REPS - Updated SWMP

2. INFORMATION REQUEST
An Information Request was received from NSC for development application MCU24/0125, OPW24/0158, and RAL24/0021 on 28 October
2024. Item 4 of the Information Request relates to Stormwater Management and is outlined below:

External Drainage Impacts and Non-Worsening

Issue

The hydraulics of this area are complicated because the pipe flows in Starboard Ave (which this site connects to) run against the road grade
immediately downstream of the site. This means that any surface flows beyond the pipe system capacity will not flow to the detention basin
but towards the sag in Starboard Ave. Unless the pipe system was designed to convey the 1%AEP, the simplistic catchment assumptions
adopted in the SWMP will be incorrect for larger events. WBNM is too simplistic for this assessment and a model which can account for the
complicated hydraulics needs to be adopted.

The assessment needs to consider the impacts from the development on all parts of the drainage network external to the site – not just the
point of discharge from the detention basin. The impact to the drainage network between the site and the detention basin needs to be
considered, as well as the other discharge direction from the site in larger events (the sag in Starboard Ave) Climate change considerations
have not been addressed in the design and are required as per PO2 of the Water Quality and Drainage Code.

Information Required

Item 9 - Provide an updated SWMP which demonstrates either non-worsening or that the QUDM performance criteria for the street network
are achieved, and which:

   a.    Accurately considers the direction of flows from the site in events of different magnitudes and considers the impacts at all sensitive
         receptors downstream from the site (not just at the discharge point from the existing detention basin);

   b.    This should include HGL long-sections and overland flow mapping for the street network downstream of the site (both directions) –
         showing that either QUDM criteria are achieved or that non-worsening (pre-subdivision) is met.

   c.    Correctly incorporates the changes in site imperviousness as a result of the development (imperviousness values adopted in the
         current SWMP could not be replicated);

   d.    Demonstrates that the existing and proposed pits along the Starboard Ave access strip, Starboard Ave and Ely Street do not surcharge
         during the 0.5EY, 10%AEP and 1%AEP events; and

   e.    Incorporates climate change through increased rainfall intensity as per current ARR advice. It is considered that SSP2-4.5 would be
         an appropriate scenario. BoM 2016 rainfall intensity should be increased for the current climate assessments as per ARR and a time
         horizon of 2100 adopted for future climate scenarios.


Response:

    a.    All flows up to the 10% AEP from the townhouse site will be captured and piped to the existing detention, which is in line with the
          current modelling in this SMP. It is acknowledged that in events greater than the 10% AEP the gap flow will not report to the
          existing detention basin and report south down Starboard Avenue. This gap flow is less than the flows from the pre-development
          catchment which reports south down Starboard Avenue and hence non-worsening is achieved (refer Section 4.3). Further to this,
          a 50m3 detention tank is proposed within the townhouse site to ensure pipe flows in the 10% AEP event are less than design flows
          determined as part of the original operational works design of the stormwater system. This will ensure that HGL levels in the
          network do not increase and as such compliance with QUDM drainage criteria.

    b.    Flows that go north down Starboard Avenue and Ely Street have reduced, with the inclusion of a 50m3 detention tank, and pipe /
          surface flows align with the original design calculations (refer Section 4.2). Flows that report south down Starboard Street have
          reduced, with no flows reporting there in events less than the 10% AEP and flow reducing significantly in the 1% AEP event (refer
          Section 4.3). It is considered that flows going north achieve the QUDM design criteria (in line with original design) and flows going
          south achieve non-worsening. No further assessment is required.

    c.    Imperviousness of each catchment area considered to be correct, with 50% impervious applied to the approved subdivision and
          90% imperviousness applied to the townhouse site. The increase in imperviousness from the townhouse site, is mostly offset by
          the 0.35 ha park within the development (this area was counted as 50% impervious in the original SMP).




73 EUMUNDI - NOOSA ROAD, NOOSAVILLE I BBNE00529_0002_0001-REP-001-2                                                                          4