River Channel Sediments

GeoPatterns

The paper presents the results obtained from a subsurface sediment grain size analysis along the upper, gravel-bed, Prahova River, Romania. Samples were taken from 20 locations over a 56 km study reach and were subsequently sieved and weighted in the laboratory. Both d 50 and d 84 characteristic diameters were plotted against distance and an overall downstream coarsening was identified. Considering the natural and anthropic context of the studied reach, hypotheses to explain this trend were launched.

Chemie der Erde, 2019

The paper presents the results of granulometric and geochemical analyses of the superficial bed sediments of the St. George branch, one of three Danube River distributaries. Three cutoff meander reaches (the Mahmudia, Dunavăţ de Sus (Upper Dunavăţ) and Dunavăţ de Jos (Lower Dunavăţ) meanders, noted here as M, DS and DJ meanders) was selected as an example to analyse the impact of the meanders rectification on the grain size and sediment quality. The cutoffs of the meanders by navigational canals since 1984-1988 caused dramatic changes in the distribution of velocities, discharge and sediment fluxes (Popa, 1997; Panin, 2003, Tiron and Provansal, 2010, Tiron Dutu et al., 2014). The correction of the studied meanders induced different hydro-sedimentary impacts: strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander in two cases (Mahmudia and Dunavăţ de Jos meanders) and slight modifications in Dunavăţ de Sus meander, where the natural channel still keeps almost its initial conveyance capacity. Sediment samples were acquired throughout several cross-sections of both natural and artificial canals of the three rectified meanders, in order to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, Skewness) show the predominance of fine fraction (silt and medium/ fine sand), relatively good sorted depending of the sample location (on the natural course of the branch, on the rectification canals or the former meanders. The statistical analysis of chemical data evidenced a high compositional variability, with concentration variation coefficients ranging between 27.4%-Sr and 178.3%-TOC (Total Organic Carbon). Generally, the highest concentrations of terrigenic compounds appear in the samples collected from the rectified natural canals. These results are validated by comparison with aDcp measurements made on each investigated profile.

38th IAHR World Congress - "Water: Connecting the World", 2019

An increasing discrepancy between surplus and lack of sediments can be observed in many river basins. This leads to an increase of flood risks and a reduction of navigation possibilities, hydropower production and negatively impacts biodiversity. Thus, a main task is to improve sediment management as well as the river morphology. To close existing knowledge gaps quantitative sediment data have been collected all along the Danube River. A Sediment Balance was prepared, which explains the problems that arise with sediment discontinuity. The Danube River basin shows, in particular due to the disbalance of the sediment regime a heavily disturbed system. The input of suspended sediments to the Black Sea has e.g. been reduced from around 60 million tons per year to about 20 million tons per year, leading to increased coastal erosion. Moreover, long sections of the Danube River have been narrowed, channelized and disconnected from floodplains as well as morphologically degraded over the last 200 years. This has caused increased bottom shear stresses, and subsequently increases sediment transport capacities. As a consequence of both, longitudinal and lateral disturbances of the sediment supply and additional impacts of the channelization, the remaining free-flowing sections are subject to various forms of river bed degradation. Such degradation or river bed incision leads to a loss of instream structures in general, with a disappearance of gravel bars at the Upper Danube, and changes of sand bars at the Lower Danube. Possible answers to these problems are provided by a catalogue of measures.

Carpathian Journal of Earth and Environmental Sciences, 2024

The Danube Delta was studied following the Geo-Eco-Mar expeditions in the summer and autumn of 2019 and 2020, respectively, from a sedimentological, faunal and environmental quality point of view. The grain size analyses of the sediments from the bed of the Chilia, Tulcea, Sulina, St. George Branches, as well as the meanders of the Dunărea Veche and Tătaru identified two types of sediments: sediments in which the sand fraction predominates, consisting, in general, of fine sand, and the fractions silt and clay have, in most cases, very low cumulative percentages, often below 1%; clay-silty sediments (muds), consisting predominantly of silt and clay, present in areas where the water velocity has low values. The presence of gravel is also noted, but subordinate to the sandy fractions. Gravel elements are an indicator of human impact in the area. In general, sediments also contain organogenic material, consisting of shells, fragments and plant remains. The interpretation of the data resulting from the granulometric analyzes (the percentages of clay, silt, sand, gravel and textural parameters) highlighted the dynamic conditions in which these sediments specific to the deltaic environment were deposited. In general, the amount of sand varies from one season to another and from one year to another, especially near the banks, with an increase in the sand content in the autumn season. Sandy sediments illustrate a faster flow regime, are well sorted, and are generally present in the central part of the Danube Branches bed.

Rivers are important resources that play the role of life sustenance in nature by providing environmental, cultural and economic benefits including municipal water use, irrigation, hydropower, navigation, fishing and recreation. Since they are the corridors connecting terrestrial environment to the ocean realm, they always transport and accumulate sediments. Knowledge and understanding of sediment characteristics, channel processes, process of sediment source and transport in river channels is vital in modeling and managing rivers in terms of how they both transport and impact engineering structures erected on rivers. Although, ongoing research is beginning to fill in some of these gaps through the use of laboratory experiments and mathematical models, this has not been matched by sufficient progress in measuring and quantifying the bifurcation process in natural river channels, very often because natural rivers are far harder to study and the technology required still remains unavailable. When the rate of sediment influx is high along river channels, reservoirs in dams are filled up with sediment which might need dredging to remove the sediments. Also turbine blades of power plants are considerably disturbed as a result of sediment particles deposited on the blades; flow in culverts and around bridge piers are exposed to erosion of the bed thereby exposing the foundation. Problems arising from sediment transport and deposition can be mitigated by selecting suitable cross sections for the measurement of sediment flow rate parameters and removal of the sediment which would lead to the reduction of negative impacts on the river structures.

This section is dedicated to the contribution of geomorphic processes in dislocation, setting in motion and removal of rock particles from the drainage system in Romania. The total volume of eroded material amounts to ca. 126 Mt/year for the entire territory of Romania. Sediments are dislodged by means of various processes including sheet erosion or rill and gully erosion, as well as mass movements (landslides, collapse, mudflows, creep, etc.). The arable lands yield approximately 80 % of the total soil loss in Romania, slightly above the European mean value of 70 %. The contribution of slope processes (sheet and rill erosion, gully erosion, landslides) was estimated at about 67 % and of fluvial processes at about 33 %. The ratio of the two major domains (slopes and river beds) in terms of sediment source contribution was 90:10 according to estimates from the 1980s. In our days, our own evaluations indicate a more balanced ratio, i.e., 67:33. We believe the tendency of adjustment of the relationship between the two sediment source domains is factual and strongly dependent on the changes of controls during the past three decades. The VII and VIII order drainage network (Strahler's system) from Romania discharges into the Danube sediments which may account for less than 1 % of the amount of sediment yielded by the hillslopes and river beds from the investigated area. At Ceatal (before the Danube enters the deltaic sector), the river transfers on average an annual volume of 20.5 Mt of sediments (from 1985 to 2006). Between 1840 and 1970 the mean amount of alluvium carried by the Danube was 57.3 Mt/year. The difference is accounted for by the sediment storage in the river basin which represents over 37.5 %. The largest sediment storage occurs in reservoirs, and the value determined by us is very similar to the global value (36.6 %).

Hydrological Processes, 2011

As with most Italian rivers, the Reno River has a long history of human modification, related also to morphological changes of the lower Po River since Roman times, but in the last decades, significant land use changes in the headwaters, dam construction, torrent control works and extensive bed material mining have caused important channel morphology and sediment budget changes. In this paper, two main types of channel adjustment, riverbed incision and channel narrowing, are analysed. Riverbed degradation is discussed by comparing four different longitudinal profiles surveyed in 1928, 1951, 1970 and 1998 in the 120 km long reach upstream of the outlet. The analysis of channel narrowing is carried out by comparing a number of cross-sections surveyed in different years across the same downstream reach. Field sediment transport measurements of seven major floods that occurred between 2003 and 2006 are compared with the bedload transport rates predicted by the most renowned equations. The current low bedload yield is discussed in terms of sediment supply limited conditions due to land use changes, erosion-control works and extensive and out of control bed material mining that have affected the Reno during the last decades.

Journal of Geographical Sciences

The accumulation of sediment in river channels is a phenomenon that is not only influenced by the channel morphology, but also by the physical and geographical characteristics and the endogenous and exogenous processes taking place in the catchment. This paper presents an analysis of the impact the changes in lithological conditions have on the morphological and morphometric parameters of the Udava River channel and their relation to the channel accumulation forms representative of the river's longitudinal profile as well as of its planform. Results document when accumulation forms occur and what is their spatial distribution within the longitudinal and cross-sectional river profiles. More resistant structures created sections with a lower degree of sedimentation, while in depression segments the degree of sedimentation was higher. With the increase in longitudinal slope, the impact of channel width on the average channel bar size increased. Also a difference in the accumulation was observed between the left and right bank which could be possibly explained by the impact of Coriolis force.

2014

The river bed of the Rhine River is subject to severe erosion and sedimentation. Such high geomorphological process rates are unwanted for economical, ecological, and safety reasons. The objectives of this study were (1) to quantify the geomorphological development of the Rhine River between 1985 and 2006; (2) to investigate the bed erosion process; and (3) to distinguish between tectonic, hydrological, and human controls. We used a unique data set with thousands of bedload and suspended-load measurements and quantified the fluxes of gravel, sand, silt, and clay through the northern Upper Rhine Graben and the Rhenish Massif. Furthermore, we calculated bed level changes and evaluated the sediment budget of the channel. Sediment transport rates were found to change in the downstream direction: silt and clay loads increase because of tributary supply; sand loads increase because of erosion of sand from the bed; and gravel loads decrease because of reduced sediment mobility caused by the base-level control exerted by the uplifting Rhenish Massif. This base-level control shows tectonic setting, in addition to hydrology and human interventions, to represent a major control on morphodynamics in the Rhine. The Rhine bed appears to be in a state of disequilibrium, with an average net bed degradation of 3 mm/a. Sand being eroded from the bed is primarily washed away in suspension, indicating a rapid supply of sand to the Rhine delta. The degradation is the result of an increased sediment transport capacity caused by nineteenth and twentieth century's river training works. In order to reduce degradation, huge amounts of sediment are fed into the river by river managers. Bed degradation and artificial sediment feeding represent the major sources of sand and gravel to the study area; only small amounts of sediment are supplied naturally from upstream or by tributaries. Sediment sinks include dredging, abrasion, and the sediment output to the downstream area. Large uncertainties exist about the amounts of sediment deposited on floodplains and in groyne fields. Compared to the natural situation during the middle Holocene, the present-day gravel and sand loads seem to be lower, whereas the silt and clay loads seem to be higher. This is probably caused by the present-day absence of meander migration, the deforestation, and the reduced sediment trapping efficiency of the floodplains. Even under natural conditions no equilibrium bed level existed. Geomorphology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / g e o m o r p h causes problems: navigational problems during low flow periods and safety problems during high flow periods (higher water levels).

Earth Surface Processes and Landforms, 2007

This paper deals with the Brenta River (the Alps, Italy), a gravel-bed river strongly affected by human impact. The aims of this work are (a) to reconstruct the channel adjustments that have occurred during the last decades and (b) to analyse the present condition of the river in terms of bedload transport and sediment sources. The study reach, which is 23 km long, was investigated using historical maps, aerial photographs, 12 monumented cross-sections, laser altimetry data and field survey. Bedload transport rates were estimated by a morphological approach, which allows the construction of a sediment budget.Dramatic channel adjustments have occurred in response to an alteration of sediment fluxes caused by different human interventions (gravel mining, dams, torrent-control works). Major channel adjustments, specifically incision (up to 8–9 m) and narrowing, took place between the 1950s and the 1980s. On the other hand, widening has become the most important process, and incision has continued only in the upper part of the study reach in more recent times (about the last 20–25 years).The estimate of bedload transport rates for the period 1984–1997 was carried out starting from the erosion and deposition volumes of the single sub-reaches. Such an estimate has shown that (a) a high spatial variability of gravel transport rates exists (from 0 up to 73 200–85 400 m3/year) and (b) most of the bedload is supplied by local sources. Bank erosion is the main source of sediments for bedload transport and its contribution can be estimated as from 9 up to 20 times higher than the upstream contribution. Finally, from a practical point of view, results from this study should be taken into account in addressing maintenance, sediment management and restoration along a dynamic and unstable river such as the Brenta. Copyright © 2007 John Wiley & Sons, Ltd.