Design of a bed load and driftwood filtering dam, analysis of the phenomena and hydraulic design

Flood protection often calls on to the realization of retention works for bed load as well as wood and debris flow. Certain relatively recent arrangements did not perform according to their intended function, what shows the complexity of the design and the implementation of such works. Adaptations were necessary to reach the security objectives. The design of a retention dam for solid materials and floating driftwood requires the consideration of numerous hydraulic and material transport processes. The analyses and design validation can be made with two approaches: physical modelling by the construction of a reduced scale model and the test realization or numerical simulation, by means of software packages such as GESMAT (1D) or TOPOFLOW (2D). The present work consists in implementing both approaches, in estimating and in comparing the answers which could be given for a bed load and debris flow filtering dam on a river with a slope of the order of 10%. Thanks to water level gauges and visual observations during tests on the physical model, the progression of the obstructions by driftwood and bed load is well understood, and the effectiveness of these obstructions proven. The tested work plays at first a role of filtering and retention and secondly a role of side overflow towards a zone with low damage potential, when the capacity of the in-stream retention space is reached. The performed numerical simulations, essentially in 1D, reproduce well the phenomena of bed load aggradation. Moreover, the potential obstruction by floating wood is considered and influences the behavior of the structure. By putting in parallel physical and numerical models, it was possible thanks to the results from the physical scale model to refine the numerical simulation tools taking into consideration additional components and behavior-type rules. These further established rules can now be used for other cases where physical modelling is not foreseen

ZSE466-3 - TOPIK-TOPIK LANJUTAN DALAM GEOFIZIK USAHACARI - APRIL 1987.

ABSTRACT: The Piano Key weir (PKW) is a hydraulically attractive alternative to linear overflow weirs, increasing the unit discharge for similar heads and spillway widths. This advantage allows operating dam reservoirs on an increased level and provides thereby an enhanced retention volume. It is a result of the non-linear nature of PKWs being folded back and forth to make repeating cycles or keys. As PKWs are relatively novel structures, only few design equations are available, so that normally physical model tests of prototypes have to be conducted assuring and optimizing their hydraulic characteristics. Nevertheless, first comprehensive and systematic model test series were conducted in several laboratories. Based on such a test series, a general design equation for A-type PKWs is proposed and discussed. Considering furthermore data of other laboratory studies, the latter is validated. It turned out that main and secondary parameters exist regarding the relative effect on the rating curve. The main parameters having a significant effect on the capacity are the relative developed crest length and the relative head. The secondary parameters of small but not negligible effect comprise the ratio of the inlet and outlet key widths, the ratio of the inlet and outlet key heights, the relative overhang lengths, and the relative height of the parapet walls.

Failure of saddle dam, Xe-Pian Xe-Namnoy project: executive summary

The Saddle Dam D at the Hydroelectric Power project Xe-Pian Xe-Namnoy in the Lao People’s Democratic Republic failed on July 23, 2018. An Independent Expert Panel (IEP) was established to investigate into and report on the failure of the embankment dam. The Executive Summary of the IEP final report (Anton J. Schleiss, Jean-Pierre Tournier, and Ahmed F. Chraibi, Report of Independent Expert Panel (IEP), Xe-Pian Xe-Namnoy Project - Failure of Saddle Dam D, Final Report. 20 March 2019) is presented here

Morphological resilience to flow fluctuations of fine sediment deposits in bank lateral cavities

Lateral cavities are built in the banks of rivers for several purposes: to create harbors, to capture sed- iment, to keep a central navigable channel (i.e., Casiers de Girardon in the Rhone river) or to promote the formation of aquatic habitats if a limited amount of sediment is captured, providing hydraulic and morphologic diversity (i.e., the case of Japanese Wandos). This work is focused on this latter purpose: promotion of hydraulic and morphologic diversity. In these scenarios, an increase in the flow discharge in the main channel may, however, re-mobilize the deposit of sediment inside these lateral embayments and cause a sudden increase of the sediment concentration and turbidity in the main channel. It is thus of interest to characterize the resistance and resilience of these sedimentary deposits when the main channel is subjected to high flow or flushing events. Laboratory tests were carried out for five different normalized geometries of the cavities installed in the banks of an open channel and for five hydrographs with different levels of unsteadiness. Water depth, sediment deposit mass, sediment concentration and area covered by the settled sediments were recorded throughout each experiment. Although sediment deposits established at equilibrium before the flushing events are different depending on the geometry of the cavities, generally, they are recovered after being flushed by the high flow events. It is shown that the resistance and resilience of the sediment deposits are strongly dependent on the flow field and the mass exchange between the main channel and the cavities. This mass exchange is governed by the geometry of the cavities and the magnitude of the hydrographs applied