POS - 7,479
APPENDIX 13: Drainage Modelling
Pre-development Flood Modelling
RPS completed a preliminary flood modelling exercise for the Ki-it Monger Brook in 2013 which included developing a hydrological and hydraulic model in the XPSWMM modelling package (refer to Appendix 5 of the LWMS for further details of the initial flood modelling). The model identified the 6 hour, 1% AEP (100 year ARI) design rainfall event as the critical event and determined the resulting flood levels along the Ki-it Monger Brook.
As part of preparing the Bullsbrook Landing Local Water Management Strategy (LWMS), the previous flood model was updated to incorporate more detailed survey information obtained specifically to inform the hydraulic modelling. This included invert levels and cross-section profiles of the watercourse as well as inverts and dimensions for all culverts within the brook. The pre- development flood levels were then re-modelled with greater accuracy.
The pre-development model adopted the parameters in Table 13-A which are consistent with the latest available guidance in the 2016 Australian Rainfall and Runoff guidelines.
Table 13-A: Pre-development Model Parameters
Catchment Type Initial Loss (mm) Continuing Loss (mm/hr) Manning’s n
Pasture 20 3.5 0.15
Uncleared 26 4.6 0.25
The results of the pre-development flood modelling are discussed in the LWMS and the flood extent and levels are provided in Figure 8 of the LWMS.
A key output of the pre-development modelling is the peak flow rates within the Ki-it Monger Brook which are to be maintained by the post-development drainage design. The modelling determined the critical design storm is the 6-hour event and that the peak 1% AEP pre- development flow rate at the discharge point from the site beneath Great Northern Highway is 12 m3/s.
Post-development Drainage Modelling
The pre-development flood model was modified to simulate the post-development drainage catchments and hydrological conditions (imperviousness, slope, run-off rates, etc.). The post- development parameters adopted in the model are provided in Table 13-B and are consistent with the most recent literature values in the 2016 Australian Rainfall and Run-off guidelines and professional judgement. The following assumptions have also been made in the drainage modelling:
• All lots will achieve Class A geotechnical classification (using imported fill where required) and thus will be suitable for on-site stormwater retention.
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• A small amount of high density lots (<300 sqm) will be included, mostly around the town centre. Lot drainage connections have been assumed for these lots in the modelling.
• Town centre / commercial areas will contain the first 15mm rainfall event on-site.
• Retirement village and primary school areas will contain all their own drainage on-site.
Table 13-B: Post-development Model Parameters
Catchment Type Initial Loss (mm) Continuing Loss (mm/hr) Manning’s n
Direct Connected Impervious 1.5 0.1 0.015
Indirect Connected 20 4 0.1
Town Centre 15 1 0.025
POS 20 4 0.2
Direct connected catchment was defined in the model as impervious catchment areas which are connected to the drainage network. This includes the impervious surface of the road reserve and lot areas that are serviced by a direct drainage connection. The indirect connected catchment refers to areas that are not directly connected to the drainage system. This includes pervious areas and impervious lot areas which drain to on-site soakage systems. The town centre and POS areas were also treated as separate catchment types with their own corresponding infiltration and roughness characteristics.
The drainage catchment boundaries were provided by the project engineers based on their earthworks assessment and design surface levels, provided as Appendix 11 of the LWMS. The catchment type breakdown was calculated by measurement of road reserve areas in the proposed structure plan and preliminary design information for lot sizes and types. The catchment type breakdown is provided in Table 13-C.
Table 13-C: Catchment Type Breakdown
Catchment Direct Connected (ha)
Indirectly Connected (ha)
Town Centre (ha) POS (ha)
1 1.652 5.458 0.000 0.960
2 1.064 3.104 0.000 0.692
3 3.346 11.264 0.000 0.000
4 0.259 1.341 0.000 0.000
5 2.527 18.943 0.000 0.000
6 1.960 7.630 0.000 0.000
7 3.178 10.752 0.000 0.000
8 1.708 6.377 0.000 0.315
9 3.451 1.479 0.000 0.310
10 2.933 2.932 0.000 0.495
11 5.735 16.365 0.000 1.150
12 2.492 6.798 0.000 0.710
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13 2.183 9.877 0.000 0.750
14 0.929 3.689 0.000 0.770
15 2.338 1.002 10.500 0.140
16 3.010 7.570 0.000 6.220
17 3.598 15.362 0.000 3.840
18 2.590 11.450 0.000 0.440
19 0.342 0.507 0.000 0.000
SC1 0.448 1.942 0.000 0.000
SC2 1.035 5.663 0.000 0.000
The post-development model determined that, with no stormwater retention or detention provided, the peak 1% AEP discharge rate for the critical (6 hour) event increases from 12 m3/s to 14 m3/s.
The model was then used to determine the required amount of flood detention storage to maintain flow rates within Ki-it Monger Brook and discharging from the site at pre-development rates. The sizing of flood storage basins was based on an assessment of the POS areas and design surface levels to determine the most suitable areas for flood storage and the likely volumes that could be accommodated within the available POS. The modelling then involved an iterative process to optimise the distribution and amount of flood storage throughout the development and confirm the required flood storage volumes.
Model Results (Drainage Design)
Graph 13-1 shows the flow hydrographs for discharge from the site beneath the Great Northern Highway. The graph shows the discharge rate for all design storm durations (1 hour to 72 hours) for three scenarios; pre-development conditions, post-development conditions without any flood storage and post-development conditions with the modelled flood storages included.
Graph 13-1 shows that without any flood storage, the post-development discharge rate increases from 12 m3/s to 14 m3/s for the critical (6 hour) event. The impact is even greater for the 1 hour event which increases from 7.5 m3/s to 12 m3/s. The graph also shows that once the modelled flood storages are included the site discharge rate is attenuated to pre-development conditions for all storm durations.
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Graph 13-1: Discharge Rate from the Site under Existing and Developed Conditions
The drainage modelling results are provided in Table 13-D which includes the modelled basin sizes, volumes, inundation areas, depths and preliminary sizing of the detention basin outlet pipes.
These details are subject to refinement during detailed design. The outline drainage design including indicative drainage basin locations and inundation areas is provided as Figures 9 to 11 in the LWMS.