Gully drop connected with manhole is one crucial structural part in several urban drainage systems. This paper analyses the flow pattern and flow hydraulics of a gully-manhole drainage structure. Analysis is done numerically using computational fluid dynamics CFD tools OpenFOAM®. Data from the Dual Drainage / Multi Link Element installation (DD-MLE) at the University of Coimbra hydraulic lab is used to validate the numerical simulations. The experimental model setup consists of a 0.5 m wide channel, a 0.6 × 0.24 × 0.32 m (L × W × D) gully, a gully outlet with an 80 mm diameter pipe and a manhole of 1 m diameter with a 300 mm inlet and outlet pipe connected. The flow pattern is observed under drainage flow conditions with different surcharge heights of the manhole. It has been observed that the intercepted flow through the gully decreases with the increase of surcharge in the manhole. The shear stress at the gully floor is found much higher than that of manhole floor. This indicates the probability that bigger sediment particle can be transported through gully but will remain deposited at the manhole floor. The flow pattern in the manhole changes with the change of surcharge height. The flow through the manhole inlet seems to disperse less at higher surcharge
