NORTH HINKSEY GROUNDWATER ASSESSMENT for NORTH HINKSEY PARISH NEIGHBOURHOOD PLAN

Home , Stanford Formation, West Walton Formation

Upton House Market Street Charlbury Oxfordshire, OX7 3PJ United Kingdom tel +44 (0)1608 810374 fax +44 (0)1608 810093 e-mail [email protected] www.gwp.uk.com

NORTH HINKSEY GROUNDWATER ASSESSMENT For NORTH HINKSEY PARISH NEIGHBOURHOOD PLAN

July 2017

GWP Consultants LLP Registered No. OC326183 Registered Office: Upton House, Market Street, Charlbury, Oxfordshire, OX7 3PJ, UK Report Title: North Hinksey Groundwater Assessment Client: North Hinksey Neighbourhood Plan

Job: HINKGW Report Number: 170707 Version: v.02 Issue Status: Final Issue Date: 31/07/17

Prepared by: Donal Neville Signature: Approved by: Clive Carpenter Date: 31/07/17

Issue History:

Issue Issue Date Description Prepared Checked Approved No. v.01 14/07/2017 Draft issued to Client DN GM CC v.02 31/07/2017 Amendments made as requested DN GM CC by David Kay, Report issued as Final

This document is based on GWP report template v1.04 and Normal template v3.07 22/01/15

This report has been prepared by GWP Consultants LLP (GWP) on the specific instructions of our Client. It is solely for our Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which GWP has not given its prior written consent, is at that person’s own risk and may be an infringement of GWP’s copyright. Furthermore the report is issued on the understanding that GWP’s general terms and conditions of engagement are accepted, copies of which are available on request. CONTENTS

1. INTRODUCTION ...... 1

2. LOCATION AND TOPOGRAPHY ...... 1

3. GEOLOGY ...... 1 3.1 Superficial Geology...... 1 3.2 Bedrock Geology...... 1

4. HYDROLOGY...... 1

5. HYDROGEOLOGY ...... 2

6. GROUNDWATER FLOOD RISK ZONE DEFINITION ...... 2

7. RECOMMENDATIONS ...... 3

8. CONCLUSIONS AND RECOMMENDATIONS...... 4

DRAWINGS

HINKGW1707-1 Site location HINKGW1707-2 Aerial view of site location HINKGW1707-3 LiDAR topography elevations contours (5m) HINKGW1707-4 Extract of British Geological Survey Geological map HINKGW1707-5 Extract of British Geological Survey Hydrogeological map HINKGW1707-6 Zone of potential groundwater flooding

North Hinksey Groundwater Assessment North Hinksey Parish Neighbourhood Plan 170707.v02 31/07/17 i NORTH HINKSEY GROUNDWATER ASSESSMENT

1. INTRODUCTION GWP Consultants LLP (GWP) has been commissioned by David Kay on behalf of the North Hinksey Parish Neighbourhood Plan Group (the NHPNPG) to identify potential groundwater flooding areas east and north-east of Cumnor Hill, Oxford.

2. LOCATION AND TOPOGRAPHY North Hinksey Parish is located directly west of Oxford (see Drawing No.HINKGW1707-1 and 2). Topographic elevations decrease from 145 metres above mean sea level (mAmSL) at the top of Cumnor Hurst Hill to 60mAmSL at North Hinksey Lane. East of North Hinksey Lane the ground is relatively flat, varying between 55 and 60mASL (see Drawing No. HINKGW1707-3).

3. GEOLOGY The geology of North Hinksey Parish is described on British Geological Survey (BGS) Geological Sheets 253 and 254 (see Drawing No. HINKGW1707-4). 3.1 Superficial Geology Two areas in the eastern most part of the Parish - directly south of Botley Road and east of the A34, are underlain by superficial sand and gravels of the Northmoor Sand and Gravel Member and alluvium comprising of a mixture of clay, silt, sand and gravel. 3.2 Bedrock Geology Low lying areas to the east and north of the Parish are underlain by the Oxford Clay Formation. The BGS characterise this formation as a “silicate-mudstone, grey, generally smooth to slightly silty”. To the south-west, the Kingston Formation overlies the Oxford Clay Formation, outcropping generally between c. 90 to 110mASL. The BGS characterise this formation as a “medium-grained quartzose sand, locally fine and coarse-grained, with carbonate cemented beds and doggers which are sandy limestone or calcareous sandstone”. The Stanford Formation overlies the Kingston Formation, outcropping generally between c. 110 to 125mASL. The BGS characterise this formation as interbedded limestone, marl and mudstones. The Ampthill Clay Formation overlies the Stanford Formation, described by the BGS as a mudstone. This formation forms the top of the geological sequence within the North Hinksey Parish Boundary. These formations are part of the Corallian Group. It is also noted that an additional geological unit, the Hazelbury Bryan Formation, outcrops along the north-west slope of Cumnor Hills (see Point A – Drawing HINKGW1707-4), but is not mapped to be present on the northern and eastern slopes. North of the B4044, the Hazelbury Bryan Formation is also mapped as part of the Corallian Group. This formation is described by the BGS as a sandy mudstone. Lastly, a number of geological slope failures - Mass Movements - are mapped in and around North Hinksey Parish (see Drawing No. HINKGW1707-4). These features are all mapped along the geological contacts between the Kingston/Hazelbury Bryan Formations and the underlying Oxford Clay Formation. Where mass movement events have occurred, slump debris sediments consisting of Hazelbury Bryan and/or Kingston Formations over lay the Oxford Clay Formation some distance downgradient of the contact, c. 200m or more (see Drawing No. HINKGW1707-4).

4. HYDROLOGY Surface water runoff from the impermeable clay cap (Ampthill Clay Formation) on top of Cumnor Hill, drains radially to the surrounding lowlands. In doing so, it is concentrated along flow paths, which in turn have sub-vertically eroded the underlying sediments to form valleys. These valleys cut down through the above mentioned geological formations, truncated at depth by the Oxford

North Hinksey Groundwater Assessment North Hinksey Neighbourhood Plan 170707.v02 31/07/17 Page 1 of 4 Clay Formation. From north to south, examples of these features are Louie Memorial Playing Fields/Hutchcomb’s Copse, Raleigh Park and the area directly north of Hinksey Heights Golf Club.

5. HYDROGEOLOGY The mudstones associated with the Oxford Clay and Ampthill Clay Formations are impermeable and act as aquicludes; prohibiting the flow of groundwater. In contrast, the Kingston, Stanford and Hazelbury Bryan Formations are permeable strata. Downward vertical groundwater flow within these formations is blocked at depth by the impermeable Oxford Clay Formation. As a result, ‘contact springs’ exist along the contact with the underlying Oxford Clay Formation where the contact is exposed, where groundwater is forced to move laterally to surface and discharge out into the surrounding areas. The BGS has inferred a regional south-east groundwater flow component within the Kingston and Stanford Formations (see Drawing No. HINKGW1707-5). It is not known if groundwater levels have been measured directly or inferred from the elevation of the top of the Oxford Clay. It is clear however that the above mentioned valleys (see Section 4), which cut down vertically through the geological succession, will intersect this groundwater flow. As a result, groundwater flow local to the valleys is diverted, flowing towards the centre of each valley. Hurst Rise Road is situated within such a valley. Residents noted that groundwater flooding to the area is more prominent along the western sections of the valley. Therefore, it is evident that regional south-easterly groundwater flow within the Corallian Group is intersected first by the western edges of the Hurst Rise Road valley, and groundwater egresses naturally into the valley, above the contact with the Oxford Clay Formation, as observed by local residents. During the site visit, groundwater seepage from the Kingston Formation was observed to occur directly above the contact with the Oxford Clay Formation in Raleigh Park. Downgradient of this contact, a holding pond is constructed to collect spring water discharge to supply water for livestock. In Louie Memorial Playing Fields/Hutchcomb’s Copse, groundwater seepage from the Kingston Formation was also observed to occur directly above the contact with the Oxford Clay Formation. Again, seepage is collected in a holding pond downgradient of the contact. Lastly, on inspection of Google Earth imagery, 2 No. large holding ponds are also constructed in Oxford Clay Formation downgradient of the Oxford Clay Formation contact with the over lying Kingston Formation, which suggests groundwater seepage is also collected, possibly for irrigation purposes. Holding ponds in Raleigh Park and Louie Memorial Playing Fields/Hutchcomb’s Copse both contained water during the site visit. Although the holding ponds also collect surface water runoff from rainfall events, given Spring to early Summer 2017 has had very little rainfall, the likely source of water maintaining water levels in the absence of rainfall, must be groundwater seepage from the Kingston Formation. Else, the herd of cattle grazing in Raleigh Park, which utilised the holding pond as a water supply, would readily deplete water reserves in the absence of groundwater seepage from the Kingston Formation. Furthermore, the presence of mass movement slumping along the contact of the Kingston and Oxford Clay Formations suggest the ‘contact springs’ present along the contact reduce fiction, enabling mass movement events to occur. The sand and gravels of the Northmoor Sand and Gravel Member are permeable and groundwater levels within them are primarily controlled by water levels of surface water features, i.e. Seacourt Stream (see Drawing No. HINKGW1707-4). Groundwater levels within the superficial sediments can exceed ground levels in the wet season, as observed by Ag MacKeith of South View House, Old Botley, Oxford, OX2 0JR.

6. GROUNDWATER FLOOD RISK ZONE DEFINITION The above information suggests that regional groundwater flow within the Corallian Group is to the south-east, intersected by sub-vertically cutting valley troughs, i.e. Louie Memorial Playing Fields/Hutchcomb’s Copse, Raleigh Park and Hinksey Heights Golf Club. As a result, groundwater

North Hinksey Groundwater Assessment North Hinksey Neighbourhood Plan 170707.v02 31/07/17 Page 2 of 4 egress to the valleys occurs above the contact with the underlying Oxford Clay Formations, along ‘contact springs’. Therefore, a ‘Zone of Potential Groundwater Flooding’ exists on or above the geological contact between the Kingston Formation and the underlying Oxford Clay Formation. The upper most limit of this zone has been assumed to be ten metres in vertical height above this contact, allowing for a 5-7m rise in groundwater levels within the Corallian Group (see Drawing No. HINKGW1707-6 – Groundwater bearing strata above Oxford Clay Formation) and a 3m deep basement excavation. Although mapped by the BGS, the actual position of the Oxford Clay Formation may vary locally, due to a lack of outcrop for geological mapping purposes, but also the presence of mass movement sediments extending further downgradient of the contact. Therefore the lower most limit of this zone was assumed to be ten metres in vertical height below this contact (see Drawing No. HINKGW1707-6 – Oxford Clay Formation – zone of uncertainty). It should be noted this zone has been estimated without the availability of groundwater level data. The zone definition should be reviewed periodically based upon available groundwater level monitoring data from future planning applications. A zone of groundwater flood risk has therefore been estimated based upon a 20m vertical height range, centres on the Oxford Clay contact with the overlying Kingston Formation. The width of this zone on the ground varies depending on the ground topographic gradient. It is typically 200-250m wide but can range from 150m just west of Hutchcombe Road to 600m on the southern boundary of the parish.

7. RECOMMENDATIONS It is advised that any future planning applications located within the ‘Zone of Potential Groundwater Flood Risk’ undertake a more rigorous groundwater assessment to confirm groundwater conditions beneath the development. Where a development excavation is proposed, the assessment should include:  A minimum of 3 No. groundwater monitoring boreholes within the planning application boundary to confirm groundwater depths and establish groundwater flow directions.  A minimum of 3 No. to 6 No. months of groundwater level monitoring, preferably monitoring annual seasonal groundwater fluctuations during the dry and wet season. Where project lead- in times permit, groundwater monitoring should be repeated over multiple years to confirm seasonal groundwater level fluctuations;  Groundwater monitoring information can be augmented or replaced by local Environmental Agency groundwater level monitoring information if found to exist;  Groundwater monitoring information should be used to investigate whether or not maximum groundwater levels will come into contact with the proposed development. If so, dewatering requirements during the construction phase should be evaluated and an impact assessment undertaken on local houses, including identification of any necessary mitigation measures;  Furthermore, an assessment of any post-construction groundwater flow truncation and subsequent groundwater rise effects should be evaluated and mitigation measures proposed if necessary to prevent flooding or damage to adjacent properties.  . The NHPNPG may wish to note that the London Borough of Camden Borough Council has introduced a Basement Impact Assessment methodology as part of its planning application process. Whilst this considers multiple issues as well as groundwater flooding, it provides a useful framework and potential guidance to developers on how to assess and demonstrate adequate consideration and mitigation of groundwater risks to surrounding properties. The Camden BIA approach and toolkit can be downloaded from the Basement Flooding section of: https://www.camden.gov.uk/ccm/content/environment/planning-and-built- environment/two/planning-policy/local-development-framework/core-strategy/evidence-and- supporting-documents/

North Hinksey Groundwater Assessment North Hinksey Neighbourhood Plan 170707.v02 31/07/17 Page 3 of 4 8. CONCLUSIONS AND RECOMMENDATIONS Publicly available geological, hydrogeological, and topography information, combined with field observations have been used to define a ‘Zone of Potential Groundwater Flood Risk’ in the North Hinksey Parish. Information suggests that areas along and up gradient of the geological contact between the Oxford Clay Formation and Kingston Formation are most prone to groundwater flooding as a result of groundwater egress along ‘contact springs’. Geological and topographic modelling techniques have been used to estimate the boundaries of a groundwater flood risk zone within the North Hinksey Parish boundary, using zones of uncertainty above and below the geological contact. It should be noted this zone has been estimated without the availability of groundwater level data. The zone definition should be reviewed periodically based upon available groundwater level monitoring data from future planning applications. It is advised that any future planning applications located within this zone undertake a more rigorous groundwater assessment to confirm groundwater conditions beneath the development. Guidance for investigating and mitigating basement groundwater impacts has been published by the London Borough of Camden Council and may be of value to local regulators and developers.

GWP CONSULTANTS JULY 2017

North Hinksey Groundwater Assessment North Hinksey Neighbourhood Plan 170707.v02 31/07/17 Page 4 of 4 N

Seacourt Stream

Reproduced from the 2017 Ordnance Survey 1:10,000 map with the permission of the Controller of Her Majesty's Stationery Office,

Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings Licence No AL100018321

LEGEND Version Revision and compilation notes Date Client A Draft 13.07.2017 North Hinksey Parish Neighbourhood Plan

North Hinksey parish boundary B Draft 14.07.2017 Project C Final Issued to Client 31.07.2017 North Hinksey Groundwater Assessment

Main water courses consultants Site location Upton House tel +44 (0)1608 810374 Market Street, Charlbury fax +44 (0)1608 810093 Date Drawn Checked Scale Oxfordshire OX7 3PJ e-mail [email protected] 31.07.2017 DN/DJM CC 1:15,000 at A3 United Kingdom web www.gwp.uk.com Drawing Ref Drawing No Version GWP Consultants LLP. Registered No. OC326183. earth & water resources Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK HINKGW1707 1 C N

LEGEND Version Revision and compilation notes Date Client A Draft 13.07.2017 North Hinksey Parish Neighbourhood Plan

North Hinksey parish boundary B Draft 14.07.2017 Project C Final Issued to Client 31.07.2017 North Hinksey Groundwater Assessment

Main water courses consultants Aerial view of site location Upton House tel +44 (0)1608 810374 Market Street, Charlbury fax +44 (0)1608 810093 Date Drawn Checked Scale Oxfordshire OX7 3PJ e-mail [email protected] 31.07.2017 DN/DJM CC 1:15,000 at A3 United Kingdom web www.gwp.uk.com Drawing Ref Drawing No Version GWP Consultants LLP. Registered No. OC326183. earth & water resources Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK HINKGW1707 2 C 115 120 LEGEND LiDAR contours (5m intervals) Main water courses North Hinksey parish boundary

110 100

120 70

90 80

125 105

115

85 90

145

75 70

140 75 85

135 65

130 125

120 115

110

105 100

115 95

110 85

105 80

100 65 60

90 85

75 55 70 55

earth & water resources

eso Revision and compilation notes Version 55

B Final Issued to Client C

Draft A 65 60

Draft 55

United Kingdom Upton House Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK GWP Consultants LLP. Registered No. OC326183. Oxfordshire OX7 3PJ Market Street, Charlbury

55 55 consultants

e-mail 60

web 55

fax

tel +44 (0)1608 810093 +44 (0)1608 810374 55 www.gwp.uk.com [email protected] 14.07.2017 13.07.2017 31.07.2017 Date Drawing Ref Date Project Client 31.07.2017 LiDAR topography elevation contours (5m) North Hinksey Groundwater Assessment North Hinksey Parish Neighbourhood Plan HINKGW1707 Reproduced from the 2017 Ordnance Survey 1:10,000 map Unauthorised reproduction infringes Crown Copyright and Drawn with the permission of Controller Her Majesty's DN/DJM may lead to prosecution or civil proceedings N Licence No AL100018321 © Stationery Office, Crown Copyright Drawing No Checked CC 3 1:15,000 at A3 Scale Version C HYB KTON NO ALL N

OXWW NO Seacourt Stream

ALL

KTON

STFD OXWW

NO ALL NDR STFD

KTON AMKC

OXWW STFD

Reproduced from the 2017 Ordnance Survey 1:10,000 map LGS with the permission of the Controller of Her Majesty's Stationery Office,

Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings Licence No AL100018321

LEGEND GEOLOGICAL ABBREVIATIONS Version Revision and compilation notes Date Client A Draft 13.07.2017 North Hinksey Parish Neighbourhood Plan SUPERFICIAL DEPOSITS BEDROCK North Hinksey parish boundary B Draft 14.07.2017 Project MM Mass Movement LGS Lower Greensand Formation C Final Issued to Client 31.07.2017 North Hinksey Groundwater Assessment AG Artificial Ground AMKC Ampthill Clay Formation and Kimmeridge Formation ALL Alluvium STFD Stanford Formation Main water courses NDR Northern Drift Formation KTON Kingston Formation Extract of British Geological Survey geology map NO Northmoor Sand and Gravel Member HYB Hazelbury Bryan Formation consultants OXWW Oxford Clay Formation and West Walton Formation Upton House tel +44 (0)1608 810374 Market Street, Charlbury fax +44 (0)1608 810093 Date Drawn Checked Scale Oxfordshire OX7 3PJ e-mail [email protected] 31.07.2017 DN/DJM CC 1:15,000 at A3 United Kingdom web www.gwp.uk.com Drawing Ref Drawing No Version GWP Consultants LLP. Registered No. OC326183. earth & water resources Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK HINKGW1707 4 C ALL

Geological Abbreviations 445000E 450000E 455000E

MG Made or worked ground 210000N ALL Alluvium

FSG Fluvial sands and gravels

LGS Lower Greensand

PLPB Portland and Burbeck Beds

CR Corallian

FSG

205000N

ALL LGS CR

LEGEND Version Revision and compilation notes Date Client A Draft 13.07.2017 North Hinksey Parish Neighbourhood Plan Project North Hinksey parish boundary B Draft 14.07.2017 C Final Issued to Client 31.07.2017 North Hinksey Groundwater Assessment

Contours on the top of the Corallian (inferred in areas of sparse data) consultants Extract of British Geological Survey hydrogeological map Upton House tel +44 (0)1608 810374 Contours on the minimum potentiometric surface of Market Street, Charlbury fax +44 (0)1608 810093 Date Drawn Checked Scale Oxfordshire OX7 3PJ e-mail [email protected] 31.07.2017 DN/DJM CC 1:50,000 at A3 the Corallian in metres relative to Ordnance Datum United Kingdom web www.gwp.uk.com (inferred in areas of sparse data) Drawing Ref Drawing No Version GWP Consultants LLP. Registered No. OC326183. earth & water resources Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK HINKGW1707 5 C N

Reproduced from the 2017 Ordnance Survey 1:10,000 map with the permission of the Controller of Her Majesty's Stationery Office,

Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings Licence No AL100018321

LEGEND Version Revision and compilation notes Date Client A Draft 13.07.2017 North Hinksey Parish Neighbourhood Plan Groundwater bearing strata 10m North Hinksey parish boundary B Draft 14.07.2017 Project above Oxford Clay Formation C Final Issued to Client 31.07.2017 North Hinksey Groundwater Assessment Zone of potential groundwater flooding Oxford Clay geological contact - zone Main water courses of uncertainty (10m vertical) consultants Zone of potential groundwater flooding Upton House tel +44 (0)1608 810374 Geological contact between Kingston Market Street, Charlbury fax +44 (0)1608 810093 Date Drawn Checked Scale & Oxford Clay Formations and Oxfordshire OX7 3PJ e-mail [email protected] 31.07.2017 DN/DJM CC 1:15,000 at A3 United Kingdom web www.gwp.uk.com groundwater seepage contact Drawing Ref Drawing No Version GWP Consultants LLP. Registered No. OC326183. earth & water resources Registered Office: Upton House, Market Street, Charlbury, Oxfordshire OX7 3PJ. UK HINKGW1707 6 C