Drinking-water Management Current Issues Update 4

Update 4 – Emergency Response Options - October 2016

The purpose of these updates is to create a knowledge environment where all members can access the best information to plan, operate and maintain drinking water networks which takes account of any of the learnings from Havelock North and generally accepted best practice.

Coping with the initial stages of an event without prior planning can leave you on the back foot in a reactive mode for weeks. Having pre-planned contingency plans can put you in the driving seat in an event rather than jumping from one issue to the next.

  • 1.Media
  • 2.Emergency Chlorine dosing
  • 3.Removing Boil Water Notice plan
  • 4.Flushing Plan
  • 5.Assistance


One of the unexpected demands that come at you during an event is coping with the media. If left to their own devices requests for interviews and information will come at you randomly throughout the day. By creating a media plan and funnelling all requests through one person you can then set times when media statements will be given and have a reasonable chance of getting a consistent message out to the public.

Media can also help aid during an event; they are set up than water utilities to contact affected users if a boil water notice needs to be issued. There is no easy way to ensure everybody is notified in such events. The use of digital motorway signs or the portable trailer digital signs on the side of the road have been effective in some cases. Some emergency management centres have a high social media following (i.e. Facebook & Twitter), the more methods that are used the better chance of notifying everyone.

Emergency Chlorine Dosing

If you have an unchlorinated supply then one of the obvious contingencies required is an emergency chlorine dosing system. There are a number of different emergency chlorine systems in place around the country, some are:

  • Packaged self-contained portable hypo-chlorite systems
  • Batch HTH chlorine plants
  • Pre- installed hypo-chlorite dosing plants that dose water in to the fluoride dilution tanks. (HFa gets turned off and removed from the same enclosure)
  • Small gas chlorinators attached to 90 kg bottles

The ability to test these systems in an unchlorinated network is always a challenge. Any stored hypo-chlorite will eventually lose its effectiveness. If online chlorine analysers are used, they typically take 24 hours to become stable when changing from a non-chlorinated supply to a chlorinated supply.

In the case of a significant seismic event, it is considered good practice to initiate chlorine dosing straight away. The reasoning behind this is both source and reticulation related; broken sewer mains may now be present near any bores or reticulation mains.

Removing Boil Water Notice Plans

Most authorities have pre-prepared boil water notices as a contingency but not many have thought about the issues involved in getting a boil water notice lifted. Identifying who the various parties are that need to be consulted and the various steps that have to be taken prior to getting back to normal are all part of this plan. In some past national events it has taken up to 6 weeks after the event before the boil water notices have been lifted and people’s lives have got back to a sense of normality. The local District Health Board officer will definitely be involved in this process.

The steps involved in removing a boil water notice are:

  • Eliminate, isolate, minimise the contamination source
  • Ensuring a non-contaminated source is available or treatment can be provided to minimise risk.
  • Ensuring the various zones within the reticulation network are isolated from each other, avoiding any risk of recontamination
  • Method of communicating requirements to those with onsite storage or private communities, large industries, dialysis patients, medical facilities.
    • Register of where the sites are and if they have been contacted.
    • Timing of when to flush, ensuring they are flushing with a non-contaminated source.
  • Creating a flushing plan if the contamination is protozoa, chemical or toxin related.
  • Communication plan to consumers
    • Notification of possible reduced pressure during flushing
    • General advice on individual residential flushing requirements
    • Where to look for notification of the boil water notice removal
  • Testing plan and requirements

Flushing Plans

The source of the contamination will determine if a flushing plan is required. If normal levels of chlorine will remove the contamination then a testing plan is all that should be required. If the contamination cannot be safely mitigated by chlorine alone then a flushing plan is required.

In previous contamination events that have occurred in New Zealand, the flushing has been carried out by systematically isolating each section of pipe work, opening hydrants for a recorded duration and where required repeating the process from the other direction, this was to ensure that no dead sections of pipes have been left behind due to hydrant location and any turbidity stirred up by the isolation and flushing actions and entrapped air has been flushed clear.

In Havelock North’s case a recently calibrated reticulation hydraulic model was available. Modellers were able to identify and model which hydrant locations needed to be left running in order to displace three times the reticulation volume through the system. The flow rates were calculated using 5 L/s per hydrant and excluded normal demand. The demand was purposely excluded as a contingency volume to further ensure flushing was achieved.

Some locations were chosen in order to minimise local surface flooding caused by flushing. There were a few dead leg sections that required to be separately flushed.

The benefits of using a hydraulic model to flush the system are:

  • That it was a continuous flush process and therefore took less time than systematically working through the network, (calculated to be approximately 4 days quicker for Havelock North for the 100mm diameter mains alone)
  • Fewer crews required for the flushing, although a number of hydrant standpipes are required.
  • Health and Safety benefits for crews from less time spent on the road
  • Less inconvenience to the consumers through less low pressure occasions and traffic management.

High level zone reservoirs with combined inlet / outlet main were isolated from the main zone with the use of RPZ’s at the pump stations supplying the zone; this was to ensure a single point of failure couldn’t cause the low level zone to become re-contaminated.


“It’s not often easy to think of draining the swamp when you are up to your arm pits in alligators”

Individuals tolerance to stress outside of their normal comfort zone all differ, some thrive on it, others become overwhelmed. Emergency events that span multiple days are not typical BAU for water utilities and they are not normally resourced to cope with long drawn out events. The Water industry in New Zealand is small, approachable and knowledge sharing, we all know others in the industry with similar issues, plants and networks.
Don’t be afraid to ask for assistance and delegate tasks to those who will assist you.

Noel Roberts
Technical Manager
Water New Zealand

Havelock North Water New Zealand