Sediment load determines the shape of rivers

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker [G. Parker, J. Fluid Mech. 89, 127–146 (1978)], that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid–sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge—a quantity notoriously difficult to measure in the field.

The Effect of Changes in Land Cover on Total Sediment Yield in Peusangan Watershed, Aceh Province

The watershed is an area above or higher than a river whose topographical boundaries cause water to flow into the same river. The river flow carries sediment particles that potentially cause silting of the estuary area. The sediment carried by the river flows from the erosion process that occurs in the watershed. Changes in land cover potentially affect the rate of sediment export to rivers due to changes in surface roughness and water infiltration rate to the ground. This study aims to identify the effect of the land cover change on the total sediment yield from the Peusangan Watershed, Aceh Province. The sediment yield is calculated from the erosion rate and the sediment delivery ratio. The erosion rate is modeled using the revised universal soil loss equation, while the sediment delivery ratio is calculated based on the function of the watershed area. From the results of the calculation, in general, the rate of erosion is at a very level where the average erosion rate in 1995 is 26,715 tons/ha/year, in 2005 it is 26,886 tons/ha/year, in 2015 it is 24,959 tons/ha/year and in 2018 amounted to 26,771 tons/ha/year. With a sediment delivery ratio value of 0.180, The total sediment yield was 1,083,148.20 tons in 1995, 1,090,047.94 tons in 2005, 1,011,920.71 tons in 2015, and 1,085,398.35 tons in 2018. The identification results show that the changes in land cover affect the total sediment yield that comes out of the watershed.

Hitchhiking Halophytes in Wrack and Sediment-Laden Ice Blocks Contribute To Tidal Marsh Development in The Upper Bay of Fundy.

Salt marshes are a type of coastal wetland that are affected by dynamic coastal processes. Ice blocks and wrack (mats of plant debris) regularly float onto northern marshes and become stranded, affecting vegetation and soil accretion. There is little research regarding the capacity of ice and wrack to transport viable plant propagules onto marshes where they can colonize, which may be particularly important at barren new salt marsh restoration sites. Contributions of sediment by ice may also be important at restoration sites to raise the marsh platform to elevations appropriate for plant colonization. We collected ice (n = 27) and wrack (n = 18) samples at marshes in the Bay of Fundy, ran germination trials with the contents, and measured the quantity of sediment in the ice. We found viable propagules from halophytic and non-halophytic species in wrack, and viable propagules of Sporobolus pumilus in ice. Additionally, we found sediment densities between 0.01 and 4.75 g·cm−3 in ice blocks that translated to 26.61 – 21,483.59 kg of total sediment per block, representing a large source of sediment. We found that the number of germinating propagules could not be predicted by wrack size, and that pH, sediment density, sediment weight in ice blocks were variable across the marsh surface, while ice salinity was negatively correlated with elevation and distance from creek. Our results indicate that ice and wrack represent a potential source for vegetation colonization at salt marsh sites and highlights their contributions to facilitating vegetation colonization through building marsh soils.

Modelling differential geomorphic effectiveness in neighbouring upland catchments: implications for sediment and flood risk management in a wetter world

In July 2007 an intense summer storm resulted in significant activation of the sediment system in the Thinhope Burn, UK. Catchment- and reach-scale morphodynamic modelling is used to investigate the geomorphic work undertaken by Thinhope Burn; comparing this with the more subdued responses shown by its neighbours. Total sediment efflux for Thinhope Burn over the 10 yr period 1998-2007 was 18, 801 m3 four times that of the larger Knar Burn catchment and fifty-four times that of the smaller Glendue Burn catchment. For a discharge of 60 m3s-1, equivalent to the July 2007 Thinhope flood, sediment efflux was 575 m3, 76 m3, and 67 m3 for Thinhope, Glendue and Knar Burns respectively. It is clear that Thinhope Burn undertook significantly more geomorphic work compared to its neighbours. Analysis of the population of shear stress for reach-scale simulations on Thinhope Burn highlighted that the final three simulations (flood peaks of 60, 90, 236 m3s-1) all produced very similar distributions, with no marked increase in the modal shear stress (∼250 Nm-2). This possibly suggests that flows >60 m3s-1 are not able to exert significantly greater energy on the channel boundary, indicating that flows in the region of 60 m3s-1 attain ‘peak’ geomorphic work. It is argued that factors such as strength resistance of the key sediment sources (e.g. paleoberms perched on terraces), structural resistance to flood waves imposed by valley form resistance, location sensitivity and transmission resistance, may all offer explanations for increased geomorphic effectiveness compared with its neighbours. With the expectation of greater rainfall totals in the winter and more extreme summer events in upland areas of the UK, it is clear that attention needs to focus upon the implications of this upon the morphological stability of these areas not least to aid future sustainable flood risk management.

Research of the Influence of Marine Residual Fuel Composition on Sedimentation Due to Incompatibility

The article shows studies of the problem of active sediment formation during mixing of residual fuels, caused by the manifestation of incompatibility. To preserve the quality and reduce sediment formation during transshipment, storage, and transportation of marine residual fuels, a laboratory method for determining the compatibility and stability of fuels has been developed, which makes it possible to determine the quantitative characteristics of the sediment formation activity. According to the method developed, laboratory studies have been carried out to determine incompatible fuel components and the influence of composition on the sedimentation process. Tests were carried out to determine the quality indicators and the individual group composition of the fuel samples. Based on the results of the studies, the dependences of the influence of normal structure paraffins in the range from 55 to 70 wt. % and asphaltenes in the range from 0.5 to 3.5 wt. % in the fuel composition on the sedimentation activity due to incompatibility were obtained. To obtain a convenient tool that is applicable in practice, a nomogram has been developed on the basis of the dependences obtained experimentally. It was also determined that, after reaching the maximum values of sediment formation with a further increase in the content of n-paraffins, saturation is observed, and the value of the sediment content remains at the same level. Maximum total sediment values have been found to depend on asphaltene content and do not significantly exceed them within 10%. The results of the research presented in this article allow laboratory and calculation to determine the possibility of incompatibility and to preserve the quality of marine residual fuels.

Effects of Dam Construction in the Wang River on Sediment Regimes in the Chao Phraya River Basin

The Wang River is one of the major tributaries of the Chao Phraya River (CPR) system in Thailand as the key riverine sediment source supplying the Chao Phraya Delta that has experienced severe shoreline retreat in the past six decades. Historical and observed river flow and sediment data measured during 1929–2019 were used to assess the variation in total sediment load along the Wang River and evaluate the effects of three major dam constructions on sediment supplied from the Wang River to the CPR. Results indicated that sediment loads increased toward downstream. Variation in long-term total sediment load (TSL) along the river suggested that construction of the Kiew Lom Dam in 1972 did not cause a reduction in sediment yield in the Wang River Basin because it impounded less than 20% of the average annual runoff, while the Mae Chang and Kiew Koh Ma Dams caused downstream sediment reduction. These three dams are located in the upper and middle river basins, and their effects on sediment load in the Wang River are ameliorated by additional sediment supplied from the lower basin. Results confirmed that construction of these three major dams in the Wang River did not greatly impact sediment supply from the Wang River to the CPR system. The dam site and sediment load variation along the river are the primary factors controlling the impact of the dam construction.

Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process

Grass strips can decrease erosion, trap sediment in silt-laden water flowing downhill, and control nonpoint source pollution. Determining the effects of different parts of grass strips on silt-laden overland flow will improve our understanding of sediment trapping by grass strips with different structures. Sediment trapping by grass strips was studied using a 5° slope, 30 L min−1 m−1 flow rate, 120 g L−1 sediment concentration, and different aboveground components of grass strips (complete grass, removed green grass, and removed green and withered grass). The whole overland flow process was monitored. Meanwhile, the runoff sediment samples at the outlet were collected and measured. Sediment trapping by aboveground grass parts was quantified at different stages. Of the soil bed surface, green grass, and withered grass, the soil bed surface dominated sediment trapping in the initial stage of the sediment-trapping process, contributing about 90% of total sediment deposition in the first 5 min. As the sediment-trapping process continued, the effect of the soil bed surface weakened, and the green grass played a major role at the later stage of sediment trapping. The ratio of the soil bed surface, green grass, and withered grass contributions to total sediment deposition at the stable stage of the experiments was approximately 3:5:2. The results will help assess the effects of vegetation restoration on sediment transport in entire watersheds.

Sediment Budget and Sediment Trap efficiency of Baglihar Hydroelectric project Reservoir – a calibrated model for prediction of longevity of the Dam

The field investigation of the reservoir area of Baglihar Hydropower project shows that the sediment budget to the reservoir is controlled by fragile rock type like shales, sandstones, phyllites and slates, soil characteristics, steep hill slopes, rainfall and landslides. The rocks are highly weathered, fissile and micaceous in nature and very sensitive to water absorption. The analysed sediments are characterised by dominance of sands, silts and clays with lower values of plasticity (14.3PL), liquidity (23.5 LL), cohesion (118) and shear strength (202 Kpa). The slope wash deposits are highly susceptible to landslides and slope failures and directly contribute to the sediment budget in the reservoir. In addition tributaries of Chenab River also bring sediments in the reservoir from the catchment area. The empirical relationship for estimating the long-term reservoir trap efficiency for large storage based on correlation between the relative reservoir size and trap efficiency was simulated in 3D model which shows that the annual sediment trap efficiency of the Baglihar reservoir is of 0.39%. The extrapolation of the calculated values shows that the total sediment load shall increase by 11% in the next 30 years and 20% in the next 50 years and correspondingly 40% in the next 100 years that shall induce corresponding decrease in the reservoir volume over the time. By applying flushing schemes, life span of the reservoir can be extended. It is estimated that after 100 years the reservoir shall lose ~35.6% storage volume. On further extrapolation, the trap efficiency will decrease from 25.5% after 30 years to 23% after 100 years. The estimated trap efficiency of Baglihar reservoir is 60%, which is greater than that based on numerical results, showing a significant overestimation.