Tamir Grodek

Fluvio-pedogenic processes in an ephemeral stream channel, Nahal Yael, Southern Negev, Israel

Abstract A detailed stratigraphic survey conducted in the alluvial fill of the channel of Nahal Yael—a small catchment in the Southern Negev Desert—indicates the existence of a continuous, compacted, red-colored unit at an average depth of 50 cm beneath the surficial grey non-cohesive alluvium. Granulometric distinction between the grey and the red alluvium is evident only within the fine (

Late Quaternary weathering, erosion, and deposition in Nahal Yael, Israel: An “impact of climatic change on an arid watershed”?

In their seminal paper in 1979, Bull and Schick proposed a conceptual model for the geomorphic response to Pleistocene to Holocene climate change, based on the hyperarid Nahal Yael watershed in the southern Negev Desert. In this model, the change from semiarid late Pleistocene to hyperarid early Holocene climates reduced vegetation cover, increased the yield of sediment from slopes, and accelerated aggradation of terraces and alluvial fans. The model is now over 30 yr old, and during this time, chronologic, paleoenvironmental, and hydrogeomorphic research has advanced. Here, we reevaluate the model using data acquired in Nahal Yael over the 30yr since the original model was proposed. Recent studies indicate that the late Pleistocene climate was hyperarid, and a transition from semiarid to hyperarid climates did not occur. The revised chronology reveals a major 35–20 ka episode of accelerated late Pleistocene sediment production on slopes (with lower rates probably already at ca. 50ka) due to increased frequency of wetting-drying cycles caused by frequent extreme storms and floods between 35 and 27 ka. Without lag time, these sediments were transported and aggraded in depositional landscape components (fluvial terraces and alluvial fans). This intensified sediment production and delivery phase is unrelated to the Pleistocene-Holocene transition. The depositional landforms were rapidly incised between 20 and 18 ka. Since and/or soon after this Last Glacial Maximum (LGM) incision, most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock. Using these new data, we propose a revision to the Bull and Schick model in this hyperarid environment. Our revision suggests that the model should include the frequent storms and floods responsible for a late Pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans. We also discuss the common use and pitfalls of using the Bull and Schick conceptual model to explain observations in diverse arid environments, usually without sufficient data on basin-specific stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.

Hydrologic processes and geomorphic constraints on urbanization of alluvial fan slopes

The natural array of processes conveying water and sediment from arid mountain catchments, through alluvial fans, into the base level below is affected by human intervention to an extent unknown until a few years ago. Previously permeable fan terrain has now been replaced by paved impermeable surfaces whose drainage becomes problematic. This problem is intensified with the growth of the town and its building density, particularly vis a vis the smaller catchments. The high sediment yields, with typically predominant bed material components that are supplied by steep catchments, create situations difficult to manage in terms of effective economical and environmentally sensitive criteria. Within the context of the general inadequacy of structural attempts to fully control the floods emanating onto an alluvial fan, a case can be made for exercising the option of local, low-key engineering intervention in protecting fan settlements. Several aspects of the considerations involved are illustrated by examples from the town of Eilat, Southern Negev Desert, Israel, a dynamically developing international tourist resort.

Claim of largest flood on record proves false

A study of paleofloods in the Gardon River in southern France reveals the occurrence of past extreme floods that were larger than any observed historically. From 8 to 9 September 2002, during the course of the study by a complete coincidence, an extreme flood claimed the lives of 21 people and caused millions of dollars worth of damage to the towns and villages along the river. This flood was larger in magnitude than any flood on record, according to gaged data since 1890. An autumn storm, which is typical of this region, struck with immense force. The rain cell migrated from the lower reaches of the basin on the evening of 8 September to the upper parts of the basin, producing 680 mm of rain in 20 hours.The floods peak discharge is preliminary estimated at 6000 m3 s−1. This flood is now considered by the media and professionals to be “The largest flood on record.” However, our research proves otherwise.