Experimental Investigation of Hydraulic Resistance Behaviour in Subsurface Drains under Downstream Backflow Conditions

Abstract

Urban flash floods can induce downstream backflow conditions that significantly alter the hydraulic behaviour of subsurface drainage systems. Although perforated and modular subsurface drains are widely used in stormwater management, their hydraulic resistance characteristics under restricted outlet conditions remain insufficiently understood. This study experimentally investigates the flow behaviour of perforated subsurface drains (PSD) and subsurface drain modules (SDM) under controlled backflow simulated using a partially open gate at a 10 cm water level. Experiments were conducted in a laboratory flume at three longitudinal slopes (1:500, 1:750, and 1:1000) to assess slope-dependent responses. The relationships between Manning’s roughness coefficient (n), velocity (v), flow depth (y), and Froude number (Fr) were analysed. At 1:500, stable inverse n–v relationships were observed, whereas correlation strength deteriorated markedly at 1:1000, indicating increasing boundary-controlled backwater influence. Weak linear associations between n and flow depth suggest that effective roughness under partial submergence cannot be described solely by depth variation. The n–Fr relationship further demonstrated regime-dependent resistance behaviour, particularly under mild gradients. SDM exhibited comparatively more stable resistance characteristics across slopes, while PSD showed greater turbulence sensitivity under partially submerged conditions. These findings highlight the necessity of incorporating downstream boundary effects in the design of resilient urban subsurface drainage systems.