Water wells are an indispensable tool for groundwater extraction. The analytical and empirical approaches available to describe the flow of groundwater towards a well are summarized. Such flow involves a strong velocity increase, especially close to the well. The linear laminar Darcy approach is, therefore, not fully applicable in well hydraulics, as inertial and turbulent flow components occur close to and inside the well, respectively. For common well set-ups and hydraulic parameters, flow in the aquifer is linear laminar, non-linear laminar in the gravel pack, and turbulent in the screen and the well interior. The most commonly used parameter of well design is the entrance velocity. There is, however, considerable debate about which value from the literature should be used. The easiest way to control entrance velocity involves the well geometry. The influence of the diameter of the screen and borehole is smaller than that of the screen length. Minimizing partial penetration can help to curb head losses.

Construction of a conveyance tunnel through rock often induces the ingress of groundwater into the tunnel, a flow that changes both the hydrogeological regime of the tunnel and its environment. To explore this key interaction, a novel modeling approach using the conduit flow process (CFP) is developed that considers both the hydraulic head and the ingress of water from the rock matrix during excavation. The resulting flow values are predicted through an adapted MODFLOW numerical model into which the tunnel is introduced with the aid of the new CFP approach. The CFP approach can simulate both laminar and turbulent flow in the tunnel whether the flow is free surface or pressurized. Several simulations, including one for which the permeability of the tunnel perimeter is assumed to be identical to the hydraulic conductivity of the surrounding rock matrix, are then used to explore the sensitivity of the predicted head and flow conditions to the permeability of the tunnel perimeter. Comparisons of the numerical results with field data from the Kerman Water Conveyance Tunnel in Iran show that the proposed approach accurately predicts the spatial variation of both groundwater ingress and hydraulic head.

Well ageing is mostly caused by mechanical and biogeochemical clogging processes, which affect the gravel pack, screen slots and casing. Clogging deposits increase head losses due to a constriction of the hydraulically effective area. For this study, clogging is mimicked by systematically reducing the gravel pack porosity, the screen open area and the nominal inner casing diameter. Groundwater flow velocity strongly increases close to the well, inducing inertial and turbulent flow components. Therefore, gravel pack head losses were calculated using the Forchheimer-Engelund equation, in conjunction with the Kozeny-Carman equation, which relates gravel pack porosity and hydraulic conductivity. Screen losses were assessed using the Orifice equation and turbulent casing losses with the Darcy-Weisbach equation. For the settings chosen here, a dramatic increase of head losses occurs when the clogging has reduced the effective porosity in the gravel pack by ~65%, the open area of the screen by ≥98%, and the casing diameter by ~50%. Since the latter two conditions are rarely reached in actual wells, the clogging of the gravel pack is the decisive parameter that controls well ageing. Regular monitoring of the well yield is therefore needed, since processes in the gravel pack are difficult to track directly. Unlike the deposits on the casing and in the screen slots, obstructions in the gravel pack are much more difficult to remove.