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Cary, NC, United States

Froehlich D.C.,303 Frenchmans Bluff Drive
Journal of Irrigation and Drainage Engineering | Year: 2012

A graphical procedure for calculating peak discharge from small catchments is developed on the basis of the standard Natural Resources Conservation Service (NRCS) rainfall-runoff hydrologic methods, in which rainfall excess obtained using the curve-number approach applied to 24-h design storms is transformed to runoff by means of triangular unit hydrographs. The solution is made dimensionless by grouping parameters, and, as a result, can be condensed into uncomplicated graphical relations that provide numerically precise peak flow rates. Mathematical expressions are also given that provide accurate approximations to the peak-discharge graphical relations. An additional result of the analysis is a comprehensive means of estimating rational-formula runoff coefficients for any location in the United States. © 2012 American Society of Civil Engineers. Source


Froehlich D.C.,303 Frenchmans Bluff Drive
River Research and Applications | Year: 2013

Stability of loose rock riprap used to protect stream banks from erosive forces because of flowing water is evaluated based on the ratio of static moments resisting overturning and those promoting overturning of a single rock particle. The ratio of moments defines a safety factor that describes the potential for riprap failure. The buoyant force acting on a particle is treated separately from the gravitational force and is further split into components that resist and promote overturning. This approach provides consistency in reasoning throughout the formulation, which results in a particle safety factor that tends to unity as rock-specific gravity approaches one. The safety factor formulation is tested using 38 onsite measurements of riprap-lined stream channels that have experienced floods approaching or exceeding the flow rates used to design the protective covers. Comparison to two other commonly used riprap-sizing methods by means of the Hanssen-Kuipers skill score and the equitable threat score shows that the particle stability procedure provides significantly better damage predictions and, for this reason, is shown to be more accurate. Based on the onsite measurements, safety factors that provide increasing levels of security against failure are suggested for use in calculating the size of loose rock riprap needed to protect stream banks against erosion by currents. © 2011 John Wiley & Sons, Ltd. Source


Froehlich D.C.,303 Frenchmans Bluff Drive
World Environmental and Water Resources Congress 2012: Crossing Boundaries, Proceedings of the 2012 Congress | Year: 2012

Stability of loose rock riprap used to protect stream banks from erosive forces due to flowing water is evaluated based on the ratio of static moments resisting overturning and those promoting overturning of a single rock particle. The ratio of moments defines a safety factor that describes the potential for riprap failure. The buoyant force acting on a particle is treated separately from the gravitational force, and is further split into components that resist and promote overturning. This approach provides consistency in reasoning throughout the formulation, which results in a particle safety factor that tends to unity as rock specific gravity approaches one. The safety factor formulation is tested using 38 onsite measurements of riprap-lined stream channels that have experienced floods approaching or exceeding the flow rates used to design the protective covers. Based on skill score assessments, the approach is shown to be a significantly more reliable riprap damage predictor than two other commonly used methods. © 2012 ASCE. Source


Froehlich D.C.,303 Frenchmans Bluff Drive | Goodell C.R.,WEST Consultants Inc.
World Environmental and Water Resources Congress 2012: Crossing Boundaries, Proceedings of the 2012 Congress | Year: 2012

Dam-breach flood analyses rarely take into account the uncertain nature of numerical model parameters when assessing the flood hazards of potential failures. Solution uncertainty is evaluated here using a point-estimate method that provides a direct and efficient computational procedure to obtain moment estimates (specifically, the means and variances) of calculated peak water-surface elevations, peak discharges, and flood peak travel times. The method is applied to Big Bay Dam to define bounds on downstream flood hazards having specified pexceedance probabilities. Comparison to peak water-surface elevations that were produced by actual failure of the dam shows the approach to provide a reasonable estimate of downstream flood hazard uncertainty. © 2012 ASCE. Source


Elliot R.C.,Northwest Hydraulic Consultants Inc. | Froehlich D.C.,303 Frenchmans Bluff Drive | MacArthur R.C.,Northwest Hydraulic Consultants Inc.
World Environmental and Water Resources Congress 2012: Crossing Boundaries, Proceedings of the 2012 Congress | Year: 2012

Potential effects of large woody debris accumulations on backwater, hydrodynamic loads on structures, and local scour at bridge piers along a section of an urban stream containing three bridge crossings were modeled using the computer program FST2DH. The two-dimensional depth-averaged flow solutions provide reliable estimates of water velocities and depths, which were used to internally calculate pressure flow and potential scour depths and hydrodynamic drag forces and impact loads on bridge piers. The effect of debris accumulations on water-surface elevations during high flows was found to be comparatively small compared to other factors such as bridge pressure flow and roughness. However, hydrodynamic drag forces acting on the blockages lead to significantly larger loads on piers. Although drift accumulations actually reduce the speeds of flows approaching piers, the most significant effect of debris accumulation on piers in this project setting is to significantly increase local scour depths. Additionally, reduced approach velocities at piers may be accompanied by increased velocities between piers, which could further increase overall bed scour. © 2012 ASCE. Source

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