Stormwater Management Plan

SC5594 – Walter Hay Dr, Noosaville                                                                12




4.3.3    Roughness Coefficients
The conduits and links used within the model are based on the ultimate design scenario for the internal
roadways and overland flow paths. Mannings “n” roughness coefficient values are specified as 0.014 for
RCP pipes and 0.011 for uPVC pipes.


For overland flow within the sub-catchment areas a Mannings “n” value of 0.014 for concrete areas and
0.035 for vegetated areas was adopted.


4.3.4    Storm Events
Storm patterns were setup using Australian Rainfall and Runoff storm patterns for Area 3 with intensity
frequency duration data taken for the local area. Storm durations of 10, 15, 20, 25, 30, 45, 60 and 90
minutes were investigated for the pre and post developed scenarios.


4.3.5    Backwater Condition
The site’s outlet node identified as ‘1.0’ has been modelled assuming a free outlet.


4.3.6    On-Site Detention
It is calculated from the development site plan that the proposed development will increase impervious
areas on the site from 0% to 55%. The proposed increase in impervious areas has the potential to increase
peak flow rates to the existing drainage system. To mitigate potential increases in flow rates it is planned to
detain stormwater flows on-site and discharge at a mitigated rate to the existing swale.


Mitigation of flow is provided by approximately 335m3 of detention volume over and above the Bio-retention
basin extended detention volume and small areas of ponding in the carpark. Flows from this storage
volume will be throttled by a reduced cross-sectional area connection at the Bio-retention basin outlet
(210mm), and an overflow weir.


Flows from minor storms up to and including the 10 year ARI are to be conveyed in the underground
drainage system. Major storm events exceeding the ARI10 year, up to and including the peak ARI 100 year
events will overtop the Bio-retention basin weir and flow overland into Council’s road reserve.