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Storm water drains collect storm water in their corresponding catchment areas during rainstorm and convey the collected water through outlets to the sea. Therefore in considering the hydraulic design of stormwater drains other than normal drainage pipe capacity to be taken into consideration one should check the backwater effect due to tidal condition at outlets if the drains are located quite close to the downstream end of outlets.
Storm water drains are normally designed to match soffit to avoid surcharging by backwater effect or when the downstream pipes are running full. Normally pipe size increases from upstream to downstream. For the case of matching drain invert when outlet pipes are fully surcharged by tidal effect of the sea or when the downstream pipes are fully filled with stormwater pipe sections immediately upstream of the outlet are also surcharged too. However for the case of matching pipe soffit the immediate upstream sections of outlet pipes are not totally surcharged even though downstream pipes are running full. However it is not always practical to maintain soffit for all pipelines because it requires sufficient drop to achieve this.
Moreover the flow of stormwater is mainly by gravity in the design of stormwater drains. In case the drains are designed to match invert then it stands a high probability that the flow in the upstream smaller pipes has to be discharged against a head.
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