The Importance of Considering Turbulent Flow when Modelling Adit Systems

Horizontal shafts, or adits, have historically been dug outward from pumping boreholes in a number of aquifers in the UK to increase yields. Such adit systems are common in the Chalk aquifer, which is a major source of public water supply for south-east England. Adit systems can be complex, individual adits can be kilometres long, and many adits are located beneath river valleys. To understand how these adited sources perform during drought, what impacts they could have on ecologically sensitive rivers, and what pumping yields can be sustained, a modelling approach is required that can represent the physical complexities of these systems and a range of flow processes, including the influence of turbulent flow on drawdown. Despite their size (diameter 2 m), high abstraction rates can mean that flow within adited systems are turbulent and, therefore, Darcian flow equations will underestimate head losses. This, in turn, can affect the estimation of yield during drought conditions. Adit systems have been investigated in the past but turbulence has not been included when modelling them. To investigate the effect of turbulence, modifications to both the numerical discrete-continuum model MODFLOW-2005 Conduit Flow Process and the Connected Linear Network Process implementation in MODFLOW-USG are applied to an idealized adit-aquifer system. Different abstraction rates (0.01 to 1.0 m3s-1) are applied to the model for both turbulent and laminar flow. Results show that head losses are significant for turbulent flow and, therefore, the inclusion of this is necessary to assess and model adit systems. The differences from Darcian flow are greater for higher abstraction rates, corresponding to higher velocities. This behaviour has also been assessed for a vertical shaft or more complex scenarios such as horizontal adits connected to vertical shafts common in adited systems. This approach will be particularly important when climate change scenarios are examined as the frequency and intensity of droughts are expected to increase in the future.