Has building the Southport Seaway made a difference to flood levels in rivers?
Yes, for the lower reaches of rivers. In the Australia Day 1974 flood, the Broadwater filled to a level of RL 1.65 metres (Australian Height Datum (AHD), or about 1.65 metres above mean tide level. Most of this water was flood water runoff from the rivers and earlier inflow from Moreton Bay. In 1974 the Broadwater had limited tidal connections to the ocean, but with the construction of the Seaway, floodwaters can now escape to the ocean. If the 1974 rainfall event was to be repeated, it is estimated that the Broadwater would be 0.3 metres to 0.65 metres less than ocurred in 1974. However, the Seaway also means that the Broadwater is now much more susceptible to storm surge than it was in 1974.
