A particular aspect of the nonstationary nature of intermittent rainfall is investigated. It manifests itself in the fact that the average rain rate varies with the distance to the surrounding dry areas. The authors call this fundamental link between the rainfall intensity and the rainfall occurrence process the ''dry drift.'' Using high-resolution radar rain-rate maps and disdrometer data, they show how the dry drift affects the structure and the variability of intermittent rainfall fields. They provide a rigorous geostatistical framework to describe it and propose an extension of the concept to more general quantities like the (rain)drop size distribution.
A stochastic rainfall simulator based on the concept of ''dry drift'' is proposed. It is characterized by a new and nonstationary representation of rainfall in which the average rain rate (in log-space) depends on the distance to the closest surrounding dry areas. The result is a more realistic transition between dry and rainy areas and a better distribution of low and high rain rates inside the simulated rainy areas. The proposed approach is very general and can be used to simulate both unconditional and conditional rain rate time series, two-dimensional fields, and space-time fields. The parameterization is intuitive and can be done using time series and/or radar rain-rate maps. Several examples illustrating the simulator's capabilities are given. The results show that the simulated time series and rain rate fields look realistic and that they are difficult to distinguish from real observations.
a b s t r a c tThe lifetime and efficiency of dams is endangered by the process of sedimentation. To ensure the sustainable use of reservoirs, many sediment management techniques exist, among which venting of turbidity currents. Nevertheless, a number of practical questions remain unanswered due to a lack of systematic investigations. The present research introduces venting and evaluates its performance using an experimental model. In the latter, turbidity currents travel on a smooth bed towards the dam and venting is applied through a rectangular bottom outlet. The combined effect of outflow discharge and bed slopes on the sediment release efficiency of venting is studied based on different criteria. Several outflow discharges are tested using three different bed slopes (i.e., 0%, 2.4% and 5.0%). Steeper slopes yield higher venting efficiency. Additionally, the optimal outflow discharge leading to the largest venting efficiency with the lowest water loss increases when moving from the horizontal bed to the inclined positions.
Reservoir sedimentation is gaining growing attention as dams are aging, due to economic and environmental consequences. Venting of turbidity currents is one of many sediment management techniques, highly recommended when water is in shortage. The venting operation is experimentally investigated using two reservoir bed slopes. The main research questions concern the opening timing of bottom outlets and the duration of venting. The timings tested are relative to the arrival of the current at the outlet. The results showed that in-time venting, synchronized with the arrival of the turbidity current at the outlet, is more efficient than early or late venting. It is recommended to start opening the gates when the turbidity current is around 300 m upstream of the outlet, so that the evacuation is synchronized with the arrival of the current at the dam. Additionally, venting should not be stopped immediately after the end of the turbidity current flow but should instead last for a certain time in order to evacuate the muddy lake depending on the outflow discharge.
Turbidity currents may be a relevant lever to manage the accumulation of fine sediments in reservoirs. In this paper, we propose to show how two different numerical codes simulate the propagation of turbidity currents. Telemac 3D and Ansys CFX 17.1 solver were chosen as they are commonly used by many research and engineering teams. The simulations are performed on two configurations. The first case aims at modeling the plunging of a turbidity current. The second model is validated based on an experimental work performed at EPFL. The latter consisted on testing turbidity current venting as a solution to manage reservoir sedimentation. A long and narrow flume was used to simulate the reservoir where a turbidity current was triggered. The advantages and limits of both approaches are discussed in order to supply guidelines for the modeling of turbidity currents in real reservoirs.
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