Paper template for HIC2004
11th ISE 2016, Melbourne, Australia
Extended Abstract
RECONSTRUCTING THE SEDIMENT DYNAMICS OF AN OVERLOADED GRAVEL BED RIVER, EAST CAPE JON TUNNICLIFFE University of Auckland School of Environment, Auckland, New Zealand IAN FULLER Institute of Agriculture & Environment, Massey University, Palmerston North, New Zealand BRETT EATON Geography Department, University of British Columbia, Vancouver, British Columbia, Canada DAVE PEACOCK Peacock D H Ltd, Makaraka, New Zealand MIKE MARDEN Landcare Research, Gisborne, New Zealand Many gravel bed rivers in New Zealand's East Coast Region underwent a phase of notable aggradation following the passage of Cyclone Bola in March, 1988, as massive quantities of sediment were eroded from steep, unstable hillslopes, gullies and large debris flow complexes. The Raparapaririki Stream in the Waiapu River catchment underwent more than 30 m of aggradation - at least 2 m yr-1 during the peak aggradation phase - burying a bridge in the process. We have used historical cross-section information and aerial photography to reconstruct the sequence of channel building, and have assessed morphologic changes to the channel as aggradation occurred. A digital terrain model of the modern river has been developed using Structure-from-Motion techniques with high resolution drone photography. We employ the resultant DEM to assess the transporting capacity of the modern river to model the likely distribution of grain sizes transferred under the prevailing flow regime. Based on this analysis, we look to assimilate the chronology of the transfer of a gravel 'wave' since Cyclone Bola, and the likely trajectory of this highly variable system into the future. 1
INTRODUCTION
In this paper we have used Structure-from-Motion (SfM) photogrammetric techniques to develop a detailed model of braided river topography along the length of the Raparapaririki River in New Zealand’s East Coast Region. From the perspective of hydraulic characterization and morphodynamic simulation of braided systems, the topography of braid plains is often difficult to capture adequately, owing to the subdued relief, and the complex multi-branched nature of the flow threads. However, braid plains tend to be highly conducive environments for photogrammetric techniques, owing to the relatively sparse vegetation, ease of access, and heterogeneous substrate, which lends itself well to the pattern-matching algorithms employed in SfM. In the present case we were fortunate to catch the river at extremely low flow levels, providing a mostly (95%) bare channel model of the study reach. The Raparapaririki Stream is a steep braided tributary of the Tapuaeroa River, which is a major branch of the Waiapu River (Figure 1). It drains a 35 km2 catchment, and is noted for its high rates of chronic mass-wasting within the headwater regions. The catchment has been impacted by clearance of the native forest in the late 1800s and early 1900s. Despite some replanting efforts in the early 1970s and early 2000s, the catchment has remained susceptible to the development of linear gullies and mass wasting complexes. The catchment is underlain by relatively erodible sandstone and mudstone units (Mazengarb and Speden [1]). The river has exhibited exceptional rates of aggradation in the last 27 years, and is thus of considerable interest from a river sedimentation perspective.
Figure 1. The East Coast study area. 2
PATTERNS OF AGGRADATION AND DEGRADATION
The East Coast Region was strongly impacted by the passage of Cyclone Bola in March of 1988. High, sustained rainfall resulted in considerable flooding and landsliding activity. A cross-section survey of the Raparapaririki Stream in October 1994 showed that the streambed at the bridge had aggraded 4.5 m since 1988, and by June 1996 had reached the top of the bridge deck, prompting construction of a new bridge 500 m downstream. In the 6 years following Cyclone Bola, 4.4x106 m3 of sediment accumulated in the surveyed reach upstream from the confluence with the Tapuaeroa River, and a further 2.6x106 m3 of gravel accumulated in the succeeding 9-year period (Liebault et al. [2]; Page et al. [3]). At the most upstream cross-section (3379 m) the streambed aggradation reached a peak of 35 m (from the 1975 baseline) in 2004, before beginning to incise again (Figure 2).
Figure 2. Long profile of the Raparapaririki Stream, showing the maximum aggraded bed levels, as of 2007. The river has begun to incise the post-1988 stratigraphy, most evidently in the upper surveyed cross-sections. A notable point regarding the downcutting trend in the Raparapaririki is that the degradation has been proceeding from the upstream end, rather than the more commonly reported pattern of upstream propagation of a degradational wave. As sediment supply is shut off, the deficit of sediment results in incision of the upstream portion of the channel, transfer of aggraded sediments to the mid-reach, and a much reduced rate of aggradation in the lower reaches. As the valley has filled, the characteristic channel planform has evolved from a steep meandering stream to a braided configuration (Figure 3). At the cross-section upstream of the old bridge in Figure 3B, the active channel width has enlarged from 83 m to 243 m. Mean channel slope has changed from roughly 0.018 to 0.024.
Figure 3. Aerial photography from (A) 1939, (B) 1988, and (C) 2013 reveals the plan-form evolution of the Raparapaririki Stream following persistent landslide sediment delivery since the passage of Cyclone Bola in 1988. The bridge in (B) was buried in the mid-1990s. The dashed line indicates the location of cross-section at 668 m in Figure 2.
3
STRUCTURE FROM MOTION SURVEYS
The Raparapaririki Stream survey was carried out using a Trimble UX5 fixed wing survey drone, equipped with a 24 MP camera. The survey area encompassed 3.0 km2, and was covered with roughly 600 photos in 5 flight lines, achieving a final maximum survey resolution of 10 cm (Figure 4). Eight ground control points were used for survey alignment and orthorectification. Dense point cloud generation was carried out using AGI Photoscan on the University of Auckland’s supercomputing cluster. The point clouds were thinned, taking minimum elevation points in a moving window. DEMs were then generated using SAGA GIS. Ground LiDAR surveys were carried out to assess the overall precision of the final models; the accuracy varied with ground cover, particularly vegetation and water, but RMS surface variance within +/- 12 cm was typically achieved, when comparing SfM and LiDAR models.
Figure 4. The confluence of the Raparapaririki and the Tapuaeroa River. In the leftmost panel, the distance across the point cloud is roughly 2.5 km. The 15 cm point cloud data (far right) provides a highly detailed picture of the channel morphology. Flows were generally less than 30 cm deep, so the river bed was mostly visible, and thus could be interpreted by the point-matching algorithm in the software. The high-resolution topography database formed a basis for evaluating a range of hydraulic variables such as mean channel widths, local slope and sinuosity, conveyance characteristics of floodplains and backwater zones, and the overall distribution of flow depths. It also provided a basis for improved computation of aggraded volumes within the valley confines. Using historical airphotos, and elevations from cross-sections, it was possible to derive comparisons with the modern surface and develop estimates of the overall deposit thickness.
4
RESULTS AND SUMMARY
Cross-sections from the SfM surveys have been used to develop a 1-d model of river evolution, employing river geometry from the surveys, and surface-based sediment transport formulae to predict the rate of incision into the aggraded deposit. The principal insight from this work is that coarse-grained inputs result in the development of a relatively narrowly-sorted bed armour (Figure 5A) across most of the floodplain surface. As the river excavates the bed in the upper reaches, finer material is released from the bed stratigraphy, enhancing the overall rate of transport downstream. The incised channels in the upper reach have relatively high bank strength, as the banks are formed of the remnant armour material. Thus, there are a number of essential interactions and feedbacks that can be captured through detailed modelling of deposit evacuation (Figure 5B), providing an idea of the timescales of response to such major valley filling episodes in steepland terrain.
Figure 5. (A) The grain size distribution for the Rapaririki Stream (lower 3 km) and for a range of landslide deposits throughout the Waiapu Catchment. (B) A schematic showing the pattern of sediment accumulation and evacuation in the Rapaririki system. Hillslope-derived material accumulates within the mid and upper reaches (t1-t2), though the narrower upper reach is the first to experience the effects of the subsequent sediment deficit (t3). The lower reach continues to aggrade. In the latter stages (t3-t4), a wave of incision moves from the upper reaches to the lower reaches.
5
ACKNOWLEDGMENTS, APPENDICES, AND REFERENCES
This work is based on extensive climate and cross-section monitoring that has been carried out by the Gisborne Regional Council. We thank Greg Hall and Ian Hughes for their assistance with data requirements and Edith Bretherton for cross-section analyses. We would also like to thank Chris McFadzean for his skilled drone piloting over the study catchment and Sina Massoud-Ansari for assistance with the cluster-based processing of the SfM surveys.
REFERENCES [1] Mazengarb, C. and Speden, I.G. (compilers), 2000. Geology of the Raukumara Area. Institute of Geological and Nuclear Sciences. 1:250,000 geological map 6. 1 sheet + 60 p. Lower Hutt, New Zealand Institute of Geological and Nuclear Sciences Limited. [2] Liebault, F., Gomez, B., Page, M.J., et al., 2005. Land-use change, sediment production and channel response in upland regions. River Res. Appl. 21, 739–756, doi:10.1002/rra.880. [3] Page, M., Marden, M., Kasai, M., Gomez, B., Peacock, D., Betts, H., Parkner, T., Marutani, T. and Trustrum, N. (2008). Changes in basin-scale sediment supply and transfer in a rapidly transformed New Zealand landscape. Developments in Earth Surface Processes, 11, 337-356.
Extended Abstract
RECONSTRUCTING THE SEDIMENT DYNAMICS OF AN OVERLOADED GRAVEL BED RIVER, EAST CAPE JON TUNNICLIFFE University of Auckland School of Environment, Auckland, New Zealand IAN FULLER Institute of Agriculture & Environment, Massey University, Palmerston North, New Zealand BRETT EATON Geography Department, University of British Columbia, Vancouver, British Columbia, Canada DAVE PEACOCK Peacock D H Ltd, Makaraka, New Zealand MIKE MARDEN Landcare Research, Gisborne, New Zealand Many gravel bed rivers in New Zealand's East Coast Region underwent a phase of notable aggradation following the passage of Cyclone Bola in March, 1988, as massive quantities of sediment were eroded from steep, unstable hillslopes, gullies and large debris flow complexes. The Raparapaririki Stream in the Waiapu River catchment underwent more than 30 m of aggradation - at least 2 m yr-1 during the peak aggradation phase - burying a bridge in the process. We have used historical cross-section information and aerial photography to reconstruct the sequence of channel building, and have assessed morphologic changes to the channel as aggradation occurred. A digital terrain model of the modern river has been developed using Structure-from-Motion techniques with high resolution drone photography. We employ the resultant DEM to assess the transporting capacity of the modern river to model the likely distribution of grain sizes transferred under the prevailing flow regime. Based on this analysis, we look to assimilate the chronology of the transfer of a gravel 'wave' since Cyclone Bola, and the likely trajectory of this highly variable system into the future. 1
INTRODUCTION
In this paper we have used Structure-from-Motion (SfM) photogrammetric techniques to develop a detailed model of braided river topography along the length of the Raparapaririki River in New Zealand’s East Coast Region. From the perspective of hydraulic characterization and morphodynamic simulation of braided systems, the topography of braid plains is often difficult to capture adequately, owing to the subdued relief, and the complex multi-branched nature of the flow threads. However, braid plains tend to be highly conducive environments for photogrammetric techniques, owing to the relatively sparse vegetation, ease of access, and heterogeneous substrate, which lends itself well to the pattern-matching algorithms employed in SfM. In the present case we were fortunate to catch the river at extremely low flow levels, providing a mostly (95%) bare channel model of the study reach. The Raparapaririki Stream is a steep braided tributary of the Tapuaeroa River, which is a major branch of the Waiapu River (Figure 1). It drains a 35 km2 catchment, and is noted for its high rates of chronic mass-wasting within the headwater regions. The catchment has been impacted by clearance of the native forest in the late 1800s and early 1900s. Despite some replanting efforts in the early 1970s and early 2000s, the catchment has remained susceptible to the development of linear gullies and mass wasting complexes. The catchment is underlain by relatively erodible sandstone and mudstone units (Mazengarb and Speden [1]). The river has exhibited exceptional rates of aggradation in the last 27 years, and is thus of considerable interest from a river sedimentation perspective.
Figure 1. The East Coast study area. 2
PATTERNS OF AGGRADATION AND DEGRADATION
The East Coast Region was strongly impacted by the passage of Cyclone Bola in March of 1988. High, sustained rainfall resulted in considerable flooding and landsliding activity. A cross-section survey of the Raparapaririki Stream in October 1994 showed that the streambed at the bridge had aggraded 4.5 m since 1988, and by June 1996 had reached the top of the bridge deck, prompting construction of a new bridge 500 m downstream. In the 6 years following Cyclone Bola, 4.4x106 m3 of sediment accumulated in the surveyed reach upstream from the confluence with the Tapuaeroa River, and a further 2.6x106 m3 of gravel accumulated in the succeeding 9-year period (Liebault et al. [2]; Page et al. [3]). At the most upstream cross-section (3379 m) the streambed aggradation reached a peak of 35 m (from the 1975 baseline) in 2004, before beginning to incise again (Figure 2).
Figure 2. Long profile of the Raparapaririki Stream, showing the maximum aggraded bed levels, as of 2007. The river has begun to incise the post-1988 stratigraphy, most evidently in the upper surveyed cross-sections. A notable point regarding the downcutting trend in the Raparapaririki is that the degradation has been proceeding from the upstream end, rather than the more commonly reported pattern of upstream propagation of a degradational wave. As sediment supply is shut off, the deficit of sediment results in incision of the upstream portion of the channel, transfer of aggraded sediments to the mid-reach, and a much reduced rate of aggradation in the lower reaches. As the valley has filled, the characteristic channel planform has evolved from a steep meandering stream to a braided configuration (Figure 3). At the cross-section upstream of the old bridge in Figure 3B, the active channel width has enlarged from 83 m to 243 m. Mean channel slope has changed from roughly 0.018 to 0.024.
Figure 3. Aerial photography from (A) 1939, (B) 1988, and (C) 2013 reveals the plan-form evolution of the Raparapaririki Stream following persistent landslide sediment delivery since the passage of Cyclone Bola in 1988. The bridge in (B) was buried in the mid-1990s. The dashed line indicates the location of cross-section at 668 m in Figure 2.
3
STRUCTURE FROM MOTION SURVEYS
The Raparapaririki Stream survey was carried out using a Trimble UX5 fixed wing survey drone, equipped with a 24 MP camera. The survey area encompassed 3.0 km2, and was covered with roughly 600 photos in 5 flight lines, achieving a final maximum survey resolution of 10 cm (Figure 4). Eight ground control points were used for survey alignment and orthorectification. Dense point cloud generation was carried out using AGI Photoscan on the University of Auckland’s supercomputing cluster. The point clouds were thinned, taking minimum elevation points in a moving window. DEMs were then generated using SAGA GIS. Ground LiDAR surveys were carried out to assess the overall precision of the final models; the accuracy varied with ground cover, particularly vegetation and water, but RMS surface variance within +/- 12 cm was typically achieved, when comparing SfM and LiDAR models.
Figure 4. The confluence of the Raparapaririki and the Tapuaeroa River. In the leftmost panel, the distance across the point cloud is roughly 2.5 km. The 15 cm point cloud data (far right) provides a highly detailed picture of the channel morphology. Flows were generally less than 30 cm deep, so the river bed was mostly visible, and thus could be interpreted by the point-matching algorithm in the software. The high-resolution topography database formed a basis for evaluating a range of hydraulic variables such as mean channel widths, local slope and sinuosity, conveyance characteristics of floodplains and backwater zones, and the overall distribution of flow depths. It also provided a basis for improved computation of aggraded volumes within the valley confines. Using historical airphotos, and elevations from cross-sections, it was possible to derive comparisons with the modern surface and develop estimates of the overall deposit thickness.
4
RESULTS AND SUMMARY
Cross-sections from the SfM surveys have been used to develop a 1-d model of river evolution, employing river geometry from the surveys, and surface-based sediment transport formulae to predict the rate of incision into the aggraded deposit. The principal insight from this work is that coarse-grained inputs result in the development of a relatively narrowly-sorted bed armour (Figure 5A) across most of the floodplain surface. As the river excavates the bed in the upper reaches, finer material is released from the bed stratigraphy, enhancing the overall rate of transport downstream. The incised channels in the upper reach have relatively high bank strength, as the banks are formed of the remnant armour material. Thus, there are a number of essential interactions and feedbacks that can be captured through detailed modelling of deposit evacuation (Figure 5B), providing an idea of the timescales of response to such major valley filling episodes in steepland terrain.
Figure 5. (A) The grain size distribution for the Rapaririki Stream (lower 3 km) and for a range of landslide deposits throughout the Waiapu Catchment. (B) A schematic showing the pattern of sediment accumulation and evacuation in the Rapaririki system. Hillslope-derived material accumulates within the mid and upper reaches (t1-t2), though the narrower upper reach is the first to experience the effects of the subsequent sediment deficit (t3). The lower reach continues to aggrade. In the latter stages (t3-t4), a wave of incision moves from the upper reaches to the lower reaches.
5
ACKNOWLEDGMENTS, APPENDICES, AND REFERENCES
This work is based on extensive climate and cross-section monitoring that has been carried out by the Gisborne Regional Council. We thank Greg Hall and Ian Hughes for their assistance with data requirements and Edith Bretherton for cross-section analyses. We would also like to thank Chris McFadzean for his skilled drone piloting over the study catchment and Sina Massoud-Ansari for assistance with the cluster-based processing of the SfM surveys.
REFERENCES [1] Mazengarb, C. and Speden, I.G. (compilers), 2000. Geology of the Raukumara Area. Institute of Geological and Nuclear Sciences. 1:250,000 geological map 6. 1 sheet + 60 p. Lower Hutt, New Zealand Institute of Geological and Nuclear Sciences Limited. [2] Liebault, F., Gomez, B., Page, M.J., et al., 2005. Land-use change, sediment production and channel response in upland regions. River Res. Appl. 21, 739–756, doi:10.1002/rra.880. [3] Page, M., Marden, M., Kasai, M., Gomez, B., Peacock, D., Betts, H., Parkner, T., Marutani, T. and Trustrum, N. (2008). Changes in basin-scale sediment supply and transfer in a rapidly transformed New Zealand landscape. Developments in Earth Surface Processes, 11, 337-356.
