Abstract:Hydraulic experiments are conducted to investigate the Manning roughness coefficient of the tunnel drainage system. Multi-pier with a rectangular cross-section is used for obstacles. Various uniform flows are achieved by adjusting the discharge and the bottom slope for each experiment to evaluate the Manning roughness coefficient easily and accurately. The Manning roughness coefficients for the multi-pier are evaluated for the various cases of longitudinal and lateral pier spacing and water depth. The results show that the Manning roughness coefficient increases with increasing water depth. It is also found that,when the water depth is fixed,the Manning roughness coefficient increases as the longitudinal pier spacing increases up to approximately the pier width. However,it decreases as the longitudinal pier spacing increases further beyond the pier width. On the other hand,when the lateral pier spacing increases,the Manning roughness coefficient decreases. The major findings in this study can be used for the derivation of an empirical formula to estimate the flow capacity of the tunnel drainage system if further experiments are conducted for various shapes and alignments of multi-pier.