29 research outputs found
Intra‐Seasonal Variability in Sediment Provenance and Transport Processes in the Brahmaputra Basin
No full textAbstractSediment composition in modern fluvial settings is commonly assessed regarding spatial but rarely temporal variability, potentially leading to a bias of unknown extent. Here, we present the grain‐size distribution, bulk chemical and mineralogical composition of a time‐series set of 36 suspended sediment samples from the Brahmaputra river, as well as clay and heavy mineral analysis of selected samples. Sampling covers the June–November 2021 period, which included two major flooding events. We show that the two flooding events are characterized by contrasting grain size, with the first event characterized by a grain‐size minimum and the second by a grain‐size maximum. Although grain sizes of the first flood and the period after the second are similar, their compositions differ significantly, highlighted by a factor‐two decrease of biotite largely compensated by an increase in quartz. By contrast, the content of garnet, clinopyroxene, sillimanite, and rutile increased compared to epidote and amphibole during the second flood event. By relating the results to spatio‐temporal rainfall and discharge patterns and basin morphology, we conclude that the first flooding primarily mobilized hydraulically pre‐sorted sediments from the exposed sandbars of the floodplains, while those sandbars are already submerged during the second flooding in a single‐channel system, resulting in higher sediment contributions from highland tributaries draining igneous and high‐grade metamorphic rocks. Such temporal variations pose constraints on the interpretation of compositional differences between individual samples regarding sediment provenance and dispersal and should be considered in studies of modern drainage basins as well as ancient sediment routing systems.Plain Language Summary: Sediment provenance, which refers to where the sediment in a river comes from, is important to understand because it can tell us about the geology of an area, various earth‐surface processes and how the landscape is changing over time. However, sediment provenance is typically studied at a spatial scale in present day river basins, and temporal variability is rarely considered. This study examines the physical, chemical and mineralogical properties of sediment in the Brahmaputra river during two major flooding events that occurred in the same season. The results show that the sediment composition varies between the events, indicating a change in the relative proportions of distinct sources. This emphasizes the importance of considering temporal variations in sediment composition when interpreting sediment provenance signals.Key Points:
Time‐series analysis of sediment composition during two major flooding events of a single monsoon season is presented
The two flooding events show contrasting grain‐size, chemical and mineralogical composition
Temporal variations in sediment composition pose constraints on the interpretation of provenance and dispersal based on individual samples
DAADGerman Ministry of Education and Researchhttps://doi.org/10.5281/zenodo.7588054http://flood.umd.edu
Analysis of Tristable Energy Harvesters Under Random Excitations
No full textThis manuscript analyzes the performance of a tristable vibration energy harvester under Gaussian white noise excitation. Broadband vibration energy harvesting has attracted significant research attention and is targeted toward obtaining large power output over a wide range of frequencies. Nonlinearity can be introduced into vibration energy harvesting systems through multi-stability. In cantilever-type vibration energy harvesters, multi-stability could be achieved by the introduction of magnetic interactions. When two external magnets are used, the harvester can have up to three stable static equilibrium positions. The harvester with two stable states has been explored widely, both theoretically and experimentally. Recently, the harvester with three stable states is shown to perform better than its bistable counterpart in the presence of a linearly increasing harmonic sweep excitation. Ambient vibrations are random in nature, and the performance of tristable energy harvesters under such excitations needs to be studied. To begin with, we study the performance of tristable energy harvesters under Gaussian white noise excitation through numerical simulations. The simulations show that beyond a certain critical amplitude of excitation, the harvesters undergo inter-well oscillations and harvest more power. This implies that if the variance of the random ambient excitation is known, then the harvester could be optimized so that the mean harvested power is maximized.</p
Fluid–Body Interactions in Fish-Like Swimming
No full textThe present study focuses on formulating a fluid–structure interaction (FSI) framework by coupling a finite element analysis (FEA) based structural solver and a lumped vortex method (LVM) based potential flow solver to study the coupled dynamics involved in the undulatory and oscillatory swimming of fishes. The caudal fin of a carangiform fish is modelled as a continuous cantilever beam with a periodic support motion. The effect of the actuation frequency on the thrust coefficient is investigated. A significant increase in the aerodynamic thrust is noticed for the support motion frequencies nearing to the structural natural frequencies of the beam. Next, the whole fish body, considering the full-body undulations, is modelled as a continuous free-free beam. This model incorporates a time-dependent actuating moment varying along the length of the body which can be attributed to the muscle moments generated by the fish. A parametric study is carried out to obtain maximum thrust output for the muscle power input in terms of the actuation moment. It is observed that the generated thrust increases significantly when the frequency of the actuation moment approaches towards the natural frequencies of the free-free beam. A comparative study of the average thrust coefficient is carried out for these two cases.</p
Field Investigation and Modeling of Rapid Subsurface Stormflow through Preferential Pathways in a Vegetated Hillslope of Northeast India
No full textEntropy and Energy Dissipation of a Braided River System
No full textAbstractThe randomness in morphology of the Brahmaputra River is very common because of its high flow variability and erodible banks. The river is severely braided with no permanent bank line and varies its width by 1.2km to 18km. During the monsoon season it follows several tortuous braided paths to dissipate its enormous energy. The river changes its braided planform in response to seasonal water and sediment waves and makes its morphology extremely complex. This paper aims to link the braided planform disorderness as a measure of entropy with the energy dissipated by the braided river system to study the river behaviour for various flood waves
Spatio-Temporal Reconstruction of MODIS NDVI by Regional Land Surface Phenology and Harmonic Analysis of Time-Series
No full textSatellite-derived Normalized Difference Vegetation Index (NDVI) is frequently obstructed by adverse atmospheric components resulting in data gaps in time series. The harmonic analysis of time series (HANTS) algorithm is a widely used technique to reconstruct missing NDVI time series. However, due to restriction of HANTS to act within temporal dimension, its direct application is bound to endure practical problems in spatiotemporal reconstruction due to large data gaps. This study proposes Moving Offset Method (MOM), a novel prefilling method applied on NDVI time series prior to application of HANTS. MOM restores the missing NDVI time series by assuming that it tends to follow a reference pattern of land surface phenology (NDVIref). The NDVIref is prepared by using a recursive search and fill algorithm (SFA) for data availability without null values. It restores null values in NDVIref at a pixel by using coefficients of linear regression with NDVIref at another pixel having identical conditions. Finally, the prefilling is prior to application of HANTS. The proposed approach is demonstrated by using MODIS 16-daily time series data for Northeast India and Bhutan region which is covered with frequent seasonal clouds. Besides direct application of HANTS, it is also compared with similar approaches which includes prefilling by inverse distance weighted (IDW) and cubic spline, prior to application of HANTS. The fitting indicators, overall reconstruction error (ORE) and normalized noise related error (NNRE) are found to be best for proposed approach in spatiotemporal comparison. Also, restoration of seasonality trait the NDVI time series better in the proposed approach. This approach is concluded to be an enhancement for HANTS that could be helpful in improving quality of NDVI reconstruction for regions with frequent seasonal obstructions around the globe
Advanced vegetation indices for sensing paddy growth via hyperspectral measurements
No full textIn this study, field-based spectroradiometer measurements were carried out in 72 plots with eight rice varieties along with three nitrogen applications. The critical wavelengths for the studied rice varieties at which the effects of nitrogen applications on spectral reflectance response were found significant, are ρ550, ρ560, ρ655, ρ750, ρ755, ρ780, ρ810, ρ840, ρ900, ρ920, ρ1000, ρ1010, ρ1020. Using the critical wavelengths, three derived vegetation indices were compared with established indices (VI and GRI) sensitive to nitrogen. VI1 and VI2 were efficiently able to discriminate the level of nitrogen treatments for most of the rice genotypes whereas VI and GRI were unable to find significant difference in nitrogen treatments for rice genotypes except two. Furthermore, nitrogen difference plots exhibited that the paddy crop growth has started earlier with N3 (150 kg/ha) than N2 (100 kg/ha) application. Quantitative analysis of rice genotypes revealed that VI2 can be applied functionally for the assessment of nitrogen stress in rice agriculture system
