Soil erosion and sediment transport in Tanzania: Part I – sediment source tracing in three neighbouring river catchments

Water bodies in Tanzania are experiencing increased siltation, which is threatening water quality, ecosystem health, and livelihood security in the region. This phenomenon is caused by increasing rates of upstream soil erosion and downstream sediment transport. However, a lack of knowledge on the contributions from different catchment zones, land-use types, and dominant erosion processes, to the transported sediment is undermining the mitigation of soil degradation at the source of the problem. In this context, complementary sediment source tracing techniques were applied in three Tanzanian river systems to further the understanding of the complex dynamics of soil erosion and sediment transport in the region. Analysis of the geochemical and biochemical fingerprints revealed a highly complex and variable soil system that could be grouped in distinct classes. These soil classes were unmixed against riverine sediment fingerprints using the Bayesian MixSIAR model, yielding proportionate source contributions for each catchment. This sediment source tracing indicated that hillslope erosion on the open rangelands and maize croplands in the mid-zone contributed over 75% of the transported sediment load in all three river systems during the sampling time-period. By integrating geochemical and biochemical fingerprints in sediment source tracing techniques, this study demonstrated links between land use, soil erosion and downstream sediment transport in Tanzania. This evidence can guide land managers in designing targeted interventions that safeguard both soil health and water quality.     

Can channel banks be the dominant source of fine sediment in a UK river?: an example using 137Cs to interpret sediment yield and sediment source

Cultivated fields have been shown to be the dominant sources of sediment in almost all investigated UK catchments, typically contributing 85 to 95% of sediment inputs. As a result, most catchment management strategies are directed towards mitigating these sediment inputs. However, in many regions of the UK such as the Nene basin there is a paucity of sediment provenance data. This study used the caesium‐137 (¹³⁷Cs) inventories of lake and floodplain cores as well as the ¹³⁷Cs activities of present day sediment to determine sediment provenance. Sediment yields were also reconstructed in a small lake catchment. Low ¹³⁷Cs inventories were present in the lake and floodplain cores in comparison to the reference inventory and inventories in cores from other UK catchments. Caesium‐137 activities in the present day sediments were low; falling close to those found in the channel bank catchment samples. It was estimated that 60 to 100% of the sediment in the Nene originated from channel banks. Pre‐1963 sediment yields were approximately 11.2 t km^?2 yr^?1 and post‐1963 was approximately 11.9 t km^?2 yr^?1. The lack of increased sediment yield post‐1963 and low sediment yield is unusual for a UK catchment (where a yield of 28 to 51 t km^?2 yr^?1 is typical for a lowland agricultural catchment), but is explained by the low predicted contribution of sediment from agricultural topsoils. The high channel bank contribution is likely caused by the river being starved of sediment from topsoils, increasing its capacity to entrain bank material. The good agreement between the results derived using cores and recently transported sediments, highlight the reliability of ¹³⁷Cs when tracing sediment sources. However, care should be taken to assess the potential impacts of sediment particle size, sediment focusing in lakes and the possible remobilization of ¹³⁷Cs from sedimentary deposits. Copyright © 2016 John Wiley & Sons, Ltd.     

Does the river run wild? Assessing chaos in hydrological systems

The standing debate over whether hydrological systems are deterministic or stochastic has been taken to a new level by controversial applications of chaos mathematics. This paper reviews the procedure, constraints, and past usage of a popular chaos time series analysis method, correlation integral analysis, in hydrology and adds a new analysis of daily streamflow from a pristine watershed. Significant problems with the use of correlation integral analysis (CIA) were found to include a continued reliance on the original algorithm even though it was corrected subsequently and failure to consider the physics underlying mathematical results. The new analysis of daily streamflow reported here found no attractor with D⩽5. Phase randomization of the Fourier Transform of streamflow was used to provide a better stochastic surrogate than an Autoregressive Moving Average (ARMA) model or gaussian noise for distinguishing between chaotic and stochastic dynamics.     

Experimental study on the effect of bottomless structure in front of a bottom outlet on a sediment ?ushing cone

One of the main problems in reservoirs is sedimentation which reduces the operating life of dams if a proper plan and analysis method are not in place.The techniques to manage sediment in reservoirs include several sustainable management techniques that route sediment through or around the reservoir.One of the main economical methods in arid and semi-arid regions is pressurized flushing using moderate drawdown of the water level of the reservoir to evacuate sediment deposited behind dams.In the current study,the effect of a new structure called a dendritic bottomless extended (DBE) outlet structure at three angles of 30°,45°,and 60° on pressurized flushing efficiency was investigated.Consequently,45 experiments were designed for three discharge rates (Q_o) ,three sediment levels (H_s),four types of structure,and a no-structure condition (reference test).The results indicated that the DBE structure with a 30° angle between the branches,a sedimentary dimensionless index of ■,and a flow dimensionless index of ■(where g is the acceleration of gravity and Do is the diameter of the bottom outlet) lead to 10-fold increase in the sediment flushing cone dimensions and sediment removal efficiency compared to the results of the reference test.Finally,according to a statistical analysis of the results,a dimensionless equation for calculating the sediment flushing cone dimensions was developed for the tested sediment characteristics.     

A comparative flood frequency study of Ganga and Brahmaputra river systems of north India — a brief appraisal

An attempt has been made to study the occurrence of floods in the two important river systems of north India, viz. Brahmaputra and Ganga. Both these river systems are located north of Lat. 22°N and lie in the longitudinal belt of north India from Long. 73°E to 97°E. Both these river systems are affected by floods during monsoon months of June–September. It has been seen after examining rainfall and floods of the period 1986–1999 that although variation of monsoon rainfall magnitudes received by these two basins differ considerably in each monsoon season, but by and large, the frequency of floods at their terminal gauge/discharge (G/D) sites at Dhubri and Farakka do not differ very much from each other.     

Stable isotope tracing of headwater sources in a river on China’s Loess Plateau

Distinguishing headwater components can provide fundamental information for water resources management; however, this is difficult with the hydrometric method for regions with a thick unsaturated zone. We sampled headwaters, precipitation and groundwater in a river on China’s Loess Plateau to determine the isotopic composition, and identified headwater components by an isotope mass balance method. The isotopic composition of precipitation varies greatly, whereas that of groundwater is almost constant, which validates the applicability of the isotope mass balance method. During the dry season, the contributions of precipitation and groundwater to headwaters are both 50% for the upper reach, while they are 20 and 80%, respectively, for the lower reach; however, during the wet season, the contributions are, respectively, 67 and 33% for the upper reach, and 43 and 57% for the lower reach. The headwaters respond quickly to rainfall but are dominated by groundwater. Groundwater protection should be of high priority to sustain the catchment-scale hydrological cycle.

EDITOR D. Koutsoyiannis ASSOCIATE EDITOR M.D. Fidelibus     

Scenarios of the spread of a waste heat discharge in a river — Vistula River case study

The problem of two-dimensional mathematical modelling of heated cooling water discharges into running waters is considered in the paper. Two models — one for the evaluation of 2D turbulent velocity field and the other, developed by authors of the study, for 2D heat transport in open-channels — were used in the calculations. Relevant scenarios of the spread of heated water discharged from a designed gas-stem power plant to be constructed at the Vistula River were presented. Environmentally most friendly variant of the discharge of the thermal pollution was selected from among four various variants.     

Ant mounds as a source of sediment in a tropical rainforest?

In Lutzito catchment on Barro Colorado Island, Panama, extraordinarily high suspended‐sediment yields of 1–2 Mg ha^?1 year^?1 were generated despite the dense forest cover coinciding with erosion‐resistant soils. We hypothesized that ant mounding activity is an important zoogeomorphological mechanism in this area, providing relevant quantities of easily transportable material at the soil surface. To test this hypothesis, all ant mound material was collected collected for dry mass determination from thirty 4 m² plots installed in the study area every 1–3 days during the 39‐day sampling period. Additionally, three ground‐nesting ant species responsible for mounds in the study area, Ectatomma ruidum, Trachymyrmex cornetzi and Strumigenys marginiventris, were identified. On the basis of the total of 1.38 kg of material collected in the wet season of 2011, the estimate for the whole 8 months wet season amounts to 725 kg ha^?1. As this value is in the same order of magnitude as sediment output, it shows that ants may act as important ecosystem engineers and contribute to sediment production here by providing large quantities of fine‐grained, readily erodible material at the soil surface for subsequent transport to the streambed. Copyright © 2014 John Wiley & Sons, Ltd.     

Ce anomalies in Gough Island lavas — Trace element characteristics of a recycled sediment component

High precision trace element data obtained by inductively coupled plasma mass spectrometry and Sr–Nd isotope analyses are presented for mafic volcanic rocks from Gough Island, South Atlantic. The new data reveal negative Ce anomalies, with Ce/Ce^? values in Gough lavas extending down to ～ 0.92. Ce is only fractionated from other rare earth elements (REE) due to formation of Ce⁴⁺ under oxidizing conditions of near-surface environments while other REE remain trivalent. Ce anomalies in convergent margin magmas have been shown to indicate a contribution of a subducted sediment component. In contrast, Ce anomalies in intra-plate basalts have been attributed to weathering processes, but can be excluded here based on element–element systematics indicating magmatic trends rather than weathering-induced element mobility. Shallow-level contamination by local marine sediments with negative Ce anomaly inherited from seawater can be excluded because Gough lavas with increasingly negative Ce anomalies do not trend towards low Ce/Pb ratios characterizing such sediments. Rather, it is argued that the negative Ce anomalies in Gough Island lavas are consistent with variable amounts of a sediment component in the mantle plume source. Mixtures between estimates of subducting sediment columns with negative Ce anomaly and mantle capable of giving rise to Gough Island magmas without Ce anomalies reproduce the Gough compositional array with the exception of highly fluid-mobile elements. The calculated trace element composition of the deeply recycled sediment in the Gough plume source is depleted in fluid-mobile elements such as Ba and Pb relative to the composition of some present-day subducting sediments. This loss is attributed to the dehydration or flushing of sediment in the subduction factory, consistent with constraints from arc magmas.     

Groundwater–surface water connectivity in a chain-of-ponds semiarid river

Semiarid rivers are often characterized by chains of small pools connected by riffles and wet meadows. The pools can be maintained by wet season surface runoff, groundwater discharge, or some combination thereof. Using synoptic surveys for several environmental tracers (δD and δ¹⁸O of H₂O, specific electrical conductance at 25°C [EC], chloride and ²²²Rn), we evaluated the groundwater—surface water connectivity of the Light River (South Australia) along an 8 km section in the vicinity of a proposed mining development. In all three surveys (representing spring, summer and winter conditions), the pools were maintained by regional groundwater discharge based on an elevated surface water EC (9–12 dS m⁻¹) similar to regional groundwater, elevated radon-222 activities (0.09–3.0 Bq L⁻¹) and low rainfall. Most pools were perennial, either because they directly received groundwater discharge or, indirectly, had an inflow originating from upstream groundwater-fed pools. The elevated salinity of regional groundwater is a key factor for the maintenance of perennial pools in the Light River because the potential for baseflow depletion by groundwater pumping is more limited.     

Sub-Saharan African Ground Water Protection—Building on International Experience

Sub-Saharan Africa faces significant challenges in dealing with ground water pollution. These countries can look to successes and missteps on other continents to help choose their own individual paths to ensuring reliable and clean supplies of ground water. In the large view, sub-Saharan Africa can define specific levels of acceptable risk in water quality that drive cleanup efforts and are amenable to acceptance across national and geographic boundaries. Ground water quality databases must be expanded, and data must be available in an electronic form that is flexible, expandable, and uniform, and that can be used over wide geographic areas. Guidance from other continents is available on well construction, sampling and monitoring, interim remediation, technical impracticability, monitored natural attenuation, and many specific issues such as how to deal with small waste generators and septic contamination of water supply wells. It is important to establish a common African view on the appropriateness of other nations’ ground water quality guidance for African issues, economic conditions, and community circumstances. Establishing numerical, concentration-based, water quality action levels for pollutants in ground water, which many neighboring African nations could hold comparable, would set the stage for risk-based remediation of contaminated sites. Efforts to gain public, grass-roots understanding and support for stable and balanced enforcement of standards are also key. Finally, effective capacity building in the region could be an eventual solution to ground water quality problems; with increased numbers of trained environmental professionals, ground water throughout the region can be protected and contaminated sites cleaned up.     

Research on the Seismic Vulnerability of Building Structure in Sichuan Province

Research on the seismic vulnerability of building structures is very important for the work of earthquake disaster preparedness and mitigation.On the basis of the related studies over a long time,this paper provides several seismic vulnerability matrices of building structure in different regions of Sichuan Province,Poor anti-seismic capability is one of the factors resulting in the earthquake disasters in the past.We can reduce economic losses caused by earthquake through improving the anti-seismic and prevention level of building structures in Sichuan Province.     

Long‐term river meandering as a part of chaotic dynamics? A contribution from mathematical modelling

In the present contribution we focus our attention on the possible signatures of a chaotic behaviour or a self‐organized criticality state triggered in river meandering dynamics by repeated occurrence of cutoff processes. The analysis is carried out examining, through some robust nonlinear methodologies inferred from time series analysis, both the spatial series of local curvatures and the time series of long‐term channel sinuosity. Temporal distribution of cutoff inter‐arrivals is also investigated. The analyzed data have been obtained by using a suitable physics‐based simulation model for river meandering able to reproduce reasonably the features of real rivers. The results are consistent and show that, at least from a modelling point of view, no evidence of chaotic determinism or self‐organized criticality is detectable in the investigated meandering dynamics. Copyright © 2010 John Wiley & Sons, Ltd.     

How does network structure and complexity in river systems affect population abundance and persistence?

Models of habitat connectivity, and how network structure and connectivity affects resident populations, are increasingly being developed for terrestrial habitat networks. Rivers, unlike many terrestrial habitat networks, follow a rigid hierarchical branching structure to form dendritic networks. It has been hypothesised that this unique structure must have implications for population processes. We developed a theoretical model to relate local-scale habitat quality and connectivity to landscape-scale population dynamics of mobile organisms (e.g. fish, aquatic invertebrates). River networks were modelled as directed graphs, with nodes being habitat patches, and edges the connections between them. Using population simulation analyses, we investigated the effects of network structure on population abundance and persistence. Network structural complexity affected landscape-scale population abundance, but the apparent effect depended upon how structure was quantified. There were no noticeable effects of dendritic network structure on population persistence. Previous research on the effects of habitat network structure on population persistence has used metapopulation patch occupancy models, which do not directly consider population dynamics. Our results show that spatially-explicit population modelling is possible, and that it provides information beyond that possible with patch occupancy models (e.g. population abundance). More importantly, it calls into question whether metapopulation models provide an adequate representation of population dynamics in dendritic habitat networks.     

Sediment fingerprinting in fluvial systems： review of tracers,sediment sources and mixing models

Suspended sediments in fluvial systems originate from a myriad of diffuse and point sources, with the relative contribution from each source varying over time and space. The process of sediment fingerprinting focuses on developing methods that enable discrete sediment sources to be identified from a composite sample of suspended material. This review identifies existing methodological steps for sediment fingerprinting including fluvial and source sampling, and critically compares biogeochemical and physical tracers used in fingerprinting studies. Implications of applying different mixing models to the same source data are explored using data from 41 catchments across Europe, Africa, Australia, Asia, and North and South America. The application of seven commonly used mixing models to two case studies from the US （North Fork Broad River watershed） and France （Bldone watershed） with local and global （genetic algorithm） optimization methods identified all outputs remained in the acceptable range of error defined by the original authors. We propose future sediment fingerprinting studies use models that combine the best explanatory parameters provided by the modified Collins （using correction factors） and Hughes （relying on iterations involving all data, and not only their mean values） models with optimization using genetic algorithms to best predict the relative contribution of sediment sources to fluvial systems.     

Experimental study on flow pattern and sediment transportation at a 90°open-channel confluence

In this paper,the evolutions of flow pattern and sediment transportation at a 90° open-channel confluence with different discharge ratios (q*) of the tributary flow to the total flow were studied.The e...     

Effect of sediment particles on the velocity profile of sediment-water mixtures in open-channel flow

The presence of sediment particles in open-channel flow has an important effect on turbulence; thus, an empirical, turbulent eddy viscosity formula was established for application in the limit for low concentrations. The current study establishes a theoretical relation for the mixture viscosity based on the two-phase mixture model. The percentage contribution of the three mechanisms of mixture viscosity,namely, fluid turbulence(FT), particle turbulence(PT), and inter-particle collisions(IPCs), w...     

Research on preservation and enrichment mechanisms of organic matter in muddy sediment and mudstone

Using multidiscipline methodologies, the differences in preservation and enrichment mechanisms of organic matter (OM) in muddy sediment and mudstone are investigated. In clay fractions, concentra- tions of TOC and chloroform bitumen “A” are significantly higher than those in coarser fractions. This indicates that clay minerals (CM) play an important role in enriching OM. The content of chloroform bitumen “A” increases obviously in the clay fraction, which reveals that dissolvable OM is the main composition of coalesce with clay minerals. Furthermore, TG and DTA data show that OM enrichment mechanisms and preservation forms have multiplicity. Several exothermic peaks in the DTA curves demonstrate that muddy sediment and mudstone contain a number of bioclasts and amorphous OM besides dissolvable OM. Through analyzing with XRD and DTA after mudstone samples were pretreated, the conclusions can be arrived at. Firstly, CM interlayer space of XRD curves and exothermic peaks of DTA curves both change as temperature increases. Secondly, the changes of CM interlayer space and exothermic peaks are concordant and stable around 350℃. All these are the features that OM enters CM interlayers to form stable organo-clay complexes. Therefore, the combination format of OM with CM is not only surface adsorption, partial OM enters CM interlayers to form stable organo-clay complexes. Finally, through the research on OM preservation forms and enrichment mechanisms in muddy sedi- ment and mudstone, the hydrocarbon-generation processes and the global carbon cycle and budget can be explained.     

Biodiversity reduction on contaminated sediments in the sea: contamination or sediment mobility?

Where contaminants in sediments correlate with biodiversity reductions, the contamination may not be the cause. Other physical factors may be responsible, and should be investigated so that any remedial action is directed properly. Two examples are given where unusual sediment mobility caused biodiversity reductions in sediments with elevated trace contaminants.