Appendix C MIKEFLOOD Model Predictions for 28 October 1998 Flood Event

Transcription

1 Appendix C MIKEFLOOD Model Predictions for 28 October 1998 Flood Event Status Issue 2 Page 99 January 2013

2 The predictions of the MIKEFLOOD model were checked against anecdotal evidence of flood inundation patterns following the aftermath of the 28 October 1998 flood (Figures 4-2 and 4-3). Figure C-1 shows the measured flood discharge hydrograph from the Ratanui gauging station for the 28 October 1998 flood event while Figure C-2 shows the inflow hydrographs used as upstream boundary conditions for the same event in the MIKEFLOOD model scaled from the measured hydrograph in Figure C-1. 8 Figure C-3 shows predicted stage hydrographs upstream of the NIMT railway and SH1 culverts on the main stream channel for the MIKEFLOOD model simulation of the 28 October 1998 flood while Figure C-5 shows corresponding discharge hydrographs at the same locations. Similarly Figure C-5 shows predicted stage hydrographs upstream of the NIMT railway and SH1 culverts on the Mangaone Overflow for the model simulation of the flood event and while Figure C-5 shows corresponding discharge hydrographs at the same locations. Figure C-7 shows the predicted flood inundation pattern across the alluvial fan surface for the 28 October 1998 flood. Figure C-4 shows a slight reduction in the peak flood discharge through the NIMT railway and SH1 culverts from about 23.0m 3 /s to 21.5m 3 /s. On the other hand Figure C-6 shows an increase in peak flood discharge through the NIMT railway and SH1 culverts on the Mangaone Overflow from about 17.4m 3 /s to 18.2m 3 /s. The increase is due to augmentation of the SH1 culvert flow by flows in the roadside drain spreading southwards towards the Mangaone Overflow from surcharging of floodwaters upstream of the SH1 culvert on the main stream channel 9. (the NIMT railway culvert at the time of the 28 October Figure C-7 indicates that significant flood ponding would have occurred upstream of the NIMT railway line which matches what can be seen in Figures 4-2 and 4-3 after the peak of the flood. It also indicates overtopping of the railway line and SH1 by the northern overland flow path at the Te Waka Road intersection with SH1 and by the School Road drain flow at the Gear Road intersection with SH1. Between the NIMT railway line and SH1, railway overtopping flows spread along the roadside drain and flowed over SH1 at several locations through Te Horo Village (as well as augmenting the flow through the SH1 culvert on the Mangaone Overflow). It is not known whether this reflects the actual observed locations of flood inundation experienced except that the predictions of shallow overflow across SH1 match anecdotal evidence of peak flood depths. In reality there would probably have been seepage of floodwaters through the track ballast on the railway line as well as overtopping. Figure C-7 also indicates shallow inundation (< 0.2m) through parts of Te Horo Village to the west of SH1. Again it is not known how accurate these predictions are of actual inundation patterns although presumably, if there was overflow of SH1 at several locations, there would also have been some minor inundation of properties through the village. 8 In Figure C-2, the leading edges of the Mangaone Stream and North-1 component inflow hydrographs have been modified to incorporate a dummy base flow to prevent the Mangaone Overflow from running dry. This was done to overcome problems of model instability. This departure from reality does not affect the results of the model verification as it is the predictions of the model at the flood peak that are of primary interest. 9 At the time of the 28 October 1998 flood, the NIMT railway culvert on the Mangaone Overflow had a smaller discharge capacity than the downstream SH1 culvert the railway culvert has since been replaced. Status Issue 2 Page 100 January 2013

3 Figure C-7 shows that the Lucinsky Overflow would have operated which again matches the photographic evidence in Figures 4-2 and 4-3. However Figure 4-2 does suggest that the source of flow for the Overflow may in fact have been the main channel just upstream of the access drive to the Lucinsky property through the line of trees in the centre of the photograph the part of the access drive across the direct line of the Overflow appears not to have been inundated. This suggests some uncertainty about the topographic detail in the MIKEFLOOD model about this location. Despite this uncertainty over the topographic detail of one small area of the DTM in the MIKEFLOOD model there is at least some evidence to suggest that the predicted flood inundation pattern for the 28 October 1998 flood broadly reflects the pattern of inundation actually experienced. If the pattern of flood inundation is overpredicted, this could be more a consequence of the uncertainty over the actual magnitude of the fled event where the NIMT railway line and SH1 cross the Mangaone Stream and Overflow. The MIKEFLOOD model is used in a comparative sense in this investigation to assess the relative effects of the Expressway compared to the existing situation when the same boundary conditions are applied. Therefore any errors in the base model will be common to both the existing and proposed situations and will affect the results of the model simulations similarly. The EPA s Technical Reviewer has queried the validity of using an approximately 15% AEP flood (estimated peak discharge 41.5m 3 /s at the SH1 stream crossing) as validation for the effects of much larger floods (peak discharges up to about twice the peak discharge of the 28 October 1998 validation event). The MIKEFLOOD model is effectively a floodplain storage model simulating the ponding effects of the transport links crossing the Mangaone Stream alluvial fan and floodplain system. Despite the two-fold increase in the magnitude of the peak flood discharge at the SH1 stream crossing between the 28 October 1998 validation flood and the 1% AEP flood adjusted for possible climate change effects to 2090 (refer Tables 2-5 and 2-6 for flood discharge values), the model predicts peak flood levels in the existing situation at the NIMT railway culvert on the main stream channel of 18.81m (Figure C-3) and 18.89m (Table 4-2) respectively, an increase of only 0.08m. Any uncertainty in the estimated magnitudes of the 1% and 0.5% AEP floods adjusted for possible future climate effects is therefore not significant in terms of the extent of predicted flood inundation. The MIKEFLOOD model as configured for the 28 October 1998 flood simulation is therefore suitable to use as a tool for assessing the effect of the Expressway on existing flood patterns across the Mangaone Stream alluvial fan and floodplain system. Status Issue 2 Page 101 January 2013

4 Discharge (m 3 /s) Discharge (m 3 /s) NZ Transport Agency Time (dd/mm/yyy hh:mm) Figure C-1 Measured flood discharge hydrograph at Mangaone at Ratanui gauging station for 28 October 1998 flood Mangaone Stream North_1 South_1 North_2 School Road Combined Discharge Time (hours) Figure C-2 Assumed inflow hydrographs for 28 October 1998 flood for input to MIKEFLOOD model Status Issue 2 Page 102 January 2013

5 Level (m MSL Wellington) Level (m MSL Wellington) NZ Transport Agency US of NIMT Railway - Chainage 6450 US of SH1 - Chainage :00:00 10:00:00 09:00:00 08:00:00 07:00:00 06:00:00 05:00:00 04:00:00 03:00:00 02:00:00 01:00:00 00:00:00 Time (hh:mm:ss) Figure C-3 Predicted stage hydrographs at NIMT railway and SH1 culverts on main stream channel for 28 October 1998 flood NIMT Railway Culvert SH1 Culvert 0 11:00:00 10:00:00 09:00:00 08:00:00 07:00:00 06:00:00 05:00:00 04:00:00 03:00:00 02:00:00 01:00:00 00:00:00 Time (hh:mm:ss) Figure C-4 Predicted discharge hydrographs at NIMT railway and SH1 culverts on main stream channel for 28 October 1998 flood Status Issue 2 Page 103 January 2013

6 Level (m MSL Wellington) Level (m MSL Wellington) NZ Transport Agency US of NIMT Railway - Chainage 6450 US of SH1 - Chainage :00:00 10:00:00 09:00:00 08:00:00 07:00:00 06:00:00 05:00:00 04:00:00 03:00:00 02:00:00 01:00:00 00:00:00 Time (hh:mm:ss) Figure C-5 Predicted stage hydrographs at NIMT railway and SH1 culverts on Mangaone Overflow for 28 October 1998 flood NIMT Railway Culvert SH1 Culvert 0 11:00:00 10:00:00 09:00:00 08:00:00 07:00:00 06:00:00 05:00:00 04:00:00 03:00:00 02:00:00 01:00:00 00:00:00 Time (hh:mm:ss) Figure C-6 Predicted discharge hydrographs at NIMT railway and SH1 culverts on Mangaone Overflow for 28 October 1998 flood (note that NIMT railway culvert was a smaller structure than the present culvert at the time of the 28 October 1998 flood) Status Issue 2 Page 104 January 2013

7 Figure C-7 Predicted peak inundation map for 28 October 1998 flood Status Issue 2 Page 105 January 2013

8 Status Issue 2 Page 106 January 2013