RFI - Attachment B - Stormwater Quality Engineering Response · 132008.1128.04

Memorandum
18-001942 - Sunrise Beach Interim RAC Water Quality



     1.   Soils in SEQ are predominantly clay and hence have low infiltration ability;

     2.   Generally there is minimal geotechnical investigations undertaken at the time of MUSIC modelling to validate any
          infiltration assumptions and hence it is conservatively assumed that there could be a shallow ground water table
          level; and

     3.   Infiltration basins are generally only suitable for treating TSS and GP pollutants as TN and TP can pass through
          the infiltration basin into the groundwater.

For the interim period, however infiltration has been modelled as per the above MUSIC model layout. The MUSIC
Modelling Guidelines allow this infiltration to be modelled following a site based assessment ‘depending on localised soil
and groundwater conditions’ (Section 4.1. Water by Design 2010. MUSIC Modelling Guidelines).

The site based assessment has confirmed that the use of an infiltration basin is suitable on the RAC site for the following
reasons:

     1.   Comprehensive geotechnical investigations have been undertaken to confirm the onsite conditions are suitable
          for the use of an infiltration basin being that the insitu sand has a high hydraulic conductivity and boreholes down
          to two (2) metres in depth revealed no presence of a shallow groundwater condition. The vegetation identified in
          the area is also consistent with a deeper ground water. The full geotechnical report has been previous provided
          to Noosa Shire Council within the overarching report 18-001942.WCSMP.01.B with relevant borehole logs
          attached to this memorandum.

     2.   Runoff from the built areas of the development are currently achieving the necessary water quality reductions for
          TN and TP with the proposed 50m2 bioretention basin. The infiltration basin has been modelled for its beneficial
          effects in filtering out TSS (ie. dirt and sediment) which is an inherent ability of infiltration basins as they filter
          flows through the insitu sand material.



3.             Results and Recommendations
After completing the required MUSIC modelling the following results were confirmed:

Firstly it was confirmed (see below Figure 2) that the built areas of the site are effectively achieving the required
reductions for TP and TN at the outlet of the bioretention basin with only TSS failing to achieve its required 80% reduction
target. This shows that the infiltration basin can be utilised for the treatment of TSS (only).




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