Magdalena M. Mrokowska

Evaluation of friction velocity in unsteady flow experiments

ABSTRACT The paper presents estimates of the friction velocity in unsteady flow using two formulas that have been derived from the two-dimensional Reynolds and one-dimensional Saint-Venant equations. Experiments on unsteady flow were performed to collect data on the spatio-temporal evolution of the water level and velocity necessary to apply the formulas. The key feature of these experiments is high temporal resolution of the data. The application of the formulas is presented with an in-depth analysis of the flow conditions, methods for estimating the input variables and an uncertainty analysis of the results. The evaluation of the input variables focuses on the spatial derivative of water depth, which is the most problematic variable in this study. A comparison of the formulas for the friction velocity shows that they are equivalent for the cases presented in this paper.

The Uncertainty of Measurements in River Hydraulics: Evaluation of Friction Velocity Based on an Unrepeatable Experiment

The chapter addresses the issue of evaluating uncertainty of measurements in river hydraulics. As replication of experiments in natural settings is not possible, the chapter highlights the case of unrepeatable experiments. Evaluation of friction velocity under flood conditions basing on the Saint-Venant set of equations serves as an example of indirect measurement of physical quantity.

Flume Experiments on Gravel Bed Load Transport in Unsteady Flow—Preliminary Results

This chapter presents settings and results of laboratory experiments on bed load transport. Experiments were performed for unsteady hydrographs and for steady flow conditions. Continuous measurements of transported mass and flow rate reveal a clockwise hysteresis of bed load rate versus flow rate. A comparison between bed load rate in unsteady and steady flow shows that it depends to a large extent on sediment supply. The results are discussed in the light of variable sediment supply, and suggestions for procedure of further experiments are presented.

Assessing Validity of the Dead Zone Model to Characterize Transport of Contaminants in the River Wkra

The objective of this chapter is to establish the validity of one-dimensional dead zone model to characterize transport of passive and conservative pollutants in the Wkra River basing on a tracer test. In the study, optimization methods prove inadequate to obtain reliable results. Therefore, uncertainty estimation and sensitivity analysis (SA) have been applied. The results indicate equifinality. Moreover, the sensitivity analysis shows that transient storage in dead zones has a small impact on model predictions.

Stratification-induced reorientation of disk settling through ambient density transition

Settling due to gravity force is a basic transport mechanism of solid particles in fluids in the Earth. A large portion of particles occurring in nature and used in technical applications are non-spherical. Settling of particles is usually studied in homogeneous ambient conditions, however, stratification is inherent of natural fluids. It has been acknowledged that stratification modifies the velocity of settling spheres and amorphous aggregates. However, the effect of particle shape on the dynamics of settling through density-stratified ambient fluid has not been recognized well enough. Here I show experimental evidence that continuous density transition markedly modifies the settling dynamics of a disk in terms of settling velocity and orientation of a particle. Settling dynamics of a disk are more complex than dynamics of spheres and aggregates studied previously. I found that in a two-layer ambient with density transition, a disk settling in a low Reynolds number regime undergoes five phases of settling with the orientation varying from horizontal to vertical, and it may achieve two local minimum settling velocities in the density transition layer. Moreover, I found that the settling dynamics depends on a density difference between upper and lower homogeneous layers, stratification strength and thickness of density transition.

Sensitivity Analysis for the Water-Air Heat Exchange Term

A term expressing heat exchange between water and air is often present in models of thermal pollution spreading in rivers. The importance of this term depends strongly on temporal and spatial scale of the process as well on meteorological and hydrological conditions. Although heat exchange between the water and atmosphere has been studied for many years, its determination is still difficult in practical cases, and different simplifications are considered in practice. The objective of this study was to verify which of the input data needed for the net heat flux calculations are of utmost importance and which of them influences its final value most significantly. The analyses have been performed for several data sets collected in natural conditions.

Laboratory studies on bedload transport under unsteady flow conditions

Abstract Two sets of triangular hydrographs were generated in a 12-m-long laboratory flume for two sets of initial bed conditions: intact and water-worked gravel bed. Flowrate ranging from 0.0013 m3 s-1 to 0.0456 m3 s-1, water level ranging from 0.02 m to 0.11 m, and cumulative mass of transported sediment ranging from 4.5 kg to 14.2 kg were measured. Then, bedload transport rate, water surface slope, bed shear stress, and stream power were evaluated. The results indicated the impact of initial bed conditions and flow unsteadiness on bedload transport rate and total sediment yield. Difference in ratio between the amount of supplied sediment and total sediment yield for tests with different initial conditions was observed. Bedload rate, bed shear stress, and stream power demonstrated clock-wise hysteretic relation with flowrate. The study revealed practical aspects of experimental design, performance, and data analysis. Water surface slope evaluation based on spatial water depth data was discussed. It was shown that for certain conditions stream power was more adequate for the analysis of sediment transport dynamics than the bed shear stress. The relations between bedload transport dynamics, and flow and sediment parameters obtained by dimensional and multiple regression analysis were presented.