Absence of a Hydraulic Threshold in Small-Diameter Surcharged Manholes: Fid-Bau Portal

Absence of a Hydraulic Threshold in Small-Diameter Surcharged Manholes

Stovin, Virginia / Bennett, Paul / Guymer, Ian

Previous research into surcharged manholes has highlighted the existence of a threshold surcharge level that separates two distinctly-different hydraulic regimes. Sharp changes in manhole energy loss and solute transport characteristics occur when the surcharge depth passes through the threshold level. With respect to solute transport, two scale-independent cumulative residence time distributions (CRTDs) have been identified, corresponding to the below-threshold and above-threshold hydraulic regimes. However, previous studies focused on large diameter manholes, in which the manhole diameter ( $Φ_{m}$ ) was at least 4.4 times greater than the pipe diameter ( $Φ_{p}$ ). This paper utilizes a validated computational fluid dynamics (CFD) modeling approach to explore the hydraulic behavior and mixing processes in small-diameter surcharged manholes ( $1.5 < Φ_{m} / Φ_{p} < 4.4$ ). It is shown that the hydraulic threshold does not exist in the small diameter manholes; instead, the flow field is characterized by short-circuiting throughout the full range of surcharge depths. Data generated at low surcharge levels suggest that the mixing effects in the below-threshold region are not independent of surcharge level, as had previously been suggested. The absence of the threshold in small-diameter manholes is explained with reference to jet theory. Several previous studies that have characterized mixing and/or energy losses in surcharged manholes are revisited, allowing findings to be generalized across a broader range of manhole configurations, including effects attributable to benching and change in outlet angle.