As part of proposed improvements to the A303 between Amesbury and Winterbourne Stoke in Wiltshire, the Highways Agency is planning to construct a 2.1 km long tunnel to conceal the road as it passes to the south of Stonehenge. The tunnel will be constructed in the Chalk and the proposed tunnelling method requires dry working conditions. In order to maintain these conditions, pumping from the Chalk aquifer will be required to temporarily lower groundwater levels to below the deepest point of the tunnel.
Although the proposed working method includes discharge of the pumped groundwater back into the Chalk aquifer locally, concerns were raised by the Environment Agency and English Nature. Their concern was principally that the scheme might allow water to move more rapidly beneath the dry valley system, potentially leading to adverse effects on summer baseflows to the River Avon, a SSSI and a candidate Special Area of Conservation.
ESI were commissioned by Halcrow Gifford Joint Venture, designers to the Balfour Beatty-Costain Joint Venture team appointed by the Highways Agency, to model the effect of the dewatering on flows to the River Avon.
A review of the available data and the existing conceptual model of the system was undertaken prior to considering the options for modelling the system. Due to the lack of detailed distributed data for the Chalk, a MODFLOW-based model was rejected as it would not be possible to adequately constrain this type of model so as to gain any benefit from the multi-dimensional flow geometry of MODFLOW.
Instead, a lumped water balance approach was chosen with the groundwater catchment simulated by a series of connected stores. Each store containing a representation of the soil zone, the unsaturated zone and the saturated zone. This approach was presented to the Environment Agency for their approval and to enable them to suggest modifications if required.
The model was then implemented in an Excel spreadsheet with outputs at each store in the form of storage change, groundwater heads and discharge to the down gradient store. The model also took into account spring flows when groundwater levels rose to surface. A recharge time series for each store was estimated using a conventional soil moisture balance method, and the results were shown to be in excellent agreement with historical time series data and flood records.
The model showed that the baseflow to the River Avon would be slightly reduced in the summer months, but that this was not significant in terms of the normal flow in the River. Both the Environment Agency and English Nature were satisfied with the results of the modelling and accepted that the pumping scheme would not adversely affect the baseflow regime of the River Avon.
ESI is recognised as a UK centre of excellence for groundwater modelling, and is contracted by both public and private organisations to carry out similar work.