DELIVERING ON REFORM Future Infrastructure Perspectives IPWEA NZ CONFERENCE 2019 WELLINGTON Impacts and implications of the 15 November 2016 rainstorm for the Wellington highway network David Stewart Principal Geotechnical Engineer WSP Opus & Jack McConchie Technical Principal - Hydrology WSP Opus Outline 1. Impact of flooding and landslides from 15 November 2016 rainstorm event 2. Geotechnical response 3. Main landslide sites 4. 15 November rainfall event and precursor events 5. Subsequent risk mitigation works. 6. Brief observations from 2017 7. Conclusions 2 Previous day — 14th November Kaikoura — Damage at CentrePort Earthquake along the wharf, — 7.8 magnitude quake Wellington — Widespread damage across Wellington region — No significant slope failures reported 15th November rainfall event (70-90mm in 12 hrs) — Landslip on SH58, Paremata Hutt River in flood (~25% AEP) Submerged Paremata roundabout Surface flooding, Lower Hutt Location of Highway Landslides — Location of slope failures and high resolution rain gauge data sites — 4 Opus Geotech teams responded from midday to slip events as part of Capital Journeys NOC role Emergency Slip Response 15 November Encountered very active slope environment! Managing traffic, eg. partial or full closure? opusinternational.com Geotechnical Response On the day - variable preparedness (eg charged phones…) Days following - Inputs into Emergency Remedial Works - Drone inspections and surveys - Detailed engineering inspections Weeks Following - Residual Risk Assessments and options reports 7 Drone Inspections / surveys Enabled : — Visibility of obscured sites — Updated site plans / overview — 3D survey models for rapid risk assessment and prelim design) 8 Main areas of slope failure: • Whenua Tapu SH1 / Pukuera Bay Main areas of slope failure: • Paremata SH58 Main areas affected: • SH2 Ngauranga to Petone SH1 to SH2 onramp Other Areas • Kaitoke slope failure After debris removed Rainfall Analysis — Location of slope failures in relation to high resolution rain gauge data November event rainfall analysis Annual Exceedance Probability for Rainfall Durations Taupo Stream at Site Maungaraki 3 B Hutt River at Te Marua A Whenua Tapu C Rainfall Rainfall Rainfall Duration AEP (%) AEP (%) AEP (%) (mm) (mm) (mm) B 10 minute 4 >50 6 >50 4 >50 C 20 minute 6 >50 7 >50 5 >50 30 minute 10 >50 9 >50 8 >50 1 hour 14 >50 17 >50 12 >50 2 hour 25 >50 26 >50 18 >50 6 hour 49 50 68 8 49 >50 A 25 5 12 hour 72 92 68 >50 (1 in 4 year) (1 in 20 year) 1 day 72 50 92 14 74 >50 Not major opusinternational.com November monthly rainfall Maungaraki 3 Taupo Stream at Hutt River at Te Marua (Lower Hutt ) Whenua Tapu (Kaitoke) Minimum 36 22 41 Mean 90 78 159 Maximum 248 225 531 November 2016 248 225 352* *Second largest total on record for this site 100 th 80 November 15 60 event 40 Rainfall (mm) Rainfall 20 0 1-Nov-2016 6-Nov-2016 11-Nov-2016 16-Nov-2016 21-Nov-2016 26-Nov-2016 Maungaraki 3 Total = 248.2mm Taupo Stream at Whenua Tapu Total = 224.5mm Hutt River at Te Marua Total = 352.0mm 220 220 200 200 180 181 November 160 161 th 140 141 15 event 120 122 100 102 API Whenua 80 83 Tapu Daily Rainfall (mm) 60 63 40 43 (Paremata/ 20 24 0 4 1-Oct-2016 11-Oct-2016 21-Oct-2016 31-Oct-2016 10-Nov-2016 20-Nov-2016 Pukuera Bay) Taupo Stream at Whenua Tapu Rainfall Total = 328.0mm Taupo Stream at Whenua Tapu API Taupo Stream at Whenua Tapu SWS 220 220 200 201 Soil moisture 180 Daily 183 accumulation 160 164 140 rain 145 120 127 Te Marua/ API 100 event 108 80 90 Kaitoke Daily Rainfall (mm) 60 71 40 52 20 34 0 15 1-Oct-2016 11-Oct-2016 21-Oct-2016 31-Oct-2016 10-Nov-2016 20-Nov-2016 Hutt River at Te Marua Daily rainfall Total = 476.0mm Hutt River at Te Marua API Hutt River at Te Marua SWS 220 220 200 200 180 181 160 161 140 141 Soil Moisture 120 121 Mangaraki 100 102 API 80 82 Indicies (Lower Hutt) Daily Rainfall (mm) 60 62 40 42 20 23 0 3 1-Oct-2016 11-Oct-2016 21-Oct-2016 31-Oct-2016 10-Nov-2016 20-Nov-2016 Maungaraki 3 Daily rainfall Total = 374.8mm Maungaraki 3 API Maungaraki 3 SWS Analysis of Soil Water Status (SWS) indices Daily Rainfall Daily SWS Rainfall AEP(%) SWS (mm) AEP(%) (mm) Maungaraki 3 72 50 150 14 Taupo Stream 14 4 at Whenua 92 (1 in 7 year 147 (in in 25 year Tapu event) event) Hutt River at Te 74 >50 168 20 Marua Remedial/ Risk Mitigation Works in week following event Remedial/ Risk Mitigation Works in week(s) following event Private property issues opusinternational.com What happened next - 2017 — Numerous slope failures throughout 2017 (some large!) – covered in detail in Stewart IPWEA 2018 paper. — Continued cumulative wet conditions exacerbating risk Effect of the Kaikoura EQ ? November 14th 7.8 magnitude earthquake Paremata post EQ inspection 14 Nov Paremata post rain inspection 15 Nov Conclusions — Accumulation of rainfall key factor in the widespread landslide occurrence rather than rainfall intensity — Antecedent precipitation indices provide a simple real-time tool for identifying periods of increased risk — Value of a mechanism for ensuring increased risk translates into preparedness of different stakeholders (incl. response teams and plant …and public) to maximise resilience of network — Value of drones for rapid up to date data capture after slip events — Consider how private property issues can be front footed through various planning inputs etc. — Jury is out on the effect of the November 14 earthquake in destabilising slopes around lower North Island Acknowledgements — NZ Transport Agency — Capital Journeys - Fulton Hogan - WSP Opus - TOC — WSP Opus Staff — Geotechnical team (Emergency Response Slips and Remedial solns) — Water Resource Team (Emergency response flooding, analysis of rainfall records) — Survey and UAV team — Property team Thank you! 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