PP2Ō Post Hydraulic Report

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PP2Ō Post Hydraulic Report 29 October 2018

T&T-DE-RPT-0017 AND 0005

Plain English Summary

Background:

The PP2Ō Expressway introduces a new corridor of infrastructure between the eastern Tararua Ranges

and Kāpiti coastal plains to the west. Water in the district primarily flows from east to west (i.e. from

the ranges to the coast).

Current infrastructure in the form of railway tracks, local roads and SH1 cause floodwater to back up

east of their alignments and create localised surface flooding in large events. These current barriers

are mitigated to allow flow and passage of flood water via a series of culverts and drains.

Design and construction of the PP2Ō Expressway is guided and regulated by a series of consent

conditions that were set out by the Board of Inquiry (BOI) process in 2013. This process

commissioned hydraulic reports covering the following:

Modelled the pre-project situation that in a nutshell captured ‘what the flood water situation

in this area is like now’.

Modelled the post-project situation based on the BOI design. This model is effectively a re-run

of the pre-project situation with the new expressway in place, and captures ‘what the flood

water situation in this area would look like after the project’.

From these reports, objectives and conditions for final design of the expressway were set out by the

BOI. These conditions require the project to meet ‘neutrality’ through its design. The models that were considered by the BOI were for the:

Ōtaki River and Mangapouri Stream,

Waitohu Stream; and

Mangaone Stream.

With final design of the PP2Ō Expressway now complete, a post-project Hydraulic Report has been

produced and models updated to reflect the final design.

This post-project modelling report tests the final design to make sure neutrality objectives set by the

BOI have been met.

Methodology for Post-Project modelling

In broad terms three models have been run:

BOI in 2013 – taking into account predictions for a 1 in 100 year event with climate change to

2090 (specimen or consenting design)

BOI in 2013 – updated and taking into account predictions for a 1 in 100 year event with

climate change to 2130 (specimen or consenting design)

Fletcher Construction Company (FCC) 2018 – post-project report taking into account

predictions for a 1 in 100 year event with climate change to 2130 (FCC design).

International modelling for the Intergovernmental Panel on Climate Change (IPCC - www.ipcc.ch) is

customised for New Zealand conditions by NIWA (www.niwa.co.nz) and this forms the basis of the

science used to allow for climate change in the project flood modelling. It is widely recognised as

best practice by local and central government.

PP2Ō Post Hydraulic Report

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The FCC 2018 models have been run for the same locations identified in the BOI reports and models:

Ōtaki River and Mangapouri Stream,

Waitohu Stream; and

Mangaone Stream.

What does a 1 in 100YR event, subject to climate change prediction, look like?

A typical day on the Ōtaki River sees peak flow running at around 30 m3

/s (cubic meters per

second)

A 1 in 2 year event sees it rise to around 900m3

/s

A 1 in 10 year event sees it rise round 1230 m3

/s (similar in magnitude to the large local flood

event in 2015)

A 1 in 100 year event with climate change sees a large increase to 2140 m3

/s.

Historically, there is an event on record in 1955 that comes close to a 1 in 100 year event with peak

flow recorded at 2320m3

/s and the next biggest occurred in 2005 when flow hit 1550m3

/s.

Results

Key themes in the final design of the PP2Ō Expressway include a series of bridges and very large

culverts to ensure we meet the BOI neutrality objectives. These help to avoid increasing existing flow

and velocity levels in very large flood events.

Overall, the post-project model results are showing the final design has achieved neutrality with the

BOI objectives or an improvement (i.e. reduced flood levels) in most areas and a slight increase in

other localised areas. In effect, some areas downstream that currently flood in large events will no

longer flood, and a number of areas already under water in very large events will now be under a little

more water.

Simply described, the report shows that in a very large event (1 in 100 years taking into account

climate change predictions) areas already prone to flooding will continue to flood.

The following plans provide a snap shot of the situation post-project in a 1 in 100 year event with

climate change for:

Ōtaki River and Mangapouri Stream,

Waitohu Stream; and

Mangaone Stream.

Yellow showing no change and that neutrality with the BOI objectives has been achieved.

Green showing areas where an improvement (i.e. reduced flood levels) to the current situation can be

expected.

Red showing where an increase of more than 100mm in a 1 in 100 year event plus climate change

can be expected (note, under the current district hazard plans these areas would appear as yellow

and already subject to flooding).

Mangapouri Stream

Racecourse Stream

Ōtaki River

LEGENDPP2Ō project work s2D m odel extentsOpen channel extents

Flood level differenceElim inated areas of flooding in post-projectPost-project flood levels > 100m m lower than pre-project<100 m m difference between pre- and post-project flood levelsPost-project flood levels >100m m higher than pre-project

- Aeria l photograph sourced from Greater W ellin gton Region a l Coun cil a n d licen sed for re-use un der the Creative Com m on s Attrib ution 3.0 N ew Zea la n d licen ce.- All levels are N ew Zea la n d Vertica l Datum 2009.- The flood m odellin g presen ted in this figure has b een com pleted for the purposes of the Deta iled Design a n d Con struction Phase of PP2Ō “the Project”, a n d m a y n ot b e suita b le for other uses. Model results m ay n ot b e accurate outside the area of in terest for the Project.

Path: T:\W ellington\TT Projects\85985\85985.0070\5000 Storm water\4 Hydraulic Modelling Schem etisation\GIS\FIGURES\20180501_PostProjectReportRev4\Fig61_1_OM_DiffPlotPostvsPre100m m _100y rCC.m xd Date: 8/05/2018 Tim e: 3:58:26 p.m .

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0Figure 61.1

N Z TRAN SPORT AGEN CYPEKA PEKA TO ŌTAKI EXPRESSW AY

Ōta ki-Ma n gapouri Model Results Com parisonPost-Project m in us Pre-Project W ater Levels - 1% AEP CC2130

Location Pla n

1:20,00085985.0070

Fig61_ 1_ OM_ DiffPlotPostvsPre100m m _ 100yrCC.m xd

KBBB May.18

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FIGURE N o.

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LEGENDPP2Ō project work s

2D m odel extentsOpen channel extents

Flood level differenceElim inated areas of flooding in post-projectPost-project flood levels > 50m m lower than pre-project<50 m m difference between pre- and post-project flood levelsPost-project flood levels >50m m higher than pre-project

- Aeria l photograph sourced from Greater W ellin gton Region a l Coun cil a n d licen sed for re-use un der the Creative Com m on s Attrib ution 3.0 N ew Zea la n d licen ce.- All levels are N ew Zea la n d Vertica l Datum 2009.- The flood m odellin g presen ted in this figure has b een com pleted for the purposes of the Deta iled Design a n d Con struction Phase of PP2Ō “the Project”, a n d m a y n ot b e suita b le for other uses. Model results m ay n ot b e accurate outside the area of in terest for the Project.

Path: T:\W ellington\TT Projects\85985\85985.0070\5000 Storm water\4 Hydraulic Modelling Schem etisation\GIS\FIGURES\20180501_PostProjectReportRev4\Fig61_2_OM_DiffPlotPostvsPre50m m _100y rCC.m xd Date: 9/05/2018 Tim e: 1:34:33 p.m .

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0Figure 61.2

N Z TRAN SPORT AGEN CYPEKA PEKA TO ŌTAKI EXPRESSW AY

Ōta ki-Ma n gapouri Model Results Com parisonPost-Project m in us Pre-Project W ater Levels - 1% AEP CC2130

Location Pla n

1:5,00085985.0070

Fig61_ 2_ OM_ DiffPlotPostvsPre50m m _ 100yrCC.m xd

KBBB May.18

0 40 80 120 160 200MetersA3 SCALE 1:5,000

N otes:

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SCALE (AT A3 SIZE)

PROJECT N o.

ARCFILE

DRAW NCHECKEDAPPROVED

FIGURE N o.

DMK May.18May.18TSRF

Green w ood Tributary

Waitohu Stream

Ō taki River

LEGENDPP2Ō project works2D m odel exten tsOpen chan n el exten tsElim in ated areas of floodin g in post-projectPost-project flood levels > 100m m lower than pre-project<100 m m differen ce between pre- an d post-project flood levelsPost-project flood levels >100m m higher than pre-project

- Aeria l photograph sourced from Greater W ellin gton Region a l Coun cil a n d licen sed for re-use un der the Creative Com m on s Attrib ution 3.0 N ew Zea la n d licen ce.- All levels are N ew Zea la n d Vertica l Datum 2009.- The flood m odellin g presen ted in this figure has b een com pleted for the purposes of the Deta iled Design a n d Con struction Phase of PP2Ō “the Project”, a n d m a y n ot b e suita b le for other uses. Model results m ay n ot b e accurate outside the area of in terest for the Project.

Path: T:\Wellin gton \TT Projects\85985\85985.0070\5000 Storm water\4 Hydraulic Modellin g Schem etisation \GIS\FIGURES\20180501_PostProjectReportRev4\Fig73_1_Wai_DiffPlotPostvsPre100m m _100yrCC.m xd Date: 9/05/2018 Tim e: 1:01:11 p.m .

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0Figure 73.1

N Z TRAN SPORT AGEN CYPEKA PEKA TO ŌTAKI EXPRESSW AYW a itohu Model Results Com parison

Post-Project m in us Pre-Project W ater Levels - 1% AEP CC2130

Location Pla n

1:15,00085985.0070

Fig73_1_ W a i_ DiffPlotPostvsPre100m m _ 100yrCC.m xd

KBBB May.18

0 0.2 0.4 0.6 0.8 1(km )A3 SCALE 1:15,000

N otes:

105 Carlton Gore Rd, N ewm arket, Auckla n dw w w.ton kin ta ylor.co.n z Rev.

SCALE (AT A3 SIZE)

PROJECT N o.

ARCFILE

DRAW NCHECKEDAPPROVED

FIGURE N o.

DMK May.18May.18TSRF

Man gaon e Stream

Man gaon e Overflow

LEGENDPP2Ō project works2D m odel exten tsOpen chan n el exten ts

Flood level differenceElim in ated areas of floodin g in post-projectPost-project flood levels > 100m m lower than pre-project<100 m m differen ce between pre- an d post-project flood levelsPost-project flood levels >100m m higher than pre-project

- Aeria l pho to gra ph so urc ed fro m Greater Wellingto n Regio na l Co unc il a nd lic ensed fo r re-use under the Crea tive Co m m o ns Attributio n 3.0 New Zea la nd lic enc e.- All levels are New Zea la nd Vertic a l Datum 2009.- The flo o d m o delling presented in this figure has b een c o m pleted fo r the purpo ses o f the Deta iled Design a nd Co nstructio n Pha se o f PP2Ō “the Pro ject”, a nd m a y no t b e suita b le fo r o ther uses. Mo del results m a y no t b e a c curate o utside the a rea o f interest fo r the Pro ject.

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0Figure 77.1

NZ TRANSPO RT AGENCYPEKA PEKA TO ŌTAKI EXPRESSWAYMa nga o ne Mo del Results Co m pa riso n

Po st-Pro ject m inus Pre-Pro ject Water Levels - 1% AEP CC2130

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