Attachment 6 - Stormwater Management Plan

R1767: NOOSA LANDFILL: CLEAN WATER DRAIN AND HES BASIN (SW5) – DESIGN REPORT & MAINTENANCE AND INSPECTION PLAN



1                INTRODUCTION
This design report applies to proposed construction of the Southern Clean Water Drain and the High-Efficiency Sediment
Basin located at outlet SW5. This design report has been provided to detail catchments, staging and operational
requirements for the proposed sediment basin and clean water drain.

2                CATCHMENT AND STAGING
The maximum catchment assumed to drain to the sediment basin is 4.5 hectares. This will occur as Cell 3.2 and 3.3 is
formed and can grade to the basin. It should be noted that to minimise the size of the basin, Cell 3.2 and 3.3 is required
to be split into quadrants to ensure the maximum catchment to the basin is not exceeded. This will allow a maximum of
half of Cell 3.2 and 3.3 to be graded to the basin.

The maximum catchment grading to the clean water drain is 6.2 hectares. This will occur once Cell 1.1 and Cell 2.2 are
capped and stabilised. The maximum catchment defined does not include the additional 4.5 hectares grading to the
sediment basin which will overtop into the final portion of the clean water drain. For assessment purposes, this additional
catchment has been included in the assessment of the downstream portion of the clean water drain.

Details on Catchments and Staging is provided in Appendix A.

3                DRAINAGE DESIGN
3.1.                DIVERSION DRAIN TO BASIN

The diversion drain to the basin has been sized for the 1 in 10 year ARI critical storm event in accordance with the
Guideline - Stormwater and environmentally relevant activities (DES, 2019). The Environmental Authority (EPPR01855314)
also specifies that the design event for temporary drainage works to convey waters to the sediment basin and around
disturbance areas is a 1 in 10 year ARI event plus 150mm freeboard for > 24 months disturbance.
To cater for the 4.5 hectare catchment and flat grade, a 3m wide turf lined drain with a minimum depth of 700mm
including freeboard is required. Refer to Section 5 for hydraulic modelling undertaken.

3.2.                INLET DRAIN FOR DOSING

To improve accuracy of flow depth measurement, the drain width at the dosing point has been reduced to 0.7m by
increasing the longitudinal grade from 0.7% to 3% and providing a concrete lining. The inlet drain has also been sized
for the 1 in 10 year ARI critical storm event in accordance with the Guideline - Stormwater and environmentally relevant
activities (DES, 2019). Refer to Section 5 for hydraulic modelling undertaken.

3.3.                LEVEL SPREADER WEIR

The level spreader is the weir in between the forebay and main cell. The level spreader has been sized for the 100 year
ARI critical storm event which is in excess of the 50 year ARI required in accordance with the Guideline - Stormwater and
environmentally relevant activities (DES, 2019). The level spreader has a width of 16 m and is concrete lined with a full
width concrete chute. The maximum water depth across the weir in a 100 year ARI is 190 mm.

Refer to Section 5 for hydraulic modelling and Appendix B for calculations undertaken.

3.4.                DECANT STRUCTURE

The decant structure has been designed in accordance with Appendix B - Sediment Basins (IECA, 2018). In accordance
with Table 5 of the document a maximum decant flow rate of 9L/s/ha has been adopted. To achieve a total of 40.5L/s
through the decant arms, a total of 13 decant arms would be required utilising the standard of 200 holes with a diameter
of 10mm. To reduce the number of arms to a pracital number, 17mm diameter holes are to be utilsied to increase the
flow rate to approximately 13L/s per arm, allowing the use of 3 arms and a single manhole structure.

3.5.                EMERGENCY SPILLWAY

The emergency spillway from the main cell has been sized for the 100 year ARI critical storm event which is in excess of
the 50 year ARI required in accordance with the Guideline - Stormwater and environmentally relevant activities (DES,



Environment. Engineering. Education.                                                                                PAGE 1