Experiments by Invited Researchers


Wave-induced bedform dynamics in mixed cohesive–noncohesive sediments

Project acronym: HyIV-Hull-06
Name of Group Leader: Dr. Joris Eggenhuisen
User-Project Title: Wave-induced bedform dynamics in mixed cohesive–noncohesive sediments
Facility: TES
Data Management Report: Report

User-Project Objectives

The principal aim of the proposed experiments is to investigate near-bed turbulence and sediment transport interactions over rippled beds of clay-sand mixtures under waves. Such cohesive mixtures may have a yield strength that affects the timing of first appearance of bedforms from a flat sediment bed, and they are subjected to demixing (or winnowing) processes, which will in turn affect the rate of bedform development which will alter the equilibrium size and shape of the bedforms. The overall aim of this project will be achieved through three specific objectives: 1. Quantify interactions of near-bed hydrodynamics, sediment transport dynamics and turbulence over rippled beds formed by waves using state-of-the-art measurement techniques. This will help to greatly improve modelling approaches by enabling rigorous model-data comparisons, representation of near-bed turbulence behaviour, and assessment of appropriate boundary conditions. 2. Quantify how clay-sand mixtures and resultant bed cohesion affect bedform evolution, boundary layer processes (including turbulence), and sediment transport, through processes of clay winnowing and near-bed turbulence modulation. The unique results, will also allow us to directly assess scaling issues in previous work. 4. Synthesise results from objectives 1 and 2 to test the predictive ability of present state-of-the-art sediment transport models in mixed sediment environments and re-calibrate such models with the trajectories from the phase space explored within the experimental results in objectives 1-3.

Short description of the work carried out

For this project a 1.65m wide wave channel was constructed with a specially constructed wave damper board which resulted in less than 10% reflected wave energy. Wave heights were recorded along the channel length using an array of wave gauges. Sediment transport was monitored using a combination of approaches: an Acoustic Backscatter system (ABS) measured continuously through the water column, Optical Backscatter Sensors (OBS) measured continuously at four heights and pumped water samples were obtained during each measurement epoch. Turbulent flow velocities were measured using an array of Acoustic Doppler Velocimeters (ADVs). At the end of each measurement epoch, detailed bed topography measurements were obtained using an array of 8 Ultrasonic Ranging Sensors (URS) which were traversed along the flume. Initially, a reference data set was collected for a pure sand mixture under wave action. Then five subsequent runs were completed with increasing proportion of clay added to the bed sediment. Particular care was taken to ensure the sediments were well mixed and the bed was re-levelled between each run. Each run was sub-divided into 15 or 30 minute epochs, depending on the rate of bedform evolution and the runs were continued until an equilibrium bed had developed. Two main difficulties were encountered during the experiments. Firstly, the time required to mix and level the bed combined with the time to scan the bed between each measurement epoch limited the total number of runs. Secondly, some of the originally proposed instrumentation was not available for the measurement campaign.

Highlights of important research results

This project has only recently been completed and detailed analysis of the results is still ongoing. However, observations during the experiments and from the initial analysis of data during these experiments have produced the following results: • Increasing the clay content of the sediment mixture appears to extend the time required for equilibrium bedform development. • Increasing the clay content has an impact on bedform shape. Bed scan data from the URS data will be used to quantify the rate of bedform development and the differences in bedform shape. Further analysis of the suspended sediment size and concentration will be used to understand how changes in the sediment mixture impact on sediment transport dynamics. ADV data will be used to determine changes in the turbulent flow structure.