Wavelet based flood forecasting models are known to perform better than conventional models, yet the effect of the way wavelet components are combined to develop a model on the forecasting ...performance, is inadequately investigated. To demonstrate this, two types of wavelet- adaptive neuro-fuzzy inference system (WANFIS), i.e. WANFIS-split data model (WANFIS-SD) and WANFIS-modified time series model (WANFIS-MS) are developed to forecast river water levels with 1-day lead time. To develop these models, first the original level time series (OLTS) is decomposed into discrete wavelet components (DWCs) by discrete wavelet transform (DWT) upto three resolution levels. In WANFIS-SD, all wavelet components are used as inputs while WANFIS-MS ignores the noise wavelet components and utilizes only the effective wavelet components. The effectiveness of the developed models are evaluated through application to two Indian rivers, Kamla and Kosi, which vary significantly in their catchment area and flow patterns. The proposed models are found to forecast river water levels accurately. On comparison, the WANFIS-SD is found to perform better than WANFIS-MS for high flood levels.
The surfaces of alluvial fans and river deltas (collectively fans) are often dissected by a small number of channels radiating from the fan apex. On long timescales, channels migrate via avulsion, ...the process of channel bed deposition and abandonment that often results in catastrophic flooding and loss of life on densely populated fans. We present results of an experimental fan that creates realistic channel patterns by avulsion. The avulsion cycle occurs with a period that is predictable from conservation of mass. Selection of a new flow path is inherently stochastic; however, once a network of 4–5 channels is established, flow oscillates among these channels indefinitely. We demonstrate that a directed random walk model with memory quantitatively reproduces these dynamics and limiting behavior, and is consistent with natural fans.
The 18 August 2008 avulsion of the Kosi River draining the parts of north Bihar in eastern India may well be regarded as one of the greatest avulsions in a large river in recent years. The Kosi River ...shifted by ~120 km eastward, triggered by the breach of the eastern afflux bund at Kusaha in Nepal at a location 12 km upstream of the Kosi barrage. This event was widely perceived as a major flood in the media and scientific circles. Although a large area was indeed inundated after this event, it is important to appreciate that this inundation was different from a regular flooding event.
Coastal sand dune sediments with associated intraformational aeolianite, palaeosols and beachrock are found throughout much of the southern African coast, and have important roles as both Quaternary ...environmental archives and in recording the interplay of sediment supply, coastal processes and sea-level change. This paper examines the stratigraphy and palaeoenvironmental significance of coastal sand dunes and associated sediments, using examples mainly from South Africa but also with reference to the sandy coasts of Namibia and southern Mozambique. Based on morphological, sedimentary and dating evidence reported in the diverse literature, the stratigraphic relationship of coastal sand dune sediments to Marine Isotope Stages (MIS) is summarized, and a chronostratigraphic diagram for the period MIS 6 to 1 along the South African coast is used to identify spatially coherent sedimentary units that correspond to different regional climatic and sea-level phases. This framework provides a stratigraphic context to better examine the relationship between coastal sand dunes and external forcing during the late Quaternary in southern Africa, and to guide future field studies.
RIVER AVULSIONS AND THEIR DEPOSITS Slingerland, Rudy; Smith, Norman D
Annual review of earth and planetary sciences,
01/2004, Letnik:
32, Številka:
1
Journal Article
Recenzirano
Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading ...floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.
We report the isotopic composition of the surface water and groundwater of the Kosi River fan on the Himalayan Foreland, India. We have collected 65 water samples from surface water (Kosi River (
n
...= 2), streams (
n
= 9), waterlogging (
n
= 29), and canal (
n
= 4)), and groundwater (
n
= 21) for δ
18
O and δ
2
H analysis during December 2019. We obtained groundwater level data measured at the observation wells from the Central Groundwater Board, India, for 1996 and 2017. The groundwater level varies from 1.0 to 8.1 m below ground level (bgl) and from 0.5 to 9.0 m bgl during 1996 and 2017, respectively. We have used water table fluctuation approach to estimate the recharge rate. The recharge rate in the Kosi Fan varies from 0.7 to 21.4 mm/year from 1996 to 2017. Further, we have used δ
18
O and δ
2
H values of water samples to identify the source and the interaction between surface water and groundwater. The δ
18
O value of groundwater shows a wide variation (from −9.3‰ to −5.6‰) compared to the surface water, i.e., streams (−7.8‰ to −6.4‰) and canals (−6.9‰ to −6.0‰), suggesting mixing in groundwater during recharge processes. Furthermore, we have used a two-component mixing model to assess the fraction contribution from streams and precipitation to groundwater. The estimated fraction contribution from stream water to groundwater ranges from 45 to 83%. We also suggest higher recharge is limited up to the depth of 6 m bgl. We suggest precipitation and surface water actively recharge groundwater. We conclude that marked spatial variation in the isotopic composition of groundwater is mainly due to the local recharge sources and interaction between surface water and groundwater.
Year-round flow consistencies of the central Himalayan springs are extremely important for addressing rural water demand. As the prediction of Himalayan spring high-flows is expected to provide ...better opportunities for the management of excess runoff, this study aims to develop a data-driven model for predicting joint-fracture and depression type spring high-flows of the Kosi watershed of central Himalaya, India. Five machine learning algorithms are used with combinations of predictors, such as standardized anomaly of rainfall, pH, electrical conductivity and water quality index of spring water. The discharge and predictor parameters are used from a total of 06 springs distributed across the watershed, and monitored during January, 2019 to December, 2020 at monthly interval. Due to asymmetric relationships between model predictors and spring discharge, model performances are tested for the predictor time lags of 0–2 (= 60 days). A total of ten experiments are carried out, and model performances during training and testing are evaluated using receiver operator characteristics. The discriminant analysis classifier, in combination with rainfall and electrical conductivity as predictors, is found to be the best model for predicting spring high-flows.
The distribution and geomorphology of aeolianite outcrop on the coast of KwaZulu-Natal, South Africa are described in the context of its preservation potential in the geological record. Aeolianite on ...the contemporary coast comprises discontinuous, widely spaced, outcrops less than 100m in longshore extent and typically 50–60m wide that reflects the combined action of subaerial weathering and physical abrasion in the coastal zone since the mid-Holocene (approximately 7000yrs). The dominant morphological component is an intertidal erosional platform that slopes gently landward. It is fronted by a notch in the mid intertidal zone (0m MSL) and contains active erosional potholes on its seaward margin. The platform surface is encrusted by a variety of marine organisms. Landward of and contiguous with the intertidal surface are small elevated outcrops of aeolianite in the supratidal zone. These comprise a 3m-high seaward scarp with a basal notch in the high intertidal zone +1m MSL. Gullies in the scarp trap coarse clasts of eroded aeolianite as well as bedrock. The upper surface of the supratidal aeolianite is planar with large potholes within which accumulations of coarse clasts and oyster shells have been cemented. The upper surface is interpreted as a remnant of an intertidal platform formed at a higher mid-Holocene sea level. The supratidal outcrop is subject to weathering and disintegration. Major joints divide the outcrop into cuboid blocks with axes of 3–4m. Eroded blocks on the seaward face that exceed 1m diameter, remain in-situ and become locked in place by repeated shaking. Smaller blocks that collapse from the seaward face accumulate in small gullies or are dispersed. The facet most likely to be preserved in the geological record is the planar intertidal platform. The morphology of aeolianite on the contemporary coast is markedly different from submerged aeolianite ridges on the adjacent shelf which are of higher relief, wider and more laterally extensive. Preservation potential of aeolianite under subaerial conditions is low and the widespread high-relief aeolianite on the shelf is attributed to rapid inundation or burial during transgression.
•Compare geomorphology of coastal and submerged aeolianites•Coastal aeolianite morphology comprises notches, intertidal platforms, and raised stacks.•Result of subaerial weathering, gravitational effects and wave abrasion•Coastal aeolianites have reduced relief and size compared to submerged equivalents.•Rapid sea level rise limits subaerial exposure and increases preservation potential.
The present work focuses on the comparison between Analytical Hierarchy Process (AHP), the most commonly used Multi-Criteria-Decision-Analysis (MCDA) model, and three bivariate models: Evidential ...Belief Function (EBF), Weights of Evidence (WoE) and Frequency Ratio (FR) to predict the flood susceptible areas in the Lower Kosi River Basin of the Ganga River Basin. Twelve flood conditioning factors, topographic (altitude, slope, aspect, curvature, and geomorphology), hydrologic (rainfall, TWI, river density), anthropogenic (LULC, distance from road), and others (distance from river, soil) have been utilized for the spatial modelling. The results suggest that geomorphology, TWI, land-use, and river density are the most dominating factors. Area-under-Receiver-Operating-Characteristic (AUROC) method was used for validation of the used models. The value of the AUROC curve for AHP is found 0.837, outstands the bivariate models- FR (0.807), EBF (0.820) and WoE (0.787), suggests the MCDA model is more accurate in predicting the flood lands than bivariate models in the plains of Ganga River Basin.
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