Modeling of Ground Water Flow to Adits

Many of the large ground water sources in the Chalk Aquifer have adits, horizontal tunnels below the ground water table, which are connected to pumped wells. The flow in an adit may be pipe or open channel flow. Adits in the United Kingdom are normally full of water, so that the adit flow is pressurized. Darcy's formula is not applicable to the adit, and conventional ground water models are inappropriate to model the aquifer‐adit system. The U.S. Geological Survey (USGS) model BRANCH simulates one‐dimensional unsteady, nonuniform, multiple‐branch interconnected open channel flow. MODBRNCH incorporates BRANCH into MODFLOW simulating open channel and aquifer interaction using deterministic responses of both systems. An aquifer‐adit model can be created by two steps. First, an integrated surface‐ground water model (MODBRNCH) enables open channel flow to be simulated. Second, introducing a fictitious narrow slot (Preissmann slot) above the adit allows pipe flow to be simulated by open channel flow equations. The slot does not affect the adit cross‐section area, and the water level in the slot represents the pressurized adit head, which can be used by MODBRNCH to calculate the water exchange between aquifer and adit according to their head difference. The approach has been tested on the Wilmington public supply source in southeast England.