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What to do about stormwater issue?

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  • What to do about stormwater issue?

    On my neighbour's property is the exit point of an underground stormwater concrete pipe culvert. Below the exit is a pool of water, which then narrows into a stream/open watercourse and runs alongside his house. The stream continues on alongside my house then past a vacant lot, then enters another underground concrete pipe culvert, allowing the stormwater to continue downstream underground.

    Recently we have been concerned about the stream bursting its banks during heavy rain events. We have spoken with the Auckland Council about this and their answer was that the watercourse has not been vested in Council (it is showing in their records as a 'Private Unlined Channel') and therefore we, as the property owners, are responsible for the watercourse. This is the case even though run-off from the nearby roads is conveyed through the watercourse.

    So we engaged an engineering company to do a hydrological modelling and catchment assessment report. This came back recommending that the watercourse be widened (though there's not much space for that on our property). One important point in the report was that the level that water would be expected to rise to during a 1% AEP flood event was the same level as the FFL (finished floor level) of our house. This means that there is no freeboard.

    Auckland Council's 'Code of Practice for Land Development and Subdivision', 1 November 2015, which refers to Section 4.3.1 of E1/VM1 (Compliance Document for New Zealand Building Code: Clause E1 Surface Water (Department of Building and Housing, 2011), states that the freeboard requirement for 1% AEP events for overland flow paths, where flow is equal to or in excess of 2m3/s, is 500mm for vulnerable activities (permanently occupying a residential property is considered a vulnerable activity).

    Our house was built in 1996. The LIM report shows that a Code Compliance Certificate was issued for the new dwelling at that time.

    In order to mitigate the flood risk, we are going to need to get a detailed design created for the channel upgrade, which will then need to go to Council for building consent. This will be further expense on our part, after having already forked out for the hydrological modelling report. So if anyone has answers to the following two questions, that would be very helpful:

    1) Auckland Council (at the time was Waitakere City Council) issued a Code Compliance Certificate for this house to be built which had less than 500mm freeboard (i.e. the finished floor level was less than 500mm above the water level in a 1% AEP event). Does this place liability on the Council to assist us with the costs of the detailed design for the channel upgrade?

    2) Because underground pipes can only cope with relatively small rain events, overland areas such as the watercourse on our property, where overflow from these pipes can run, are important. Overland flowpaths are designed to convey the rainwater generated in an extreme storm event and stop it from flooding houses. So upgrading this watercourse is in Council's best interests because as well as stopping our house from flooding it would reduce risks upstream and downstream.

    Is it likely that Council would assist us with the costs of the detailed design for the channel upgrade due to this second reason, or is the first reason a better bet?

  • #2
    the CCC is not related to flood levels that is a RMA issue.

    The building has been there 20 years how many floods have there been? None then code compliant?

    If you cover the ditch with a drain you will get the amenity of useful land area so the value is with you.

    overland flowpaths are related to surface water and catchments not the open drain that serves the catchment.

    you bought with the open drain so caveat emptor applies?


    • #3
      Hi John
      Thanks for your comments. You say that Auckland Council would not take flood levels into account when issuing a CCC as this is an RMA issue. But the compliant height of the finished floor level in a house design is these days based on the 1 in 100 year (or 1% AEP) flood event. So the finished floor level is checked as part of the issuing of the CCC. What I don't know is if finished floor levels were part of the compliance checking process in 1996.
      Even if flood levels are an RMA issue, who administers the RMA? As far as I know, administration of government acts are done by the relevant local authority, i.e. Auckland Council.
      Water can definitely get into the crawl space under the house and this is what we need to prevent.
      The relevant provision of the building code in respect of floor levels, in clause E1 "Surface water", is:
      E1.3.2 Surface water, resulting from a storm having a 2% probability of occurring annually, shall not enter buildings.
      During a 1% AEP or 2% AEP (1 in 50 year) flood event, water would definitely flood the crawl space under our house and my aim is to prevent this by getting a detailed design created for the channel upgrade.
      My question is, what can we say that will put the onus on Council to assist us with the cost of this?


      • #4
        are you now saying that water is entering the subfloor?

        How does fixing the drain lower the flood level?

        when you say council should pay you do of course mean me as a ratepayer?

        Why should i pay for your home improvements?
        Last edited by John the builder; 21-06-2018, 03:45 PM.


        • #5
          Yes, water will enter the subfloor during a 1% or 2% AEP flood event.

          Widening and deepening the open drain/stream will prevent this because the water will have a larger space in which to flow, meaning that it will take longer to fill the stream to capacity and the risk of it bursting its banks is much reduced.

          Council is only supposed to let the stormwater network run past a house via an open drain if there is no risk of water entering the building during a 2% AEP flood event.

          Runoff from nearby roads drains into the stream via the road drainage gutters. Litter that collects in these gutters also flows into the stream. The stream, being part of the stormwater network, is performing an essential service, however the fact that the stream is undersized for larger flood events means that if one of these events were to occur, we would be sacrificing our house for the sake of this section of stormwater network.

          So are we wrong in asking if the Council can assist with the cost of a detailed design for the stream upgrade?


          • #6
            The capacity of a drain system is the capacity of the drain which is downstream of you.

            If you are subject to inundation from 1 or 2 % storms then this must be a common occurrence for you as these are not uncommon?

            You are not wrong but I object as a ratepayer in paying for your upgrade