33522 Stormwater report018-01 SWQA issued 180704 · MCU18/0001

                                                                                    CIVIL ENGINEERING PROJECT MANAGEMENT
                                                                                           SEDIMENT AND EROSION CONTROL




              INFILTRATION TANK SIZING AND BACKGROUND


The Aquaintel report (2014) recommended that underground infiltration tanks be installed within the
property to minimise the required area for infiltration. These tanks were subsequently design and installed as
per the recommendations outlined in the report.

The original infiltration tank sizes were calculated based on geotechnical information provided by Tectonic
Geotechnical Pty Ltd and supplied in the Aquaintel report (2014). This information expected 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.

Two alternate methods of sizing the original infiltration tanks were adopted and compared. These methods -
The Hydrological Effectiveness Method and the Design Storm Method are 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. It 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.
The Design Storm Method sizes the detention capacity based on the difference between the volume of runoff
from the catchment during the design event and the volume that could be infiltrated during a rainfall event. It
requires the values of the saturated hydraulic conductivity of the insitu soil and the dimensions of the
proposed tank in order to complete the calculations.

The Aquaintel report (2014) adopted the Design Storm Method and generated volumes of proposed runoff
using the WBNM model for the 1% AEP event. This value based on the original 70% fraction impervious has
been reproduced below in Table 2. It is demonstrated that the Design Event Method provides a larger tank
size and was adopted for design and construction.


Table 2 – Required Infiltration Tank Volumes

 Catchment             Area (ha)                                   Volume
                                   Hydrological Effectiveness Method         Design Event Method
                                                      3
    Lot 696             0.0506                  5.06m                                6.36m3
(Aquaintel 03.07.14)


With an increase in the impervious fraction up to 90%, the original WBNM model was regenerated and
provided an increase in the volume for the Design Event Method from 6.36m3 to 6.94 m3.




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