Application of MODFLOW-USG to simulate borehole and adit yields

Application of MODFLOW-USG to simulate borehole
and adit yields in the Chalk of the South Downs
Simon Cook1, Will Witterick2, Tim Power1, Rob Soley2, Sorab Panday3
[email protected]
1AMEC Environment & Infrastructure UK Ltd, 17 Angel Gate, City Road, London EC1V 2SH, UK
2AMEC Environment & Infrastructure UK Ltd, Canon Court, Abbey Lawn, Abbey Foregate, Shrewsbury, SY2 5DE, UK
3AMEC Environment & Infrastructure , Los Angeles, 4005 – Irvine, CA, 20151, USA
Abstract
Modelling the South Downs Chalk Aquifer
The development of regional scale numerical groundwater models is a well established practice in the United
Kingdom for the management of water resources. Most existing models have been developed as relatively
coarse, regularly gridded finite difference models. Whilst these are adequate for regional water resources
planning, sub grid scale effects often cannot be adequately captured. A regional model of the Chalk aquifer of the
South Downs which supports public supplies to Brighton and Worthing is currently under development. This model
must also be capable of accurately simulating pumped water levels and saline intrusion risks at abstraction wells
and extensive large diameter shaft and adit systems in order to simulate Deployable Output constraints. Partially
developed by AMEC,MODFLOW Unstructured Grids (USG) is being used for the first time in the United Kingdom
to meet this aim. A combination of local refinement in MODFLOW-USG and explicit representation of adits shafts
and wells as discrete features is being utilised to simulate water levels in the context of the regional groundwater
A Groundwater flow model of the South Downs aquifer has been developed by AMEC on behalf of
Southern Water. The unstructured model area is approximately 45 x 15km and incorporates around
300,000 cells varying in size from 200m to 12m across the Brighton and Worthing Chalk Blocks.
The model is intended to be used for two purposes :
•
Assessing the effects of abstractions upon flows to an ephemeral spring catchment under a
national environment programme investigation
•
Prediction and optimisation of abstraction yields and resilience for future water resources
management planning under a possible range of future climate conditions
Simulation of heads and flows in adit and borehole sources
system. The refined model will used for the assessment of winterbourne spring flows as part of a national
environment programme investigation and to optimise future source and aquifer management.
Local refinement of the model grid
The South Downs Chalk Aquifer
in MODFLOW- USG, here we have
employed Quad Tree refinement
around boreholes and adits,
although any arbitrary mesh is
possible.
Adit Source
Borehole Source
The South Downs Chalk aquifer is the principal
water supply for the major conurbations of Brighton
and Hove, Lancing and Worthing surface water
supplies are scare and comprise highly saline tidal
estuaries unsuitable for use.
Factors influencing the hydrogeology of the Chalk aquifer
Geomorphology, Sea Level Change,
Saline Water
Fractures, Flints
and Stratigraphy1
Primary Porosity
& Sedimentation
Adits and Karst
Adits and boreholes are represented as discrete 3D coupled linear networks (CLNs)
which intersect the model cells. The exchange of water is determined by a leakance term
that can be varied along the element length
The position of flowing horizons and major fissures can be used to refine the leakance parameters on individual
boreholes. Where available the results of aquifer testing can also inform parameter choice to simulate well
Mapping the distribution of flows and fractures in the Chalk
Worthing Chalk Block
0
0
60
20
20
10
10
-10
-20
-40
-60
-80
-100
0
Upper aquifer
poorly developed
owing to a
synformal barrier
Elevation (m AOD)
0
Elevation (m AOD)
Frequency
20
40
hydraulics.
Brighton Chalk Block
-10
-20
-40
Frequency
20
40
Borehole yield is often
determined by to the position of
Present base level
and adit systems
Low stand -28mOD
Low stand -32mOD
-140
-180
-180
-220
-220
relative to pumped water levels.
Understanding the vertical
-80
-140
productive fissure horizons
Low stand -56mOD
-60
-100
60
Flow Horizons from impeller logs
Fissures from CCTV, Caliper Logs, Adit
logs
variation in fracture distributions
is vital for accurate modelling of
Adit sources are characterised by low drawdown owing to additional storage
volume and productive fissures, though often the adits may become dewatered
borehole yield
MODFLOW vs MODFLOW-USG
The USGS finite different code MODFLOW 2 is the de-facto standard
software in the United Kingdom for regional scale water resources
modelling. The accuracy of MODFLOW is a function of grid size Local
refinement is not possible, thus simulation of small scale effects by global
grid refinement is computationally inefficient.
Borehole sources typically show higher drawdown owing to well losses
MODFLOW USG3 is a significant revision of the MODFLOW code, that
Summary
supports various styles of unstructured grid. As part of this project we
The combination of local mesh refinement coupled with representation of abstraction sources as discrete three
have further enhanced MODFLOW-USG through the addition of the
dimensional features within the model mesh provides a significant at reduced numerical cost compared with earlier
Environment Agency Developed module to simulate Vertical Variations in
models. Complicated adit sources, as present within the South Downs Chalk aquifer can also be appropriately
hydraulic parameters with depth (VKD)4 to USG functionality.. The 4R
simulated. Final refinement of the unstructured model is currently underway and will be used for both environmental
Recharge model has also been upgraded allow unstructured grids5,6.
assessment and for the prediction of future yields
References
1) Molyneux, I., 2012, Hydrogeological characterisation of the Chalk: with specific reference to unsaturated zone behaviour, PhD thesis, University of Brighton 2) Harbaugh, A.W., 2005. MODFLOW-2005, the U.S. Geological Survey modular ground-water model -- the Ground-Water Flow Process ( No. 6-A34), U.S. Geological Survey Techniques and
Methods. United States Geological Survey. 3) Panday, Sorab, Langevin, C.D., Niswonger, R.G., Ibaraki, Motomu, and Hughes, J.D., 2013, MODFLOW–USG version 1: An unstructured grid version of MODFLOW for simulating groundwater flow and tightly coupled processes using a control volume finite-difference formulation: U.S. Geological
Survey Techniques and Methods, book 6, chap. A45, 66 p. 4) Environment Agency, 2003. Enhancements to MODFLOW-User Guide for MODFLOW VKD–A modified version of MODFLOW 96 to include variations in hydraulic properties with depth (NC/00/23). National Groundwater & Contaminated Land Centre., 5) Heathcote, J.A., Lewis, R.T.,
Soley, R.W.N., 2004. Rainfall routing to runoff and recharge for regional groundwater resource models. Quarterly Journal of Engineering Geology and Hydrogeology 37, 113–130. 6) Lewis, R.T., Davison, P., Witterick, W., 2011. Spatio-temporal recharge and runoff estimation for unstructured grids. Presented at the MODFLOW and More 2011:
Integrated Hydrologic Modelling, Golden, Colorado.,

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