CONSIDERATIONS FOR DYNAMIC FLOOD-BORNE DEBRIS IMPACT LOADS

R. Shafiei, J. Speedy, A. Dickson & A. Harvey (Beca Ltd), R. Smedley (Auckland Council)

ABSTRACT

Flood flows have the potential to carry fast-moving debris, posing significant risks to both human life and downstream infrastructure. The nature of flood-borne debris can vary depending on upstream catchment characteristics, encompassing a wide range of types, shapes, and sizes. These debris impact loads can be classified into two categories: static and dynamic. Static loads involve the accumulation of debris, such as tree branches, which increase drag resistance and overall hydraulic loads on structures. Dynamic loads occur when flood flow forces floating debris onto structures. While established guidelines like the Australian Standard 5100 Bridge Design (AS 5100) provide guidance for assessing static loads, there is a scarcity of resources available for calculating and evaluating dynamic debris impact loads.

This paper presents an assessment framework that specifically addresses the dynamic loads resulting from the impact of flood-borne debris on buildings and low-level bridges located in flood-prone areas. The scope of the study included a comprehensive review of the available literature to explore various approaches for estimating impact loads induced by floating debris, the application of these approaches to carefully chosen case studies, the comparison of debris impact loads with existing flood and wind loads, and the formulation of an appropriate assessment framework tailored to buildings at risk of experiencing significant loads due to flood-borne debris.

The three most common approaches for estimating the debris impact load on a structure are contact-stiffness, impulse-momentum, and work-energy. For each approach, the load is a function of the mass and velocity of the debris. An additional parameter is required depending on the approach: effective contact stiffness for contact-stiffness, the stopping time for impulse-momentum, and the stopping distance for work-energy. Based on a comparative analysis of different approaches, it was determined that Haehnel and Daly’s impulse-momentum equation is the most suitable method for integrating debris impact load into the proposed framework. The case studies yielded valuable insights into the criticality of debris loading. It was observed that debris raft loading had a greater impact compared to flood loading in both building and bridge cases. Furthermore, the structural response to dynamic debris impact loads has the potential to result in localised damage to structural elements, including deformation, cracking, or even failure.

Based on the research findings, the following framework for debris impact loading assessment in flood-prone areas is proposed.

• Conduct site-specific debris hazard assessment to identify an appropriate design debris size and mass.
• Assess the debris raft loading based on the AS5100 guidelines.
• Assess the debris impact load in accordance with Haehnel and Daly's Impulse-Momentum approach and using the recommended factors.
• Evaluate the structural response and potential damage resulting from debris impact. The debris raft and debris impact loads should not be considered concurrently. A preliminary suggestion is to utilise a load factor of 1.0.
• Develop mitigation measures based on the analysis and assessment.