Appendix D - Engineering Report · REC15/0011

Proposed Subdivision
1 George Tewantin




    Unitywater indicated their preference was for the whole of the site to be commanded
    by a gravity sewer with the sewer mains being located on their standard alignment
    within the road reserve, but acknowledged that if Council wished to retain trees
    within the road reserve then they would consider alternative alignments.

    We have investigated the use of bio swales and grass lined swales in lieu of bio
    retention basins and found that bio swales are less effective than bio retention
    basins due to the lack of detention time of water over the filter media. This results
    in a greater area of swale being required compared to a bio retention basin and
    thus the long term maintenance requirements for Council would effectively be
    greater not less.

    Our concept design therefore proposes bio retention basins to treat the stormwater
    run-off from the site, however instead of on larger basin, two smaller basins have
    been proposed to minimise impacts on existing trees and to make use of the natural
    lay of the land. Following discussions with Council at the prelodgement meeting,
    water from the lots is conveyed to the bio retention basins via a conventional piped
    interlot drainage system. A gravel access is proposed down the eastern and
    western side of the site to provide access for maintenance of the bio retention
    basins.

    In regard to run off from the existing road and footpath area of George Street,
    Council has indicated a desire retain the flush kerb detail instead of replacing this
    with a conventional kerb and channel. As such it is proposed to provide a grass
    lined swale drain on the low side of the existing footpath. This will convey run off
    from the road from the crest about mid-way along the frontage to each end of the
    site.

    At the western end this drain will be directed into the Council reserve and ultimately
    to Wooroi Creek. At the eastern end the drain will be directed into a field inlet and
    the stormwater piped to the rear of the battle axe blocks where it will be discharged
    and dispersed as overland flow to the existing gully that drains to Wooroi Creek, as
    is currently the case. It is proposed that future driveways servicing the new lots be
    constructed with pipe culvert crossing of the swale drain in lieu of a standard urban
    driveway profile. This has been depicted on the concept plans.




    J/N 15513                                                                    May 2015
     Rev B                                                                      Page 2 of 3