Estimating mean flow velocity in channel and floodplain areas and its use for explaining the pattern of overbank deposition and floodplain retention

Abstract

Stage-discharge curves for out-of-bank flows are derived from direct discharge measurements and theoretical calculations of channel and floodplain conveyance at a station, and comparisons of volumes of flood waves at successive stations. A method was developed which permits estimation of the mean flow velocity in the channel and floodplain zones at a given total discharge taken from a rating curve. Out-of-bank channel flows are estimated under the assumption that the interface planes between the channel zone and the floodplain are included in the wetted perimeter for the channel subarea. The floodplain flows are then calculated as the residual values after subtracting the flow in the channel zone from the total discharge. Results obtained using the method are presented for three cross-sections located on mountain, foothill and piedmont rivers within the upper Vistula drainage basin, southern Poland, and are considered in relation with the pattern of floodplain sedimentation caused by a major flood of July 1997. High rates of lateral thinning and fining of overbank deposits were observed for the piedmont Vistula River which is characterized by a marked contrast in mean velocity between the channel and floodplain flows. On the mountain Skawa River and the foothill Skawinka River mean velocity of the floodplain flow was high, both in absolute values and when compared with that of the channel flow; overbank deposits were generally thin there and showed minor changes in thickness, mean size and sorting across the floodplains. This study shows that the proposed method of estimating mean flow velocity in channel and floodplain areas may yield valuable information about hydraulics of out-of-bank flows, which may be used for explaining the observed pattern of floodplain deposition. The results also demonstrate that the floodplain of the Vistula acts mainly as a storage reservoir at low out-of-bank flows but its efficiency in carrying flood water grows when flow exceeds that of 12-year frequency. On the other hand, retention potential of the Skawa floodplain is very low but flow becomes increasingly stored after water starts to spread rapidly onto the higher terrace at a discharge of 14-year recurrence interval.