6.4 Sensitivity Tests

Transcription

1 6.4 Sensitivity Tests Sensitivity of Floodplain Inundation to Width of Dry Culvert The preliminary design for the Expressway based on consideration of possible future climate effects to 2090 assumed a 40m wide dry culvert through the Expressway embankment to provide continuity for the secondary flow path along the floodplain, parallel with and to the north of the right stopbank system protecting Ōtaki Township. This included the definition of dimensions and levels for the secondary flood containment bund to prevent the spread northwards of stopbank overtopping flows into the Mangapouri Stream. A sensitivity test was carried out to check the effect on flood levels within the flood ponding area formed by the secondary flood containment bund of increasing the width of the proposed dry culvert through the Expressway embankment. The culvert width was increased from 40m to 50m. Figure 6-18 shows a graph of predicted flood levels along the alignment of the proposed secondary flood containment bund for floods of varying magnitudes overtopping the Chrystall s Bend extended stopbank Level (m MSL) Offset (m) 0.2% AEP - 40m wide culvert 0.2% AEP + CC m wide culvert 0.2% AEP + CC m wide culvert 0.2% AEP + CC m wide culvert Breach Scenario - 0.2% AEP + CC m wide culvert Ground level Figure 6-18 Predicted peak flood level profiles along alignment of proposed secondary flood containment bund on Ōtaki River floodplain for floods overtopping Chrystall s Bend stopbank. Status Issue 1 Page 78 September 2013

2 For the 0.2% AEP +CC 2130 flood (average climate change scenario), the peak flood in the containment bund area with a 50m wide dry culvert is only marginally (0.05m) lower than with a 40m wide dry culvert. For a flood of this magnitude, operation of the overflow weir on the Expressway embankment therefore is clearly the predominant control on flood levels in the containment bund basin. Figure 6-18 also indicates that the nominated crest level of the secondary containment bund of 16.05m (MSL Wellington (1953) datum) from Smith and Webby (2013d) would need to be raised slightly by about 0.3m. The vertical profile of the overflow weir on the Expressway embankment would also need to be modified slightly Sensitivity of Floodplain Inundation to Floodplain Surface roughness Peak flood depths across the Ōtaki River floodplain resulting from overtopping of the Chrystall s Bend extended stopbank are affected by the surface roughness of the floodplain. This is represented in the MIKEFLOOD model by means of Manning s n surface roughness values. Table 6-7 summarises the standard set of Manning s n values assumed for the model simulations of overland flow resulting from stopbank overtopping. It was considered appropriate to consider the sensitivity of the peak flood depths across the floodplain for increased values of surface roughness for each roughness category. Table 6-7 also summarises these increased values. Table 6-7 Manning's n surface roughness values for Ōtaki River floodplain. Manning s n Roughness Value Surface Roughness Type Standard Set Increased Set Areas covered by buildings Vegetated areas Grass-covered areas Figure 6-19 shows a peak flood depth difference map between the two model simulations with the different sets of floodplain roughness values for the 0.2% AEP +CC 2130 flood (average climate change scenario). Peak flood levels across the floodplain resulting from the increased surface roughness are all less than +0.10m Sensitivity of Floodplain Inundation to Flood in Mangapouri Stream All flood simulations of overland flow across the Ōtaki River floodplain described previously have deliberately avoided the complication of a simultaneous flood in the Mangapouri Stream on the northern edge of the floodplain. This strategy was adopted so as to isolate the effects of the Expressway on secondary flows across the floodplain arising from stopbank overtopping. A further sensitivity test was therefore carried out for the following flood combination: Status Issue 1 Page 79 September 2013

3 a 0.2% AEP +CC 2130 (average climate change scenario) flood overtopping the Chrystall s Bend extended stopbank (with the existing SH1 bridge removed and replaced). A 1% AEP +CC 2130 flood in the Mangapouri Stream. This choice of flood combination reflects the fact that the annual exceedance probability of the flood in the Mangapouri Stream is likely to be smaller than that of the simultaneous flood in the Ōtaki River. Figure 6-20 shows a peak flood depth difference map for the proposed situation for the 0.2% AEP +CC 2130 flood with and without the 1% AEP +CC 2130 flood in the Mangapouri incorporated. This figure illustrates the incremental effect of adding the 1% AEP +CC 2130 flood in the Mangapouri Stream to the floodplain inundation pattern and peak flood depths. The increases in peak flood depth across the floodplain are confined to the area of the two flood storage basins on the Mangapouri Stream, the area upstream of the Expressway embankment between Rahui Road (along the southern edge of the two storage basins) and the secondary flood containment bund, and a short distance along the course of the Mangapouri Stream downstream of the SH1 culvert. There is negligible change across the rest of the floodplain, indicating that the Mangapouri Stream flows downstream of the SH1 culvert would largely be confined to the channel. The maximum increase in peak flood level along the course of the Mangapouri Stream is predicted to be less than 0.1m. Status Issue 1 Page 80 September 2013

4 Figure 6-19 Changes in peak flood depth across Ōtaki River floodplain due to increased surface roughness for 0.2% AEP +CC 2130 (average climate change scenario) flood with existing SH1 bridge removed. Status Issue 1 Page 81 September 2013

5 Figure 6-20 Changes in peak flood depth across Ōtaki River floodplain for 0.2% AEP +CC 2130 (mid-high climate change scenario) flood in proposed situation with addition of 1% AEP +CC 2130 flood in Mangapouri Stream. Status Issue 1 Page 82 September 2013

6 6.5 Duration of Ponding in Secondary Containment Bund Area The duration of ponding in the secondary containment bund area in the proposed situation would be very similar to that indicated by Figure 5-13 in Technical Report TR9-C (Smith and Webby 2013d). Floodwaters in this area could pond for a period of about 4 hours for the 0.2% AEP +CC 2130 flood (average climate change scenario) although this would be influenced by the duration of the flood discharge hydrograph in the river. 6.6 Interpretation of Effects for Floodplain Increases in peak flood depth across the Ōtaki River floodplain in the proposed situation due to stopbank overtopping by the 0.2% AEP + CC 2130 flood (average and mid-high climate change scenarios) are confined to the area contained by the Expressway embankment, the Chrystall s Bend extended stopbank, and secondary flood containment bund. Elsewhere across the floodplain, peak flood depth changes area well within the estimated accuracy of the flood depth predictions. This implies that there is effectively no change between the proposed and existing situations based on the 0.2% AEP +CC 2130 flood (average or mid-high climate change scenarios). Peak flood depths (levels) across the floodplain are fairly insensitive to changes in surface roughness and very insensitive to a concurrent flood in the Mangapouri Stream. The duration of ponding in the secondary flood containment bund area in the proposed situation for the 0.2% AEP + CC 2130 flood (average climate change scenario) could be likely to be in the order of 4 hours although this would be influenced by the shape of the flood discharge hydrograph in the river. 6.7 Impact on River and Floodplain Crossing Design As for the Waitohu Stream crossing, the design of the Ōtaki River and floodplain crossing is predicated on the assumption of predicted peak flood levels for the 1% AEP + CC 2090 flood (average climate change scenario). The predicted peak flood levels in Table 6-1 for the 1% AEP + CC 2130 flood (average climate change scenario) indicate that the effect on the river crossing design (i.e. in setting design levels) will be minimal with an increase in peak level at the Expressway bridges of only about 0.1m. The floodplain crossing design incorporates a very wide culvert through the Expressway embankment and an emergency overflow weir. This design is robust enough not to need any significant modification except for some minor adjustments to vertical profile at the northern end of the weir section to contain overtopping floodwaters. The design crest level of the secondary flood containment bund will need to be raised by about 0.25m. Status Issue 1 Page 83 September 2013

7 7. Effects of Mangaone Stream Crossing 7.1 Peak Flood Level and Discharge Predictions Flood hydrographs for the estimated 1% AEP +CC 2130 floods based on average and mid-high climate change scenarios were routed across the Mangaone Stream alluvial fan past the proposed Expressway, existing NIMT railway and existing SH1 transport links. Table 7-1 compares predicted peak levels at key locations across the fan within the area of interest between the existing and proposed situations for both flood cases. includes locations on the Mangaone Stream and on the Mangaone Overflow. Table 7-1 Predicted peak flood levels for existing and proposed situations for different 1% AEP + CC 2130 Location Mangaone Stream floods (red text indicates proposed culverts, black text indicates existing culverts). Local link road (eastern side) Expressway NIMT railway - existing SH1 - existing Local link road (western side) Mangaone Overflow Local link road (eastern side) Expressway NIMT railway - existing SH1 - existing School Rd Drain School Rd / Gear Rd intersection Local link road culvert Lucinsky Overflow Local link road (western side) Peak Flood Level (m MSL Wellington (1953) datum) CC to 2090 (average scenario) Existing Proposed CC to 2130 (average scenario) Existing Proposed This CC to 2130 (mid high scenario) Existing Proposed US US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS US DS * with Lucinsky Overflow unblocked sensitivity test for CC to 2090 scenario US DS US DS US DS US * Status Issue 1 Page 84 September 2013

8 Similarly, Table 7-2 compares predicted peak discharges at the same locations between the existing and proposed situations or both flood cases. Peak flood levels and discharges in the existing and proposed situations for the 1% AEP +CC 2090 flood from Technical Report 9-D (Smith and Webby, 2013e) are also given in Table 7-1 and Table 7-2 for comparison. Table 7-2 Predicted peak flood discharges for existing and proposed situations for different 1% AEP + CC 2130 floods (red text indicates proposed culverts, black text indicates existing culverts). Peak Flood Discharge (m 3 /s) Location Mangaone Stream Local link road (eastern side) Expressway NIMT railway - existing SH1 - existing Local link road (western side) Mangaone Overflow Local link road (eastern side) Expressway NIMT railway - existing SH1 - existing School Rd Drain School Rd / Gear Rd intersection Local link road culvert CC to 2090 (average scenario) Existing Proposed CC to 2130 (average scenario) Existing Proposed CC to 2130 (mid high scenario) Existing Proposed culvert 1.8 road overflow culvert 1.0 road overflow culvert 1.2 road overflow culvert 1.2 road overflow 26.8 culvert 1.6 road overflow 25.6 culvert 1.5 road overflow 26.5 culvert 2.8 road overflow 25.6 culvert 1.52 road overflow 26.5 culvert 2.8 road overflow Status Issue 1 Page 85 September 2013

9 The peak flood level and discharge predictions in Table 7-1 and Table 7-2 need to be interpreted in conjunction with the peak flood depth inundation maps and peak flood depth difference maps in Figure 7-1 to Figure 7-6. However, examination of Table 7-1 and Table 7-2 gives rise to the following observations. The increases in peak flood level between the proposed and existing situations upstream of the local link road culverts on both the main stream channel and on the overflow in each 1% AEP +CC 2130 flood case reflect the partial barrier effect of the local link road embankment with respect to overland flow. Similarly, the increases in peak flood level between the proposed and existing situations upstream of the Expressway culverts on the main stream channel and on the overflow in both flood cases reflect the flood containment function of the bund upstream of the proposed Expressway. The flood level increases are greatest upstream of the Expressway culvert on the overflow where ground levels are naturally lower and the preferential flow path for overland flow down the fan is located. Peak flood levels upstream of the NIMT railway culvert on the main stream channel are slightly reduced between the proposed and existing situations in both 1% AEP +CC 2130 flood cases (-0.10m and -0.11m). Peak flood discharges through the railway culvert are also reduced between the existing and proposed situations in both flood cases due to the throttling effect of the Expressway / NIMT railway / SH1 culvert system and the flood containment effect of the Expressway bund (the flow areas of the existing railway and SH1 culverts are about 70% of the flow area of the proposed Expressway culvert). Peak flood levels upstream of the NIMT railway culvert on the Overflow are increased very slightly between the proposed and existing situations for each 1% AEP +CC 2130 flood case (+0.15m and +0.17m respectively). This reflects the fact that ground levels are locally slightly lower along the course of the overflow due to this flow path being the former course of the Mangaone Stream. Peak flood discharges through the railway culvert are increased very slightly between the proposed and existing situations (+0.6m³/s and 0.4m³/s respectively). However these discharges are substantially attenuated by the existing SH1 culvert immediately downstream which has a flow area of only about 60% of the new railway culvert upstream and the proposed Expressway culvert. The proposed modifications to the Lucinsky Overflow (including a lowered bank area along the main stream channel adjacent to the Overflow entrance and a 5m wide by 1m high box culvert through the western approach embankment to the local link road overbridge) appropriately replicate the original discharge characteristics of this secondary flow path. 7.2 Peak Flood Depth Maps Figure 7-1 and Figure 7-2 show peak flood depth inundation maps for the existing and proposed situations respectively across the Mangaone Stream alluvial fan for the 1% AEP +CC 2130 (average climate change scenario) flood. Both figures show that the 1% AEP +CC 2130 flood breaks out of the main stream channel and starts to flow overland more than 700m upstream of the Expressway crossing of the fan. In the existing situation flood ponding would occur upstream of the NIMT railway line over a distance of about 560m along the railway line and extending about 70m up the fan. The maximum depth of flood inundation above natural ground levels would be about 1.0m at the railway culvert on the main stream channel and 1.4m at the railway culvert on the overflow. Status Issue 1 Page 86 September 2013

10 In the proposed situation, flood ponding would occur upstream of the local link road (over a distance of about 440m along the road and extending about 75m up the fan) and upstream of the proposed Expressway (over a distance of about 640m along the road and covering most of the area enclosed by the Expressway and local link road, and an additional area to the north of the local link road overbridge crossing). The maximum depth of inundation in the flood ponding area upstream of the local link road would be less than 2m. The maximum depth of inundation above natural ground levels in the flood ponding area upstream of Expressway would be about 1.3m at the culvert on the main stream channel less than 2m at the culvert on the overflow. The two flood ponding areas upstream of the local link road and the Expressway represent the primary effects of the Project. The flood ponding areas affect only pastoral land, much of it within the proposed designation for the Project. None of this land features any buildings that will not be acquired by the Project. This flood inundation would be very infrequent, of limited duration and confined to this limited area. Therefore the effects of flood inundation will be minimal and acceptable. Figure 7-7 compares flood discharge hydrographs through the Expressway culvert in the proposed situation and through the NIMT railway culvert on the main stream channel for the 0.2% AEP +CC 2130 (average climate change scenario) flood. Similarly, Figure 7-8 compares flood discharge hydrographs through the Expressway culvert in the proposed situation and through the NIMT railway culvert in the existing situation on the Mangaone Overflow for the same flood case. Both Figure 7-7 and Figure 7-8 provide a comparative measure of the duration of the flood along each primary flow path. The shape of the flood hydrograph used for routing purposes is shown in Figure 2-4 of Smith and Webby (2013e). The hydrograph is an extended duration one with three peaks. The total flood discharge for the 1% AEP +CC 2090 flood exceeds 14m³/s (the estimated flow at which breakout from the main stream channel towards the Overflow is estimated to occur) for up to 20 hours. The pattern of the discharge hydrographs in Figure 7-7and Figure 7-8 suggest that the duration of ponding in the flood ponding area upstream of the Expressway could be at least 12 hours. However this is heavily influenced by the shape of the base hydrograph for the flood. A single peaked flood hydrograph would cause the duration of ponding to be reduced. Figure 7-3 shows changes in peak flood depth across the alluvial fan between the proposed and existing situations for the 0.2% AEP +CC 2130 (average climate change scenario) flood. This peak flood depth difference map confirms that the major changes in flood depths will occur in the two identified flood ponding areas, upstream of the local link road and upstream of the Expressway. The maximum change in peak flood depth is about 1.3m in the local link road ponding area while the maximum change is less than 1m in the Expressway ponding area. Upstream of the two flood ponding areas, the flood inundation patterns shown in Figure 7-1 for the existing situation and Figure 7-2 for the proposed situation are much the same. The apparent differences in peak flood depth over this upstream are within the estimated accuracy of the model predictions so that there is effectively no change between the proposed and existing situations. Downstream of the existing SH1, Figure 7-3 and Figure 7-6 both show a broad central strip (250m wide in the case of the average climate change scenario flood and smaller in the case of the mid-high climate change scenario flood) of light pink shading (indicating less than a +0.05m change in flood depth) with flanking strips either side of light green shading (indicating less than a -0.05m change in flood depth). Again these changes Status Issue 1 Page 87 September 2013

11 in peak flood depth over both the central strip and the flanking strips are within the estimated accuracy of the model predictions. To the south of this wider strip, peak flood depths along an old remnant channel are reduced slightly in the proposed situation due to the elimination of overtopping of the NIMT railway and SH1 in the existing situation seen in Figure 7-1. Another rail and road overflow path at Te Waka Road to the north is also eliminated (this is just off the peak flood depth map in Figure 7-2). Along Te Horo Beach Road between SH1 and the local link road tee intersection, peak flood depths are also reduced slightly in the proposed situation. Similarly to the north of the local link road western overbridge approach embankment, peak flood depths are also reduced slightly due to confinement of SH1 road overflow by the bridge approach embankment at the site of the SH1 culvert on the main stream channel. The partial damming effect of the western approach embankment to the local link road overbridge causes slightly increased peak flood depths within the area between the main stream channel and the approach embankment. Overall the impact of the Project on the inundation area to the west of the existing SH1 is no worse than in the existing situation even though peak flood discharges through the SH1 culvert and over SH1 on the Mangaone Overflow are marginally increased (see Table 7-2). Figure 7-4 and Figure 7-5 contain peak flood inundation depth maps for the existing and proposed situations respectively for the 1% AEP +CC 2130 flood (mid-high climate change scenario). These figures show a similar flood inundation pattern to Figure 7-1 and Figure 7-2, albeit with slightly higher maximum depths. The proposed bund upstream of the Expressway to provide flood containment remains sufficiently high to contain this flood with an 8% larger peak than the 1% AEP +CC 2130 (average climate change scenario) flood. Similarly the peak flood depth difference map in Figure 7-6 shows a very similar pattern to that in Figure 7-3. The previous conclusions that the effects of flood inundation outside of the two primary food ponding areas (upstream of the local link road and upstream of the Expressway) are no worse than in the existing situation remain valid for this larger flood. Over most of the area downstream of SH1, peak flood level differences between the proposed and existing situations are within ±0.05m which is within the estimated accuracy of the flood level predictions. Status Issue 1 Page 88 September 2013

12 Figure 7-1 Flood peak depths across Mangaone River alluvial fan in existing situation for 1% AEP +CC 2130 flood based on average (mid-range) climate change scenario. Status Issue 1 Page 89 September 2013

13 Figure 7-2 Peak flood depths across Mangaone Stream alluvial fan in proposed situation for 1% AEP +CC 2130 flood based on average (mid-range) climate change scenario. Status Issue 1 Page 90 September 2013

14 Figure 7-3 Changes in peak flood depths across Mangaone River alluvial fan between proposed and existing situations for 1% AEP +CC 2130 flood based on average (mid-range) climate change scenario (note: pink and red shading indicates areas of increased flow depths, green shading indicates areas of decreased flow depths). Status Issue 1 Page 91 September 2013

15 Figure 7-4 Peak flood depths across the Mangaone Stream alluvial fan in existing situation for 1% AEP +CC 2130 flood based on mid-high climate change scenario. Status Issue 1 Page 92 September 2013

16 Figure 7-5 Peak flood depths across the Mangaone Stream alluvial fan in proposed situation for 1% AEP +CC 2130 flood based on mid-high climate change scenario. Status Issue 1 Page 93 September 2013

17 Figure 7-6 Changes in peak flood depths across the Mangaone Stream alluvial fan between proposed and existing situations for 1% AEP +CC 2130 flood based on mid-high climate change scenario (note: pink and red shading indicates areas of increased flow depths, green shading indicates areas of decreased flow depths). Status Issue 1 Page 94 September 2013

18 Discharge (m 3 /s) Existing 5 Proposed Time (hours) Figure 7-7 Comparison of flood discharge hydrographs on the main stream channel through the Expressway culvert in the proposed situation and the NIMT railway culvert in the existing situation (1% AEP +CC 2130 flood average climate change scenario). Status Issue 1 Page 95 September 2013

19 Discharge (m 3 /s) Existing Proposed Time (hours) Figure 7-8 Comparison of flood discharge hydrographs on Mangaone Overflow through Expressway culvert in the proposed situation and the NIMT railway culvert in the existing situation (1% AEP + CC 2130 flood average climate change scenario). Status Issue 1 Page 96 September 2013