Evaluating flash-flood warnings at ungauged locations using post-event surveys: a case study with the AIGA warning system

Abstract

This article presents a comparison between real-time discharges calculated by a flash-flood warning system and post-event flood peak estimates. The studied event occurred on 15 and 16 June 2010 at the Argens catchment located in the south of France. Real-time flood warnings were provided by the AIGA (Adaptation d’Information Géographique pour l’Alerte en Crue) warning system, which is based on a simple distributed hydrological model run at a 1-km² resolution using radar rainfall information. The timing of the warnings (updated every 15 min) was compared to the observed flood impacts. Furthermore, “consolidated” flood peaks estimated by an intensive post-event survey were used to evaluate the AIGA-estimated peak discharges. The results indicated that the AIGA warnings clearly identified the most affected areas. However, the effective lead-time of the event detection was short, especially for fast-response catchments, because the current method does not take into account any rainfall forecast. The flood peak analysis showed a relatively good correspondence between AIGA- and field-estimated peak values, although some differences were due to the rainfall underestimation by the radar and rainfall–runoff model limitations.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Javelle, P., Demargne, J., Defrance, D., Pansu, J. and Arnaud, P., 2014. Evaluating flash-flood warnings at ungauged locations using post-event surveys: a case study with the AIGA warning system. Hydrological Sciences Journal, 59 (7), 1390–1402. http://dx.doi.org/10.1080/02626667.2014.923970     

Analytical description of physical librations of saturnian coorbital satellites Janus and Epimetheus

Janus and Epimetheus are famously known for their distinctive horseshoe-shaped orbits resulting from a 1:1 orbital resonance. Every 4 years these two satellites swap their orbits by a few tens of kilometers as a result of their close encounter. Recently Tiscareno et al. (Tiscareno, M.S., Thomas, P.C., Burns, J.A. [2009]. Icarus 204, 254-261) have proposed a model of rotation based on images from the Cassini orbiter. These authors inferred the amplitude of rotational librational motion in longitude at the orbital period by fitting a shape model to Cassini ISS images. By a quasi-periodic approximation of the orbital motion, we describe how the orbital swap impacts the rotation of the satellites. To that purpose, we have developed a formalism based on quasi-periodic series with long- and short-period librations. In this framework, the amplitude of the libration at the orbital period is found proportional to a term accounting for the orbital swap. We checked the analytical quasi-periodic development by performing a numerical simulation and find both results in good agreement. To complete this study, the results obtained for the short-period librations are studied with the help of an adiabatic-like approach.     

Coherent structure dynamics and sediment particle motion around a cylindrical pier in developing scour holes

Recent investigations on the dynamics of the turbulent horseshoe vortex system (THV) around cylindrical piers have shown that the rich coherent dynamics of the vortical structures is dominated by low-frequency bimodal fluctuations of the velocity field. In spite of these advances, many questions remain regarding the changes of the flow and sediment transport dynamics as scour progresses. In this investigation we carry out laboratory experiments to register the development of the scour hole around a cylindrical pier in a fine-sand bed (d ₅₀ = 0.36 mm). We use the bathymetry measured in the experiment to simulate the flow field employing the detached-eddy simulation approach (DES), which has shown to resolve most of the turbulent stresses around surface-mounted obstacles. From these simulations we compare the dynamics of the THV to the flat-bed case, and analyze the effects on particle transport and sediment flux using the Lagrangian particle model of Escauriaza and Sotiropoulos (2011b) to study the impact of the changes of the flow on the sediment dynamics.     

Observing and Simulating Wind-Turbine Wakes During the Evening Transition

Wind-turbine-wake evolution during the evening transition introduces variability to wind-farm power production at a time of day typically characterized by high electricity demand. During the evening transition, the atmosphere evolves from an unstable to a stable regime, and vertical stratification of the wind profile develops as the residual planetary boundary layer decouples from the surface layer. The evolution of wind-turbine wakes during the evening transition is examined from two perspectives: wake observations from single turbines, and simulations of multiple turbine wakes using the mesoscale Weather Research and Forecasting (WRF) model. Throughout the evening transition, the wake’s wind-speed deficit and turbulence enhancement are confined within the rotor layer when the atmospheric stability changes from unstable to stable. The height variations of maximum upwind-downwind differences of wind speed and turbulence intensity gradually decrease during the evening transition. After verifying the WRF-model-simulated upwind wind speed, wind direction and turbulent kinetic energy profiles with observations, the wind-farm-scale wake evolution during the evening transition is investigated using the WRF-model wind-farm parametrization scheme. As the evening progresses, due to the presence of the wind farm, the modelled hub-height wind-speed deficit monotonically increases, the relative turbulence enhancement at hub height grows by 50%, and the downwind surface sensible heat flux increases, reducing surface cooling. Overall, the intensifying wakes from upwind turbines respond to the evolving atmospheric boundary layer during the evening transition, and undermine the power production of downwind turbines in the evening.     

TOPOGRAPHIC EFFECTS ON VENTIFACT DEVELOPMENT,MOJAVE DESERT,CALIFORNIA

I nvestigations into processes of ventifact formation in the east-central Mojave Desert, California confirm the importance of topography as a control in the location, orientation, and intensity of ventifact abrasion. Ventifacts in the region appear to be relict in nature and probably formed during a period that ended several thousand years ago. Comparison of groove orientations with available wind data shows that regional flow direction has not changed in the recent past. Although west to northwest winds are the most frequent and intense, and therefore dictate the regional erosion pattern, low to moderate southeasterly flow is recorded on ventifacts near the crests of hills owing to the effects of velocity acceleration. Two conditions that affect ventifact development are considered in this paper: (1) wind acceleration through topographic constrictions; and (2) wind acceleration up the windward flanks of hills. Constrictions in the Barstow-Bristol trough allowed velocity increases that resulted in ventifaction on 70–90 percent of all exposed cobbles and boulders. Velocity acceleration towards the crests of hills caused a marked increase in sand transport and resultant abrasion, as expressed by increases in groove and pit dimensions. [Key words: Aeolian geomorphology, ventifacts, Mojave Desert, California.]     

Effects of the October 25, 2010, Mentawai tsunami in La Réunion Island (France): observations and crisis management

On October 25, 2010, a Mw = 7.7 magnitude earthquake was recorded in the Kepulauan Mentawai archipelago, in Indonesia. Following the earthquake, the Pacific Tsunami Warning Center issued a tsunami bulletin to the countries of the Indian Ocean informing them that a local tsunami watch was in effect for Indonesia. La Réunion Island, a French territory located 5,000 km southwest of the seismic source, did not trigger its tsunami early warning protocol, but did suffer the impact of this tsunami 7h20 post-earthquake. This paper presents the post-tsunami survey conducted in La Réunion Island a few days after the event, providing run-up measurements that reach 1.72 m locally as well as information about the material losses and the crisis management. It provides a time sequence of the observations along the coast and the reactions of the population and authorities in dealing with this crisis. This paper also shows the local authorities’ lack of action in managing this event—thus increasing human and material vulnerabilities—the consequences for victims and the impact on the credibility of the local early warning system. These results provide geophysicists with information to better describe the source of this tsunami through far-field inversion and provide local decision makers with a comprehensive assessment of the weaknesses of the local early warning system and protocols.     

Fisheries mismanagement

We analysed the extent to which European politicians have adhered to scientific recommendations on annual total allowable catches (TACs) from 1987 to 2011, covering most of the period of the Common Fisheries Policy (CFP). For the 11 stocks examined, TACs were set higher than scientific recommendations in 68% of decisions. Politically-adjusted TACs averaged 33% above scientifically recommended levels. There was no evidence that the 2002 reform of the CFP improved decision-making, as was claimed at the time. We modelled the effects of such politically-driven decision-making on stock sustainability. Our results suggest that political adjustment of scientific recommendations dramatically increases the probability of a stock collapsing within 40 years.In 2012 European fisheries policy will undergo a once-a-decade reform. Ten years ago radical reforms were promised but the changes failed to improve sustainability. It is likely that the 2012 reform will be similarly ineffective unless decision-making is changed so that catch allocations are based on science rather than politics.