Where the piped stormwater network joins with the open channel network can often be a point at which numerous issues arise, particularly during flood events. At the upstream end of the system there can be large natural catchments which bring down large volumes of water and debris (gravel and vegetation) which result in blockage of intakes. At the downstream end of the piped stormwater network there will be a discharge point into either a natural stream or river or the coast. The dynamic nature of these boundaries, particularly with regard to aggrading bed levels can significantly affect the performance of the network.
In the large flood events in the Wellington Region in April/May 2015 there were significant issues at both the upstream and downstream ends of the piped stormwater network with intakes and outfalls being blocked or buried with gravel. At the upstream end of the system the common issue was gravel and vegetation that was bought down during the event blocked culvert intakes. At the downstream end of the network a number of stormwater outlets were blocked due to longer term build of river bed levels.
The combined effect of the upstream and downstream blockages was significant overflows, surcharging and inundation of a large number of residential and commercial properties including most of the Porirua CBD.
This event highlighted the need for more active management of sediment and river bed levels in both the upstream and downstream channels to ensure the performance of the urban network is not compromised when it is most needed to function effectively.
In response to this event Wellington Water has initiated investigations to install sediment traps at all of the key locations where large upstream catchments meet with the stormwater network. A more active monitoring approach for critical reaches of the downstream river systems is also being developed in collaboration with Greater Wellington Regional Council to ensure that outlets remain functional.