Impact of SEQ flooding on waterway health
13 January 2011
The unusually high, intense rainfall occurring in South East Queensland (SEQ), and the above average rainfall received since mid November, is causing major high flows throughout the region.
While soil acts as a natural sponge soaking up water, the current rainfall events have left the whole of SEQ completely saturated, and there is no longer any natural capacity for water to soak into our soils. As a result, the water has no choice but to runoff.
Impact of flows in rural areas
- Along creeks where trees, shrubs and grass are absent, the creek banks will suffer erosion as there is no protection for the soil and creek bank material to be retained.
- Places with existing eroded gullies in paddocks or people’s backyards will continue to erode.
- In any grazed paddocks where the grass coverage is poor, surface soil will also be lost.
- Soil has accumulated in the beds of many creeks from historic erosion and those materials are now being re-suspended and moved along the creeks. Eventually theses soil materials will end up in the estuarine and bay areas.
- Even in well protected areas with good tree and grass cover, the saturated soil, moisture and high speed of overland flow will result in some natural soil loss. In less protected areas, soil losses will be higher. This is the natural process of soil movement that happens in unusually high intensity rain events.
Impact on waterway health
- Eroded soil will increase the turbidity of all waterways in the region. Soil particles will have nutrients attached, or will dissolve and become part of the creek flow. For many creeks and streams, increased turbidity is likely to remain for many weeks.
- When the creek and river flows reduce back to normal levels, a lot of the soil will settle and smother the creek and river bottoms.
- In the big river flows such as the Brisbane River, large plumes of soil will move out into the marine areas and, with time, settle out as new mud deposits across Moreton Bay. Elevated soil deposits will smother any aquatic plants growing in the creek and bay areas.
- Following the flood, increased organic matter (such as litter) may result in localized fish kills.
Benefits to waterway health
- These greater volumes of water will increase water levels in our local creeks, ensuring habitats for aquatic wildlife.
- During some of the high flows, particular fish will migrate upstream to seek refuge and to spawn when the conditions are suitable.
- Local flooding of some riparian corridors will be highly beneficial for subsequent plant growth.
- After peak flows reduce, the bubbling of water over shallow areas from ongoing flows will result in increased levels of oxygen in creeks.
- Our underground water storage reserves (acquifiers) in the region are being refilled. They will provide major benefits for flows in future dry periods by supplying critical base flows for local creeks.
Moving forward
At this early stage, it is difficult to determine the exact level of impact that the flooding will have on the health of our waterways. Further analysis of ecosystem health monitoring data will be conducted and more information will be released in the coming weeks.
SEQ flood assistance information:
- Flood support and assistance – http://www.qld.gov.au/floods/support.html
- List of suburbs affected by flooding – http://www.brisbane.qld.gov.au/
- Road closures – http://highload.131940.qld.gov.au/
- Assistance with property damage or flooding within property –
- Report downed trees outside your property boundary or issues with stormwater drains –
- Report fallen powerlines
- Bureau of Meteorology – www.bom.gov.au
- Donate to the flood relief appeal – http://www.qld.gov.au/floods/donate.html
