2016-09-06 - Planning & Environment Committee Agenda

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        5.       EFFECTIVENESS OF DEBRIS REMOVAL FOR FLOOD PROTECTION

        In most cases it is generally not considered appropriate to remove debris from a natural floodplain
        for the purpose of flood management. While increased levels of debris will generally have some
        impact on peak flood levels, on a wide flat floodplain such as that of Wooroi Creek, these increases
        would generally be expected to be minor.
        There are a range of issues with clearing debris from a natural creek system, with one of these
        being that clearing debris will often cause more debris to become available. The reason for this is
        that debris acts to slow flood waters down. Removal of debris will speed up flood waters which will
        in turn have potential to increase erosion and cause more debris to become available (e.g. trees
        more likely to fall as creek banks erode).
        It is considered standard practice for local governments to leave natural debris in place in a natural
        creek system. Without extenuating circumstances, debris should typically only be removed for the
        purpose of flood management when it is nearby to hydraulic structures (e.g. culverts) which have
        potential to become blocked.

        6.       IMPACT OF THE LOMANDRA PLANTINGS

        Lomandra plants are commonly used for planting along water courses because they are a hardy
        plant tolerant of wet and dry periods and they are an effective soil stabiliser. Generally, the planting
        of Lomandras in a bushland floodplain setting is appropriate however care should be taken to
        minimise the potential for these plantings to obstruct flow paths which may impact upon properties
        or infrastructure. While it is unlikely that these plantings will have a large impact during large floods
        because their resistance to flow becomes drowned out as leaves are pushed over with floodwaters,
        it is possible that dense plantings of Lomandras could impact upon the frequency of flooding during
        minor / moderate events and could therefore potentially cause a nuisance, rather than a hazard.
        While the current Lomandra plantings are not considered a significant issue in relation to flood
        hazard, it is recommended that the risk from future plantings be mitigated by:
            1. Avoiding dense planting in areas know to be active flow paths nearby to any properties or
                infrastructure that may be sensitive to flooding;
            2. Allow breaks in any continuous rows of planting;
            3.  Align any rows of planting parallel to flow paths rather than perpendicular to flows.

        7.       HISTORIC RAINFALL ANALYSIS

        In light of community concerns that the frequency of flooding through the parklands can be
        attributed to high level of debris, HWMC has undertaken an analysis of historic rainfall which has
        been recorded by the Bureau of Meteorology at the Mount Tinbeerwah Alert rainfall gauge.
        A response time for a catchment of this size would be expected to be between 1 to 2 hours. What
        this means is that, for example, steady rainfall over a period of a day is unlikely to produce flood
        levels that are as high as those produced by intense rainfall that falls over a 1 to 2 hour period.
        Therefore, when considering peak flood events for the estate, rainfall intensities recorded over a 2
        hour period are of most relevance. Peak two hour rainfall intensities recorded every month for the
        full historic record are shown on an enclosed chart.
        This chart shows that since 2008 there has been 4 separate occasions where greater than 150mm
        has been recorded over a 2 hour period at the Mount Tinbeerwah Alert rainfall gauge.      Rainfall
        intensities of this magnitude would be expected to cause flows to break out from creek banks and to
        flow across the floodplain, regardless of debris levels.

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