Due to the complexity of fine sediment dynamics in tidal estuaries a large number of parameters is necessary to describe the processes for the formation of temporally and spatially variable bottom conditions. By virtue of the Van-der-Waals forces, which dominate below a critical distance against repulsive forces, fine sediment mixtures with clay contents above ~5-10% tend to aggregate to flocs. The time- and location dependent physical, chemical and biological environmental conditions significantly influence the cohesiveness of the individual particles and thus also the transport dynamics of the particles. Furthermore, tidal asymmetries and estuarine circulation lead to the formation of an estuarine turbidity maximum zone (ETM). The location of ETMs is often linked with the mixing zone of salt and fresh water, where the environmental conditions change periodically (tidal cycles,...) and non-periodically (outflow, wind,...). As a result, high concentrations of suspended solids lead to the formation of stationary suspensions, resulting in near-bed lutoclines with partly non-newtonian behavior. The long-term accumulation of the sole is attributed to dynamic stratification and may rely on fresh water discharge when tidal energy is low during neap-tide cycles slack time.In order to ensure the nautical depth, highly expensive regular maintenance dredging is necessary in estuaries where the ETM dynamics lead to accumulation of cohesive sediments. A previous study has shown improvements of modelling the dynamics of sediment transport for representing the Weser ETM by implementing a 2-Layer-concept for modelling sediment transport and bed exchange. Limitations in the model are due to missing crucial information on (fractious) insitu behavior of cohesive sediments in terms of consolidation, erosion and deposition fluxes. The main force leading to erosion is the flow, which contains turbulent and non-turbulent proportions. In contrast, critical bottom-shear-stress is seen as a key factor to model erosion resistance, which is naturally affected by various parameters like organic content, consolidation, stress history and ambient chemical conditions. Key factors to model consolidation are deposition and stress history. A sediments density is a cross-process parameter related to erosion resistance and the state of consolidation. The goal of the research started with hereby presented project FAUST (For An improved understanding of Sediment Transport) is filling gaps in knowledge by determining erosion, deposition and consolidation characteristics of natural soils from northern German estuaries (Weser) in laboratory experiments to improve the overall understanding of bed exchange of these sediments.The methodology proposed is of a four-step schedule which consists of sediment sampling during slack water, onsite soil characterization (erosion tests, density measurements), lab sedimentation and erosion experiments and finally, the results lead to the development of an INTERCOH - Bottom Shear, Erosion and Bed Exchangeupdated bed exchange concept. Sediment samples from a site (Blexen) within the Weser ETM are drawn with a soil core sampler (cylindric core of 120cm height vs. 20 cm diameter) developed at IRCE in 2019 to cover the lower water body as well as upper soil layers. On board, a set of erosion and density measurements investigate the quasi-insitu natural sediments. Further on, a set of sediment cores are seperated into layerwise samples for further analysis, such as sieve/sedimentation analysis or TOC determination. The natural layer structure stays intact by a straight up transport of the sediment cores. They are stored at 6 °C. In the lab, we perform two types of analysis. First of all, the aim is to reproduce the previously quasi insitu determined characteristics of the sediment stratification in settling experiments. Therefore, a settling column of 2.5 m height and 20 cm diameter was developed which is consistent to the sediment cores geometry. A set of settling experiments is carried out to analyse depth dependent density development. Subsequently, depth- and time-dependent critical erosion shear stresses as well as erosion rates will be determined by means of erosion tests. These tests are carried out by eroding the beforehand generated sediment samples. With this methodology, the effect of influencing parameters on erosion characteristics will be investigated (visually, density, sediment proposition). The erosion device used is a modified gust erosion chamber which is able to produce defined shear stresses on a soil samples surface to determine erosion rates at different flow states. To estimate erosion rates ultrasonic sounders measure the change in soil sediment height over time. For redundancy, the turbidity within the water column above soil is measured as well. The ultrasonic sounders are used beforehand to measure the generated flow field in the erosion chamber to estimate induced bottom shear stresses on the soil and to estimate mean measure settling velocities. For a combined analysis of erosion and settling experiments, the erosion chamber induces tidal flow field characteristics to investigate long-term sediment accumulation behaviors. Finally, the result will be further knowledge on how the natural sediment characteristics behave under certain conditions. An adapted bed exchange model specified for the examined estuary will be provided to improve existing numerical models.LiteratureGUST,G.;MÜLLER,V.(1997): Interfacial hydrodynamics and entrainment functions of currently used erosion devices. In: Cohesive Sediments, S. 149–174.HESSE,R.F.;ZORNDT,A.;FRÖHLE,P.(2019): Modelling dynamics of the estuarine turbidity maximum and local net deposition. In: Ocean Dynamics 69 (4), S. 489–507. DOI: 10.1007/s10236-019-01250-w.TORFS,H.;MITCHENER,H.;HUYSENTRUYT,H.;TOORMAN,E.A.(1996): Settling and consolidation of mud/sand mixtures. In: Coastal Engineering 29 (1-2), S. 27–45. DOI: 10.1016/S0378-3839(96)00013-0.
15th INTERNATIONAL CONFERENCE on COHESIVE SEDIMENT TRANSPORT PROCESSES13 - 17 OCTOBER 2019 • ISTANBUL / TURKEYYILDIZ TECHNICAL UNIVERSITY - YILDIZ CAMPUS99P1002 - CONCEPTUAL DESIGN FOR INVESTIGATIONS ON NATURAL COHESIVESEDIMENTS FROM WESER ESTUARYBottom Shear Erosion and Bed ExchangeJustus Patzke 1, Roland Hesse 1, Edgar Nehlsen 1, Anna Zorndt 2, Peter Fröhle 1Tu Hamburg, Institute for River and Coastal Engineering (irce), Hamburg-Germany 1Federal Waterways Engineering and Research Institute, Hydraulic Engineering In CoastalAreas, Wedel-Germany 2Keywords: microcosm, bottom shear, erosion, Weser, ElbeDue to the complexity of fine sediment dynamics in tidal estuaries a large number of parameters is necessary to describe the processes for the formation of temporally and spatially variable bottom conditions. By virtue of the Van-der-Waals forces, which dominate below a critical distance against repulsive forces, fine sediment mixtures with clay contents above ~5-10% tend to aggregate to flocs. The time- and location dependent physical, chemical and biological environmental conditions significantly influence the cohesiveness of the individual particles and thus also the transport dynamics of the particles. Furthermore, tidal asymmetries and estuarine circulation lead to the formation of an estuarine turbidity maximum zone (ETM). The location of ETMs is often linked with the mixing zone of salt and fresh water, where the environmental conditions change periodically (tidal cycles,...) and non-periodically (outflow, wind,...). As a result, high concentrations of suspended solids lead to the formation of stationary suspensions, resulting in near-bed lutoclines with partly non-newtonian behavior. The long-term accumulation of the sole is attributed to dynamic stratification and may rely on fresh water discharge when tidal energy is low during neap-tide cycles slack time.In order to ensure the nautical depth, highly expensive regular maintenance dredging is necessary in estuaries where the ETM dynamics lead to accumulation of cohesive sediments. A previous study has shown improvements of modelling the dynamics of sediment transport for representing the Weser ETM by implementing a 2-Layer-concept for modelling sediment transport and bed exchange. Limitations in the model are due to missing crucial information on (fractious) insitu behavior of cohesive sediments in terms of consolidation, erosion and deposition fluxes. The main force leading to erosion is the flow, which contains turbulent and non-turbulent proportions. In contrast, critical bottom-shear-stress is seen as a key factor to model erosion resistance, which is naturally affected by various parameters like organic content, consolidation, stress history and ambient chemical conditions. Key factors to model consolidation are deposition and stress history. A sediments density is a cross-process parameter related to erosion resistance and the state of consolidation. The goal of the research started with hereby presented project FAUST (For An improved understanding of Sediment Transport) is filling gaps in knowledge by determining erosion, deposition and consolidation characteristics of natural soils from northern German estuaries (Weser) in laboratory experiments to improve the overall understanding of bed exchange of these sediments.The methodology proposed is of a four-step schedule which consists of sediment sampling during slack water, onsite soil characterization (erosion tests, density measurements), lab sedimentation and erosion experiments and finally, the results lead to the development of an updated bed exchange concept. Sediment samples from a site (Blexen) within the Weser ETM are drawn with a soil core sampler (cylindric core of 120cm height vs. 20 cm diameter) developed at IRCE in 2019 to cover the lower water body as well as upper soil layers. On board, a set of erosion and density measurements investigate the quasi-insitu natural sediments. Further on, a set of sediment cores are seperated into layerwise samples for further analysis, such as sieve/sedimentation analysis or TOC determination. The natural layer structure stays intact by a straight up transport of the sediment cores. They are stored at 6 °C. In the lab, we perform two types of analysis. First of all, the aim is to reproduce the previously quasi insitu determined characteristics of the sediment stratification in settling experiments. Therefore, a settling column of 2.5 m height and 20 cm diameter was developed which is consistent to the sediment cores geometry. A set of settling experiments is carried out to analyse depth dependent density development. Subsequently, depth- and time-dependent critical erosion shear stresses as well as erosion rates will be determined by means of erosion tests. These tests are carried out by eroding the beforehand generated sediment samples. With this methodology, the effect of influencing parameters on erosion characteristics will be investigated (visually, density, sediment proposition). The erosion device used is a modified gust erosion chamber which is able to produce defined shear stresses on a soil samples surface to determine erosion rates at different flow states. To estimate erosion rates ultrasonic sounders measure the change in soil sediment height over time. For redundancy, the turbidity within the water column above soil is measured as well. The ultrasonic sounders are used beforehand to measure the generated flow field in the erosion chamber to estimate induced bottom shear stresses on the soil and to estimate mean measure settling velocities. For a combined analysis of erosion and settling experiments, the erosion chamber induces tidal flow field characteristics to investigate long-term sediment accumulation behaviors. Finally, the result will be further knowledge on how the natural sediment characteristics behave under certain conditions. An adapted bed exchange model specified for the examined estuary will be provided to improve existing numerical models
