Bed-material entrainment in a mountain river affected by hydropeaking.

Hydropeaking, by artificially generated flow peaks, influences hydro-sedimentary dynamics on rivers and, consequently, affects bed material entrainment and transport. This study examines the onset of motion of sediment particles in four sections of a Pyrenean gravel-to-cobble bed river exposed to frequent hydropeaking (once per day, on average). Five criteria of particle entrainment have been used to assess the prediction of the initiation of grain motion at-a-section scale. Theoretical entrainment conditions were validated using real observations of mobility by means of tracers. It was found that the maximum flow discharged by the hydropower plant mostly affects the furthest downstream section, located almost 17 km downstream, in which the finer fractions of the bed are entrained. The mobile grain sizes include up to coarse gravels (≈ 30 mm). Differences in sediment supply (imposed by tributaries), the value of the bed slope and the structure of the coarse surface layer decisively control the downstream variability of incipient particle motion between sections. Results from a 17 km study segment indicated that hydropeaking generate partial transport, that is, a partially size-selective transport that occurs downstream from the hydropower plant and winnows the sand and small gravel further downstream, increasing armouring and depleting fine sediments.

A review of the impacts of dams on the hydromorphology of tropical rivers.

A major programme of dam building is underway in many of the world's tropical countries. This raises the question of whether existing research is sufficient to fully understand the impacts of dams on tropical river systems. This paper provides a systematic review of what is known about the impacts of dams on river flows, sediment dynamics and geomorphic processes in tropical rivers. The review was conducted using the SCOPUS® and Web of Science® databases, with papers analysed to look for temporal and geographic patterns in published work, assess the approaches used to help understand dam impacts, and assess the nature and magnitude of impacts on the flow regimes and geomorphology ('hydromorphology') of tropical rivers. As part of the review, a meta-analysis was used to compare key impacts across different climate regions. Although research on tropical rivers remains scarce, existing work is sufficient to allow us to draw some very broad, general conclusions about the nature of hydromorphic change: tropical dams have resulted in reductions in flow variability, lower flood peaks, reductions in sediment supply and loads, and complex geomorphic adjustments that include both channel incision and aggradation at different times and downstream distances. At this general level, impacts are consistent with those observed in other climate regions. However, studies are too few and variable in their focus to determine whether some of the more specific aspects of change observed in tropical rivers (e.g. time to reach a new, adjusted state, and downstream recovery distance) differ consistently from those in other regions. The review helps stress the need for research that incorporates before-after comparisons of flow and geomorphic conditions, and for the wider application of tools available now for assessing hydromorphic change. Very few studies have considered hydromorphic processes when designing flow operational policies for tropical dams.

Downstream changes of particle entrainment in a hydropeaked river.

Hydropeaking, through artificially generated flow peaks, affects hydro-sedimentary dynamics on rivers. The frequency and magnitude of such artificial flow pulses impact sedimentary process and, inevitably, affects bed-material entrainment. This study analyses the entrainment of particles in six sections of a Pyrenean river under frequent hydropeaking. Three equations of particle entrainment with contrasting behaviours, validated for particle mobility from tracer data, have been used to predict the initiation of motion in each section. Results show that the peak discharge generated by the hydropower station mostly affects the section immediately downstream from the hydropower plant, where the finer fractions of the bed are mobilised. The mobile grain sizes include fine to medium gravels (< 20mm). Channel geometry and higher slopes also have an effect on particle entrainment. Entrainment of the finer size fractions of the bed is termed partial transport, i.e. a partially size-selective transport that occurs downstream from the hydropower station and winnows the sand and small gravel further downstream.

Mapping water and sediment connectivity.

Connectivity has become a key issue in the study of processes acting in hydro-geomorphic systems and has strong implications on the understanding of their behaviour. Given the high complexity of hydro-geomorphic systems and the large variety of the processes controlling the efficiency of water and sediment transfer through a catchment, mapping hydrological and sediment connectivity is fundamental to understand the linkages between different parts of the system and the role played by system configuration, natural landforms and man-made structures in favouring or obstacolating the continuity of runoff and sediment pathways. Furthermore, the analysis of changes on connectivity through time can help to investigate the effect of both natural and anthropic disturbance on water and sediment fluxes and associated processes. This special issue aimed to shed light on the latest advances inmapping water and sediment connectivity by means of field measurements, modelling and geomorphometric approaches along with quantitative methods for the analysis of connectivity temporal evolution.The special issue is composed of twenty-one papers presenting a huge variety of topics dealing with hydrological and sediment connectivity and their changes through time in different geographical andclimatic regions of the world, at different spatial and temporal scales. This special issue highlights the importance of connectivity assessment to properly address sediment and water-related issues and to improve management strategies in hydro-geomorphic systems.

Effects of biofilm on river-bed scour.

Biofilm acts stabilising river-bed sediments, interfering with particle entrainment and, consequently, preventing bed disturbance. In this paper we present the results of a series of experiments carried out in indoor channels, aimed to understand biofilm alteration of bed material motion and topographic changes in stream channels. We analysed the erosion patterns and bedload rates in non-cohesive sediments in channels colonised by biofilms and compared them to biofilm-free others. All the channels had the same conditions of light irradiance, temperature, slope, and particle size (sand). Discharge and water surface slope were modified to create a range of hydraulic conditions, with pairs of colonised and non-colonised channels subjected to the same flows. We observed that biofilm slightly modified bed roughness and flow hydraulics, but that highly influenced bed disturbance. Biofilm caused bed scour to occur in patches unevenly distributed along the channel length, as a result of localised weaknesses of the biofilm. Once biofilm was ripped up it was transported in chunks, and sand grains were observed attached to these chunks. In non-colonised sediments the erosion was more homogeneous and the formation and movement of bedforms were observed. On average, bedload rates were 5 times lower when biofilm was present. Overall, the protective effect of the biofilm prevented generalised erosion of the channel and delayed the entrainment and transport of sand grains. Results emphasised the important role of biofilm in the incipient motion of bed-material in stream channels; this role may affect the magnitude and frequency of subsequent river bed processes, notably the onset of bedload and associated channel morpho-dynamics.

A review of sediment quantity issues: examples from the River Ebro and adjacent basins (Northeastern Spain).

Sediment flows naturally through the drainage network, from source areas to deposition zones. Sedimentary disequilibrium in rivers and coastlines is related to the imbalance within the fluvial system caused mostly by dams, instream mining, and changes in land use. This phenomenon is also responsible for ecological perturbations in rivers and streams. A broad need exists to establish comprehensive management strategies (soft measures) that would go beyond site-specific engineering practices (technical measures) typically taken to solve particular problems. Long-term programs are also required to monitor sediment transport in river basins, in order to assess the magnitude and variability of sediment transfer and potential deficits. This paper shows examples of rivers with important sediment disequilibrium in the Ebro and adjacent basins. These basins, like most in the Iberian Peninsula, experience sediment discontinuity in the catchment-river-coast system. Reservoir siltation is the main quantitative issue. Land use change and especially gravel mining downstream from dams accentuate the process. We also present and discuss recent developments on water and sediment management undertaken to improve the morphosedimentary dynamics of rivers.