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Inflow & Infiltration

What is Inflow & Infiltration?

Inflow is rainwater or groundwater entering the wastewater network via a range of defects on public and private property.

Typical inflow sources may consist of:

  • Roof downpipe with incorrect connections
  • Low gully traps that act as low drainage points
  • Damaged or improperly constructed gully traps
  • At-surface manhole defects like holes in the lid or rim, particularly where surface ponding occurs.

Infiltration is when rainwater or groundwater drains into the ground enters the wastewater network. Ways that this can happen include:

  • Cracked public sewer or private sewer pipes
  • Open and moved joints in either public or private sewer pipes
  • Cracks or construction joint leaks in manholes, and other wastewater structures

The wastewater network is designed to only handle wastewater we all produce.Stormwater inputs limit how much our wastewater pipes can hold.  

If enough stormwater enters the wastewater network it could cause issues like wastewater network overflows, sending wastewater onto people’s property and into the environment.  

It can alsoinundate or effect the wastewater treatment plants, which could further increase the number of,or volume of overflows in the wastewater network, as well as reducing the level of treatment the wastewater receives before being discharged into the environment.   

Flooding the network with inflow and infiltration also increases the cost associated with wastewater pumping and treatment.  

This is not a new issue in Central Hawke’s Bay, but it is something council must address to reinforce durable infrastructure and allow for smart growth.  

We have set aside funding in our Long Term Plan (LTP) to address this issue. In the current LTP period, we have been supported by Central Government by securing Central Government funding from the Three Waters Reform programme for this project. This has meant we have been able to invest more resources into investigation of our network. 

Other actions we are taking include: 

  • We are using CCTV cameras to identify any leaking pipes 

  • Inspecting manholes for any defects 

  • Looking at private properties to identify pipes sending stormwater into the wastewater network 

  • Fixing issues with existing assetse.g.jetting pipes to clear infiltration and fat build up, replacing or sealing broken pipes and manholes. 

When we find a defect, or someone reports an overflow, we are fixing, repairing, or replacing pipes and manholes. Our scheduled pipe renewals also play a part in this. Find out more about our renewals programme here: 

  • Monitoring flows 

Flow monitoring studies, and hydraulic modelling of the network helps us determine where stormwater comes from and how much flow there is. We also investigate issues that arise on the network that can help pinpoint where stormwater is entering. 

Look at the stormwater and wastewater connections on your property. If you find any defects, there are steps to take to fix them and ensure everything is running correctly. 


What the defect causes 


Downpipe connected to gully trap   

Stormwater will enter wastewater network 

Connect downpipe to appropriate stormwater outlet    


Downpipe to ground    

Water ends up ponding on the ground, and can end up under the house, resulting in rising dampness 

Connect downpipe to appropriate stormwateroutlet    

Gully trap level with the ground    

Ponding water can overtop into the gully traps because the sides are not high enough    

Raise gully trap or put sealed barrier around the gully trap. Gully traps should be raised 10cm from the ground   


Cracked or leaking gully trap   

Ponding water can get into gully trap    

Replace gully trap   

Terminal vent missing cover   

Vermin can get into your wastewater pipes   

Protect your terminal vent with a vent pipe cover   

No spouting, leaky spouting  

Water ends up ponding on the ground, and can end up under the house, resulting in rising damp  

The water can also end up getting into the pipework on your property if ponding occurs in the same area as the pipework 

Install spouting and downpipe, connect to appropriate stormwater outlet    

Broken downpipe   

Allows stormwater to spill onto ground with the same impact as the downpipe to ground   

Fix the broken downpipe   

No back on gully trap    

Ponding water can get into gully trap   


Install a new concrete surround, or replace gully trap   

Covered gully trap    

Covered gully traps mean sewage might back up into the house    

The gully trap is an overflow point. If the lateral to the Council network becomes blocked, the wastewater would normally come out the gully trap. By covering it, the wastewater would come out the next lowest point   

Educate residents to let them know why they shouldn’t cover them    

Please let us know if you spot any overflows from the wastewater network. These can be caused by stormwater getting into the network or a system blockage, which requires immediate attention. 

Contact your local plumber or drainlayer if you find a defect on your property. They will determine if there is a problem on your propertyor inournetwork. 

If you are unsure if you have a defect, you can send us a picture via our Send Snap Solve App, and we can advise. 

You can also contact our team or call us on 06 857 8060. 


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