Question
Why is it best to design storm water drains to match soffit?

Answer
Storm water drains gather storm water in their corresponding catchment areas throughout rainstorm and express collected water through outlets to sea. So, in considering hydraulic designing of storm water drains, previous than regular drainage pipe ability to be taken into consideration, one have to check backwater effect due to tidal condition at outlets if drains are situated quite close to downstream end of outlets.

Storm water drains are usually calculated to match soffit to keep away from surcharging by backwater result or when downstream pipes are organization full. Usually pipe size increase from upstream to downstream. For in case of matching drain invert, while outlet pipes are fully surcharged by tidal effect of sea or while downstream pipes are fully filled with storm water, pipe sections right away upstream of outlet are as well surcharged .though, for in case of matching pipe soffit, immediate upstream sections of outlet pipes are not entirely surcharged although downstream pipes are running full. But, it is not always practical to uphold soffit for all pipelines since it requires enough drop to achieve this.

Furthermore, flow of storm water is mostly by gravity in plan of storm water drains. In case drains are designed to equal invert, next it stands a high prospect that flow in upstream smaller pipes has to be free next to a head.