Ronald D. Townsend

Smoothed-Particle Hydrodynamics Numerical Modeling of Structures Impacted by Tsunami Bores

AbstractRecently, major tsunami events have resulted in considerable damage to low-lying coastal developments as well as significant loss of life. This is owing to the fact that coastal communities and associated infrastructure are not able to withstand the extreme hydrodynamic forces induced by the overland progression of large tsunami waves. This paper reports the findings of a study, which is part of an ongoing comprehensive (hydraulic-structural engineering) research program initiated at the University of Ottawa in Canada. The goals of the latter are to contribute to the understanding and quantification of extreme hydrodynamic impacts on structural elements, and to the development of design guidelines for structures located in tsunami-prone areas. In this study, a single-phase three-dimensional (3D) weakly compressible smoothed-particle hydrodynamics (WCSPH) model is used to investigate the hydrodynamic forces, induced by the impact of rapidly advancing tsunami-like hydraulic bores, on a freestanding ...

3D Flow and Sediment Dynamics in a Laboratory Channel Bend with and without Stream Barbs

AbstractA series of laboratory flume experiments were performed to study the effect of stream barbs on flow field dynamics and sediment erosion in a 135° mobile-bed channel bend. Stream barbs (also known as spur dikes or submerged groynes) are low-profile linear rock features that redirect high velocity flow away from the outer bank of channel bends. Unlike emergent groynes, the submerged nature of these structures creates a unique combination of horizontal shear (plunging type flow) and vertical shear (at the groyne tip). Spatially dense, high frequency velocity data were collected and analyzed to describe the pattern and magnitude of three-dimensional (3D) velocity throughout the bend and in the vicinity of the stream barbs. This paper demonstrates that the outer bank region (particularly between barbs) may still be at risk of erosion (or even increased erosion greater than the same case without barbs) if stream barbs generate excessive secondary velocities (because of their size and layout) that are op...

Influence of Channel Aspect Ratio and Curvature on Flow, Secondary Circulation, and Bed Shear Stress in a Rectangular Channel Bend

AbstractFlow within an alluvial channel bend is significantly affected by channel geometry, including curvature ratio (bend radius/channel width, R/B) and aspect ratio (channel width/flow depth, B/H). High curvature bends (R/B≤3) can experience substantially more erosion than milder curvature bends. This study employs a three-dimensional Reynolds-Averaged Navier-Stokes (RANS) model to investigate the effects of curvature ratio and aspect ratio on bend flow with respect to a high curvature (R/B=1.5) base case in a 135° bend. Experimental data are used to validate the RANS model predictions for the high curvature base case with a flat bed (FB) and an equilibrium deformed bed (DB). Five curvature ratios (1.5, 3, 5, 8, and 10) and four aspect ratios (5.00, 6.67, 9.09, and 12.50) are investigated. Results show that a decrease in R/B or B/H for the FB cases results in a strong increase in total circulation of the regions associated with the primary cell of cross-stream circulation (Γ+), an increase in maximum b...

Numerical Modeling of the Impact with Structures of Tsunami Bores Propagating on Dry and Wet Beds Using the SPH Method

In this paper, the simulation of the impact with structures of tsunami-like bores rapidly advancing on dry and wet beds is performed using a three-dimensional numerical model based on the Smoothed Particle Hydrodynamics (SPH) method. Firstly, to validate the numerical model, physical experiments are simulated and a quantitative comparison between experimental and numerical results is presented. Secondly, numerical results of the propagation of bores on dry and wet beds are compared to the analytical solution of the shallow water equations. Furthermore, the resulting time-histories of the pressures and net force acting on a square column and a vertical wall due to the impact of these bores are compared qualitatively. To better understand the development of the hydrodynamic forces, a detailed analysis of the velocity field and of the water surface elevation is also incorporated. This study is part of a comprehensive interdisciplinary research program whose purpose is to help develop design guidelines for tsunami-prone structures.

A model for routing unsteady flows in compound channels

A mathematical model for routing unsteady flows through compound channels is described. The new model, RUFICC, accounts for flood plain contributions to system conveyance and also for lateral momentum transfer between adjacent deep and shallow zones of compound flow fields. In the modeling approach composite channel sections are divided into representative deep (main channel) and shallow (flood plain) zones. Continuity and momentum equations are written separately for the different sub-sections and summed to yield the composite section equations. The one-dimensional model equations, which are a modified form of the St. Venant equations, are solved using a four-point implicit finite-difference scheme. The initial model validation tests reported herein used laboratory data from a European study on unsteady flow in open channels. For the various model flood events simulated a good agreement was obtained between predicted and observed stage and discharge hydrographs. RUFICCs performance was compared to those...

A semi-permanent method for fixing sand beds in laboratory flumes

A new flume bed-sediment hardening method employing a surficial layer of plaster of Paris coated by spar urethane has been developed and tested for sand beds. Unlike existing bed-hardening techniques, this method is gentle enough to be used in and subsequently removed from a laboratory flume made of Plexiglas, yet is durable enough to last several days under flowing water. Tests showed that artificially fixed beds withstand an estimated bed shear stress of up to 0.251 N/m2 for 52 h. The effects of the hardening technique on bed roughness were evaluated. A mean absolute difference in the bed roughness estimate between fixed and loose beds of 0.166 mm was found when tested on a sand bed. Although roughness estimates seemed reasonable, standard deviations were high. Further testing under a wider range of flow conditions and sediment sizes is required to assess the effect of the hardening technique on roughness.

Three-dimensional Flow In A Barb Field

Barbs (or submerged groynes) are low-profile linear rock structures that are primarily used to prevent the erosion of stream banks. They are a variation of a groyne, similar to spur dikes and bendway weirs. Barbs are typically anchored, in series, to the outside bank in stream bends and extend in an upstream direction from the bank into the flow. This configuration redirects flow away from the outer stream bank and disrupts the velocity gradient close to the outer bank, encouraging sediment deposition adjacent to the barb. These interactions between the flow pattern and the sediment transport around the barb prevent erosion of the bank. Furthermore, vortices generated by the barb create local scour holes that can enhance aquatic habitat. A three-dimensional numerical model, Sediment Simulation in Intakes with Multiblock option (SSIIM), was used to examine the turbulent flow field and associated scour and depositional patterns due to a series of barbs in a mobile-bed channel bend. Previous physical model observations were used for model calibration and verification. Barb design was optimized to minimize scour. The parameters considered in the optimization were channel bend angle, barb alignment angle, and barb spacing. In this paper the spatial distribution of the three-dimensional flow field through a series of barbs in a 90o channel bend are mapped and the impact on the flow field of subtle differences in barb geometry are examined. Special emphasis is placed on the influence of barb design on secondary currents in the meander bend and the generation of three-dimensional vortices, and the consequent impact on scour.

Design of stream barbs for field scale application at Sawmill Creek, Ottawa.

A three-dimensional numerical model Simulation in Intakes with Multiblock option (SSIIM) was used to model the effects of placing a series of barbs along an unstable section of Sawmill Creek, a small urban stream in the city of Ottawa, Canada. Stream barbs (a type of submerged groyne) are low-profile linear rock structures that prevent the erosion of stream banks by redirecting high velocity flow away from the bank. As they can be built at a relatively low cost and also provide significant ecological benefit, these structures are an important and relatively new method of stream bank protection. The numerical model was used to assess various design alternatives for a series of seven stream barbs at two consecutive channel bends requiring stabilization measures along their outer banks. Design criteria were principally based on the goals of reducing local velocities, shear stress and subsequent erosion at the outside bank of each bend, and on moving the thalweg away from the (existing) outside bank regions towards the centre of the channel. This paper reports on (i) the unique site conditions and environmental protection requirements, (ii) the results of the numerical simulation, and (iii) the design methodology.

Stream barb performance in a semi-alluvial meandering channel

A series of seven stream barbs were installed at two consecutive channel bends in Sawmill Creek, a semi-alluvial stream located in Ottawa, Canada. Stream barbs (also known as submerged groynes) are low-profile linear rock structures that extend from the outside bank regions of channel bends in an upstream direction, to redirect the attacking currents and prevent erosion of the bank. As well as providing bank protection, these structures promote vegetated stream banks, create resting pools and scour holes for fish habitat, and increase biodiversity for aquatic species. Despite these benefits, because of their relative novelty as river training structures, stream barbs are not a common means of stream bank protection in Canada and are possibly non-existent for semi-alluvial or clay channels. Three years of monitoring and measurement of flow conditions (discharge, water velocity and depth) and bathymetry, before (2 years) and after (1 year) the construction of the barbs, have been collected at the Sawmill Creek study site, providing valuable data for understanding their performance in a semi-alluvial channel. Sawmill Creek has a predominately clay bed and banks, presenting a rare opportunity to study the unique dynamics between flow and sediment transport within a clay channel. This paper reports on (i) the unique site conditions and monitoring methodology; (ii) preliminary results of the 3 year monitoring program; and (iii) recommendations for future design and implementation of these structures.

On Inert Tracer Modeling In A Compound Open Channel

This study investigates the impact on the pollutant transport and mixing processes of strong lateral momentum transfer effects associated with severely compound flow fields. Consideration was limited to conservative, non-buoyant material represented by a (dye) tracer. The experiments include measurements of dye concentrations downstream from steady-injection point source(s). In general, it is not possible to obtain analytical solutions to the dispersion equation in natural waterways with arbitrary boundary conditions. However, a variety of simplified solutions exists for idealized situations, which can be useful in obtaining order-of-magnitude estimates. These solutions were applied to our experimental data. Also, the 2-D depth-averaged advection-diffusion equation was solved numerically and applied to the data sets. Comparison between measured and numerically predicted concentration curves indicates a reasonably good agreement in regards to general shape, peak concentrations and time to peak. Transactions on Ecology and the Environment vol 17,