Sensitivity analysis of the response of one-dimensional modeling of 90° bends in urban drainage networks to different manhole sizes

This study investigates the hydraulic behavior of 90° manholes in urban drainage networks and evaluates the reliability of 1D modeling in reproducing flow conditions characterized by significant energy losses. Computational fluid dynamics (CFD) modeling was applied to simulate the 3D flow within manholes under realistic open-channel conditions. A local sensitivity analysis was performed to assess the effect of manhole size on the accuracy of 1D simulations. The results show that 1D modeling effectively reproduces flow behavior in small manholes but progressively loses reliability as the size increases, with large chambers functioning similarly to storage units. For circular manholes, the Nash–Sutcliffe efficiency decreased from 1.00 at 0.6 m diameter to 0.74 at 10 m, while for rectangular manholes it declined from 1.00 at a 1 m side length to 0.61 at 10 m. The first notable degradation in representativeness occurred at 6 m, where NSE dropped to 0.94 (circular) and 0.92 (rectangular). These thresholds indicate the conditions under which 1D formulations become inadequate for capturing the combined effects of energy dissipation and internal storage. Polynomial equations were derived, providing practitioners with quantitative tools for identifying critical locations in sewer networks where energy losses cannot be reliably represented by 1D modeling approaches.