Report No. : GEO Report No. 292

Report Title : Review of Methods in Estimating Surface Runoff from Natural Terrain (2013), 98 p.

Author : Fugro Scott Wilson Joint Venture

Abstract

Slope surface drainage plays an important role in preventing erosion and improving slope stability. Inadequate slope surface drainage could lead to serious landslides and/or flooding. Guidelines on estimating surface runoff for design of slope drainage works in Hong Kong are given in various manuals and guidance notes issued by the Geotechnical Engineering Office of Civil Engineering and Development Department (GEO/CEDD), Highways Department and Drainage Services Department.

This Report reviews the prevailing practices in surface runoff estimation in Hong Kong and various overseas countries. It includes an assessment of the stream gauge data collected by the Water Supplies Department for four local watersheds to enhance the understanding of the surface runoff characteristics in natural terrain. Various methods of estimating surface runoff have been reviewed and the continued use of the Rational Method with the Bransby-Williams equation is recommended. However, it was found that the peak runoffs estimated by the Rational Method were exceeded by recorded flows in the watersheds in some rainstorms with return periods of 17 years or less, which were much less than the design return period of 200 years. Based on this finding and the review, this Report discusses and recommends the use of weighted average runoff coefficient for natural terrain catchments to take account of different ground conditions within the watersheds, and an increase in the runoff coefficient of steep natural terrain by at least 50% to allow for the effect of antecedent rainfall.

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