DEVELOPING DEBRIS FLOW HAZARD ASSESSMENT METHODOLOGY IN HONG KONG, CHINA

The Hong Kong government has proactively initiated a strategy to address the many natural hillside catchments susceptible to potential landslides which are proximal to buildings and public facilities. To that effect the Geotechnical Engineering Office (GEO) is now requiring a Natural Terrain Hazard Study for new development and implementing a Landslip Prevention and Mitigation Program (LPMP) for existing developments that are subject to natural terrain hazards. The LPMP is a long term program that characterizes slope hazards, establishes design events for those hazards and provides mitigation recommendations.

Debris flow hazards are of particular concern due to their long mobility and potential to entrain large volumes of material. An emerging procedure for the assessment of debris flow hazards combines detailed field mapping and aerial photo assessment of the slope geomorphology, surficial geology, hillslope processes, and landslide inventory compilation. Regions of potential landslide susceptibility are first delineated objectively using slope angle and landslide density. Each area of susceptibility is sub-divided by hazard, terrain type, and assigned a relative landslide risk ranking. The potential landslide source magnitude is characterized by assessing the volumes of recognized landslides in various geomorphic terrains and surficial geologic units.

The entrainment potential of channelised debris flows is estimated using a channel yield approach. Drainage lines are divided into chainages of similar gradient, channel dimensions, morphology and regolith type. Detailed cross sections are drawn at each chainage interval, and field estimates made for the depth of possible entrainment. Entrainment is considered likely where chanelisation ratios are less than 5:1 (w:d), channel gradient >15°, and entrainable material is present in the channel bottom and sides. Debris flow run-out is assessed empirically by mapping of colluvial fans and determination of credible debris flow paths. A design event is established in each catchment by combining the largest recognized landslide source from the magnitude analysis within the regions of high landslide risk with the total volume of potential entrainment.