ALBERT

Description: [1]  Travel distance and residence time probability distributions are the key components of stochastic models for coarse sediment transport. Residence time for individual grains is difficult to measure and residence time distributions appropriate to field and laboratory settings are typically inferred theoretically or from overall transport characteristics. However, bed elevation time series collected using sonar-transducers and LIDAR can be translated into empirical residence time distributions at each elevation in the bed and for the entire bed thickness. Sediment residence time at a given depth can be conceptualized as a stochastic return time process on a finite interval. Overall sediment residence time is an average of residence times at all depths weighted by the likelihood of deposition at each depth. Theory and experiment show that when tracers are seeded on the bed surface, power-law residence time will be observed until a timescale set by the bed thickness and bed fluctuation statistics. After this time, the long-time (global) residence time distribution will take exponential form. Crossover time is the time of transition from power-law to exponential behavior. The crossover time in flume studies can be on the order of seconds to minutes while that in rivers can be days to years.

Description: [1]  Erodibility describes the inherent resistance of soil to erosion. Hillslope erosion models typically consider erodibility to be constant with depth. This may not be the case after wildfire because erodibility is partly determined by the availability of non-cohesive soil and ash at the surface. This study quantifies erodibility of burned soils using methods that explicitly capture variations in soil properties with depth. Flume experiments on intact cores from three sites in western United States showed that erodibility of fire-affected soil was highest at the soil surface and declined exponentially within the top 20 mm of the soil profile, with root density and soil depth accounting for 62 % of the variation. Variation in erodibility with depth resulted in transient sediment flux during erosion experiments on bounded field plots. Material that contributed to transient flux was conceptualized as a layer of non-cohesive material of variable depth ( d nc ). This depth was related to shear strength measurements, and sampled spatially to obtain the probability distribution of non-cohesive material as a function of depth below the surface. After wildfire in southeast Australia, the initial d nc ranged from 7.5 to 9.1 mm, which equated to 97 - 117 Mg ha -1 of non-cohesive material. The depth decreased exponentially with time since wildfire to 0.4 mm (or 〈 5 Mg ha -1 ) after three years of recovery. The results are organized into a framework for modeling fire effects on erodibility as a function of the production and depletion of the non-cohesive layer overlying a cohesive layer.

Description: [1]  In this study, the natural process of river meandering is captured in a computational model that considers the effects of bank erosion, the process of land accretion along the inner banks of meander bends, and the formation of channel cutoffs. The methodology for predicting bank erosion explicitly includes a sub-model treating the formation and eventual removal of slump blocks. The accretion of bank material on the inner bank is modeled by defining the time scale over which areas that are originally channel become land. Channel cutoff formation is treated relatively simply by re-computing the channel alignment at a single model time step when migrating banks meet. The model is used to compute meandering processes in both steady and unsteady flows. The key features of this new model are a) the ability describe bank depositional and bank erosional responses separately, b) to couple them to bed morphodynamics, and thus c) describe co-evolving river width and sinuosity. Two cases of steady flow are considered, one with a larger discharge (i.e. “bankfull”) and one with a smaller discharge (i.e. “low flow”). In the former case the shear stress well above the critical shear stress, but in the latter case it is initially below it”). In at least one case of constant discharge, the planform pattern can develop some sinuosity, but the pattern appears to deviate somewhat from that observed in natural meandering channels. For the case of unsteady flow, discharge variation is modeled in the simplest possible manner by cyclically alternating the two discharges used in the steady flow computations. This model produces a rich pattern of meander planform evolution that is consistent with that observed in natural rivers. Also the relationship between the meandering evolution and the return time scale of floods are investigated by the model under the several unsteady flow patterns. The results indicate that meandering planforms have different shapes depending on the values of these two scales. In predicting meander evolution, it is important to consider the ratio of these two time scales in addition to such factors as bank erosion, slump block formation and decay, bar accretion and cutoff formation, which are also included in the model.

Description: [1]  Anthropogenic and climatic forces have modified the geomorphology of tidal wetlands over a range of timescales. Changes in land-use, sediment supply, river flow, storminess, and sea level alter the layout of tidal channels, intertidal flats, and marsh plains; these elements define wetland complexes. Diagnostically, measurements of net sediment fluxes through tidal channels are high-temporal resolution, spatially integrated quantities that indicate 1) whether a complex is stable over seasonal timescales, and 2) what mechanisms are leading to that state. We estimated sediment fluxes through tidal channels draining wetland complexes on the Blackwater and Transquaking Rivers, Maryland, USA. While the Blackwater complex has experienced decades of degradation and been largely converted to open water, the Transquaking complex has persisted as an expansive, vegetated marsh. The measured net export at the Blackwater complex (1.0 kg/s or 0.56 kg/m 2 /y over the landward marsh area) was caused by northwesterly winds, which exported water and sediment on the subtidal timescale; tidally forced net fluxes were weak and precluded landward transport of suspended-sediment from potential seaward sources. Though wind forcing also exported sediment at the Transquaking complex, strong tidal forcing and proximity to a turbidity maximum led to an import of sediment (0.031 kg/s or 0.70 kg/m 2 /y). This resulted in a spatially averaged accretion of 3.9 mm/y, equaling the regional relative sea-level rise. Our results suggest that in areas where seaward sediment supply is dominant, seaward wetlands may be more capable of withstanding sea-level rise over the short term than landward wetlands. We propose a conceptual model to determine a complex's tendency towards stability or instability based on sediment source, wetland channel location, and transport mechanisms. Wetlands with a reliable portfolio of sources and transport mechanisms appear better suited to offset natural and anthropogenic loss.

Description: [1]  Runoff during intense rainstorms plays a major role in generating debris flows in many alpine areas and burned steeplands. Yet compared to debris-flow initiation from shallow landslides, the mechanics by which runoff generates a debris flow are less understood. To better understand debris-flow initiation by surface water runoff, we monitored flow stage and rainfall associated with debris flows in the headwaters of two small catchments: a bedrock-dominated alpine basin in central Colorado (0.06 km 2 ), and a recently burned area in southern California (0.01 km 2 ). We also obtained video footage of debris-flow initiation and flow dynamics from three cameras at the Colorado site. Stage observations at both sites display distinct patterns in debris-flow surge characteristics relative to rainfall intensity ( I ). We observe small, quasi-periodic surges at low I ; large, quasi-periodic surges at intermediate I ; and at high I , a single large surge is followed by small-amplitude fluctuations about a more steady high flow. Video observations of surge formation lead us to the hypothesis that these flow patterns are controlled by upstream variations in channel slope, in which low-gradient sections act as “sediment capacitors”, temporarily storing incoming bedload transported by water flow and periodically releasing the accumulated sediment as a debris-flow surge. To explore this hypothesis, we develop a simple one-dimensional morphodynamic model of a sediment capacitor that consists of a system of coupled equations for water flow, bedload transport, slope stability, and mass flow. This model reproduces the essential patterns in surge magnitude and frequency with rainfall intensity observed at the two field sites and provides a new framework for predicting the runoff threshold for debris-flow initiation in a burned or alpine setting.

Description: [1]  A large fraction of soil erosion in temperate climate systems proceeds from gully headcut growth processes. Nevertheless, headcut retreat is not well understood. Few erosion models include gully headcut growth processes, and none of the existing headcut retreat models have been tested against long-term retreat rate estimates. In this work the headcut retreat resulting from plunge pool erosion in the Channel Hillslope Integrated Landscape Development (CHILD) model is calibrated and compared to long-term evolution measurements of six gullies at the Bardenas Reales, Northeast Spain. [2]  The headcut retreat module of CHILD was calibrated by adjusting the shape factor parameter to fit the observed retreat and volumetric soil loss of one gully during a 36 year period, using reported and collected field data to parameterize the rest of the model. To test the calibrated model, estimates by CHILD were compared to observations of headcut retreat from five other neighboring gullies. The differences in volumetric soil loss rates between the simulations and observations were less than 0.05 m 3  year -1 , on average, with standard deviations smaller than 0.35 m 3  year -1 . These results are the first evaluation of the headcut retreat module implemented in CHILD with a field data set. These results also show the usefulness of the model as a tool for simulating long-term volumetric gully evolution due to plunge pool erosion.

Description: [1]  We present a two-dimensional Glacier Drainage System model (GlaDS) that couples distributed and channelized subglacial water flow. Distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness. Channelized flow occurs through Röthlisberger channels that can form on any of the edges of a prescribed, unstructured network of potential channels. Water storage is accounted for in an englacial aquifer and in moulins, which also act as point sources of water to the subglacial system. Solutions are presented for a synthetic topography designed to mimic an ice-sheet margin. For low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge, the model exhibits a channelization instability which leads to the formation of a self-organized channel system. The random orientation of the network edges allows the channel-system geometry to be relatively unbiased, in contrast to previous structured grid-based models. Under steady conditions, the model supports the classical view of the subglacial drainage system, with low pressure regions forming around the channels. Under diurnally varying input, water flows in and out of the channels, and a rather complex spatiotemporal pattern of water pressures is predicted. We explore the effects of parameter variations on the channel-system topology and mean effective pressure. The model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model. The results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings.

Description: [1]  Barchan dunes are bedforms found in many sedimentary environments with a limited supply of sediment, and may occur in isolation or in more complex dune fields. Barchans have a crescentic planform morphology with horns elongated in the downflow direction. To study flow over barchan dunes, we performed large-eddy simulations in a channel with different interdune spacings at a flow Reynolds number, Re ∞  ≃ 26000 (based on the free stream velocity and channel height). The largest interdune spacing (2.38 λ , where λ is the wavelength of the barchan dune) presents similar characteristics to a solitary dune in isolation, indicating that at this distance the sheltering effect of the upstream dune is rather weak. Barchan dunes induce two counter-rotating streamwise vortices, one along each of the horns, which direct high-momentum fluid toward the symmetry plane and low-momentum fluid near the bed away from the centerline. The flow close to the centerline plane separates at the crest, but away from the centerline plane and along the horns, flow separation occurs intermittently. The flow in the separation bubble is directed towards the horns and leaves the dune at its tips. The internal boundary layer developing on the bed downstream of the reattachment region develops similarly for various interdune spacings; the development slows down 14.5 dune heights downstream. The turbulent kinetic energy budgets show the importance of pressure transport and mean-flow advection in transferring energy from the overlying wake layer to the internal boundary layer over the stoss side. For closely-spaced dunes, the bed shear-stress is 30% larger than at the largest spacing, and instantaneous coherent high- and low-speed streaks are shorter but stronger. Coherent eddies in the separated-shear layer are generated more frequently for smaller interdune spacing, where they move farther away from the bed, towards the free surface, and remain located between the horns.

Description: [1]  This paper addresses an important question of modeling stream dynamics: how may numerical models of braided stream morphodynamics be rigorously and objectively evaluated against a real case study? Using simulations from the CAESAR Reduced-Complexity Model (RCM) of a 33 km reach of a large gravel-bed river (the Tagliamento River, Italy) this paper aims to: (i) identify a sound strategy for calibration and validation of RCMs; (ii) investigate the effectiveness of multi-performance model assessments; (iii) assess the potential of using CAESAR at meso spatial and temporal scales. The approach used has three main steps: firstly sensitivity analysis (using a screening method and a variance-based method), then calibration and finally validation. This approach allowed us to analyze 12 input factors initially and then to focus calibration only on the factors identified as most important. Sensitivity analysis and calibration were performed on a 7.5 km sub-reach, using a hydrological time series of 20 months, while validation on the whole 33 km study reach over a period of eight years (2001 - 2009). CAESAR was able to reproduce the macro morphological changes of the study reach and gave good results as for annual bedload sediment estimates which turned out to be consistent with measurements in other large gravel-bed rivers, but showed a poorer performance in reproducing the characteristics of the braided channel (e.g. braiding intensity). The approach developed in this study can be effectively applied in other similar RCM contexts, allowing the use of RCMs not only in an explorative manner but also to obtain quantitative results and scenarios.

Description: ABSTRACT [1]  Spatial patterns of soil often do not reflect those of topographic controls. We attempted to identify possible causes of this by comparing observed and simulated soil horizon depths. Observed depths of E, Bt, BC, C1 and C2 horizons in loess-derived soils in Belgium showed a weak to absent relation to terrain attributes in a sloping area. We applied the soil genesis model SoilGen2.16 onto 108 1x1 m 2 locations in a 1329 ha area to find possible causes. Two scenarios were simulated. Model 1 simulated soil development under undisturbed conditions, taking slope, aspect and loess thickness as the only sources of variations. Model 2 additionally included a stochastic submodel to generate tree uprooting events based on the exposure of trees to the wind. Outputs of both models were converted to depths of transitions between horizons, using an algorithm calibrated to horizon depths observed in the field. Model 1 showed strong correlations between terrain attributes and depths for all horizons, although surprisingly regression kriging was not able to model all variations. Model 2 showed a weak to absent correlation for the upper horizons but still a strong correlation for the deeper horizons BC, C1 and C2. For the upper horizons the spatial variation strongly resembled that of the measurements. This is a strong indication that bioturbation in the course of soil formation due to treefalls influences spatial patterns of horizon depths.