You have the option to apply additional storage to the shaft and chamber
of the manhole in any proportion. So you could apply all the storage to
the shaft, or the chamber, or a percentage to both.
- Shaft Storage - Applying
the additional storage to the shaft will mean that the storage has no
hydraulic effect until the water level reaches above the level of the
chamber roof. In most cases this will be
appropriate. The additional small pipes that feed into a manhole from
nearby properties often flow in close to the ground surface, and not at
the level of the main sewers.
- Chamber Storage - Applying
the additional storage to the chamber will mean the storage has hydraulic
effects at all flow levels, with the hydrograph being attenuated as it
passes through the manhole. This may be
appropriate if the additional storage comes from small pipes that flow
directly into the main sewers, rather than joining at the manhole.
In most cases, it will be appropriate to apply the additional storage
in the shaft. However, you may choose to apply it in the chamber, or split
between the two, depending on the actual construction of the manhole and
surrounding pipes.