Sediment Transport Capacity Research Articles

Investigating effects of mining on sedimentary properties of Lisar River (Guilan Province, Iran) using HEC_RAS model

ABSTRACT Increasing land development projects lead to demand for riverine mining, followed by erosion and deposition. The aim of the research was to assess the sedimentary impacts of mining activities on the Lisar River. The research material comprised environmental, hydraulic, and sedimentary information, mining history, and dimensions. Detailed topographic and TIN maps, and 55 cross-sections of the Lisar River mining, were prepared. This study simulates flow patterns in a quasi-unsteady condition and sediment transport capacity using the HEC-RAS model. The maximum change along the longitudinal profile of the river mining is 3 m. The spatial map of the mining in different river sections was determined based on the maximum allowable depth of mining. The present research recommends the Yang function use in simulations of the rivers with sandy-gravel texture accompanied with or without riverine mining and steep slope. Results indicate the current mining volume is up to three times the allowed capacity for the extraction from the Lisar River. The present research concluded that the management plan for spatial mining and measures for monitoring based on the spatial distribution of depositional and erosional sites is necessary for this and other such areas to conserve natural resources.

THE IMPACT OF CLIMATE CHANGE ON SEDIMENT TRANSPORT CAPACITY: A CASE STUDY OF THE BOUBO COASTAL WATERSHED

For soil erosion modeling, determining sediment transport capacity (Tc) is essential because it plays a key role in sediment detachment, transport, and deposition research. This paper provides insights into the seasonal spatial distribution of sediment transport capacity, excess runoff depth in response to the distribution of precipitation, and land use at a watershed scale, using SCN Curve Number (CN) method, Remote Sensing (RS), and Geographic Information Systems (GIS). Spatial distribution in runoff production on hillslopes and sediment transport are explained. We integrated the effect of slope gradient in the curve number to model the landscape effect on sediment transport. The findings show that seasonal variation in sediment transport capacity is influenced by climate change. During June and October, the transport capacity is higher and coincides with channel areas in the Boubo watershed. Potential applications of this map may help the decision-maker to deal with problems associated with watershed development and management.

Development of a Quantitative Approach for Morphodynamic Assessment of Extreme Flood Event in Lokoja

Abstract – This study developed a quantitative approach for the morphodynamic assessment of River Niger at Lokoja. The study involves the collection of twenty-one years daily discharge and water level at the gauging station. The data were collated and outliers were removed. The missing data were fitted using the Least Square Method. The flood generating processes were investigated by computing the bankfull discharge of selected reach as well as collecting river bed samples at six (6) different locations along the river reach. Based on these, the specific gravity and mobile D50 and D90 of the bed materials were obtained. The preliminary investigation of the river morphodynamics was studied by computing the sediment transport parameters such as Shield parameter, bed shear velocity, sediment fall velocity amongst others. These parameters are significantly important due the high flow rate associated with the river. Based on criteria by two well-known sediment transport capacity formula, the river reach is characterized as sandy-bed River in line with Eugelund and Hansen’s Formula (E-H-F). The entire flow generation parameters and mobile river bed materials failed the Meyer and Peter- Muller criteria. Hence, the sediment transport capacity of the river was computed using (E-H-F). The results show that understanding the morphodynamics of the rivers play a significant role in the understanding of flooding.

Erosion of four Brazilian coastal deltas: how dam construction is changing the natural pattern of coastal sedimentary systems.

Reservoir-building on rivers is still "the Brazilian energy culture", such that many other dams are being constructed and planned, with many other rivers potentially suffering the same impact in their sediment flux. In this study, we evaluated the impacts of reservoir-building on the erosion of the São Francisco, Jequitinhonha, Doce, and Paraíba do Sul river deltas. A time series of Landsat images from 1973 to 2020 was analyzed, with three key highlighted moments (1973, 1997 and 2020) that summarize the erosion processes on these deltas. In addition to the images, continuous river water discharge, sediment discharge and basin rainfall data were analyzed between 1940 and 2020, providing river parameters over a period exceeding that of the satellite data. The findings suggest that coastal erosion has progressed in the four deltas, with higher estimated losses in the São Francisco and Paraíba do Sul over the 47 years of available satellite images. However, despite minimal overall estimated gains, the Jequitinhonha and Doce experienced high erosion at their river mouths, as in the other two rivers, compensated by accretion in distal coastal areas. These results can be explained by reductions in river flux and consequent sediment transport capacity due to reservoirs.

植被和梯田对黄土高原小流域泥沙连通性的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 植被和梯田对黄土高原小流域泥沙连通性的影响 DOI: 10.5846/stxb202204211096 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金面上项目(42177319) Effects of terrace and vegetation on sediment connectivity in a small watershed on the Loess Plateau Author: Affiliation: Fund Project: The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan)（42177319） 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:黄土高原经过长期水土流失治理,泥沙问题得到有效解决,但植被恢复和梯田修建对泥沙空间输移的影响尚未完全揭示。通过土地利用数据和Landsat影像确定梯田分布和植被覆盖情况,使用能表征植被和梯田对泥沙输移的阻滞作用的植被覆盖与作物管理因子和工程因子计算泥沙连通性指数(IC),分析泥沙连通性的时空动态变化,量化植被与梯田共同影响下黄土高原第三副区典型流域泥沙连通性。调整权重因子模拟不同情景下泥沙连通性指数的计算,分析植被与梯田对泥沙连通性的单独影响。结果表明由于植被快速恢复和梯田大面积修建,IC在35年间(1986-2020年)从-3.42到-9.17显著下降。情景模拟发现,在1986-1999年间,梯田为泥沙连通性减弱主要驱动因素,1999年后植被与梯田共同作用。植被影响下的IC从1986年到2020年下降73.05%,在1986-2020年间梯田影响下IC下降26.75%。在空间上梯田及高植被覆盖度区域呈弱泥沙连通性,中下游主沟及部分支沟区域为强泥沙连通性。总体而言,植被恢复和梯田修建减弱泥沙输移能力,流域输沙路径集中于主沟中下游及部分支沟区域。研究结果有助于深化黄土丘陵区小流域植被恢复和梯田修建影响下泥沙变化研究。 Abstract:After long-term soil erosion control on the Loess Plateau, the sediment problem has been effectively solved, but the impact of vegetation restoration and terrace construction on the spatial transport of sediment has not been fully revealed. This study uses land use data and Landsat images to determine the distribution of terraces and vegetation coverage, and uses vegetation coverage and crop management factors and engineering factors, which could characterize the resistance of vegetation and terraces to sediment transport, to calculate sediment. The index of connectivity (IC) was applied to analyze the temporal and spatial dynamic changes of sediment connectivity, the combined effects of vegetation and terraces on sediment connectivity of typical watersheds in the third subregion of the Loess Plateau are quantified. Weighting factors were adjusted to simulate the calculation of the sediment connectivity index under different scenarios and analyze the separate effects of vegetation and terraces on sediment connectivity. The results showed that IC significantly decreased from -3.42 to -9.17 in 35 years (1986-2020) due to the rapid vegetation restoration and the large-scale construction of terraces. The scenario simulation found that the terraced fields were the main driving factor for the weakening of sediment connectivity between 1986 and 1999. After 1999, vegetation and terraced fields worked together. The IC under the influence of vegetation decreased by 73.05% from 1986 to 2020, and the IC under the influence of terraced fields decreased by 26.75% from 1986 to 2020. In space, terraces and areas with high vegetation coverage showed weak sediment connectivity, while the main ditch and some branch ditch areas in the middle and lower reaches have strong sediment connectivity. In general, vegetation restoration and terrace construction decreased the sediment transport capacity, and the sediment transport paths in the basin were concentrated in the middle and lower reaches of the main ditch and some branch ditch areas. The results of this study can help to deepen the study of sediment changes under the influence of vegetation restoration and terrace construction in small watersheds in the loess hilly area. 参考文献 相似文献 引证文献

Phosphorus spatial distribution and mass balance in the Itaipu lagoon (Rio de Janeiro, Brazil)

The degradation of tropical coastal lagoon systems in urban areas of the least developed countries has been associated with an increase in impermeable areas and poor domestic sewage treatment, increasing land-based runoff of nutrients and suspended solids from catchments. This study aimed to assess the biogeochemical changes caused by human interventions through the analysis of the spatial distribution of sedimentary phosphorus (P) and its mass balance in the Itaipu lagoon, located on the east coast of the state of Rio de Janeiro. Human intervention in the Itaipu lagoon system has caused severe imbalances in biogeochemical cycles over the past decades. Watercourses have been channeled to normalize the hydrological regime and increase hydraulic energy, improving sediment transport capacity. In this context, the increase in runoff from the coastal urban basin into the Itaipu lagoon has buried an increasing amount of phosphorus in the sediment. Recently, a regional increase in storm events caused a series of landslides and floods, which have been reported as possible consequences of global climate change. In recent decades, the synergy between landslides and river channeling has increased TP loads, accelerating phosphorus settling and changing P spatial distribution in surface sediments. This has accelerated siltation of the lagoon with an accumulation of nutrients and organic matter, leading in some cases to sediment anoxia. The lagoon has undergone strong eutrophication, changing its trophic state from meso- to hypertrophic in less than 30 years, even though P loads are not as high as in other coastal lagoons. Our findings confirm that human intervention impacts nutrient loads, which in turn disrupt the balance of biogeochemical cycles, compromising coastal water resources. This leads to the collapse of ecosystem services, another step towards degrading planetary boundaries.

Future Sediment Transport Ability and its Consequences in the Urbanized River Basin

The urbanization process of the Kathmandu Valley has a significant impact on LULC change, river runoff, and sediment transport capability. The historical sediment flow pattern indicates that the sediment transport capacity of the basin has increased even when precipitation and river discharge decreased. So, the sediment regression model is developed in this study in relation to discharge, precipitation, and built-up area change. Model parameters are calibrated and validated through the measured sediment discharge of the basin and the performance of the model is evaluated through NSE, PBIAS, and R2. In the future, the sediment transport capacity of the channel is projected for average monthly, maximum, and minimum flow conditions by +4.33%, +6%, and -2.66% respectively per decade due to the rise in the urban area (+6% per decade). Increasing the rigid ground surface through urbanization reduces the sediment generation through the watershed and balances the sediment transport capability, excess erosion is produced in the river channel causing a change in the river morphology. The findings of this study will be useful for planning and management of the river basin and the river structures.

Soil Erosion across Scales: Assessing Its Sources of Variation in Sahelian Landscapes under Semi-Arid Climate

Soil erosion varies in space and time. As the contributing surface area increases, heterogeneity effects are amplified, inducing scale effects. In the present study, soil erosion processes as affected by the observation scale and the soil surface conditions are assessed. An experimental field scale setup of 18 plots (1–150 m2) with different soil surface conditions (bare and degraded, cultivated) and slopes (0.75–4.2%) are used to monitor soil losses between 2010 to 2018 under natural rainfall. The results showed that soil loss rates range between 2.5 and 19.5 t.ha−1 under cultivated plots and increase to 12–45 t.ha−1 on bare and degraded soils, which outlines the control of soil surface conditions on soil erosion. At a larger scale (38 km2), soil losses are estimated at 2.2–4.5 t.ha−1, highlighting the major contribution of scale. The scale effect is likely caused by the redistribution of sediments in the drainage network. These findings outline the nature and contribution of the emerging and dominant soil erosion processes at larger scales. At the plot scale, however, diffuse erosion remains dominant, since surface runoff is laminar and sediment transport capacity is limited, resulting in lower soil erosion rates.

Methodological Contribution to the Assessment of Generation and Sediment Transport in Tropical Hydrographic Systems

The efficiency and useful life of reservoirs are directly related to the production and input rates of sediments resulting from erosive processes at the edges and those resulting from the action of surface runoff in contribution areas and transported via tributary channels. Knowledge of the intensity, as well as the relationship between generation and input, allows more precise identification of critical environments, helping in the decision-making process and allowing the definition of mitigating measures. This work aims to relate the spatial variability of soil loss with the respective sediment transfer potential in two sub-basins tributary to the HPP Batalha reservoir in the Midwest region of Brazil. The methodology comprised the bivariate analysis between estimates of soil loss in areas of contribution and the Declivity-Extent Relationship along the channels. The results point to the configuration of four spatial patterns, indicating different levels of criticality in terms of sediment generation potential and transport capacity. In addition, they highlight basins with high potential and greater proximity to the reservoir, which constitute priority areas for monitoring, especially the conditions of soil cover and management, to contribute to the reduction of sediment inputs and prolong the efficiency of these structures.

Experimental evidence that rill-bed morphology is governed by emergent nonlinear spatial dynamics

Past experimental work found that rill erosion occurs mainly during rill formation in response to feedback between rill-flow hydraulics and rill-bed roughness, and that this feedback mechanism shapes rill beds into a succession of step-pool units that self-regulates sediment transport capacity of established rills. The search for clear regularities in the spatial distribution of step-pool units has been stymied by experimental rill-bed profiles exhibiting irregular fluctuating patterns of qualitative behavior. We hypothesized that the succession of step-pool units is governed by nonlinear-deterministic dynamics, which would explain observed irregular fluctuations. We tested this hypothesis with nonlinear time series analysis to reverse-engineer (reconstruct) state-space dynamics from fifteen experimental rill-bed profiles analyzed in previous work. Our results support this hypothesis for rill-bed profiles generated both in a controlled lab (flume) setting and in an in-situ hillside setting. The results provide experimental evidence that rill morphology is shaped endogenously by internal nonlinear hydrologic and soil processes rather than stochastically forced; and set a benchmark guiding specification and testing of new theoretical framings of rill-bed roughness in soil-erosion modeling. Finally, we applied echo state neural network machine learning to simulate reconstructed rill-bed dynamics so that morphological development could be forecasted out-of-sample.

Prospects of sediment deposition at small watershed scale in the black soil region of Northeast China: A mini review

Sediment deposition is one of the most significant processes in small watersheds characterized by gentle long hillslopes in the black soil (Mollisol) region of Northeast China, as indicated by severe ephemeral gully and gully erosion on hillslopes and very low sediment concentrations in river systems. Few reviews have been conducted to summarize the related research in this region. The objectives of this review were to identify the potential factors influencing sediment deposition, review related studies, and propose future research needs in the black soil region of Northeast China. Sediment deposition is controlled by the deficit between sediment transport capacity of flow and sediment load. Hence, all factors affecting flow transport capacity and sediment load directly affect sediment deposition. For a specific small watershed, the change in slope gradient along the flow path is the key factor affecting sediment deposition. Shelterbelts, ridge tillage systems, terraces, grass strips, road distribution, ponds and reservoirs, and land-use patterns also influence the spatial distribution and rate of deposition. The trace method has been widely used to quantify sediment deposition in this region. The results of cesium-137 (137Cs), lead-210 (210Pb), and magnetic susceptibility reveal that serious deposition occurs on the back and foot slopes. Distinct deposition occurs in front of contour shelterbelts. Future studies should focus on the methodology, spatial and temporal variations, dominant influencing factors and their mechanisms, and the potential effects on land productivity within specific small watersheds and across the black soil region. This review provides insights into the sediment deposition process in small watersheds characterized by gentle, long hillslopes.

Experimental determination of sediment transport capacity of rill flow over sandified loess slope

Rill erosion is affected by the sand particle content in soil, especially in the wind and water erosion transition region of the Loess Plateau. The sediment transport capacity (STC) is a key parameter in rill erosion research, assessing the impact of aeolian sand intrusion on the STC of rill flow is of importance for a better understanding of rill erosion. This study aimed to assess the effect of aeolian sand intrusion on the STC on sandified loess slopes, with typical slopes and flow discharges, using a flume system which consisting of a sediment-feeding and a sediment-supply/settlement flume. The sediment feeding flume was jointed by 10° higher than that of the sediment measurement flume section. Three flow discharges (2, 4, and 8 L min−1) and four slope gradients (5°, 10°, 15°, and 25°) were used to represent the natural hydrological conditions under three intrusion rates (SIR) of aeolian sands (10%, 20%, and 50%). The results show that STC increased with slope gradient and flow discharge, and the relationship between the STC and the SIR was significantly affected by the slope gradient; the STCs decreased with the SIR on a slope of 5° but increased with the SIR on steep slopes of 15°–25°, implying a significant impact of slope gradient on the relationship between SIR and STC. The SIR of 50% resulted in the highest sediment concentration nearly 1200 kg m−3 on slopes of 25°. On sandified loess slopes of 10%, 20%, and 50% SIR, the STC were about 30%, 46%, and 57% higher than on loess slopes, indicating an increased erosion rate by sand particle intrusion into loess soil. These results highlight the impact of sand intrusion on STC of rill flow and provide deeper insights into the soil loss process on the sandified loess slope.

Analysis of the evolution trends and influential factors of bankfull discharge in the Lower Yellow River

After the Xiaolangdi Reservoir (XLDR) was put into operation, new water and sediment conditions have improved the silting condition of the Lower Yellow River (LYR), but against this background, the flow capacity of the LYR has become more complex. In this research, the measured water and sediment data of seven hydrological stations in the LYR from 1950 to 2020 were systematically collated. The evolution trend and development period of the bankfull discharge in this area were studied based on the wavelet analysis method, and the main factors influencing the evolution of the bankfull discharge were explored. The results indicate that the evolution process of bankfull discharge in the LYR has experienced two phases in the last 70 years. XLDR has been impounded since October 1999. Before XLDR operation, the bankfull discharge of the LYR has a main time scale of about 26 years. After XLDR operation began, it has a time scale of about 10 years. The bankfull discharge of the LYR shows two phases of evolution, and these phases are mainly influenced by the factors of water and sediment conditions. This research is needed for a deeper understanding of flow-bed and river discharge and sediment transport capacity under the water and sediment conditions in the LYR.

Hydrodynamic Model-Based Evaluation of Sediment Transport Capacity for the Makhool-Samarra Reach of Tigris River

This paper implemented an HEC-RAS-based steady two-dimensional hydrodynamic model to evaluate the hydraulic characteristics and Sediment Transport Capacity (STC) of the Makhool-Samarra Reach of the Tigris River, where this part is not already investigated. This model was prepared with the aid of topographical surveys, hydraulic measurements, and laboratory tests, as well as recorded data. The main findings show that the flow velocities within most of the studied reach are considered high compared to the local and worldwide flow velocity ranges in natural rivers. Also, the Toffaleti sediment transport function with the Van Rijn fall velocity and the effective depth-to-width computation methods are the most compatible and suitable methods for computing the Sediment Transport Potential (STP) in the ‎studied reach. With the maximum flow, the computed STP was less than the average (5.7×106 tons/day) in 45% of the reach length, where the maximum STP is 8.7×106 tons/day. However, with the minimum flow, the STP intermittently fluctuates below the average (0.2×106 tons/day) in 48% of the reach length, where the minimum STP is 0.14×106 tons/day. Generally, these STPs are little more than the Tigris River in Mosul and much higher than that in Baghdad. Also, these STPs are considered high on a global scale, especially during high flow. Medium gravel is the larger grain gradation class that can be transported. It constitutes less than 0.00005 % of the STP. Furthermore, the STC computations clearly show that the suspended sediment is the dominant part of the STC and the bed material constitutes less than 0.002% of the STC of the total load. However, most of the STC ‎concerns the clay and very fine silt.

Absence of well‐developed floodplains along the lowland rivers and controls of the hydrogeomorphic conditions in the Western Ghat, India

AbstractLowland rivers across the world are characterized by accumulation of sediment due to a decrease in the channel slope. Despite the antiquity of the topography, the lowland part of the west‐flowing rivers of Western Ghat, India is characterized by poorly developed floodplains except for the southern part of the Ghat. This poses a question of the sediment transportation and deposition capacity in these lowland rivers, which in turn governs geomorphic characteristics. In this study, we adapted a novel method to integrate hydrological model‐derived discharge estimates, channel slope estimates from digital elevation models, bankfull channel width estimates from high‐resolution Google Earth satellite images and field observations. We modelled the downstream distribution of total stream power ( ), specific stream power ( ) and shear stress ( ) for four west‐flowing river systems from different lithological and climatic settings. The 75th percentile discharge from monthly time series data was used in modelling to assess channel processes. The peak values of are distributed all along the longitudinal profile, whereas the peak values of and are consistently higher in the escarpment. Scaling relationships between hydrogeomorphic parameters were also established to understand the inherent controls on the distribution of these driving parameters. We inferred that is sufficient to transport large grain sizes even in the low‐gradient lowland stretches. Hence sediments are mostly absent in the channel, and floodplains are not well developed. Despite having a low gradient, the lowland coastal rivers can efficiently evacuate most of the eroded sediments to the offshore region. Finally, the lithological control is manifested by the variability of channel width regulating the sediment transport in the coastal rivers of the Western Ghat.

Establishment of a sediment transport capacity equation on loessal slope via experimental investigation

A reliable sediment transport capacity function provides response against challenges of soil erosion prediction on the Loess Plateau of China. The popular sediment transport capacity functions are questionable on loess slopes because the experimental conditions from which they were derived, like bed materials, gentle slopes, and surface roughness, are different from soil erosion processes on the loess slopes. Due to the foregoing uncertainty, a suitable sediment transport capacity function was developed based on a flume experiment investigation. The erodible bed experiment was implemented for five unit width discharges (1.3 × 10−4–3.3 × 10−4 m2/s) and ten slopes (5.2%–57.7%). The selected sediment transport capacity equations were tested in an applicability evaluation. The results of this evaluation found that the Govers (1990) function had the best performance (P.O.0.5-2.0 = 76%, R2 = 0.93, RMSE = 0.03 kg/(m·s)), followed by the Yalin (1963) function (P.O.0.5-2.0 = 80%, R2 = 0.81, RMSE = 0.05 kg/(m·s)), and finally by the Low (1989) function (P.O.0.5-2.0 = 72%, R2 = 0.79, RMSE = 0.04 kg/(m·s)), where P.O.0.5-2.0 is the percentage of estimated values with logarithmic discrepancies with the measured value between 0.5 and 2.0; R2 is the coefficient of determination and RMSE is the roof mean squares error. The cohesive sediment incipient motion velocity is introduced into the Govers (1990) function, and the sediment particle parameter is determined from measured experimental data. The developed sediment transport capacity equation has good performance with predicted values corresponding to measured values (P.O.0.5-2.0 = 98%, R2 = 0.98, RMSE = 0.006 kg/(m·s)). The research results provide an improved sediment transport capacity function on the basis of experiments for steep loessal slopes.

Impact of Sediment Deposition on Flood Carrying Capacity of an Alluvial Channel: A Case Study of the Lower Indus Basin

Impacts of climate change and human-made interventions have altered the fluvial regime of most rivers. The increasingly uncertain floods would further threaten the flow delivery system in regions such as Pakistan. In this study, an alluvial reach of the Indus River below Kotri barrage was investigated for the geomorphologic effects of sediments deposited over the floodplain as well as the influences on the downstream flood-carrying capacity. The hydrodynamic modeling suite HEC-RAS in combination with ground and remotely sensed data were used to undertake this analysis. Results suggest that the morphology of the river reach has degraded due to depleted flows over a long period and hydrological extremes that have led to excessive sediment deposition over the floodplain and an enhancement in flood water extension possibility over the banks. A deposition of 4.3 billion cubic meters (BCM) of sediment increased the elevation of the channel bed which in turn reduced a 17.75% flood-carrying capacity of the river reach. Moreover, the excessive deposition of sediments and the persistence of low flows have caused a loss of 48.34% bank-full discharges over the past 24 years. Consequently, the river’s active reach has been flattened, with a live threat of left levee failure and the inundation of the populous city of Hyderabad. The study would gain insights into characterizing the impacts associated with a reduction in the flood-carrying capacity of the alluvial channel on account of the inadequate sediment transport capacity after heavy flow regulations.

Estimating the debris-flow magnitude using landslide sediment connectivity, Qipan catchment, Wenchuan County, China

In an earthquake disturbed basin, the earthquake-induced landslides’ connectivity with debris-flow channels determines the landslide sediment volume that is transported into debris flows, and the debris-flow magnitude as well. In this study, the index of sediment connectivity (IC) was used to analyze the evolution of landslide sediment transport capacity. A normalized IC was developed to quantify the movable volume of the landslide sediment available for debris flows. Combined with a landslide empirical relationship, sediment connectivity calculation, particle model, and numerical algorithm, two debris-flow events after the 2008 Wenchuan earthquake were simulated for the Qipan catchment. The results show good agreement between the simulated and observed debris-flow magnitudes of peak discharges, inundated areas, and depths with approximately 9% average error, which indicates that the normalized landslide connectivity index is suitable for estimating the movable volume. Moreover, the IC and landslide characteristics (area and location) showed significant negative correlations, indicating that the spatial connectivity of landslide sediment generally decayed over time, with a corresponding reduction in debris-flow magnitude.

River avulsions and megafan development triggered by high recurrent explosive volcanism

Megafans are ever-evolving landforms and sedimentary bodies produced by rivers that regularly change their courses through avulsions, in which the processes that cause the avulsions and their rates are tightly linked to the formation and evolution of the fans. The commonly cited condition required for the formation of megafans is a highly variable climate setting (monsoonal) which results in variable discharge and sediment transport capacity. However, some megafans occur in tropical settings in rivers with lower discharge variability but high sediment loads due to input from volcanic or tectonically active sources. We examine the Santa Clara Megafan (Costa Rica) to establish a model of explosive volcanism-driven megafan evolution, an alternative model to the climate-driven one. Studying chronological, archaeological, historical and remote sensing data we infer that major avulsions and shifts in the main rivers of this megafan are temporally and spatially correlated to strong eruptions (VEI ≥ 3). Furthermore, a period of ca. 1700 years with few changes in the fluvial system of the eastern sector of the megafan coincides with a low volcanic activity in its source area (i.e. Turrialba Volcano). The megafan avulsion rate was estimated as ∼1 major avulsion per century in the last five centuries, when four strong eruptions (∼0.8 eruptions/century) took place. These shifts in the rivers in response to the volcanic activity produced the activation or abandonment of megafan sedimentary lobes at a secular pace. The record of these changes will support future archaeological studies and lahar hazard assessments. Furthermore, the Santa Clara Megafan provides a model to explain the formation of megafans on Earth or on other planets through non-climatic processes.

MORPHOMETRIC CHARACTERISTICS OF A TROPICAL RIVER BASIN, CENTRAL KERALA, INDIA USING GEOSPATIAL TECHNIQUES

The Thutapuzha watershed (TW) is one among the major tributaries of Bharathapuzha – the largest west flowing river in Kerala, India. Morphometric analysis was carried out to determine the spatial variations in the drainage characteristics and to understand the prevailing geologic variation, topographic information and structural setup of TW using Survey of India topographic maps and ASTER-DEM. Geoprocessing techniques has been used for the delineation and calculation of the morphometric parameters of the watershed. The TW sprawled over 1107 Km2 and the study revealed that the watershed includes a sixth order stream and lower order streams mostly dominate the basin with a drianage density of 1.36 m/Km2exhibiting highly resistant subsoil, dense vegetation, and low relief of surface nature. The study indicate that rainfall has a significant role in the drainage development whereas the drainage pattern is controlled by structure and relief. The watershed of TW is moderate to well-drained and exhibited a geomorphic maturity in its physiographic development. The shape parameters revealed the elongated nature of TW having less prone to flood, lower erosion and sediment transport capacities and drainage network development in the watershed. This study strongly brings to light that the drainage morphometric parameters have the enormous potentiality to unveil the hydro-morphologicalcharacteristics of the river basins. Integrating hydro-morphological characteristics with conventional watershed assessment methods would have a beneficial effect on judicious watershed management, which helped to formulate a comprehensive watershed management plan.