Hydrometeorological analysis and forecasting of a 3 d flash-flood-triggering desert rainstorm

Yair Rinat,Moshe Armon,Elyakom Vadislavsky,Asher Metzger,Marcelo Rosensaft,Pavel Khain,Yoav Levi,Francesco Marra,Efrat Morin

doi:10.5194/nhess-21-917-2021

Abstract

Abstract. Flash floods are among the most devastating and lethal natural hazards. In 2018, three flash-flood episodes resulted in 46 casualties in the deserts of Israel and Jordan alone. This paper presents the hydrometeorological analysis and forecasting of a substantial storm (25–27 April 2018) that hit an arid desert basin (Zin, ∼1400 km2, southern Israel) claiming 12 human lives. This paper aims to (a) spatially assess the severity of the storm, (b) quantify the timescale of the hydrological response, and (c) evaluate the available operational precipitation forecasting. Return periods of the storm's maximal rain intensities were derived locally at 1 km2 resolution using weather radar data and a novel statistical methodology. A high-resolution grid-based hydrological model was used to study the intra-basin flash-flood magnitudes which were consistent with direct information from witnesses. The model was further used to examine the hydrological response to different forecast scenarios. A small portion of the basin (1 %–20 %) experienced extreme precipitation intensities (75- to 100-year return period), resulting in a local hydrological response of a high magnitude (10- to 50-year return period). Hillslope runoff, initiated minutes after the intense rainfall occurred, reached the streams and resulted in peak discharge within tens of minutes. Available deterministic operational precipitation forecasts poorly predicted the hydrological response in the studied basins (tens to hundreds of square kilometers) mostly due to location inaccuracy. There was no gain from assimilating radar estimates in the numerical weather prediction model. Therefore, we suggest using deterministic forecasts with caution as it might lead to fatal decision making. To cope with such errors, a novel cost-effective methodology is applied by spatially shifting the forecasted precipitation fields. In this way, flash-flood occurrences were captured in most of the subbasins, resulting in few false alarms.

Highlights

Flash floods are rapidly evolving events characterized by a sudden rise in stream water level and discharge (Sene, 2013)
We addressed the following questions. (a) What was the severity of the storm and flood and how did it vary spatially? (b) What was the time scale of the flashflood response? (c) What was the operational predictability of the rainfall and the resulting flash floods and can it be improved? To answer these questions, we combined datasets and tools, including radar rainfall data, operational rainfall forecasts, rainfall and flood-frequency analyses and their spatial variations, a grid-based hydrological model, and unique direct field observations during the event
Knowledge of desert rainstorms and flash floods is limited despite their devastating potential

Summary

Introduction

Flash floods are rapidly evolving events characterized by a sudden rise in stream water level and discharge (Sene, 2013). These events often result in casualties and damage (Barredo, 2007; Borga et al, 2019; Doocy et al, 2013; Grodek et al, 2012; Petrucci et al, 2019; Vinet et al, 2019) and are ranked among the most devastating natural hazards worldwide (Barredo, 2007; Borga et al, 2014; Doocy et al, 2013; Gaume et al, 2009; Gruntfest and Handmer, 2001; Marchi et al, 2010; Sene, 2013). Rinat et al.: Hydrometeorological analysis and forecasting of a 3 d flash-flood-triggering desert rainstorm flood-forecasting skills in these areas is limited due to poor measurements, sparse documentation, and a relatively small number of studies (Armon et al, 2018; Nicholson, 2011; Simmers, 2003; Yang et al, 2017; Zoccatelli et al, 2019)

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