Showing papers on "Stream power published in 2016"

A theoretical assessment of microplastic transport in river catchments and their retention by soils and river sediments

Abstract: The presence of microplastics (MPs) in the environment is a problem of growing concern. While research has focused on MP occurrence and impacts in the marine environment, very little is known about their release on land, storage in soils and sediments and transport by run-off and rivers. This study describes a first theoretical assessment of these processes. A mathematical model of catchment hydrology, soil erosion and sediment budgets was upgraded to enable description of MP fate. The Thames River in the UK was used as a case study. A general lack of data on MP emissions to soils and rivers and the mass of MPs in agricultural soils, limits the present work to serve as a purely theoretical, nevertheless rigorous, assessment that can be used to guide future monitoring and impact evaluations. The fundamental assumption on which modelling is based is that the same physical controls on soil erosion and natural sediment transport (for which model calibration and validation are possible), also control MP transport and storage. Depending on sub-catchment soil characteristics and precipitation patterns, approximately 16-38% of the heavier-than-water MPs hypothetically added to soils (e.g. through routine applications of sewage sludge) are predicted to be stored locally. In the stream, MPs < 0.2 mm are generally not retained, regardless of their density. Larger MPs with densities marginally higher than water can instead be retained in the sediment. It is, however, anticipated that high flow periods can remobilize this pool. Sediments of river sections experiencing low stream power are likely hotspots for deposition of MPs. Exposure and impact assessments should prioritize these environments.

Impacts of rainfall intensity and slope gradient on rill erosion processes at loessial hillslope.

Abstract: Rill erosion constitutes one of the mechanisms of soil loss by water on agricultural land. However, studies on hillslope rill erosion characteristics and its intrinsic mechanisms are still unclear. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on hillslope rill erosion processes, rill flow hydraulic characteristics and dynamic mechanisms. A soil pan (10 m long, 1.5 m wide and 0.5 m deep and with an adjustable slope gradient of 0–30°) was subjected to rainfall simulation experiments under three rainfall intensities (50, 75 and 100 mm h −1 ) of representative erosive rainfall and three typical slope gradients (10, 15 and 20°) on the Loess Plateau of China. The results showed that rill erosion exhibited significant contributions to hillslope soil erosion, occupying 62.2–84.8% of hillslope soil loss. The equation between the rill erosion rate with rainfall intensity and slope gradient was generated, which indicated that the impacts of rainfall intensity on hillslope rill erosion were greater than those of slope gradient. For the experimental treatments, the mean headward erosion rates varied between 2.2 and 8.2 cm min −1 , and they increased with an increase in either rainfall intensity or slope gradient. Most rill flow belonged to turbulent and subcritical flow regimes. The critical shear stress, the critical stream power, and the critical unit stream power of rill occurrence were 0.986 Pa, 0.207 N m −1 s −1 , and 0.002 m s −1 , respectively. Additionally, hillslope rill erosion was sensitive to rill flow velocity and stream power. In a word, rainfall intensity and slope gradient exhibited important impacts on rill erosion processes and its hydrodynamic characteristics. Therefore, preventing rainfall erosion and weakening slope gradient effects through conservation tillage are useful for reduction of rill erosion at loessial hillslopes.

The Use of Evolutionary Trajectories to Guide ‘Moving Targets’ in the Management of River Futures

Abstract: River histories provide important guidance with which to inform river management. Evolutionary trajectories and appraisals of system responses to changing flux conditions and disturbance events can be used to determine the range of potential future states and associated behavioural regimes, assessing the likelihood that that these states will be attained over a given timeframe. In these analyses, natural or historical reference reaches may not provide a realistic basis to set target conditions for management actions, as what has gone before does not necessarily provide a complete and reliable picture of prospective future conditions. This paper outlines the use of a conceptual tool, the river evolution diagram, as a geomorphic platform to assess river history and the potential range of river futures for any given system. Evolutionary adjustments of a sand bed river in southeastern Australia are used to demonstrate the application of this approach. Applying adaptive management principles, ‘moving targets’ for river management are framed in relation to the range of likely future states and trajectories of adjustment. Copyright © 2015 John Wiley & Sons, Ltd.

River profile response to normal fault growth and linkage: an example from the Hellenic forearc of south-central Crete, Greece

Abstract: . Topography is a reflection of the tectonic and geodynamic processes that act to uplift the Earth's surface and the erosional processes that work to return it to base level. Numerous studies have shown that topography is a sensitive recorder of tectonic signals. A quasi-physical understanding of the relationship between river incision and rock uplift has made the analysis of fluvial topography a popular technique for deciphering relative, and some argue absolute, histories of rock uplift. Here we present results from a study of the fluvial topography from south-central Crete, demonstrating that river longitudinal profiles indeed record the relative history of uplift, but several other processes make it difficult to recover quantitative uplift histories. Prior research demonstrates that the south-central coastline of Crete is bound by a large ( ∼ 100 km long) E–W striking composite normal fault system. Marine terraces reveal that it is uplifting between 0.1 and 1.0 mm yr−1. These studies suggest that two normal fault systems, the offshore Ptolemy and onshore South-Central Crete faults, linked together in the recent geologic past (ca. 0.4–1 My BP). Fault mechanics predict that when adjacent faults link into a single fault the uplift rate in footwalls of the linkage zone will increase rapidly. We use this natural experiment to assess the response of river profiles to a temporal jump in uplift rate and to assess the applicability of the stream power incision model to this setting. Using river profile analysis we show that rivers in south-central Crete record the relative uplift history of fault growth and linkage as theory predicts that they should. Calibration of the commonly used stream power incision model shows that the slope exponent, n, is ∼ 0.5, contrary to most studies that find n ≥ 1. Analysis of fluvial knickpoints shows that migration distances are not proportional to upstream contributing drainage area, as predicted by the stream power incision model. Maps of the transformed stream distance variable, χ, indicate that drainage basin instability, drainage divide migration, and river capture events complicate river profile analysis in south-central Crete. Waterfalls are observed in southern Crete and appear to operate under less efficient and different incision mechanics than assumed by the stream power incision model. Drainage area exchange and waterfall formation are argued to obscure linkages between empirically derived metrics and quasi-physical descriptions of river incision, making it difficult to quantitatively interpret rock uplift histories from river profiles in this setting. Karst hydrology, break down of assumed drainage area discharge scaling, and chemical weathering might also contribute to the failure of the stream power incision model to adequately predict the behavior of the fluvial system in south-central Crete.

Large scale terrain generation from tectonic uplift and fluvial erosion

Abstract: At large scale, landscapes result from the combination of two major processes: tectonics which generate the main relief through crust uplift, and weather which accounts for erosion. This paper presents the first method in computer graphics that combines uplift and hydraulic erosion to generate visually plausible terrains. Given a user-painted uplift map, we generate a stream graph over the entire domain embedding elevation information and stream flow. Our approach relies on the stream power equation introduced in geology for hydraulic erosion. By combining crust uplift and stream power erosion we generate large realistic terrains at a low computational cost. Finally, we convert this graph into a digital elevation model by blending landform feature kernels whose parameters are derived from the information in the graph. Our method gives high-level control over the large scale dendritic structures of the resulting river networks, watersheds, and mountains ridges.

Tectonic geomorphology at small catchment sizes – extensions of the stream-power approach and the χ method

Abstract: . Quantitative tectonic geomorphology hinges on the analysis of longitudinal river profiles. The model behind almost all approaches in this field originates from an empirical relationship between channel slope and catchment size, often substantiated in the form of the stream-power model for fluvial incision. Significant methodological progress was recently achieved by introducing the χ transform. It defines a nonlinear length coordinate in such a way that the inherent curvature of river profiles due to the increase of catchment sizes in the downstream direction is removed from the analysis. However, the limitation to large catchment sizes inherited from the stream-power approach for fluvial incision persists. As a consequence, only a small fraction of all nodes of a digital elevation model (DEM) can be used for the analysis. In this study we present and discuss some empirically derived extensions of the stream power law towards small catchment sizes in order to overcome this limitation. Beyond this, we introduce a simple method for estimating the adjustable parameters in the original χ method as well as in our extended approaches. As a main result, an approach originally suggested for debris flow channels seems to be the best approximation if both large and small catchment sizes are included in the same analysis.

Role of hydrological regime and floodplain sediments in channel instability of the Bhagirathi River, Ganga-Brahmaputra Delta, India

Abstract: This research deals with the surface dynamics and key factors – hydrological regime, sediment load, and erodibility of floodplain facies – of frequent channel shifting, intensive meandering, and lateral instability of the Bhagirathi River in the western part of the Ganga-Brahmaputra Delta (GBD). At present, the floodplain of the Bhagirathi is categorized as a medium energy (specific stream power of 10–300 W m−2), non-cohesive floodplain, which exhibits a mixed-load and a meandering channel, an entrenchment ratio >2.2, width–depth ratio >12, sinuosity >1.4, and channel slope <0.02. In the study area, since 1975, four meander cutoffs have been shaped at an average rate of one in every 9–10 years. In the active meander belt and sand-silt dominated floodplains of GBD, frequent shifting of the channel and meander migration escalate severe bank erosion (e.g. 2.5 × 106 m3 of land lost between 1999 and 2004) throughout the year. Remote sensing based spatio-temporal analysis and stratigraphic analysis reve...

Modelling sediment transport capacity of rill flow for loess sediments on steep slopes

Abstract: Sediment transport is an important aspect of soil erosion, and sediment transport capacity (Tc) is a key to establishing process-based erosion models. A lot of studies exist that have determined Tc for overland flow, however, few studies have been conducted to determine Tc for loess sediments on steep slopes. Experimental data for this region are thus needed. The objectives of this study are to formulate new equations to describe Tc and evaluate the suitability of these equations for loess sediments on steep slopes. The slope gradients in this study ranged from 10.51% to 38.39%, and flow discharges per unit width varied from 1.11 × 10− 3 m2 s− 1 to 3.78 × 10− 3 m2 s− 1. Results showed that Tc increased as a power function with flow discharge and slope gradient, with R2 = 0.99 and Nash–Sutcliffe model efficiency (NSE) = 0.99. Tc was more sensitive to flow discharge than slope gradient. Tc increased as a power function with mean flow velocity, which was satisfied to predict Tc with R2 = 0.99 and NSE = 0.99. Shear stress (R2 = 0.89, NSE = 0.88) was also a good predictor of Tc, and stream power (R2 = 0.96, NSE = 0.96) was a better predictor of Tc than shear stress. However, unit stream power was not a good predictor to estimate Tc in our study, with R2 = 0.63 and NSE = 0.62. These findings offer a new approach for predicting Tc for loess sediments on steep slopes.

Inflow Rate Impact on Hillslope Erosion Processes and Flow Hydrodynamics

Abstract: Inflow water from upslope is an extremely important factor that influences downslope erosion processes. Little information is available concerning how inflow water affects downslope erosion processes in the Chinese Mollisol region, where a large amount of runoff is generated from upslope. The purpose of this study was to determine the effects of inflow rate on erosion processes and flow hydrodynamic parameters. A soil pan (10 m long, 1.5 m wide, and 0.5 m deep) was subjected to rainfall simulation and inflow experiments under one rainfall intensity (50 mm h⁻¹), two slope gradients (5 and 10°), and five inflow rates (50, 100, 150, 200, and 300 L min ⁻¹). The result showed that when upslope inflow was included, soil loss increased 12 to 1950 times compared with no upslope inflow. When rill erosion dominated, rill erosion accounted for 52 to 90% of the total soil loss as the inflow rate increased from 50 to 300 L min⁻¹. An increase in the inflow rate from 50 to 300 L min⁻¹ caused the flow velocity to increase 0.6 to 7.8 and 1.7 to 12.9 times at 5 and 10° slopes, respectively, while the Darcy–Weisbach coefficient decreased from 13.5 to 95.4%. For sheet-dominated erosion, significant linear regressions were fitted between shear stress, stream power, unit stream power, and inflow rate; but for rill-dominated erosion, power functions were established between these flow hydrodynamic parameters and the inflow rate.

A theoretical model for fluvial channel response time during time-dependent climatic and tectonic forcing and its inverse applications

Abstract: The fluvial response time dictates the duration of fluvial channel adjustment in response to changing climatic and tectonic conditions. However, when these conditions vary continuously, the channel cannot equilibrate and the response time is not well defined. Here I develop an analytical solution to the linear stream power model of fluvial incision that predicts the channel topography as a function of time-dependent climatic and tectonic conditions. From this solution, a general definition of the fluvial response time emerges: the duration over which the tectonic history needs to be known to evaluate channel topography. This new definition is used in linear inversion schemes for inferring climatic or tectonic histories from river long profiles. The analytic solution further reveals that high-frequency climatic oscillations, such as Milankovitch cycles, are not expected to leave significant fingerprints on the long profiles of fluvially incised detachment-limited rivers.

Landslide dam formation susceptibility analysis based on geomorphic features

Abstract: Global climate change has increased the frequency of abnormally high rainfall; such high rainfall events in recent years have occurred in the mountainous areas of Taiwan. This study identifies historical earthquake- and typhoon-induced landslide dam formations in Taiwan along with the geomorphic characteristics of the landslides. Two separate groups of landslides are examined which are classified as those that were dammed by river water and those that were not. Our methodology applies spatial analysis using geographic information system (GIS) and models the geomorphic features with 20 × 20 m digital terrain mapping. The Spot 6 satellite images after Typhoon Morakot were used for an interpretation of the landslide areas. The multivariate statistical analysis is also used to find which major factors contribute to the formation of a landslide dam. The objective is to identify the possible locations of landslide dams by the geomorphic features of landslide-prone slopes. The selected nine geomorphic features include landslide area, slope, aspect, length, width, elevation change, runout distance, average landslide elevation, and river width. Our four geomorphic indexes include stream power, form factor, topographic wetness, and elevation–relief ratio. The features of the 28 river-damming landslides and of the 59 non-damming landslides are used for multivariate statistical analysis by Fisher discriminant analysis and logistic regression analysis. The principal component analysis screened out eleven major geomorphic features for landslide area, slope, aspect, elevation change, length, width, runout distance, average elevation, form factor, river width, stream power, and topography wetness. Results show that the correctness by Fisher discriminant analysis was 68.0 % and was 70.8 % by logistic regression analysis. This study suggests that using logistic regression analysis as the assessment model for identifying the potential location of a landslide dam is beneficial. Landslide threshold equations applying the geomorphic features of slope angle, angle of landslide elevation change, and river width (H L/W R) to identify the potential formation of natural dams are proposed for analysis. Disaster prevention and mitigation measures are enhanced when the locations of potential landslide dams are identified; further, in order to benefit such measures, dam volume estimates responsible for breaches are key.

Streambank sediment loading rates at the watershed scale and the benefit of riparian protection.

Abstract: Streambank erosion is a pathway for sediment and nutrient loading to streams, but insufficient data exist on the magnitude of this source. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. The objective of this research was to quantify watershed-scale streambank erosion and estimate the benefits of riparian protection. The research focused on Spavinaw Creek within the Eucha-Spavinaw watershed in eastern Oklahoma, where composite streambanks consist of a small cohesive topsoil layer underlain by non-cohesive gravel. Fine sediment erosion from 2003 to 2013 was derived using aerial photography and processed in ArcMap to quantify eroded area. ArcMap was also utilized in determining the bank retreat rate at various locations in relation to the riparian vegetation buffer width. Box and whisker plots clearly showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks, statistically significant based on non-parametric t-tests. The total soil mass eroded from 2003 to 2013 was estimated at 7.27 × 107 kg yr.−1, and the average bank retreat was 2.5 m yr.−1. Many current erosion models assume that fluvial erosion is the dominant stream erosion process. Bank retreat was positively correlated with stream discharge and/or stream power, but with considerable variability, suggesting that mass wasting plays an important role in streambank erosion within this watershed. Finally, watershed monitoring programs commonly characterize erosion at only a few sites and may scale results to the entire watershed. Selection of random sites and scaling to the watershed scale greatly underestimated the actual erosion and loading rates. Copyright © 2016 John Wiley & Sons, Ltd.

Coupling slope–area analysis, integral approach and statistic tests to steady-state bedrock river profile analysis

Abstract: . Slope–area analysis and the integral approach have both been widely used in stream profile analysis. The former is better at identifying changes in concavity indices but produces stream power parameters with high uncertainties relative to the integral approach. The latter is much better for calculating channel steepness. Limited work has been done to couple the advantages of the two methods and to remedy such drawbacks. Here we show the merit of the log-transformed slope–area plot to determine changes in concavities and then to identify colluvial, bedrock and alluvial channels along river profiles. Via the integral approach, we obtain bedrock channel concavity and steepness with high precision. In addition, we run bivariant linear regression statistic tests for the two methods to examine and eliminate serially correlated residuals because they may bias both the estimated value and the precision of stream power parameters. We finally suggest that the coupled process, integrating the advantages of both slope–area analysis and the integral approach, can be a more robust and capable method for bedrock river profile analysis.

Particle selectivity of sediment deposited over grass barriers and the effect of rainfall

Abstract: Particle selectivity of the sediment deposited over vegetative barriers is of importance to predict sediment transport and particulate pollutant load into surface waters. Grassed barriers with 20%, 40%, 60%, 70% and 90% covers at 15° slope were subjected to silt-laden inflows in the presence and absence of simulated rainfalls to investigate the sediment deposition processes. The results show that re-grass of steep croplands can effectively trap eroded sediment from upslope, and the rowed grass barriers can strengthen sediment deposition. The deposition order of sediment particle sizes (μm) follows (>50)> (25-50)>(10-25)=( (2-10), and the particle selectivity weakens with increasing grass covers. Clay particles had a similar deposition efficiency to overall sediment, implying the effectiveness of re-grass in controlling soil nutrient loss. The contribution of grass to total overland flow resistance is almost equivalent to the percentage of grass cover. For steep grassed slopes, raindrop impact significantly decreases sediment deposition, but limitedly affects particle selectivity of deposited sediment and overland flow hydraulics. Both raindrop kinetic energy and stream power available for surface soil contribute to sediment deposition in net deposition areas of grass barriers. These imply that rainfall effect on sediment delivery over vegetated barriers derives from the additional raindrop energy, rather than the variation in runoff hydraulics. These results can help to clarify the effect of raindrop impact on sediment transport and to evaluate the benefit of re-vegetation in decreasing sediment yield and its particulate nutrient load into surface waters. This article is protected by copyright. All rights reserved.