Assessing thresholds for fluvial entrainment with instrumented particles

<p>Sediment transport is considered to be the governing process in many applications around the fields of geosciences and engineering as well as infrastructure and environment monitoring. Of a special interest to which scientists and engineers have dedicated a lot of time and experimental studies in the last century is the conditions for initiation of sediment entrainment, or incipient motion. In the literature, there are different criteria for determining the conditions that can result in initiation of sediment entrainment. Among these criteria, the impulse (or energy) criterion [1-2] captures the actual physics of sediment entrainment since it accounts for both the magnitude and the duration of the turbulent flow events that can result in initiation of a particle’s motion. The experimental and field studies of incipient motion use relatively expensive tools, like Particle image velocimetry (PIV) or Acoustic Doppler velocimetry (ADV), with indirect methods to determine flow parameters that could be related to predicting sediment entrainment. However, technological developments in recent decades has made it possible to assess sediment entrainment directly. Recently, a number of research studies [3-4] have suggested linking micro-electromechanical system (MEMS) recordings that consist of accelerometers, gyroscopes and magnetometer as well as an internal digital motion processor that are interconnected forming inertial measurement units (IMUs) to the probability of entrainment of individual particles. The particles have been presented provide a direct, non-intrusive, low-cost and accessible method for assessing the probability of entrainment of individual sediment particles rather than inferred using near bed flow diagnostics. In this work, an instrumented particle of 3cm in diameter [5] is used to investigate experimentally the conditions that can result in initiation of sediment entrainment for a range of flowrates that represent the near threshold conditions. The data is used to derive metrics like frequency of entrainment that could be linked to the probability of entrainment of individual sediment particles which could be used as an indicator of the risk of riverbed destabilization based on well-established theories in hydraulic engineering. Additionally, the novelty of this work is explicitly linking the probability of entrainment to the flow hydrodynamics. In addition to that, a stochastic analysis is performed to identify the relevance of certain flow structures (sweeps) to the incipient entrainment of the instrumented particle.</p><p>[1] Valyrakis M., Diplas P., Dancey C.L., Greer K., Celik A.O. (2010). Role of Instantaneous Force Magnitude and Duration on Particle Entrainment. JGR, 115, 1-18.</p><p>[2] Valyrakis M., Diplas P., Dancey C.L. (2013). Entrainment of Coarse Particles in Turbulent Flows: An Energy Approach. JGR- Earth Surf., 118, 42-53.</p><p>[3] Valyrakis, M., Alexakis, A. (2016). Development of a “smart-pebble” for tracking sediment transport. River Flow 2016, MO, USA.</p><p>[4] Al-Obaidi, K., Xu, Y., Valyrakis, M. (2020). The Design and Calibration of Instrumented Particles for Assessing Water Infrastructure Hazards. JSAN, 9, 3, 36.</p><p>[5] Al-Obaidi, K., Valyrakis, M. (2020). A sensory instrumented particle for environmental monitoring applications: development and calibration. IEEE sensors journal (accepted).</p>

Effect of a surface biofilm on sediment transport implemented in a 1D numerical model

<p>The transport of sediment shapes rivers and deltas, and has a huge impact on natural fluvial processes and human interaction within these environments. Conservation and hydraulic engineering applications in river basins crucially depend on understanding the processes of scour, transport and deposition of sediments. The sediment entrainment process in mathematical models are typically based on laboratory experiment using clean (abiotic) sediments. However, natural sediments are rich in biological communities, often forming visible biofilms which include sticky Extracellular Polymeric Substances (EPS). The presence of biological communities has been shown to significantly increase the critical shear stress of sediment entrainment compared with clean sediment, and these communities are recognized as ‘ecosystem engineers’ as they act as bio-stabilizers. Furthermore, biofilms provide stability, such that only the most energetic conditions can remove them in a sudden catastrophic way. In this study, a one-dimensional (1D) morphodynamic model for rivers is implemented to account for the development and growth of a surface biofilm subject to variable hydrodynamic disturbances (e.g. tidal forces) and with a biofilm-dependent erodibility. The 1D form of the shallow water equations are simplified with the aid of the quasi-steady approximation and the Exner equation expressing the conservation of bed material is used to compute the changes in channel bed elevation. The effect of geochemical drivers such as light, temperature and nutrients, which affect the presence or absence and growth of a biofilm, is accounted for in the model. Previous studies have shown that when sediments are covered by biofilms, entrainment occurs via biomat failure and the carpet-like detachment of biofilm-sediment composites. Different hydrodynamic conditions are tested to investigate their role in eroding the biofilm and detaching it from the sediment surface.</p>

Development, calibration and testing of a miniaturized instrumented particle for the study of entrainment of solids in turbulent flows

<p>Infrastructure damage due to riverbed and bank destabilisation or localised scour may result in considerable financial costs and even loss of life. As the risk to infrastructure keeps increasing due to climate change, the need to directly monitor it becomes crucial. Typically, hazards assessments for infrastructure near water are performed using relatively expensive and indirect methods that require field visits to remote and harsh environments to obtain mean flow measurements, using acoustic Doppler velocimetry [1], laser Doppler velocimetry [2] or water level stations along with discharge hydrographs [3]. In this work, a miniaturized instrumented particle that can provide a direct, non-intrusive and accessible method for the assessment of coarse sediment particles entrainment is developed, calibrated and tested. The particle has a diameter of only 3cm and is fitted with inertial microelectromechanical sensors (MEMS) that enable recording its three-dimensional displacement [4, 5]. The sensor is capable of recording acceleration, angular velocity and orientation at a rate of up to 1000Hz and has deployment time of at least one hour. The data can be transferred and downloaded to a PC or an SD card at a fast transfer rate and in easy format for further analysis. The calibration process of the sensor consisted of simple physical motions and the results of the calibration show that the uncertainties in the calibration experiments and in the accelerometer’s and gyroscope’s readings are deemed acceptable. The uncertainty quantification and noise estimation for the sensors, provide the input of the appropriate fusion filter that is applied to the raw data to achieve uncertainty reduction. The testing process consisted of moving the particle on a micro-bed topography and using a camera to record the distance it moved. The orientation of the instrumented particle during testing is determined by inertial sensor fusion of the raw readings of the 3 sensors. The results show that the instrumented particle’s motion could be detected accurately and therefore it could provide a method for direct assessment of the sediment entrainment due to hydrodynamic forces at low cost and in a non-intrusive and direct manner. The instrumented particle presented has a potential of use in a wide range of future applications around the fields of geosciences and environmental and infrastructures monitoring where sediment entrainment [5] and transport [6] is considered to be the governing process.</p><ol><li>Liu, D., Valyrakis, M., Williams, R. 2017. Flow Hydrodynamics across Open Channel Flows with Riparian Zones: Implications for Riverbank Stability.</li> <li>Diplas, P., Celik, A.O., Valyrakis, M., Dancey C.L. 2010. Some Thoughts on Measurements of Marginal Bedload Transport Rates Based on Experience from Laboratory Flume Experiments.</li> <li>Koursari, E., Wallace, S., Valyrakis, M., Michalis, P. 2019. Remote Monitoring of Infrastructure at Risk due to Hydrologic Hazards and Scour.</li> <li>Valyrakis, M. & Pavlovskis, E. 2014. "Smart pebble” design for environmental monitoring applications.</li> <li>Valyrakis M., A. Alexakis. 2016. Development of a “smart-pebble” for tracking sediment transport.</li> <li>Valyrakis, M., Farhadi, H. 2017. Investigating coarse sediment particles transport using PTV and “smart-pebbles” instrumented with inertial sensors.</li> </ol>