A NUMERICAL STUDY OF BANK STORAGE AND ITS CONTRIBUTION TO STREAMFLOW

Floodplains mitigate against extreme annual hydrologic phenomena by st oring substantial volumes of water which would otherwise increase hood volumes. Later floodplains gradually release this water which serves to maintain baseflows. This phenomenon, called bank storage, has impor tant physical and ecological ramifications which in addition to reduci ng flood peaks, include sustaining riparian vegetation and improving w ater quality. We developed a model of bank storage based upon the earl ier work of Neuman and Witherspoon (Wat. Resour. Res., 6 (1990) 889, 1 376; 7 (1971) 611). This model (WaTab2D) treats the flow of water thro ugh floodplain soils for a general two-dimensional case: non-symmetric al valleys, non-symmetrical channel banks, non-uniform hydraulic geome try and non-zero boundary fluxes. The total volume that can potentiall y be released from bank storage is nearly proportional to the width of the floodplain, the height of the bank, and the specific yield; the d uration over which water is released from bank storage increases with increasing floodplain width and decreases with hydraulic conductivity. Drainage of water from the floodplain with a drop in channel water le vel occurs over a period of days in gravel, weeks to a few years in sa nd, years in silt, and decades in clay. The rate of drainage decreases in an exponential-like manner. (C) 1997 Elsevier Science B.V.