RFI - C - Site Based Stormwater Management Plan

 STORMWATER MANAGEMENT

          STORMWATER QUANTITY MANAGEMENT

          Infiltration Tank Sizing and Background

It is recommended that underground infiltration tanks be installed within the property to minimise the flows
being sent to the historical point of discharge (i.e. the infiltration system within the adjacent council owned
property).


The proposed infiltration tank sizes are calculated based on geotechnical investigations of the property
showing material as sand. This stratum typically demonstrates a saturated hydraulic conductivity of
k = 2.6x10-3 m/s (9216 mm/hr) and assumed that a groundwater table would be at least 1.0m below the base
of any infiltration system installed.


The sizing of the proposed infiltration tanks is based on the Hydrological Effectiveness Method as outlined in
Section 7 of “The WSUD Technical Design Guidelines for South East Queensland” (2006).


The Hydrological Effectiveness Method sizes an infiltration trench based on the specified proportion of the
mean annual runoff volume that will infiltrate in to the soil profile. In brevity the methodology states that for
the geotechnical information provided, an infiltration tank 1.0m deep with an area equal to 1% of the
catchment will infiltrate 100% of the mean annual runoff.


Based on the generated volume of proposed runoff for the 1% AEP event and the Hydrological Effectiveness
Method the proposed volume is reproduced below in Table 1. The detention volume calculated using the Insite
Water software (with a permissible discharge volume for non-worsening and orifice plate) is also provided as
comparison. It is demonstrated that Hydrological Effectiveness Method provides a larger tank size and was
adopted for design and construction.



Table 1 – Required Infiltration Tank Volumes

 Catchment       Area (ha)                                      Volume
                              Hydrological Effectiveness Method                  In-site Water
    Lot 22        0.0582                   5.82m3                                   4.30m3



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