Haltas I.,Northwest Hydraulic Consultants Inc. |
Kavvas M.L.,University of California at Davis
Journal of Hydrologic Engineering | Year: 2010
There is a strong analogy between fractal geometries and scale invariant processes. Fractal geometries are self-similar at different scales. Similar to fractal geometriessolutions of scale invariant processes at different space-time scales are self-similar. This unique property of scale-invariant processes can be employed to find the solution of the processes at a much larger or smaller space-time scale based on the solution calculated on the original scale. Herewe investigate scale invariance properties of hydrologic processes as initial-boundary value problems in one-parameter Lie group of point transformations framework. Scaling (stretching) transformation has unique importance among other Lie group of point transformationsas it leads to the scale invariance or scale dependence of a process. Scale invariance of a process allows using the same mathematical model for the process at different scales and facilitates finding the solution at any scale using the solution at the original scale. In this studythe process parameters and source/sink terms are regarded as state variables of some (secondary) processes that underlie or couple with the original process. Then under the scaling transformationsthe invariance conditions for the resulting system of processes at time-space scales that are different from the original time-space scales are investigated. The conditions to be satisfied by the form of a governing equation and its parametersas well as the initial and boundary conditions of the processare established in order for the process to be scale invariant. Alsothe self-similarity of the solution of an invariant process is demonstrated by various numerical example problems. © 2011 ASCE.
Curran J.C.,Northwest Hydraulic Consultants Inc. |
Waters K.A.,University of Virginia |
Cannatelli K.M.,Inter Fluve Inc.
Geomorphology | Year: 2015
Real-time measurements of bed changes over a reach are a missing piece needed to link bed morphology with sediment transport processes during unsteady flows when the bed adjusts quickly to changing transport rates or visual observation of the bed is precluded by fine sediment in the water column. A new technique is presented that provides continuous measurement of sediment movement over the length of a flume. A bedload monitoring system (BLMS) was developed that makes use of pressure pillows under a false flume bottom to measure sediment and water weights over discrete flume channel sections throughout a flow event. This paper details the construction of the BLMS and provides examples of its use in a laboratory setting to reconstruct bed slopes during unsteady flows and to create a real-time record of sediment transport rates across the flume channel bed during a sediment transporting flow.Data gathered from the BLMS compared well against techniques commonly in use in flume studies. When the BLMS was analyzed in conjunction with bed surface DEMs and differenced DEMs, a complete transport and bed adjustment picture was constructed. The difference DEMs provided information on the spatial extent of bed morphology changes. The BLMS supplied the data record necessary to reconstruct sediment transport records through the downstream channel, including locations and time periods of temporary sediment storage and supply. The BLMS makes it possible to construct a continuous record of the spatial distribution of sediment movement through the flume, including areas of temporary aggradation and degradation. Exciting implications of future research that incorporates a BLMS include a more informed management of river systems as a result of improved temporal predictions of sediment movement and the associated changes in channel slope and bed morphology. © 2015 Elsevier B.V.
Curran J.C.,Northwest Hydraulic Consultants Inc. |
Waters K.A.,University of Virginia
Journal of Geophysical Research: Earth Surface | Year: 2014
The surface structure of static armor layers generated from water-worked gravel bed channels was investigated with primary focus on the influence of sand content and flow rate. Flume experiments were conducted in which four sediment mixtures with sand contents between 1% and 38% were armored under one of three different flow rates. First- and second-order statistical analyses were applied to digital elevation models of unarmored, armored, and clustered bed surface areas to identify changes in surface structure. Results were combined with data from previous research to create an extended data set of armored bed surfaces. Water-worked, unarmored bed surfaces established under a dynamic equilibrium flow rate impacted the topographic variability and structure of the armored beds. Surface complexity decreased with armor formation as surface grains preferentially aligned with the flow direction. The bed surface became smoother, and where sediment mixture sand content was constant, there was greater smoothing of the surface during higher armoring flows as grains rearranged more easily. As bulk sand content increased, statistical analyses of the expanded data set showed that beds with very little sand content developed static armor layers that remained rough and had greater topographic variability than armor layers from sediments with higher sand contents. The bulk sediment sand content exerted a stronger influence over the change in surface roughness and structure upon armoring than that of the flow rate during armor formation. When combined with the knowledge of the local flow regime, the sand content may aid in predictions related to armored bed surface structure. ©2014. American Geophysical Union. All Rights Reserved.
Waters K.A.,University of Virginia |
Curran J.C.,Northwest Hydraulic Consultants Inc.
Water Resources Research | Year: 2015
Flume experiments were conducted to measure bed morphology adjustments in sand/gravel and sand/silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales. Sediment composition and hydrograph flow magnitude greatly influenced channel morphology, which impacted sediment yield, hysteresis, and transport predictions. Bed load yields were larger and more variable for the sand/silt mixture, as gravel in the sand/gravel sediment inhibited grain entrainment, limited bed form growth, and acted to stabilize the bed. Hysteresis patterns varied due to bed form and surface structure adjustments, as well as the stabilizing effect of antecedent low flows. Using half the data set, a dimensionless fractional transport equation was derived based on excess shear stress. Dimensionless reference shear stresses were estimated in two ways: as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph. For the other half of the data set, transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses. Thus, hydrograph limb-dependent dimensionless reference shear stress links changing bed morphology and sediment transport, providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes. This approach represents a framework with which to develop site-specific transport relationships for varying flow regimes, particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time. Key Points: Bed morphology and sediment transport studied during unsteady flow sequences Separate limb reference shear stresses improved sediment transport predictions Reference shear stress effectively linked transport to changing bed morphology © 2015. American Geophysical Union. All Rights Reserved.
Oberhagemann K.,Northwest Hydraulic Consultants Inc. |
Hossain M.M.,Bangladesh Water Development Board
Geotextiles and Geomembranes | Year: 2011
Since the late 1990s, riverbank revetments constructed of sand-filled geotextile bags (geotextile bags) have been developed in Bangladesh in response to the lack of traditional erosion-protection materials, particularly rock. After independence in 1971 and the related loss of access to quarries, rock was replaced by concrete cubes, but those are expensive and slow to manufacture. Geotextile bags on the other hand, first used as emergency measures during the second half of the 1990s, can be filled with local sand and therefore provide the opportunity to respond quickly to dynamic river changes. Geotextile bags also provide the potential for substantial cost reduction, due to the use of locally available resources. The use of the abundant local sand reduces transport distance and cost, while local labor is used for filling, transporting, and dumping of the 75-250. kg bags. Driven by the need for longer protection, the idea of using geotextile bags for permanent riverbank protection emerged in 2001. Eight years of experience have enabled systematic placement of geotextile bag protection along about 12. km of major riverbanks at a unit cost of around USD 2. M per km. By comparison, concrete-block revetments cost around USD 5. M per km. In addition, there are strong indications that geotextile bags perform better than concrete blocks as underwater protection, largely due to their inherent filter properties and better launching behavior when the toe of the protected underwater slope is under-scoured. This article reports the outcome of the last eight years of development work under the ADB-supported Jamuna-Meghna River Erosion Mitigation Project (ADB, 2002), implemented by the Bangladesh Water Development Board. Besides substituting geotextile bags for concrete blocks as protective elements, the project involved development of a comprehensive planning system to improve the overall reliability and sustainability of riverbank protection works. © 2010 Elsevier Ltd.
Papanicolaou A.N.,University of Iowa |
Elhakeem M.,Abu Dhabi University |
Wardman B.,Northwest Hydraulic Consultants Inc.
Journal of Hydraulic Engineering | Year: 2010
The predictive capability of a two-dimensional (2D)-hydrodynamic model, the finite-element surface water modeling system (FESWMS), to describe adequately the flow characteristics around emergent bendway weir structures was evaluated. To examine FESWMS predictive capability, a sensitivity analysis was performed to identify the flow conditions and locations within the modeled reach, where FESWMS inputs for Manning's n and eddy viscosity must be spatially distributed for to better represent the river bed flow roughness characteristics and regions where the flow is highly turbulent in nature. The sensitivity analysis showed that high flow conditions masked the impact of Manning's n and eddy viscosity on the model outputs. Therefore, the model was calibrated under low flow conditions when the structures were emergent and had the largest impact on the flow pattern and model inputs. Detailed field measurements were performed under low flow conditions at the Raccoon River, Iowa for model calibration and verification. The model predictions were examined for both spatially averaged and distributed Manning's n and eddy viscosity model input values within the study reach for an array of emergent structures. Spatially averaged model inputs for Manning's n and eddy viscosity provided satisfactory flow depth predictions but poor velocity predictions. Estimated errors in the predicted values were less than 10% for flow depth and about 60% for flow velocity. Distributed Manning's n and eddy viscosity model inputs, on the contrary, provided both satisfactory flow depth and velocity predictions. Further, distributed inputs were able to mimic closely the recirculation flow pattern in the wake region behind the bendway weir structures. Estimated errors in the predicted values were less than 10 and 25% for flow depth and velocity, respectively. Overall, in the case of distributed model inputs, FESWMS provided satisfactory results and allowed a closed depiction of the flow patterns around the emergent bendway weirs. These findings suggest that 2D models with spatially distributed values for Manning's n and eddy viscosity can adequately replicate the velocity vector field around emergent structures and can be valuable tools to river managers, except in cases when detailed three-dimensional flow patterns are needed. The study was limited to the examined low flow conditions, and more field data, especially under high flow conditions, are necessary to generalize the findings of this study regarding the model prediction capabilities. © 2011 ASCE.
Ham D.,Northwest Hydraulic Consultants Inc. |
Church M.,University of British Columbia
Earth Surface Processes and Landforms | Year: 2012
We investigate large-scale morphologic changes over a 65-year period in a major sedimentation zone within the 50-km long wandering gravel-bed reach of lower Fraser River, British Columbia. This reach remains in a relatively pristine state compared with many major rivers in populated regions even though various forms of human interference have occurred over the past century to mitigate flooding and erosion concerns. A template of channel evolutionary development is identified, consisting of extended periods of orderly accretion of unit bars by deposition of gravel sheets, controlled by the major riffles in the river. When this process constricts channel conveyance an avulsion occurs and the major bar complexes are reorganized. The recognition of fairly predictable short-term behavior can potentially aid the development of models of sediment transport and channel evolution. Accordingly, results of the detailed descriptive analysis are compared with a GIS-based probabilistic approach developed from transition-state analysis. This reveals that the river has a far stronger tendency to retain its existing morphology than to change its form within a roughly decennial time scale - regardless of the magnitude of intervening flows - but that this tendency gradually declines over time as major bar and island complexes are re-worked by sediment exchange. The timescale for major bar development appears to be of the order 30years, while complete sediment exchange occurs within a century. © 2012 John Wiley & Sons, Ltd.
Nelson A.,Northwest Hydraulic Consultants Inc. |
Dube K.,Watershed GeoDynamics
Earth Surface Processes and Landforms | Year: 2016
We exploit a natural experiment caused by an extreme flood (~500year recurrence interval) and sediment pulse derived from more than 2500 concurrent landslides to explore the influence of valley-scale geomorphic controls on sediment slug evolution and the impact of sediment pulse passage and slug deposition and dispersion on channel stability and channel form. Sediment slug movement is a crucial process that shapes gravel-bed rivers and alluvial valleys and is an important mechanism of downstream bed material transport. Further, increased bed material transport rates during slug deposition can trigger channel responses including increases in lateral mobility, channel width, and alluvial bar dominance. Pre- and post-flood LiDAR and aerial photographs bracketing the 2007 flood on the Chehalis River in south-western Washington State, USA, document the channel response with high spatial and temporal definition. The sediment slug behaved as a Gilbert Wave, with both channel aggradation and sequestration of large volumes of material in floodplains of headwaters' reaches and reaches where confined valleys enter into broad alluvial valleys. Differences between the valley form of two separate sub-basins impacted by the pulse highlight the important role channel and channel-floodplain connectivity play in governing downstream movement of sediment slug material. Finally, channel response to the extreme flood and sediment pulse illustrate the connection between bed material transport and channel form. Specifically, the channel widened, lateral channel mobility increased, and the proportion of the active channel covered by bars increased in all reaches in the study area. The response scaled tightly with the relative amount of bed material sediment transport through individual reaches, indicating that the amount of morphological change caused by the flood was conditioned by the simultaneous introduction of a sediment pulse to the channel network. © 2016 John Wiley & Sons, Ltd.
Axworthy D.H.,Northwest Hydraulic Consultants Inc.
BHR Group - 12th International Conference on Pressure Surges | Year: 2016
Surge alleviation strategies are descnbcd I) to prevent over-pressurization of two large diameter transmission mains sith little allowance for surge following turbine load rejection and closure of pressure reducing valves, and 2) to prevent column separation in a third transmission main after pump power failure The sometimes competing priorities of each surge control device were accommodated for the proposed bi-dwectional operation of the transmission mains. The results of the transient analyses show both the inability of the existing surge control devices and the effectiveness of the additionally proposed surge protection measures to control pressure surges to acceptable levels in the transmission mains. © BHR Group 2015.
Khan L.A.,Northwest Hydraulic Consultants Inc.
Air and Waste Management Association - Guideline on Air Quality Models 2016: The New Path | Year: 2016
A three-dimensional (3-D) computational fluid dynamics (CFD) model simulating the transport, dispersion, and interaction of flue gas plumes from three adjacent stacks is presented in this paper. The modeling approach considers the flue gas as a single-phase, miscible mixer of air and 2,000 ppm of nitrogen dioxide (NO2), the pollutant of interest. The plumes at 277°C are discharged to the atmosphere by a 50-m and two 70-m high stacks with exhaust velocity varying from 7.5 to 15 m/s. The model indicates that the exit momentum and buoyancy are sufficiently high to prevent stack- Tip downwash. The vertically rising plumes are deflected by the ambient wind and dissipated by turbulence. These plumes merge a short distance downstream and form a cross-sectional shape resembling that of an inverted kidney, as reported in previous studies. The entrainment of ambient air dilutes the NO2 concentration to 20 ppm and reduces the temperature by 250°C in a horizontal distance of 250 m. At the same time, the plume rises by 50 m and the NO2 concentration and temperature increases on the roof of a 50 m high building are undetectable. The CFD model study confirms effectiveness of the stacks in dispersing and diluting the flue gases.