Graduate Thesis Or Dissertation
 

Laboratory Observations of Tsunami-Driven Debris Damming with Comparison to ASCE 7-22 Minimum Design Loads

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4t64gx19p

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  • Tsunami inundation of coastal communities can impose a wide array of forces on the built environment. Forces generated by tsunami-driven debris damming have the potential to cause failure of coastal structures and further accumulate flow-entrained debris. Since tsunami-resilient design standards were adopted by ASCE in 2016, debris damming considerations have not been thoroughly evaluated in comparison to physical model studies. Much existing debris damming literature comes from hydraulic engineering in which steady flow conditions are commonly used. Recent laboratory experiments have shifted towards unsteady, transient flow modeling, however most use small-scale debris and limited flume widths relative to the structural specimen. It remains unclear how steady flow results can be translated to coastal engineering applications and how debris damming load estimates in current structural standards compare to experimentally observed forces. As such, the purpose of this thesis is to compare current ASCE 7-22 load prediction components to experimental debris damming results. This experiment was performed in the Large Wave Flume of the O.H. Hinsdale Wave Research Laboratory at Oregon State University. Incident waves propagated as a tsunami-like bore over a flat wet-bed test section, entraining debris elements from a debris source platform and freely accumulating these elements against a column array representing an elevated coastal structure. Two different lengths of 1:20 geometric scaled shipping containers were considered in this study and three different column densities were modelled, representing both Open and Closed structures per ASCE 7-22 definitions. In addition to horizontal debris damming loads, submerged projected areas of in-situ experimental debris dams were analyzed via a new photogrammetric method for comparison to minimum closure ratios proposed in ASCE 7-22. Results of this study indicate that two alternative methods of tsunami horizontal load prediction may yield widely varying levels of structural design conservatism. ASCE 7-22 Equation 6.10-1, a simplified equivalent uniform lateral static pressure approach, yields reasonable design factors of safety for high Froude (Fr > 0.8) regime inundation, but becomes overconservative and likely impractical for design under lower velocity flows. Conversely, ASCE 7-22 Equation 6.10-2, a detailed hydrodynamic lateral force approach, is often unconservative and seems to be exacerbated by high Froude (Fr > 0.8) regime inundation and larger debris elements. However, this approach does seem reasonable for design for dense column configurations under low Froude (Fr < 0.7) regime inundation. Assumed minimum closure ratios due to debris accumulation and overall structure drag coefficients both serve as inputs to the load prediction methods described above. Results indicate that for closed structures, the assumed minimum closure ratio of 0.7 is conservative under almost all tested conditions. The assumed minimum closure ratio for open structures of 0.5 is often unconservative and is exceeded under all three incident wave conditions. This under-conservatism is also exacerbated by large debris elements. Finally, results indicate that ASCE 7-22 drag coefficients for rectilinear structures may not capture free-surface effects and intercolumn interactions induced by flow around and through elevated coastal structures. For this reason, recent tsunami literature has adopted the use of “resistance coefficient” to capture both form drag and unbalanced hydrostatic force components. It was found that experimental resistance coefficients exceed ASCE 7-22 drag coefficients for rectilinear structures for all tested column configurations. It is proposed that a bulk resistance coefficient – intended to capture form drag and free surface effects as well as intercolumn flow interaction – may provide an improved dimensionless measure of flow resistance for surface-piercing column arrays.
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  • This research was funded by the National Science Foundation (NSF) Division of Civil, Mechanical, and Manufacturing Innovation (CMMI) and Natural Hazards Engineering Research Infrastructure (NHERI) through Grants #2203131, #2203116, and #2037914.
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