David M. Admiraal

Entrainment response of bed sediment to time-varying flows

Unsteady flows are ubiquitous in nature. In order to understand the behavior of sediment when subjected to unsteady flows, a set of experiments was performed in a rectangular duct with a mobile bed. A computer-operated valve governed the velocity of the water in the duct, and the flow velocity, wall shear stress, and vertical distribution of suspended sediment were simultaneously measured. Beds composed of 120 μm and 580 μm diameter sand were investigated. Both quasi-steady flows and pulse flows were simulated in the duct. For the pulse flows the water was accelerated at a constant rate to a peak velocity and then decelerated at a constant rate to zero velocity. Phase lags were observed between the bed shear stress and the upward flux (entrainment) of sand from the bed. The phase lags were larger for tests with fine sand than for tests with coarse sand. Differences were attributed to differences in bed roughness and flow Reynolds numbers. Relations based on flow acceleration and sediment size were developed for predicting the entrainment phase lag. Large phase lags can have a considerable impact on the amount of sediment transported by boat wakes, waves, and other unsteady flows.

Vortex trajectory hysteresis above self-formed vortex ripples

Particle image velocimetry (PIV) using fluorescent tracer particles has been used to measure the velocity field of the fluid above self-formed vortex ripples in an oscillatory flow with suspended sediment. Measured velocities were used to find distributions of phase-averaged velocity and vorticity. Using the distributions, discrete vortices were then identified and tracked. Vortices that originated in the wakes of ripples were carried by the flow to adjacent ripples where they recombined with vortices being generated during the next half-cycle of the flow. Two possible states were observed for similar flow conditions, where vortex migration was either 0.9 or 1.5 ripple wavelengths before recombination. Evidence indicates that this may be the result of two possible ripple wavelengths: one that is approximately twice the flow amplitude, and one that is approximately 1.4 times the flow amplitude. The 2-D circulation strength of discrete vortices, defined as the integral of vorticity over the area of the vortex, was numerically estimated from PIV measurements. Maximum dimensionless values of circulation tend to increase as the Reynolds number of the oscillatory flow increases and to decrease as the mobility parameter (a dimensionless number that quantifies the sediment transport capacity of the oscillatory flow) increases. This is interpreted as an effect of enhanced dissipation of vorticity due to the presence of increasing concentrations of entrained sediment as the mobility number increases.

Yard waste compost as a stormwater protection treatment for construction sites.

Runoff water quality improvement from three yard waste compost erosion control treatments were compared with two conventional treatments and an untreated control on plots of 3:1 slope during two growing seasons, using natural events and simulated rainfall. Runoff volume, suspended solids, nutrients, biomass, turf shear strength, and turfgrass color scale were monitored. The most effective compost treatment, a 5-cm thick blown compost blanket, produced 12.7 times less runoff and 9.8 times less sediment load than a straw mat and silt fence treatment. The compost treatments generated eight times more biomass than the straw mat treatments. Root development was significantly better on the compost treatments based on turf shear strength measurements. Tilled-in compost was not as effective as a compost blanket at reducing sediment loss, particularly before the establishment of grass on the plot. The cost of compost treatments was similar to that of straw mat with silt fence treatments.

Energy Dissipation in Culverts by Forced Hydraulic Jump Within a Barrel

Riprap and concrete stilling basins are often built at culvert outlets to keep high-energy flows from scouring the streambed. Two simple alternatives to large basins are examined: a horizontal apron with an end weir and a drop structure with an end weir. The two designs are intended to reduce the flow energy at the outlet by inducing a hydraulic jump within the culvert barrel without the aid of tailwater. This research examines the jump geometry and the effectiveness of each jump type and proposes a design procedure for practicing engineers. The design procedure is applicable to culverts with approach Froude numbers from 2.6 to 6.0. Both designs are effective in reducing outlet velocity 0.7 to 8.5 ft/s (0.21 to 2.59 m/s), momentum 10% to 48%, and energy 6% to 71%. The design layouts allow easy access for maintenance activities.

Exploratory Study of Oscillatory Flow over a Movable Sediment Bed with Particle-Image-Velocimetry (PIV)

Sand ripples develop spontaneously in an oscillatory flow over a movable sediment bed. The mechanics of sediment suspension in the presence of ripples is complex and its direct observation is complicated unless a technique can be used to capture the flow field and particle motion above and around the ripples. This work presents the results of an exploratory study conducted in a U-tube. The goal of the study was to use Particle-Image-Velocimetry (PIV) to capture the interaction between turbulent flow structures, suspended particles, and ripples and other bed instabilities. The PIV technique was chosen because of its potential to simultaneously determine the fluid velocity and the solid-particle velocity fields. Exploratory measurements indicate that the mechanisms responsible for the resuspension of bed particles in the presence of ripples are more complicated than anticipated. As water flows past each ripple, separation takes place at the crest of the ripple. Vortical structures generated downstream of the crest entrain sediment particles into the flow. When the flow reverses direction, each structure, along with the sediment it has suspended, is carried away from the ripples and into the outer flow field. As the structure dissipates, the sediment suspended in it is released. The PIV techniques implemented in this study provide information about velocity, vorticity, and turbulence distributions, helping to explain this complex flow field.

Modeling Beaver Dam Effects on Ecohydraulics and Sedimentation in an Agricultural Watershed

Populations of North American beaver (Castor canadensis) have increased in recent decades throughout the agricultural Midwestern U.S., leading to an increase in the frequency of beaver dams in small streams. The impact of beaver dams on channel structure in this region is not known. Our field observations indicate that beaver dams are too dynamic and their affects on channel structure occur over longer time frames than is practical to study with field measurements. Modeling is therefore needed to determine if beaver dams will help stabilize and aggrade incised streams. The objective of this paper is to determine how a channel evolution model (CONCEPTS) might be used to predict the impact of beaver dams on channel structure.

ADCP Water Velocity Measurements in a Hydropower Tailwater Impoundment: A Case Study

An acoustic Doppler current profiler (ACDP) was used to characterize water velocities and flow patterns in a hydropower tailwater impoundment in western Nebraska. The impoundment, a 100 hectare L-shaped lake formed by a diversion dam located 3000 meters downstream of the hydropower dam, regulates flow into a supply canal. A trout fishery in the tailwater lake has recently experienced stresses, likely caused by some combination of low dissolved oxygen and/or toxicity (e.g., from ammonia or hydrogen sulfide). In addition, macrophytes attached to the lake bed in shallow areas may be exacerbating the stresses by restricting water flow and causing “stagnant” areas. Finally the mode of operation of the hydropower facility may contribute to the fishery stress by discharging low-DO water in the evening when macrophyte respiration is further reducing DO. The hydropower facility operation regime, in conjunction with fairly constant outflows through the supply canal, causes the water level in the tailwater lake to fluctuate more than one meter per day, resulting in complex flows within the lake. To help characterize water quality dynamics (e.g., dissolved oxygen, toxic constituents) in the tailwater lake, and to support physical and computer models of the lake, water velocities were measured throughout the tailwater lake using an ADCP. The ADCP was mounted on the bow of an outboard boat. Transects were made with the boat at numerous locations on the lake. Transects were made during both filling and draining cycles and were designed to capture the effects of the hydropower operation, the lake bathymetry, and the macrophytes on flow patterns. Measured velocities ranged from greater than 4 m/sec to less than 0.05 m/sec. Overall, the ADCP worked well for collecting the water velocity data needed for this study. Difficulties identified with the use of the ADCP in this study include loss of velocity and bottom-tracking data in areas with dense macrophytes, poor ADCP performance on transects that had drastically varying depths, and difficulty in collecting near-shore velocity data. The physical and computer models make use of the ADCP data to characterize circulation patterns in the lake and to predict concentrations of dissolved oxygen (and other constituents) under various remedial and operational scenarios for the lake.

Characterization of flow circulation in a tailwater reservoir using an Acoustic Doppler Current Profiler (ADCP) and a 1: 230 scale physical model

An Acoustic Doppler Current Profiler (ADCP) was used to measure flow circulation patterns in Lake Ogallala, a tailwater reservoir of Lake McConaughy in western Nebraska. Velocity transects were gathered at four locations in a shallow (less than 3 m deep) basin of the lake. The tests indicate that despite significant flow rates through the basin, large regions of the basin are stagnant, and do not have substantial oxygen exchange with the rest of the lake. ADCP measurements were verified with an Acoustic Doppler Velocimeter (ADV) and with a dye study of the basin. After a few seconds of time averaging, ADV and ADCP measurements were within 10 percent even for water velocities as low as 10 cm/s. The dye study confirmed that the north part of the Keystone Basin of Lake Ogallala experiences limited oxygen exchange with the bulk of the water flowing through the basin – a result that can also be inferred from ADCP measurements. Results of the ADCP tests will also be used to calibrate a computer model and a 1:230 scale physical model of Lake Ogallala that is nearing completion. Finally, all test results will be used in a multifaceted study of Lake Ogallala intended to help improve Dissolved Oxygen (DO) levels for an existing recreational trout fishery.

Atmospheric Contributions of Nitrate to Stormwater Runoff from Two Urban Watersheds

AbstractTwo independent methods were used to assess if atmospheric deposition was a major source of nitrate nitrogen (NO3-N) to stormwater in two small urban watersheds located in Lincoln, NE. Firs...

Tracking the Sources of Antibiotic Resistance Genes in an Urban Stream during Wet Weather using Shotgun Metagenomic Analyses

Stormwater runoff has been known to cause increases in bacterial loadings in urban streams. However, little is known about its impacts on antibiotic resistance genes (ARGs) in urban watersheds. This study was performed to characterize the ARG composition of various environmental compartments of an urban watershed and to quantify their contributions of microbes and ARGs to an urban stream under wet weather conditions. Shotgun metagenomic results showed that the ARG abundance in wet weather flow was significantly higher than in base flow. Multidrug resistance genes were the most common ARG type across environmental samples. Vancomycin resistance genes were abundant in embankment soil and street sweeping samples. Analyses using SourceTracker estimated storm drain outfall water to be the biggest contributor of microbes (54-57%) and ARGs (82-88%) in the urban stream during wet weather flows. Furthermore, results on street sweepings showed that wash-off from streets was the biggest known contributor of microbes (41-45%) and ARGs (92-96%) in storm drain outfall water. Pantoea and Pseudomonas were associated with the highest numbers of ARGs and were most abundant in stormwater-related samples. Results from this study can advance our knowledge about ARGs in urban streams, an important medium linking environmental ARGs to the general public.