13 th International Conference on Urban Drainage, Sarawak, Malaysia, 7–12 September 2014 1 Self-Cleansing Design of Rectangular Open Storm Sewer Charles Hin Joo BONG 1,3* , Tze Liang LAU 2 , Aminuddin AB GHANI 3 1 Civil Engineering Department, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia 2 School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300, Nibong Tebal, Penang, Malaysia 3 River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300, Nibong Tebal, Penang, Malaysia *Corresponding author Email: bhjcharles@feng.unimas.my ABSTRACT The aim of the present study is to develop incipient motion equation and design chart based on critical velocity approach for the self-cleansing design of rectangular open storm sewer. Data from experimental work on incipient motion from rectangular flumes with two different widths, namely 0.3 m and 0.6 m were subjected to multiple linear regression analysis. Selected regression models for critical velocity were further subjected to performance test by using the data from three different authors in the literature. The best regression model was proposed as the critical velocity equation and was used to develop a self-cleansing design chart. The design chart could be used to determine the minimum slope required for the purpose of self-cleansing design for the respective standard open storm sewer size available from the manufacturer of open concrete rectangular sewer. KEYWORDS Critical velocity, incipient motion, open storm sewer, self-cleansing design INTRODUCTION Open sewer system has been frequently used in developing country like Malaysia to convey storm water runoff (Bong et al., 2013). Though open sewer system could be quite effective in rapid surface runoff removal; sediment tends to build up after a period of time. For the purpose of self-cleansing design to reduce sedimentation, a minimum average flow velocity of not less than 0.6 m/s has been recommended for lined open drain by “Urban Stormwater Management Manual for Malaysia” (DID, 2000) which was replaced later by “Urban Stormwater Management Manual for Malaysia – 2 nd Edition” (DID, 2012). However, the adopted minimum velocity value appear to be developed from experience without underlying research or theoretical justification and no account is taken of the sediment characteristics and hydraulic aspects of the channel (Butler et al., 1996). A more viable approach is to incorporate some aspects of sediment and channel characteristics into self-cleansing design through incipient motion equation (Novak and Nalluri, 1984; El-Zaemey, 1991). The self-cleansing design criteria for closed conduit sewer are widely available in the literature. Design charts relating the flow discharge with the sewer gradient and pipe diameter (Nalluri and Ab. Ghani, 1996; Butler et al., 2003; Bizier, 2007) have been developed for