Sabine Chamoun

Stochastic simulation of intermittent rainfall using the concept of “dry drift”†

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 simulators 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.

Nonstationarity in Intermittent Rainfall: The “Dry Drift”

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.

Venting of turbidity currents approaching a rectangular opening on a horizontal bed

ABSTRACT Reservoir sedimentation is a worldwide problem hampering the sustainable use of reservoirs and the sediment balance of impacted rivers. Various techniques are applied for sediment mitigation. However, as turbidity currents are a major source of sediments in reservoirs, venting them through outlets reduces sedimentation. The sediment release efficiency associated with venting turbidity currents on a horizontal bed is experimentally investigated in this paper. The outflow discharge and duration of venting are the main parameters assessed. Venting efficiency is studied based on two different concepts: (1) a global venting efficiency comparing inflow and outflow sediment fluxes during the total venting operation and (2) a local venting efficiency comparing masses starting at the arrival of the turbidity current to the outlet and taking into account deposited sediment masses. An efficiency indicator accounting for water losses is also introduced. Results can be used to improve the efficiency of venting by employing adequate outlet discharges.

Experimental investigation on turbidity current venting under restrained outflow discharges

Reservoir sedimentation is a worldwide problem affecting sustainable storage use as well as sediment transport downstream of dams. Various techniques are applied for sediment mitigation, among which, venting of turbidity currents. This paper reviews, discusses and evaluates venting of turbidity currents based on an experimental approach. Data acquisition during the tests is accomplished using five different measuring instruments. The efficiency of venting is analyzed by varying the relative outflow discharge. The study reveals the potential of optimizing venting operations in terms of water losses by employing adequate outlet discharges during venting.

Management of turbidity current venting in reservoirs under different bed slopes

The 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.

Hybrid Modelling Approach to Study Scour Potential at Chancy-Pougny Dam Stilling Basin

Chancy-Pougny is a run-of-river dam on the Swiss-French border constructed in the early 1920s. Since its inauguration, the operation of the four spillway gates was responsible for a progressive erosion of the stilling basin. Thus, a hybrid modelling was performed to study the scour potential and to determine adequate solutions to maintain future scour within acceptable levels. Visual observations on the physical model identified a strong recirculation in the non-symmetrical basin, that combined with the outflow from the spillway gate, lead to an enhanced specific discharge and to the formation of a turbulent vortex impacting the rocky foundation. Some measures to limit the recirculation, including a free-standing wall and various configurations of concrete prisms for scour protection, were tested on both the physical and numerical model. The pressure and water depth measurements resulting from the physical model were used within the numerical model to determine the corresponding scour potential for each of the tested mitigation measures. A solution containing a layer of randomly distributed concrete prisms laid on the basin’s current bottom was identified through this study, proving the importance of both numerical and physical approaches in hydraulic engineering

Protection de la fosse de dissipation du barrage de Chancy-Pougny avec prismes en béton

Davide WÜTHRICH, Sabine CHAMOUN, Giovanni DE CESARE, Anton J. SCHLEISS 1 Laboratoire de Constructions Hydrauliques (LCH), Ecole Polytechnique Fédérale de Lausanne, Suisse Station 18, 1015 Lausanne, Switzerland – davide.wuthrich@epfl.ch 2 Laboratoire de Constructions Hydrauliques (LCH), Ecole Polytechnique Fédérale de Lausanne, Suisse Station 18, 1015 Lausanne, Switzerland – sabine.chamoun@epfl.ch 3 Laboratoire de Constructions Hydrauliques (LCH), Ecole Polytechnique Fédérale de Lausanne, Suisse Station 18, 1015 Lausanne, Switzerland – giovanni.decesare@epfl.ch 4 Laboratoire de Constructions Hydrauliques (LCH), Ecole Polytechnique Fédérale de Lausanne, Suisse Station 18, 1015 Lausanne, Switzerland – anton.schleiss@epfl.ch