What is the problem?
When stormwater (surface runoff of rainwater) gets into the sewer network it creates a big problem – the pipes have to carry much more liquid than they were designed for and then more has to be pumped and treated as well. In Opotiki, the pipes can’t cope and they overflow.
In a perfect world, there shouldn’t be a difference in how much sewage there is when it is raining and when it isn’t (we don’t suddenly flush the toilet more because it is raining!). But in Opotiki, our monitoring shows that the flow in the pipes when it is raining is much higher than when it isn’t. So clearly, the rainwater is getting in there somewhere.
There are two ways this can happen – inflow and infiltration.
What is inflow and infiltration?
(click on the image to see a larger version).
Inflow is rain water directly entering the system through flooded gully traps, stormwater cross connections, roof down pipes, submerged manhole lids etc. Infiltration is water entering the system through leaky pipes and manholes.
What is an illegal connection?
A property owner is responsible for stormwater on their own property and it is illegal to drain stormwater into the sewers. For example, you need to make sure that the downpipe from your roof goes into a soak hole or other appropriate way of managing the stormwater. If it is connected to the gully trap or drains into the sewer in another way, it is an illegal connection.
Auckland Council has some great ideas on their website about ways you can manage stormwater on your property – soak holes, rain gardens, retention ponds, rain tanks and more.
What has the project found out so far?
During 2014, we installed 8 groundwater monitoring sites and 9 sewer monitoring sites. We also started live monitoring of rainfall and river water levels, pump station inflows and outflows and treatment plant inflow and pond water levels.
In addition, in 2013/14 we carried out private sewer lateral investigations, assessing the condition of private sewer lateral lines in the Opotiki Township (you may remember CCTV and sewer smoke tests).
Taken together, this data is indicating that a big part of the problem may be from inflow and infiltration issues.
To better pinpoint the extent of the issue and the sort of solutions we may be able to use, we have selected a pilot suburb to work through a ‘find and fix’ programme.
We have selected the area north of Elliot Street for the pilot are where it is all serviced by Pump Station 3. We will visit each property in the area and where possible, fix stormwater cross-connections, gully traps and lateral defects, repair or replace laterals that were not checked with CCTV last year, check drop bends and seal all road manholes.
With these improvements, we will monitor the situation during heavy rain events in winter 2016 and that will give us a good understanding of how much improvement we get for the cost of the changes. This, in addition to further investigation into the condition of the larger sewer mains, will inform the Council of the best solution for the whole town.