Sealed Judgement

Figure 3-2 Post-Development Lumped Catchment Plan


3.4.1         Stormwater Quantity Management Strategy

As part of the stormwater quantity management strategy, it is proposed to incorporate detention
within the Site to ensure that the peak flow discharged from the Site does not increase as a result
of development.

Given the proportion of undeveloped area within the catchments, it is proposed to deliver this
detention via tanks. In the case of cabins, a portion of the rainwater tank provided to each cabin will
be allocated to detention storage (draining after each rainfall event). A detention tank will also be
provided beneath the car parking area.
Using this approach, there is no need for the bioretention system proposed to treat stormwater
runoff to also provide a detention function. This provides flexibility with respect to the discharge of
roof runoff from the cabins, as it is only necessary to drain low flows from the rainwater tanks to
the bioretention system and overflows can be directed to the ground adjacent to the cabins and
allowed to runoff from with the remainder of the (undeveloped) catchment.
To assess this, the WBNM catchments were further split into carpark, roof and ground areas as listed
in Table 3-5 and shown in Figure 3-3. With reference to Figure 3-3, the following assumptions were
made:
x   Roof Areas within Sub-catchment A1 (i.e., Cabins 4/5 and 6/7)

    Each of these roof areas drain to a 3.0 kL rainwater tank (i.e., a combined tank capacity of 2x
    3.0 kL = 6.0 kL). The tanks nominally drain via a pipe to the proposed bioretention basin, with
    overflows allowed to flow to the ground as no further detention is required.




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