ROLE OF FLOOD DURATION ON TRANSIENT BANK STORAGE IN ALLUVIAL AQUIFERS

The role of flood duration, especially one longer than 10 days, on bank storage was studied for an unconfined alluvial aquifer composed of sand and gravel adjacent to a wide river. The purpose was to estimate transient bank storage using a constant versus a transient groundwater hydraulic gradient and to calculate difference between the two corresponding results. A convolution-integral method was used to estimate flux across the stream-aquifer boundary and bank storage due to such flood. The bank storage due to flood wave was simulated using both constant and transient hydraulic gradients. For the transient gradient simulation, an aquifer was first completely saturated and then drained half of its groundwater to match aquifer storage for the two simulations at the beginning. Water table, thus obtained, was found slightly different from that of a constant gradient. Results show that bank storage for both simulations was approximately equal until the critical time (t_c=W²/5D; W=aquifer width, D= aquifer diffusivity). But, the two bank-storage became significantly different thereafter resulting excessive groundwater exfiltration in use of the constant gradient. The difference between two results increased as aquifer diffusivity was increased. But, for different aquifer diffusivities, the difference between two bank-storage at corresponding times remained apparently constant when expressed in dimensionless time (W²t/D; t=time). Flood duration longer than the t_c requires consideration of a transient hydraulic gradient for better estimation of bank storage, especially for narrow but high transmissivity alluvial aquifers adjacent to a wide river.