Relationship between bedload and suspended sediment in the sand-bed Exu River,in the semi-arid region of Brazil

Abstract

Suspended sediment and bedload discharges in sand-bed rivers shape semi-arid landscapes and impact sediment delivery from these landscapes, but are still incompletely understood. Suspended sediment and bedload fluxes of the intermittent Exu River, Brazil, were sampled by direct measurements. The highest suspended sediment concentration observed was 4847.4 mg L^-1 and this value was possibly associated with the entrainment of sediment that was deposited in the preceding year. The bedload flux was well related to the stream power and the river efficiently transported all available bedload with a mean rate of 0.0047 kg m^-1 s^-1, and the percentage of bedload to suspended sediment varied between 4 and 12.72. The bed sediment of Exu River was prone to entrainment and showed a proclivity for transport. Thus, sand-bed and gravel-bed rivers of arid environments seem to exhibit the same mobility in the absence of armour layer.

Editor D. Koutsoyiannis; Associate editor B. Touaibia

Citation Cantalice, J.R.B., Cunha Filho, M., Stosic, B.D., Piscoya, V.C., Guerra, S.M.S., and Singh, V.P., 2013. Relationship between bedload and suspended sediment in the sand-bed Exu River, in the semi-arid region of Brazil. Hydrological Sciences Journal, 58 (8), 1789–1802.     

New method for estimating reference evapotranspiration and comparison with alternative methods in a fruit-producing hub in the semi-arid region of Brazil

Theoretical and Applied Climatology - Analysing alternative methods for estimating reference evapotranspiration (ET0) is essential for improving the management of water resources, especially where...     

Radioactivity backgrounds in ZEPLIN-III

We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75 ± 0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05 ± 0.5 nuclear recoils with energy 5-50 keV per year, which translates to an expectation of 0.4 events in a 1 yr dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light from Compton electrons and from β activity internal to photomultipliers, which can increase the size and lower the apparent time constant of the scintillation response. Another challenge is posed by multiple-scatter γ-rays with one or more vertices in regions that yield no ionisation. If the discrimination power achieved in the first run can be replicated, ZEPLIN-III should reach a sensitivity of ∼1 × 10⁻⁸pb · yr to the scalar WIMP-nucleon elastic cross-section, as originally conceived.     

Organotin levels in Nazaré canyon (west Iberian Margin, NE Atlantic) and adjacent coastal area

Organotin compounds (OTs) are ubiquitous in the marine environment and high concentrations (μg g(-1) range) in sediments from different coastal areas around the world have been reported. However, few reports have described the OTs contamination status in the offshore and deep sea environment. This work investigated organotin levels in Nazaré canyon for the first time. Levels of monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT), diphenyltin (DPT), triphenyltin (TPT), dioctyltin (DOT) and trioctyltin (TOT) were quantified in sediment samples from the upper flanks of the canyon and from the adjacent coastal area. TBT levels detected in the canyon flanks are about two to three orders of magnitude lower than those found in the coastal area. Nevertheless, when quantifiable, TBT levels in the canyon samples were higher than the Environmental Assessment Criteria set for TBT in sediments by the OSPAR Commission indicating that at those locations negative ecological impacts are likely to occur.     

Evaluation of surface sediment contamination by polycyclic aromatic hydrocarbons in the "Saco do Laranjal" - (Patos Lagoon, Brazil)

This paper evaluates the concentration of Polyaromatic hydrocarbons - PAHs in the Estuarine area of the Patos Lagoon, more precisely in a local named "Saco do Laranjal" in Pelotas City (southern Brazil). The samples were collected in May 2008. The concentrations of 16 US-EPA priority polycyclic aromatic hydrocarbons (PAHs) were determined by gas chromatography with a mass spectrometry detector (GC/MS). The sum of PAHs concentration ranged from 7.3 to 92.8μgkg(-1) in dry weight. The positive correlation (Pearson test) between the total organic material (TOM) and the total PAH concentration (0.98) suggests that TOM plays an important role in controlling the PAHs levels in the sediments. According to the observed ratios of individual PAHs, the contamination in the studied area is originated, predominantly pyrolytic sources. The values found are considered under normal levels and indicates a not impacted area.     

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal,western Iberia)

We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected D_e values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected D_e of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (～Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (～MIS9-6); T5 is 136–75 ka (～MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2).     

Stratigraphy and allogenic controls of the western Portugal Cretaceous: an updated synthesis

The stratigraphy of the western Portugal on-shore Cretaceous record (western Iberian margin, Lusitanian Basin) is described, including formal units and a selection of informal units prevailing in the geological literature. This paper is a synthesis based on a review of previous works, but with an innovative emphasis on the interpretation of eustatic and tectonic controls. The sedimentary record is dominated by siliciclastics and comprises fluvial and deltaic coastal marine siliciclastic systems, as well as extensive deposits of shallow marine carbonate platforms, both open and rimmed. Several regional unconformities and transgressive/regressive cycles are identified and the allogenic controls interpreted, namely the geodynamic events along the boundaries of the Iberian plate. Above the Berriasian deposits belonging to the Upper Jurassic cycle, the five main unconformity-bounded units are: (1) upper Berriasian–lower Barremian, (2) upper Barremian–lower Aptian, (3) upper Aptian–uppermost Cenomanian, (4) mid lower Turonian–lower Campanian and (5) middle Campanian–Maastrichtian. These units show transgressive peaks in the lower Hauterivian, lower Aptian, base of the upper Cenomanian and mid lower Turonian. The general trend of the Lower Cretaceous reflects the transition from late rifting to passive margin, with the last break-up unconformity dated as late Aptian. The Lusitanian Basin achieved full infill by the Cenomanian, when a large carbonate platform extended far inland. The later deposits were preserved only in the northern sector and the accompanying unconformities reflect transpressive intraplate stresses generated in boundaries of the plate with Africa and Eurasia. With very low accommodation being created throughout the Late Cretaceous, fluvial deposits were dominant, including a few marine levels related with eustatic rises in the early Turonian, the Coniacian, the early Campanian and the Maastrichtian.