Local, Science/Environment

Could the ‘one in one hundred year’ Townsville floods happen on the Coffs Coast?

Given what we’ve recently seen at and around Townsville should we on the Coffs Coast, and especially Bonville, be concerned ? 

By ‘Noah’

The climate is changing and there’s an increase in global floods now occurring. Townsville is now formally recognised as being subject to 1 in 100 year flood events and many more parts of Australia are likely to suffer from extreme weather events.

As we know, Bonville sits in a geographical flood plain and is within the 1 in 100 year flood extent.  Significant areas of Bonville are not immune to recurring flooding and we should all remember what happened in 2009 when we had 3 major ‘once in a century’ flooding events.

This 2009 map shows the Boambee flood prone areas

For those with an interest, here are some definitions:

1.         A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year. The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%

2.    The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year.  Likewise, the term “100-year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring

Recurrence intervals for the annual peak stream flow at a given location changes if there are significant changes in the flow patterns at that location, possibly caused by an ‘impoundment’ or diversion of flow.  The effects of development (conversion of land from agricultural uses to residential uses) on peak flows, is generally much greater for low-recurrence interval floods than for high-recurrence interval floods.  During larger floods, the soil is saturated and does not have the capacity to absorb additional rainfall.  Under these conditions, all of the rain that falls, whether on paved surfaces or on saturated soil, runs off and becomes stream flow.  This is evidenced by seeing water from saturated lands spilling over local roads.

This issue has been known about as far back as 2009 as this article from the Coffs Coast Advocate shows https://www.coffscoastadvocate.com.au/news/former-mayor-says-boambee-a-bigger-problem/417686/

‘Noah’ is known to the editor as someone with expertise and experience in town and country planning

5 Comments

  1. Surely there are other areas which could fulfill the need for rural residential land without the risk or loss of such lush agricultural land as Bonville has. Was the move to rezone this land a result of demand from residents or an initiative of government?

  2. Last year’s biggest 2-day rain event saturated the soils of land released for residential. Even after it stopped raining, water was still flowing off the land and over a local road. Got the photos to show it.

  3. The Scarlet Pimpernel

    There is a long history of flooding in Coffs Harbour. Significant flood events have occurred in 1917, 1938, 1950,1963 1973, 1977, 1989, 1991, 1996 and 2009. And it appears that some flood-prone areas have been rezoned for residential when they should not have been.

  4. Ivegotmewellieson.

    Whether you are a climate cycle or climate change believer it is not important right now, what we do know is we are seeing extremes of weather in all its forms. From history we know that Coffs Area is susceptible to flooding and to see high density residential blocks on former flood plains or flood prone areas seems crazy. There are many examples in Australia, rows and rows of new single level solid homes on concrete slabs, as most recently seen in Townsville that flood and become quickly inundated, we do not need to find out the hard way here. Just looking at the topography and low lying nature of the land now being offered in Bonville inundation will happen, a flood which we will all pay for thru insurance rises to all, or the inability of people to afford future premiums in these known low areas.The simple solution would be if we want to see Coffs grow and prosper, leave the low lying coastal land as public space and consider more intense development west of the highway. We have great traffic infrastructure in place in the form of highway on/off ramps that are able to support growth and provide people with safe places to raise their families without the risk of inundation now or in the future.

  5. So this is the effect and concern: With dwellings forecast to be built at Bonville, the effect of rooftops, increased hard surfaces such as concrete pathways and new roads, will seriously impact on where rain water will drain and flow to. Remaining exposed lands will become more swamped, surface water levels will rise and streamflow will increase. According to Flood Specialists, flood modelling makes this abundantly clear.

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