Soluble reactive phosphorus concentration values were not consistent with the WFD standard to support good status (0.095 and 0.098 mg/l for the two sites respectively) for fish and invertebrates for a lowland high alkalinity river. Elevated soluble reactive phosphorus concentrations were observed throughout the two records, indicative of WFD moderate status (0.222 and 0.227 mg/l) at both sites; prior to 2008 there are also values indicative of WFD poor status (0.56 mg/l) at Gaters Mill. 72 Appendix B Hydrology and Physical Environment Assessment Figure 34 Soluble reactive phosphorus concentration at River Itchen Bishopstone against WFD status bands 1.2 Bad 1.1 1.0 0.9 0.8 0.7 0.6 Poor 0.5 0.4 0.3 Soluble Reactive Soluble Reactive Phosphorus mgP/l 0.2 0.1 Mod Good 0.0 High 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow Figure 35 Soluble reactive phosphorus concentration at River Itchen Gaters Mill against WFD status bands 1.2 Bad 1.1 1.0 0.9 0.8 0.7 0.6 Poor 0.5 0.4 0.3 Soluble Reactive Soluble Reactive Phosphorus mgP/l 0.2 0.1 Mod Good 0.0 High 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow B.4.2.4. Reach C-D – Itchen Estuary (part of Southampton Water WFD waterbody) Water quality analysis for this reach has been undertaken based on the data available at River Itchen at Cobden Bridge (G0003787) and River Itchen and Kemps Boatyard (G0003781). The salinity and turbidity conditions for these two sites are presented in Table 16. 73 Appendix B Hydrology and Physical Environment Assessment Table 16 Summary of salinity and turbidity statistics 2005-2016 dataset Site: River Itchen at River Itchen at Cobden Bridge Kemps Boatyard Salinity (ppt) Min 0.34 2.19 Mean 5.82 22.66 Max 18.70 32.91 Turbidity (suspended solids mg/L) Min 3.00 3.00 Mean 7.29 6.12 Max 23.10 29.10 Dissolved Inorganic Nitrogen Dissolved Inorganic Nitrogen (DIN) is a measure of the dissolved fractions of ammonia, nitrate and nitrite present in the water column. DIN standards are specific to each site, being expressed in micromoles/litre and calculated based on the average annual turbidity and salinity data24. Owing to the lack of comprehensive data on the three chemical fractions which characterise DIN, it is not possible to calculate the EQS specific to the sites in the Itchen Estuary (which forms part of the Southampton Water WFD transitional water body). However, the current WFD DIN status for Southampton Water is moderate25 and the available DIN data (expressed in mg/l) from the two estuarine water quality monitoring sites are presented in Figures 36 and 37. Figure 36 Dissolved inorganic nitrogen (DIN) concentration at River Itchen at Cobden Bridge (Itchen Estuary) 24 The Water Framework Directive (Standards and Classification) Directions (England and Wales) 2015. Available at http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf. Accessed 21/11/2017. 25 Environment Agency. Catchment Data Explorer – Southampton Water. Available at http://environment.data.gov.uk/catchment-planning/WaterBody/GB520704202800. Accessed 21/11/2017 74 Appendix B Hydrology and Physical Environment Assessment Figure 37 Dissolved inorganic nitrogen (DIN) concentration at River Itchen at Kemps Boatyard (Itchen Estuary) Dissolved Oxygen Concentrations The dissolved oxygen concentration EQS for estuarine waters is also site-specific and have been calculated for the estuarine monitoring sites based on the specific average salinity data, in accordance to the methodology presented in Section 3 of the Water Framework Directive Directions 201526. The results are presented in Figures 38 and 39 and show that the two sites in the Itchen Estuary achieve the ‘high’ WFD status for dissolved oxygen concentration in line with the overall current WFD status classification for Southampton Water (EA, 2015). Figure 38 Dissolved oxygen concentration at River Itchen at Kemps Boatyard (Itchen Estuary) 26 The Water Framework Directive (Standards and Classification) Directions (England and Wales) 2015. Section 3. Available at http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf. Accessed 21/11/2017. 75 Appendix B Hydrology and Physical Environment Assessment Figure 39 Dissolved oxygen concentration at River Itchen at Kemps Boatyard (Itchen Estuary) B.4.2.1. Water quality summary Assessment of the risk of water quality deterioration as a result of the Lower Itchen sources drought orders has been undertaken considering the available water quality data and the hydrological and hydrogeological impact within the affected reaches as presented earlier. The findings are summarised in Table 17. Table 17 Summary of water quality deterioration risks for Lower Itchen sources drought orders Soluble reactive Reach Ammonia Dissolved oxygen phosphorus Chalk aquifer Negligible A-B Negligible Negligible Low B-C C-D Negligible for all relevant water quality parameters There is little groundwater quality monitoring data, but water quality impacts on the Chalk aquifer are predicted to be negligible. Total ammonia concentration values were consistently in line with the WFD standard to support high status for fish and invertebrates on the River Itchen. Therefore, the risk of water quality deterioration with respect to total ammonia is assessed as negligible in all reaches Dissolved oxygen saturation values were consistently in line with WFD high status for all the sites on the River Itchen. Therefore, the risk of water quality deterioration with respect to dissolved oxygen is assessed as negligible across all reaches. Soluble reactive phosphorus (SRP) concentrations fluctuated between high and good status in the majority of the River Itchen sites, with isolated instances in which SRP concentrations were indicative of moderate or poor status and which were linked to low flow conditions. Therefore, the risk of water quality deterioration with respect to SRP is assessed as low in Reaches A to B and B to C. 76 Appendix B Hydrology and Physical Environment Assessment B.4.3. Environmental pressures B.4.3.1. Abstraction pressures During a drought, abstractions put pressure on groundwater levels and surface water flow, potentially exacerbating natural low flow conditions. This section considers the other groundwater and surface water abstractions that may impact on the reaches affected by the Lower Itchen sources Drought Orders. The information used in this assessment relating to abstraction was received from the Environment Agency in January 2017. Groundwater abstractions Abstractions that are located within the hydrogeological radius of influence defined in Section B.2.3 are listed in Table 18 (excluding the Southern Water Lower Itchen sources abstraction licences). The Lower Itchen sources drought order has the potential to impact on these groundwater users due to the reduction in groundwater levels. However, the risk of derogation cannot readily be quantified. A small change in water level could be significant if the pump is already close to the bottom of the well. On the other hand, a large change in water level may not derogate others if the well is deep and there is the potential to lower the pump. The data on well depth and pump intake location is unknown, and therefore the assessment of risk can only be made based on distance from the Southern Water proposed drought action abstraction. Given these uncertainties, these abstractions have been conservatively assessed as being at a medium to high risk of derogation. Table 18 Other groundwater abstractors within the area of influence Maximum Maximum Licence daily annual Industry Use description HOF Number quantity quantity (Ml/d) (Ml) Potable Water 11/42/25.2/50 PWS 31.5 7,487 n/a Supply - Direct Industrial, Commercial Evaporative 11/42/21/1 0.4 46 n/a And Public Cooling Services Cessation flow Industrial, condition when Commercial Spray Irrigation - Allbrook and 11/42/22.6/95 0.1 3.95 And Public Direct Highbridge is Services 240 Ml/d from May to November General Farming & 31/107 Agriculture 0.1 13.5 n/a Domestic General Farming & 31/108 Agriculture 0.1 13.5 n/a Domestic Data source: EA data request in January 2017 Note: Northbrook and Lower Upham sources (Portsmouth Water) are listed under the same licence (11/42/25.2/50) 77 Appendix B Hydrology and Physical Environment Assessment Surface water abstractions Surface water abstractions that are within the hydrological and hydrogeological radius of influence defined in Section B.2.3 are listed in Table 19 (excluding the Southern Water and Portsmouth Water Lower Itchen abstraction licences). The Lower Itchen sources drought order has the potential to impact on these surface water abstractors through the reduction in surface water flow with the result that they may be unable to abstract all of the licensed quantity. Not all of the surface water licences within the area of influence are impacted by this drought order, in particular those surface water abstractions on the lower River Test or on tributaries of the lower River Itchen where there will be no groundwater or surface water impact. Table highlights those licences that may be impacted. There is some uncertainty over the abstraction volumes for some sources. The data returned by the Environment Agency records ‘0’ for both daily and annual abstraction limits for some licences. Furthermore, the units applied to the flow constraint in licence 31/110 are unclear as both l/s and m3/s are referenced for the same number. Of the five abstraction licences with HOF conditions, four have a flow constraint that is higher than that applicable to the Southern Water Lower Itchen sources (198 Ml/d). Therefore, abstraction from these sources would already have been reduced or ceased by the point the drought order was implemented, and therefore there is negligible risk to these abstractions. There are six other surface water abstractions that may be impacted by the drought order. The abstraction rates for four of these are unknown. Of the remaining two, there is only one reasonably large abstraction. This is for fish farming, upstream of Otterbourne SW (31/086). There is the potential this licence could be impacted as a result of groundwater abstraction from the Lower Itchen sources inducing a reduction in baseflow. However, because the licence is located on the very edge of the simulated area of impact and that the maximum abstraction limit is less than 10% of the Q95, the risk to this licence is deemed to be low, especially given the precautionary nature of the assessment used to generate that boundary. B.4.3.2. Water quality pressures Discharges put pressure on water quality during a drought as lower than normal river flows are experienced. Discharges can however increase river flows to help ameliorate the effect of drought and drought orders. Discharge data were requested from the Environment Agency in order to understand the possible environmental impacts of discharges made into the hydrological zone of influence of the Lower Itchen sources drought orders. In total, 633 discharge permits were identified within the radius of influence defined in Section B.2.3. However, not all of these discharges will affect the hydrology or water quality of the impacted reaches of the Lower Itchen, in particular, discharges to the River Test. Furthermore, many of these discharges are below 0.5 Ml/d and therefore are not anticipated to individually have any material environmental effect on the river reaches. Those discharges over 0.5 Ml/d (either dry weather flow or max daily flow) are identified in Table 20. The largest discharge relates to the WwTW at Eastleigh, which has a permitted dry weather flow of 32 Ml/d. B.5. Cumulative impacts The simulated radius of influence for Southern Water’s other drought management options have been compared against that of the Lower Itchen sources. No other drought management options are predicted to lead to cumulative impacts with the Lower Itchen sources drought orders. 78 Appendix B Hydrology and Physical Environment Assessment Portsmouth Water has also confirmed that it does not have any other planned drought management option that would have any cumulative impacts with the Lower Itchen sources drought orders27. Therefore no cumulative assessment is required. B.6. Cumulative impacts No other drought order or permit options or other drought management measures will lead to any cumulative adverse impacts with the Lower Itchen sources drought orders. The effects of the Candover Augmentation Scheme drought permit will be beneficial to river flows downstream of the flow augmentation scheme discharge location on the River Itchen. HRA and WFD screening assessments have indicated that there would be no cumulative or in-combination effects on European sites or WFD water bodies downstream of the River Test (i.e. European sites and WFD water bodies in Southampton Water) with other drought orders, including drought order/permit options on the Isle of Wight. 27 pers comm. J Burke, Southern Water, February 2017. 79 Appendix B Hydrology and Physical Environment Assessment Table 19 Other surface water abstractors within the area of influence Maximum Maximum daily annual Reach Licence number Industry Use description HOF condition Comment quantity quantity (Ml/d) (Ml) River Spray Irrigation - Reach not impacted Test 11/42/18.16/442 Agriculture Direct 2.2 104.6 (on the River Test) River Potable Water Based on flow in the Reach not impacted Test 11/42/18.16/546 PWS Supply - Direct 136.4 49915.1 River Test (on the River Test) River Fish Farm/Cress Reach not impacted Test 11/42/18.16/547 Agriculture Pond Throughflow 45.5 16592.9 (on the River Test) Reach Fish Farm/Cress Reach impacted by A - B 31/086 Agriculture Pond Throughflow 10.9 4000.0 GW abstraction only Transfer Between Reach Sources (Post Reach impacted by A - B SO/042/0031/003 Environmental Water Act 2003) 0.0 0.0 GW abstraction only Abstraction reduced to Transfer Between 6 Ml/d when flow at Reach Sources (Post Allbrook and Highbridge Reach impacted by A - B SO/042/0031/006 Environmental Water Act 2003) 0.0 0.0 is less than 248 Ml/d GW abstraction only Transfer Between Reach Sources (Post Reach impacted by A - B SO/042/0031/010 Environmental Water Act 2003) 0.0 0.0 GW abstraction only Abstraction ceased when flow at Allbrook and Reach Production Of Hydroelectric Highbridge is less than Reach impacted by A - B SO/042/0031/018 Energy Power Generation 119.2 0.0 257 Ml/d GW abstraction only Reach Reach impacted by A - B SO/042/0031/035 Water Supply Heat Pump 0.1 47.4 GW abstraction only 80 Appendix B Hydrology and Physical Environment Assessment Maximum Maximum daily annual Reach Licence number Industry Use description HOF condition Comment quantity quantity (Ml/d) (Ml) Transfer Between Reach impacted by Reach Sources (Pre GW and SW A - B SO/042/0031/020 Environmental Water Act 2003) 0.0 0.0 abstraction Transfer Between Reach impacted by Reach Sources (Post GW and SW A - B SO/042/0031/023 Environmental Water Act 2003) 0.0 0.0 abstraction Abstraction ceased when Transfer Between flow at Allbrook and Reach impacted by Reach Sources (Post Highbridge is less than GW and SW B - C 31/110 Environmental Water Act 2003) 1.0 365.0 400 m3/s (l/s?)* abstraction Abstraction reduced to Transfer Between 6 Ml/d when flow at Reach impacted by Reach Sources (Post Allbrook and Highbridge GW and SW B - C SO/042/0031/005 Environmental Water Act 2003) 0.0 0.0 is less than 248 Ml/d abstraction Reach not impacted Reach Spray Irrigation - (on Lower River B - C 11/42/22.10/120 Agriculture Direct 0.1 2.3 Itchen tributary) * Units uncertain - referenced as m3/s and l/s. 81 Appendix B Hydrology and Physical Environment Assessment Table 20 Discharge permits within the area of influence Reach Permit number Use Receiving Dry weather Max daily Consideration of watercourse flow (Ml/d) flow (Ml/d) water quality pressure risk (during baseline low flow conditions) Domestic property (single) (incl Groundwater via A-B EPR/EB3496WK Unknown Unknown Negligible farm house) soakaway WwTW/Sewage Treatment Works B-C A00154 The River Itchen 32 Unknown Low (water company) WTW/water Tributary of River B-C P06057 Unknown 2.3 Negligible collection/treatment/supply Itchen Fish + aquaculture/fish farm/cress Bow Lake stream D-B G00171 Unknown 2.2 Negligible farm via 200mm pipe Fish + aquaculture/fish farm/cress A tributary of Bow D-B NPSWQD005981 Unknown 2.0 Negligible farm Lake Bow Lake and a Fish + aquaculture/fish farm/cress D-B EPRNB3235AG tributary of Bow Unknown 1.0 Negligible farm Lake WwTW/Sewage Treatment Works River Itchen Negligible (on Itchen n/a W00314 27.7 Unknown (water company) Estuary estuary) Empress dock, Negligible (on Itchen n/a NPSWQD002578 Undefined or other tributary of River Unknown 3.9 estuary) Itchen Negligible (Reach WTW/water n/a A00691 The River Test Unknown 2.0 not impacted - on collection/treatment/supply River Test) 82 Appendix B Hydrology and Physical Environment Assessment