As　urbanization　grows　in　size,　the　problems　of　flash　floods　and　water　pollution　are　expected　to　worsen,　so　viable　and　cost-effective　solutions　are　essential　to　reduce　the　impacts.　The　Bio-Ecological　Drainage　System　(BIOECODS)　was　developed　to　demonstrate　the　use　of　＇control　at　source＇　approaches　to　urban　stormwater　management.　This　research　attempts　to　analyse　the　optimal　design　of　a　subsurface　conveyance　system　(modular　conduit)　that　is　available　in　a　case　study　with　a　BIOECOD　project.　This　modelling　exercise　uses　a　novel　technique　to　merge　the　surface　and　online　subsurface　flow.　Through　the　InfoWorks　SD　software,　the　BIOECODS　model　has　been　calibrated　and　validated　using　rainfall　events　with　different　intensities,　rainfall　amounts,　and　durations.　The　developed　model　was　then　further　analysed　to　obtain　the　optimum　design　of　online　subsurface　modular　conduit　in　the　BIOECODS　project,　according　to　different　scenarios.　The　results　show　that　the　subsurface　modular　conduit　in　the　study　area　has　an　optimal　size　of　0.45　m　height　and　is　capable　of　attenuating　peak　flow　at　the　downstream　outlet　by　more　than　60%.　It　is　expected　that　the　innovative　modelling　technique　and　the　optimum　design　of　an　online　subsurface　conveyance　system　can　be　of　interest　to　the　community.