Appendix P Drainage, Flooding, Stormwater Management Part 5

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Flooding, Drainage and Stormwater Management Sanctuary Villages Details of XP-RAFTS developed model catchment data are summarised in Appendix B. Node locations used in the XP-RAFTS developed model are identical to those for the existing model and are shown in Figure 3. The percentage of impervious area for the development site has been adopted as 60%. This value has been adopted to represent the relative contribution of various land types including; road reserves, commercial, residential and parkland. This assumption is valid for a regional level study but a more detailed breakdown of land use will be considered at the local strategy level. 3.2.3 Developed peak discharges Predicted developed flow rates within Congewai Creek at Millfield Bridge for the 1, 5, 10, 20, 50 and 100 year ARI storm events are summarised in Table 3-4. Also shown are existing peak flow rates at this location and the percentage change in flow resulting from development. Runoff hydrographs for the critical duration storm that produced the largest peak flow estimate are provided in Appendix E for developed conditions. The critical storm duration that produced the peak flow rate at node CON.15 was the 720 minute storm for the 1 year ARI event and 2160 minute storm for all other events. Table 3-4: Developed peak flow estimates within Congewai Creek at Millfield Bridge (Node CON.15) ARI Existing Peak Flow Rate (m 3 /s) Developed Peak Flow Rate (m 3 /s) Percentage Change 1 Year 62 62 0% 5 Year 221 221 0% 10 Year 296 295 -0.3% 20 Year 392 391 -0.2% 50 Year 571 571 0% 100 Year 762 761 -0.1% Table 3-4 illustrates that the proposed development results in a negligible change in the critical duration peak flow rate at Millfield Bridge for the 1, 5, 10, 20, 50 and 100 year ARI storm events. Examination of Figure 3 shows the proposed development area to be located near the outlet of the analysed catchment, which extends as far as Millfield Bridge. The introduction of impervious surfaces at this location increases the rate of runoff from the lower portions of the catchment allowing runoff to enter Congewai Creek before the overall catchment flood peak. This is further supported by examination of the developed runoff hydrograph for the 1 year critical duration storm event contained within Appendix E, which shows a small spike on the rising limb of the hydrograph when compared to the existing runoff hydrograph at this location. A comparison of existing and developed peak flow rates for the 1, 10 and 100 ARI storm events run for a range of storm durations is shown in Figure 5. Examination of Figure 5 reveals negligible change in peak flow rate for the 10 and 100 year ARI storm events at Millfield Bridge for all storm durations. However, there is a minor increase in peak flow rate for the shorter duration 1 year ARI storm events. This increase in flow rate will require detention storage to reduce the potential for scour of existing drainage lines. PARSONS BRINCKERHOFF 2122870A-PR0116 Page 13
Flooding, Drainage and Stormwater Management Sanctuary Villages This study has considered the impact of the development on the major (regional) flood events. Future stormwater strategies should consider the impact of the development on local stormwater runoff flow rates. The results of the future stormwater management strategy are likely to indicate the need for stormwater detention at local outfalls, which should also resolve the impact of the development on shorter duration 1 year ARI regional storm events. Details of stormwater management measures that may be considered in future detailed stormwater management strategies are provided in Section 5. PARSONS BRINCKERHOFF 2122870A-PR0116 Page 14
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Appendix P Drainage, Flooding, Stormwater Management Part 5

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