Geomorphological baseline assessment in the vicinity of the Whanganui rivermouth Technical Report
- Author
- Shand, R.
- Year
- 2016
- Publisher / Organisation
- Coastal Systems Limited
- Pages
- 85
- Summary
- A Geomorphological Baseline Study is required as part of the Lower Whanganui River Management Strategy project being carried out by the Wanganui District Council to manage channel alignment and riverbank stability and to provide an information base for other works. In particular, the study is required to provide a historical context of changes in the lower Whanganui River to assist in understanding the current processes, planning and design work, and predicting future changes both (a) naturally occurring, and (b) as a result of any future management modifications. Tonkin + Taylor Ltd (T+T) have been commissioned by the Whanganui District Council to undertake a Lower Whanganui River Management Strategy (the Project) and Coastal Systems Ltd (CSL) http://coastalsystems.co.nz/ have been subcontracted to prepare the Geomorphological Assessment (henceforth referred to as the Baseline Study) for which the full Terms of Reference are attached as Appendix A. Briefly, the study is to extend from the Cobham Bridge, some 3.7 km upstream, to the rivermouth and include approximately 2 km of coast to either side, with the primary focus being the estuary and port area and the rivermouth/coast being the secondary focus. The time of interest is from the late 1800s to the present, with particular emphasis since the 1990s. The study will identify temporal morphology, port and river control structures, notable extreme events, review the literature and undertake higher level (broad) analyses. The study will also summarise available (driving) process information. Rather than begin with a stand-alone literature review, the various consultant, resident engineer and academic contributions are incorporated within the relevant sections. Sections 2 and 3 cover the primary focus area with Section 2 chronologically describing the morphological nature and management-associated modifications up to the present day, and Section 3 providing the high-level (broad) analysis of these changes. Note, a SUMMARY of harbour development works, morphological change and analysis is included as Section 3.3. The rivermouth and adjacent coast are described in Section 4. Section 5 summarises the energy and sediment regimes (sediment, tides, river hydrology, waves and wind) that drive morphological change. Conclusions and recommendations are contained in Section 6. Pertinent information sources used in the study are summarised in Appendix B, and archive materials are listed in Appendix C with electronic files also provided. Contoured bathymetries between 1893 and 2015 are included as Appendix D and Appendix E contains the sampling density for the two digital surveys (2006 and 2015).