Greater Wellington Regional Council

Final Report

May 2015

Jim Cooke Park Stopbank Upgrade

Hydraulic Modelling

gwrc_waikanae jim cooke memorial park design flood levels update.docx / plw / 2015-05-05

Greater Wellington Regional Council Final Report

May 2015

This report has been prepared under the DHI Business Management System certified by DNV to comply with ISO 9001 (Quality Management)

DHI Water and Environment Ltd• Level 6, 173-175 Victoria Street• Wellington• New Zealand• Telephone: +64 21 238 7515 • • info.nz@dhigroup.com• www.dhigroup.com

Jim Cooke Park Stopbank Upgrade

Hydraulic Modelling

Prepared for Greater Wellington Regional Council Represented by Kees Nauta

Jim Cooke Memorial Park and Stopbank

Photo courtesy of Kees Nauta, GWRC

Project manager Philip Wallace

Project number 44800665-2

Approval date 5 May 2015

Revision Final 2.0

Classification Open

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CONTENTS

1 Introduction ................................................................................................................. 1

2 Hydraulic model .......................................................................................................... 1

3 Recalibration ............................................................................................................... 3

4 Scenarios ..................................................................................................................... 6

5 Design levels ............................................................................................................... 7

6 Impacts of stopbank upgrade ..................................................................................... 8

7 Floodmaps ................................................................................................................... 9

8 Implications for previous design results ................................................................. 12

9 References ................................................................................................................. 13

FIGURES Figure 1 Model study area and main channels in model ..........................................................................2 Figure 2 Recorded and estimated peak flood levels, January 2005 event ................................................3 Figure 3 Assumed relationship between rainfall depth increase and flow increase for climate

change ......................................................................................................................................6 Figure 4 Model predictions and design levels for JCP stopbank ...............................................................7 Figure 5 Stopbank locations ....................................................................................................................8 Figure 6 Increase in flood depth on floodplain due to raised flood defences in Jim Cooke Park

(standard 1% AEP + climate change scenario) ..........................................................................9 Figure 7 Increase in flood depth in river channel due to raised flood defences in Jim Cooke Park

(standard 1% AEP + climate change scenario). .........................................................................9 Figure 8 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks ....................... 10 Figure 9 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks with

breach at Chillingworth stopbank ............................................................................................. 11 Figure 10 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks with

breach at upstream end of JCP ............................................................................................... 12

TABLES Table 1 Revised calibration results .........................................................................................................5

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APPENDICES

APPENDIX A – Previous Calibration Results

APPENDIX B – River Cross-Section Locations

APPENDIX C – Additional Calibration Information

APPENDIX D – Design Stopbank Levels

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1 Introduction Greater Wellington Regional Council (GWRC) intends to upgrade the Jim Cooke Memorial Park (JCP) stopbank, alongside the Waikanae River, in accordance with the commitment made in the Waikanae Floodplain Management Plan. This stopbank protects residential areas on the right (north) bank of the Waikanae River, but requires upgrading to provide the design level of protection against risk of overtopping or breach failure. In January 2005, the stopbank came close to being overtopped in what is estimated as being about a 1 in 80 year flood event. With expected climate change impacts, the risk is increased and the need for upgrading will be greater.

GWRC requested that design levels for the JCP stopbank be provided, sufficient to contain a 1 in 100 year flood event with provision for forecast climate change to 2115. The design levels were required to take into account the observations from the 2005 flood.

Design levels for a preliminary alignment of the JCP stopbank were provided to GWRC in an earlier version of this report, dated 5 August 2014. The alignment was subsequently finalised by GWRC (Opus drawing JCP-OPT3-PLAN, Sheet OP3 Rev 1-15). The model has been rerun with this final alignment and the design levels are updated in this current report.

2 Hydraulic model A MIKE FLOOD model of the Waikanae River and floodplain has previously been developed. The model is described in reports of 2010 (River Edge Consulting (REC), 2010) and 2013 (REC & SKM, 2013), but in summary the Waikanae River has been modelled from the Water Treatment Plant upstream of State Highway One to the sea. The other main branches of the model include the Mazengarb Drain to the south and the Waimeha and the Ngarara Streams to the north. The floodplain modelled is roughly bounded to the south and east by Kapiti Road and SH1 (although extending up to the Water Treatment Plant on the Waikanae River) and extends as far north as the Wastewater treatment ponds in the Ngarara catchment (Figure 1). The floodplain is modelled with a rectangular grid of 4m cell size.

Despite some isolated variations, overall the model previously calibrated satisfactorily to the January 2005 event over the model area. The average of the absolute differences between model results and recorded flood levels was 23 cm. (Table 1 of the 2013 report, reproduced in, Appendix A here) Notably however, the model underpredicted in the vicinity of JCP, by 350 mm according to a documented level at cross-section 260. (Cross-section locations are shown in Appendix B.)

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Figure 1 Model study area and main channels in model

Further anecdotal reports from GWRC staff suggested that the stopbanks were within 200-300 mm of overtopping, implying underprediction by around 500mm, but there was no documentation of those made following the flood. However, recent discussions with GWRC staff (Graeme Campbell, pers. comm.) indicated that they were confident that levels were indeed higher than the documented levels. Appendix C provides further documentation of observed 2005 flood levels in the vicinity. Figure 2 shows the originally documented levels used in the earlier calibration, as well as two anecdotal levels and a series of sample spot levels

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derived from an assumption that flood levels were 250 mm below the top of the bank between cross-sections 290 and 300.

GWRC explicitly requested that the model be recalibrated to match the higher levels.

Figure 2 Recorded and estimated peak flood levels, January 2005 event

3 Recalibration In the prior modelling, the main channel Mannings n value was set to 0.050 in the vicinity of JCP, in an attempt to calibrate the model. This is higher than what might normally be expected, given the nature of the bed material and the channel. Yet the model still undercalibrated. Some possible explanations included:

• There is uncertainty in the estimated river flows. NIWA (McKerchar, 2009) notes that gauging have only been made up to around 140 m3/s (i.e. less than 40% of the estimated flow in the January 2005 event), and that typically the error range at the top end of rating curves is ±10% at the 95% confidence interval

• Debris marks, while very useful, are not always well defined and sometimes are not perhaps indicative of general peak water level – e.g. afflux against a tree, or wave action pushing debris above the more typical flood level. Some degree of subjectivity is involved in recording debris marks.

• Records show that the level runs made in measuring the debris levels were not closed in several instances. The possibility that errors were made in the measurements cannot therefore be discounted in those instances.

• Confusion with stormwater/internal floodwaters seems to have occurred at some locations. (REC, 2010).

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Notwithstanding the already high Mannings value assumed and the possible explanations for underprediction, further effort was made to calibrate to the 2005 observations. Floodplain resistance values were refined (between Otaihanga and SH1, more detail was added by including additional small areas of higher resistances and more generally small increases were made to the left floodplain resistance) , although these changes only had a minor impact. Some attempt was also made to explicitly model bend losses between cross-sections 220 and 240, although again this made little difference to results and bend losses were not included in the final model. Channel resistance values were increased further. Ultimately however, even after pushing resistances to higher than earlier, up to n = 0.060, predicted levels were still around 300mm too low in JCP. To raise predicted levels by that much would require much higher resistance values.

Accepting that the recollections of observed flood levels in JCP are correct, there was presumably some unknown and unmodelled phenomenon that occurred to raise levels nearly to the top of the stopbank. Possible explanations include bed forms (dunes) being present in the river, a slug of gravel raising the river bed, or some significant debris blockage against trees across the berms in JCP. As the model overpredicts the additional levels just downstream (i.e. levels 8.2m and 7.63m shown in Figure 1), the phenomenon (or phenomena) could have been quite localised.

Rather than persevering with unrealistic (or speculative) model assumptions in an attempt to match observed flood levels, the model was left giving levels 300 mm short but the recommended freeboard was increased from the standard 600mm to 900mm along that reach.

Table 1 shows the calibration results achieved in this current exercise. Overall, the results are still generally acceptable, with an average absolute error similar to that obtained previously (Appendix A).

A small flood event occurred in January 2008. In the previous calibration work, the model had predicted levels consistent with limited anecdotal flood level information in the vicinity (REC & SKM, 2013). That event has not been rerun through the model in this current exercise; the 2005 event is considered more relevant (it being closer to the design flow scenario).

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Table 1 Revised calibration results

Location (approximate) Recorded level Model Difference Commentxs 410 (RB) 20.792 20.349 -0.443 xs 400 (RB) 19.880 19.450 -0.430 xs 390 (RB) 21.178 19.168 -2.010 Recorded level in error? xs 380 (RB) 17.475 18.220 0.745xs 345 (RB) 16.172 15.756 -0.416xs 340 (RB) 14.868 15.018 0.150xs 320 (RB) 13.019 13.194 0.175xs 310 (RB) 12.377 12.305 -0.072xs 40 (RB) 1.635 1.928 0.293

Jim Cooke Park stopbank (xs 260) 10.287 10.125 -0.162Near xs 210 (RB) 6.537 6.543 0.006Greenaway Rd 6.672 6.358 -0.314Greenaway Rd 6.002 Dry Localised stormwater?

Near xs 175 (RB) 5.325 5.422 0.097Near xs 155 (RB) 5.014 4.804 -0.210

Footbridge, Otaihanga Domain (LB) 3.76 3.851 0.091end of ROW Otaihanga Rd 4.266 4.340 0.074end of ROW Otaihanga Rd 4.196 4.337 0.141end of ROW Otaihanga Rd 4.251 4.312 0.061Otaihanga Rd at substation 3.514 3.174 -0.340

Mid-way between xs 70 & 80 (RB) 2.677 3.092 0.415Toilet block, Otaihanga Domain 3.8 3.854 0.054

73 Makora Rd (inside house) 4.586 4.387 -0.199River side of flood wall 3.792 3.886 0.094

11 Toroa Rd 4.325 4.388 0.0633 Toroa Rd 4.424 4.388 -0.036

21 Makora Rd (inside house) 3.124 3.459 0.33517?? Makoroa Rd (inside garage) 2.559 2.602 0.043

Driveway to 46 Makora Rd 4.002 Dry Localised stormwater? Couldn't flood from river 42 Makora Rd (garage) 4.012 Dry Localised stormwater? Couldn't flood from river

42 Makora Rd (inside house) 3.94 Dry Localised stormwater? Couldn't flood from river 42 Makora Rd (inside house) 3.487 Dry Same location as point above. Error?

1 Makora Rd 2.203 2.403 0.200

Observations GWRC staff: 200-300mm below TOB. Assume 250mm, take sample locations as per Figure 2:

XS300 RB (2010 survey pt27) 11.6 11.265 -0.3352010 survey pt 11 10.932 10.427 -0.5052010 survey pt 15 10.974 10.637 -0.3372010 survey pt 23 11.393 11.104 -0.289

Observation McIntyre 7.63 8.065 0.435Observation McIntyre 8.2 8.42 0.220

Average -0.012Average (absolute) 0.236

Points close by, difference may reflect local afflux or subjectivity of recording

Highlighted cells only

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4 Scenarios The Waikanae River stopbanks are required to provide protection from a 1% AEP flood event. GWRC policy is to include allowance for climate change in design levels. Present GWRC policy is that design shall include an allowance for 20% increase in peak rainfall intensities and 800mm sea level rise.

Figure 3 shows results from recent work correlating increases in rainfall due to climate change with increases in river flow, for the Waikanae catchment. NIWA (2009) considered that design flow increases would be of the order of 10% and 20% for 7.2% and 16.8% increases in flow respectively. In work for the M2PP project, a linear fit to these data points (going through the origin – i.e. zero rainfall increase gives zero flow increase) was considered appropriate for extrapolating, despite the coarseness of the NIWA recommendations (Levy, 2013). For this current exercise, some slight rounding has been made to the extrapolated results, and, it is assumed that a 20% increase in peak rainfall intensity would lead to a 25% increase in peak flood flow.

GWRC has recognised that it may be more difficult to top up flood defences further in future if climate change increases more than expected. It would be easier to make at least some allowance now (e.g. providing the ability to raise walls by allowing sufficient foundation etc). GWRC therefore requested an additional scenario with a 40% increase in rainfall and 1.5m sea level rise. Again, the relationship shown in Figure 3 has been applied, and with some slight rounding 40% increase in rainfall is assumed to result in a 50% increase in peak flow.

Figure 3 Assumed relationship between rainfall depth increase and flow increase for climate change

Two stopbank breach scenarios have also been modelled: at the upstream end of Jim Cooke Park and at the Chillingworth stopbank. These locations were originally those modelled by GWRC in 1997. At that time, the former site was considered the more likely to breach. Since that time, the river has moved away from the Jim Cooke Park stopbank, and flood protection works including rock groynes have been constructed in the river, so that the likelihood of a breach there will have diminished. The Chillingworth stopbank is a well-constructed bank and is set back from the river, so the likelihood of a breach there is considered small.

Stopbank breach scenarios were also modelled at these locations as part of the M2PP investigations (and again in 2013 for GWRC). The Chillingworth location was shown to lead to the greatest flood extent downstream. An additional stopbank breach was modelled further downstream in Jim Cooke Park at that time, but results showed that outflows did not extend very

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0% 5% 10% 15% 20% 25% 30% 35% 40% 45%

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Rainfall depth increase due to CC

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M2PP assumptions (linear fit and extrapolation to NIWA values)

Assumed, for 2014 Waikanae modelling

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far due to the relatively high land behind the stopbanks at that location (REC, 2011). Hence these current investigations have not included a breach at that location.

The breach assumptions are described in REC & SKM (2013).

5 Design levels Figure 4 shows the model predictions for the standard design scenario, i.e. 1% AEP plus a 20% increase in rainfall as allowance for climate change, at specified numbered locations along the stopbank (Figure 5).

Design levels are also plotted, including a 900mm freeboard allowance on the predicted 1% AEP plus 20% increase in rainfall scenario levels. The design levels however are smoothed (with the result that there is a minimum of 900 mm freeboard but more in some places).

Model predictions are also shown for the scenario with 40% increase in rainfall intensity, showing levels on average 200 mm higher than for the 20% increase scenario.

Results are tabulated in Appendix D.

Figure 4 Model predictions and design levels for JCP stopbank

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43 47 51 55 59 63 67 71 75 79 83 87 91 95 99 103 107 111

Leve

l (m

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Location (Figure 5)

Model prediction Q100CC (20% increase in rainfall)

Model prediction Q100CC (40% increase in rainfall)

Design

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Figure 5 Stopbank locations

6 Impacts of stopbank upgrade Raising the flood defences along Jim Cooke Park will result in additional flow being kept in the floodway in the design and overdesign events. The impacts of this are shown in Figures 6 and 7. The slight reduction in the floodway width, particularly at the upstream end of JCP, and the prevention of JCP stopbank overflows is predicted to lead to an increase in depth of around 130 mm on the undeveloped floodplain on the left bank opposite JCP. However, to put that in context, even in the existing situation the predicted depths in that area range up to 3 m (averaging 1.2 m).

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Figure 6 Increase in flood depth on floodplain due to raised flood defences in Jim Cooke Park (standard 1% AEP + climate change scenario)

Figure 7 Increase in flood depth in river channel due to raised flood defences in Jim Cooke Park (standard 1% AEP + climate change scenario).

7 Floodmaps Floodmaps for modelled scenarios, all with the existing stopbank, are given in Figures 8-10, showing flooding on the true right (town) side only.

Figure 8 shows the predicted flood extent for the standard 1% AEP climate change scenario (i.e. 20% increase in rainfall). To allow for the fact that the modelled underpredicted by 300 mm around JCP, this floodmap was produced by running a scenario with the existing Jim Cooke

350 345 340 330 320 310 300 290 280 270 260 250 240 230 220

-0.06-0.04-0.020.000.020.040.060.080.100.120.14

45000 45200 45400 45600 45800 46000 46200 46400 46600 46800 47000

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Park and Chillingworth stopbanks (i.e. between Figure 5 locations 43 and 111) lowered by 300 mm in compensation.

Figures 9 and 10 show the predicted flood extent for the two breach scenarios (again, with the standard 1% AEP climate change scenario).

Note that maps as such have no freeboard. Any required freeboard will therefore need to be added to minimum floor levels (for instance) within and adjacent to the mapped flood extent.

Figure 8 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks

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Figure 9 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks with breach at Chillingworth stopbank

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Figure 10 Predicted flood depths, 1% AEP flood with climate change, existing stopbanks with breach at upstream end of JCP

8 Implications for previous design results The 2013 hydraulic modelling and mapping report (REC & SKM, 2013) presented flood maps and peak river levels for design scenarios. With the changes made to the calibration in this

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current modelling exercise, updates to those previous model results could be warranted. Furthermore, for at least the 1% AEP scenarios, the floodmaps could be reproduced with the Jim Cooke Park stopbank lowered by 300 mm to compensate for the calibration underprediction as discussed above.

However such updates need not be a high priority for several reasons:

• Firstly, results in the lower reaches of the river and floodplain (around and below Makora Road/Otaihanga) with the updated calibration would be slightly lower than the 2013 results, due to the extra attenuation caused by the higher resistances and extra spilling around the Jim Cooke Park area. This can also be seen by comparing Table 1 results with those in Appendix A. The updated calibration also gives slightly lower levels in the upstream reaches of the river (upstream of cross-section 380). Hence, for much of the river, the earlier values would in fact be slightly more conservative.

• While results in the Jim Cooke Park area are higher, the increased flood depths and extents for the design scenario would only be a temporary result until the proposed stopbank upgrade is completed.

• The 2013 model results made lesser allowance for climate change. GWRC now plans for a 20% increase in peak rainfall intensity.

• The 2013 model study was in part initiated for emergency management planning. The main difference between the 2013 and the current model results is in the Jim Cooke Park area and adjacent floodplain in a 1% AEP scenario. Previous results for breach scenarios and a PMF event should be sufficient to guide emergency management planning.

Nonetheless, the current model results for the 1% AEP scenario, with the standard climate change allowance (20% increase in peak rainfall intensity, 800 mm sea level rise) and with the existing JCP stopbank, should be used until such time as the JCP stopbank upgrade is completed. (As noted earlier, freeboard will need to be added to results shown in Figure 7, in line with the method described in REC & SKM (2013)).

9 References GWRC (2013); Wellington Region – Flood Vulnerability and Climate Change Impacts Scoping Study. Report 13.720 to Environmental Wellbeing Committee, by Alistair Allan, 10 September 2013

GWRC (2014); Jim Cooke Park Stopbank Reconstruction. Newsletter #1, Feb 2014

Levy, G. (2013); Waikanae River and Muaupoko Stream 2115 Hydrology -Revised Assessment. Memorandum to Kyle Christensen, 23 September 2013. Reference M2PP-26K-D-MEM-0002. MacKays to Peka Expressway Alliance

McKerchar, A. (2009); Review of the flood hydrology for the Waikanae and Otaki Rivers. NIWA Client Report CHC2008-158.

River Edge Consulting Limited (2010); Waikanae River Hydraulic Model Update 2010. Report to Greater Wellington, May 2010.

River Edge Consulting Limited (2011); Hydraulic Impacts of M2PP Expressway (Mazengarb Stream to Waikanae River). Report prepared for the M2PP Alliance, November 2011.

River Edge Consulting Limited & SKM (2013); Waikanae River Hydraulic Modelling and Mapping. Report prepared for Greater Wellington Regional Council

Wallace, Philip (2009); Waikanae River Hydraulic Model Update 2009. Report to GW, August 2009. River Edge Consulting Limited.

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Wellington Regional Council (2013); Waikanae Floodplain Management Plan: The Community’s Plan for the Waikanae River and its Environment. Reprinted 2013. Publication No. (2013) GW/FP-G-12/271

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APPENDICES

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APPENDIX A

Previous Calibration Results

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A Previous Calibration Results

Table A-1 Previous calibration results for January 2005 flood (REC & SKM, 2013)

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APPENDIX B

River Cross-Section Locations

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B River Cross-Section Locations

Figure B- 1 River cross-section locations

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APPENDIX C

Additional Calibration Information

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C Additional Calibration Information

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APPENDIX D

Stopbank Design Levels

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D Stopbank Design Levels Table D- 1 Stopbank design levels (m)

Location Design20% increase in Rainfall 40% increase in rainfall (smoothed)

111 13.503 13.797 14.40110 13.443 13.706 14.34109 13.371 13.630 14.27108 13.249 13.490 14.21107 13.220 13.445 14.16106 13.195 13.429 14.12105 13.190 13.392 14.09104 13.130 13.320 14.03103 13.032 13.223 13.94102 12.965 13.140 13.87101 12.878 13.117 13.78100 12.807 13.034 13.71

99 12.734 12.965 13.6398 12.660 12.893 13.5697 12.588 12.823 13.4996 12.506 12.738 13.4195 12.303 12.475 13.2494 12.212 12.363 13.1193 12.088 12.230 12.9992 11.878 12.100 12.7891 11.638 11.779 12.5890 11.530 11.692 12.4489 11.438 11.608 12.3488 11.313 11.497 12.2187 11.177 11.344 12.0886 11.004 11.202 11.9285 10.866 11.087 11.7984 10.773 10.995 11.6983 10.708 10.945 11.6182 10.641 10.877 11.5481 10.553 10.807 11.4680 10.487 10.743 11.3979 10.386 10.634 11.3078 10.316 10.563 11.2277 10.168 10.367 11.0776 9.958 10.181 10.87

Model Prediction 1%AEP + CC ("Q100CC")

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Table D- 2 Stopbank design levels (m) (cont)

Location Design20% increase in Rainfall 40% increase in rainfall (smoothed)

75 9.802 10.015 10.7374 9.716 9.817 10.6273 9.577 9.922 10.4872 9.251 9.445 10.3071 9.075 9.260 10.1670 9.004 9.195 10.0569 8.930 9.113 9.9468 8.876 9.052 9.8567 8.876 9.045 9.7866 8.496 8.641 9.6165 8.447 8.647 9.4964 8.361 8.569 9.3963 8.354 8.563 9.2962 8.330 8.540 9.2461 8.316 8.539 9.2260 8.300 8.521 9.2059 8.289 8.506 9.1958 8.285 8.505 9.1957 8.285 8.504 9.1956 8.290 8.509 9.1955 8.093 8.310 9.1054 8.089 8.310 9.0553 8.088 8.310 9.0152 8.088 8.308 9.0051 8.085 8.306 8.9950 8.081 8.301 8.9849 8.052 8.290 8.9548 8.006 8.265 8.9247 7.982 8.191 8.8846 7.928 8.148 8.8445 7.902 8.110 8.8044 7.827 8.077 8.7343 7.763 7.981 8.66

Model Prediction 1%AEP + CC ("Q100CC")