Asset Management

Detention Basins

  • 1.  Detention Basins

    Posted 07 August 2018 00:32
    This is a question that came up in our North East Asset Management Group  (VIC)  and If anyone out there is willing to share their knowledge, experience on this it would be much appreciated.

    Does anyone fence detention Basins?
    Have you done a risk management assessment on whether to fence these or not?
    Do you have different rules for those in urban and rural areas?
    Do you also consider slope as well as water levels in this determination?
    Do you consider these a Land Asset or a Drainage Asset?

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    Ashley Bishop
    Asset Management Coordinator
    Benalla Rural City Council
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  • 2.  RE: Detention Basins

    Posted 08 August 2018 01:03
    ​Ashley & other practicing Drainage Engineers,

    As a matter of principle, it has become a reasonably well accepted practice that Councils could not afford to fence all basins, wetlands, engineered channels and natural watercourses in their care and control. I understand the emotion behind the call for fencing, but the logistics of it would be prohibitive and there is good reason not to prevent passive recreation activities along our waterways.

    Exceptions to this are where there are obvious (identifiable) and quantifiable  risks which must be mitigated - especially around hard engineering structures. Risk mitigation strategies should aim to minimise exposure and that may mean the provision of suitable fencing to prevent accidental entry in the following situations:
    • where the batters are steep and do not allow safe walk-in-walk out entry and exit for small children;
    • there is no clear vegetative barrier (spiky and prickly Australian natives are a good deterrent) to prevent children falling into water where they cannot be safely reached from the shore;
    • where there are hard edges and the height difference between the walking surface and the bed (at the edge of a basin or pond) is greater than say 0.9 - 1.2 m ( i.e. an adult could not reach over and safely grab an unconscious child);
    • if ponding depths in the edge zone of ponds and wetlands are likely to be greater than 1.2 m;
    • in locations where there are pipe inlets and outlets with concentrated flows with localised velocities greater than those recommended as being safe for children;
    • inlets in parks and basins that lead into an underground drainage system with buried junctions, chambers and changes in pipe configuration - (i.e. where you can get sucked in and not come out unscathed within 5 - 10 seconds) - such areas should be protected with a higher priority;
    • areas in the vicinity of schools should also have a priority weighting factor applied - the exposure class increases;
    • areas adjacent to artificial (engineered) channels (especially in urban areas) probably need to be fenced or provided with barrier edge planting to prevent accidental entry to fast flowing water;
    • facilities in rural areas could get a lower ranking (lower exposure class) but beware that many rural dams and basins end up being right in the middle of developing urban areas and could require expensive refurbishment and re-engineering to mitigate those risks sooner than expected.
    Note: Some Councils have a policy of excluding certain play equipment and activities from concentrated flow paths in reserves. (Again, a risk based approach is recommended based on a rigorous methodology).
    Also note that you will never exclude deliberate entry and if they get in they need to be able to get out just as easily.

    Good guidance on accepted safety standards for drainage facilities is included in QUDM 2016.
    It is recommended that all Councils complete a thorough safety audit of their drainage assets and list all potential risk mitigation works with the exposure class and risks consistently evaluated and ranked. If you have an adopted policy based on sound advice and consistently applied engineering principles it will be easier to publicly defend and easier to attract budget support.  It may even reduce you and your Council's exposure to future litigation.

    The question of whether the asset is a land asset or a water asset and how it is valued varies. My view has always been that if a basin/channel/ etc. is an asset provided to cater for an adopted (desired) standard of service and you need to spend money to ensure it continues to provide the same level of service, it should be on your drainage asset register. The land has an intrinsic value but your land managers are not going to spend money to ensure the continued amenity of an asset that they are not responsible for.

    Rule One: The asset register and the assets on it should be maintained by those responsible for providing the service - follow the line of delegated responsibility and the money!





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    Allan Herring
    Senior Engineer | Design Manager
    CPO Flood Management Team
    Brisbane City Council
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  • 3.  RE: Detention Basins

    Posted 09 August 2018 22:19
    Edited by intouch 09 August 2018 22:19
    It might be worth while looking at the Queensland Urban Drainage Manual (QUDM) Ch 12. Safety aspects, which includes a risk assessment. [Editor's note: QUDM is available for purchase here: https://www.ipweaq.com/qudm]

    Section 12.4 looks specifically at fencing:



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