Showing papers on "Stream power published in 2018"

Rill erosion processes on a steep colluvial deposit slope under heavy rainfall in flume experiments with artificial rain

Abstract: Understanding rill erosion processes is important in the prediction of soil erosion and the prevention of soil loss. However, limited information is available concerning the impacts of rainfall on rill erosion on steep slopes. Colluvial deposits with steep slopes make up the packed material underlying the collapsing walls in benggang, which collapse due to hydraulic pressure and gravity. They contain loose materials and large amounts of coarse particles. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on the rill erosion process, rill development and rill flow dynamic mechanisms on the steep slopes of colluvial deposits. The colluvial soils were subjected to simulated rainfall in a 5-m2 (5-m by 1-m) flume at heavy rainfall intensities (100, 120, and 140 mm h−1) and on five steep slopes (20, 25, 30, 35, and 40°). Rill erosion contributed significantly to colluvial slope erosion; on average, rills accounted for 61% of the soil loss, and the effects of slope gradient were greater than those of the rainfall intensity. After rill development, rill density, rill length, width, and depth all significantly increased. Correspondingly, the soil loss rate sharply raised and irregularly fluctuated. Moreover, the collapse of rill heads or sidewalls tended to increase the relative contribution to rill erosion and rill development. The rill flow was characterized by transitional and subcritical flow regimes. The rill flow velocity was the most sensitive hydraulic parameter, and the unit stream power provided the optimal hydrodynamic parameter to characterize the dynamic mechanisms of rill erosion on colluvial deposits. The collapse of rill heads or sidewalls could result in negative values for critical shear stress, critical stream power, and critical unit stream power of rill erosion, which were −19 Pa, −5.3 N m−1 s−1, and −0.09 m s−1, respectively. These results provide a better understanding of the mechanism of rill erosion on steep slopes.

How concave are river channels

Abstract: For over a century, geomorphologists have attempted to unravel information about landscape evolution, and processes that drive it, using river profiles Many studies have combined new topographic datasets with theoretical models of channel incision to infer erosion rates, identify rock types with different resistance to erosion, and detect potential regions of tectonic activity The most common metric used to analyse river profile geometry is channel steepness, or ks However, the calculation of channel steepness requires the normalisation of channel gradient by drainage area This normalisation requires a power law exponent that is referred to as the channel concavity index Despite the concavity index being crucial in determining channel steepness, it is challenging to constrain In this contribution, we compare both slope–area methods for calculating the concavity index and methods based on integrating drainage area along the length of the channel, using so-called “chi” ( χ ) analysis We present a new χ -based method which directly compares χ values of tributary nodes to those on the main stem; this method allows us to constrain the concavity index in transient landscapes without assuming a linear relationship between χ and elevation Patterns of the concavity index have been linked to the ratio of the area and slope exponents of the stream power incision model ( m∕n ); we therefore construct simple numerical models obeying detachment-limited stream power and test the different methods against simulations with imposed m and n We find that χ -based methods are better than slope–area methods at reproducing imposed m∕n ratios when our numerical landscapes are subject to either transient uplift or spatially varying uplift and fluvial erodibility We also test our methods on several real landscapes, including sites with both lithological and structural heterogeneity, to provide examples of the methods' performance and limitations These methods are made available in a new software package so that other workers can explore how the concavity index varies across diverse landscapes, with the aim to improve our understanding of the physics behind bedrock channel incision

Combined effects of simulated rainfall and overland flow on sediment and solute transport in hillslope erosion

Abstract: Soil water erosion and nutrient transport, which are strongly affected by rainfall intensity and inflow rate, often lead to severe land degradation and unexpected water eutrophication. However, little research has been conducted on the combined effects of rainfall and runoff on soil erosion. This study investigated soil erosion and solute transport in runoff under the simultaneous influence of rainfall and runoff and evaluated the relationship between soil erosion and hydraulic parameters. The rainfall simulation and inflow experiments were conducted on a slope of 10° under inflow (10, 15, and 20 L min−1) alone and combination of inflow and rainfall (60, 90, and 120 mm h−1). Flow velocity (V), Reynolds number (Re), Froude number (Fr), stream power (ω), shear stress (τ), Darcy–Weisbach friction (f), and the ratio of the Manning roughness coefficient to average flow depth (n/h) were also measured. Both rainfall intensity and inflow rate played important roles in runoff generation, soil loss, and solute transport in surface runoff. Increasing inflow rate substantially increased runoff rate and sediment concentration in runoff either with or without rainfall impact. The combination of rainfall splash and water scouring increased runoff transport, whereas did not causally increase sediment transport. Notably, the sediment concentration or cumulative sediment load under 20 L min−1 inflow rate alone was higher than those of combinations of rainfall and inflow. Most flow belonged to turbulent and supercritical flow regimes. Solute transport in runoff was also affected by both rainfall intensity and inflow rate, and bromide concentration in runoff decreased as a power function of duration on the red soil slope. Both the Reynolds number and the average flow velocity were good indicators of unit sediment load and solute transport in runoff. In a multiple regression model, the Reynolds number was found to be the best predictor of unit sediment load with an exponential relationship. Of the studied hydraulic parameters, average flow velocity was found a better parameter than the Reynolds number when estimating solute transport in runoff under combination of rainfall and inflow.

Outburst floods provide erodability estimates consistent with long-term landscape evolution

Abstract: Most current models for the landscape evolution over geological timescales are based on semi-empirical laws that consider riverbed incision proportional to rock erodability (dependent on lithology) and to the work performed by water flow (stream power). However, the erodability values obtained from these models are entangled with poorly known conditions of past climate and streamflow. Here we use the erosion reported for 82 outburst floods triggered by overtopping lakes as a way to estimate the outlet erodability. This avoids the common assumptions regarding past hydrology because water discharge from overtopping floods is often well constrained from geomorphological evidence along the spillway. This novel methodology yields values of erodability that show a quantitative relation to lithology similar to previous river erosion analyses, expanding the range of hydrological and temporal scales of fluvial incision models and suggesting some consistency between the mathematical formulations of long-term and catastrophic erosional mechanisms. Our results also clarify conditions leading to the runaway erosion responsible for outburst floods triggered by overtopping lakes.

Evaluation of shear stress and unit stream power to determine the sediment transport capacity of loess materials on different slopes

Abstract: This study aims to evaluate the relationship between loess soil-based sediment transport capacity and the most well-known and extensively used shear stress and unit stream power for different steep slopes. This study also determined the suitability of shear stress- and unit stream power-based transport capacity functions for rill flow on non-erodible bed. Loess soil was collected from Ansai County, which is located in a typical loessial region in China’s Loess Plateau. The median diameter of the loess soil was 0.04 mm. The experiment was conducted in a rill flume with a soil-feeding hopper. The slope gradients in this study ranged from 10.51 to 38.39%, and the flow discharges per unit width varied from 1.11 × 10−3 to 3.78 × 10−3 m2 s−1. The sediment transport capacity was measured for each combination. Results showed that Tc can be effectively described by the power function shear stress-based equations for various slope gradients with R 2 > 0.94 and P < 0.01. Shear stress was a good predictor of Tc for different slope gradients with the Nash–Sutcliffe model efficiency (NSE) from 0.94 to 0.99. Moreover, shear stress was better in predicting Tc when the slope gradient was above 21.26%. Tc can be efficiently described by the power function unit stream power-based equations for various slope gradients with R 2 > 0.95 and P < 0.01. Unit stream power was a good predictor of Tc for different slope gradients with NSE that ranged from 0.95 to 0.99. The unit stream power predicted Tc better when the slope gradient was above 26.79%. Unit stream power was more satisfied than shear stress for predicting Tc under different slope gradients. The unit stream power-based LISEM, which was multiplied by 0.62 (i.e., the correction coefficient), predicted well the sediment transport capacity of the rill flow in our experiment, where NSE = 0.93. The shear stress-based Zhang model, which was multiplied by the correction coefficient of 0.77, adequately predicted the sediment transport capacity of rill flow in our experiment, where NSE = 0.81. By performing the controlled rill flume experiments, this study showed that shear stress and unit stream power strongly influenced Tc for certain slope gradients under non-erodible conditions.

Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland)

Abstract: . On 24 July 2014, an exceptionally large flood (recurrence interval ca. 150 years) caused large-scale inundations, severe overbank sedimentation, and damage to infrastructure and buildings along the Emme River (central Switzerland). Widespread lateral bank erosion occurred along the river, thereby entraining sediment and large wood (LW) from alluvial forest stands. This work analyzes the catchment response to the flood in terms of channel widening and LW recruitment and deposition, but also identifies the factors controlling these processes. We found that hydraulic forces (e.g., stream power index) or geomorphic variables (e.g., channel width, gradient, valley confinement), if considered alone, are not sufficient to explain the flood response. Instead, the spatial variability of channel widening was first driven by precipitation and secondly by geomorphic variables (e.g., channel width, gradient, confinement, and forest length). LW recruitment was mainly caused by channel widening (lateral bank erosion) and thus indirectly driven by precipitation. In contrast, LW deposition was controlled by channel morphology (mainly channel gradient and width). However, we also observed that extending the analysis to the whole upper catchment of the Emme River by including all the tributaries and not only to the most affected zones resulted in a different set of significant explanatory or correlated variables. Our findings highlight the need to continue documenting and analyzing channel widening after floods at different locations and scales for a better process understanding. The identification of controlling factors can also contribute to the identification of critical reaches, which in turn is crucial for the forecasting and design of sound river basin management strategies.

Parsimonious sediment transport equations based on Bagnold's stream power approach

Abstract: It is increasingly recognized that effective river management requires a catchment scale approach. Sediment transport processes are relevant to a number of river functions but quantifying sediment fluxes at network scales is hampered by the difficulty of measuring the variables required for most sediment transport equations (e.g. shear stress, velocity, and flow depth). We develop new bedload and total load sediment transport equations based on specific stream power. These equations use data that are relatively easy to collect or estimate throughout stream networks using remote sensing and other available data: slope, discharge, channel width, and grain size. The new equations are parsimonious yet have similar accuracy to other, more established, alternatives. We further confirm previous findings that the dimensionless critical specific stream power for incipient particle motion is generally consistent across datasets, and that the uncertainty in this parameter has only a minor impact on calculated sediment transport rates. Finally, we test the new bedload transport equation by applying it in a simple channel incision model. Our model results are in close agreement to flume observations and can predict incision rates better than a more complicated morphodynamic model. These new sediment transport equations are well suited for use at stream network scales, allowing quantification of this important process for river management applications.

Identifying sediment transport capacity of raindrop-impacted overland flow within transport-limited system of interrill erosion processes on steep loess hillslopes of China

Abstract: Interrill erosion processes typically involve such scientific issues as detachment-limited and transport-limited erosion behaviour. An accurate estimation of the sediment transport capacity (Tc) by raindrop-impacted overland flow is critical for interrill erosion modelling and for evaluating sediment budgets under erosion-limiting conditions. Simulated rainfall experiments with rainfall intensities from 0.8 to 2.5 mm min−1 over a three-area soil pan with slope gradients from 12.7% to 46.6% were conducted to identify the transport-limited cases and determine Tc by raindrop-impacted overland flow within the transport-limited systems of interrill erosion processes. Results indicated that Tc increased as a power function of rainfall intensity and slope gradient (R2 = 0.84, NSE = 0.75), and Tc was more sensitive to rainfall intensity than to slope gradient. In terms of R2 and NSE, stream power was the key hydraulic parameter that influenced Tc among flow velocity (R2 = 0.64, NSE = 0.39), shear stress (R2 = 0.53, NSE = 0.23), stream power (R2 = 0.76, NSE = 0.52) and unit stream power (R2 = 0.49, NSE = 0.16). The addition of rainfall physical parameters in response equations of Tc in addition to hydraulic parameter, could improve an accuracy of Tc modelling. Stream power combined with rainfall kinetic energy can best describe the Tc of raindrop-impacted overland flow within the transport-limited system of interrill erosion processes by a power-exponent function (R2 = 0.90, NSE = 0.72). Rainfall kinetic energy can reduce the Darcy-Weisbach resistance coefficient of raindrop-impacted overland flow and thus benefit sediment transporting. This study provides another method for directly identifying the Tc of raindrop-impacted overland flow in interrill erosion processes on steep loess slopes, and points out that rainfall impacts should be particularly considered when studying Tc by raindrop-impacted overland flow.

Longitudinal variability of geomorphic response to floods

Abstract: Morphodynamic response of channels and floodplains to flooding reflects interactions of erosive and resistive forces with sediment transport capacity and supply at multiple scales. Monotonic relationships between reach-scale response to floods with independent variables such as flood stream power and channel confinement can be confounded by longitudinal variations in these variables at longer scales. In these cases, channel response depends on both local and upstream drivers. Using high resolution preand post-flood digital elevation models, we calculate reach-scale (0.5 to 1 km) and segment scale (10 km) longitudinal variations in channel widening and sediment balance. We relate these responses to longitudinal variations of unit stream power and channel confinement for selected streams impacted by the 2013 Colorado Front Range regional flood event. These streams transition from steep and relatively confined in the canyons of the foothills to less steep and unconfined on the high plains. The channel widening response is more closely linked with reach scale gradients in unit stream power: abrupt widening typically occurred within reaches where a large drop in unit stream power occurred relative to upstream. Sediment balance followed segment scale trends in unit stream power, exhibiting a net erosional trend within the foothills that switches to a net depositional trend within the transition to the plains. These findings indicate that unit stream power gradients mediate channel response at reach to segment scales. Predictive modeling of stream response to floods and fluvial hazards assessments that only consider absolute values of reach-scale stream power may under-estimate fluvial hazards in some settings by ignoring unit stream power gradients. © 2018 John Wiley & Sons, Ltd.

Characterization of channel planform features and sinuosity indices in parts of Yamuna River flood plain using remote sensing and GIS techniques

Abstract: River planform features and sinuosity are widely acknowledged as important geomorphological indices that control the channel hydraulics and stream power which determines the flow velocity and sediment supply to downstream reaches. Despite their significance, there has been little study about these indices in mighty Yamuna River for understanding the topographic control and hydraulic regime. This paper investigates the channel planform features and sinuosity by employing topographic map, digital elevation model (DEM), and satellite imageries to elucidate the morphological, sedimentological, and hydrological characteristics of the channel. Based on sinuosity index (varies between 1.02 and 1.41), the river segments are categorized into straight (segments 1, 3, 4, 5, 6), sinuous (segments 2, 7, 8, 9, and 11), and meandering (segment 10). The parameters analyzed to find out the influencing factors on sinuosity indicate geomorphological and anthropologic control in their development. Tectonic control of sinuosity in the studied stretch is ruled out as it is flowing through an alluvial plain with low rate of discharge.

A simulation of rill bed incision processes in upland concentrated flows

Abstract: Quantifying rill bed incision provides fundamental information for process-based erosion modeling; while the morphodynamic and hydrodynamic mechanism in bed incision processes are still unclear. Thus, experiments were conducted to examine rill bed incision processes in upland concentrated flows. DEMs (2 mm × 2 mm resolution) obtained by photogrammetry were used for rill bed morphology analysis. Rill channel (2.0 m-long, 0.08 m-wide and 0.15 m-deep) with two slope gradients (15° and 20°) were subjected to four overland flow rates (1.0, 2.0, 3.0 and 4.0 L min−1). The results showed that sediment delivery, rill bed incision rate and average rill depth increased with inflow rate and bed slope. Sediment delivery increased from 0.060 to 0.226 kg min−1 per 1 L min−1 inflow increment and from 0.043 to 0.207 kg min−1 when bed slope increased from 15° to 20°. In a well-developed rill channel, rill bed incision could be divided into three phases: pre-headcut formation (dominated by rill flow shear stress), headcut incision (dominated by headcut advancing) and post-headcut incision (dominated by rill flow shear stress). Headcut incision phase, which only accounted for 65% of rill bed sediment. In the pre-headcut formation phase, rill flow velocity, shear stress and stream power increased with increases of inflow rate and slope gradient. Conversely, flow velocity showed no evident trend with increased inflow rate and bed slope during headcut incision phase. Initial headcut advancing rate could be predicted by a non-linear function based upon soil characteristics, rill flow shear stress and headcut height. Sediment delivery showed a power function with the product of inflow rate and squared bed slope. Because rill bed incision is dominated by headcut advancement and incision, practices for controlling headcut initiation should be implemented to decrease hillslope soil loss.

Flow Resistance Interactions on Hillslopes With Heterogeneous Attributes: Effects on Runoff Hydrograph Characteristics

Abstract: An improved modeling framework for capturing the effects of space and time‐variant resistance to overland flow is developed for intensively managed landscapes. The framework builds on the WEPP model but it removes the limitations of the "equivalent" plane and time‐invariant roughness assumption. The enhanced model therefore accounts for spatiotemporal changes in flow resistance along a hillslope due to changes in roughness, in profile curvature, and downslope variability. The model is used to quantify the degree of influence—from individual soil grains to aggregates, “isolated roughness elements,” and vegetation—on overland flow characteristics under different storm magnitudes, downslope gradients, and profile curvatures. It was found that the net effects of land use change from vegetation to a bare surface resulted in hydrograph peaks that were up to 133% larger. Changes in hillslope profile curvature instead resulted in peak runoff rate changes that were only up to 16%. The stream power concept is utilized to develop a taxonomy that relates the influence of grains, isolated roughness elements, and vegetation, on overland flow under different storm magnitudes and hillslope gradients. Critical storm magnitudes and hillslope gradients were found beyond which the effects of these landscape attributes on the peak stream power were negligible. The results also highlight weaknesses of the space/time‐invariant flow resistance assumption and demonstrate that assumptions on landscape terrain characteristics exert a strong control both on the shape and magnitude of hydrographs, with deviations reaching 65% in the peak runoff when space/time‐variant resistance effects are ignored in some cases.

Morphometric Parameters and Neotectonics of Kalyani River Basin, Ganga Plain: A Remote Sensing and GIS Approach

Abstract: The drainage basin of the Kalyani river, a tributary of Gomati river has been mapped and delineated using Survey of India toposheets (1:50,000 scale) and remote sensing satellite data. The digitization, slope map preparation and statistical calculations have been carried out with the help of geographical information system (Arc GIS 10). Kalyani a fifth order river exhibits meandering behavior having 2.45 sinuosity index (SI). The Kalyani river basin has about 1235 km2area with NW-SE sloping trend. The total number of first, second, third, and fourth order streams are 373, 71, 12 and 2 respectively, showing dominance of first order streams in the basin. The mean bifurcation ratio (Rb) of the entire basin is 4.8, which indicates that the drainage is not much influenced by geological structures and exhibits dendritic drainage pattern. Relief ratio (Rr) indicates low to medium surface run-off, and low stream power for erosion. The analysis of river bank height ‘r’ (escarpment) and longitudinal profile of the river closely reveals neotectonic activity at some locations in the basin. To prepare a comprehensive watershed development and management plan, it is important to understand the topography and drainage characteristics of the region.

Four-topic correlation between flood dendrogeomorphological evidence and hydraulic parameters (the Portainé stream, Iberian Peninsula)

Abstract: Torrential floods are hazardous hydrological phenomena that produce significant economic damage worldwide. Flood reconstruction is still problematic in ungauged mountainous areas due to the lack of systematic data, so indirect techniques are required. This paper presents an integrated palaeoflood study of a Pyrenean stream that combines fluvio-torrential geomorphology, dendrogeomorphology, palaeoflood discharge estimation and flow hydraulics. The use of a total station and airborne LiDAR data allows detailed topography for geomorphological mapping and running a one-dimensional hydraulic model. Based on the height of scars on several damaged trees, we obtained palaeodischarges of 316 m3 s− 1 and 314 m3 s− 1 for the 2008 and 2010 floods. The hydraulic parameters were related to the geomorphic position of trees, showing a positive relation between most energetic geomorphic elements and both flow depth and velocity values. The most affected trees are located in intermediate energy geomorphic positions. Analysing variation in scar height and flow stage differences, we suggest that most reliable trees for peak discharge estimation correspond to those in areas related with fluvio-torrential processes of intermediate energy. This multidisciplinary palaeohydrological study relates flood hydrodynamics with the damage to trees and their geomorphological characteristics, focusing on the hydraulic parameters of the peak flow (depth, velocity and unit stream power), which has never been performed before. The proposed approach shows strong potential for palaeoflood analysis in ungauged mountain catchments with scarce non-systematic data.

Deriving and evaluating hydraulics and detachment models of rill erosion for some calcareous soils

Abstract: Rill erosion plays an important role in the amount of soil detachment and the transport sediment load on hillslope. Quantification of the soil erosion requires a more precise understanding of the processes and development of rill erosion models. The objective of this study was to derive and evaluate hydraulics and detachment models of rill erosion in calcareous soils of northwestern Iran. Rill erosion experiments were carried out at 55 locations with three replications under field conditions. At each point, the rill plots were created with a 0.2 m width and 4 m length on agricultural soils. The inflow rates were 4, 12, 20 and 30 l min−1 with varying slope from 4 to 25.5%. The results indicated that all conditions of flow regimes including sub- and super-critical (laminar and turbulent) were observed in created rills by overland flow. The mean flow velocity and rill depth have been described well by flow rate and slope gradient, while rill width and flow depth have been explained well by flow rate. The prediction detachment rate by rill flow based on stream power model by non-linear regression yielded the best results (R2 = 0.545 and RMSE = 0.00213 kg m−2 s−1) for all combinations of slope classes. However, there are no significant differences between prediction accuracy of linear and non-linear models, when individual slope classes were considered.

Sediment selective behaviors of colluvial deposit materials on steep slopes and under heavy rainfall conditions in southeastern China

Abstract: Characterizing sediment selectivity during transport will improve understanding of soil erosion processes, but few studies have focused on steep slopes and soil with loose, coarse material. This study examined colluvial deposits, which contain loose materials with large amounts of coarse particles. These soils were subjected to simulated rainfall in a 5 m2 (5 m by 1 m) box at heavy rainfall intensities (1, 1.33, 1.67, 2, and 2.33 mm min−1) and on five steep slopes (20°, 25°, 30°, 35°, and 40°). During the experiments, the particle size of the eroded material gradually coarsened, then stabilized and often showed short-term fluctuations as the rainfall progressed. During interrill erosion processes, the sediment became enriched with finer particles because of the low runoff energy of interrill flow; the dominant transport mechanism was the suspension-saltation transport of sediment with a grain size finer than 0.537 mm. However, the sediment was coarser after rill development than before, and increased stream power meant that bed-load transport by rolling of the larger sediment grains (coarser than 0.957 mm) became an increasingly important mechanism; bed-load transport also increased with rainfall intensity, so the particle-size distribution for high-intensity rainfall resembled that of the original soil. Finally, during rill erosion processes, rill bank collapse occured and the original soil contained coarse, heavy particles that could more easily be rolled on steep slopes. These factors could increase the relative importance of bed-load transport. The results of this study have important implications for assessment and modeling of erosion processes in disturbed soils with large amounts of coarse particles.

Laboratory studies on bedload transport under unsteady flow conditions

Abstract: Abstract Two sets of triangular hydrographs were generated in a 12-m-long laboratory flume for two sets of initial bed conditions: intact and water-worked gravel bed. Flowrate ranging from 0.0013 m3 s-1 to 0.0456 m3 s-1, water level ranging from 0.02 m to 0.11 m, and cumulative mass of transported sediment ranging from 4.5 kg to 14.2 kg were measured. Then, bedload transport rate, water surface slope, bed shear stress, and stream power were evaluated. The results indicated the impact of initial bed conditions and flow unsteadiness on bedload transport rate and total sediment yield. Difference in ratio between the amount of supplied sediment and total sediment yield for tests with different initial conditions was observed. Bedload rate, bed shear stress, and stream power demonstrated clock-wise hysteretic relation with flowrate. The study revealed practical aspects of experimental design, performance, and data analysis. Water surface slope evaluation based on spatial water depth data was discussed. It was shown that for certain conditions stream power was more adequate for the analysis of sediment transport dynamics than the bed shear stress. The relations between bedload transport dynamics, and flow and sediment parameters obtained by dimensional and multiple regression analysis were presented.

A Potential Link Between Waterfall Recession Rate and Bedrock Channel Concavity

Abstract: The incision of bedrock channels is typically modeled through the stream power or the shear stress applied on the channel bed. However, this approach is not valid for quasi-vertical knickpoints (hereafter waterfalls), where water and sediments do not apply direct force on the vertical face and waterfall retreat rate is often modeled as a power function of drainage area. These different incision modes are associated with two measurable exponents: the channel concavity, $\theta$, that is measured from the channel topography and is used to evaluate the exponents of drainage area and slope in the channel incision model, and $p$, that is measured from the location of waterfalls within watersheds, and evaluates the dependency of the waterfall recession rate on drainage area. To better understand the relations between channel incision and waterfall recession we systematically compare between the exponents $p$ and $\theta$. These parameters were computed from digital elevation models (30 m SRTM) of 12 river basins with easily detectable waterfalls. We show that $p$ and $\theta$ are: (1) similar within uncertainty, (2) come from a similar distribution, and (3) covary for networks with a large number of waterfalls (>9). In the context of bedrock incision models this hints that the same processes govern waterfall retreat rate and the incision of non-vertical channel reaches in the analyzed basins, and/or that downstream incision can dictate waterfall retreat rate.

Tidal stream energy resource assessment in the Qiantang River Estuary, China

Abstract: A tidal stream energy resource assessment can be achieved through direct measurements of tidal elevations or flow velocities, theoretical formulas, and numerical models. This paper first described the development of renewable energy in China. Then, the tidal stream energy resource in the Qiantang River Estuary was assessed. The present work established a 2D numerical model for calculating the power density in the estuary and validated it with in situ water level, tidal current, and suspended sediment concentration measurements. Three possible sites found near the Hangzhou Bay Bridge were selected for detailed resource assessment. The results indicated that if only 50 turbines are deployed to each site, the tidal stream energy output by all turbines would be 7.5 GWh/year. Finally, building a tidal stream power generation device into an existing bridge structure was proposed at the Qiantang River Estuary.

The fate of suspended sediment and particulate organic carbon in transit through the channels of a river catchment

Abstract: Particulate organic matter (POM) transiting through rivers could be lost to overbank storage, stored in‐channel, added to by erosion or autochthonous production, or turned over to release greenhouse gases to the atmosphere (either while in the water column or while stored in the channel). In the UK, a net loss of POM across catchments has been recorded, and the aim here was to investigate the balances of processes acting on the POM. This study considered records of suspended sediment and POM flux in comparison to stream flow, velocity, stream power, and residence time for the River Trent (English Midlands, 8,231 km2). We show that for the lower two thirds (106 km) of the River Trent, 2% is lost to overbank storage; 10% is lost to the atmosphere in the water column; and 31% is turned over while in temporary storage. Permanent in‐channel storage is negligible, and for the lower course of the river, material stored in‐channel will have a residence time of the order of hundreds of days between the last flood hydrograph of one winter and the first winter storm of the next winter (usually in the same calendar year).When considered at the scale of the UK, 1% POM in transit would be lost to overbank sedimentation; 5% turned over in the water column, and 14% turned over while in temporary storage. In the upper third of the study river channel, there is insufficient stream power to transport sediment and so in‐channel storage or in‐channel turnover over to the atmosphere dominate. The in‐channel processes of the River Trent do not conform to that expected for river channels as the headwaters are not eroding or transporting sediment. Therefore, the source of sediment must be lower down the channel network.