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 2 Appendix B Hydrology and Physical Environment Assessment 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 3 Appendix B Hydrology and Physical Environment Assessment 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