ESI, now part of Stantec, were appointed to develop an appropriate tool for assessing the potential impact of proposed dewatering on a Habitats Directive Site in relation to the planning of a tunnel passing Stonehenge.
The Highways Agency were in the planning stages of a 2.1km tunnel to conceal the A303 passing Stonehenge. The tunnel was to be constructed in the Chalk, with the proposed tunnelling method requiring dry working conditions. To maintain these dry conditions pumping from the Chalk aquifer was required to temporarily lower groundwater levels below the deepest point of the tunnel.
Although this proposed working method included discharge of the pumped groundwater back into the Chalk aquifer locally, the Environment Agency and English Nature raised concerns that the scheme may have adverse effects on baseflows to the River Avon, a Habitats Directive site.
ESI were commissioned to model the effect of dewatering on flows to the River Avon. Following review of available data and the existing conceptual model, a MODFLOW-based model was rejected due to the lack of detailed distributed data for the Chalk. To adequately constrain the model to gain benefit from the multi-dimensional flow geometry was not possible.
An alternative lumped water balance approach was taken, with groundwater catchment simulated by a series of connected stores containing a representation of the soil, unsaturated and saturated zones. A recharge time series for each store was estimated using a conventional soil moisture balance method, with results in excellent agreement with historical time series data and flood records.
The model showed that baseflow to the River Avon would be slightly reduced in the summer months, but not significant in terms of the normal flow. Both the Environment Agency and English Nature were satisfied with the results and accepted that the pumping scheme would not adversely affect the baseflow regime of the River Avon.
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