Two dredges now pumping salty water from weir
Tuesday 29 August 2017
Council has now got two dredges working to pump salty water from the lower depths of the Bray Park Weir.
Together they are pumping about 380 litres a second out from depths up to 12 metres and discharging it over the weir wall and downstream.
The dissolved solids content from 12 metres is just shy of 3000mg/litre.
Tweed’s normal dissolved solids content for the drinking water it treats at Bray Park Water Treatment Plant is 160mg/litre. Levels currently in the mains pipes are at about 360mg/litre, indicating residents may still taste the salt for a few days’ yet. However, water treated at the plant and due for release in the next few days is at near-normal levels of 250gm/litre.
The weir is 19 metres at its deepest point and salt concentrations at the bottom of that hole are likely to be very high as they have been built up over many years with heavier salty waters sinking. It is not known yet whether the current dredging operation will manage to pull water from that depth.
Meanwhile Council is continuing to harvest water from the weir up to about two metres deep and refresh it with a continual release of 50 megalitres a day from Clarrie Hall Dam.
“We are continuing to get good quality water into the Bray Park Water Treatment Plant but it will still be several days before the salty water has been removed through household taps with normal water consumption,” Manager Water and Wastewater Anthony Burnham said.
The water is good to drink but should not be used on salt-sensitive plants or in freshwater fish tanks.
Meanwhile the Bureau of Meteorology has confirmed that a number of climatic factors came together in a rare event resulting in the overtopping of the weir wall on the night of Monday 21 August.
First, there was a run of high tides up to 420mm higher than predicted. Second, sea waters were saltier than normal. Third, ocean temperatures were higher than normal. And fourth, a ‘trapped’ wave travelled north up the coastline to the Tweed.
“Under normal operating procedures, Council uses tidal information together with downstream river flows to manage the risk of the weir wall overtopping during high tides,” Mr Burnham said. “If the information indicates overtopping is likely, we raise the weir wall with sandbags. However, that data failed us this time given the ocean anomalies that were in play.”
Dredging of the lower depths of the weir will continue for the remainder of the week.
