Regional MODFLOW 6 modelling in an area of complex faulted geometry: Cannock Chase, UKAuthors: 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 Staffordshire Water3, Environment Agency4, Wood plc5

BACKGROUND

REGIONAL GEOLOGY RECHARGE MODEL (SWAcMOD)

3D GEOLOGICAL MODEL

Groundwater model (MODFLOW6)

Reference

UNSTRUCTURED GRID

Cannock Chase is a Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC) in Staffordshire. The Sherwood Sandstone aquifer underlying the Chase is used by both South Staffordshire Water (SSW) and Severn Trent Water Ltd. (STWL) for public water supply and also provides baseflow to a number of streams and rivers, including the River Sow, River Penk and River Trent.

A number of the watercourses in the vicinity of Cannock Chase have been investigated for low flows as part of the 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 abstractions at STWL1 and STWL2 on the Watercourse 2 and Watercourse 3.

The Cannock Chase groundwater model is a collaborative project commissioned jointly by SSW and the Environment Agency (EA) to provide a robust tool to support future planning and regulation of water resources in the area. The model has been developed by Stantec in partnership with Groundwater Science.

Feature Initial mesh refinement Revised mesh refinement

Adits 10 m 10 mAbstraction wells 10 m 50 mRivers/streams 50 m 100 mCanals 50 m N/AFaults 50 m 100 mGWDTE (edge) 50 m 75 mGWDTE (interior) 50 m 150 mBackground cell size (sandstone) 200 m 200 mBackground cell size (mudstone) 200 m 500 mCell count 453,379 156,082Run time (Intel i7 CPU @ 1.90 GHz, 16 GB RAM) c. 12 hours c. 1h 30m

Three options for spatial discretisation are available in MODFLOW6:

� 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).

The bedrock geology of the Cannock Chase area is complex: the Triassic Sherwood Sandstone aquifer and overlying Mercia Mudstone have been “chopped” into multiple fault blocks, and the Sherwood Sandstone rests with angular unconformity on Carboniferous strata that are themselves folded and faulted. The Chase itself occupies a horst bounded by major faults.

Superficial cover is limited, with the greatest development of superficial deposits being in the Coven area in the south west (glacial till and glacio-fluvial sand/gravel), and along the River Trent (alluvium and river terrace sand/gravel deposits).

The recharge model was created using Groundwater Science’s SWAcMOD software, allowing it to use the same Voronoi mesh as that used for the groundwater model. SWAcMOD is an open source recharge modelling tool with a flexible modular design, allowing different processes to be represented in different climatic regions (Black and Lagi, 2017), and has been developed with unstructured grids specifically in mind. The flowchart illustrates the processes that have been included in the recharge model for Cannock Chase.

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.

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, Birmingham.

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 datasets: surface geology (bedrock & superficial) and land use. The surface geology (BGS 50k) was used as a proxy for soil type rather than use proprietary soil type data, and the land use was mapped using freely available CORINE Land Cover data. Each model node was mapped to a combined soil/land use class and given parameters based initially on those for other EA models in the Midlands.

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.

The Cannock Chase groundwater model is based on a 3D model of the geology prepared using Seequent’s Leapfrog software. Data for the geological model were taken from digitised BGS borehole records, published geological maps and memoirs and the BGS UK3D cross-section dataset. Additional cross-sections produced for SSW as part of its AMP5 NEP investigations were also used.

During development of the 3D geological model, Leapfrog Viewer was used to share the model with the wider project team, who were then able to review it and check the representation of the geology against maps, cross-sections and other data.

Table 1 Comparison of run times and grid refinement

Total AE Rainfall and PE

Recharge to saturated zone

Recharge attenuation

Interflow reservoir

Runoff Surface flowSurface flow attenuation

Soil moisture calculations (FAO methodology)

RCH package for MF6

Stream flow routing for SFR package

Inte

rflo

w

Macropore bypass

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

Contains Ordnance Survey data. © Crown copyright and database right 2019. Contains public sector information licensed under the Open Government Licence v3.0.

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

Land use

Surface geology Recharge

Actual Evapotranspiration (AE)