Effect of surcharge on gully-manhole flow

In an urban drainage system, surface system and buried system are connected through several linking elements like gullies and manholes. To model an urban flood through a 1D or 2D routing model, proper representation of these linking elements is mandatory. This paper focuses on the effects on inline manhole head loss coefficient and gully flow due to change in manhole surcharges to get knowledge of their characteristics, flow patterns and influence in the overall flow. The solver interFoam with VOF capability under open source CFD toolbox OpenFOAM® is used for the numerical simulations. The CFD model created is a real scale model following the experimental setup installed at the University of Coimbra. The CFD model results were compared with discharge and water depth data at the manhole and velocity data at the gully, measured from the physical model. The manhole head loss coefficient was found very high at low surcharge level and reaching a constant value of 0.3 for higher surcharge levels. The gully flow was found to follow typical orifice flow equation with different discharge coefficients for different water depth conditions of the manhole. Three different surcharged zones were identified at the manhole on which discharge coefficients are dependent. The shear stress pattern at the manhole floor is also found varying due to surcharge depth as well as the gully flow. The variation showed distinctive configuration at different surcharge zones.