DIVERGENCE IN BANK STORAGE METRICS: CONSTANT VERSUS TRANSIENT HYDRAULIC GRADIENT

The hydraulic gradient along stream/aquifer boundary is one of the most variable parameters in a stream/aquifer system. The regional hydraulic gradient (RHG) towards a stream had been often neglected or considered linear and static while estimating bank storage. In this study, I used a non-linear, transient RHG to estimate the bank storage in response to a hypothetical flood wave. The purpose of this study was to compute bank storage separately using a constant vs a transient RHG and estimate the difference between two corresponding results. For a constant RHG, it was considered that head at the valley wall was H unit higher than the head at the stream throughout the simulation. However, for a transient RHG, the water level in the aquifer was instantaneously raised by H units throughout the aquifer width, then the groundwater was drained under natural gradient until the transient RHG at the stream/aquifer boundary matches with the constant RHG for both fully penetrating streams with and without streambed sediments. Water table thus obtained was slightly different from that of the constant RHG. A convolution-integral method was used to estimate flux across the stream/aquifer boundary. Results showed that bank storage was higher and baseflow started later in time for the transient RHG than that for the constant RHG. Furthermore, the difference in bank storage between the two RHG diverged significantly and increased after the critical time. Flood duration longer than the critical time requires consideration of a transient RHG for better estimation of bank storage, especially for narrow but high transmissive alluvial aquifers.