Graduation of Han Speerstra

18 June 2018 16:00 - Location: Room 2.98, Faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Estimating bed roughness of muddy beds"| Professor of graduation: Prof. dr. ir. P.M.J. Herman, supervisors: ir. Bas van Maren (TU Delft), ir. Irene Colosimo (TU Delft), ir. Tjerk Zitman (TU Delft)

Bed roughness is an important parameter for the prediction of sediment transport as well as defining flow conditions at near the bed. Numerical models are extensively used to make these predictions and represen- tative values for the bed roughness are key. The bed roughness is being estimated using a dataset from an ongoing pilot project in the Dutch western Wadden sea. This dataset consists of 40 days of velocity measure- ments using ADV (8 Hz) and ADCP (1 Hz) instruments and measurements of the concentration of SPM using OBS instruments.

In order to estimate the bed roughness, the dataset is analyzed using four methods for calculating the bed shear stress. The logarithmic profile, turbulent kinetic energy, vertical turbulent kinetic energy and the Reynolds stress method.

The processed data from the data analysis is averaged per tidal phase for application of a statistical anal- ysis. From this analysis, it is found that concentration of SPM at 6 cm increases with increasing wind speed. Furthermore, the bed roughness increases with increasing concentration of SPM measured at 6 cm above the bed. This leads to the hypothesis that suspended sediment making the bed rougher is not primarily governed by horizontal advection but local resuspension.

A 1DV numerical model is used in which horizontal advection is excluded to test this hypothesis. Simu- lations are performed with stationary boundary conditions. For this, combinations of waterdepth (0.2 to 2.8 m) and velocities (5 cm/s to 70 cm/s) are simulated. These simulations are imposed with an initial homo- geneous concentration. Per combination, the concentration is gradually increased until the concentration profile becomes L-shaped.

Besides stationary conditions, a simulation with one tidal cycle is simulated using time series of water- depth and velocity. The time series is chosen such that it starts at low water and ends at the next low water.

All simulations performed with the numerical model are without wind and waves. Furthermore, no water- bed exchange is assumed.

From the simulations with stationary boundary conditions the roughness is calculated using the LP method. It is found that bed roughness increases with increasing initial homogeneous concentration. From the sim- ulation of one tidal cycle, it is found that the roughness increases towards the turn of the tide. After the turning of the tide, the concentration profile becomes L-shaped and the roughness is decreased. After a cer- tain velocity and waterdepth, the concentration profile changes to a more homogeneous profile again and the roughness increases. During the tidal cycle, it is possible to have a collapsed concentration profile, which indicates a lower bed roughness and thus a smoother bed.

  • For more information, please send your questions to Han Speerstra