[1] Desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below, while gravel bed streams in humid and snowmelt areas typically have well-armored surfaces. The degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size. A set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0.5 to 2.4 and with an average value of 1.2. The armor ratio of snowmelt-fed gravel bed streams in the same set ranges from 2 to 7, with an average value of 3.4. The reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes. Thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions. The experiments have two limiting cases: a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods. All constant hydrograph experiments developed a well-armored structured surface, while short asymmetrical hydrographs did not result in substantial vertical sorting. All symmetrical hydrographs show some degree of sorting, and the sorting tended to become more pronounced with longer duration. Sediment supply appears to be a first-order control on bed surface armoring, while the shape of the hydrograph plays a secondary role.
Geomorphological studies of braided rivers still lack a consistent measurement of the complexity of the braided pattern. Several simple indices have been proposed and two (channel count and total sinuosity) are the most commonly applied. For none of these indices has there been an assessment of the sampling requirements and there has been no systematic study of the equivalence of the indices to each other and their sensitivity to river stage. Resolution of these issues is essential for progress in studies of braided morphology and dynamics at the scale of the channel network.A series of experiments was run using small-scale physical models of braided rivers in a 3 m × × × × × 20 m flume. Sampling criteria for braid indices and their comparability were assessed using constant-discharge experiments. Sample hydrographs were run to assess the effect of flow variability.Reach lengths of at least 10 times the average wetted width are needed to measure braid indices with precision of the order of 20% of the mean. Inherent variability in channel pattern makes it difficult to achieve greater precision. Channel count indices need a minimum of 10 cross-sections spaced no further apart than the average wetted width of the river. Several of the braid indices, including total sinuosity, give very similar numerical values but they differ substantially from channel-count index values. Consequently, functional relationships between channel pattern and, for example, discharge, are sensitive to the choice of braid index. Braid indices are sensitive to river stage and the highest values typically occur below peak flows of a diurnal (melt-water) hydrograph in pro-glacial rivers. There is no general relationship with stage that would allow data from rivers at different relative stage to be compared. At present, channel count indices give the best combination of rapid measurement, precision, and range of sources from which measurements can be reliably made. They can also be related directly to bar theory for braided pattern development.
[1] Physical models of gravel braided rivers were used to investigate the adjustment of braiding intensity to step changes in channel-forming discharge and the mechanisms by which channel pattern adjustment and maintenance occurs. A braided channel developed at low discharge was subjected to two step increases in discharge between which the channel was given time to develop stable average braiding intensity in response to each steady discharge. Active (with visible bed material movement) and total channel networks were mapped throughout the experiment. Total braiding intensity exceeded active braiding intensity and both adjusted to a stable, average value at each discharge, indicating that channel pattern adjustment to total discharge involves both the active and the total network. Only portions of the total braided channel network developed at a given time, and it formed progressively by migration and avulsion of the (less extensive) active network. At equilibrium, the ratio of active to total braiding intensity stabilized at about 0.4. This stable value may increase with relative mobility of the bed material (stream power relative to grain size). The stable value was achieved via gradual increase of total braiding intensity while active braiding intensity adjusted very quickly to the increased flow. These adjustments are controlled by partial avulsion of the main active channel associated with changes in its sinuosity, and allocation of flow and bed load to secondary anabranches. Braided channel pattern dynamics is closely tied to, and explained by, the local dynamics and symmetry/asymmetry of bifurcations and avulsions.Citation: Egozi, R., and P. Ashmore (2009), Experimental analysis of braided channel pattern response to increased discharge,
Micro-estuarine ecosystems have a surface area < km and are abundant in Mediterranean 17 regions. As a result of their small size, these systems are particularly vulnerable to effects of 18 chemical pollution. Due to fluctuating flow conditions of base-flow dominated by treated 19 wastewater effluents and flood events transporting rural and urban non-point-source pollution, 20 micro-estuaries are under a dynamic risk regime, consequently, struggling to provide 21 ecological services. This two-year study explored the occurrence and risks of pharmaceutical 22 contamination in the Alexander micro-estuary in Israel. Pharmaceuticals were detected in all 23 samples (n=280) at as high as 18 µg L -1 in flood events and 14 µg L -1 in base-flow.Pharmaceutical mixtures composition was affected by flow conditions with carbamazepine dominating base-flow and caffeine dominating flood events. Median annual risk quotients for fish, crustaceans and algae were 19.6, 5.2, and 4.5, indicating that pharmaceuticals pose high risk to the ecosystem. Ibuprofen, carbamazepine and caffeine were contribute most to the risk quotients. The current work highlights that micro-estuary ecosystems, like the Alexander estuary, are continuously exposed to pharmaceuticals and most likely to other pollutants, placing these ecologically important systems under an elevated risk, in comparison to the more frequently studied large estuarine systems.
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