Rivers flood and urban rivers behave differently to natural waterways. One of the most significant flood causes in most river systems is the reduction in channel capacity caused by development and management. This slows down the release of urban water, exaccerbating the impact of the rural catchment. The Brisbane River recently flooded upstream to at least AHD 7.5 but in Newstead it was AHD 2.5 (at most, others will have more accurate information). Trying to manage the urban river as a natural stream and encouraging mangroves in areas they did not traditionally inhabit in its urban history, has caused siltation of the river and potential debris dams to develop, all of which has a major effect on flood levels. Mangroves are ecologically good but marginally so in the urban reaches of the river. In rural areas, changes in stream capacity just means other areas are flooded. In urban rivers poor management can result in property damage. We can improve the design of buildings but it is better to be proactive than entirely reactive. We cannot solve the flooding problem but rational management would help.
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