Avoiding the Pitfalls in Using Cascading Manholes

Cascade drop manholes are used (where permitted by the local authority) to avoid crossing services; in an attempt to expend energy within the drainage network; or to reduce pipe grades. Hydraulics through drop manholes is not widely documented, and research such as documented in Granata, de Marinis et al, (2011) indicates that drop manholes can have a significant, unexpected, deleterious effect on the hydraulic capacity of the system.

CFD analysis allows for a cost effective assessment of the hydraulics of structures such as drop manholes without having to construct and monitor scale models or ‘take the plunge’ and construct and monitor full sized trial systems. 

CFD models for a number of manhole geometries (square and circular); inlet geometries (centre cascade, offset cascade and external dropper) and outlet geometries (benched, unbenched, centre outlet and offset outlet) have been modelled. The result of this modelling has been charted for energy loss. The use of this chart will allow the designer to assess the projected energy loss and expected flow regime within the manhole for the purposes of design or 1d modelling.