Debris flow is one of the most devastating types of landslide which surges downslope at high velocity and damages infrastructure along with loss of human life. To intercept debris flows, countermeasures are installed along the predicted debris flow paths. Installation of baffles is imperative to attenuate debris and boulders velocities and impact force on the downstream rigid barriers. The current design guidelines of rigid barrier (Kwan 2012) and baffles (Kwan et al., 2018) propose a semi-empirical hydrodynamic approach to calculate debris impact. The hydrodynamic co-efficient 2.5 is recommended to account for the impact of boulder diameter up to 0.5 m in the debris and superposition of debris and boulder impacts is not necessary. However, the downstream rigid barrier is designe...[ Read more ]

Debris flow is one of the most devastating types of landslide which surges downslope at high velocity and damages infrastructure along with loss of human life. To intercept debris flows, countermeasures are installed along the predicted debris flow paths. Installation of baffles is imperative to attenuate debris and boulders velocities and impact force on the downstream rigid barriers. The current design guidelines of rigid barrier (Kwan 2012) and baffles (Kwan et al., 2018) propose a semi-empirical hydrodynamic approach to calculate debris impact. The hydrodynamic co-efficient 2.5 is recommended to account for the impact of boulder diameter up to 0.5 m in the debris and superposition of debris and boulder impacts is not necessary. However, the downstream rigid barrier is designed without considering the impact force reduced by baffles. Baffles are installed prior to the downstream barrier on experience-based with no scientific recommendations. The objective of this research is to investigate the impact of two-phase debris flow against baffles and the downstream rigid barrier by 28-meter flume modeling. The flume has a width of 2 m and depth of 1 m. Debris flows with and without boulders are adopted. A debris composition representing debris flow events in Hong Kong and Korea is used in this study. The debris comprises of sand, aggregates (up to 20 mm) and Kaolin clay mixed with water. Bouldery debris has granite boulders (200 and 400 mm) to study the influence of boulder impacts. The debris impact on the downstream rigid barrier is investigated by varying the number of rows of baffles to provide hydrodynamic coefficient for designing. The bouldery debris mix impact on the baffles and barrier is also studied and compared with the current design guidelines.

It is found that two rows of baffles can attenuate 31% velocity and four rows of baffles diminish 15% more velocity of debris. The velocity of the leading 200 mm boulder is diminished 79% by four rows of baffles. Similarly, four rows of baffles for debris mix with 200 mm boulders attenuate the boulders and debris impact forces by 95% and 86% on the downstream barrier by reducing the velocity by 80% and 68% as compared to the test without installing any baffles. The impact force of the boulders on baffles depends on location and contact area. The Hertz equation that is suggested by current design guideline (Kwan, 2012) may estimate boulder impact twice more than the measured impact on the baffles.

To design the first row of baffles, hydrodynamic co-efficient is suggested as 0.2 and superposition of debris and boulder force are required to estimate bouldery debris impact. For designing downstream barrier against debris impact with one, two, three and four rows of baffles, the hydrodynamic co-efficient is suggested as 0.80, 0.67, 0.68 and 0.69 respectively. Similarly, the hydrodynamic coefficient for debris impact only, is suggested as 1.05, 1, 0.95 and 0.9 for the downstream rigid barrier design against the bouldery (diameter 200 mm) debris by installing single, two, three and four rows of baffles respectively. The design basis for the downstream barrier with single and three rows of baffles are suggested based on the results of two and four rows of baffles investigation. Further evaluation and comparison are recommended to robust the design basis. Moreover, by using four rows of baffles the downstream rigid barrier design does not require superposition of debris and boulders impact.