Storm Rains Test Region's Contour Banks
Feb 18, 2015
Across the Burnett Mary region high intensity storms have tested the capacity of contour banks this summer. Although isolated, there have been a number of very high intensity storms which have delivered up to 100mm per hour. This type of rain intensity is hard to handle with any structure as it exceeds the design capacity of most standard soil conservation works. The rain also fell when most farms were still bare of crop or only recently planted. This scenario is a double threat as the dry soil, worked to a fine tilth ready for planting, tends to surface seal when the rainfall is very heavy with large droplets. The result is a lot of runoff very quickly and invariably some contour banks overtop.
The only real solution to minimising these erosive bank breakages is to ensure that the banks are topped up regularly and have the capacity to carry the water they were originally designed to carry. A standard bank design with a depth of 0.5 metres will have a cross section of approximately 7 m2 on a land slope of 2%. See Fig. 1 for details on checking bank capacity.
Fig. 1. Capacity in Metres squared = (width X depth) ÷ 2
As the steepness of the land increases the capacity decreases. The same 0.5 metre bank ona 5% slope will have a 2 m2 capacity. If the depth of your banks are below 0.5 metres the capacity will also be drastically reduced. A depth of 0.3 metres on 2% slope will give a capacity of only 2.5 m2, less than half the desired capacity.
The discussion has highlighted just how important it is to maintain contour banks at the design depth to provide the intended protection during the storm season. This is particularly relevant on the steeper slopes where bank slumping is more common due to the action of mechanical disturbance and the normal gravity effects.
The overall summer rainfall this year is below average in most places however even in these conditions the storm intensity has been quite high. The moral of the story is the best protection is prevention with regular maintenance. With farm-over systems it is wise to check bank depths annually at strategic spots where the banks are most likely to fail, such as old gully lines or close to the outlet ends. For banks which are not farm-over checking every second year is wise.
For more information on contour bank specifications and maintenance check the fact sheets, “Maintaining Contour Banks” L202 www.qld.gov.au/dsitia/assets/soil/maintaining-contour-banks.pdf. and “Contour Bank Specifications” L205 www.qld.gov.au/dsitia/assets/soil/contour-bank-specifications.pdf. or hard copy from the Burnett Mary Regional Group office at Wondai.
For information and advice on contour bank systems or gully erosion rehabilitation contact John Day, Soil Conservation Officer based at Wondai BMRG office on 41690720 or mobile 0490071147.
Contour bank failure causes significant soil loss