Geomorphology-based index for detecting minimal flood stages in arid alluvial streams

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

A Hydrological and Geomorphometric Approach to Understanding the Generation of Wadi Flash Floods

“…R shows weak to very weak correlation with Q p /A and T p under the variant applied storms, which is consistent with the results of other studies [72]. However, R and Q p /A are directly proportional, whereas R and T p are inversely proportional.…”

“…One of the unique features of WFFs is that the dry natural wadi channel does not have a consistent geometry like humid rivers, where the drainage network can be enhanced and maintained to receive water without flooding. Conversely, wadi channels can be flooded by any amount of discharge [72]. Furthermore, the absence of efficient mitigation strategies and unmanaged development, as discussed in the introduction, could increase the flood vulnerability and risk, even for low-stage floods.…”

Response to referee #1

“…Rainfall occurs from October to May, with no rain during summer (Goldreich, 2003). Intensities and duration of extreme events differ dramatically within the study area, with the relative frequency of high rainfall intensities increasing as the mean annual precipitation decreases (Marra et al, 2017;Marra and Morin, 2015;Morin et al, 2009;Sharon and Kutiel, 1986).…”

“…8e) were examined. The rel-ative frequency of high-intensity rainfall increases with the reduction of annual rainfall amounts (Goodrich et al, 1995;Marra et al, 2017;Marra and Morin, 2015;Sharon and Kutiel, 1986), and orographic effects lead to enhanced rainfall generation (Houze, 2012;Sharon and Kutiel, 1986;Warner, 2004;Wheater et al, 1991). Both phenomena are reflected in the characteristics of the rain cells of Mediterranean lows: (i) mean rain intensities are generally increasing with the degradation of mean annual rainfall southward towards drier regions.…”

Assessment of environmental flows under limited data availability: case study of the Acheloos River, Greece

“…Thus, apart from a monthly schedule of minimum maintenance flows, it also accounts for the so-called "bankfull flow", which represents the dominant discharge in channels at dynamic equilibrium and it is usually calculated as the 1.5-year flood (this is a generally accepted value, although a wider range of estimates of the corresponding return period have been reported in the literature, particularly in semi-arid climates; cf. Shamir et al 2013), as well as the "maximum flow", estimated as the 25-year flood. The method is now incorporated within the Spanish regulation implementing the obligations of the WFD.…”