Abstract
Abstract. Identification of a geomorphic index to represent lower thresholds for minor flows in ephemeral, alluvial streams in arid environments is an essential step as a precursor for reliable flash flood hazard estimations and establishing flood warning systems. An index, termed Alluvial wadi Flood Incipient Geomorphologic Index (AFIG), is presented. Analysis of data from an extensive field survey in the arid ephemeral streams in southern and eastern Israel was conducted to investigate the AFIG and the control over its value across the region. During the survey we identified distinguishable flow marks in the lower parts of streams' banks, such as niches, vegetation line, and change in bank material, which are indicative of low flows. The cross-sectional characteristics of the AFIG were studied in relationship with contributing drainage basin characteristics such as lithology, topography, and precipitation. Drainage area and hardness of the exposed lithology (presented as a basin-wide index) are the preferred descriptors to be used in estimating a specific AFIG in unsurveyed sites. Analyses of discharge records from seven hydrometric stations indicate that the recurrence interval of the determined AFIG is equal to or more frequent than 0.5 yr.
Highlights
In mildly sloped humid regions, which are entrenched with perennial streams, the economic damage and fatalities from floods are caused mainly by water overtopping banks of rivers to cause floodplain inundation
This study is concerned with identifying a field-based geomorphic index that signifies low flows in ephemeral arid environments
In arid areas of Israel, as in other arid environments, the characteristics of floods are considerably different from perennial streams in temperate environments
Summary
In mildly sloped humid regions, which are entrenched with perennial streams, the economic damage and fatalities from floods are caused mainly by water overtopping banks of rivers to cause floodplain inundation. Bankfull flow – i.e., a flow in which the water level in a stream or a river is at the top of its banks and further rise would result in inundation of the floodplain (Leopold, 1994) – often serves in temperate to humid areas as a geomorphic threshold in flood warning systems to indicate the incipience of minor flooding (e.g., Carpenter et al, 1999; Reed et al, 2002; Georgakakos, 2006; Shamir et al, 2013). In conjunction with hydrologic models, these marks provide a continuous and dynamic risk assessment that identifies the short-term hydrologic conditions that can lead to these flows given continuously changing antecedent conditions Such a modeling framework provides a tool for forecasters to assess short-term forecasts and issue flash flood watches and warnings for specific locations (e.g., Georgakakos, 1987; Reed et al, 2002; Shamir et al, 2013). We present an assessment of the AFIG cross-sectional hydraulic properties with respect to various properties of the drainage area of the basins, and derive estimates of the AFIG recurrence intervals
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