Low Discharge Regime Research Articles

Variability of Cohesive Particle Characteristics in an Energetic Estuary: Flocs vs. Aggregates

Floc size, density, and settling velocity were investigated in the Connecticut River estuary over 3 years spanning varying fluvial discharge regimes to determine the role of cohesive suspended particle characteristics in the sediment-transport patterns of an energetic estuary. Concurrent measurements of flow, bed stress, salinity, and suspended sediment concentration were used to identify primary controls on floc size variability. Water discharge ranged from 202 to 910 m3/s between the three sampling campaigns, and the timing of major sediment-discharge events preceding measurement periods from 23 to 162 days. Two distinct particle populations were observed under high and low sediment-discharge regimes. With abundant fluvial sediment input, flocculation occurred resulting in large, loosely packed flocs dominating the suspended signal (median sizes of 194–209 μm; median excess densities of 13–17 kg/m3). Following an extended period of low sediment discharge, small, dense aggregates resuspended from the bed were observed throughout the water column (median size of 171 μm and excess density of 60 kg/m3). The timing and partial decoupling of water and sediment discharge led to inter-annual patterns of particle packaging controlled by fresh sediment supply. When the estuary is “charged” with sediment following high discharge events, the characteristic large, less dense flocs with lower settling velocities primarily bypass the estuary. The similar disaggregated grain size distribution of the suspended material of the two regimes suggests the same source sediment is reintroduced to the estuary with the intrusion of the salt wedge, which extends farther up-estuary during low-discharge regimes. The fines repackaged as dense aggregates ultimately supply the channel margins and off-channel coves currently experiencing sediment accumulation.

Saltwater intrusion in a subtropical estuary

River discharge, precipitation, wind, water level and salinity measurements were used to investigate saltwater intrusion in a subtropical estuary. The upstream limit of saltwater intrusion was defined by the position of the 2 g/kg isohaline (X2) as it changed each tidal cycle. Observations were obtained from November 2014 to November 2015 along the Suwannee River, at Florida's Big Bend. Observations provided information to construct a first-order autoregressive Markov-chain model and predict X2. The model predicts X2 from its position in the previous tidal cycle, from river discharge, from water level, from precipitation and from wind velocity. In order to capture the seasonality of the Suwannee River discharge, the analysis was done separately for high and low river discharge regimes. The high discharge regime for the particular year of observations coincided with the cold season and had a mean value of 360 m3/s. The low discharge regime overlapped with the warm season and had a mean of 198 m3/s. As expected, saltwater intrusion was inversely related to river discharge. Values of X2 were 0.4 km for a discharge of 377 m3/s, and 6 km for a discharge of 195 m3/s. The autoregressive model was able to reproduce the observed seasonality of saltwater intrusion in the Suwannee River. The model was then used to project values of X2 for different scenarios of climate change as represented by possible combinations of river discharge and water levels.

A comparative analysis on the effects of river discharge on trophic interactions in two tropical streams

Discharge-mediated seasonal patterns of food web interactions were investigated in two streams in Sri Lanka; Eswathu Oya (a perennial wet-zone stream) and Yan Oya (a seasonal dry-zone stream). Based on volumetric proportions of diet composition, relative abundance of fish species and their daily food rations, the mean cumulative consumption of each prey taxon was estimated for each fish population. Food web diagrams were prepared using trophic index of fish, trophic class of prey and feeding interactions between fish and prey. Both streams showed seasonal patterns of discharge due to rainfall, but no significant effect was evident in the trophic index of most fish species. In both streams, cumulative consumption of prey taxa was highest during low discharge regime due to increased abundance of both prey taxa and consumers. In Eswathu Oya, diversity of prey taxa was higher during the low discharge regime, but in Yan Oya, high diversity occurred during the high discharge regime. Herbivorous and/or detritivorous fish species were rare in Eswathu Oya but dominant in Yan Oya. Complex food web structure in Yan Oya due to high fish species richness and high diversity of prey categories made it less sensitive to discharge extremes in contrast to relatively simple food web structure in Eswathu Oya. This study, therefore, highlights the importance of maintaining the quality of riparian environments for conservation of biodiversity.

Surface flows from images: ten days of observations from the Tiber River gauge-cam station

Fully remote surface flow measurements are crucial for flow monitoring during floods and in difficult-to-access areas. Recently, optics-based surface flow monitoring has been enabled through a permanent gauge-cam station on the Tiber River, Rome, Italy. Therein, a system of lasers and an internet protocol camera equipped with two optical modules afford video acquisitions of the river surface every 10 minutes. In this work, we establish a standard video-processing protocol by analyzing more than 10 Gb of footage data captured during low discharge regime from May 2nd to 11th, 2015, through particle tracking velocimetry (PTV). We show that good image-based velocity data can be obtained throughout the day – from 6 am to 8 pm – despite the challenging experimental settings (direct sunlight illumination, mirror-like river surface, and overlying bridge shadow). Further, we demonstrate that images captured with a 27° angle of view optical sensor lead to average velocity measurements in agreement with available radar data. Consistent with similar optical methods, PTV is not applicable in case of adverse illumination and at night; however, it is more robust for dishomogeneous distributions of floaters in the field of view.

Impact of river discharge on phytoplankton bloom dynamics in eutrophic estuaries: A model study

Field observations in estuaries reveal that phytoplankton blooms are strongly affected by advection processes related to river flow. To gain quantitative insight into this dependence, experiments were performed with a new idealised model that couples physical and biological processes. Advection of phytoplankton and nutrient by subtidal flow was explicitly accounted for, as well as longitudinal and vertical mixing processes. Results show that the idealised model is capable of reproducing the observed bloom. The specific spatial distribution of phytoplankton population emerges because the latter is suppressed in the upper reach by the advection processes, and the growth is limited in the lower reach by low nutrient concentrations. A sensitivity study of model results to different river discharges reveals the presence of three regimes. In the low discharge regime, blooms form because growth is faster than decay due to advection processes. In the high discharge regime, the situation is opposite and no blooms form. If time scales of growth and advection are comparable (in moderate discharge regime), phytoplankton population increases significantly slower compared to the low discharge regime. Results of additional model runs, in which water depth and the e-folding length scale of estuarine width convergence were varied, revealed that the three regimes occur in all these cases.

Habitat dynamics in the bed sediments of an intermittent upland stream

The effects of the hydrological regime on temporal changes to physical characteristics of substratum habitat, sediment texture of surface sediments (<10 cm), were investigated in a sub-tropical headwater stream over four years. Surface discharge was measured together with vertical hydraulic gradient and groundwater depth in order to explore features of sediment habitat that extend beyond the streambed surface. Whilst the typical discharge pattern was one of intermittent base flows and infrequent flow events associated with monsoonal rain patterns, the study period also encompassed a drought and a one-in-a-hundred-year flood. Rainfall and discharge did not necessarily reflect the actual conditions in the stream. Although surface waters were persistent long after discharge ceased, the streambed was completely dry on several occasions. Shallow groundwater was present at variable depths throughout the study period, being absent only at the height of the drought. The streambed sediments were mainly gravels, sand and clay. Finer sediment fractions showed a marked change in grain size over time, although bedload movement was limited to a single high discharge event. In response to a low discharge regimen (drought), sediments characteristically showed non-normal distributions and were dominated by finer materials. A high-energy discharge event produced a coarsening of sands and a diminished clay fraction in the streambed. Particulate organic matter from sediments showed trends of build-up and decline with the high and low discharge regimes, respectively. Within the surface sediment intersticies three potential categories of invertebrate habitat were recognised, each with dynamic spatial and temporal boundaries.

SnO 2 negative electrode for lithium ion cell: in situ Mössbauer investigation of chemical changes upon discharge

In situ 119Sn Mössbauer study of an SnO 2 electrode was performed during discharge of a lithium ion cell. The first step is lithium intercalation into the SnO 2 host structure. This lithium intercalation results in reinforcement of the SnO 2 lattice instead of direct decomposition of the oxide upon reduction. This first step is followed by the reduction of tin dioxide into unusual tin species (possibly “ exotic” forms of Sn(II) or Sn(0)). The last step of the discharge consists in Li–Sn alloy formation. However, non-reduced SnO 2 is present nearly up to the end of the discharge despite a very low discharge regime. It seems highly probable that this fact is related both to slow Li diffusion and disconnection of SnO 2 particles due to Li 2O formation. The working electrode appears to be rather far from equilibrium during continuous discharge, which means that ideal succession of well-defined stages cannot describe the real phenomena involved in the operating battery.

Modélisation sur vingt ans (1976–1995) de la production phytoplanctonique en baie de Seine

This paper presents an ecological modelling of the Bay of Seine (Eastern Channel) over the last twenty years, chosen as a typical case of eutrophication in a river plume. In the physical sub-model, the Bay is divided into 42 boxes and water fluxes between them are calculated automatically using Ifremer's “Elise” software. A two-layer, vertical thermohaline model is then linked with the horizontal circulation scheme in order to take vertical stratification into account. The biological submodel deals with two chemical elements, nitrogen and silicon, and splits phytoplankton into diatoms and flagellates. Results from this ecological model point out the spatial concordance of highest phytoplanktonic concentrations with the river plume spreading in the bay. Contrary to diatoms, flagellate production appears to be mainly confined to the eastern bay, due to the vertical haline stratification in front of the river mouth. As far as the whole bay is concerned, the interannual fluctuations of diatom production are related to the level of spring insolation, whereas silicon inputs regulate diatom production in the river plume. The flagellate summer production in the plume is enhanced by high water temperature and high N/Si ratios, which appear during dry years with low discharge regimes. Finally, interannual increase of flagellate production could be related to gradual increase of nitrogen loadings, contrasting with silicon loadings, which remained stable for twenty years.

Trace metal distribution, partition and fluxes in the Seine estuary (France) in low discharge regime

Samples collected in surface waters of the Seine estuary (France) during a low discharge period were analyzed for dissolved and particulate trace metals (Cd, Co, Cu, Mn, Ni, Pb and Zn). The studies showed that these trace elements are reactive in the estuary: an addition to the dissolved phase is observed for Cd, Co, Cu, Mn, Ni and Zn and a removal for Pb. Cd, Cu, Ni and Zn are released from seaward fluxing riverine particles or estuarine sediments directly in the water column, while the dissolved Co-salinity distribution, similar to Mn distribution suggests a preliminary release in the sediment pore water. Rapid adsorption onto the suspended load plays a major role in Pb removal from the dissolved phase. Partition coefficients for the dissolved and particulate phase ( K D) decrease in the order: Pb > Co > Cu, Zn > Ni > Cd, i.e. a sequence similar to those reported from others European areas. Gross riverine inputs to the estuary and net efflux of dissolved trace metals to the English Channel are estimated. Values show that the Seine river and its estuary constitute a significant source of trace metals for the North Sea.