<p>Soil erosion due to rainfall and overland flow can be detrimental to agricultural management and long-term agricultural sustainability. Although numerous conservation measures and planning strategies have greatly reduced the amount of sediment moving within the landscape, there are still unresolved questions concerning initiation of particle motion, susceptibility to erosion, total soil loss, sediment transport and general measurement theory. Within agricultural fields, ephemeral erosion is particularly harmful because these sources can accelerate sediment transport, often yield more sediment than interrill sources and are more challenging to mitigate. In this study, terrain data were collected by aerial photogrammetry using an unmanned aerial system (UAS) following planting and approximately one month later, while climate variables during the period were collected using NexRad radar. Imagery was captured within seven agricultural fields (six in Iowa and one in Minnesota), ranging in size from 0.6 to 3.6 hectare (1.6 to 8.8 acre). Considering the small scale in topographic variation between two surveys, extreme efforts were applied to image processing and geospatial registration. Advanced models for camera calibration utilizing Micmac open-source photogrammetry software package were used to account for complex distortion patterns in the raw image data set. The undistorted images were then processed using Agisoft Photoscan for camera alignment, model georeferencing and dense point cloud generation (millions to billions of points per survey), from which digital elevation models (DEMs; 10 to 57 million cells) were produced. A physically-based finite element hydrodynamic and sediment transport model (CCHE2D, developed at the National Center for Computational Hydroscience and Engineering) was applied to simulate hydrological (runoff), sediment detachment (raindrop splash, sheet flow, and concentrated flow erosion) and sediment transport/deposition landscape evolution processes. Simulated geomorphological and sediment budget results over time were compared to field observations for model input parameter adjustment and consequently quantification of estimates. Integration of high-resolution spatial and temporal topographic measurements with physically-based numerical models support the development and validation of dynamic landscape evolution models needed for accurate prediction and quantification of gully initiation, evolution and impact on total soil loss and effective conservation management planning.</p>
To understand more fully the fluid and sediment dynamics of upper stage plane beds, laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed‐elevation fluctuations on mobile upper stage plane beds reveal millimeter‐high bed waves. Vertical profiles of flow velocity, mixing length, and eddy viscosity (diffusivity) are represented well by the law of the wall. For the mobile bed, von Kármán/s κ ≈ 0.33 and equivalent sand roughness to mean bed‐grain size varies from 9 to 17 because of the presence of bed load and low‐relief bed waves. For fixed beds with no sediment transport, κ ≈ 0.41 and equivalent sand roughness is equal to the mean bedgrain size. The decrease in κ for mobile beds is related to the relative motion of grains and fluid. Mobile‐bed turbulence intensities are greater than those for sediment‐free fixed beds because of enhanced wake formation from the lee side of near‐bed grains and low‐relief bed waves. Sediment diffusivities ε s calculated in a similar way to fluid diffusivities ε indicate that ε s ≈ε. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to ε in near‐bed regions (y/d < 0.3) but are larger than ε higher in the flow, suggesting that suspended‐sediment concentration higher in the flow is not closely related to mean fluid turbulence. Sediment diffusivities calculated for high‐magnitude ejection events are comparable to those calculated using the diffusion‐settling balance for y/d > 0.3, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.
Soil erosion and soil degradation are critical concerns worldwide. Developing comprehensive techniques to assess soil erodibility is in direct response to the need for quantifying this characteristic. Soil erosion rate is commonly estimated by the excess shear stress, the amount of shear stress acting on a soil surface in excess of critical shear stress (τc), and conditioned by the erodibility coefficient (Kd). The submerged jet erosion test (JET) is a widely used technique to measure the excess shear stress parameters based upon the impinging jet theory. Recent studies, however, have shown that the JET result is highly dependent on test conditions and jet hydrodynamics. Therefore, this study seeks to investigate the effects of changing test conditions on soil erodibility. We designed a series of experiments using one type of soil and intended to record the resulting scour depth for a narrow range of impinging heights and nozzle velocities. The main objective of the study is to improve the JET methodology and to quantify the effect of variable test conditions. The scour depth was quantified using point gauge reading and photogrammetry technique. Data collected at set time intervals for a maximum of seven hours from the beginning of the JET test. The results demonstrate that current JET methods should be revisited to make them consistent and independent of the test conditions.
The author is grateful to the discussers for their interesting comments about the contribution of dead load to the buckling of arches and in particular for their bringing to his attention the experimental work of Lind and Deutsch and their own analytical
Abstract The time‐temperature dependence of the cohesive fracture energy is deduced from experiments on a centrally cracked sheet of butadiene‐acrylonitrile‐acrylic acid viscoelastic terpolymer crosslinked with an epoxy curing agent. Analytic results based upon a cylindrical flaw model of the crack permit the segregation of the fracture energy time dependence from that of the relaxation modulus.
Fish passage design could greatly benefit from knowledge of the movement and path choices of fish in complex, turbulent environments, but empirical information required to construct and validate agent-based models is generally not available. To address this issue, an image-based data analysis scheme is proposed to measure turbulent flow and fish motion simultaneously using a small keystone species to exemplify the procedures. Using the same video images collected in a laboratory channel, large-scale particle tracking velocimetry is employed to compute the instantaneous Eulerian flow field, and a fish detection model and background-foreground subtraction method are used to identify fish positions. The fish detection model uses deep learning with an architecture based on a faster regional convolutional neural network (faster R-CNN). This methodology and architecture are shown to have a success rate greater than 93% in identifying fish positions. Data are then presented to illustrate the utility of this technique as it relates to agent-based models for fish movement. These results show that when velocities surrounding a swimming fish are spatially averaged, the values derived from time-averaged velocities are markedly different when compared to those derived from instantaneous velocities, as evidenced by relatively large root-mean-square errors. A swimming fatigue analysis also shows that, in general, the fish would swim for relatively short durations when experiencing fatigue in contrast to swim trial data. The experimental and data analysis methods proposed here offer a new approach to examine fish-flow interactions in complex hydrodynamic environments and provide a potentially important tool for assessing and evaluating fish passage design and river restoration projects.
Ephemeral gully erosion can cause severe soil degradation and contribute significantly to total soil losses in agricultural areas. Physically based prediction technology is necessary to assess the magnitude of these phenomena so that appropriate conservation measures can be implemented, but such technology currently does not exist. To address this issue, a conceptual and numerical framework is presented in which ephemeral gully development, growth, and associated soil losses are simulated within the Annualized Agricultural Non-Point Source (AnnAGNPS) model. This approach incorporates analytic formulations for plunge pool erosion and headcut retreat within single or multiple storm events in unsteady, spatially varied flow at the sub-cell scale, and addresses five soil particle-size classes to predict gully evolution, transport-capacity and transport-limited flows, gully widening, and gully reactivation. Single-event and continuous simulations demonstrate the model's utility for predicting both the initial development of an ephemeral gully and its evolution over multiple runoff events. The model is shown to recreate reasonably well the dimensions of observed ephemeral gullies in Mississippi. The inclusion of ephemeral gully erosion within AnnAGNPS will greatly enhance the model's predictive capabilities and further assist practitioners in the management of agricultural watersheds.
