Wervelend water

Civil Engineering and Geoscience

Horizontal mixing in shallow flows

Shallow shear flows play an important role in the transverse transport of mass and momentum in rivers. In order to understand structure of the flow and the mechanism resulting in the particular turbulence properties, three types of shallow mixing layers are investigated each with a different cause for the velocity difference: inflow conditions, bed level and bed roughness. It is shown that mixing layer properties are highly affected by the 3D turbulence generated in the bottom boundary layer. This implies that effects of subtle geometric properties like roughness variation and bed level changes should be incorporated in predictive modelsHydraulic EngineeringCivil Engineering and Geoscience

Turbulente ondiepe stromingen

Hydraulic EngineeringCivil Engineering and Geoscience

Numerical and physical modelling of rapidly varying flow over weir-like obstacles in rivers and floodplains

Hydraulic EngineeringCivil Engineering and Geoscience

Flow resistance of vegetated oblique weir-like obstacles during high water stages

At high water stages obstacles in the flood plains of a river contribute to the flow resistance and hamper the conveyance capacity. In particular the elevated vegetated parts are expected to play an important role. The objective of this research work is to determine the form drag due to vegetated oblique weir-like obstacles. The experiments have been performed to measure the energy head losses for a range of subcritical flow conditions, varying discharges and downstream water levels. The energy head loss caused by the submerged vegetated weir-like obstacle has been modeled using an expansion loss form drag model that has been derived from one dimensional momentum conservation equation and accounts for the energy loss associated with a deceleration of the flow downstream of a sudden expansion. The results have been compared with the experimental data and showed an overall good agreement.Hydraulic EngineeringCivil Engineering and Geoscience

Flow resistance of vegetated oblique weir-like obstacles during high water stages

At high water stages obstacles in the floodplains of a river contribute to the flow resistance. In particular the elevated vegetated parts are expected to play an important role. The objective of this study is to estimate and parameterize the form drag due to vegetated oblique weir-like obstacles. An experimental study has been carried out in the Laboratory of Environmental Fluid Mechanics of Delft University of Technology. Measurements for energy head losses were carried out for a range of discharges and downstream water levels covering submerged and subcritical flow conditions. The Head loss due to submerged vegetated dikes has been modeled by an expansion loss form drag model. Expansion loss form drag model has been derived from one dimensional momentum conservation equation and accounts for the energy loss associated with a deceleration of the flow downstream of a sudden flow expansion. The results have been compared with experimental data.Hydraulic EngineeringCivil Engineering and Geoscience

The form drag due to vegetated weir-like obstacles interpreted as expansion losses

The Objective of this study is to estimate the form drag due to vegetated weir- like obstacles. Head loss due to submerged vegetated dikes and groins has been modeled by expansion loss form drag model (Energy and momentum balance principles) and it has been compared with experimental data. An experimental study has been carried out on a scaled physical model in the Laboratory of Environmental Fluid Mechanics of Delft University of Technology. The prototype dike and groin has been modeled as a weir and vegetation has been represented as cylinders on the crest of the weir with varying blockage area. Different cases have been studied for emerged and submerged cylinders. Measurements for energy head losses were carried out for a range of discharges and down stream water levels covering submerged and subcritical flow conditions. Agreement of theoretical results predicted by an expansion loss form drag model is good with experimental data for the case of a weir with cylinders. So the expansion loss model is of practical use for the prediction of head losses of a vegetated groin or dike.Hydraulic EngineeringCivil Engineering and Geoscience

Transverse Momentum Exchange Induced by Large Coherent Structures in a Vegetated Compound Channel

In floodplains of vegetated channels, transverse exchange processes of mass and momentum are of primary importance as these are directly linked to the river bank stability, sedimentation, and nutrient transport. Despite its importance, knowledge about this phenomenon is still incomplete especially in the context of the presence of the large horizontal coherent structures (LHCSs). As a result, although various exchange models have been developed, their applicability in different circumstances is still unclear as their validity is usually restricted to a narrowly ranging experiment data set. A proper model for this exchange in a compound channel geometry with or without vegetation is lacking. In order to obtain more insight, a laboratory experiment of a shallow flow field in a compound vegetated channel has been conducted. A quadrant analysis of the Reynolds shear stresses has been applied to study the connection between the cycloid flow events induced by the LHCSs and transverse momentum exchange in the channel. It is suggested that local variability leads to differences in the transverse exchange of momentum. Furthermore, the experimental data were used to verify state-of-the-art momentum exchange models. As the limitations of those models were analyzed, for the first time a hybrid eddy viscosity model based on the occurrence of LHCSs and the presence of vegetation was proposed and validated using a variety of experimental data sets. The results suggest that the transverse momentum exchange can be well modeled with the new eddy viscosity model for quite a range of different setups and scenarios by varying only a coefficient of proportionality β, which is related to the transverse slope between the main channel and the floodplain.Environmental Fluid Mechanic

Cycloid flows induced by the Large horizontal coherent structures in the vegetated compound channel

Vegetation in general and mangrove, in particular, plays a significant role in the protection of the coastal and estuarine regions from erosion. In particular, estuarine mangrove forests can effciently reduce the impact of a strong along-bank flow during high tides and high river discharge, creating shelter regions for the development of the ecological system. As estuarine mangrove is usually inundated during high tides and exposed to strong tidal flows, the hydrodynamic of estuarine mangrove forest is similar to that of a vegetated compound channel. In order to gain more insight into this field of the research, a hydraulic laboratory experiment of a shallow flow field in a vegetated compound channel has been conducted. Experimental results confirm a pronounced existence of large horizontal coherent structures (LHCSs). Although the presence of the large horizontal coherent structures in the vegetated channel has been well recognized, it is still unclear how they affect the flow field, mass and momentum exchange in the vegetated compound channel. Detailed analyses of the experimental data reveal more about the role of the large horizontal coherent structures. The flow field under the effect of the large coherent structures shows a spatially and temporally cycloid motion, contributing a large part to the momentum and mass transfer. The quadrant analysis of the Reynolds shear stresses has been applied to examine the connection between the motion of the large horizontal coherent structures and their associated cycloid flow events.Coastal EngineeringEnvironmental Fluid Mechanic

Preliminary Evaluation of Existing Breaching Erosion Models

The ability to estimate the erosion rate along an underwater breach face is crucial to understand the evolution of breaching failure. To this end, breaching erosion models were developed and applied in numerical models. However, these erosion models have never been validated, owing to the scarcity of direct measurements of turbidity currents that accompany breaching. The aim of this study is to evaluate the existing breaching erosion models using direct measurements of recently performed laboratory experiments on breaching flow slides. We found out that the erosion model put forward by Mastbergen & Van Den Berg (2003) provides good agreement with the data and performs better than the one proposed by Van Rhee (2015). The latter tends to overestimate the erosion rate, particularly at steeper slopes.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Environmental Fluid Mechanic