Riffle-pool morphometry and stage-dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen-Anhalt, Germany)

摘要

Riffle-pool sequences are a common feature of gravel-bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross-sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen-Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage-dependant morphometric changes to riffle and pool maintaining processes. In accordance with previous authors, pool cross-sections of the Mulde River are narrow and riffle cross-sections are wide suggesting that they should rather be addressed as two general types of channel cross-sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Q_(bf_ (40% bankfull discharge). An index for the longitudinal amplitude of riffle-pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes. According to the observed dynamics of bed amplitudes, maintenance of riffle-pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post-flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility. The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload.