ALBERT

Description: ABSTRACT Single-thread, gravel-bed streams of moderate slope in the northern Negev are characterized by three channel units: bars exhibit steeper than average slopes and poorly sorted mixtures of small–medium cobbles and coarse–very coarse pebbles; flats are associated with more gentle slopes and well-sorted medium–fine pebbles and granules; and transitional units have intermediate slopes and grain size. In general, all three units are planar, span the full channel width and have well-defined boundaries. Bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres, forming sequences that are reminiscent of the riffle–pool structure commonly observed in humid-temperate gravel-bed rivers. A notable contrast is the absence of significant bed relief: bars lack crests and flats lack depressions. The relative lack of bed relief in bar–flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport. This reduction of bed relief lowers channel roughness, which in turn increases flow velocity and, therefore, the ability of the channel to transmit the large sediment loads it receives. Although our analyses pertain to a semi-arid river system, the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments. Copyright © 2012 John Wiley & Sons, Ltd.

Description: The applicability of a portable, commercially available surface velocity radar (SVR) for non-contact stream gauging was evaluated through a series of field-scale experiments carried out in a variety of sites and deployment conditions. Comparisons with various concurrent techniques showed acceptable agreement with velocity profiles, with larger uncertainties close to the banks. In addition to discharge error sources shared with intrusive velocity-area techniques, SVR discharge estimates are affected by flood-induced changes in the bed profile and by the selection of a depth-averaged to surface velocity ratio, or velocity coefficient ( α ). Cross-sectional averaged velocity coefficients showed smaller fluctuations and closer agreement with theoretical values than those computed on individual verticals, especially in channels with high relative roughness. Our findings confirm that α = 0.85 is a valid default value, with a preferred site-specific calibration to avoid underestimation of discharge in very smooth channels (relative roughness ∼ 0.001) and overestimation in very rough channels (relative roughness 〉 0.05). Theoretically derived and site-calibrated values of α also give accurate SVR-based discharge estimates (within 10%) for low and intermediate roughness flows (relative roughness 0.001 to 0.05). Moreover, discharge uncertainty does not exceed 10% even for a limited number of SVR positions along the cross-section (particularly advantageous to gauge unsteady flood flows and very large floods), thereby extending the range of validity of rating curves. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Whilst clay and silt matrices of gravel-bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms, their impact on sediment entrainment and transport has been explored less. A recent increase of such a matrix in the bed of Nahal Eshtemoa, an ephemeral river of the northern Negev, has more than doubled the boundary shear stress needed to initiate bedload, from 7 N m -2 (τ *  = 0.027) during the flash floods of 1991–2001 to 15 N m -2 (τ *  = 0.059) during those of 2008–2009. The relation between bedload flux and boundary shear stress continues to be well-defined, but it is displaced. The matrix now contains a significant amount of silt and clay size material. The reasons for the increased entrainment threshold of bedload are explored. Large-scale laser scanning of the dry bed reveals a reduction in grain-scale morphological roughness, while artificial in situ tests of matrix integrity indicate considerable cohesion. The implications for adopting bed material sampling strategies that account for matrix development are assessed. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Single-thread, gravel-bed streams of moderate slope in the northern Negev are characterised by three channel units: bars exhibit steeper than average slopes and poorly sorted mixtures of small-medium cobbles and coarse-very coarse pebbles; flats are associated with more gentle slopes and well-sorted medium-fine pebbles and granules; and transitional units have intermediate slopes and grain size. In general, all three units are planar, span the full channel width and have well-defined boundaries. Bars and flats are much more common than the transitional units and alternate downstream for distances of several hundred metres, forming sequences that are reminiscent of the riffle-pool structure commonly observed in humid-temperate gravel-bed rivers. A notable contrast is the absence of significant bed relief: bars lack crests and flats lack depressions. The relative lack of bed relief in bar-flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport. This reduction of bed relief lowers channel roughness, which in turn increases flow velocity and, therefore, the ability of the channel to transmit the large sediment loads it receives. Although our analyses pertain to a semi-arid river system, the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments. Copyright © 2011 John Wiley & Sons, Ltd.

Description: ABSTRACT Braiding has been widely studied in humid-temperate climates though rarely in arid environments. Morpho-texture of braided streams refers to the morphological-textural organization in a braid-cell (stream section including bars, anabranches and chutes) that may strongly relat to different processes and hydrological regimes. The objective of this study is to compare the morpho-texture of braided streams governed by diverse flow recessions in different climates. Measurements were conducted in the wadis Ze'elim and Rahaf, SE hyper-arid Israel, in the Mediterranean Barranca-de-los-Pinos, Central-Spain and in humid-temperate braided systems, the La-Bléone River, Haute-Provence (France), Saisera and Cimoliana torrents, NE Italy. Terrestrial Laser Scanning was used to produce point clouds and high resolution Digital Elevation Models of the braid-cells. Wet braid-cells in humid-temperate environments were surveyed by a Total Station. Roughness and the upper tail of grain size distributions were derived from the scanned point clouds or from Wolman sampling. We found that anabranches are commonly finer-grained than the bars in dryland systems and in semi-arid sandy braided systems, contrary to the humid-temperate braided systems. In both climates, chutes are similar or coarser-grained than the bars which they dissect, in accordance with their steeper gradients due to the considerable bar-anabranch relief. The Saisera's morpho-texture is similar to that of the dryland braided channels, despite the very humid-temperate environment in which it is located, due to its short-lived, ephemeral type hydrograph. Hydrograph shape, specifically the duration of flow recession, typical of a climate but not confined to it, determines the morpho-texture of braided streams and the textural differentiation between a depositional bar and the adjacent anabranches. The morpho-texture of chutes and bars results also from local erosional processes affected by local topography, i.e., unsteady longitudinal profiles, and is not solely determined by flow recession. This new morpho-textural model enables identifying primary depositional and erosional braiding processes. This article is protected by copyright. All rights reserved.

Description: Abstract The Dead Sea is a closed lake, the water level of which is lowering at an alarming rate of about 1 m/yr. Factors difficult to determine in its water balance are evaporation and groundwater inflow, some of which emanate as submarine groundwater discharge. A vertical buoyant jet generated by the difference in densities between the groundwater and the Dead Sea brine forms at submarine spring outlets. To characterize this flow field and to determine its volumetric discharge, a system was developed to measure the velocity and density of the ascending submarine groundwater across the center of the stream along several horizontal sections and equidistant depths while divers sampled the spring. This was also undertaken on an artificial submarine spring with a known discharge to determine the quality of the measurements and the accuracy of the method. The underwater widening of the flow is linear and independent of the volumetric spring discharge. The temperature of the Dead Sea brine at lower layers primarily determines the temperature of the surface of the upwelling, produced above the jet flow, as the origin of the main mass of water in the submarine jet flow is Dead Sea brine. Based on the measurements, a model is presented to evaluate the distribution of velocity and solute density in the flow field of an emanating buoyant jet. This model allows the calculation of the volumetric submarine discharge, merely requiring either the maximum flow velocity or the minimal density at a given depth.

Description: Water budget analyses are important for the evaluation of the water resources in semi-arid and arid regions. The lack of observed data is the major obstacle for hydrological modelling in arid regions. The aim of this study is the analysis and calculation of the natural water resources of the Western Dead Sea subsurface catchment, one which is highly sensitive to rainfall resulting in highly variable temporal and spatial groundwater recharge. We focus on the subsurface catchment and subsequently apply the findings to a large-scale groundwater flow model to estimate the groundwater discharge to the Dead Sea. We apply a semi-distributed hydrological model (J2000g), originally developed for the Mediterranean, to the hyper-arid region of the Western Dead Sea catchment, where runoff data and meteorological records are sparsely available. The challenge is to simulate the water budget, where the localized nature of extreme rainstorms together with sparse runoff data results in few observed runoff and recharge events. To overcome the scarcity of climate input data we enhance the database with mean monthly rainfall data. The rainfall data of two satellites are shown to be unsuitable to fill the missing rainfall data due to underrepresentation of the steep hydrological gradient and temporal resolution. Hydrological models need to be calibrated against measured values, hence the absence of adequate data can be problematic. Therefore, our calibration approach is based on a nested strategy of diverse observations. We calculate a direct surface runoff of the Western Dead Sea surface area (1801 km 2 ) of 3.4 mm/a and an average recharge (36.7 mm/a) for the 3816 km 2 subsurface drainage basin of the Cretaceous aquifer system.