AbstractThe methodology commonly adopted for sewer design employs Manning’s equation for hydraulic calculations considering constant surface roughness values, thus neglecting the presence of biofilms, sediments, and obstacles under real operating situations. This paper analyzes the experimental results from several sewers and sewer design criteria and standards adopted around the world to mathematically model 3,964 representative sewage operation data points from the literature and 312 new pieces of data obtained in experiments carried out on a pilot scale and in actual sewer systems in Rio de Janeiro State (Brazil), encompassing different diameters, slopes, and materials. After statistical analysis and model validation, new design criteria are proposed for roughness values under real operating conditions of gravity sewers expressed using two novel roughness equations. These results lead to the recommendation to adopt Manning’s roughness equal to 0.013 (which amounts to a 1-mm roughness height) for early operation, 0.015 (or 3 mm) for long-term operation, and 0.024 (or 36 mm) for both internally corrugated pipes and sewers retaining deposited sediments or containing obstacles.

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