Tsunami Debris Damming Forces and Associated Coefficients for Elevated Coastal Structure Columns: Experimental Comparison to ASCE 7-22 Minimum Design Loads

Abstract

Debris damming forces of 1∶20-scale shipping containers freely accumulated against elevated coastal structure columns were experimentally determined to evaluate ASCE 7-22 tsunami-resilient design standards. Three inundation conditions were generated to represent Froude regimes estimated in posttsunami field studies. Three different column array densities and two different shipping container sizes were evaluated. A photogrammetric method was employed to estimate the submerged projected area of in situ transient debris dams from two synchronized camera perspectives. Relative to this experimental data, it was found that the ASCE 7-22 equation for simplified equivalent uniform lateral static pressure is conservative by a mean factor of safety of 14.6 and performs as intended given the prescribed scope. Similarly, the ASCE 7-22 equation for detailed hydrodynamic lateral forces yielded a lower mean factor of safety of 2.4 but maintained design conservatism across all tested experimental conditions, also performing as intended. Minimum closure ratios and overall structure drag coefficients serve as input values for these detailed hydrodynamic lateral design loads. The proportion of closure coefficients prescribed by ASCE 7-22 tend to be reasonably conservative in general, and any instances of experimental exceedance of these design values did not appear to affect the design conservatism of Eq. (3). Finally, drag coefficients for rectilinear structures prescribed by ASCE 7-22 appear unrepresentative of coastal structures, which tend to generate column-flow interactions and unbalanced hydrostatic conditions. It is therefore suggested that the flow resistance of such structures be quantified via a bulk resistance coefficient, indicated by recent literature as a more appropriate measure applicable to surface-piercing flow obstructions. Since the 2016 adoption of tsunami-resilient design standards in ASCE 7-16, debris damming design loads have yet to be thoroughly examined. The results of this experiment indicate that the application of hydrodynamic loading equations in ASCE 7-22 Section 6.10 is conservative across all tested experimental conditions. Debris accumulation on the seaward face of the modeled structure is generally conservative relative to the design proportion of closure coefficients, and instances of exceedance do not result in unconservative load prediction. Finally, drag coefficients for rectilinear structures may not capture phenomena associated with surface-piercing flow obstructions such as column-flow interactions and unbalanced hydrostatic forces. It is suggested that a bulk resistance coefficient be adopted to account for both form drag and surface effects of flow around elevated coastal structure columns. Accurate quantification of tsunami-induced loads is crucial to the design of critical and essential infrastructure located within tsunami inundation zones, especially vertical evacuation refuge structures.