Jonathan B. Laronne

Bed Load Sediment Transport in an Ephemeral Stream and a Comparison with Seasonal and Perennial Counterparts

Bed load sediment flux in an ephemeral channel, the Nahal Yatir, is shown to be a comparatively simple function of stream power and to reach levels that are several orders of magnitude higher than maxima measured at similar levels of stream power in perennial counterparts. Channel average submerged unit flux rates are recorded as high as 4.3 kg s−1 m−1, while at the center of the channel, the highest rate recorded is 6.5 kg s−1m−1. Transport efficiency is at least an order of magnitude higher than in other channels for which there are comparable data and, on average, as much as 400 times that of Oak Creek. These differences are explained by the fact that the bed of the Yatir is not armored. It is surmised that the unvegetated nature of this desert watershed provides ample supplies of sediment of all sizes and that this, together with the rapid recession of the flash flood hydrograph and the extended periods of no flow, discourages the development of an armor layer. The flux rates are not sediment supply-limited, as they are in perennial channels. Nahal Yatir and Oak Creek represent two ends of a spectrum, between which come seasonal and less well armored perennial streams. Transport efficiency is shown to vary considerably for each stream and from one stream to another, suggesting that it may not be possible to incorporate it easily into bed load equations in order to improve levels of prediction.

The non-layering of gravel streambeds under ephemeral flood regimes

Abstract The two-layer format common to perennial streambeds, in which a relatively coarse armour overlies a finer subarmour, develops as a function of both the ingress and subsequent near-surface winnowing of interstitial material and the selective non-entrainment or slower transport velocity of coarse clasts. Ephemeral streams appear to lack such vertical layering or are characterized by weak layer development. Some of this may be due to the degree of mixing associated with the scour-and-fill process. However, continuous monitoring of bedload discharge in the Nahal Yatir in the northern Negev Desert reveals that sediment transport rates are extremely high so that the chance of armour layer development through selective non-entrainment is much reduced. Indeed, a comparison of the bedload and bed material size-distributions confirms a high degree of similarity and hints at equal mobility regardless of clast size. The monitoring programme also indicates that the bed becomes highly mobile at comparatively modest fluid shear, so that practically all floods are associated with high transport rates. Consequently, the winnowing that might be brought about by low transport-rate events does not occur. Even within a single event, winnowing is precluded by the rapid nature of flow recession that is so characteristic of flash-floods. The high degree of bed material mobility is attributable, in part, to the lack of strength that would otherwise be a corollary of armour development. However, it also highlights the divergent nature of the feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems.

Microbial and Chemical Characterization of Underwater Fresh Water Springs in the Dead Sea

Due to its extreme salinity and high Mg concentration the Dead Sea is characterized by a very low density of cells most of which are Archaea. We discovered several underwater fresh to brackish water springs in the Dead Sea harboring dense microbial communities. We provide the first characterization of these communities, discuss their possible origin, hydrochemical environment, energetic resources and the putative biogeochemical pathways they are mediating. Pyrosequencing of the 16S rRNA gene and community fingerprinting methods showed that the spring community originates from the Dead Sea sediments and not from the aquifer. Furthermore, it suggested that there is a dense Archaeal community in the shoreline pore water of the lake. Sequences of bacterial sulfate reducers, nitrifiers iron oxidizers and iron reducers were identified as well. Analysis of white and green biofilms suggested that sulfide oxidation through chemolitotrophy and phototrophy is highly significant. Hyperspectral analysis showed a tight association between abundant green sulfur bacteria and cyanobacteria in the green biofilms. Together, our findings show that the Dead Sea floor harbors diverse microbial communities, part of which is not known from other hypersaline environments. Analysis of the water’s chemistry shows evidence of microbial activity along the path and suggests that the springs supply nitrogen, phosphorus and organic matter to the microbial communities in the Dead Sea. The underwater springs are a newly recognized water source for the Dead Sea. Their input of microorganisms and nutrients needs to be considered in the assessment of possible impact of dilution events of the lake surface waters, such as those that will occur in the future due to the intended establishment of the Red Sea−Dead Sea water conduit.

Flash-flood and bedload dynamics of desert gravel-bed streams

Comparatively little is known about the hydrology of desert flash-floods despite the extent of the worlds drylands. There is even less known about their sedimentary behaviour and particularly about the movement of coarse material as bedload. The results of an intense field monitoring programme carried out on an ephemeral gravel-bed stream in the northern Negev Desert are presented. In this semi-arid setting, flow duration analysis indicates that the channel is hydrologically active for 2% of the time, or about seven days per year, and that overbank flow can be expected for only 0·03% of the time—about three hours per year. Multipeaked flood hydrographs are the norm, reflecting many factors including the arrival of separate slugs of discharge from contributing subcatchments. The passage of the initial flood bore is surprisingly slow, but the rising limb of the flood hydrograph is rapid with a median time of rise of 10 minutes, in keeping with expected flash-flood behaviour. Bedload flux is high, averaging 2·67 kg s−1 m−1 during the period that the channel carries flow. This gives very high bedload sediment yield despite the infrequent and short duration of flood flows and matches the high yield of suspended sediment. The relationship between bedload flux and boundary shear stress is simple, in contrast with perennial gravel-bed streams, and the exponent of the log–log relationship is 1·52. Of great value is that the behaviour of the Nahal Eshtemoa corroborates a pattern established by the authors previously in a smaller tributary stream.

Continuous monitoring of bedload flux in a mountain gravel-bed river

An automatic bedload monitoring station has been established on the perennial Tordera River, a Mediterranean mountain gravel-bed stream located in NE Spain. Bedload fluxes were obtained automatically at 1-min intervals using two Birkbeck-type pit bedload samplers aligned across the width of the channel. Flow depth and water-surface slope were both measured continuously and synchronously with bedload. Data for five flow events show that bedload flux varies considerably. Bedload flux is high when it is compared with other perennial streams. For a 15-min interval, maximum channel-average bedload flux was 0.83 kg m−1 s−1. Bedload records also show that there is no direct relationship between bedload flux and grain shear stress. The low correlation between bedload flux and hydraulics arises due to the different thresholds of initiation of motion for each flow event and the variable character of the bed between events. For two individual floods, there is a reasonable relationship between bedload flux and grain shear stress, in part, as a consequence of individual particle entrainment and patch movement over a stable coarse gravel-bed during moderately high discharges. These results demonstrate that the Birkbeck system functions well in a Mediterranean mountain gravel-bed stream with a longitudinal slope of 2%.

The continuous measurement of bedload discharge in a large alpine gravel bed river

The objective of this paper is to analyze the efficiency of a bedload-measuring trap for continuous sediment measurements. The trap is implemented into the river-bed of the Drau, located in Southern Austria. At the trap site the mean annual discharge is 64 m S, the slope is about 0.002 and the mean subsurface grain diameter is about 28 mm. Two criteria, hydraulic efficiency and sampling efficiency, are used to evaluate the performance of the trap. The hydraulic efficiency as well as the sampling efficiency are shown to be high. The advantage of the trap is that continuous and automatic bedload transport measurements can be obtained even in wide, alpine gravel bed rivers. We demonstrate that continuous bedload data are useful with respect to identifying initiation of motion. The comparison with formulas shows significant deviations between measurements and calculations. In such a supply limited river-bed, where bedload often moves over an armour layer, it is apparent that local hydraulic conditions explain only a fraction, about one third of the variation in the weak bedload flux.

Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel-bed streams

Indirect bedload transport measurements have been made with the Swiss plate geophone system in five gravel-bed mountain streams. These geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed. To calibrate the geophone system, direct bedload transport measurements were undertaken simultaneously. At the Erlenbach in Switzerland, a moving-basket sampler was used. At the Fischbach and Ruetz streams in Austria, a Helley–Smith type bedload sampler provided the calibration measurements. A Bunte-type bedload trap was used at the Rofenache stream in Austria. At the Nahal Eshtemoa in Israel, Reid-type slot bedload samplers were used. To characterize the response of the geophone signal to bedload particles impacting on the plate, geophone summary values were calculated from the raw signal and stored at one second intervals. The number of impulses, i.e. the number of peaks above a pre-defined threshold value of the geophone output signal, correlated well with field measured gravel transport loads and was found to be a robust parameter. The relations of impulses to gravel transport loads were generally near-linear, but the steepness of the calibration relations differed from site to site. By comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments, it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient. The analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload.

Scour chain employment in gravel bed rivers

Abstract The dynamics of river beds can be assessed fromscour and fill data obtained from scour chains. Chains have not been used extensively in gravel bed rivers due to a variety of employment difficulties. Chain insertion, anchoring and relocation can be determined with minimal manpower by utilising a set of tested employment methodologies. These are decribed in sufficient detail to allow fluvial geomorphologists, ecologists and river engineers to more commonly utilise chains in studies of river stability and river response.

Bed load texture in an alpine gravel bed river

The spatial and temporal variations in bed load texture measured in a gravel bed river with a Helley-Smith sampler are compared with its bed material texture. Most measurements were undertaken at low excess shear stress, with low bed load discharge over an armored bed. Cross-sectionally averaged fractional bed load discharge increases with water discharge, but the explanation of the variance in fractional discharge is low (20–40%). Cross-sectional distribution of shear velocity explains in part the spatial variation in bed load texture. Because local hydraulic parameters are weak predictors of fractional bed load discharge for the granule-pebble range at nonequilibrium conditions, it reinforces the hypothesis that this size range, which comprises the bulk of bed load, derives from upstream sources. Temporal variations in the cross-sectional distribution of bed load texture are shown to depend on variations in bed topography and local shear stress. We demonstrate that the dominance of granule-pebble fractions is in part related to the hiding behavior of smaller fractions.

The variation of water-surface slope and its significance for bedload transport during floods in gravel-bed streams

Water-surface slope is usually assumed to be constant when predicting bedload sediment transport in rivers despite its significance as a determinant of shear stress and the impact that variability would have on calculated sediment flux. This is pragmatic. It recognises that confirmatory data are unlikely to be available, especially during flood flows, and it is an appropriate assumption where discharge is steady. Where discharge is unsteady, water-surface slope varies and an expected pattern of hysteresis in the relation between watersurface slope and flow depth emerges from datasets collected in four gravel-bed streams, two ephemeral, one seasonal and one perennial. When water-surface slope is treated as a variable in applying a bedload equation, it is shown that flood bedload yields are about 8 percent higher than those derived with the same equation but with water-surface slope held constant and approximating the slope of the channel bed. It is concluded that, in engineering design, accounting for the ...