Influence of lateral embayments on suspended sediment transport under unsteady flow conditions

Abstract

Local widening in a channelized river is a common practice in restoration projects. The lateral embayments built for this purpose in the river banks are partially filled up by fine sediments. This allows the formation of aquatic habitats with hydraulic and morphologic diversity. However, the design of these lateral cavities may be compromised by the fluctuations in the water discharge. To address this problem, systematic experimental investigations have been carried out with five different fluctuating hydrograph scenarios. Water depth, sediment concentration and area covered by the settled sediments are analysed in each experiment. The process of sedimentation in the lateral embayments proved to be, in general, resilient to flow fluctuations. However, there were several differences observed during the high flow phase fluctuation phase depending on the geometric configuration of the embayments and the applied hydrograph: (i) a higher peak in discharge means that more sediments are resuspended and a complete remobilization of the in-cavity sediment deposits is possible. (ii) Long and short cavities are more resilient to high flow events than cavities with an intermediate length.

Poison in paradise: increase of toxic effects in restored sections of two rivers jeopardizes the success of hydromorphological restoration measures

“…As this is the first study to report an increase in toxicity as well as estrogenic and dioxin-like activity in restored river sections, this may explain the lack of success of many other in-stream restoration projects [13,29,[31][32][33]. A possible explanation for the significantly worse results within the restored sections is the transport and deposition of polluted fine sediments within the restoration measures [18,[119][120][121]. Since the restored sections are characterized by a higher flow diversity, it is conceivable that polluted fine particulate matter, e.g., introduced by soil erosion from surrounding fields [122][123][124][125] or by WWTPs [107,125], may settle in the flow-calmed zones [119,[126][127][128].…”

“…A possible explanation for the significantly worse results within the restored sections is the transport and deposition of polluted fine sediments within the restoration measures [18,[119][120][121]. Since the restored sections are characterized by a higher flow diversity, it is conceivable that polluted fine particulate matter, e.g., introduced by soil erosion from surrounding fields [122][123][124][125] or by WWTPs [107,125], may settle in the flow-calmed zones [119,[126][127][128]. The reduced flow velocity might result in an increased exchange between sediment and water, so that formerly sediment associated substances are remobilized and affect the local invertebrate fauna [96,97,122,129].…”

Much effort, little success: causes for the low ecological efficacy of restoration measures in German surface waters

“…A possible explanation for these results was proposed by [76], who linked the two aspects of deposition of fine sediments and increased toxicity in restored river sections: in contrast to the unrestored sections with their straightened structure and uniform flow pattern, the restored sections are characterized by a higher flow diversity. This allows the deposition of contaminated fine particulate matter in the flow-calmed zones [153]. In these zones the reduced flow velocity supports an increased exchange between sediment and water, so that sediment-bound chemicals are more likely to be remobilized and may affect the macrozoobenthic community [154,155].…”