Synergetic CT, XRF and Geoelectric Imaging for Non-Destructive Soil and Sediment Stratigraphic Study

This paper presents the application of three non-destructive techniques in the study of an agricultural area on the west coast of Peloponnese, Greece. The applied methods include (a) electromagnetic geophysical research using a handheld EM profiler (EMP-400 GSSI), (b) computed tomography (CT) with coring data, and (c) X-ray Fluorescence (XRF) scanning. As electrical conductivity is mainly influenced by the bulk soil, including water content, clay content and mineralogy, organic matter, and bulk density, a comparison of the three applied techniques indicates the same soil stratification and same soil properties with depth. Moreover, the ground-truthing by the undisturbed soil and sediments core retrieved in the centre of the site as well as the laboratory analyses of soil and sediment properties confirm the reliability of the geophysical research and the revealed soil/sediment stratification.

Sensitivities of Bottom Stress Estimation to Sediment Stratification in a Tidal Coastal Bottom Boundary Layer

The bottom friction velocity (U*), which controls seabed erosion and deposition, plays a critical role in sediment transport in tidal coastal bottom boundary layers. Approaches have been proposed to calculate U*, including the log profile (LP) estimation, the direct covariance (COV) measurement, and the turbulent kinetic energy (TKE) method. However, the LP method assumes homogeneous flow and the effects of stratification need to be taken into account. Here, field investigations of hydrodynamics and sediment dynamics were carried out on the Jiangsu Coast, China. Two acoustic Doppler velocimeters (ADV) velocity measurements at 0.2 and 1 m above the seabed have been used to estimate U*, based on the aforementioned three methods. The COV and TKE methods provided reasonable estimations of U*, while a pronounced overestimation was identified when using the LP method. This overestimation can be attributed to the stratification effects associated with the vertical suspended sediment concentration (SSC) gradient near the bottom. Then, three models were utilized to correct the overestimation, in which the gradient/flux Richardson number was modified with empirical constants α, β, and A to parameterize the stratification effects in the logarithmic velocity distribution. The values of α, β, and A derived from the observation are smaller than the results from previous investigations. These modified logarithmic velocity distribution models can be applied in numerical simulations when sediment stratification is important.

Modelling of sedimentation processes inside Roseires Reservoir (Sudan)

Roseires Reservoir, located on the Blue Nile River in Sudan, is the first trap to the sediments coming from the vast upper river catchment in Ethiopia, which suffers from high erosion and desertification problems. The reservoir has already lost more than one-third of its storage capacity due to sedimentation in the last four decades. Appropriate management of the eroded soils in the upper basin could mitigate this problem. In order to do that, the areas providing the highest sediment volumes to the river have to be identified, since they should have priority with respect to the application of erosion control practices. This requires studying the sedimentation record inside Roseires Reservoir in order to assess when and how much sediment is deposited and to identify its source. This paper deals with the identification of deposition time and soil stratification inside the reservoir, based on historical bathymetric data, numerical modelling and newly acquired soil data. The remoteness of the study area and the extreme climate result in coring campaigns being expensive and difficult. Therefore, these activities need to be optimised and coring locations selected beforehand. This was done by combining bathymetric data and the results of a depth-averaged morphodynamic model recording the vertical stratification in sediment deposits. The model allowed for recognising the areas that are potentially subject to neither net erosion nor bar migration during the lifespan of the reservoir. Verification of these results was carried out by analysing sediment stratification from the data collected during the subsequent field campaign.

Modelling of sedimentation processes inside Roseires Reservoir (Sudan)

Roseires Reservoir, located on the Blue Nile River, in Sudan, is the first trap to the sediments coming from the upper catchment in Ethiopia, which suffers from high erosion and desertification problems. The reservoir lost already more than one third of its storage capacity due to sedimentation in the last four decades. Appropriate management of the eroded area in the upper basin could mitigate this problem. In order to do that, the areas providing the highest sediment volumes to the river have to be identified, since they should have priority with respect to the application of erosion control practices. This requires studying the sedimentation record inside Roseires Reservoir, with the aim of identifying when and how much sediment from a certain area is deposited. The identification of deposition time is derived from soil stratification inside the reservoir. This requires expensive coring campaigns that need to be optimized. The most promising sampling coring areas were therefore selected beforehand by combining bathymetric data and the results of a depth-averaged morphodynamic model able to record vertical stratification in sediment deposits. The model allowed recognising the areas that are potentially neither subject to net erosion nor to bar migration during the life span of the reservoir. Verification of these results was carried out by analysing sediment stratification from the data collected in subsequent field campaign.

The effects of flow stratification by non-cohesive sediment on transport in high-energy wave-driven flows

The two-way effects of the time-varying suppression of turbulence by gradients in suspended sediment concentration have been investigated using a modified form of the Generalized Ocean Turbulence Model (GOTM). Field measurements of fluid velocities and sediment concentrations collected under high-energy conditions (mobility number ≈ 900) have been simulated both including and neglecting the feedback between sediment and turbulence. The results show that, when present, this feedback increases the wave-coherent component of transport relative to the mean component of transport, which can even change the direction of transport. Comparisons between measured and simulated time series of near-bed sediment concentrations show great coherence (0.95 correlation) and it is argued that the differences in net transport rates may be partially explained by the use of a uniform grain size in the simulations. It is seen that the effects of sediment stratification scale with orbital velocity divided by sediment setting velocity, um/ws, for all grain sizes.