Experimental Evaluation of Loads from Inundation-Driven Debris Fields

Abstract

Inundation events, such as tsunamis and storm surges, pose a significant threat to coastal communities and infrastructure globally. Damage caused to communities is not only caused by flowing water, but also by debris from collapsed structures, vegetation, and whatever might be moved by an inundation flow. Previous studies have investigated wave- and flow- induced loading, but have not investigated the influence of debris fields (i.e., multiple pieces of debris) carried by inundation-type flows. The impacts from dense debris fields are chaotic in nature and it can be difficult to predict how the disparate debris will interact with a coastal structure. While single-debris impacts are fairly predictable and reproducible, debris fields require a statistically-driven approach. This study presents the results of an experimental program in the Large Wave Flume at the NHERI Wave Research Lab conducted to generate a statistically-representative data set of various multi-debris scenarios and provide measurements to inform numerical modeling efforts. This thesis analyzes streamwise forcing via in-line load cells attached to a test structure to quantify reaction forces induced by high-density polyethylene (HDPE) debris fields, while varying: (i) number of debris, (ii) debris orientation, (iii) debris field density, and (iv) individual debris size. All tests within this scope were subjected to a single, repeatable wave that remained unbroken throughout the test environment. In general, peak forcing increases at a decreasing rate with the number of debris, suchthat the highest impact forces observed (with 24 debris) are approximately 3.25 times the magnitude of the maximum impact of a single debris oriented with its long axis in the direction of flow. Additionally, increasing debris field density tends to yield higher measured forces. A clear distinction between a first peak loading event and a second peak loading event is revealed in forcing time histories. Via analysis of video data, hypothesized explanations for the first and second peak are proposed: the first peak represents the initial contact of the debris field with the structure, while the second peak occurs up to 2 seconds later as randomized impacts after the initial debris field has broken apart. Correlation is also found between first peak impact and: 1) orientation of individual debris, such that debris oriented in the direction of flow has higher first peak values than transverse counterparts; and 2) cohesion of the mass within the debris field at the time of impact. Most tests, however, see absolute maxima during the second peak period, indicating that chaotic impacts after the initial impact represent significant loading. Further, low frequency forcing after impacts (i.e., “damming”) is also present. This is hypothesized to be due to debris becoming caught on the test structure and has high variability between tests. Configurations with smaller individual debris pieces tend to have lower damming forces, despite the mass and density of the debris field being held constant. Damming reaction forces can be as high as 20% of peak loading values: the sustained forcing may have significant structural implications. Ultimately, this thesis will add to a large-scale study working to inform how a flow-driven debris field may interact with the coastal built environment during large-scale inundation events, and seeks to add research to assist coastal community resilience in the face of natural hazards.