Tracking Adjustments in Fan and Floodplain Storage in a Braided

Tracking Adjustments in Fan and Floodplain Storage in a Braided Channel Following Major Sedimentary Disturbance, East Cape, NZ Tunnicliffe, J.1, Leenman, A.1,2, Eaton, B.C.2 and Fuller, I.C.3 1School of Environment, University of Auckland, Auckland, NZ; 2Geography Department, University of British Columbia, Vancouver, BC, Canada Abstract: 3Institute of Agriculture & Environment, Massey University, Palmerston North, NZ We reconstruct the trajectory of a coarse-grained gravel-bed network in recovery from a major sedimentary disturbance, driven by mass-wasting exacerbated by ex-tropical Cyclone Bola in 1988. A strong trend of aggradation in the Tapuaeroa Catchment between Longitudinal Evolution The net balance of erosion and deposition from 1988 and 2013 culminated in a valley fill with a thickness of up to 35 m. A record of cross-section surveys spanning 58 years, and high 2015-2016 is shown in Fig 4b (blue filled line). resolution Structure-from-Motion surveys spanning 2015-2016, show the complex nature of recovery; the upper tributary sections 2013 The system shows a modest surplus on the a) 6 2007 2014 6 exhibit incision and terrace-building while the lower reaches and the trunk channel exhibit a mix of aggradation and degradation. 2016 Recent, Minor Aggradation Raparapaririki Fan (1-3 km), with more Tributary fans provide a record of changes to intermediate storage, depending on local supply conditions in the trunk channel. A 5 5 pronounced accumulations 4-9 km downstream. sediment budget model is developed to elucidate the complex nature of fan and floodplain evolution over annual and decadal 2003 4 1998 4 timescales following major disturbance. The pattern of deposition shows three evident 1993 3 3 peaks in sediment accumulation, associated with 1988 The East Cape Context S 1982 i 2 2 tributary streams. From approximately 9 km to i k i r 1975 1976 2 a r 1968 r a p 1964 The Tapuaeroa River (326 km catchment p a the Waiorongomai confluence there is a R a 1 1963 1 > 1961 area) is a major tributary of the Waiapu Historric Bed Change (m) trend of more erosion than deposition. 0 1958 0 R 1 River; it joins the Mata River to form one of ) u 3 b) It is instructive to compare this recent p Cumulative Change the largest river systems in New Zealand’s a N m Net Accumulation, a p u a i 3 survey with the historical mean bed levels 2 T a a e r o a Raparapaririki Aggradation a R East Coast Region (1,582 km in total). W 0.5 Stream Net Removal 0.5 (x10 (Fig 4a). The locus of aggradation has The gravelly river is wide and braided, due Ruatoria > historically been near 4 km; we now R Landslide- 6 primarily to a continuing surplus of coarse m a prone 0 0 begin to see a deficit there, and a catchments 3 sediment. Landsliding in the headwaters t ) a Survey M extents accumulation toward 8 km. As sediment drives a remarkable regime of sediment Study Area transfer. The river has one of the highest 5 km Net Change (x10 -0.5 -0.5 supply diminishes and degradation Figure 6: Looking SW, at Mt. Wharekia rising over the Tapuaeroa River 1 XS 531 1 Mokoiwi XS 532 Figure 1: Study Area c) Mangapoi begins, the locus of gravel accumulation ) XS 529 suspended sediment concentrations in XS 530 3 > Mangawhairiki shows diffusive downstream Perspective New Zealand [1]. Raparapaririki Stream (Fig. 1), in particular, is recovering from m Deposition 4 0.5 Mangahoanga 0.5 significant sediment loading following the passage of Cyclone Bola in 1988. Material in Error/Noise movement. The results from the 2015-2016 surveys demonstrate the Dry Bed, temporary storage within this valley will continue to feed the fan at its terminus, and Deposition immense potential for monitoring river change and establishing an 0 0 Submerged C upper-middle sections of the Tapuaeroa. The study area covers a 12 km reach of river River Bed annual sediment budget. The difference map provides a minimum estimate of > that has shown dramatic response to wide-spread gullying Dry Bed, B Erosion change, since numerous bedload transport events have occurred in the 1-year and landslides in the tributary headwaters. -0.5 Erosion -0.5 Error/Noise interval, but we assume that most of the load is captured within the 12-km river We have employed Structure-from-Motion drone > corridor. surveys in order to characterise the current sediment Change (x10 Volume -1 -1 Downstream migration of meander bends gives rise to scouring of new - and transfer regime, and explore linkages between 0 2 4 6 8 10 12 Distance Downstream (km) fill of older - channels. Most of the scour and fill is generally found within close > tributaries and the mainstem channel. Figure 4: Longitudinal changes in storage (a) 1958-2016, and (b,c) 2015-2016. proximity of former channel bounds. This has important implications for > sediment budget assessment, since the abdandonnment and refilling of channels 2000 > 136 > > accounts for a significant proportion of the annual budget, particularly in the A 135 Mangapoi > > lower part of the study area. It is therefore important to take added care with Stream Mangawhairiki 134 B Stream submerged topography. 5000 133 2015 Survey 7000 3.2 m > 4000 (m) Elevation Eroded Fan Toe There is significant, focused erosion at distal fan edges, and the peaks of 3000 > > 132 > 6000 > 8000 deposition in Figure 4c attest to the strong influence of tributaries on the 2016 Survey A 131 mainstem river evolution. This is a unique snapshot of river connectivity, XS-530 130 XS-530 0 20 40 60 80 100 120 140 160 > Distance (m) highlighting the cycle of filling and fan-building within tributaries between 2015 Wetted Channel > floods, followed by evacuation during major storms. 2016 > Ground Control Point Mokoiwi Uncertainty Stream GDC Cross-Section > > C > Depth of Project Aims: Error and Uncertainty Erosion/Deposition -1 m -0.6-0.2 0.2 0.6 1 m - Quantify changes to mainstem channel sediment stores in the span of one year. The active alluvial surface was divided into cells at 100 m intervals along its length. > - Determine the nature of tributary fan interaction with the mainstem river. Landslips within tributaries are The erosion/deposition raster surface showed a standard deviation of 0.54 m, indicating the main sediment sources: which tributaries are promoting aggradation? most of the change observed was within this range, though local channel avulsion and - Assess the mechanisms of sediment recruitment and deposition, e.g. bank switching, and trimming of fan deposits resulted in excavation or fill depths of 1.5-3 m. 9000 erosion vs. channel scour, channel fills vs. overbank deposition. Waiorongomai We assume that cut and fill values < 30 cm are subject to error (e.g. [2,3]), and these uncertainty River - Explore sources of error in broad-scale SfM difference maps. bounds are presented as a fringe of fainter blues/reds in the diagram of volumetric change (Fig. 4c). > > XS-529 Methods Summer river levels were quite low: water covered 8% of the bed in 2015; 16% of the bed in 2016. There 11000 12000 were nevertheless some deep pools >1 m depth that could not be captured using photogrammetric methods. 10000 > - A fixed-wing UAV (Trimble UX5; Figure 2) with a 12MP Sony NEX-5T Optical proxy methods (e.g., [4]) were ineffective, because bed substrate and sediment concentrations varied > camera flown at a height of roughly 80 m along 12 km of river. Conclusions > longitudinally and temporally. Thus, portions of the survey, particularly downstream, tend to underestimate - Mid-February of 2015 and 2016 during very low flows; roughly 3,000 > Figure 2: Trimble UX5 fixed-wing fills and erosion (Fig 5). This is reflected in the band of 'submerged' volumes shown in Fig 4c, which are Two high-resolution Structure-from-Motion surveys were carried out along a 12 km length photos were captured in 5 flight legs over the study area each time. survey drone, ready to launch. assumed to have greater error, given 111 of braided river floodplain. The pattern of erosion and deposition is consistent with the historical trend, and - 27 ground control mats (1 x 1 m) were laid throughout the study area, and SfM Survey, 16 Feb, 2016 provides a detailed picture of the ongoing evolution of this very active system. Submerged topography under surveyed with differential GPS, in order to link the aerial photography with precise geographic coordinates our subjective depth estimates. GDC Survey, 25 Jan, 2016 110 and elevation. >0.5 m of water is not resolved in the surveys, which introduces a key element of uncertainty. Most cut and fill Figure 5: Cross-section near Waiorongomai can be attributed to channel erosion and fills; trimming of lateral fans is highlighted as a key sediment source. - Surveys lasted 2 days; one day for ground control layout and flights, and one day for GPS surveys. River (529) monitored by Gisborne District 109 - The survey point clouds and elevation models were generated using Agisoft Photoscan; the dense point Council (GDC) shows the limitations of SfM Acknowledgements 2 in deep flows; we were able to resolve cloud comprised 705 million topographic points (Figure 3), with a model density of 32.8 points per m . channel bed features in flows <0.5 m deep. 108 Thanks to: Chris McFadzean (Epiphany Mapping), Dave Peacock (Peacock, Ltd.) and GDC staff (Ian Hughes, Greg Hall), Tui Warmenhoven, 250 200 150 100 050 -50 -100 - The 2015 and 2016 models were gridded to generate a 30-cm digital elevation model (DEM) and these Manu Caddie and friends in Ruatoria, Sina Massoud-Ansari (U of A eResearch) for assistance with the cluster-based processing of the SfM surveys.

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