Steven R. Abt

A Comparison of Coarse Bedload Transport Measured with Bedload Traps and Helley-Smith Samplers

Gravel bedload transport rates were measured at eight study sites in coarse-bedded Rocky Mountain streams using 4-6 bedload traps deployed across the stream width and a 76 by 76 mm opening Helley Smith sampler. Transport rates obtained from bedload traps increased steeply with flow which resulted in steep and well-defined transport rating curves with exponents of 8 to 16. Gravel transport rates measured by the Helley- Smith sampler started with much higher transport rates during low flows and increased less steeply, thus fitted bedload rating curves were less steep with exponents of 2 to 4. Transport rates measured with both samplers approached similar results near or above bankfull flow, but at 50 % of bankfull, transport rates from the bedload traps were 2-4 orders of magnitude lower than those obtained from the Helley-Smith sampler. The maximum bedload particle sizes also differed between the two samplers. They were smaller in the bedload traps than the Helley-Smith sampler at low flows, while at higher flows bedload traps collected larger particles than the Helley-Smith sampler. Differences in sampler opening size and sampling time contribute to the measured differences, but the biggest effect is likely attributable to the bedload traps being mounted on ground plates thus avoiding direct contact between the sampler and the bed and preventing involuntary particle pick up.

Predicting Peak Outflow from Breached Embankment Dams

From 1977 to 1995, a series of regression relations were developed to predict the peak discharge from breached embankment dams. In 2008, a study was conducted to review previous investigations, expand the working database of dam failures, and evaluate the appropriateness of the existing relationships. Forty-four case studies were collected and added to the existing database, resulting in a composite database of 87 case studies. The composite database was evaluated and a statistical analysis was performed using linear, curvilinear, and multiple-regression techniques. Enhanced simple- and multiple-regression expressions were developed to predict peak-breach discharge as a function of the height and/or volume of water behind the dam. A comparison of selected historical and newly derived expressions indicates that three previously developed relations remain valid for conservative peak outflow predictions. Newly developed expressions relating peak outflow as a function of the height (H) and/or volume (V) of wa...

Equilibrium Scour Downstream of Three-Dimensional Grade-Control Structures

To promote grade-control, bank stability, and fish habitat enhancement, three-dimensional structures installed within river channels are becoming increasingly popular choices for rehabilitation. A-, U-, and W-shaped rock weirs are commonly used in streams, yet three-dimensional scour holes develop downstream of the weir crest and often undermine the weir foundation. Current approaches for the prediction of scour depth and geometry are generally extrapolated from the case of two-dimensional flow. Furthermore, data from the literature regarding structures inciting significant lateral flow components are sparse. A series of laboratory experiments were conducted in which data from A-, U-, and W-shaped weirs were collected and evaluated using scour-prediction methodologies, focusing on their applicability to predict maximum scour depths. New dimensionless expressions were developed for the prediction of scour depth downstream of A-, U-, and W-shaped weirs on the basis of laboratory data that incorporate specif...

Enhanced Predictions for Peak Outflow from Breached Embankment Dams

A study was conducted to enhance the historic regression relations that predict the peak discharge from breached embankment dams. Forty-four dam breach case studies were collected and added to an existing database resulting in a composite database of 87 case studies. The composite database was evaluated and a statistical analysis performed using regression techniques. Peak outflow ( Qp ) prediction expressions from breached embankment dams were developed as a function of the height of the dam (H) , the volume of water behind the dam (V) , the embankment length (L) , the average embankment width ( Wave ) , and a combination of these variables. The multivariate regression analysis indicated that a series of expressions may be formulated relating peak outflow as a function of H⋅V⋅L and H⋅V⋅ Wave . The newly developed expressions derived from the expanded database appear to reduce the conservatism in predicting the peak discharge from a breached embankment, reduce the prediction error, and reduce the uncertai...

VERTICAL CONTRACTION SCOUR AT BRIDGES WITH WATER FLOWING UNDER PRESSURE CONDITIONS

One of the areas of bridge scour estimation technology that hydraulic engineers, researchers, and scientists generally agree needs more understanding is what happens to the bottom of the channel under a bridge when the low chord becomes inundated and the flow through the opening changes from free surface flow to pressure flow. The vast majority of what is known about local scour in bridge openings has been derived for bridges with water flowing with a free surface. The literature shows that no previously conducted research is capable of being used to predict the component of vertical contraction scour that would be encountered under a full range of sediment transport conditions. In an attempt to fill part of this gap in the field of bridge scour estimation technology, a comprehensive program was developed to investigate pressure flow scour in bridge openings. Data were collected so that a relationship could be developed to predict the scour that would occur across the channel as a result of a vertical flow contraction. An equation is presented that can be used to estimate the magnitude of vertical contraction scour that could occur at a bridge with water flowing under pressure conditions. The equation presented has a coefficient of determination of 0.89 and has readily obtainable independent variables. In addition to the equation that can be used to estimate the magnitude of vertical deck scour that may occur, techniques and guidance that could be applied by the practicing hydraulic engineer to estimate the effects of pressure flow conditions on local scour in bridge openings are also presented.

Effects of Bendway Weir Characteristics on Resulting Flow Conditions

Natural processes associated with meandering river systems have been shown to lead to degradation of channel beds, erosion on channel banks, and bend migration. Attempts at mitigating the local instability associated with meandering systems, such as transverse and longitudinal structures, armor protection, and bio-engineering techniques have been utilized worldwide. One type of structure, bendway weirs, are rock structures placed in series along the outer bank of a bend in an effort to alleviate stresses, as well as create habitat. Many aspects associated with the bendway weir indicate the structure to be a viable alternative to longitudinal stone toe for controlling migration and bank erosion. While bendway weirs have been successfully used in many applications, quantifiable design guidance has not been available. Past projects utilizing bendway weirs have relied heavily on field experiences, site-specific flume studies, and engineering judgment, but have lacked general design guidelines. To accurately model flow conditions resulting from the placement of bendway weirs, an undistorted 1:12 Froude scale, hard boundary model was constructed at the Hydraulics Laboratory of the Engineering Research Center at Colorado State University. The model contained two bends, which exhibited unique geometric characteristics representative of those found in the Middle Rio Grande reach. Three-dimensional velocities and water surface profiles were recorded for a series of tests including variations of weir length, spacing, and angle. Using the data obtained from the test series, a relationship of the reduction of velocities relative to pre-weir conditions, due to the variation of bendway weir characteristics, was developed. The location of the strongest eddies found in a bend as a function of weir length was also examined. Bendway weir testing resulted in the creation of design criteria. Utilization of a known set of baseline conditions for a given bend and design parameters for bendway weirs, a designer is enabled to predict maximum velocities found in the bend after the installation of bendway weirs.

Discharge Correction for Longitudinal Settlement of Parshall Flumes

ABSTRACT A3-in. Parshall flume was installed at slopes of 0, + 1.9, +5.8, +7.8, +10.2, -1.2, -5.5, -7.8, — 9.4, and —22.8%. The flume rating, or measured discharge, was compared to the actual discharge for each flume slope with free outfall conditions. The results indicated that the Parshall flume accuracy is slope dependent. The flume rating was in error 32% at slopes of ±10%. A method for aligning the measured discharge with the actual discharge is presented for variable slope conditions.

TESTING LEVEE SLOPE RESILIENCY AT THE NEW COLORADO STATE UNIVERSITY WAVE OVERTOPPING TEST FACILITY

Resilient levee design requires guidelines developed using controlled experiments conducted at full scale to avoid significant scale effects related to grass strength and soil erosion. Colorado State University was commissioned by the New Orleans District, Corps of Engineers to (1) design, build, and calibrate a large Wave Overtopping Simulator, and (2) conduct extensive resiliency tests of grass slopes and other erosion protection alternatives subjected to very high irregular wave overtopping rates. The landward-side levee slope was replicated for the experiments using “planter boxes” prepared in the same manner as actual levee slopes. The grass proved to be surprisingly resilient at average discharges up to 370 l/s per m without failure, but dormant grass did fail at lower discharge rates. Bare clay exhibited little resiliency to wave overtopping. Hydraulic measurements quantified the degree of air entrainment at the lower end of the levee slope, and the measurements indicated a centrifugal force contribution at the transition between the levee and toe berm slopes.

Supercritical flow measurement using a small Parshall flume.

An experimental program was conducted to determine if a Parshall flume, developed to accurately measure open-channel subcritical flow, could also be used to measure discharge in a supercritical flow regime. Fifteen experimental configurations were tested using two small Parshall flumes [6-in. (15.2-cm) and 9-in. (22.9-cm) crest width] with varying approach channel slopes, approach channel roughness, and flume convergence. It was determined that a single Parshall flume can be used to measure flow (within ±5%) for both supercritical and subcritical flow regimes for a specified range of flows. The original Parshall flume equation was then modified to incorporate crest width, channel slope, channel roughness, and convergence in the prediction algorithm. Unique expressions were developed for both supercritical and subcritical flow regimes to estimate the discharge. A single expression does not appear feasible for accurate discharge measurement for both flow regimes in a Parshall flume at this time.

One-dimensional Modeling Techniques for Energy Dissipation in U-weir Grade-control Structures

Generally constructed with materials from the surrounding environment and promoting a more natural aesthetic within a stream, U-weirs have recently become popular alternatives for channel grade control, bank stabilization, and fish habitat enhancement. U-weirs are constructed with segments extending at angles to the channel banks, inciting three-dimensional, rapidly-varied flow over the structure. Compared to a one-dimensional flow scenario, energy expenditure through the structure reach is increased due to strong velocity gradients at plunging jet boundaries and turbulence losses. Complex equilibrium scour geometries are interrelated with, and contribute to, the redirection of streamlines from a predominantly downstream direction. Accordingly, modeling methodologies utilizing a one-dimensional flow assumption are not directly applicable for representation of hydraulics through rock weirs. Using data collected from a physical model an approach altering parameters and cross-section configuration within a commonly implemented one-dimensional model, HEC-RAS, to approximate hydraulic conditions associated with a specific type of grade-control structure, the U-weir, was developed. The approach utilizes generated cross sections based upon weir geometry parameters and select survey data along with adjusted Manning roughness values was developed. Compared to data collected in a physical model, total mechanical energy head conditions and water surface elevations were approximated to a mean reach-averaged cross-section absolute error of less than 0.01 m.