D. C. Dermisis

Investigating the Role of Clasts on the Movement of Sand in Gravel Bed Rivers

The bed morphology of mountain rivers is characterized primarily by the presence of distinguishable isolated roughness elements, such boulders or clasts. The objective of this experimental study was to provide a unique insight into the role of an array of clasts in regulating sand movement over gravel beds for low relative submergence conditions, H/dc<1, and flow depth, H, to the diameter of the clast, dc, a process that has not been studied thoroughly. To assess the role of clasts in controlling incoming sand movement, detailed flume experiments were conducted by placing 40 equally spaced clasts atop a well-packed glass bead bed for replicating the isolated roughness flow regime. The experiments were performed for moderate (∼2.50τcr* where τcr* is the critical dimensionless bed shear stress) and high (∼5.50τcr*) applied bed shear stress conditions, representative of gravel bed rivers. For comparison purposes, experiments were also repeated for nearly identical flow conditions but without the presence of ...

Fish passage over hydraulic structures in Midwestern Rivers of the USA

Abstract The main objective of this study was to examine the hydraulic performance of knickpoint‐control structures (i.e., riprap weirs, grouted‐riprap weirs, baffled and non‐baffled fish ladders) to facilitate unimpeded fish passage. The hydraulic performance of these structures, located in Midwestern streams, was evaluated by determining the mean and turbulent flow characteristics in the vicinity of the structures via detailed field tests. Non‐intrusive mean flow measurements were performed using a 3‐dimensional sidelooking Flowtracker handheld Acoustic Doppler Velocimeter (ADV) and by utilizing the newly developed Large Scale Particle Image Velocimetry (LSPIV) technique. Non‐intrusive turbulent flow measurements were performed using a field ADV with a high sampling frequency. Results from the mean flow measurements illustrated that the mean flow depth (Y) and velocity (V) atop the structures were critical for evaluating the performance of the structures. In addition, the present study suggested that the (Y, V) should be viewed in conjunction with the drainage area (DA), slope (S) of the structure, and structure type. The DA size affected (Y, V) in the streams, S affected the power of the water action, and the structure type could cause flow acceleration/deceleration. Results from the turbulent flow measurements illustrated that the flow approaching the structures became 3‐dimensonal, while the size of eddies in the baffled fish ladder were found to disorient fish in the transverse direction due to sudden constriction. In short, the results presented herein illustrated that a complete assessment of the hydraulic performance of structures should include not only the conventional permissible criteria for (Y, V) but also the turbulent flow characteristics at the locations atop the structures.

The coupling of WEPP and 3ST1D numerical models for improved estimation of runoff and sediment yield at watershed scales

One of the major problems in watershed hydrology and sedimentology is to accurately simulate the transport of water and sediment from their sources to the watershed outlet. Current numerical models have been extensively used to determine upland erosion, but their application is primarily limited to the field/hillslope scale without providing an estimation for the sediment delivery to the main channels. Along the same lines, hydrodynamic and sediment transport models of the in-stream channel processes have been developed assuming that the channel system is isolated from its surrounding hills. This lack of connectivity between the upland erosion and the in-stream channel processes introduces significant error in the water and sediment yield estimates along the channel network. The main objective of our study is to provide a modeling framework to evaluate transport of water and sediment from the fields to the main channels. To meet this objective, two numerical models will be coupled; the well established WEPP model, which is a continuous process-based upland erosion simulation model capable of accounting for the effects of crop rotation, and the one-dimensional 3ST1D model which is used to calculate flow and sediment transport within the channels. The main advantage of 3ST1D is that it can handle transcritical flows without violating the flow continuity equation, it is applicable for both cohesive and non-cohesive sediments and includes various formulas for determining the sediment transport capacity as well as incipient motion criteria. It is envisaged that the coupled model will improve water and sediment yield estimates at the watershed scale, thus making it possible to evaluate the efficiency of various erosion prevention Best Management Practices, currently being evaluated primarily at the hillslope scale.

Fish Passage Over Weirs In Midwestern Streams

The objective of this investigation is to employ the state-of-the-art large scale particle image velocimetry (LSPIV) technique for free surface flow measurements around and within the vicinity of hydraulic structures (e.g. weirs, ladders) used for fish passage in the loess region of Western Iowa, USA. Knowledge of the two dimensional (2-D) free surface velocity allows engineers and biologists to determine the hydraulic performance of the above structures as it relates to fish passage. The magnitude of the velocity will provide unique information about the level of turbulence around and within the vicinity of the structures, the approach velocity magnitude, the dissipation rate and the forces acting on juvenile and adult fish passing through the structures. The LSPIV technique is founded on similar principles with those of the particle image velocimetry (PIV). LSPIV is a powerful and efficient technique for measuring river surface velocities especially in natural -large scale- systems which are of the primary interest of this research. It is a cheap method as it needs, basically, an inexpensive video equipment (camera) and a geodetic survey to describe the region of interest (ROI). The water velocity vector magnitude and direction are calculated by dividing the estimated displacement of the particles with the time interval between two sequential images. The LSPIV technique does not require calibration and it is well-suited for measuring in very shallow flows quickly and accurately; also can be used to estimate the discharge in conjunction with bathymetry information and assumed velocity distribution over the depth. Validation of the LSPIV measurements will be attained with the use of a SonTek flow tracker.

A mechanistic model to simulate rill erosion.

The key objective of this paper is to develop a mechanistic, 1-D hydrodynamic and sediment transport model namely, RILL 1D, that handles transcritical flows over abrupt changes and predicts changes in rill bed elevation, without violating the flow continuity equation. Replication of rill conditions necessitated the use of an enhanced TVD-MacCormack scheme with implementation of Tsengs surface gradient method, to provide an oscillation free solution over formed pool crests. The performance of the model is evaluated by comparing its results with rill observations from a flume study with fixed bed geometry and two field investigations with mobile beds. It is shown that the enhanced TVD-MacCormack scheme approximates, satisfactorily, depth and velocity for a fixed bed rill with steps and pools, except near the pool headwalls where a backroller forms with negative velocities. With field data the code is tested for its ability to reproduce measured values of sediment transport rates. RILL 1D performs reasonably well in these simulations in terms of sediment prediction rates and faired adequately in terms of replicating rill bed morphology. A sensitivity analysis is performed to assess the effects of cell size and critical erosional strength in the predictive ability of the model.

Determining the 2-D Surface Velocity Field around Hydraulic Structures with the Use of a Large Scale Particle Image Velocimetry (LSPIV) Technique

The objective of this investigation is to employ the state-of-the-art large scale particle image velocimetry (LSPIV) technique for free surface flow measurements around and within the vicinity of hydraulic structures (e.g. weirs, ladders) used for fish passage in the loess region of Western Iowa, USA. Knowledge of the 2-D free surface velocity allows engineers and biologists to determine the hydraulic performance of the above structures as it relates to fish passage. The magnitude of the velocity will provide unique information about the level of turbulence around and within the vicinity of the structures, the approach velocity magnitude, the dissipation rate and the forces acting on juvenile and adult fish passing through the structures. The LSPIV technique is founded on similar principles with those of the particle image velocimetry (PIV). LSPIV is a powerful and efficient technique for measuring river surface velocities especially in natural -large scale- systems which are of the primary interest of this research. It is a cheap method as it needs, basically, an inexpensive video equipment (camera) and a geodetic survey to describe the region of interest (ROI). The water velocity vector magnitude and direction are calculated by dividing the estimated displacement of the particles with the time interval between two sequential images. The LSPIV technique does not require calibration and it is well-suited for measuring in very shallow flows quickly and accurately; also can be used to estimate the discharge in conjunction with bathymetry information and assumed velocity distribution over the depth. Validation of the LSPIV measurements will be attained with the use of a SonTek flow tracker.