ICM's default representation of manholes could be more accurate. The limitations of current setup allows accurate representation of either the shaft or the benching but not both. Thus most people tend to apply chamber areas to invert which overestimates the volume prior to surcharge or the shaft to top of benching which overestimates the volume at the surface.
Because we can now define the benching in the database, it would be better if the default dimensions for manholes were altered to represent 4 critical levels, invert, top of benching, roof, and cover. With associated areas as channel, chamber and shaft.
In this setup given a fully benched manhole, the channel volume is marginal since it is already accounted for in the connecting pipes. The chamber volume would start at top of benching and shaft from roof to cover.
This would also allow better representation of ancillary structures without the need to use storage ponds.
Further to this, the default chamber area calculations could also be improved by applying a specification database field (or date of construction) to the manhole, thus sewers for adoption or CP2005 etc. The specification would drive the default chamber area / shaft calculations based on the rules set out in these specs. This is important as the current default calculations underestimate the volumes of manholes on small diameter systems. Since these systems make up the vast majority of any network, the combined loss in system volume overall is large and contributes to overpredictions of flow and flood volume in extreme events.
