Sediment Mobilization Research Articles

Modelling localized sources of sediment in mountain catchments for provenance studies

AbstractA hydrology–sediment modelling framework based on the model Topkapi‐ETH combined with basin geomorphic mapping is used to investigate the role of localized sediment sources in a mountain river basin (Kleine Emme, Switzerland). The periodic sediment mobilization from incised areas and landslides by hillslope runoff and river discharge is simulated in addition to overland flow erosion to quantify their contributions to suspended sediment fluxes. The framework simulates the suspended sediment load provenance at the outlet and its temporal dynamics, by routing fine sediment along topographically driven pathways from the distinct sediment sources to the outlet. We show that accounting for localized sediment sources substantially improves the modelling of observed sediment concentrations and loads at the outlet compared to overland flow erosion alone. We demonstrate that the modelled river basin can shift between channel‐process and hillslope‐process dominant behaviour depending on the model parameter describing gully competence on landslide surfaces. The simulations in which channel processes dominate were found to be more consistent with observations, and with two independent validations in the Kleine Emme, by topographic analysis of surface roughness and by sediment tracing with 10Be concentrations. This research shows that spatially explicit modelling can be used to infer the dominant sediment production process in a river basin, to inform and optimize sediment sampling strategies for denudation rate estimates, and in general to support sediment provenance studies. © 2020 John Wiley & Sons, Ltd.

Swash zone morphodynamic modelling including sediment entrained by bore-generated turbulence

In this paper we introduce a mathematical model of sediment entrainment due to bore-generated turbulence in a shallow water context. In this model, the entrainment is assumed to be proportional to the energy decay rate across a bore on a mobile bed. The energy decay rate across a bore on a mobile bed is derived analytically. This model is incorporated into the one dimensional morphodynamic model developed by Zhu and Dodd (2015), which includes bed- and suspended load transport. This results in new shock conditions, which allow for sediment entrainment at a shock. With it we investigate the effects of sediment entrainment due to bore-generated turbulence on beachface evolution under a single swash event driven by a solitary wave. The simulation results imply that sediment entrainment by bore turbulence at the incoming bore dominates over sediment mobilisation by bed shear stress. In contrast, the backwash bore is dominated by bed shear stress related processes. The morphodynamic impact of bore turbulence on this swash event is primarily erosion of the sea bed seaward of the initial shoreline. Any sediment remaining in the water column seaward of this point is then available to be transported by subsequent events. It is shown that the bed step is primarily a bed load related feature, with sediment entrained as suspended load counteracting the bed step growth.

Reservoir Construction Has Reduced Organic Carbon Deposition in the East China Sea by Half Since 2006

AbstractRiver‐dominated marginal seas are important carbon sinks on Earth. However, their carbon sequestration capacities are changing due to increased anthropogenic perturbations. Herein, we employ substantial datasets to study the characteristics of organic carbon (OC) from the East China Sea (ECS) in 2006 and 2018 and reveal the impacts of reservoir construction on the OC deposition in this region. We show that the distribution of sedimentary OC in the ECS is primarily controlled by riverine input and seabed erosion processes. Hydrodynamic processes influence the OC deposition due to both the scouring of fine‐grained sediments and selective degradation of OC associated with the sediment mobilization. The deposition flux of OC in the ECS decreased by 48% after reservoir construction. These findings demonstrate that reservoir construction seriously affected the OC deposition in the ECS and may be applicable to river‐dominated continental shelves worldwide.

Application of Unmanned Aerial Vehicle (UAV)-Acquired Topography for Quantifying Typhoon-Driven Landslide Volume and Its Potential Topographic Impact on Rivers in Mountainous Catchments

Landslides are highly erosional processes that dominate sediment mobilization and reshape landscapes in orogenic belts. Therefore, quantifying and characterizing landslide volume is essential to disaster prevention and understanding landscape evolution in mountainous rivers. Progressive development of the structure-from-motion (SfM) and multi-view stereo (MVS) photogrammetric techniques and Unmanned Aerial Vehicles (UAV) provides low-cost and high-resolution digital elevation models (DEMs), compared to traditional aerial photogrammetry at the same resolution. In this study, we quantified landslide volume and change in river channel volume at meter-scale accuracy for the Laishe River catchment of southern Taiwan from 2009 to 2015, which provides reliable data for discussing sediment transport and morphological response. The observations indicate that Typhoon Morakot in August 2009, induced a landslide volume of 31.63 million (M) m3, which is equal to 87% of the six-year sediment production. Typhoon Morakot also caused the deposition of 8.2 M m3 in the Laishe River. Additionally, this study demonstrates the feasibility of using UAVs to quantify the migration of landslide material and changes in channel area and volume, and the detection of landslide dams. In conclusion, two sources of images, especially those by UAVs, were used to decipher the consequence and potential hazard, social impact, and morphological changes in a mountainous river.

Evidence of episodically accelerated denudation on the Namche Barwa massif (Eastern Himalayan syntaxis) by megafloods

Megafloods cause widespread mobilization of sediments that have strong potential to control the geodynamic evolution of the Namche Barwa (NB) massif by changing the landscape morphology within a short time. However, the impact of catastrophic events on the denudation of the massif over the longer time scale has not been explored in detail. This contribution assesses the impact of catastrophic floods on the denudation style of the Namche Barwa (NB) massif over millennial time scales. Towards this, 21 optical ages and ∼1700 detrital thermochronometric ages of zircon and apatite minerals extracted from paleoflood sediments downstream of the massif, were integrated with decadal sediment load and aggradation data to estimate the minimum denudation rate during paleoflood events. The results indicate that the 7 ka flood eroded ∼71 m of material from the NB massif. This denudation is equivalent to the material that would be produced over ∼8000 years of erosion at a rate of ∼9 mm/a. The 3.5 ka flood eroded ∼24 m of material equaling ∼2500 years worth of erosion at a rate of ∼9 mm/a. Such relatively instantaneous denudations are three orders of magnitude higher than averaged denudation rates over million-year time scale. This study, therefore, provides the first evidence of the strong interplay of catastrophic flood events and tectonics for the rapid exhumation of the Namche Barwa massif. Megafloods from the failure of dammed lakes provide valuable information to understand the flood surge risk for human settlements in the Siang and the Assam valleys.

A seismic monitoring approach to detect and quantify river sediment mobilization by steelhead redd‐building activity

AbstractThe role of spawning salmonids in altering river bed morphology and sediment transport is significant, yet poorly understood. This is due, in large part, to limitations in monitoring the redd‐building process in a continuous and spatially extended way. A complementary approach may be provided through the use of a small seismic sensor network analysing the ground motion signals generated by the agitation of sediment during the redd‐building process. We successfully tested the viability of this approach by detecting and locating artificially generated redd signals in a reach of the Mashel River, Washington State, USA. We then utilize records of 17 seismic stations, in which we automatically detected seismic events that were subsequently manually checked, yielding a catalogue of 45 potential redd‐building events. Such redd‐building events typically lasted between 1 and 20 min and consisted of a series of clusters of 50–100 short energetic pulses in the 20–60 Hz frequency range. The majority (>90%) of these redd‐building events occurred within 11 days, predominantly during the early morning and late afternoon. The seismically derived locations of the signals were in agreement with independently mapped redds. Improved network geometry and installation conditions are required for more efficient detection, robust location and improved energetic insights into redd‐building processes in larger reaches. The passive and continuous nature of the seismic approach in detecting redds and describing fish behaviour provides a novel tool for fish biologists and fisheries managers, but also for fluvial geomorphologists, interested in quantifying the amount of sediment mobilized by this ecosystem engineer. When complemented with classic approaches, it could allow for a more holistic picture of the kinetics and temporal patterns (at scales from seconds to multiple seasons) of a key phase of salmonid life cycles. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Seismic Imaging of Collapsed-Anticline Diapirs in Central Tunisia

Central Tunisia is characterized by major fault systems, being ornamented by salt diapirs of the Late Triassic evaporite-rich interval. Salt movement began in the Late Triassic and continues to the present-day. Lasting halokinesis create pillows, diapirs, salt wall and salt-cored folds, sometimes, with collapse graben above. Salt diapir structures have been mapped and many regions collapsed diapirs are, perhaps, associated to salt removal. These structures were eroded resulting exposure and sub-aerial erosion by the Late Cretaceous to Paleogene uplift and post-Middle Miocene. The initial studies and findings related to salt structures in Tunisia suggesting a clear correlation between diapiric uplift and local tectonic structures. In particular, the N‒S and NW‒SE shortening trends during the Neogene and Quaternary folding strongly controlled the development of rows of diapirs and salt walls. The salt activities are controlled by tectonics as well as differential loading in the Triassic sediments over long distances, displaying that the mobilization of Triassic sediments covered the whole region and not just the zones near the single diapiric structures. Here we will focus also in the salt related plays and the associated petroleum systems with particular emphasis on trap types, reservoir and hydrocarbon migration pathways. Obviously, all these elements are linked together to certain degree and controlled by the salt tectonics.

Unveiling the provenance of sediments in the moraine complex of Aldegonda Glacier (Svalbard) after glacial retreat using radionuclides and elemental fingerprints

In the changing glacierized areas of the maritime Arctic region, fluxes of sediments are of high intensity during the Arctic melting season. On the Western Spitsbergen Island around Grønfjord, several glaciers have declined rapidly over the last century. The Aldegonda Glacier has retreated about 2 km since 1911 when the glacier tongue reached the seashore which lead to new exposed surfaces in the proglacial area supplying sediments to the fjord during the arctic summer. We explored if the geochemical signature of sediments can be used to discriminate sediment provenance in proglacial environments during different stages of the melting season. To this purpose a sampling campaign was undertaken in July 2015 in the Aldegonda Glacier area, to obtain 24 composite surface samples of moraine materials and sediments at the glacier surface and 12 sediment mixtures. Fingerprinting techniques were applied to determine the sediment provenance and tracers were selected using a novel conservative index and consensus ranking method. FingerPro model identified bottom moraines as predominant source in all cases (74%), recent moraines amounted to 15% and sediment on ice contributed 11%. Radionuclide signatures were of great value to identify sediment provenance, and 137Cs and 210Pbex were among the most selected tracers. Variations in source contributions depended on the location and type of the sediment mixtures. Differences in source contributions also varied temporarily suggesting different sediment mobilization processes during snow and ice melt. Snow and ice melt are the main drivers of sediment mobilization contributed from moraines during summer. Moreover, sediments accumulated on the glacier surface including cryoconite, deliver radionuclides and heavy metals associated to fine particles that have an impact on terrestrial and sea arctic ecosystems. Such deleterious impact could be one main effect of climate change on glacierized environments worldwide.

Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River, Massachusetts, USA

AbstractCenturies‐long intensive land‐use change in the north‐eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest‐clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6‐m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17‐m‐tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium‐137 (137Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment as historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long‐term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source. © 2020 John Wiley & Sons, Ltd.

Development of a water erosion tracer using industrial residue as a source of rare earth elements

This study was aimed at developing a water erosion tracer from montmorillonite (MMT) chemically modified with rare earth elements (REE) as chemical signatures to understand the landscape sediment mobilization dynamics. A new route of development involving the use of an industrial residue from the REE extraction process was evaluated for its REE incorporation efficiency in the clay mineral to enable further tracer applications on a field scale. This industrial residue-modified MMT tracer was compared with a reference tracer based on the chemical modification of MMT with a commercial REE precursor. These REE-modified MMT tracers were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence spectroscopies, inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffractometry, scanning electron microscopy (SEM), and N2 physisorption measurements were performed to verify whether the MMT tracer modified with the industrial residue had the same chemical signature (REE) content and chemical stability as those of the commercial REE precursor-based tracer. Chemical stability tests under different pH conditions were carried out to evaluate possible REE content variations in the tracers, mainly over the targeted soil pH range (pH = 4.0–7.7). The clay modified with the industrial residue was further submitted to laboratory horizontal mobility tests using a soil flume. The chemical modification method was effective in producing a chemically stable MMT-based tracer that also displayed multi-chemical signature due to the use of the ClLa40-INB solution, which will potentially enhance the tracer detection in sediments. The mobility tests confirmed the suitability of the modified clay as a tracer.

Mud flow levitation on Mars: Insights from laboratory simulations

Sediment mobilisation occurring at depth and ultimately manifesting at the surface, is a process which may have operated on Mars. However, the propagation behaviour of this mixture of water and sediments (hereafter simply referred to as mud) over the martian surface, remains uncertain. Although most of the martian surface is below freezing today, locally warmer surface temperatures do occur, and our current knowledge suggests that similar conditions prevailed in the recent past. Here, we present the results of experiments performed inside a low pressure chamber to investigate mud propagation over a warm (∼295 K) unconsolidated sand surface under martian atmospheric pressure conditions (∼7 mbar). Results show that the mud boils while flowing over the warm surface. The gas released during this process can displace the underlying sand particles and hence erode part of the substrate. This “entrenched” flow can act as a platform for further mud propagation over the surface. The escaping gas causes intermittent levitation of the mud resulting in enhanced flow rates. The mud flow morphologies produced by these phenomena differ from those produced when mud flows over a frozen martian surface as well as from their terrestrial counterparts. The intense boiling removes the latent heat both from the mud and the subsurface, meaning that the mud flow would eventually start to freeze and hence changing again the way it propagates. The diverse morphology expressed by our experimental mudflows implies that caution should be exercised when interpreting flow features on the surface of Mars and other celestial bodies.

Assessing paraglacial processes at Nexpayantla Gorge (Popocatépetl volcano, Central Mexico) using OSL and14C

AbstractWe evaluate the paraglacial activity in Nexpayantla, a subtropical mountainous gorge in Popocatépetl volcano (Central Mexico), fully deglaciated in the 20th century. Glacial advances are evidenced by the presence of moraines. Fluvio‐glacial terraces and an alluvial megafan resulted from the gorge deglaciation. Current reworking of the glacigenic material is done by landslides and debris flows produced on the moraines and terraces. To study the different phases of mobilization of glacigenic sediment, we used an approach based on the study of the optically stimulated luminescence (OSL) signals obtained from a portable OSL (POSL) reader in samples extracted from both glacigenic and paraglacial deposits. The luminescence (POSL) results obtained at moraines increase as altitude decreases, which is expected for deglaciated valleys where the oldest moraines are located at lower elevations. We evaluate the grade of luminescence signal reset of the glacigenic sediments during the proglacial stage, and the subsequent deglaciation phases. Our results indicate that there is a marked transition between glacial and fluvially dominated processes at Nexpayantla Gorge. We find that the grade of luminescence signal resetting in the paraglacial deposits is a good indicator to trace paraglacial stages and the beginning of exhaustion of the paraglacial activity in mountain areas. OSL ages confirm that the oldest fluvio‐glacial terraces found at the middle sector of Nexpayantla Gorge are ~2 ka, which is also supported by an AMS14C age. OSL dating was found challenging, since quartz grains have low sensitivity because of their volcanic origin; POSL signals, however, are in good agreement with the location and distribution of geomorphic markers. We propose that luminescence data obtained from the POSL unit can be useful to provide information about sediment mobilization in paraglacial environments during different climatic pulses – even for the case where mineral grains have low sensitivity, such as in volcanic sediments. © 2020 John Wiley & Sons, Ltd.

Microcosm based analysis of arsenic release potential of Bacillus sp. strain IIIJ3-1 under varying redox conditions.

The role of indigenous bacteria in mobilization of sediment bound arsenic (As) into groundwater is investigated using subsurface sediment from Brahmaputra River Basin (BRB) and the Bacillus sp. strain IIIJ3-1, an indigenous species to BRB. Anaerobic sediment microcosms with varying organic carbon sources and terminal electron acceptors (TEAs) are used to illustrate the role of the test bacterium in As mobilization. The aquifer sediment shows an asymmetric distribution of As and Fe in its different phases. Among the TEAs added, NO3 amendment promotes higher cell growth, oxalic acid production and maximum release of sediment bound As. X-ray diffraction analysis further suggests that weathering of As bearing secondary minerals through bacterial action enhances As bioavailability, followed by dissimilatory reduction and thus promotes its mobilization into aqueous phase. Co-release pattern of other elements from the sediment indicates that release of As is decoupled from that of Fe. This study confirms that microbe-mediated mineral weathering followed by respiratory reduction of As facilitates mobilization of sediment hosted As into aqueous phase, and provides a better insight into the catabolic ability of groundwater bacteria in mobilization of sediment hosted As in BRB region.

Sediment mobilization study on Cretaceous, Tertiary and Quaternary lithological formations of an external Rif catchment, Morocco

ABSTRACT Soil erosion is a serious ecological problem in Mediterranean areas. The IntErO model based on the erosion potential method (EPM) and the modified universal soil loss equation (MUSLE) have been used to assess soil erosion in several basins. This study aimed to assess and evaluate the effectiveness of these methods for evaluating sediment production and deposition rates in the Arbaa Ayacha basin, Morocco, in order to estimate sediment fluxes on a catchment scale. Our findings suggest that the basin is strongly exposed to erosion owing to geological formations, slope and land use, with average losses of about 28.4 t ha−1 year−1. Erosion processes were evaluated at the erosion production (Eocene marly formations) and sedimentation zones (Quaternary terraces). The results of these models may be useful to address soil and water management in this region and to assess the impact of a river dam that will be built in the basin.

Landscape evolution of Punta Arena sand spit (SE Baja California Peninsula, NW Mexico): Implications of ENSO on landscape erosion rates

Coastal bars are deltaic landforms resulting from sediment mobilization from source to sink. The formation, evolution and extinction of coastal bars is of broad interest for Quaternary studies because these are natural archives of former and present climatic conditions where complex ecological and biological relations take place. The Pacific coast of Mexico contains many coastal bars, like the strand-plain of Nayarit (~1400 km2) which is one of the largest around the world. We present the first ages of the largest sand spit (~8.5 km2) of the SE part of Baja California Peninsula, named Punta Arena. OSL ages indicate that the innermost bar is 0.26 ± 0.02 ka (1755 ± 20 AD). Using OSL ages and data obtained from Landsat images we estimate a mean sedimentation rate of ca. 7.7 m yr –1. Results from satellite images spanning for 32 years (1984 to 2016) confirms that sand spit is accreting at ca. 0.6 m yr –1 along the bar formation zone and to ca. 0.7 m yr –1 along the longshore drift zone, such rates however, are significantly lower compared other formation periods between 1755 and 1984 CE when sedimentation rates were ~ 8 to ~ 52 m yr –1. The progradation of the sand spit is consistent with the apparent development and density of vegetation which dramatically drops from inland towards the coast. The analysis of rainfall data spanning from 1950 to 2015 indicates that the largest periods of sedimentation along the sand spit are related with extended non-rainfall months occurring during ENSO conditions. Our data indicate that climate in SE Baja California Peninsula has remained more or less stable since the LIA and the periods of landscape erosion, leading to the formation of bars and sand spit accretion, mainly occur under the warm phase of ENSO (La Niña) which produces extended periods of landscape dryness and intense rainfalls during the winter.

Sediment sources and transport by the Kahiltna Glacier and other catchments along the south side of the Alaska Range, Alaska

AbstractErosion related to glacial activity produces enormous amounts of sediment. However, sediment mobilization in glacial systems is extremely complex. Sediment is derived from headwalls, slopes along the margins of glaciers, and basal erosion; however, the rates and relative contributions of each are unknown. To test and quantify conceptual models for sediment generation and transport in a simple valley glacier system, we collected samples for 10Be analysis from the Kahiltna Glacier, which flows off Denali, the tallest mountain in North America. We collected angular quartz clasts on bedrock ledges from a high mountainside above the equilibrium line altitude (ELA), amalgamated clast samples from medial moraines, and sand samples from the river below the glacier. We also collected sand from nine other rivers along the south flank of the Alaska Range. In the upper catchment of the Kahiltna drainage system, toppling, rockfall, and slab collapse are significant erosional processes. Erosion rates of hundreds of millimeters per thousand years were calculated from 10Be concentrations. The 10Be concentrations in amalgamated samples from medial moraines showed concentrations much lower than those measured from the high mountainside, a result of the incorporation of thick, and effectively unexposed, blocks into the moraine, as well as the incorporation of material from lower-elevation nearby slopes above the moraines. The 10Be sediment samples from downstream of the Kahiltna Glacier terminus showed decreasing concentrations with increasing distance from the moraine, indicating the incorporation of material that was less exposed to cosmic rays, most likely from the glacier base as well as from slopes downstream of the glacier. Taken together, 10Be concentrations in various samples from the Kahiltna drainage system indicated erosion rates of hundreds of millimeters per thousand years, which is typical of tectonically active terrains. We also measured 10Be concentrations from river sediment samples collected from across the south flank of the Alaska Range. Calculation of basinwide weighted erosion rates that incorporated hypsometric curves produced unrealistically high erosion rates, which indicates that the major source of sediment was not exposed to cosmic rays and was primarily derived from the base of glaciers. Moreover, the apparently high erosion rates suggest that parts of each drainage system are not in erosional steady state with respect to cosmogenic isotope accumulation.

The representation of sediment source group tracer distributions in Monte Carlo uncertainty routines for fingerprinting: An analysis of accuracy and precision using data for four contrasting catchments.

Previous studies comparing sediment fingerprinting un‐mixing models report large differences in their accuracy. The representation of tracer concentrations in source groups is perhaps the largest difference between published studies. However, the importance of decisions concerning the representation of tracer distributions has not been explored explicitly. Accordingly, potential sediment sources in four contrasting catchments were intensively sampled. Virtual sample mixtures were formed using between 10 and 100% of the retrieved samples to simulate sediment mobilization and delivery from subsections of each catchment. Source apportionment used models with a transformed multivariate normal distribution, normal distribution, 25th–75th percentile distribution and a distribution replicating the retrieved source samples. The accuracy and precision of model results were quantified and the reasons for differences were investigated. The 25th–75th percentile distribution produced the lowest mean inaccuracy (8.8%) and imprecision (8.5%), with the Sample Based distribution being next best (11.5%; 9.3%). The transformed multivariate (16.9%; 17.3%) and untransformed normal distributions (16.3%; 20.8%) performed poorly. When only a small proportion of the source samples formed the virtual mixtures, accuracy decreased with the 25th–75th percentile and Sample Based distributions so that when <20% of source samples were used, the actual mixture composition infrequently fell outside of the range of uncertainty shown in un‐mixing model outputs. Poor performance was due to combined random Monte Carlo numbers generated for all tracers not being viable for the retrieved source samples. Trialling the use of a 25th–75th percentile distribution alongside alternatives may result in significant improvements in both accuracy and precision of fingerprinting estimates, evaluated using virtual mixtures. Caution should be exercised when using a normal type distribution, without exploration of alternatives, as un‐mixing model performance may be unacceptably poor.

Hydraulic, wash-off and sediment transport experiments in a full-scale urban drainage physical model

This paper presents a dataset obtained from hydraulic and sediment transport experiments performed in a full-scale urban drainage physical model of 36 m2. The study seeks to accurately measure sediment mobilization through the different parts of the model (surface, gully pots and pipe system), also obtaining a precise characterization of water flow and using realistic rainfall simulator to ensure the transferability of the results. Three different rain intensities and five sediment granulometries were tested in 6 hydraulic and 23 wash-off and sediment transport experiments. The following experimental data were produced: surface elevations and 2D runoff velocities measured by visualization techniques; surface and in-pipe water depths; flow discharges, total suspended solids concentrations and particle size distribution at the entrance of the gully pots and at the pipe system outlet; and sediment mass balances. This data is optimal for developing and validating wash-off and sediment transport formulations in urban drainage models, towards better treatment and management techniques for minimizing the impact of urban surface pollutants on the environments of towns and cities.

Sensitivity of source apportionment predicted by a Bayesian tracer mixing model to the inclusion of a sediment connectivity index as an informative prior: Illustration using the Kharka catchment (Nepal)

Long-chain saturated fatty acid (LCSFA) isotopic composition in tandem with Bayesian isotope mixing models (BIMM) can provide insight into land use-based sediment sources in catchment systems. Apportioning sediment sources robustly, however, requires careful consideration of how additional factors including topography, surface cover and land use practices interact to influence contributions from individual sources. Prior knowledge can be used in BIMM; however, the full capacity of this functionality has not been thoroughly exploited yet in conjunction with sediment fingerprinting. In response, we propose an approach for applying a state-of-the-art BIMM incorporating a sediment connectivity index (SCI) as an informative prior for sediment source apportionment in a highly hydrodynamic catchment in Nepal. A library of LCSFA carbon isotopic composition was constructed for surface soils collected from mixed forest, upland and lowland terraces in the Kharka micro-catchment. δ13C values of LCSFA of time-integrated suspended bulk (<2 mm) sediment were depleted by 4‰ compared to the fine (<0.063 mm) sediment fraction. Conventional source apportionment for fine sediment samples without the SCI informative prior suggested that 66% of the sediment is derived from forest soils followed by lowland (19%) and upland (15%) terraces. Incorporation of the SCI as an informative prior in BIMM, however, modified the original source apportionment estimates to 90%, 9% and 1% respectively. The lower contributions from agricultural terraces are explained by landscape complexity comprising small levelled terraces that reduce hillslope-to-channel sediment connectivity. This study demonstrates the sensitivity of BIMM posterior distributions to incorporation of an informative prior based on a SCI. Inclusion of SCI linked to land use and management can provide a more physically-grounded approach to estimating sediment source contributions from biogeochemical tracers, and critically one which generates results better reflecting what makes good environmental sense in the context of land management and visual evidence of sediment mobilisation and delivery.

Morphological and sedimentological responses of small stream channels to extreme rainfall and land use in the Darjeeling Himalayas

Three pairs of small, fourth- to sixth-order catchments (approximately 5–80 km2), draining the margin of the Darjeeling Himalayas into the piedmont, were selected for a comparison of the land-use impact on the morphology and sedimentology of their stream channels. Each pair experienced similar annual rainfall and comprised similar metamorphic bedrock, steep topography, and brown soils; the members of each pair contrast with respect to their land-use structure as they comprise large forest cover (forest >90%) and a significant contribution of agricultural land with tea plantations (forest <63%), respectively. A set of rainfall, cross-sectional, and sediment data were collected for the characterization of the stream channels. The obtained results revealed that, under extreme rainfall and high denudation rates as well as frequent flash floods, the geomorphic response to land-use changes involving agricultural expansion at the expense of forest cover was less pronounced in the mountain channels than in the piedmont. Significant differences between the studied pairs of mountain streams draining forested and agricultural catchments were observed only at bankfull width and baseflow depth in fourth-order streams and baseflow depth in fifth-order streams. Local environmental factors such as bedrock channel boundaries, steep topography, high material supply to river networks, and selective sediment mobilization during extreme rainfall and floods override the effects of land use and exert dominant control over stream channels in mountain catchments. However, the effects of agricultural activity in mountains are propagated downstream. In the agricultural catchment in the piedmont, a local rapid decrease in river slope and an increase in water infiltration into alluvia facilitates the deposition of eroded material. In addition, the stream in the agricultural catchment exhibited a rise in the riverbed with the expansion of a braided channel over a dozen kilometers below the Himalayan front in the piedmont, while the streams in the forested catchment revealed a distinct tendency towards incision. The present-day channel morphologies and sediment patterns observed in forested catchments are most likely similar to those that existed in the studied rivers prior to the development of tea plantations and settlements in the mid-19th century.