080624 2383-01 FINAL Engineering Report for DA.pdf [As Published] · 132008.1128

BLUE CARE SUNRISE BEACH
ENGINEERING REPORT FOR DEVELOPMENT APPLICATION


    On-site detention storage is required to reduce the peak discharges from the
    development to values which are less than or equal to those from the undeveloped site.
    This storage is proposed to be provided, within each lot, in conjunction with the
    proposed bio-retention basins required for stormwater quality management.

    2.6.2          Proposed Detention Storages

    The locations and general arrangement details of the proposed bio-retention basins and
    detention storages are shown on Drawing No. 2383-01 SK01. The operation of the bio-
    retention basins is discussed in Section 2.6.3.

    To minimise their footprints and to maximize the storage capacity of the basins, it is
    proposed that they are constructed with rock-filled wire gabion retaining walls, rather
    than with un-retained, excavated batters. The external faces of the walls of the basins
    will be essentially vertical and the internal faces will have an effective slope of
    1(V):0.5(H). These structures are subject to further detailed design, geotechnical and
    hydraulic investigations.
    The bio-retention basins will be constructed within the base of each detention basin, as
    shown on Drawings 2383-01 SK03 and SK04. An overflow grate, at a level 0.4m above
    the surface of the bio-retention basin filter medium, will be provided within each basin.
    The height of the grate above the filter is the proposed extended detention depth of the
    bio-retention basin. The grate will provide the outlet from the detention basin and will
    be connected to discharge pipework extending through the basin wall.

    Concentrated discharges from the detention basin outlets will require respreading as
    sheet flow before entering the national park. This will be accomplished using spreader
    channels, as shown on Drawings 2383-01 SK03 and SK04. The proposed basins and
    spreader channels are located within the site development area boundary or within the
    10m wide fire break adjacent to the buffer area to the national park.

    To assess the required capacities and outlet configurations of the detention basins, a
    dynamic, numerical, hydrological and hydraulic model of the site catchments, the basins
    and discharge pipes was set up using the modelling package XP-SWMM.

    The hydrology in the XP-SWMM model was adjusted, as required, to ensure
    consistency between the catchment peak discharges estimated using the model and
    those calculated using the Rational Method.

    The model was run for rainfall events with average recurrence intervals of 2, 10, and
    100 years with durations of up to 360 minutes to determine the duration which produced
    the peak discharges from the basins.

    The catchment areas allocated to each basin and used for the initial assessment of
    basin performance were set to the areas of the respective lots, with the Grasstree Court
    road catchment discharging to Lot 1. The resulting catchment areas were: Lot 1 and
    road, 4.275ha; Lot 6, 0.7774ha.

    For this arrangement, it was found that the Lot 6 basin was capable of accepting runoff
    from a larger catchment area and the Lot 1 basin could not accept the runoff from the
    assigned catchment area, without peak discharges exceeding those from the
    undeveloped site. A number of trial divisions of the catchments were modelled to
    obtain an arrangement which best utilised the capacity available within each basin. In
    addition to peak discharges, it was also necessary to consider peak water levels within
    each basin.

    The adopted catchment division assumed that runoff from the Grasstree Court road
    catchment (0.235ha) and from 0.8ha from the north-eastern corner of the development,



Blue Care Property Development Unit                 Version 1.1                             24 June 2008
2008 06 24 2383-01 Engineering Report for DA   Commercial in Confidence                          Page 11