Graham Askey suggests that Rocky Creek catchment above the proposed Dunoon Dam (locally called the DuD) would deliver 10% of the water flowing from the entire catchment above Lismore despite being only 3.57% of the land area.  

There are several problems with this claim:

1. Rocky Creek is minor compared with the other streams from the high-rainfall Nightcap Range.  Goolmangar, Tuntable, Terania, Wilsons and Coopers Creeks all send flood water to Lismore but not via the proposed DuD. 

In the last flood, 930 mm was recorded in 18 hours at the top end of Terania Creek.  That is a record for NSW.  We can assume that the other creeks also recorded astronomical amounts in their upper reaches.  However, until we have accurate data collected from all the high-rainfall areas it is not possible to guess which creeks contributed most – all we have is their relative catchment areas.

2. Ballina also receives water from high rainfall streams not just on the Nightcap Range but also from the Border Ranges (as shown on the attached map).  Lismore’s catchment (1400 sq. km) is 20.4% of Ballina’s catchment (6862 sq. km), so the catchment of Rocky Creek above the DuD is only 0.73% of Ballina’s catchment.   Even if it could be established that Rocky Creek above the DuD supplies a disproportionate amount of flood water it is still negligible on the scale of the floodplain.  

3. "Rain bombs”, eg the 431 mm that hit Alstonville in late March 2022, can occur anywhere.  Extreme rainfall is not restricted to mountain creeks.

4. To mitigate floods, dams need to be empty or low but dams are never low during flood events. Emptying or lowering dams in anticipation of extreme weather events is never simple and “success” is never guaranteed.  The recriminations about the Wivenhoe Dam releases during the Brisbane floods are a good example. 

5. Emptying the DuD might be technically possible but would those in an already catastrophically flooded downstream landscape agree?  Imagine telling people that you are adding to their flood problems by releasing the dam because there might be another flood event hard on the heels of the first one.  And that there might be a third flood which would require emptying the dam immediately yet again? 

Climate change caused by human-induced global heating has now rendered even the most sophisticated probability estimates unreliable at best.  We need to admit that we don’t know how big or how frequent the coming floods will be.   

In the end, it is irrelevant whether the correct figure is 3.57% or 10%.   Debating the relative size of stream contributions to floods is like the occupants of a small boat with storm-waves crashing over the side debating whether a tea-cup or a coffee mug is better for bailing.