Council monitoring blockage at creek mouth
04 October 2018
High tides may avert need to excavate
For the past week, the mouth of Mooball Creek at Pottsville has been blocked with sand, resulting in very limited tidal flushing.
The creek has become blocked due to: a very large beach width to the south of the creek mouth’s rock training walls; and, the formation of a sand bar east of and across the creek mouth.
The mouths of a number of other creeks in the Tweed are undergoing similar conditions, including both Cudgera and Cudgen creeks. None of these, however, have become blocked.
Council is assessing whether the creek mouth should be cleared and when and how this could be done given that State Government approvals for the works would be required.
Council’s Waterways Program Leader Tom Alletson said small coastal estuary mouths do become blocked with sand from time to time.
“Mooball Creek last became blocked in 2007,” Mr Alletson said. “Prior to the construction of training walls and floodplain drains, the systems would have spent a much greater proportion of time separated from the ocean. The aquatic ecosystems adapt to it, but the difference these days is water quality. Runoff from the catchment, including both urban and agricultural development, means we need to ensure that levels of dissolved oxygen do not fall so low that aquatic fauna is at risk, or that bacteria levels do not climb so high that swimming conditions become unsafe.”
Council has been sampling the water quality in Mooball Creek for the past week and to date those samples met safe levels for the protection of aquatic ecosystems and swimming.
“Water in the creek is becoming a little greener. This is due to the growth of phytoplankton (single-celled, non-toxic green algae), which is building up due to reduced tidal flushing. This is natural and typical of water quality further up the creek where tidal flushing is always more limited.”
Over the weekend, forecast high tides of up to 1.75 meters and a south swell of up to 2.1 metres should flush the entrance bar and refresh waters within the lower reaches of the creek.
This may reduce the size of the entrance bar and result in scouring of the channel through to the ocean.
The forecast for the weekend is for relatively cool weather and rainfall, reducing the risk of high temperatures that could cause a sudden reduction in dissolved oxygen levels.
On Monday, Council will reassess the situation at the creek mouth to determine if intervention is needed.
“The entrance conditions could change significantly over the weekend. There is also a high possibility that a channel dug artificially to allow the creek to drain would be closed off on the following tide due to sand settling straight back into it, so if we can let nature do the job for us, we will achieve a better outcome.”
“We may see beach and entrance sand bar conditions like this persist for several weeks or more, and we can’t open the creek every day, so caution, monitoring and preparedness must be the approach.”
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Caption: Mooball Creek at High Tide on 3 October 2018