Measurement and modeling of solitary wave induced bed shear stress over a rough bed

Bed shear stresses generated by solitary waves were measured using a shear cell apparatus over a rough bed in laminar and transitional flow regimes (~7600 < Re < ~60200). Modeling of bed shear stress was carried out using analytical models employing convolution integration methods forced with the free stream velocity and three eddy viscosity models. The measured wave height to water depth (h/d) ratio varied between 0.13 and 0.65; maximum near- bed velocity varied between 0.16 and 0.47 m/s and the maximum total shear stress (sum of form drag and bed shear) varied between 0.565 and 3.29 Pa. Wave friction factors estimated from the bed shear stresses at the maximum bed shear stress using both maximum and instantaneous velocities showed that there is an increase in friction factors estimated using instantaneous velocities, for non-breaking waves. Maximum positive total stress was approximately 2.2 times larger than maximum negative total stress for non-breaking waves. Modeled and measured positive total stresses are well correlated using the convolution model with an eddy viscosity model analogous to steady flow conditions (nu_t=0.45u* z1; where nu_t is eddy viscosity, u* is shear velocity and z1 is the elevation parameter related to relative roughness). The bed shear stress leads the free stream fluid velocity by approximately 30° for non-breaking waves and by 48° for breaking waves, which is under-predicted by 27% by the convolution model with above mentioned eddy viscosity model

Measurements and modeling of direct bed shear stress under solitary waves

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Measurement and modelling of bed shear induced by Solitary waves

Tsunamis are a geo-hazard that have a high potential to devastate majority of infrastructure in their path. Considering the threat from possible tsunami around the Australian coast, CSIRO Australia, under the Wealth from Ocean Flagship program, initiated studies on tsunami induced hazards to the submarine infrastructure along the northwest Australian coast. This thesis is part of the research work initiated by CSIRO Australia. Experimental and numerical model studies are carried out at UQ to understand the tsunami effects on the seabed especially continental slope and continental shelf. Solitary waves are considered to represent the leading waves of a tsunami. Breaking and non-breaking solitary waves over smooth and rough beds (d50=0.2mm) were generated in the laboratory and the corresponding shear stresses were measured using a Shear Plate apparatus. A physical model was set up in the laboratory that represented the deep horizontal ocean floor, inclined continental slope and the shallow horizontal continental shelf. Measurements of bed shear stress, surface elevation and flow velocities were carried out. Periodic waves were also generated and the bed shear stresses measured over a horizontal bed were found to be comparable with the earlier studies and theoretical estimates. The total force (sum of bed shear stress and pressure gradient force) measured using the shear plate is important in determining the stability of submarine sediment and in sheet flow regimes. The bed shear stresses generated by breaking and non-breaking solitary waves were in laminar and transitional flow regimes (~10^4 < Re < ~10^5), and showed reversal of sign during the deceleration phase of the solitary wave, although the flow did not change its direction. The non-breaking solitary wave height to still water depth ratio over the horizontal smooth bed varied between 0.12 and 0.68. The maximum near bed velocity varied between 0.16 m/s and 0.51 m/s and the peak positive total shear stress varied between 0.386 N/m^2 and 2.06 N/m^2. The maximum positive total shear stress magnitudes over smooth bed were observed to increase up the slope and further on the shelf for a given h/d ratio. For rough bed cases, the peak positive total shear stress was an order of magnitude larger than that of smooth bed cases, with increasing peak positive total shear stress as the wave propagated up slope. Wave friction factors are found to vary depending on the choice of normalizing velocity used in the drag law, i.e., either using the maximum velocity, as is conventional, or the instantaneous velocity corresponding to the shear stress. To understand the impact of the phase difference of the velocity and shear stress on friction factors, friction factors were estimated from bed shear stress at different instances over the wave, viz., time of maximum positive total shear stress, maximum bed shear stress and at the time of maximum velocity, using both the maximum velocity and the instantaneous velocity at that phase of the wave cycle. Friction factors are consistent with previous data for monochromatic waves, and are found to vary inversely with the square-root of the Reynolds number. The phase difference between the maximum bed shear stress and the maximum velocity was about 30° for the smooth horizontal bed and it was about 38° for the rough horizontal bed. The median phase difference for the sloping bed was between 24° to 28° and for the horizontal bed beyond the slope was about 34°. A convolution model forced with the measured free stream acceleration is used to predict total and bed shear stresses. The peak positive value of the total and bed shear stresses are considered for comparison between measurements and model results. Modelled and measured peak positive bed shear stresses correlated well with the measurements for the horizontal bed data. However, for the sloping bed region, the model generally resulted in under estimation of the bed shear stress, few reasons could be attributed to changing eddy viscosity during upslope flow propagation or could be due to excessive estimation of the pressure gradient forces. For the rough bed cases, the model behaved similarly to that over the smooth bed, with appropriate roughness incorporated in the model. However, the model did not predict well for breaking wave conditions. Due to the inherent formulation of the convolution model, it is found that the model fails for steady flow cases. Further work related to a comprehensive model including steady flow and unsteady flow would be required. The convolution model and conventional drag law model to estimate the shear stress are applied to tsunami induced flow over the slope and shelf region of the northwest Australian (NWA) coast and the south east Indian coast. MIKE3 - a 3D hydrodynamic numerical model was used to obtain the tsunami induced flow field. The results suggest that tsunami is highly likely to induce significant sediment mobility at shallower depths. However, to identify the hotspots of high sediment mobility which could trigger submarine slides, careful study is required since the tsunami induced flow field is site specific

A numerical study on tsunami induced sediment transport in vicinity of a submarine canyon off southeast coast of India

Tsunamis often devastate the coastal infrastructure and also result in large transport of sediments in the coastal regions. Qualification of sediment transport across and nearshore is attempted from the available sediment transport formulae that use free stream velocities as input. DHI-MIKE numerical model is used to obtain velocities due to tsunami event of December 2004, on the southeast coast of India. Sediment transport using estimated velocity is calculated in the study region. Due to the large water depths and small tsunami wave heights in the offshore, the sediment transport is assumed to be predominantly a bed load transport outside surfzone. Bed load sediment transport equations outside the surfzone are used in this study. This paper attempts to estimate the sediment transport due tsunami induce velocities in the near shore region in the vicinity of a submarine canyon

Measurement and modeling of bed shear stress under solitary waves

Direct measurements of bed shear stresses (using a shear cell apparatus) generated by non-breaking solitary waves are presented. The measurements were carried out over a smooth bed in laminar and transitional flow regimes (similar to 10(4

Modelling of wave propagation over a submerged sand bar using SWASH

1177-1182<span style="font-size:11.0pt;font-family: " times="" new="" roman";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-gb;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-GB">A non-hydrostatic numerical model ‘SWASH’ (Simulating WAves till SHore) is used to study the wave propagation over a submerged sand bar in a wave flume. The SWASH model is calibrated and further used to validate the wave propagation for two different cases. The wave heights and wave induced velocities obtained from the model and the laboratory experimental resultsare compared. The model without the morphology feedback provided good correlation with the measurements for case of low wave energy, whereas for the case of a moderately high wave energy, due to significant variations in the bed morphology, the model under-performed towards the later part of the simulation. However, incorporating a modified bathymetry considering the variation in the bed morphology, the model results were reasonable.</span

Effect of submarine canyons on tsunami heights, currents and run-up off the southeast coast of India

Tsunami numerical model studies are mostly focused on inundation and run-up onto the coast. Fewer studies have been aimed at investigating the role of submarine canyons on tsunami heights, currents and run-up. The tsunami hydrodynamics in the vicinity of submarine canyons and ridges in the Palar-Cauvery region off the southeast coast of India on 26 December 2004 is considered in this study. Numerical modelling was carried out to study tsunami heights and currents in the vicinity of the submarine canyons as well as the variation of tsunami heights at 10 m water depth. Comparisons between the tsunami wave energy density at 10 m depth and the onshore run-up height observations showed good correlation for select locations, with the run-up heights being about 3% of the wave energy density. However, the local topography in the run-up zone also strongly influences the local run-up, which reduces direct correlations between run-up and nearshore tsunami height

Modelling of wave propagation over a submerged sand bar using SWASH

A non-hydrostatic numerical model ‘SWASH’ (Simulating WAves till SHore) is used to study the wave propagation over a submerged sand bar in a wave flume. The SWASH model is calibrated and further used to validate the wave propagation for two different cases. The wave heights and wave induced velocities obtained from the model and the laboratory experimental resultsare compared. The model without the morphology feedback provided good correlation with the measurements for case of low wave energy, whereas for the case of a moderately high wave energy, due to significant variations in the bed morphology, the model under-performed towards the later part of the simulation. However, incorporating a modified bathymetry considering the variation in the bed morphology, the model results were reasonable

Shoreline changes along Tamil Nadu coast: A study based on archaeological and coastal dynamics perspective

1167-1176Available geophysical survey data confirm submergence of a large area comprising of building complex, which are possible remains of a submerged township. A global sea level rise estimate of 1-2 mm per year would inundate up to several hundred meters of coast line over a period of 2000 years. Shore line changes have been calculated to about 497 m and 380 m at Poompuhar and Tranquebar during the last 75 years.  Apart from prevailing waves and currents, past sea level change estimates, tectonic movement induced subduction, erosion from storms and palaeo-tsunamis events, are plausible reasons for the shoreline retreat. It can be said that the coastal erosion due to invasion of sea has played a major role in submergence of these structures. The present paper deals with the shoreline retreat estimates resulting from underwater explorations, past sea level changes and extreme events along the Tamil Nadu coast. </span