The Cottingham groundwater source is complex, comprising two pumping shafts, 17 other shafts and 1000 m of operational adit. Understanding the interaction between aquifer, adit and shafts is very important to assess the yield and vulnerabilityof the source. Numerical models are good tools to assemble and analyse field data and formulate ideas about system dynamics. An integrated groundwater,pipe and open-channel flow computer code has been developed by a combination of a Preissmann slot and MODBRNCH, and used to simulate the aquifer-aditsystem. Introducing a fictitious narrow slot (Preissmann slot) above the adit makes the equations for open-channel flow also valid for pipe flow. A model ofCottingham was calibrated against multiple targetsincluding initial and transient conditions, aquifer and adit heads, the head profiles for the initial condition, and the temporal changes of heads during a pumping test. Three alternative conceptual models were tested, each with different hydraulic assumptions about interactionbetween aquifer, adit and shafts. Only one model was well calibrated to all targets, and had a variable ratio of inflows to shafts and adit. The study developed a new understanding of the hydraulics and hydrogeology of the complex Cottingham adit and shaft system.

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