Review of a Semi-Empirical Modelling Approach for Cohesive Sediment Transport in River Systems

In this paper, a review of a semi-empirical modelling approach for cohesive sediment transport in river systems is presented. The mathematical modelling of cohesive sediment transport is a challenge because of the number of governing parameters controlling the various transport processes involved in cohesive sediment, and hence a semi-empirical approach is a viable option. A semi-empirical model of cohesive sediment called the RIVFLOC model developed by Krishnappan is reviewed and the model parameters that need to be determined using a rotating circular flume are highlighted. The parameters that were determined using a rotating circular flume during the application of the RIVFLOC model to different river systems include the critical shear stress for erosion of the cohesive sediment, critical shear stress for deposition according to the definition of Partheniades, critical shear stress for deposition according to the definition of Krone, the cohesion parameter governing the flocculation of cohesive sediment and a set of empirical parameters that define the density of the floc in terms of the size of the flocs. An examination of the variability of these parameters shows the need for testing site-specific sediments using a rotating circular flume to achieve a reliable prediction of the RIVFLOC model. Application of the model to various river systems has highlighted the need for including the entrapment process in a cohesive sediment transport model.

Macroinvertebrate assemblages in mountain tributaries of glacial-fed and rain-fed rivers in eastern Nepal

While river macroinvertebrates are the most widely used form of bioindicators, their baseline information, although crucial, is scarce in Nepal. The main objective of this study was to assess the macroinvertebrate assemblages in mountain tributaries of the glacial-fed Tamor and rain-fed Kamala rivers. A total of eight sites were sampled during March 2015 (Spring), November 2015 (Autumn), January 2016 (Winter), and May 2016 (Summer). Altogether, 49 Families of macroinvertebrates belonging to 15 Orders were identified with 39 Families and 12 Orders in Tamor’s tributaries, and 33 Families and 10 Orders in Kamala’s tributaries. Non-metric multi-dimensional scaling (NMDS) revealed different assemblages between the two river systems. The most dominant Order in the Tamor was Ephemeroptera and it was Trichoptera in the Kamala. EPT (Ephemeroptera Plecoptera Trichoptera) assemblages were the most abundant in all four seasons for both the river systems and higher % EPT in Tamor’s tributaries indicate better water quality than in the Kamala tributaries. Typical cold water adapted Families such as Rhyacophilidae and Stenopsychidae were observed in Tamor’s tributaries whereas in Kamala’s tributaries, warm water adapted Families like Naididae and Thiaridae were found, reflecting a difference in the abiotic variables such as temperature, dissolved oxygen attributed to each of the catchments. This baseline data can serve as the foundation for further bioassessment including those of climate change impacts on aquatic biodiversity.

Improving Carbonate Equilibria-Based Estimation of pCO2 in Anthropogenically Impacted River Systems

Estimating riverine carbon dioxide (CO2) emissions has been constrained by lacking field measurements of the partial pressure of CO2 (pCO2) and inaccuracies in calculating pCO2 using carbonate equilibria-based models such as CO2SYS. To evaluate potential errors in applying the carbonate equilibria-based pCO2 calculation to river systems affected by monsoon rainfall and water pollution, we compared pCO2 values calculated using CO2SYS and those measured by headspace equilibration in five Asian rivers (Ganges, Mekong, Yangtze, Yellow, and Han rivers) undergoing various water pollution stages. Across the five rivers, calculated and measured pCO2 values exhibited larger discrepancies during the monsoon season, particularly in the low pH range, while in the Han River mismatches were also noticeable during the dry season. In the Han River, pH was negatively correlated with dissolved organic carbon (DOC) during the monsoon, indicating organic acids flushed from soils during rainfalls as a key factor for overestimated pCO2 at sites with low pH and alkalinity, whereas dry-season overestimation of pCO2 may be ascribed to non-carbonate alkalinity including organic acids and inorganic anions delivered by wastewater effluents or sporadic rainfalls. The four large rivers exhibited a positive correlation between pH and DOC in tributaries during the monsoon season, indicating that DOC flushed from soils may be diluted by monsoonal floods to such a degree as to exert little influence on pH and hence pCO2. Therefore, the monsoonal overestimation of pCO2 at sites with low pH and alkalinity warrants further investigation of other factors than non-carbonate alkalinity to explain the increased sensitivity of pCO2 to subtle changes in acidity and buffering. These results illustrate the importance of direct measurements of pCO2 in highly polluted rivers, especially during the monsoon season. For river systems lacking pCO2 measurements, we suggest that carbonate equilibria-based models be complemented with corrective measures: 1) presenting pCO2 values calculated from low pH values (pH < 6.5 for monsoon and pH < 6.3 for dry season) together with the pH range to warn potential overestimation; 2) using pre-established regressions between measured pCO2 and environmental variables to correct pCO2 values, particularly during wet periods when large changes in pH and acid buffering are expected.

A database system for querying of river networks: facilitating monitoring and prediction applications

The increasing availability of real-time in situ measurements and remote sensing observations have the potential to contribute to the optimization of water resources management. Global challenges such as climate change, intensive agriculture and urbanization put a high pressure on our water resources. Due to recent innovations in measuring both water quantity and quality, river systems can now be monitored in real time at an unprecedented spatial and temporal scale. To interpret the sensor measurements and remote sensing observations additional data for example on: the location of the measurement, upstream and downstream catchment characteristics, … are required. In this paper, we present a data management system to support flow-path related functionality for decision making and prediction modelling. Adding meta data sets and facilitating (near) real-time processing of sensor data questions are key concepts for the systems. The potential of the database framework for hydrological applications is demonstrated using different applications for the river system of Flanders. In one, the database framework is used to simulate the daily discharge for each segment within a catchment using a simple data-driven approach. The presented system is useful for numerous applications including pollution tracking, alerting and inter-sensor validation in river systems, or related networks.

Evolution of eastern Tibetan river systems is driven by the indentation of India

The main rivers that originate from the Tibetan Plateau are important as a resource and for the sedimentary and biogeochemical exchange between mountains and oceans. However, the dominant mechanism for the evolution of eastern Tibetan river systems remains ambiguous. Here we conduct geomorphological analyses of river systems and assess catchment-average erosion rates in the eastern Tibetan Plateau using a digital elevation model and cosmogenic radionuclide data. We find that major dividing ranges have northeast oriented asymmetric geometries and that erosion rates reduce in the same direction. This coincides with the northeastward indentation of India and we suggest this indicates a primarily tectonic influence on the large-scale configuration of eastern Tibetan river systems. In contrast, low-level streams appear to be controlled by fluvial self-organization processes. We propose that this distinction between high- and low-order channel evolution highlights the importance of local optimization of optimal channel network models in tectonically active areas.