Henrietta Fraser’s letter last week raises some common questions about the breakwaters and rock walls at Brunswick Heads. These works were done in the early 1960s to provide a safe harbour for the local fishing fleet, which was nearly wiped out by cyclones in the mid-1950s at Byron Bay.
The Brunswick River entrance and estuary were wide and shallow, and the purpose of the works was to create a narrow channel that would hopefully be kept deep by tidal flows. Hence the breakwaters to shelter a narrow channel at the bar and the internal rock walls to concentrate tidal flows into a narrow and deep channel to the boatharbour, and also to protect Brunswick Heads from coastal erosion and storm surges.
The east-west rock wall across the entrance to Marshalls Creek was part of this work, and originally had an opening at the eastern end only. Apparently the channel to the boat harbour kept silting up, so an opening was made at the western end of the wall and the internal north-south wall in Readings Bay was built in the late 1960s or early 1970s to increase the tidal flow in the main river channel to the boat harbour.
This north-south rock wall was originally the same height as the other rock walls, but after the 1972 flood its height was reduced to about three quarters of high tide level, apparently in 1973. I don’t know whether the effect of all these works on flooding was considered when they were built, but I suspect not. The narrowing of the channels has probably increased flood levels from floods generated by heavy rainfall, but also reduced flooding from storm surges and cyclones by restricting the inflow of ocean water into the estuary.
The north-south rock wall in Readings Bay is often thought to increase flooding in Marshalls Creek, but there is no evidence to support this theory. In 1989 modelling was done on removing the north-south wall and lowering the east-west wall to about the current height of the north-south wall. The effect in a one per cent (1-in-100 year) flood was an increase in flood levels downstream of Orana Bridge of 34mm, and a reduction in levels at New Brighton of 14mm and at Ocean Shores of 10mm.
I have a photo of the north-south wall in a flood in 1989, and the wall is clearly having no effect on the water flow, although there was a strong flow in the channel past Torakina Park and out through the breakwaters. The reason for this is, I believe, that the main river and Simpsons and Marshalls Creeks all join together at the same point, and their combined flow is too great for the channel past Torakina Park, creating a choke point right at the exit point of Marshalls Creek.
All these issues will be looked at in a new and more detailed assessment of the 2022 flood, although it is taking a very long time to get any reports on that event. As for the mythical ‘clogged rivers’, that might have to wait for another day.
