March 2017
BERKYN MANOR (POYLE) QUARRY (WESTERN EXTENSION)
Groundwater Flow Modelling to Assess Quarrying and Restoration Impacts
Submitted to: Peter Brett Associates Caversham Bridge House Waterman Place Reading RG1 8DN
Report Number 1656400.500/A.2 Distribution:
REPORT Peter Brett Associates - 1 copy (pdf) Golder Associates (UK) Ltd - 1 copy
BERKYN MANOR, POYLE (WESTERN EXTENSION) GROUNDWATER MODEL
Table of Contents
1.0 INTRODUCTION ...... 1
1.1 Background...... 1
1.2 Study Context ...... 1
1.3 Objectives ...... 1
2.0 SITE SETTING ...... 2
2.1 Site Location ...... 2
2.2 Geological Setting ...... 2
2.2.1 Regional Geology...... 2
2.2.2 Local Geology ...... 2
2.3 Hydrogeology...... 3
2.3.1 Regional Hydrogeology ...... 3
2.3.2 Groundwater Levels and Flow ...... 5
2.4 Hydrology ...... 8
3.0 MODEL SETUP ...... 10
3.1 Numerical Model ...... 10
3.2 Model Domain...... 10
3.3 Horizontal Grid ...... 10
3.4 Vertical Layering ...... 10
3.5 Top and Bottom Layer Elevation ...... 10
4.0 BOUNDARY CONDITIONS ...... 12
4.1 Southern Boundary ...... 13
4.2 Eastern Boundary ...... 13
4.3 Western Boundary ...... 14
4.4 Northern Boundary ...... 14
4.5 Internal Boundaries ...... 14
5.0 AQUIFER PROPERTIES ...... 16
5.1 Hydraulic Conductivity ...... 16
6.0 SOURCES AND SINKS ...... 19
6.1 Model Domain Internal River Boundaries ...... 19
6.2 Lakes ...... 19
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6.3 Groundwater Abstractions ...... 19
6.4 Drains ...... 20
6.5 Recharge ...... 21
6.6 Injection Wells (Recharge Trenches) ...... 22
7.0 MODEL CALIBRATION ...... 23
7.1 Calibration Results ...... 24
8.0 PREDICTIVE SCENARIOS: NO MITIGATION ...... 28
8.1 Scenario 1...... 28
8.2 Scenario 2...... 30
8.3 Scenario 3 ...... 31
8.4 Scenario 4 ...... 33
9.0 PREDICTIVE SCENARIO WITH MITIGATION ...... 35
9.1 Scenario 1A ...... 36
9.2 Scenario 2A ...... 37
9.3 Scenario 3A ...... 38
9.4 Scenario 4A ...... 39
10.0 SENSITIVITY ANALYSIS ...... 40
10.1 Hydraulic Conductivity ...... 40
10.2 Recharge ...... 40
10.3 River Bed Conductance ...... 41
11.0 CONCLUSIONS ...... 43
11.1 Effects of Dewatering – Without Mitigation ...... 43
11.2 Effect of De-watering - Proposed Mitigation ...... 43
12.0 REFERENCES ...... 45
TABLES Table 1: Site Geology ...... 3 Table 2: Groundwater Level Statistics ...... 7 Table 3: List of Borehole Logs accessed via BGS used for Model Base Modification...... 11 Table 4: Southern and Eastern Boundary Conditions ...... 13 Table 5: List of Boreholes used for Northern Boundary Conditions ...... 14 Table 6: The Details of the Rivers Making Up the Internal Model Boundaries...... 15 Table 7: Hydraulic Conductivity Pre-calibration Values for 2016 Poyle Quarry (Western Extension) Model...... 17 Table 8: Licensed Groundwater Abstraction Locations and Licensed Rates ...... 20
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Table 9: Recharge Zones and Values Used in the Pre calibration Model...... 22 Table 10: Review of Published Gravel Aquifer Hydraulic Conductivity Values ...... 23 Table 11: Post-Calibration Hydraulic Conductivity Values...... 24 Table 12: Post Calibration Effective Recharge Values used for 2016 Model...... 24 Table 13: Groundwater Level Calibration Details...... 25 Table 14: Sensitivity Analysis - Groundwater Head to Changes in Hydraulic Conductivity of the Gravel Aquifer ...... 40 Table 15: Sensitivity Analysis - Groundwater Head to Recharge ...... 41 Table 16: Sensitivity Analysis - River Bed Conductance ...... 41
FIGURES Figure 1: Regional Chalk Groundwater Contour Map, NERC 2003...... 4 Figure 2: Groundwater elevation and monthly rainfall at the Site, 2015-2016...... 5 Figure 3: Groundwater elevation and monthly rainfall at the Site, 2003 to 2004 ...... 6 Figure 4: Groundwater Levels, June 2016 ...... 8 Figure 5: Model boundary conditions, constant head boundaries are shown in maroon, river boundaries in blue...... 12 Figure 6: Location of historical and authorised landfill sites within the model domain (Environment Agency, 2016). Historical landfills are shown in pink and authorised landfills in brown...... 18 Figure 7: Final Model calibration results to June 2016 dataset: calibrated versus observed groundwater heads average (left) and June 2016 (right)...... 25 Figure 8: Regional calibrated groundwater heads (For detail in the Site vicinity, see Figure 9; Berkyn Manor Site shown in Blue) ...... 26 Figure 9: Local calibrated groundwater heads (June 2016 heads) (Berkyn Manor Site shown in Blue) ...... 27 Figure 10: Difference in groundwater head (shown as a red contours) between the base case scenario model and Scenario 1 model, predicted groundwater level shown in blue. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 29 Figure 11: Difference in groundwater head (shown as a red contours) between the base case scenario model including 2019 heads (shown as a blue contours) and Scenario 2 model. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 30 Figure 12: Difference in groundwater head (shown as a red contours) between the base case scenario model including 2020 predicted heads (shown as a blue contours) and Scenario 3 model. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 32 Figure 13: Groundwater differences (shown as drawdown) between base case model and scenario 4. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 33 Figure 14: The position of the recharge trench along the western tip and northern Site boundary...... 35 Figure 15: The position of the drainage trench (in green) along the northern and eastern Site boundary and recharge trench along the southern Site boundary (in blue)...... 36 Figure 16: Comparison between non-mitigated Scenario 1 (left) and mitigated Scenario 1A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 37
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Figure 17: Comparison between non-mitigated Scenario 2 (left) and mitigated Scenario 2A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown. Negative drawdown denotes a rise in groundwater level...... 38 Figure 18: Comparison between non-mitigated Scenario 3 (left) and mitigated Scenario 3A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 39 Figure 19: Comparison between non-mitigated Scenario 4 (left) and mitigated Scenario 4A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown...... 39 Figure 20: Head difference resulting in applying of pre and post calibration river bed permeability values...... 42
APPENDICES DRAWINGS Drawing 1 – Site Location Plan Drawing 2 – Geology Drawing 3 – Hydrology and Surface Water Gauging Stations APPENDIX A Borehole Logs APPENDIX B River Level Data
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1.0 INTRODUCTION 1.1 Background Peter Brett Associates (PBA) has commissioned Golder Associates (UK) Ltd (Golder) to complete a groundwater modelling study to quantify the hydrogeological impacts of the proposed development of the Berkyn Manor (Poyle) Quarry (referred to as ‘the Site’), and to evaluate the efficacy of the proposed groundwater management mitigation measures.
This hydrogeological modelling study for the Site is intended to support an Environmental Impact Assessment prepared as part of a Planning Application for quarrying and restoration. It is proposed to extract gravels at the Site in five phases commencing in the northern part of the Site and progressing southwards. Extraction in each phase will be followed by progressive restoration through backfilling with inert waste. 1.2 Study Context Planning permission for quarrying and landfill restoration for the Site has been granted previously, but lapsed in December 2015 as pre-development conditions had not been fully discharged and development had not proceeded. A hydrogeological impact assessment, including groundwater flow modelling, was undertaken in 2004 by WRc Plc on behalf of RMC (UK) Ltd to support the original planning application for the Site.
This current study was commissioned primarily because conditions surrounding the Site have changed since the original planning application was made. For the purpose of this study, Golder has been instructed to assume that there will be no changes to the previously submitted operational phasing plan.
The assessment aims to address the impacts that the gravel extraction is likely to have on close-by features during operation and backfilling, including the expected effects of changes in groundwater flow regime due to the continued gravel extraction directly west from the Site at Horton Brook Quarry, operated by Jayflex Construction Ltd (Jayflex), hereafter known as ‘the Horton Brook’ site.
Golder has prepared a number of numerical steady-state simulations to examine the gravel extraction and backfilling at various stages while incorporating the influence of the neighbouring Horton Brook site. A similar study has previously been completed for the projected development of the Horton Brook site. This study (Golder, 2007a) has been considered within the current work in so far as it presents an alternative conceptualisation of the area in comparison to the WRc (2004) study, and presents information on the design and projected performance of mitigation measures implemented at the Horton Brook site as described in Golder (2007b). 1.3 Objectives The overall objectives of the work were to develop a calibrated numerical model of the area around the Site to enable the following assessments: Quantification of the predicted magnitude and extent of drawdown as a result of the proposed quarry dewatering; Assessment of the impact on groundwater by backfilling with inert materials; Assessment of the impact on receptors including residential properties, nearby groundwater abstractions and the neighbouring extraction operation to the proposed development (both dewatering and restoration); and Evaluation of the performance of the proposed groundwater control measures.
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2.0 SITE SETTING 2.1 Site Location The Site is located between Colnbrook village to the north, Berkyn Manor Farm to the south and property owned by Thames Water (also known as the Eric Mortimer Rayner Memorial Lakes) to the southwest. The western Site perimeter is marked by a ditch separating it from the adjacent Horton Brook site. Further to the west and west-north-west lies the public water supply reservoir known as Queen Mother Reservoir. The neighbouring Wraysbury Reservoir is located approximately 600 m to the south west of the Site and has been designated since 1999 as a Site of Special Scientific Interest (Natural England, 2016).
The Site covers an area of approximately 21.5 ha, of which approximately 16.5 ha will be used for extraction. It is situated in a semi-rural area composed of lakes, streams, farmland and active and completed gravel quarries. The processing plant for extracted materials is located approximately 500 m east from the proposed area of extraction. The Site lies between the following coordinates: Easting: 501800 to 503300 and Northing: 175800 to 177100.
Topographic elevation at the Site ranges from 18.6 metres above Ordnance Datum (mAOD) in the south-west to 19.5 mAOD in the north-eastern corner. The current land-use is farmland.
The Site location and setting has been shown on Drawing 1. 2.2 Geological Setting 2.2.1 Regional Geology The geology of the shallow aquifer was described in the previous groundwater modelling report for the Site (WRc, 2004) and subsequent Environmental Impact Assessment (Wyn Thomas Gordon Lewis, 2004) to comprise primarily unconsolidated Quaternary river terrace gravels and alluvium deposits (Maidenhead Formation) of the River Thames and its tributaries. These are underlain by the Eocene London Clay Formation. Regionally, the study area is located within the northern limb of the London Basin syncline structure, in which London Clay lies unconformably over the Cretaceous Upper Chalk (Morgan-Jones et al,1984).
From the interpretation of the geological map (Drawing 2, BGS (2016a)) it can be seen that the river terrace deposits are the dominant regional surficial lithology and cover the majority of the model domain area. The Shepperton Gravel deposits, identified in the central and south eastern portion of the domain area, are irregularly overlain by younger superficial river alluvium (reported to be predominantly silt, sandy silty and gravelly clay) within the eastern portion of the model domain area and north from King George VI Reservoir.
Sediments of the older Taplow Gravel terrace are present mainly in the northern portion of the domain area and are unconformably overlain by alluvial deposits containing clay and silt (known as Langley Silt Member) present beyond the Slough Arm canal (north of which the northern model boundary has been set). These take the form of shallow channels or floodplain deposits, and are generally thin deposits.
Morgan-Jones et al (1984) indicated that the superficial deposits in this region occur essentially in horizontal layers as terraces at various elevations. The top/subsoil in the area is of varying thickness, and generally found to be up to 0.2 m thick in the available Site borehole logs. Regionally, the thickness of the gravels ranges from 0 m to 12 m in the area, according to borehole logs accessed via British Geological Survey’s (BGS) Geoindex Onshore service (BGS, 2016b). 2.2.2 Local Geology It is interpreted from borehole logs examined by Golder that, while the elevation of London Clay increases from approximately 9.8 mAOD the near the River Thames to approximately 28 mAOD along the Slough Arm canal near the northern model boundary, there is no indication of a significant reduction of gravel thickness in this direction.
The variations that occur throughout the region appear to be independent of present day topographic control, and is considered to primarily reflect the irregular erosion of London Clay base and to lesser extent, irregular erosion and variable thickness of drift deposits. It is also concluded that Quaternary erosion processes have
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not exposed London Clay within the study area, but it is considered the London Clay is present at shallow depths in north-eastern corner of the model area. It is inferred that the shallow clay impedes groundwater flow, particularly in the transition area between River Thames flood terrace and first terrace to the north from northern Site boundary where the top of the London Clay is above the groundwater level.
The Site stratigraphy has been confirmed by the evaluation of 9 boreholes drilled in 2003. The borehole logs for these boreholes are included in Appendix A. The typical geological succession observed on the Site is presented in Table 1. Table 1: Site Geology Thickness Description (m) Topsoil Gravelly topsoil* 0.0 – 0.2 Subsoil Dark grey, sandy, gravelly clay* 0.8 – 1.80 Quaternary Gravels Fine to coarse angular gravel, partly made up of flint, with coarse 1.9 – 3.3 (river terrace deposits) sand at some locations, medium dense to dense, orange-brown* Stiff to firm grey silty clay, occasionally brown at the top of the unit and multi-coloured at depth, with occasional thin horizons of weak London Clay > 45.9** grey claystone and pebbly at depth; one 3 m thick fine sand layer encountered at 42 m below ground level in one location** * soil description taken from borehole logs drilled for RMC Aggregates 2003 (provided by the client) ** taken from Golder Associates (2007a) *** The base of London Clay has not been proved in any boreholes drilled on Site. 3 out of 19 boreholes advanced into London Clay to ~50 m below ground in adjacent Horton Brook site (information given in 2004 WRc report).
Given the nature of sedimentation processes that led to deposition of soils within the shallow subsurface, it is considered that the lithologies encountered at the Site, as summarised in Table 1, are likely to exhibit a relatively high degree of uniformity with regard to their properties and anisotropy and some variation with regard to thickness. The maximum combined thickness of top and sub-soils at the site could be up to 2.0 m, but is generally approximately 1.5 m in thickness. 2.3 Hydrogeology 2.3.1 Regional Hydrogeology Given the geological and topographical Site setting, along with an interpretation of typical groundwater table elevations observed on Site and illustrated in Figure 2, the aquifer system has been characterised as a uniform, unconfined single-layer unit receiving recharge from the top surface and discharging mainly into the River Thames.
Underlying the unconsolidated Quaternary drift deposits is the London Clay, which is considered to act as an aquiclude to the gravel aquifer. The marginal sections of the gravel aquifer toward the perimeter of the gravel deposits (located approximately 15 to 20 km north and west from the Site and along Misbourne and Alder Bourne valleys) are reported to be directly underlain by the Chalk and assumed to be in hydraulic continuity with the underlying aquifer (Morgan-Jones et al, 1984). The Chalk aquifer is reported to form an important contribution to river baseflow (NERC, 2003).
NERC (2003) presented groundwater contours for the Chalk aquifer, which are reproduced in Figure 1. This suggests that on the broad regional scale groundwater flow direction is mainly controlled by the River Thames and indicates the direction of the flow to be perpendicular to the River Thames.
Based on the regional interpretation, in the model domain area, groundwater flow in the Quaternary deposits is assumed to be occurring in a general direction from north to south. Groundwater contours exhibit some degree of deviation from the inferred regional groundwater head distribution in proximity to the outcrops of the Langley Silt Member (Brickearth) overlying London Clay in the northern portion of the Site domain.
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It is inferred that the presence of Queen Mother Reservoir locally diverts groundwater flow in the gravel aquifer. The general groundwater head contours and flow path lines (in the Chalk aquifer) are shown on BGS map presented on Figure 1. The groundwater head gradients measured at the Site (Golder, 2004) and the adjacent Cemex Site (WRc, 2004) suggest that groundwater enters the region from the north, local scale groundwater flow based on more recent data is discussed below.
Site
Figure 1: Regional Chalk Groundwater Contour Map, NERC 2003.
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2.3.2 Groundwater Levels and Flow Data for groundwater levels measured in monitoring wells installed in the gravel aquifer at the Site was supplied by Summerleaze Ltd. Groundwater level data for the adjacent Horton Brook site was also obtained from the Environment Agency.
Groundwater elevations at the Site for the 2015 and 2016 period are shown on Figure 2 and for 2003 and 2004 in Figure 3. Well locations are shown in Figure 4. It is considered likely that groundwater heads measured in the recent period in monitoring wells WG06, WG07 and GW08 have been affected by the dewatering taking place at the adjacent Horton Brook site as levels in these wells show a marked recovery in 2016. Groundwater levels in 2015 and 2016 in wells on the east of the Site (WG01 to WG03) are generally approximately 1 m lower across the Site than in 2003/2004. It is considered probable that this change is largely due to short term climate fluctuation and associated river level changes, as it is unlikely that dewatering at the Horton Brook site would have unilaterally affected wells to the east of the Site.
The historical groundwater elevation data indicate that seasonal groundwater fluctuations are of approximately 0.5 m in magnitude. Comparison of groundwater fluctuations and monthly rainfall (Figure 2) suggests that seasonal change in rainfall has limited impact on groundwater fluctuations at the Site. It is proposed that groundwater levels in the gravel aquifer are in part controlled by fluctuations in river levels in the major water courses bounding the aquifer.
Figure 2: Groundwater elevation and monthly rainfall at the Site, 2015-2016.
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Figure 3: Groundwater elevation and monthly rainfall at the Site, 2003 to 2004
Groundwater level summary statistics for the adjacent Horton Brook site are presented in Table 2.
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Table 2: Groundwater Level Statistics Groundwater Level (m AOD) June 2016* Borehole Average min max (m AOD) (2014-2016) Berkyn Manor (Poyle) Site (Oct 2015 to June 2016) WG01 17.97 18.10 17.80 17.92 WG02 17.85 17.97 17.61 17.82 WG03 18.24 18.37 18.04 18.20 WG04 17.67 17.83 17.16 17.83 WG06 16.77 17.08 16.17 17.08 WG07 16.76 17.57 16.29 17.57 WG08 16.66 17.72 15.63 17.72 Horton Brook Site (Feb 2014 to Jul 2015) BH1 17.06 16.59 17.65 17.4 BH2 16.27 15.60 17.21 17.41 BH3 15.99 15.05 17.47 16.5 BH4 17.13 16.24 18.00 17.41 BH10 17.74 17.14 18.38 17.71 BH12 16.92 16.42 18.00 17.42 BH15 17.71 17.37 18.18 17.93 BH16 17.76 16.69 18.27 18.33
It is noted that, whilst the groundwater level fluctuations are seen to be greater on Horton Brook site, the most recent groundwater elevation data from June 2016 appears to be largely comparable with 2014 to 2016 averaged dataset available for both monitored sites. This is found to be within 0.5 m difference for most instances. The greatest discrepancies are observed in WG07, WG08 along the western Site perimeter (eastern Horton Brook site boundary) i.e. areas subjected to impact from dewatering activities and subsequent reinjection of pumped water through recharge ditch resulting from ongoing gravel extraction at that site.
Groundwater contours for June 2016 for both the Site and the adjacent Horton Brook site are shown on Figure 4 below. Groundwater contours indicate that groundwater flow in the Site vicinity occurs from the northeast to the southwest, toward the River Thames. This is consistent with the groundwater flow direction within the Berkyn Manor site based on 2003/2004 monitoring data, which also indicates flow to the southwest.
Current groundwater levels in the vicinity of the Horton Brook site are likely to be influenced by dewatering operations and this may be influencing local groundwater flow directions. Based on planned phasing (Golder, 2007a), it is assumed that dewatering is currently occurring in Phase 9 of the Horton Brook site.
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Figure 4: Groundwater Levels, June 2016 2.4 Hydrology The River Thames to the south and River Colne to the east of the Site are important watercourses that are likely to interact with the aquifer. Other watercourses present within the model domain i.e. Colne Brook, Poyle Channel and Wraysbury River are considered likely to have bed sediments that exhibit resistance in regards to interactions between surface water and groundwater. This is inferred from direct comparison of groundwater elevations in monitoring boreholes along the eastern Site perimeter (i.e. not likely to be impacted by dewatering activities at Horton Brook site) with nearby surface water levels obtained from the Site Topographical Survey drawing (SLR, October 2015), which indicates a significant difference in between river levels and groundwater levels.
A river stage of 18.96 m AOD was measured approximately 70 m to the north from the monitoring well WG03, in which groundwater level was measured concurrently to be 18.04 m AOD. Similarly, groundwater elevation was reported to be 17.61 m AOD in WG02, whilst the stage at the nearby surface water gauging point was reported as 18.94 m AOD for a corresponding time period. This trend is observed to be consistent in historical data available for this assessment.
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On average, surface water levels are found to be approximately 1.1 m above the groundwater elevation in surrounding monitoring wells, suggesting that the relationship between river and groundwater levels in this area is approximately constant and does not vary seasonally. It is considered that surface water exhibits reduced hydraulic connection to underlying groundwater either by low permeability bed sediments or because the river bed rests on the superficial clay deposits within the drift sequence. As a consequence, the surface waters local to the Site area are considered to contribute to groundwater recharge but have limited influence in controlling groundwater levels. It is noted that the earlier study by WRc (2004) assumed that the groundwater system receives recharge mainly through rainfall.
The Site is located in the Western London Reservoir Complex with two reservoirs, the Queen Mother Reservoir and the Wraysbury Reservoir, situated inside the model domain. These have very little connection to the aquifer since they are isolated from the surrounding gravel aquifer by engineered clay bunds (Pawsey and Humphreys, 1976) through which negligible flow is likely to occur. The clay bunds are keyed into the underlying London Clay and have a typical maximum design permeability of 1 x 10-10 m/s (Golder, 2007). The water level in the reservoir varies throughout the year but is assumed to be consistently higher than the surrounding groundwater.
The gravel aquifer is considered to directly interact with the lakes (flooded gravel pits) which are frequent across the modelled area. These lakes have resulted from restoration to open surface waters in previously quarried areas, and hence are considered to be with good connection to the aquifer. It is assumed that these lakes generally extend to the bottom of the gravel layer.
The hydrological features of the study along with the surface water gauging stations from which surface water elevation were derived from are shown in Drawing 3.
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3.0 MODEL SETUP 3.1 Numerical Model The model has been designed on the basis that the system response can be adequately represented by a steady state simulation and that the aquifer is unconfined and can be simplified to one layer. This is consistent with the previous model (WRc, 2004).
The industry standard numerical finite difference groundwater flow model MODFLOW has been used for setting up the initial baseline model and development of predictive scenarios, with use of Visual MODFLOW (Waterloo Hydrogeologic, 2016) as the pre and post processor. 3.2 Model Domain The model domain covers an area of approximately 10 km by 12 km (NGR 497000 170000 to NGR 507000 182000), of which the central area (approximately 7 km by 9 km) contains active cells (Figure 5). The domain has been set at a regional scale to encompass regional flow, and was positioned so the refined portion of the grid was centralised on the domain. The grid is aligned in a north to south orientation, and the orientation of the grid also is therefore largely aligned to the principal direction of regional groundwater flow.
Groundwater control measures at the Horton Brook site (assumed to be as proposed in Golder (2007b)) were also taken into consideration in the development of boundary conditions, which are unaffected by the groundwater abstractions and associated mitigation measures present within the model domain.
It should be noted that the Wraysbury and Queen Mother Reservoirs have been defined as zones of inactive flow, i.e. the surface area covered by the reservoirs is excluded from calculation. This is because the single layer aquifer represented by the model is physically absent in the area of the reservoirs, and the lakes are isolated from the aquifer by marginal clay bunds (Pawsey and Humphreys, 1976). 3.3 Horizontal Grid The total model domain has been discretised into a finite difference grid with 414 rows and 438 columns. The largest cells are 50 m by 50 m in areas of the model distant from the site, and the smallest cells of 5 m by 5 m were positioned to contain the Site and the Horton Brook site at the centre of the domain. High spatial resolution was required at the sites to examine the details of the impact from the current operations at the Horton Brook site, including the effects of dewatering, the influence of backfilling previously quarried phases, and the operation of the existing drainage system. 3.4 Vertical Layering Conceptually, based on review of the regional geology, it is evident that one layer is considered to be an appropriate representation of the gravel aquifer system. Due to the high hydraulic conductivity and relative thickness of the gravels as well as typical groundwater level and its natural fluctuations, the existing top- and sub-soils are not considered to influence groundwater flow. 3.5 Top and Bottom Layer Elevation Surfaces that define the vertical extent of the aquifer are required for the correct functioning of the model. The aquifer thickness is defined by the top and basal surfaces defined in the model, and elevation gradients within the aquifer are defined by these surfaces. Topographic data was obtained from LIDAR Composite Digital Terrain Model published by Environmental Agency (2014) as a 2 m spatial resolution grid to accurately represent the ground level at and around the Site.
Since the gravel aquifer is underlain by London Clay, the model base (base of the gravel layer) for the entire model domain was taken from the London Clay surface indicated on the 1:50,000 geological map (solid and drift version) of the area (BGS, 1999). Some alterations and refinements to the basal elevation of Quaternary deposits were made. It was considered important, in particular, due to regional geology to represent variation in the top elevation of London Clay in the northern portion of the modelled area. This was achieved through a review of historical boreholes accessed via BGS Onshore Geoindex (BGS, 2016b). The borehole logs reviewed and used as the basis of the top of London Clay surface modifications are listed in Table 3.
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Table 3: List of Borehole Logs accessed via BGS used for Model Base Modification.
BGS borehole number Inferred depth of London Clay top (m AOD)
TQ07NW306 20 TQ08SW20 29.5 TQ08SW11 22 TQ08SW10 20 TQ08SW15 28.6 TQ08SW42 30 TQ08SW9 30.5 TQ08SW40 36.6 TQ08SW45 29 TQ07NW129 21.7 TQ08SW37 23.3 TQ08SW269 32 TQ08SW107 28.12 TQ08SW383 29.46 TQ08SW26 23.3 TQ07NW652 19.4 TQ07NW287 16.9 TQ07NE365 20.9 TQ07NE366 22 TQ08SE11 26 TQ08SE186 25.8 *Based on the 2016 BGS Geoindex Onshore on line platform.
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4.0 BOUNDARY CONDITIONS Boundary conditions were chosen to be analogous to the previous WRc model (2004) and are set at natural hydraulic features defining the model’s lateral extent. The boundary conditions selected for this modelling are described below and are shown on Figure 5.
Figure 5: Model boundary conditions, constant head boundaries are shown in maroon, river boundaries in blue.
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4.1 Southern Boundary The southern boundary has been set as river cells along the River Thames. The river gauging station elevations were obtained from the Flood Warning Information service (2016) administrated by the Environment Agency. Gauging station elevations relevant for the Windsor Park and Bell Weir stations were taken as the typical low (averaged low) values for a long term (three year) monitoring timeframe available (River Level and Flood Warnings, 2016).
A linear gradient was applied between the initial point (at Windsor) and final point (at Bell Weir). The width of the river was estimated from aerial imagery. The depth and river bed thicknesses were established based on available publications relating to River Thames modelling (Tomes et al, 2006).
The Sediment Impact Analysis (Tomes et al, 2006) indicates that the River Thames bed material is generally dominated by fine-medium gravel (70-95%) and medium coarse sand (5-25%), although occasionally some samples analysed returned fine-medium sand to coarse gravel dominated gradation curves. Since the Thames as a major river is generally considered to be in relatively good connection with the aquifer, the vertical permeability of the river bed sediments was chosen to be roughly one order of magnitude lower than the aquifer permeability.
The river’s width, depth and vertical permeability of the river bed sediments were used to automatically determine the conductance of the river in Visual MODFLOW. The details of the southern and eastern model boundary are summarised in Table 4. Table 4: Southern and Eastern Boundary Conditions River Bed Initial Final Initial Final Width Depth Vertical Stage* Stage* NGR NGR (m) (m) Permeability (m AOD) (m/s) (m AOD) River Thames 498066, 501759, 80 7 1 x 10-4 16.22 14.46 (Windsor Park) 177159 172020 River Thames 501759, 503577, 80 7 1 x 10-4 14.46 12.70 (Bell Weir) 172020 171304 River Colne at Iver 504908, 505399, 10 2 1 x 10-4 27.27 24.39 (Section 1) 182204 179083 River Colne at 505399, 504187, West Drayton 10 2 1 x 10-4 24.39 20.21 179083 175205 (Section 2) River Colne at 504187, 503514, Stanwell Mill 10 2 1 x 10-4 20.21 14.91 175205 172356 (Section 3) River Colne at Staines Trading 503514, 503461, 10 2 1 x 10-4 14.91 14.05 Estate 172356 171982 (Section 4) River Colne at 503461, 503305, Thames River 10 2 1 x 10-4 14.05 13.03 171982 171479 (Section 5) * Based on typical low level (Jan 2013 to Jul 2016) 4.2 Eastern Boundary The eastern boundary is assigned along the River Colne running from West Drayton to the confluence with the River Thames. The surface water levels were obtained from Flood Information Service (2016). The width was also estimated by measurement of the aerial imagery. As with the southern boundary, the conductance of the river was calculated from the depth, width and riverbed sediment permeability. The details of the eastern boundary are summarised in Table 4.
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4.3 Western Boundary The western boundary is defined as a no flow boundary and runs from the northern end of the River Thames (which forms the southern boundary) to the northern boundary. The location and type of this boundary is the same as that used in previous modelling studies for the area and it is considered appropriate given the regional groundwater contours for the area. 4.4 Northern Boundary The northern boundary was set as a constant head boundary, with the groundwater head set on 23 m AOD. Head estimates for the constant heads were identified from borehole logs, accessed via the BGS Onshore Geoindex (BGS, 2016b). The list of boreholes, which the groundwater contour map for the northern part of the model domain was completed, is presented within the Table 5. This data was cross checked against groundwater contour map included in NERC (2003) Baseline Report shown on Figure 1 and groundwater contour map included in Hydrogeological Effects of Gravel Winnings report (1984) to inform the degree of uncertainty. Table 5: List of Boreholes used for Northern Boundary Conditions
Borehole Scan Number Borehole Northing Borehole Easting
TQ08SW11 180490 500190 SU98SE72 180330 499520 SU98SE51 180370 499360 SU98SE121 180320 499160 TQ08SW420 180170 501600 TQ07NW591 179520 502830 TQ07NE184 178443 505252 *Based on the 2016 BGS Geoindex Onshore on line platform.
The boundary was positioned north from the western extension of the Grand Union Canal. It should be highlighted that the exact relationship between the Grand Union Canal and the aquifer remain unknown, but canals in this area were generally lined with puddle clay. Therefore, the canal itself has not been incorporated in the model as a hydraulic boundary and is assumed to have no effect. Given the distance of the canal from the area of interest, this assumption is considered to have minimal influence on the model predictions in the area of interest.
The application of a constant head boundary permits the model to generate an effectively infinite supply of water from the north. The use of constant heads in models can cause unrealistic solutions by allowing large volumes of water to enter or exit the model with no limit. To address this issue, estimates of flow volumes across the northern boundary have been calculated using Darcy’s law and compared to the volumes reported by the constant head boundary to ensure that the flows reported as realistic. In this way, the calculations undertaken by MODFLOW has been verified and validated. 4.5 Internal Boundaries Based on the conceptualisation developed, the reservoirs inside the model domain have virtually no connection to the aquifer (Section 2.4), the model cells in the extent of the Wraysbury and Queen Mother Reservoir have been assigned as inactive. The model therefore does not permit flow through the reservoir area and the reservoir perimeter forms a no-flow boundary.
According to conceptualisation of the geological setting of the area (in particular lithology of the top soils) as well as interpretation of interactions between surface water and groundwater, it is considered that typically low permeabilities are appropriate to represent the beds of internal rivers in proximity to the Site. It is also assumed that the permeability of the river beds increases towards major water courses i.e. River Thames and River Colne as these are understood to be providing drainage to the gravel aquifer via river baseflow.
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In proximity to the Site vertical permeability of the River Colne bed was estimated to be at least 1x10-7 m/s and 1x10-6 m/s for Wraysbury River, whereas sections of the rivers directly adjacent to the main rivers to reflect river bed permeability of the rivers providing the drainage i.e. 1x10-5 m/s.
The details including parametrization of the model domain internal surface water features are summarized in the Table 6 below; source data is presented in Appendix B (used in conjunction with gauge elevations from the Flood Warning Information Service). The Flood Warning Information Services gauge locations, Site gauge location, and water courses described are shown in Drawing 3. Table 6: The Details of the Rivers Making Up the Internal Model Boundaries.
River Bed Initial Final Initial Width Depth Vertical Final NGR Stage Stage NGR (m) (m) Permeability (m/s) (m AOD) (m AOD) Colne Brook Section 1 504228, 503362, -5 (from the model boundary 10 1.5 24.00* 21.22 178473 177413 1 x 10 to Colne Brook at the A4 at Colnbrook) Colne Brook Section 2 503362, 502540, (from Colne Brook at the 10 1 1x10-7 21.22 18.94** 177413 176400 A4 at Colnbrook to Site gauging station) Colne Brook Section 3 502540, 501862, -7 10 4 18.94** 14.70* (from Site gauging station 176400 172466 1 x 10 to Hythe Weir) Colne Brook Section 4 501862, 501905, -5 (from Colne Brook at Hythe 10 4 14.70* 14.50* 172466 171993 1 x 10 Weir to confluence at River Thames) Wraysbury River Section 1 503969, 503268, -6 (from the confluence to 10 4 21.15* 14.49* 176502 171831 1 x 10 River Colne to Wraysbury River at Staines Moor) Wraysbury River Section2 503268, 503278, -6 (from Wraysbury River at 10 4 14.49* 13.00* 171831 171539 1 x 10 Staines Moor to confluence with River Thames) Poyle Channel Section 1 504979, 503250, -7 (from the confluence with 10 4 23.00* 20.52* 177830 176423 1 x 10 River Colne to Poyle channel at Poyle station) Poyle Channel Section 2 503250, 502540, -7 (from Poyle channel at 10 4 20.52* 18.94** 176423 176400 1 x 10 Poyle station to Site surface gauging station) * Based on typical low level (Jan 2013 to Jul 2016) ** River water levels were obtained from site topographical survey map (October 2015, SLR) provided by the client.
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5.0 AQUIFER PROPERTIES 5.1 Hydraulic Conductivity The gravel aquifer within the study area has received relatively little attention in published literature, and as a consequence, limited peer-reviewed published information is available regarding the hydrogeological characteristics of this formation.
The hydraulic conductivities obtained from the slug-tests conducted at Horton Brook site by Golder (2007) ranged from 1.5 x 10-4 m/s to 1 x 10-3 m/s. As a pre-calibration initial value for the gravel aquifer hydraulic conductivity, the mean value of 4.9 x 10-4 m/s derived from a number of slug tests was used.
The Langley Silt Member, as discussed in Section 2.2, primarily consists of silt and clay and is often referred as a Brickearth. This unit is shown on the geological map (Drawing 2) and is dominant in the north-eastern portion of the model domain. Based on the review of the boreholes drilled into the Langley Silt Member (BGS Geoindex, 2016), it was concluded that this formation largely directly overlies the London Clay formation. Given the lithological characteristics of this formation, it is considered to exhibit a hydraulic conductivity intermediate between the gravel terraces and London Clay. The hydraulic conductivity was therefore adjusted in the area where Langley Silt has been mapped and value of 1 x 10-6 m/s was assigned accordingly.
There are numerous historical landfill sites resulting from previous gravel extraction present within the model area (Figure 6). These have been used for the disposal of a range of materials including inert waste once the gravel working ceased. Based on the conceptualisation presented in the published report regarding Kingsmead landfill (Arup, 2014), the hydraulic conductivity of the inert waste was estimated to be in the order -7 of 7.30 x 10 m/s.
In areas of the adjacent Horton Brook site, where placement of inert waste has occurred, then a value of -7 1 x 10 m/s was applied as a first order estimate for model calibration (minimum requirement for lining for inert waste under the Landfill Directive (1999/31/EC)).
The starting values for hydraulic conductivities used in the model pre-calibration process are summarised in Table 7.
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Table 7: Hydraulic Conductivity Pre-calibration Values for 2016 Poyle Quarry (Western Extension) Model. Horizontal Vertical Hydraulic Hydraulic Zone Description Rationale/Source Conductivity Conductivity (m/s) (m/s) River Terrace Includes Shepperton 4.9 x 10-4 4.9 x 10-4 Adopted for calibration Gravels and Taplow Terrace based on the mean of slug gravel deposits. test results. -6 -6 Langley Silt Also referred as a 1.0 x 10 1.00 x 10 Assumed for calibration Member Brickearth (interpreted based on typical lithology. from BGS Geoindex website). Present at the central portion of the Northern Site boundary. Reported to consist primarily of clay and silt material. -7 -7 Inert backfill Used for Horton Brook 1 x 10 1 x 10 Assumed for calibration, (engineered site based on Landfill Directive landfill site) requirements for inert waste. -7 -7 Inert backfill Used for Historical and 7.30 x 10 7.30 x 10 Assumed for calibration (for historical Authorised Landfills based on hydraulic landfill sites) listed by Environment conductivity used for Agency website withine Landfill Hydrogeological the Site domain. Risk Assessment of former gravel extraction site within the study area (Arup, 2014).
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The location of historical and authorised landfills is shown on the Figure 6.
Kingsmead Landfill
Figure 6: Location of historical and authorised landfill sites within the model domain (Environment Agency, 2016). Historical landfills are shown in pink and authorised landfills in brown.
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6.0 SOURCES AND SINKS 6.1 Model Domain Internal River Boundaries Since surface elevation data is available for dedicated gauging stations for internal rivers/drains, linear approximation was used to calculate the gradient and assign heads to the Colne Brook, Poyle Channel, and Wraysbury River. All watercourses are considered likely to interact with the aquifer, but it is interpreted that the influence of these watercourses (whether they are gaining or losing to groundwater) varies across the Site and regional area.
Watercourses running in close proximity to the Site are considered to encounter resistance in the interactions with groundwater due to the presence of clays in the shallow superficial geology. This results in a head difference observed between the river and the groundwater levels. River levels are locally above groundwater levels, and the rivers are therefore interpreted to recharge the gravel aquifer i.e. provide additional water to the system. This conceptualisation is represented in the model by assigning low conductance for the sections of the rivers for which historical borehole logs indicated were underlain by an approximately 1.0 m thick clay layer clay close to surface. The river dimensions and the vertical hydraulic conductivity of the river bed sediments were used to calculate the assigned river bed conductance. It was considered the internal rivers indicated that both have the same approximate dimensions as the River Colne.
Given that the presented data relating to groundwater heads and detailed surface water elevation from the recent topographic survey exhibit no uncertainty, no adjustment to the surface water level was made. The details of the simulation parameters used to describe the internal rivers are detailed in Table 6. 6.2 Lakes Surface water elevations are not available for most of the open waters features present within the model domain. Therefore, these have been defined as evapotranspiration cells to include potentially significant amount of water that is being removed from the system by direct evaporation. Lakes formed by gravel extraction are considered likely to represent ‘groundwater outcrops’ with a limited role in groundwater recharge or discharge.
Site survey data provided by the client includes surface water elevation details for two lakes resulting from previous gravel extraction at the Poyle site. Since the lakes are considered as being in direct contact with the aquifer, the conductivity of the lake sediments was set to be equal to the hydraulic conductivity of the aquifer, and the water levels in the lakes were set according to surveying data. 6.3 Groundwater Abstractions Data for groundwater abstractions within a radius of 3 km of the Site was obtained from the Environment Agency. Abstractions from the Chalk aquifer and those in the shallow aquifer that are located beyond the model domain, were not included in the model.
Four existing licensed groundwater abstractions within a radius of 3 km of the Site have been included in the model. These are situated to the north north-east and south of the Site. Details of the abstractions are included in Table 8 and their locations are shown on Figure 5.
The data obtained for the abstraction points states the maximum allowed abstraction rate/annum granted under the licence. It is considered unlikely that maximum allowed abstraction rates has been routinely utilised for these abstraction wells; therefore, an abstraction at 75% of the maximum permitted abstraction rate has been applied within the model.
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Table 8: Licensed Groundwater Abstraction Locations and Licensed Rates Maximum Licensed Abstraction Rate used NGR Abstraction Rate in Model (m3/d) (m3/d) Brett Aggregates Limited 503440 174480 5000 3750* Licence No.: 28/39/28/0067 Rayner & Sons Ltd, Berkyn Manor (Borehole A Horton Brook 501900, 175900 10.92** Farm) Licence No.: 28/39/28/0204 27.3 Rayner & Sons Ltd, Berkyn Manor 503526,178127 16.38** (Borehole B Colnbrook Farm) Licence No.: 28/39/28/0204 0 (It is assumed water is Cemex UK Materials Ltd, not being abstracted 502630, 176660 2500 Poyle Quarry since the Site is not currently operational) 2500 (gravel 1875* Cemex UK Materials Ltd, extraction) Kingsmead Quarry 501805, 175325 1957 82 (concrete Licence No.: TH/039/0028/010 82* production) * Taken as 75% of maximum licence permitted abstraction rate. ** According to the pumping regime indicated in abstraction licence.
For the purposes of modelling, it is assumed that water removed by licensed abstractions is lost to the groundwater system, and if returned within the model domain, will be returned to surface water courses with relatively little groundwater interaction.
Dewatering at the Horton Brook site does not occur under an abstraction licence and is represented via a drain boundary at a fixed elevation in the model. This abstraction is described in Section 6.4. 6.4 Drains A drain boundary was used to simulate the removal of water in the Horton Brook site model by a groundwater interception ditch at the northern Site boundary. The drain was set at a depth of between 17.30 m AOD and 14.497 m AOD as indicated in Golder (2007a). Installation of this interception ditch (drain) was designed to control the groundwater levels at the perimeter of the Horton Brook site and act as a point of discharge for water being extracted to facilitate gravel workings in the active phase.
For the purpose of the developing predictive scenario for the Site, the drain along the southern part of the western and eastern Horton Brook site perimeter was not included for two reasons: Firstly, because the drain is being used simultaneously for groundwater control and groundwater reinjection and the location of the transition between these conditions is uncertain; and Secondly, to ensure that a fixed head boundary along the margin of the Horton Brook site would not exert excessive control on the propagation of drawdown in the direction of the Site subject to modelling.
Drain boundaries permit discharge from the model domain where groundwater levels reach the elevation of the drain, but do not permit inflow to the model domain.
A drain was also used in the model to simulate the groundwater dewatering process within the currently worked extraction phase at Horton Brook site by fixing head at the approximate level of the bottom of the gravel layer.
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6.5 Recharge The aquifer has been simulated as an unconfined system with capacity to receive recharge from the surface. Since the topography within the Site does not indicate significant potential for runoff, and groundwater fluctuations are observed to be minor (approximately 0.5 m) and are not responsive to rainfall patterns, it is considered that only a small proportion of the potential groundwater recharge available percolates though the shallower layers of superficial deposits to reach the unconfined aquifer.
An examination of borehole logs for the model domain indicates the presence of shallow clay horizons within the drift sequence overlying the primary gravel aquifer. These low permeability deposits are considered to limit recharge to the underlying aquifer.
A number of sources have been used to guide the definition and calibration of the recharge value in the model: WRc (2004) proposed the recharge to be distributed based on the geology of superficial materials. Adopted recharge values are reported to be ranging from 36.5 mm/year assigned to flood plain gravel aquifer to 1.46 mm/year for alluvial materials. Golder (2007a) incorporated the calculation of effective annual rainfall recharge taking into account potential evapotranspiration values for the study area. Following minor correction for calibration, the recharge value of 150 mm/year was accepted by the Environment Agency. ESI (2014) presents a Groundwater Risk Assessment report for a site of similar hydrogeological characteristics located west of the Queen Mother Reservoir. The report identifies that the Environment Agency estimation of effective rainfall is generally much higher than the hydrologically effective rainfall calculated by MORECS. Effective recharge estimations by the two methods (Environment Agency and MORECS) were 214 mm/year and 87.5 mm/year respectively (ESI, 2014).
The recharge provided by Environment Agency is reportedly derived based on average rainfall (assumed 603.4 mm/year based on Averaged Met Office rainfall data), catchment area of 8,120 km2 and mean flow data from a gauging station on the Thames at Staines at 54.99 m3/s. It appears however it is likely to be overestimated, because infiltration intercepted by shallow clays in the superficial cover is likely to be directed to surface water and therefore would report in the balance calculated by the Environment Agency, without having passed through the sand and gravel aquifer.
Based on the sources considered and conceptualisation of the system, an average of 160 mm/year of recharge was applied to the model for calibration. This value was selected as a compromise between 214 mm/year suggested by the Environment Agency, 87.65 mm/year resulted from MORECS calculations (ESI, 2014) and 150 mm/year used by the Golder (2007a).
Recharge values for the area covered by alluvium deposits were initially assigned to be 170 mm/year i.e. slightly higher than flood plain gravel to the west pertaining presence of thinner layer of low conductivity materials overlying the gravel aquifer.
Effective recharge was reduced for Langley Silt outcrops within the model area due to high silt and clay content of this geologic formation. As per pre-calibration value stage, 10% of the initial average recharge was applied i.e. 16 mm/year.
Recharge rates were also reduced for the historical landfill sites present within the model area to 40 mm/year contributing 25% of the averaged recharge estimated.
The recharge values assigned to the different zones are summarised in Table 9.
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Table 9: Recharge Zones and Values Used in the Pre calibration Model. Zone Recharge (mm/yr) River Terrace Deposits 160 Alluvium 170 Langley Silt Member 16 London Clay 1** Backfilled and capped Horton Brook quarry 5* Historical landfills 40 *Golder, 2007a, post-calibration value. **Pre-calibration estimate based on hydraulic properties
6.6 Injection Wells (Recharge Trenches) Lines of injection wells were used to simulate recharge trenches. The injection wells were placed on both sides of the currently dewatered extraction phase in the adjacent Horton Brook site. The amount recharged by these wells was calculated based on the amount of groundwater pumped out for dewatering and/or intercepted by the drainage ditch in the base case model (i.e. model scenarios with mitigation measures absent).
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7.0 MODEL CALIBRATION The model calibration process was undertaken by evaluating the model response to changes in the following model parameters:
1) The hydraulic conductivity of the gravel aquifer;
2) The hydraulic conductivity of engineered and backfilled Horton Brook site; 3) The hydraulic conductivity of other historical landfill sites present within the model area; and
4) The aquifer effective recharge.
The model was calibrated to June 2016 groundwater levels in monitoring wells surrounding the Site and the currently operating adjacent Horton Brook site. Recent groundwater levels were selected because operations at the Horton Brook site have migrated over the monitoring period, and the objective of the calibration was to fit spatial variations in hydrogeological conditions in close Site proximity that are likely to have an impact on future Site groundwater management. The fit to mean groundwater levels was also evaluated as part of the calibration process.
During calibration, in order to reduce the hydraulic gradient at the Site, the hydraulic conductivity was increased by assigning a value of 1x10-3 m/s across the gravel aquifer extent within the model area. This value represents the upper end of the range in permeability values obtained from slug test results and provides an acceptable fit between observed and model computed heads.
The calibration of hydraulic conductivity was also guided by the data for the Berkyn Manor, Horton Brook and other local sites for which studies are in the public domain including: the Kingsmead and Riding Court Farm site, which is located directly adjacent to the study area.
The collated hydraulic conductivity values are summarised in Table 10. The calibrated value selected for this study is similar to or slightly below the values used in other studies. Table 10: Review of Published Gravel Aquifer Hydraulic Conductivity Values Hydraulic Reference Comment Conductivity (m/s) 1.5x10-3 Golder Horton Brook (2007a) Result of iterative calibration. The modelled post calibration hydraulic conductivity value was reported to be consistent within the 2.3x10-3 WRc Berkyn Manor (2004) range of historic field values obtained from 42 site specific samples from Kingsmead area. Kingsmead Groundwater 2.89x10-3 Pumping test data result (average). Model (ESI, 2014b) Riding Court Farm 1.74x10-3 Groundwater Flood Risk Modelled hydraulic conductivity. Assessment (2014)
The gravel layer is considered to be isotropic with regard to hydraulic conductivity; therefore, the same value was applied for vertical conductivity.
Hydraulic conductivity of 5x10-7 m/s was used for the portion of the Horton Brook (Jayflex site) Quarry that is understood to have undergone gravel extraction followed by construction of low permeability liner and placement of inert waste within. The chosen value (similar to WRc 2004 model) resulted from iterative calibration process and it was increased to minimise excessive groundwater mounding on the top of the landfilled area.
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An initially proposed hydraulic conductivity value for inert waste infill in historical landfill sites was increased following calibration to 1x10-5 m/s in order to mitigate unrealistically high groundwater levels and extreme gradients within the backfilled areas.
Post-calibration hydraulic conductivity values for the model are summarised in the Table 11. Table 11: Post-Calibration Hydraulic Conductivity Values Horizontal Hydraulic Vertical Hydraulic Zone Description Conductivity (m/s) Conductivity (m/s)
River Terrace Includes Shepperton and Taplow -3 -3 Gravels Terrace gravel deposits. 1.0 x 10 1.0 x 10 Present at the central portion of Langley Silt the Northern Site boundary. -6 -6 Member Reported to consist primarily of 1.0 x 10 1.0 x 10 clay and silt material. Used for Historical and Authorised Inert backfill Landfills listed by Environment (for historical landfill -5 -5 Agency website within the model 1.0x10 1.0x10 sites) domain. Inert backfill 7 7 (engineered landfill Used for Horton Brook site. 5.0x10- 5.0x10- site)
In order to further improve the model calibration, recharge rates were also adjusted and were lowered to reduce heads in part of the Site where recharge on backfilled areas caused excessive mounding in the model. The calibration of the groundwater recharge value was guided by the conceptualisation of the Site and previously published studies as described in Section 6.5. In the process of iterative calibration, recharge rates were reduced to 80 mm/yr in the River Terrace Gravels, similar to the effective recharge calculation provided in ESI (2014). Table 12: Post Calibration Effective Recharge Values used for 2016 Model. Zone Recharge (mm/yr) River Terrace Gravel 80 Alluvium 85 Langley Silt member 16 London Clay 1 Backfilled and capped Horton Brook quarry 5 Historical Landfills 40
7.1 Calibration Results Given the objectives of this model, which are to evaluate the impact of the proposed development given the dewatering activities ongoing at adjacent Horton Brook site, it was considered necessary for the model accuracy to include calibration targets at boreholes present at both Poyle and Horton Brook site. In the absence of long term overlapping groundwater level data for all calibration targets and in order to account for variable hydrodynamic conditions prevailing at Horton Brook site due to its progressively changing pumping regime, the emphasis was put on most recent groundwater elevation dataset from June 2016 in calibration. However, calibration to statistical averages was also performed to inform the level of uncertainty associated with the use of a single time point in calibration.
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The calibration to statistical groundwater average elevations yielded the residual mean of 0.619 m with the absolute residual mean of 0.706 m (see Figure 7 - right). This is higher than residuals of 0.235 m and 0.501 m respectively for calibration to most recent groundwater elevation dataset (Figure 7 – left). It is considered that groundwater elevation change due to groundwater interception and water discharge at Horton Brook site is not reflected by the average groundwater levels, and resulting in greater residual values for observed versus calculated heads.
The fit of the calibrated model to the observation groundwater heads is shown graphically in Figure 7 (left) with calibration details listed in Table 13.
Figure 7: Final Model calibration results to June 2016 dataset: calibrated versus observed groundwater heads average (left) and June 2016 (right).
Table 13: Groundwater Level Calibration Details. June 2016 level Simulated GW level Residual Borehole (m AOD) (m AOD) (m) BH1 17.4 17.80 0.40 BH10 17.71 17.97 0.26 BH12 17.42 16.71 -0.71 BH15 17.93 17.54 -0.39 BH16 18.33 17.48 -0.85 BH2 17.41 17.36 -0.05 BH3 16.5 17.07 0.57 BH4 17.41 17.41 0.00 WG01 17.92 18.56 0.64 WG02 17.82 18.64 0.82 WG03 18.2 18.86 0.66 WG04 17.83 18.48 0.65 WG06 17.08 18.15 1.07 WG07 17.57 17.90 0.33 WG08 17.72 17.82 0.10
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The regional and local calibrated heads are further depicted on Figure 8 and 9 below.
Figure 8: Regional calibrated groundwater heads (For detail in the Site vicinity, see Figure 9; Berkyn Manor Site shown in Blue)
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Figure 9: Local calibrated groundwater heads (June 2016 heads) (Berkyn Manor Site shown in Blue)
Calibration of the model is considered to have resulted in a reasonable fit to observed conditions with modification of parameters within an acceptable range from initial estimates. The calibrated model is considered to sufficiently reflect the processes occurring on and around the Site to provide an appropriate basis for comparative assessment of the impact of future changes to the Site on the groundwater regime.
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8.0 PREDICTIVE SCENARIOS: NO MITIGATION A number of predictive scenarios were developed using the calibrated model to assess groundwater conditions at the Site as a combination of groundwater impacts resulting from proposed quarrying and restoration works at the Site and the operations at the adjacent Horton Brook site.
It is understood that operations will commence in 2018 and will progress according to the phasing scheme set out in the Poyle Quarry Extension RMC South East Non-Technical Summary (Wyn Thomas Gordon Lewis, 2004). It is assumed that the adjacent Horton Brook site will still be operating at this time.
The following scenarios were simulated as models constructed on the basis of the calibrated base model:
Scenario 1: dewatering of Phase 1 and Phase 2; ongoing operation in Phase 9 at the Horton Brook site. Scenario 2: dewatering of Phase 3 and Phase 4; gravel extraction and backfilling completed at Phase 1 and 2; ongoing operation in Phase 10 at Horton Brook site.
Scenario 3: dewatering of Phase 5; gravel extraction and backfilling completed at Phase 1 to 4; Horton Brook site infilled. Scenario 4: The Site is infilled; and Horton Brook site completed (i.e. both sites fully restored). The effect on groundwater heads imposed by dewatering and/or restoration works undertaken at the Berkyn Manor (Poyle) site has been quantified by calculating a head difference in comparison to base case model assessed individually for each scenario commensurate with the progression of dewatering operations and infilling southward at the Horton Brook site over time.
Predicted drawdown is provided to inform the magnitude of change to 0.5 m accuracy as this is typical of the average natural groundwater fluctuations in the study area. Each scenario for the Site was modelled with and without the groundwater mitigations to assess the effect of the proposed mitigation measures. The ‘without mitigation’ assessments are summarised below. Results with mitigation are presented in the following section (Section 9.0).
Engineering measures at the Horton Brook site to mitigate the impacts of both dewatering and restoration comprise a pipe in an enclosed trench drain installed around the site perimeter. Based on the planned progression of works and groundwater control design (Golder, 2007a; Golder, 2007b) and observed groundwater levels around the northern perimeter, this drain is assumed to already be in place around the northern, eastern and western perimeter of the Horton Brook site. It is assumed to be used for groundwater control in the northern part of the site, and groundwater reinjection in the southern part of the site. 8.1 Scenario 1. In this scenario, both Phase 1 and Phase 2 are dewatered simultaneously to simulate the combined effect of gravel extraction from these phases. It is assumed that at the time of initiation of gravel extraction at the Site, quarrying and subsequent dewatering at adjacent Horton Brook site will be taking place within Phase 9 on the southern portion of the Horton Brook site. As above, the groundwater control feature is also assumed to be in place around the Horton Brook site. The objective of this assessment is to evaluate the influence of operation of the Site, therefore only this aspect of the predicted changes in groundwater elevation are discussed. In order to distinguish an effect upon groundwater heads imposed by dewatering of the Site only, the modelled groundwater heads were compared against the base case model that incorporated dewatering solely within Phase 9 at Horton Brook site.
A spatial plot illustrating the difference in groundwater head (shown as a red contours) between the base case scenario model and Scenario 1 model is presented in Figure 10 below. The predicted 2018 groundwater head distribution is also shown (blue contours).
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Figure 10: Difference in groundwater head (shown as a red contours) between the base case scenario model and Scenario 1 model, predicted groundwater level shown in blue. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
Without any mitigation measures in place at Berkyn Manor, the model indicates that dewatering of Phases 1 and 2 of the Site simultaneously (a worst case scenario) may result in lowering groundwater level of between 0.5 m and 1.0 m beneath the Colnbrook village to the north from the northern Site perimeter and less than 0.5 m beneath Colnbrook landfill located north from Colnbrook. The model calculates that abstraction of approximately 4635 m3/day would be required in this scenario in order to lower groundwater level to the base of the gravel layer i.e. approximately 14.0 mAOD.
Mitigation measures are included for this scenario to limit the impact of groundwater level reduction in Colnbrook and the private groundwater abstraction located north of the northern Site boundary.
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8.2 Scenario 2. In this scenario, it is simulated that gravel extraction and dewatering has progressed to Phases 3 and 4 of the Site, while Phase and 1 and 2 have been subsequently backfilled with relatively low permeability waste (or waste with a relatively low permeability liner). It was assumed that dewatering activities would be taking place within Phase 10 of the Horton Brook site, and that groundwater control measures around the north of the Horton Brook site would be operational. The predicted groundwater heads resulting from the dewatering of both sites were compared against simulated groundwater heads for the operation of Horton Brook (Phase 10, restored areas and groundwater control trench) in isolation.
A spatial plot illustrating the difference in groundwater head (shown as a red contours) between the base case scenario model Scenario 2 model is presented in Figure 11 below. The predicted 2019 groundwater head distribution is also shown (blue contours).
Figure 11: Difference in groundwater head (shown as a red contours) between the base case scenario model including 2019 heads (shown as a blue contours) and Scenario 2 model. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
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The results for this scenario indicate that progressing quarrying and dewatering of consecutive phases in the southern direction at the Site and backfilling of Phase 1 and 2 results in a decrease of combined drawdown propagation in the northern direction compared to Scenario 1. In addition to that, the model results indicate that shifting the dewatering operation further to the south moves the radius of influence away from the Colnbrook area which is no longer affected by groundwater reduction according to this predictive scenario.
Since the lake located directly east from the Site acts as a constant head boundary, this prohibits propagation of the drawdown in that direction. The model predicts propagation of the drawdown to be occurring mainly to the west and south from the dewatering area. It is also considered the drawdown imposed by dewatering of combined Site phase 3 and phase 4 is likely to reach the private abstraction at Horton Brook village but it is predicted to be of less than 0.5 m magnitude.
It is noted, the magnitude of groundwater level reduction and spacial distribution of groundwater heads in the vicinity of the Phase 10 undergoing dewatering on Horton Brook site in this scenario is considered to be relatively unaffected by Site dewatering operation because a similar effect was predicted by simulating the removal of groundwater from Phase 10 by Golder 2007 model.
Mitigation measures may need to be considered to minimise an effect on the Rayner Farm well abstraction in Horton Brook village. 8.3 Scenario 3 Scenario 3 considers dewatering operations taking place only within the Phase 5 of the Site with the preceding Phase 1, Phase 2, Phase 3 and Phase 4 already restored. For the purpose of this scenario, it was assumed that the Horton Brook site is fully restored with inert material and active dewatering is no longer undertaken at that site. The groundwater control trench is assumed to be in place surrounding the Horton Brook site, modelled as a drain feature around the northern, eastern and western perimeter, and a zone of recharge along the southern perimeter.
A spatial plot illustrating the difference in groundwater head (shown as a red contours) between the base case scenario model and Scenario 3 model is presented in Figure 12 below. The predicted 2020 groundwater head distribution is also shown (blue contours).
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Figure 12: Difference in groundwater head (shown as a red contours) between the base case scenario model including 2020 predicted heads (shown as a blue contours) and Scenario 3 model. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
Note: In Figure 12, wells along the southern perimeter of the Horton Brook site are associated with modelled reinjection of water from the groundwater control trench.
For this unmitigated case, the modelled scenario shows further reduction in impact in the vicinity of the Site following cessation of operation of this site and site restoration. It can be also seen that the water table drawdown to the north, east and west from the Site is largely consistent with predictions of Scenario 2. A slight reduction of drawdown in the direction of Horton Brook site is predicted due to rebound of groundwater heads related to cessation of groundwater pumping at Horton Brook site. The modelled extent of the drawdown in the southern direction is shown to have increased compared to previous scenarios leading to groundwater reduction of approximately 1.0 m in the vicinity of private water groundwater abstraction in Horton Brook village.
Mitigation measures may need to be considered to minimise the effect on the Rayner Farm abstraction well in Horton Brook village.
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8.4 Scenario 4 The scenario 4 considers both the Poyle site and Horton Brook site to be fully restored with inert waste. Given the objective of this predictive scenario is to identify areas that have potential to be flooded due to groundwater mounding hydraulically upgradient from the infilled voids and areas where groundwater may be lowered in the ‘shadow’ of the filled area downgradient, the difference in groundwater heads for this Scenario 4 has been calculated by comparison to groundwater heads under undisturbed (pre-development) conditions.
A spatial plot illustrating the difference in groundwater head (shown as a red contours) between the base case scenario model and Scenario 4 model is presented in Figure 13 below. The predicted post-restoration groundwater head distribution is also shown (blue contours).
Figure 13: Groundwater differences (shown as drawdown) between base case model and scenario 4. Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
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As in Scenarios 1 to 3, the groundwater control system proposed for Horton Brook site (slotted pipe laid around the entire perimeter of the site) is assumed to be installed and operational following restoration of the development in Scenario 4.
The modelled scenario shows that upon completion of backfilling both sites with inert material (assumed to be of permeability lower than the surrounding aquifer), a water table decrease in comparison to pre-development conditions of between approximately 0.5 m to 1.5 m is predicted to occur north from both sites in the proximity of Colnbrook and south from the Site in the vicinity of Horton Brook village. This drawdown occurs because the groundwater control system installed around the Horton Brook site (as modelled), assuming its continued maintenance and functionality, not only reduces groundwater mounding north from the Horton Brook site but exerts sufficient groundwater control to prevent mounding north of the Berkyn Manor (Poyle) site. The engineering around the Horton Brook site (as modelled), results in a lowering in groundwater heads below pre-development conditions across a relatively large area, capturing water from north of the Berkyn Manor Site.
The modelling suggests that backfilling of both sites will not result in any significant rise in groundwater beneath developed areas to the north and south of the quarries. However, long term groundwater control may result in a reduction in groundwater levels of more than 0.5 m from pre-development conditions beneath both Horton and Colnbrook villages.
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9.0 PREDICTIVE SCENARIO WITH MITIGATION The objective of the ‘with mitigation’ scenarios is to evaluate the efficacy of proposed engineering measures to mitigate drawdown impacts identified by the predictive scenarios without mitigation measures.
The proposed mitigation is similar to that in use at the Horton Brook site: a groundwater control trench (or similar - detailed design to be considered by others) to be installed initially around the northern side of the Site and ultimately around the whole site, which would, initially be used for reinjection of water from the dewatered void and following restoration would act to passively control groundwater heads by promoting groundwater flow around the Site.
The control feature is modelled as a series of closely spaced injection wells (simulating a trench) during the injection phases in the MODFLOW model, and as a drain boundary in the control phases, similar to previous studies (e.g. Golder (2007a)). As in the base case model, the rate of reinjection was determined based on the predicted rate of abstraction in the unmitigated case. The position of the proposed trench in Scenarios 1A to 3A is illustrated in Figure 14.
Figure 14: The position of the recharge trench along the western tip and northern Site boundary.
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Whilst Scenario 4 indicates that engineering Site to control groundwater mounding post-restoration would not be required due to the existing engineering surrounding the Horton Brook site, Golder has nonetheless considered the construction of a groundwater control trench around the Site. It is proposed that this feature will be constructed to reduce reliance on engineering at the adjacent site and allow for over-estimation of the drain performance due to the idealised representation of the control feature in the groundwater flow model. The position of the proposed trench simulated in Scenario 4A is illustrated in Figure 15.
Figure 15: The position of the drainage trench (in green) along the northern and eastern Site boundary and recharge trench along the southern Site boundary (in blue).
9.1 Scenario 1A In order to mitigate the effect of dewatering of combined Phase 1 and Phase 2 (shown as drawdown in Figure 10 in Chapter 8.1), Scenario 1A included a recharge trench along the western and northern site perimeter. In this simulation, the volume of water being removed by the drain boundary for Phase 1 and Phase 2 was reinjected via injection wells (simulating the presence of recharge trench) around the northern and north-western boundary. As in Scenario 1, dewatering and perimeter groundwater control were simulated to be active at the Horton Brook Site.
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Figure 16: Comparison between non-mitigated Scenario 1 (left) and mitigated Scenario 1A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
The comparison between unmitigated and mitigated scenarios can be seen in Figure 16. The results indicate that injection of this volume upgradient is likely to cause some localised groundwater mounding (the outer red contour is - 0.5 m), however groundwater levels are not predicted to intercept the ground surface (green contour) hence are not considered to be causing flooding local to the Site. The model shows the proposed engineering mitigates the propagation of the drawdown that would have been affecting the private abstraction well and dwellings near Colnbrook. 9.2 Scenario 2A In this scenario, the effect of the mitigation scheme on the impact of dewatering of Phases 3 and 4 and backfilling of Phases 1 and 2 with inert waste was evaluated. In order to simulate the mitigation scheme, a recharge trench along the southern and south eastern boundary was included to recharge the aquifer downgradient of the excavation and to reduce the risk of potential derogation of the private groundwater well located to the south from the Site. A drainage ditch (modelled at the location of the recharge trench in Scenario 1A) was included upgradient of Phases 1 and 2 as a mitigation measure to prevent excessive groundwater mounding, and was extended along the eastern site boundary.
As in Scenario 2, dewatering and perimeter groundwater control were simulated to be active at the Horton Brook Site.
The comparison between the unmitigated and mitigated scenarios can be seen in Figure 17 with the unmitigated scenario shown on the left and mitigated scenario shown on the right. The mitigation results indicate some groundwater mounding (red contour) in the proximity of the line of water discharge (modelled as injection wells), but the rise is not sufficient to bring the water table up to intersect ground surface (shown by the green contour in Figure 17). It is inferred therefore that local groundwater flooding is not predicted to occur. The proposed length therefore is considered likely to be adequate in terms of receiving capacity.
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Figure 17: Comparison between non-mitigated Scenario 2 (left) and mitigated Scenario 2A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown. Negative drawdown denotes a rise in groundwater level.
9.3 Scenario 3A In this scenario, the effect of the mitigation scheme on the dewatering of Phase 5 and backfilling of Phases 1 to 4 is considered. Mitigation measures similar to those proposed in Scenario 2A were applied to this scenario i.e. a recharge trench along the southern and south eastern site boundary and drainage ditch upgradient of Phases 1 and 2 and along the eastern site boundary. As in Scenario 3, (passive) perimeter groundwater control was simulated around the Horton Brook Site.
The comparison between the unmitigated and mitigated scenarios is shown in Figure 18 with the unmitigated scenario shown on the left and mitigated scenario shown on the right. The results indicate that when mitigations are in place (injection wells to re-inject dewatering water volumes to the south of the operational phase) the dewatering impact is reduced (the 0.5 m contour has moved around 140 m north and the 1 m contour now lies north of the built area), but still potentially has a small impact in terms of more than a 0.5 m drawdown in the vicinity of the private abstraction and buildings at the very edge of Horton village.
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Figure 18: Comparison between non-mitigated Scenario 3 (left) and mitigated Scenario 3A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown. 9.4 Scenario 4A In this scenario, the effect of the proposed mitigation scheme (drainage ditch around the northern and eastern Site perimeter) was modelled against the groundwater impact imposed by infilling both Poyle and Horton Brook site with inert waste. As in Scenario 4, (passive) perimeter groundwater control was simulated around the Horton Brook Site. Drawdown shown in Figure 19 represents the difference in predicted groundwater heads in comparison to groundwater heads under undisturbed (pre-development) conditions
The comparison between unmitigated and mitigated scenarios is shown in Figure 19 with the unmitigated scenario shown on the left and mitigated scenario shown on the right. Based on the model predictions, it appears that the drainage ditch present in the mitigated scenario has virtually no impact on the groundwater head distribution in this scenario compared to the unmitigated scenario. The existing groundwater control system around the Horton Brook site is interpreted to provide sufficient drainage capacity to relieve the additional groundwater mounding at the Berkyn Manor (Poyle) site, such that installation of an additional trench has minimal effect. It is noted that this scenario assumes that the groundwater control system around the Horton Brook site continues to operate as designed.
Figure 19: Comparison between non-mitigated Scenario 4 (left) and mitigated Scenario 4A (right). Outer red contour denotes a drawdown in groundwater level of 0.5m, contours increase inwards at 0.5m contour intervals of drawdown.
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10.0 SENSITIVITY ANALYSIS Sensitivity analysis was undertaken on the Scenario 1 model as this was judged to be the scenario that may have the most significant effect on groundwater heads and groundwater flow at the Site. Individual key parameters were varied and the effect on the model results investigated. The sensitivity of predicted groundwater heads to parameter variation was analysed at following locations: Monitoring well BH16 located north west from the Site, upgradient to the Site; Monitoring well WG02 located east from the Site, cross gradient to the Site; and Monitoring well WG06 located south from the Site, downgradient to the Site. The well locations are shown in Figure 3. The observation wells were selected so that they are located beyond the predicted pumping radius of influence of both quarries and represent any change to groundwater heads that would have occurred if different model parametrization was used instead. 10.1 Hydraulic Conductivity The sensitivity analysis considered variation in the hydraulic conductivity of the gravel aquifer (Shepperton Gravel Formation) only as the predicted impacts to groundwater heads are considered to be occurring locally and within the unit. The details of sensitivity analysis to hydraulic conductivity of the gravel aquifer is presented in Table 14. Table 14: Sensitivity Analysis - Groundwater Head to Changes in Hydraulic Conductivity of the Gravel Aquifer
Hydraulic conductivity BH16 (m AOD) WG02 (m AOD) WG06 (m AOD) (m/s)
1x10-4 No convergence* No convergence No convergence 5x10-4 17.62 18.49 17.69 1x10-3 17.44 18.47 17.53 5x10-3 16.95 18.43 17.34 1x10-2 16.87 18.42 17.33 Reported (base case) model value shown in bold *In this scenario, the model solution was unstable and a convergent solution was not obtained, no results could be reported.
The model calculated groundwater heads appear to be responsive in a very narrow range to variable hydraulic conductivity values in the sand and gravel aquifer based on the calculated change in monitoring well WG02 and WG06. An increase in hydraulic conductivity of an order of magnitude results in a negligible (0.05 m) change in calculated groundwater heads in cross location WG02. At the upgradient location BH16, the calculated change in groundwater head with an order of magnitude increase in hydraulic conductivity is 0.57 m, and at the downgradient location WG06, it is 0.2 m. A decrease of hydraulic conductivity by an order of magnitude results in model instability. A decrease in hydraulic conductivity of half an order of magnitude, results in an increase in calculated groundwater heads in all locations and calculated to be 0.18 m in BH16, 0.02 m in WG02 and 0.16 m in WG06.
It is concluded that the upgradient borehole is generally more sensitive to changes in hydraulic conductivity than cross and downgradient boreholes. 10.2 Recharge An effective recharge value of 80 mm/year was applied to the gravel aquifer. This value was changed in 25% intervals to give a range from 40 mm/year to 250 mm/year. The resulting heads are summarized in Table 15.
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Table 15: Sensitivity Analysis - Groundwater Head to Recharge
Recharge (mm/year) BH16 (m AOD) WG02 (m AOD) WG06 (m AOD)
40 17.29 18.47 17.51 60 17.39 18.47 17.52 80 17.44 18.47 17.53 100 17.48 18.47 17.54 120 17.52 18.47 17.54 140 17.55 18.47 17.55 160 17.58 18.47 17.55 180 17.60 18.47 17.56 200 17.61 18.47 17.57 225 17.64 18.47 17.58 250 17.66 18.47 17.59 Reported (base case) model value shown in bold Data presented in Table 14 shows some weak positive correlation between recharge and groundwater head in upgradient borehole BH16 and to a lesser extent in downgradient borehole WH06. It is noted, a change in recharge value by 50 % for BH16 corresponds with a change of groundwater head of approximately 0.08 m. If the recharge value of 250 mm/year was applied to the model, this would have resulted in groundwater increase in BH16 by 0.22 m. The change in cross gradient and downgradient boreholes are shown to be negligible.
The model results are not considered to be strongly influenced by recharge within reasonable bounds in the value of this parameter. 10.3 River Bed Conductance It is noted that for river bed conductance MODFLOW calculates the river bed conductance in m2/s in all cells to which a river boundary is assigned as a function of the dimensions of the cells and the river bed permeability. The sensitivity analysis for river bed conductance has therefore carried out by modifying different values for river bed conductance rather than re-assigning of river bed permeability as new boundaries. The river bed conductance of Colne Brook was readjusted during the model calibration to account for low permeability materials identified at the top of the lithological profile and to facilitate water heads differences between groundwater and surface water heads in this part of the Site.
The resulting head changes due to the chosen values in the three boreholes are shown in Table 16. Table 16: Sensitivity Analysis - River Bed Conductance
River Bed Permeability BH16 (m AOD) WG02 (m AOD) WG06 (m AOD) (m/s)
1x10-8 17.44 18.46 17.52 1x10-7 17.44 18.47 17.53 1x10-6 17.49 18.56 17.59 1x10-5 17.56 18.71 17.67 1x10-4 17.58 18.84 17.75 1x10-3 17.58 18.84 17.75 Reported (base case) model value shown in bold
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The sensitivity analysis indicates a negative correlation between recharge and groundwater head at all the borehole locations chosen for analysis. The closer the borehole is to the river boundary, the stronger the correlation. This is demonstrated in Table 16 where it can be seen that the total change in head decreases with the radial distance from the river boundary (0.14 m in BH16, 0.38 m in WG02 and 0.23 m in WG06). It is also noted that the change of an order of magnitude in river bed permeability from 1x10-6 m/s to1x10-5 m/s represents the most sensitive change and is calculated to be 0.15 m for WG02 and 0.08 m for WG06. The head difference between the pre-calibrated model with river bed permeability assigned to be 1x10-4 m/s and post calibrated model with the river bed permeability assigned to be 1x10-7 m/s is calculated to be 0.37 m and 0.22 m respectively. The sensitivity of the model to river bed conductance is shown spatially in Figure 20. The drawdown represents the groundwater head difference resulting in applying pre and post calibration river bed permeability values. The influence is greatest in the area to the northeast of the Site.
Figure 20: Head difference resulting in applying of pre and post calibration river bed permeability values.
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11.0 CONCLUSIONS Using a 2-dimensional finite difference numerical groundwater flow model, a number of predictive scenarios have been developed to assess groundwater conditions at the Site as a result of simultaneous quarrying and restoration works at the Site and the adjacent Horton Brook site. A calibrated base case model was developed, and predictive scenarios developed for future conditions from this base case. 11.1 Effects of Dewatering – Without Mitigation The predictive modelling for the unmitigated scenarios shows that progressive dewatering and backfilling at the Site results in the drawdown initially extending north beyond the Site boundary, with a progressive reduction in the extent of drawdown to the north as works progress in a southerly direction.
Without the mitigation measures in place, the model indicates that dewatering of Phases 1 and 2, may result in lowering groundwater levels between 0.5 m and 1.0 m (in comparison to projected conditions based on the existing permitted development) beneath the Colnbrook village to the north and less than 0.5 m beneath Colnbrook landfill located north of Colnbrook village. In comparison, modelling indicates that dewatering of the combined Phase 3 and Phase 4 is not predicted to affect Colnbrook area.
The model predicts that a reduction in groundwater levels (in comparison to projected conditions based on the existing permitted development) would occur at the Rayner Farm abstraction at Horton Brook village. It is predicted to be less than 0.5 m during the dewatering of combined Phases 3 and 4. A slight reduction in extent of drawdown in the direction of the Horton Brook site is predicted as works progress, due mainly to the rebound of groundwater levels following the cessation of groundwater pumping at Horton Brook site.
The modelled extent of the drawdown increases as dewatering progresses south, leading to a lowering of groundwater levels of approximately 1.0 m in the vicinity of the Rayner Farm abstraction in Horton Brook village when the most southern phase of the Site (Phase 4) is undergoing dewatering.
Upon completion of backfilling of both the Site and the Horton Brook site with inert material, a lowering of the water table of between 0.5 m to 1.5 m in comparison to pre-development conditions (for both sites) is predicted to occur north from the Site in the proximity of Colnbrook and south from the Site in the vicinity of Horton Brook village. This long term drawdown in comparison to pre-development conditions is a function of passive drainage around the two sites via the Horton Brook perimeter drainage (groundwater control) system, which is planned to remain following restoration to mitigate the risk of groundwater mounding upgradient of the filled void.
The unmitigated model also shows the groundwater control system installed around the Horton Brook site has the capacity to reduce and control groundwater in the vicinity of the Site when no active dewatering at the Site is taking place. 11.2 Effect of De-watering - Proposed Mitigation The predictive scenarios incorporating the proposed mitigation measures indicate that the installation of a recharge trench (or other groundwater control method) along the western and northern Site perimeter will potentially provide a sufficient level of mitigation against the extension of drawdown north from the Site in the Scenario 1 model (early operation).
As the quarrying progresses, a drainage ditch (at the location of the recharge trench in Scenario 1) was included upgradient of Phases 1 and 2 to mitigate groundwater mounding upgradient of the Site; this feature was extended along the eastern Site boundary.
Whilst the re-injection of the pumped water along the southern boundary of the Site during the dewatering of Phase 3 and 4 may cause groundwater mounding, the modelling indicates that maximum groundwater levels will remain below ground surface i.e. would not cause local flooding.
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For dewatering of Phase 5, the model indicates that when mitigation is in place (injection wells to re-inject dewatering water volumes to the south of the operational phase) the dewatering impact is reduced (the 0.5 m contour has moved around 140 m north and the 1.0 m contour now lies north of the built area of Horton Brook), but still has a small potential impact in terms of a drawdown of more than 0.5 m in the vicinity of the Rayner Farm abstraction and buildings at the very edge of Horton village. During operation of Phase 5, drawdown to the north does not extend as far as the village of Colnbrook and is within the Site area and the adjacent quarry.
As the magnitude of groundwater fluctuations observed on Site and in adjacent areas lies within the range of the predicted effects of dewatering, it is considered that the proposed mitigation is adequate to reduce the potential effects of dewatering and backfilling of the Site to an acceptable level.
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12.0 REFERENCES 1) Arup, 2014. Kingsmead Landfill Hydrogeological Risk Assessment Report. Reference KIN EP Vol 5, Revision A, January 2014. Accessed online via: http://www.rbwm.gov.uk/pam/docservlet?docId=129660049&filename=1611804... 2) British Geological Survey, 1999. 1:50,000 Geological Map of Windsor (Solid and Drift Geology), sheet 269. 3) British Geological Survey, 2016a. Superficial Geology Map, Geoindex Onshore 2016. Accessed online from: http://mapapps2.bgs.ac.uk/geoindex/home.html in August 2016. 4) British Geological Survey, 2016b. http://bgs.ac.uk/geoindex/ accessed August 2016. 5) Environment Agency, 2014. LIDAR Composite Digital Terrain Model (DTM). Data accessed online via: http://environment.data.gov.uk/ds/survey/#/survey?grid=TQ07 in August 2016. 6) ESI, 2014. Groundwater Risk Assessment for Riding Court Farm.. Accessed on line via: www.rbwm.gov.uk/pam/docservlet?docId=131099999&filename=1668168 7) Flood Information Warning, 2016. Accessed online via: https://flood-warning- information.service.gov.uk/river-and-sea-levels in August 2016. 8) Golder Associates (UK) Ltd.2007a. Groundwater Flow Modelling Land East of Horton Road. Accessed via: www.rbwm.gov.uk/pam/docservlet?docId=6819391&filename=759477.TIF 9) Golder Associates (UK) Ltd. 2007b. Groundwater Control System for Land East of Horton Road ref. 04519507.510/A.1, January 2007. Accessed on line via: www.rbwm.gov.uk/pam/docservlet?docId=6819412&filename=759484.TIF 10) Morgan-Jones, M., Bennett, S., Kinsella, J.V.. 1984. Hydrogeological Effects of Gravel Winnings in an Area West of London. United Kingdom. Accessed on line via: http://info.ngwa.org/gwol/pdf/841030462.PDF on August 2016. 11) National Environmental Research Council (NERC), 2003. Baseline Report Series: 6. The Chalk of the Colne and Lee River Catchments,. Accessed online via: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/290909/scho0207blyd-e- e.pdf in August 2016. 12) Pawsey, D.B.H, Humprey, A.W., 1976, The Queen Mother Reservoir, Datchet – Some Aspects of its Design and Construction. Ground Engineering, October 1976. 13) River Levels UK, 2016. Accessed online via https://www.riverlevels.uk/map#.V8V_-v72aB8 in August 2016. 14) Tomes, I.M., 2006. Sediment Impact Analysis for the Lower Thames Flood Strategy Study, Proceeding to the Eight Federal Interagency Sedimentation Conference, USA 2006. Accessed on line via: http://pubs.usgs.gov/misc/FISC_1947-2006/pdf/1st-7thFISCs-CD/8thFISC/Session%203A- 4_Thorne.pdf on August 2016. 15) Waterloo Hydrogeologic Inc. 2016. Visual Modflow Manual 4.6.0.167. 16) Wraysbury Reservoir SSSI, Natural England 2016. Map viewed via: http://magic.defra.gov.uk/MagicMap.aspx?startTopic=Designations&activelayer=sssiIndex&query=HYP ERLINK%3D%272000374%27 on September 2016. 17) WRc, 2004. Modelling of Berkyn Manor Farm, Poyle. In: Wyn Thomas Gordon Lewis 2004, Vol.2 (Appendices). Report No. UC6637, November 2004. 18) Wyn Thomas Gordon Lewis, 2004. Extraction of sand and gravel from Poyle Quarry Extension (Part of Preferred Area 12) with restoration to agriculture: Environmental Impact Assessment. Vol. 1-4 and Non-Technical Summary, December 2004.
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BERKYN MANOR, POYLE (WESTERN EXTENSION) GROUNDWATER MODEL
Report Signature Page
GOLDER ASSOCIATES (UK) LTD
David Hybert Gareth Digges La Touche Project Manager Senior Hydrogeologist
Date: 16 March 2017
BK/HG/GDLT/DH/ab
Company Registered in England No.1125149 At Attenborough House, Browns Lane Business Park, Stanton-on-the-Wolds, Nottinghamshire NG12 5BL
VAT No. 209 0084 92 Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation.
March 2017 Report No. 1656400.500/A.2
BERKYN MANOR, POYLE (WESTERN EXTENSION) GROUNDWATER MODEL
DRAWINGS Drawing 1 – Site Location Plan Drawing 2 – Geology Drawing 3 – Hydrology and Surface Water Gauging Stations
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APPENDIX A Borehole Logs
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BERKYN MANOR, POYLE (WESTERN EXTENSION) GROUNDWATER MODEL
APPENDIX B River Level Data
March 2017 Report No. 1656400.500/A.2
Caption Text
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