Reference Document MCU22/0102 Stormwater Management Plan

    4. Design                                                     4.3 BYPASS

                                                                  The SPELBasin® system is designed for flows that
                                                                  exceed the design treatment flow rate (TFR) to be
    Unlike traditional biofilter design, it is not necessary to   bypassed externally. This is typically achieved by
    utilise drainage, transition and filter media layers. The     ensuring that when the inlet cannot carry any additional
    SPELBasin® system is delivered with the proprietary           flow, flow is re-directed to a nearby/adjacent inlet pit.
    media pre-mixed ready for installation into the               If the SPELBasin® requires an underground pipe inlet
    prefabricated chamber. Of importance, however, is that        type, the bypass can be formed into the inlet chamber,
    the geotextile protecting the outer geogrid and internal      or as a separate pit.
    collection pipes is not damaged during the media
    installation.                                                 4.4 INLET AND OUTLET CONFIGURATIONS

    4.1 OUTLET INVERT LEVEL (SATURATED ZONE)                      Multiple configurations are possible with the
                                                                  SPELBasin® depending on the site constraints. Some
    The SPELBasin® system is designed to accommodate              examples are indicated below.
    up to 300mm reservoir of water as a saturated/
    anaerobic layer at the bottom. The levels from this
    outlet to the receiving drainage network should be
    checked to ensure the outlet is free-draining.

    As a secondary guide, the Facility for Advancing Water
    Biofiltraton (FAWB, 2009) recommend calculating the
    saturated zone with the following equation;

    D = 8x t

    Where: D = depth of saturated zone (mm)
    			    t = average of the longest annual dry period
    			 (days)

    This level tends to determine the inlet level that runoff
    can enter the SPELBasin®.

    4.2 INLET INVERT LEVEL & TYPE

    The inlet level of the SPELBasin® system depends on
    the type of inlet structure;

    • Kerb inlet;
    • Grated inlet; and/or
    • Pipe inlet.

    Kerb and grated inlets have up to 150 mm of flexibility
    to match with surface levels. This also enables up to
    150mm of extended detention within the SPELBasin®
    system.

    Piped inlets have ~600 mm of flexibility to ensure the        FIGURE 5.
                                                                  SPELBASIN® INLET AND OUTLET CONFIGURATIONS
    inlet pipe and outlet discharge to a free water surface.
    The exact dimensions will depend on the inlet pipe
    diameter, depth/cover and class.




6        Technical Design Guidelien SPELBasin