Regional MODFLOW 6 modelling in an area of complex faulted geometry: Chase, UK

Authors: Philip Hubbard1, Sam Bishop1, Alastair Black2 and William Witterick2 Stantec1 Groundwater Science2 With grateful acknowledgements to the Steering Group: Peter Greenaway3, Adam Janman4, John Davis4, Rob Soley5 South Water3, Environment Agency4, Wood plc5

Table 1 Comparison of run times and grid refinement BACKGROUND Feature Initial mesh refinement Revised mesh refinement Cannock Chase is a Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC) in Staffordshire. The Adits 10 m 10 m Sherwood Sandstone aquifer underlying the Chase is used by both Water (SSW) and Severn Trent Abstraction wells 10 m 50 m Water Ltd. (STWL) for public water supply and also provides baseflow to a number of streams and rivers, including the Rivers/streams 50 m 100 m , and . Canals 50 m N/A Faults 50 m 100 m A number of the watercourses in the vicinity of Cannock Chase have been investigated for low flows as part of the GWDTE (edge) 50 m 75 m National Environment Programme (NEP) in the past two decades. SSW’s investigations have focussed on the Watercourse 1 which runs close to its pumping stations at SSW1 and SSW2, while STWL has investigated the impact of its GWDTE (interior) 50 m 150 m abstractions at STWL1 and STWL2 on the Watercourse 2 and Watercourse 3. Background cell size (sandstone) 200 m 200 m Background cell size (mudstone) 200 m 500 m The Cannock Chase groundwater model is a collaborative project commissioned jointly by SSW and the Environment Cell count 453,379 156,082 Agency (EA) to provide a robust tool to support future planning and regulation of water resources in the area. The model Run time c. 12 hours c. 1h 30m has been developed by Stantec in partnership with Groundwater Science. (Intel i7 CPU @ 1.90 GHz, 16 GB RAM)

REGIONAL GEOLOGY RECHARGE MODEL (SWAcMOD)

The bedrock geology of the Cannock Chase area is complex: the The recharge model was created Total AE Rainfall and PE Triassic Sherwood Sandstone aquifer and overlying Mercia Mudstone using Groundwater Science’s Surface flow SWAcMOD software, allowing it Runoff Surface flow have been “chopped” into multiple fault blocks, and the Sherwood attenuation Sandstone rests with angular unconformity on Carboniferous strata to use the same Voronoi mesh as that used for the groundwater that are themselves folded and faulted. The Chase itself occupies a Stream flow routing Soil moisture calculations horst bounded by major faults. model. SWAcMOD is an open source for SFR package recharge modelling tool with a (FAO methodology) Interflow Superficial cover is limited, with the greatest development of flexible modular design, allowing different processes to be represented superficial deposits being in the Coven area in the south west (glacial Interflow reservoir Macropore bypass till and glacio-fluvial sand/gravel), and along the River Trent (alluvium in different climatic regions (Black and Lagi, 2017), and has been and river terrace sand/gravel deposits). Recharge attenuation developed with unstructured grids specifically in mind. The flowchart illustrates the processes that have Recharge to saturated zone been included in the recharge model

for Cannock Chase. RCH package for MF6

Land use Actual Evapotranspiration (AE) Rainfall and potential evapotranspiration (PE) rates were obtained from 1 km gridded timeseries datasets (CEH-GEAR and CHESS- PE respectively). Parameterisation of the model was largely based on two spatial 3D GEOLOGICAL MODEL datasets: surface geology (bedrock & superficial) and land use. The surface geology (BGS 50k) was used as a proxy for soil type The Cannock Chase groundwater model is based on a 3D model of the geology prepared using Seequent’s Leapfrog rather than use proprietary soil type data, software. Data for the geological model were taken from digitised BGS borehole records, published geological maps and and the land use was mapped using freely memoirs and the BGS UK3D cross-section dataset. Additional cross-sections produced for SSW as part of its AMP5 NEP available CORINE Land Cover data. Each investigations were also used. model node was mapped to a combined soil/ land use class and given parameters based During development of the 3D geological model, Leapfrog Viewer was used to share the model with the wider project initially on those for other EA models in the team, who were then able to review it and check the representation of the geology against maps, cross-sections and Surface geology Recharge Midlands. other data. Spatial outputs of the recharge model provide a useful sense check of model performance, as can be seen in the figures below. A limited process of calibration was undertaken against flow gauges in the model area, with further refinement to be carried out as part of the groundwater model calibration.

Contains Ordnance Survey data. © Crown copyright and database right 2019. Geology C13/05-CCSL BGS © NERC. All rights reserved.

UNSTRUCTURED GRID Groundwater model (MODFLOW6)

Three options for spatial discretisation are available in MODFLOW6: The 3D geological model and 2D mesh design were used to create a 3D model mesh for use with MODFLOW 6. The groundwater model is still under development and is currently undergoing calibration. „„ DIS – a traditional rectilinear grid „„ DISV – an unstructured 2D mesh superimposed on a traditional layered model, with horizontal connections within layers only „„ DISU – a completely unstructured grid The Cannock Chase model has been built using the DISU approach, which allows maximum flexibility in terms of inter-cell connections. The model represents faulted contacts and pinching out of layers.

One of the key features of using an unstructured grid is the ability to refine locally around areas of interest such as abstractions, watercourses and groundwater dependent terrestrial ecosystems (GWDTEs). However, such refinement should only go as far as is necessary to meet the objectives of the model, a lesson learned in this project as the initial grid contained a higher degree of complexity than was required, giving a correspondingly high cell count and long run times (see Table 1).

Reference

Black, A. and Lagi, M., 2017. Surface Water Accounting Model, SWAc – linking surface flow and recharge processes to groundwater models. 15th meeting of the Groundwater Modellers’ Forum, 8th November 2017, . Contains Ordnance Survey data. © Crown copyright and database right 2019. Geology C13/05-CCSL BGS Contains Ordnance Survey data. © Crown copyright and database right 2019. Contains public sector information © NERC. All rights reserved. licensed under the Open Government Licence v3.0.