Scour Envelope Curve (SEC), Negev Desert, Israel

An upper limit for long-term scour in ephemeral streams has not been studied, although it is necessary to know it for flow regime determination, paleohydrology, and sediment budget evaluations. The available data on scour derive from short investigations lasting a few decades at most, but these are not representative of long-term processes, which are crucial for the understanding of alluvial geomorphic, pedologic, and hydrologic conditions in arid regions. A new investigation in several desert streams in the Negev Desert of Israel found a distinctive unit characterized by pedogenic features beneath the active channel bed. This unit, which has some initial attributes of a desert soil, such as horizonation, calcium carbonate deposition, iron oxide release, and accumulation of fine-grained material, has been termed a Fluvio-Pedogenic-Unit (FPU) due to its formation under stable conditions by water of fluvial origin.The long time-period needed for the FPU formation suggests stable and limited depth of the active alluvial layer and, therefore, a long-term steady-state sediment budget within a given watershed and, hence, the existence of an upper limit to scour and an indication for a period of a steady hydrological regime. The FPU, easily recognized in comparison to the laborious traditional methods, facilitates data collection of the depth and spatial distribution of maximum scour in streams of various catchment sizes.Comparing the FPU depth with measured and paleo-flood peak discharges for several watersheds in the Negev indicates a maximum scour of 0.5 m for the small catchments (Nahal Yael, 0.5 km²), and a scour not exceeding 1.3 m in the largest catchment (Nahal Zin, 1205 km²). The concept of envelope curves for maximum measured discharges has been adapted for maximum scour evaluations and a Scour Envelope Curve (SEC) is suggested. Using the SEC with the Regional Envelope Curve (REC) supplies a more complete understanding of maximum possible discharges and the maximum possible scour that can occur in ungauged watersheds in the arid Negev.