By Stephen Martin, Water and Wastewater, Townsville City Council.
Between 27 January and 8 February 2019, Townsville received 1250mm of rain. Ross River Dam rose to 250%, the highest level ever recorded and only 2m from the top of the wall, with all gates fully open and 1200m3/s of water being released into the Ross River.
There was extensive flooding around the Ross River and other minor rivers and creeks around Townsville. 3300 homes were fully inundated with flood waters and another 8500 were flood affected. 16,000 properties lost power during the event and roads and rail lines were cut to the north, south and west of Townsville.
The monsoon event had a huge impact on Townsville’s water and wastewater assets. The Ross River Dam suffered extensive erosion around the outlet gates, which forced the Townsville Water and Wastewater Department (TWW) to close the gates as soon as it was practical to do so. That in turn created a large fish kill in front of the dam gates as thousands of fish were trapped in shallow pools as the water receded. The fish kill took well over a week for excavators and vacuum trucks to clean up.
BMD was bought in to dump thousands of tonnes of rock into the eroded sections of the dam. This stabilised the area and allowed us to open the gates and cone valves again to reduce the water level in the dam back to 100%.
Our sewer treatment plants were inundated with stormwater and all six plants bypassed significant amounts of raw (screened) sewerage over 11 days of the event.
We had a major break in a 750mm DICL main at the secondary screens at our Cleveland Bay Plant, which meant the whole plant had to be bypassed for 11 days while the break was repaired. H2S had eaten the DICL pipe away so that the top third of the pipe was non-existent. It was only when the flows into the plant were 11 times higher than ADWF that the holes in the pipe were discovered.
Townsville City Council (TCC) workers and contractors rebuilt the manifold and pipeline using GRP pipe, which conveniently has the same outside diameter as DICL. TCC is now inspecting all DICL mains in sewer treatment plants to ensure we do not have similar problems.
We also had a major break on a 600mm DICL sewer rising main, again due to H2S attack. That break took over four weeks to repair because we could only isolate it for about five hours per day, and we had to replace a 55m length plus one air valve chamber. We have inspected every air valve (high points) along that rising main and have calculated that we will need to replace well over 200m of main in the next couple of years.
Sections of the main that are fully flooded at all times are in excellent condition. It is only those areas of the pipe that have an air/gas space when the pumps are not running that will need replacing. We will replace those areas with poly or GRP pipe.
We had approximately 66 of our 189 sewer pump stations lose power during the event. Eight of those pump stations had their electrical boards covered by flood water and were therefore out of action for over a week. The rest were restored fairly quickly or had generator power available.
At the peak of the event, 75 sewer pump stations were flooded (42%) and 90% of pump stations experienced more than five times ADWF. Townsville has had severe inflow/ infiltration problems for quite a number of years. Extensive sewer relining programs have not stemmed the problems.
There were more than 600 homes that had sewerage discharged either inside the house or into the yard. And every environmental overflow was opened, discharging raw sewerage into creeks, stormwaters and parkland.
TCC is now inspecting all manholes in the flood zones as these were the most likely source of infiltration. We have identified a number of manholes where the top ring floated, allowing flood water to drain into the sewer. We are also placing overflow relief gully (ORG) caps in all properties where the yards were under water. This will prevent stormwater from draining back through the ORGs.
Townsville has always allowed for swimming pools to be backwashed into the sewers. This exacerbated our inflow problems during and after the rain event. We are now looking at changing our planning standards to stop this from happening.
Townsville experienced a number of major water main breaks during the event. There was a 375mm DICL break under the Ross River. It took our crews some time to isolate this main on the night that it broke because all of our plans for the main were wrong and there were a number of valves covered by recent road works. But we managed to stop emptying a critical reservoir on that night and everyone went home relieved. About five days later we discovered that one of our largest reservoirs, Mt Louisa was also dropping rapidly so we started investigating the possible cause. It turned out that there was a 250mm cross connection to that same 375mm main that did not appear on any plan. Once that was isolated the reservoirs recovered quickly.
There was also a 250mm break under a lake in the flooded suburb of Annandale. All the valves for that main were also underwater at the time, but the local reservoir was dropping fast and a large number of customers would have been without water if we did not act fast. We had Army assistance to take our crews into the flooded area to the GPS coordinates for those valves. Our crews got a little wet but they were able to find the valves and isolate the mains.
Five of the water main breaks during this time were due to the fact that no thrust blocks had been installed during construction. With the ground becoming so soft, the flexible joints gave way. This becomes very hard to fix in an emergency situation because you have to wait for the new thrust blocks to set, and hope that the sodden ground holds them, before you can recharge the main. In a lot of cases we were forced to isolate the break section and return when the ground had dried out.
Old AC mains and old poly mains and services constituted the majority of the breaks received at this time.
There was a break on our 450mm steel mains that runs from Crystal Creek to the Northern Water Treatment Plant. This was caused by a log being washed downstream that sheared off a scour tee. This took two weeks to repair, mainly because it was about 10 days before we could get into the area to find and assess the break. What has come from this incident is a detailed study of our Mt Spec pipeline to discover all the vulnerable points where similar incidents can happen. There are over 30 creek or river crossings on that critical pipeline, so we need to develop a plan to protect it during similar rain events.
This article appeared in the April 2019 edition of Queensland Source.