Te Puru Stream flood protection scheme - service level review

TR 2019/26
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Report: TR 2019/26

Author: Duncan Grant

Abstract

A service level review has been undertaken of the flood protection assets on the Te Puru Stream. The process has included a reassessment of catchment hydrology and design flows, a complete rebuild of hydraulic models based on the available topographic datasets, and a comparison of revised flood profiles against surveyed asset crest levels (stopbanks and floodwalls). The review indicates the scheme can convey the design flow safely although there are some shortfalls in freeboard levels in the lower scheme. The shortfalls are possibly associated with aggradation of sediment in the lower stream channel near the mouth as captured in topographic datasets. Aggradation could result in less in-channel capacity which may increase flood levels. It is recommended that re-survey of the channel cross-sections is undertaken to compare with past records and rerun the hydraulic model to determine appropriate bed levels. Given the fixed nature of the assets (primarily floodwalls or stopbanks within confined areas) the recommended solution to maintain the service level and improve performance in the lower stream is to undertake channel excavation/maintenance to allow sufficient in-channel capacity and freeboard for the design condition.

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Contents
Abstract iv
1 Introduction 1
1.1 Purpose 1
1.2 Background 1
2 Hydrology and coastal water levels 3
2.1 Catchment characteristics 3
2.2 Design rainfalls 4
2.3 Design discharge 5
2.3.1 Rational method 6
2.3.2 TM61 6
2.3.3 Regional method 7
2.3.4 Flow scaling 7
2.3.5 Direct rainfall modelling 9
2.3.6 Summary of design discharge estimates 9
2.4 Design hydrograph 10
2.5 Tide levels 11
3 Flood protection scheme 13
4 Survey 15
4.1 Datum 15
4.2 Ground survey 2014 15
4.3 LIDAR 2013 15
4.4 Te Puru Local Datum offsets 16
5 Hydraulics 18
5.1 MIKE11 model 18
5.1.1 MIKE11 model development 18
5.1.2 MIKE22 model results 21
5.1.3 MIKE11 model summary 24
5.2 MIKE21 2D model 25
5.2.1 MIKE21 model development 25
5.2.2 MIKE21 model scenarios 27
5.2.3 MIKE21 model results 27
5.2.4 MIKE21 model summary 33
5.3 Modelling discussion 33
6 Service level review of flood protection assets 34
7 Conclusions 43
8 Recommendations 44
References 45
Appendix A - Comparison of ground survey cross-sections (2014 - local datum) and LIDAR DTM slices (2012 - AVD-46) 46
Appendix B - Comparison of 2004 and 2014 ground survey cross-sections 63
Appendix C - Conquest service level data 79