Northland Bridges - Kaeo 9 August 2019 Revision: A Hydraulic Modelling Technical Specialist Report Reference: 254914 NZ Transport Agency Document control record Document prepared by: Aurecon New Zealand Limited Level 4, 139 Carlton Gore Road Newmarket Auckland 1023 PO Box 9762 Newmarket Auckland 1149 New Zealand T +64 9 520 6019 F +64 9 524 7815 E [email protected] W aurecongroup.com A person using Aurecon documents or data accepts the risk of: a) Using the documents or data in electronic form without requesting and checking them for accuracy against the original hard copy version. b) Using the documents or data for any purpose not agreed to in writing by Aurecon. Document control Report title +\GUDXOLF0RGHOOLQJTechnical Specialist Report Document ID 5(3:' Project number 254914 File path Client NZ Transport Agency Client contact Laura Devcich Rev Date Revision details/status Prepared Author Verifier Approver by A 9 August 2019 Draft IRU1=7$ 15&FRPPHQW M.Ho M.Ho R.Kelly D.Hughes Current revision A Approval Author signature Approver signature Name Marcia Ho Name David Hughes Title Civil Engineer Title Technical Director Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Contents 1 Introduction 1 2 Project description 1 2.1 Project Background 1 2.2 Purpose of this Report 2 2.3 Existing Environment 2 3 Modelling Methodology 3 3.1 Model Scenarios 3 4 Results 5 4.1 Existing Scenario 7 4.2 Design Scenario 1 10 4.1 Design Scenario 2 14 4.2 Sensitivity Analysis 15 4.3 Results Summary 16 5 Conclusions 16 6 References 16 Figures Figure 1 Kaeo Bridge Project Location Plan 2 Figure 2 Locations of flood levels extracted from the modelling results. 6 Figure 3 Existing scenario 10-year ARI maximum water depth 8 Figure 4 Existing scenario 100-year ARI maximum water depth 9 Figure 5 Design scenario 1 10-year ARI maximum water depth 10 Figure 6 Design scenario 1 100-year ARI maximum water depth 11 Figure 7 Difference map between Design Scenario 1 and the Existing Scenario in 10 Year ARI 13 Figure 8 Difference map between Design Scenario 1 and the Existing Scenario in 100 Year ARI 14 Tables Table 1 Summary of Model Scenarios 4 Table 2 Approximate property locations 6 Table 3 Other locations at which results were extracted for analysis 7 Table 4 Flood levels and depths at property locations – Existing Scenario 7 Table 5 Flood levels and depths at other key locations – Existing Scenario 8 Table 6 Flood levels and depths at property locations – Design Scenario 1 12 Table 7 Flood levels and depths at other key locations – Design Scenario 1 12 Table 8 Flood levels and depths at other key locations – Design Scenario 2 14 Table 9 Flood levesls and depths at other key locations – Design Scenario 2 15 Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Page i Appendices Appendix A – Kaeo River Catchment Appendix B – Sub-model Development and Hydraulic Modelling Methodology Appendix C – Design Scenario 2 - Difference Maps Appendix D – Sensitivity Scenarios - Difference Maps Appendix E – Meeting record with Northland Regional Council Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Page ii Executive Summary Aurecon has been engaged by New Zealand Transport Agency (NZTA) to design the realignment of State Highway 10 and a new two-lane bridge to replace an existing one-lane bridge across Kaeo River. Flood modelling has been undertaken to assess the impacts on flood levels as a result of the proposed bridge and embankment. An existing hydrologic and hydraulic model of the Kaeo River Catchment, developed by GHD in 2008, received from Northland Regional Council in 2017, was used to develop a sub-model for this project. This report discusses: The existing flooding extents in the 10-year and 100-year 12-hour duration ARI storm events; Modelled flooding scenarios for the Project in the 10-year and 100-year 12-hour duration ARI storm events and a summary of impacts of the Project on any affected habitable floors; The proposed flooding and stormwater management measures for the Kaeo Bridge Project will cause an afflux of up to 200mm directly upstream of the bridge and embankment which decreases further upstream in the 100-year ARI 12 hour duration event. Impacts in a 10-year ARI 12-hour duration event are up to 80mm which is limited to upstream of the road embankment. Seven habitable properties were identified as potentially affected in the area. The largest impacts on existing dwellings is a 90 mm increase in water level at property 2 in a 100-year ARI 12-hour duration event and 40 mm increase in water level at property 3 in the 10-year ARI 12-hour duration event. In the existing scenario, or the base case, these particular properties are already affected by a flood depth of 1.0m at property 2 and 0.8m at property 3 in the 100-year ARI and 10-year ARI respectively. Four of the seven properties are already affected by flooding in the 10-year ARI event; and all properties are affected by flooding in the 100-year ARI. Considering that most of the properties are already affected by considerable flood depths (by 1 metre or more) in the existing scenario, increases in flooding at these properties of less than 100mm for the proposed bridge are considered less than minor. It is also noted that impacts less than 50mm are regarded to be within modelling error margins. Impacts less than 50mm are therefore considered to be less than minor. Survey of habitable floor levels of potentially affected properties is currently in process to confirm the flood impacts and consultation with key landowners is currently being undertaken. Model assumptions were discussed with Northland Regional Council prior to modelling of final scenarios. Further modelling of the design scenario in the 1000-year ARI and 25-year ARI will be required to obtain flood levels in accordance with the NZ bridge manual. Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Page iii 1 Introduction This report has been prepared as supporting information for the Assessment of Environmental Effects (AEE) report for Resource Consent applications to Northland Regional Council (NRC) for stormwater and flood management associated with the improvement works. 2 Project description 2.1 Project Background SH10 is an important link in the Northland State Highway network and is part of the Paihia to Kaitaia section of the Twin Coast Discovery Route. The Kaeo bridge is a one lane bridge located along SH10 (asset reference 448), at the intersection with Whangaroa Road, crossing the Kaeo River. The bridge and the connecting sections of Whangaroa Rd and SH10 have been identified by the Transport Agency to have risks to driver safety. Refer to figure below for the bridge location. The Project proposes to improve driver safety in this area by realigning the state highway geometry, replacing the existing bridge with a two-lane bridge, and connecting the bridge to a new roundabout on Whangaroa Rd. The key elements of the Project include: Realignment of SH10 through rural land, including constructing a new highway embankment to connect the highway to the new bridge; Construction of a two-lane road bridge over Kaeo River, including piles within the Kaeo River; Construction of a SH10 roundabout intersection with Whangaroa Road; Construction of cross culverts under SH10 and earthworks to improve resilience during flooding; and Decommissioning of the existing single lane Kaeo River bridge and western embankment The Project will improve highway capacity and safety through: Enhanced vertical and horizontal alignment; Increased sight distances on approach to the proposed bridge from SH10 and the Whangaroa Road intersection; and Introducing safety barriers along the length of the alignment. A full description of the Project, including its components and construction, is contained in the Assessment of Environmental Effects (AEE) for the Project. Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Page 1 Figure 1 Kaeo Bridge Project Location Plan 2.2 Purpose of this Report This report is one of a suite of technical reports that has been prepared to inform the AEE for the Project. The report outlines the flood modelling methodology, assumptions and parameters used and discusses the flood modelling results. The particular focus of this report is an assessment of the effects of the Project on stormwater flooding. 2.3 Existing Environment The Kaeo River catchment is approximately 11,560 hectares and typically slopes steeply down from its eastern and southern boundaries toward Kaeo River. Refer to Appendix A for the Kaeo River Catchment Boundary. The catchment, except for the Kaeo Township and its surrounding area, is generally covered by bush or plantation forestry with steep topography. The basin areas and valley bottoms typically comprise of pasture and rural land (GHD, 2013). The Kaeo River drains to the Whangaroa Harbour and is therefore influenced by tide levels. For a detailed analysis of the Kaeo River Catchment refer to the report ‘Kaeo River Catchment Flood Model Upgrade Report (GHD August 2013)’. Project 254914 File Kaeo Stormwater Technical Specialist Report.docx 9 August 2019 Revision A Page 2 3 Modelling Methodology A hydraulic model for the Kaeo River Catchment, developed by GHD in 2008 was received from NRC in 2017 to assist the flood modelling for this project. This model comprised a rainfall-runoff model with a one-dimensional MIKE 11 model of the Kaeo River and its tributaries coupled to a two-dimensional MIKE 21 hydraulic model of the floodplain.