Effectiveness of exceptional rainstorms on a small Mediterranean basin Tamir Grodek a, ⁎, Yael Jacoby a , Efrat Morin a , Oded Katz b a Geography Department, The Hebrew University of Jerusalem, Mt. Scopus, Jerusalem 91905, Israel b Geological Survey of Israel, 30 Malkhe Israel St. Jerusalem 95501, Israel abstract article info Article history: Received 2 March 2011 Received in revised form 4 March 2012 Accepted 20 March 2012 Available online 6 April 2012 Keywords: Exceptional rainstorm Geomorphic processes Landslides Debris flows Channel incision Factor of safety A comprehensive investigation of rainstorms and their consequent impacts on landscape evolution is geomor- phologically important, but only scant information may be available on exceptional events, because parame- ters on synoptic conditions, rainstorm, landforms and hydrology for such events may be incomparable with previous knowledge. We studied an exceptional storm on April 2, 2006, in the Ramot Menashe region, Israel. Our investigation of rainfall, landslides, debris flows and channel suggests the effectiveness of such an event on the development of basin-scale morphology. The storm caused damage and casualties although it covered relatively narrow strips. Neither direct rainfall nor runoff measurements exist for the most severely affected area of Ramot Menashe, but the geomorpholo- gic evidence combined with high-resolution meteorological radar data provides the basic understanding of the processes and hazardous conditions which prevailed at the time. In the storm core, based on estimation from meteorological radar data, 263 mm of rain fell within 3 h with a maximum intensity of 220 mm h -1 for 10 min, triggering both sporadic landslides at the soil/bedrock contact on the upper slopes and widespread landslides at the fractured/massive bedrock contact on the lower slopes. The 1st order channels on the alter- nation of chalk and marl also underwent erosion, and the produced sediment deposited on alluvial fans at the confluence with the main channel. The specific peak discharges for catchment size of 0.3–10 km 2 were 11 to 73 m 3 s -1 km -2 , higher than any recorded floods in the Mediterranean climatic region of Israel. The effec- tiveness of the flood for geomorphic work, represented by shear stress and stream power per unit boundary area reached 87–398 N m -2 and 212–2134 W m -2 , respectively. This kind of analysis can be applied to haz- ard prediction in other areas under similar geomorphological conditions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction An exceptional rainstorm higher than ever observed in a given re- gion can shed light on the geomorphic processes that shape the land- scape. In the long standing debate of which event magnitudes are more significant in long term landscape evolution, i.e., frequent moderate-size rain events (Wolman and Miller, 1960) or exceptional rain events (Hack and Goodlett, 1960), less is known on the latter. Such rain events happen somewhere in the world every year (Morgan et al., 1997) and a recently compiled data on European flash floods (Gaume et al., 2009) will bring valuable information. Exceptional rain events may differ from extreme or catastrophic events (large, sudden, and rare on human timescales; Jacobson et al., 1989) as in many cases they are related to minor landscape changes, new to the observer's eye, but only within an occurrence of 10–100 years (e.g., Wolman and Gerson, 1978; Newson, 1980; Inbar, 1987; Benito et al., 1998; Hicks et al., 2005; Greenbaum and Bergman, 2006; Sheffer et al., 2008). The term “cataclysmic event” (Baker et al., 1988) may better refer to exceptional storms that (i) ex- ceed the regional rainfall/flood maxima (e.g., Morgan et al., 1997), or (ii) cause geomorphic changes incomparable with other events in the region. Wolman and Gerson (1978) relate extreme magnitudes to re- currence intervals of 500 years or longer resulting in irreparable transformation of valley landforms. Baker and Costa (1987) suggested quantifying geomorphic effectiveness of flood by unit boundary shear stress (τ) or stream power per unit area (ω) rather than peak dis- charge and frequency (see also Magilligan, 1992). The common evi- dence for such events is fluvial deposition, including 1) imbrications and clusters of large particles, bars, and boulder berms (e.g., Schumm, 1977; Allen, 1982; Carling, 1989); 2) debris flow as levees composed of coarse poorly sorted bed material along the channel sides; 3) U-shaped debris flow channels, with a ratio of width/ depth b 10 (Costa and Jarrett, 1981); 4) reverse imbrications and coarse poorly sorted sediments, and 5) terminal lobes at the fan end with a steep front built by coarse bed sorted materials (Costa, 1984, 1988). Slope instability is another phenomenon likely to be affected by exceptional rainfall; landslides including single/multiple slope fail- ures and rilling, usually shallow, occur frequently in steep, soil man- tled landscapes (e.g., Rodine and Johnson, 1976; Kirkby, 1987; Webb et al., 1988; Benda and Cundy, 1990; Selby, 1994; Matsushi et Geomorphology 159-160 (2012) 156–168 ⁎ Corresponding author. Tel.: + 972 548820060; fax: + 972 5820549. E-mail address: T.grodek@gmail.com (T. Grodek). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2012.03.016 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph