A simple analytical formula is developed to calculate transient discharge inflow rates into a tunnel or a well under constant drawdown. The agreement with the classical, but cumbersome diffusion-equation-based solution of Jacob and Lohman is excellent throughout the range of dimensionless times. By using only a straightforward logarithmic function, this explicit solution may therefore be used with great computational benefits in practice, and also when further mathematical manipulations such as differentiation or integration are required.Resumen Una fórmula analítica sencilla fue desarrollada para calcular el grado de descarga transitoria hacia un tfflnel, o un pozo, bajo condición de un abatimiento constante. Existe una concordancia excelente, desde el comienzo hasta el final, en el rango de los tiempos adimensionales, con la solución clµsica pero complicada, de Jacob y Lohman, basada esta fflltima en la ecuación de difusión. Solamente mediante el uso de una función logarítmica simple, esta solución explícita puede por tanto ser usada en la prµctica con grandes ventajas computacionales, y tambiØn cuando se necesitan manipulaciones matemµticas adicionales, tales como diferenciación o integración.RØsumØ On a dØveloppØ une formule analytique simple pour le calcul du flux infiltrØ en rØgime transitoire dans un tunnel ou un puits en supposant le rabattement constante. Les rØsultats sont en accord avec la solution plus compliquØe de Jacob-Lohman de l'Øquation de diffusion, sur tout l'intervalle de temps adimensionel considØrØ. En utilisant une fonction logarithmique ce solution explicite peut Þtre utilisØe sans un grand effort de calcul dans la pratique courante, ainsi que dans les situations o il est nØcessaire à dØriver ou intØgrer l'expression du rabattement.
Keywords Well hydraulics · Tunnel discharge
Classical SolutionConfined horizontal flow towards a fully penetrating well in a semi-infinite, homogeneous aquifer of constant thickness is classically described by the radial form of the diffusion equation. Assuming uniform hydrostatic initial heads and a sudden, constant drawdown at the well, an analytical expression for the resulting transient discharge rates was first published by Jacob and Lohman (1952) who applied the heat conduction solution of Smith (1937) to groundwater dynamics.Consider the radial form of the diffusion equationwith the initial and boundary conditions sðr; 0Þ ¼ 0;sðr o ; tÞ ¼ s o ;sð1;tÞ ¼ 0 ð2Þwhere the symbols stand for aquifer transmissivity (T), storage coefficient (S), time (t), radial coordinate (r), well radius (r o ), drawdown (s) and drawdown at the well (s o ). The above problem was analysed by Smith who resorted to Green's functions and integral transforms but could only find a solution for @s=@r at the origin. Using this result, Jacob and Lohman deduced the flow rate at the origin (i.e. into the well) aswithwhere a is dimensionless time, J o and Y o are first and second kind zero-order Bessel functions respectively, and u is a dummy variable. The above solution has bec...