Attachment 6 - Stormwater Management Plan

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



2019). The spillway has a width of 15 m and is concrete lined with a rock chute and dissipater. The maximum water depth
across the weir in a 100 year ARI is 170 mm.

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

3.6.                CLEAN WATER DIVERSION DRAIN

The clean water diversion drain 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).
To cater for the maximum 6.2 hectare catchment and flat grade, a 3m wide turf lined drain with a minimum depth of
500mm to 700mm including freeboard is required. Refer to Section 5 for hydraulic modelling undertaken.
3.6.1.           MANNINGS ROUGHNESS SENSITIVITY ANALAYSIS
An analysis of the sensitivity of flood depths with the clean water drain was undertaken based on a fully turfed drain vs
a turf lined base with vegetated batters. A Manning’s roughness of 0.15 was adopted as a worst case scenario for the
planted batters.

The sensitivity analysis indicate a flood depth increase ranging from 30mm to 80mm in the drain, however, all flows will
be contained within the drain indicating no concern in regards to planting batters from a flow conveyance position.

4                BASIN VOLUME AND SIZE
4.1.                EA REQUIREMENT

The Environmental Authority (EPPR01855314) specifies that site erosion protection measures and sediment control
measure must be implemented to have the hydraulic effectiveness to treat stormwater for 90% of the annual runoff for
each stormwater catchment.

4.2.                ASSESSMENT APPROACH

An assessment of a Type A High-Efficiency Sediment basin was undertaken utilising the MUSIC (Model for Urban
Stormwater Improvement Conceptualisation) software as a water balance model to determine the percentage of annual
runoff discharged from the decant structure rather than over the emergency spillway. In accordance with IECA (2018),
Type A basins are sized to allow the design event runoff to discharge (treated) from the decant structure with any runoff
overtopping the emergency spillway to be considered over-design. The MUSIC analysis was undertaken for three Type
A sediment basin design events (and corresponding basin sizes):

       +      1 year ARI, 24 hour (short-term design event in accordance with IECA (2018))
       +      2 year ARI, 24 hour
       +      5 year ARI, 24 hour (long-term design event in accordance with IECA (2018))

In addition, a comparison of the effectiveness of a 10 year 24 hour traditional sediment basin was undertaken using a
water balance model to determine the percentage of annual runoff that would be captured and treated by the basin.
This assessment is not required to determine the size of the Type A sediment basin, however, provides some additional
justification for achieving a 90% annual runoff treatment target.

4.3.                HISTORIC RAINFALL PERIOD

The models utilised 10 years of historic rainfall from the Cooroy Composite Station (Station ID 40059) for the period
between 1/1/1973 – 31/12/1983. This period is considered representative of historic rainfall and is in accordance with the
Water By Design MUSIC Modelling Guidelines. A 6 minute timestep was utilised in the HES basin assessment, where a
daily rainfall total was utilised from the Station data for the traditional basin assessment.

4.4.                CATCHMENT

A 1 hectare catchment was assessed. A range of catchment sizes were also assessed for the HES basin model as a
sensitivity analysis. It was found that that catchment size did not significantly affect the annual runoff treatment
performance.




Environment. Engineering. Education.                                                                                 PAGE 2