Appendix P Drainage, Flooding, Stormwater Management Part 5

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Flooding, Drainage and Stormwater Management Sanctuary Villages A summary of existing peak discharges at Millfield Bridge (node CON.15) for the 1, 5, 10, 20, 50 and 100 year ARI storm events is provided in Table 3-2. Also provided are peak discharge estimates documented in the Patterson Britton and Partners (PBP) flood study (April 2004) at the same location. The critical storm duration that produced the peak discharge for all events except the 1 year ARI event was the 2160 minute storm. The peak discharge for the 1 year ARI event occurred for the 720 minute storm. Estimated hydrographs at Millfield Bridge (node CON.15) for the 1, 5, 10, 20, 50 and 100 year ARI critical duration storm event are provided in Appendix D. 800 4,000 700 3,500 600 3,000 Discharge (m 3 /s) 500 400 300 2,500 2,000 1,500 Storage (ML) 200 1,000 100 500 0 0 110.5 111 111.5 112 112.5 113 113.5 114 Height (m AHD) PB H-Q Relationship PBP H-Q Relationship PB H-S Relationship PBP H-S Relationship Figure 4: Height-Storage-Discharge Relationship for Ellalong Lagoon Table 3-2: Existing peak discharges at Millfield Bridge (Node CON.15) ARI Existing Peak Discharge (m 3 /s) Critical Storm Duration (min) PBP Peak Discharge Estimate (m 3 /s)^ 1 Year 62 720 - 5 Year 221 2160 255 10 Year 296 2160 - 20 Year 392 2160 434 50 Year 571 2160 - 100 Year 762 2160 654 ^ PBP peak discharge estimate has been provided for comparative purposes only. PARSONS BRINCKERHOFF 2122870A-PR0116 Page 11
Flooding, Drainage and Stormwater Management Sanctuary Villages Existing peak discharges at Millfield Bridge for the 5 and 20 year ARI storm events are generally in close agreement with estimates documented in the Wollombi Valley Flood Study prepared by Patterson Britton and Partners (April 2004). The predicted 100 year discharge is 17% larger than that documented by PBP. This may be due to variations in the height-storage-discharge relationship adopted by both Consultants. A summary of existing peak discharges, peak storage and peak water level at Ellalong Lagoon (node QUO.7) for the 1, 5, 10, 20, 50 and 100 year ARI storm event is provided in Table 3-3, and will be used as a baseline comparison for evaluating the impacts of the development on existing flow rates, as well as for estimating the 100 year ARI flood levels within the study area. Table 3-3: ARI Existing peak discharge, peak storage and peak water level in Ellalong Lagoon Existing Peak Discharge (m 3 /s) Critical Storm Duration (min) Peak Storage (ML) Peak Water Level (m AHD) 1 Year 16 720 473 111.23 5 Year 64 2160 1,560 112.35 10 Year 80 2160 1,560 112.84 20 Year 141 2160 2,820 113.34 50 Year 231 2160 3,030 113.51 100 Year 305 2160 3,160 113.62 3.2 Developed hydrology 3.2.1 Potential hydrological impacts of development Urban development typically results in an increase in the impervious surfaces within a catchment and engineering works to collect and convey runoff. Without appropriate management, several potential impacts result from development. These could include: increased frequency of runoff events; increased runoff volume, peak flow and velocity; and decreased groundwater base flows and lowering of the watertable. These impacts may in turn lead to: inundation of downstream property and infrastructure; change in existing channel form and sediment transport; damage to riparian and aquatic vegetation; and damage to existing aquatic ecosystems and natural wetlands. 3.2.2 Model parameters The effect of urbanisation, of the proposed development site, was modelled using XP- RAFTS and involved the inclusion of impervious areas within the existing model. The model was then used to estimate developed peak flow rates at Millfield Bridge (node CON.15). PARSONS BRINCKERHOFF 2122870A-PR0116 Page 12
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Appendix P Drainage, Flooding, Stormwater Management Part 5

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