Gomez and Purdie Progress in Earth and Planetary Science (2018) 5:50
https://doi.org/10.1186/s40645-018-0208-3

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Point cloud technology and 2D
computational flow dynamic modeling for
rapid hazards and disaster risk appraisal on
Yellow Creek fan, Southern Alps of New
ZealandChristopher Gomez1*

and Heather Purdie2

Abstract
Glacial recession in Alpine Valleys uncorks low-altitude tributary valleys that spill trapped sediments in the sedimentcascade, in turn generating sediment-related hazards and flood hazards. Within this complex, the present contribution
investigates the transition zone between the tributary and the main valley at Fox Glacier, New Zealand, where glacierrecession has allowed the development of large debris flow fans. As the valley is narrow and its geomorphology is
rapidly evolving, 3D point cloud technologies based on aerial photogrammetry from UAV or ground and airborne
laser technologies are essential to understand the geomorphology and generate boundary conditions to run hazard
simulations such as debris flows. Using airborne structure from motion photogrammetry with ground control points
collected by RTK-GNSS, we investigated (1) the geomorphology of the valley to understand its evolution and (2) therole of this recent evolution on the flood hazards at the Yellow Creek debris flow fan. The geomorphology of the Fox
River valley is a typical U-shape valley with steep valley walls that command the valley by more than 1000 m. The
valley walls are connected to the bottom of the valley by active sediment aprons and debris flow fans at the exit of
the tributaries. The slopes present several topographic steps or noses on both the aprons and the debris flow fans. By
comparison with the glacial recession, they can be related to past locations of the limits of the glacier. On Straight
Creek fan, the ridges are perpendicular to the valley and divide the fan between an overgrown half downstream theFox Valley and an upstream section thinner and smaller. This pattern was also created by the presence of the glacierthat stopped the development of the fan on its right (upstream half) while developing the true left hand side of thefan. On the other side of the valley, the debris flow fan of Yellow Creek presents a series of deep trough that control
water flows. Flood simulations show that these controls can be overcome and instead of being channeled in othersub-channels, the flow tends to spread on the true right (downstream) end of the fan as a sheet flow. In the 2014 stateof the fan, the safest location was thus on the true left half of the fan towards the glacier. Such information is essential
as tourists occupy the valley and walk towards the glacier the whole year.
Keywords: Fox Valley, Debuttressing, Structure-from-motion, 2D computational fluid dynamics, Debris flow fans,
Floods, Climate change* Correspondence: christophergomez@bear.kobe-u.ac.jp
1
Graduate School of Maritime Sciences, Laboratory of Volcanic Risks at Sea,
Kobe University, Higashinada-ku, Fukaeminami-machi 5-1-1, Kobe 658-0022,
Japan
Full list of author information is available at the end of the article© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Gomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

Introduction
Presentation of the problem

The southern half of the South Island of New Zealand ishome of Alpine Valleys connected to the West Coast,
where the landscape is dominated by Quaternary
Glaciation products. In the present climate, the ice has
been retreating at the Quaternary scale and it has been
moving forward and backward at the historical scale, with ageneral retreating trend. Those multiscale changes are thenat the origin of several challenges for the population livingon the West Coast of New Zealand. Indeed, as Quaternary
scale large glaciers have “disappeared,” a postglacial
rebound, or debuttressing, has been occurring through thetemperate mountain environments, generating mass movements and sediment releases in the valleys (Fig. 1). Thiseffect is accentuated with glacial retreat, which impacts
sediment transport and downstream hydrology. To thisgeneral framework, the West Coast glaciers havepositioned their terminus at low altitude because of theimportant precipitation in altitude. Consequently, theconnected tributaries in the lower parts of the glacial valleys have accumulated debris flow and fluviatile sediments
behind the ice, as the processes of these tributaries were
not governed by glacial processes but by more “temperate”
processes. As the glaciers are now receding to their lowest
extend for centuries, the sediments trapped in the tributaries are released, sucked in the main valleys, because of thesudden lowering of the base level—the previous base level
was the glacier surface elevation, now it is the river bed—
and eventually because of the unwelding of rocks and ice,

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due to ice melting. These processes then in turn create
issues downstream with sediment accumulation and transfer, threatening infrastructures and settlements like at Franz
Josef (Davies et al. 2013). It is therefore critical to be able togenerate topographic dataset of those areas in a very shortperiod of time.
Sedimentary setting

Periods of transition in landscape construction might justbe the norm compared to any form of stability, even if
geomorphologists have tried to define landform evolution
and transformation linked to sets of processes, suggestingthat there exists a format for a given landform, even astate in the evolution if one refers to the early twentiethcentury Davisian theories of geomorphology and landscape evolution.
Nevertheless, glacier valleys and the sediment transported through them are in constant evolution; often difficult to predict, because one needs to place present
processes and landforms in a continuum of past processes
and landform legacies, in order to understand present day
events. Of particular interest in the present contribution is
the role of the Quaternary glacier that have left importantmoraine deposits, organized in arcs in valleys andfloodplains, acting as dams for sediment, which will tend toinflate in those areas (Fig. 2). This legacy influences sediment deposition, but what we have known in the past few
decades is likely to change again with the progressive recession of glaciers, which are freeing sediments with new regimes, in turn generating floods and other sediment-ladenFig. 1 Statement of one of the present issues in the deglaciating valleys of the West Coast of New ZealandGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

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Fig. 2 Conceptual idea of the interplay between different sediment generations during the Quaternary

flow hazards (Fig. 2). Some of those pressure points areplaces where sediments can be stopped, such as the backof moraines (1 in Fig. 2), at slope changes between forming fans and aggrading floodplains (2 in Fig. 2), where themain valley slope changes because either of sedimentaccumulation or new stretch of valley opening with glacierrecession (3 in Fig. 2) and finally because of modified
geometries of existing or newly created fans, mostly
because of changed sediment transports, but also due tohuman influences (4 on Fig. 2).
Debris flows and debris flow fans

Debris flows are saturated mixtures of water and dry material ranging from fine sediments to large boulders, eventually
including bio-debris (such as trees), which flow rapidly bygravity, waxed by the mélange of fluid and fine sediments(e.g., Vallance 2005; Hungr et al. 2014; Takahashi 2014).
Debris flows are recurrent events that can have cause
severe damages (Cui et al. 2015; Badoux et al. 2016), andtheir frequency notoriously increases after earthquakes
because of the material readied for remobilization (Tang etal. 2009) and after mountain fires.
The water content in debris flows can originate fromdifferent sources (lake outburst, integration from riverwater, etc.), although in mountain areas, rainstorms dominate debris flow triggering and occurrence (Wieczorek
1987; Wieczorek and Glade 2005), making them difficult
to predict (Marra et al. 2017), especially because the roleof predisposing factors such as antecedent rainfall can bedifficult to quantify (Bel et al. 2017). Consequently, recent
research has shown that climate change will have impact
on their amplitude and frequency (Gariano and Guzzetti
2008; Dietrich and Krautblatter 2017).

One of the common countermeasure for debris flows
are different types of check dams, slit dams and deceleration baffles, the most famous series of which being the
SABO dam systems developed by the Japanese, such as
the ones that belt Unzen Volcano on Kyushu Island inJapan (Gomez and Wassmer 2015).
When debris flows leave the mountain areas for thesurrounding plain, their sudden loss of energy generates
sediment deposition, creating semi-circular structures
named fans or cones. Sedimentary fans are conic landforms
that connect a steep valley to an area of lower slope gradient. The sediment accumulation is related to the slope
gradient change and the consequent loss of energy (Harvey2011). Fans can be either dominated by stream-flow, inwhich case they are named alluvial fans, or dominated bydebris flow processes, in which case they are named either
debris flow fan (the terminology used in the present publication) or debris flow cones. Debris flow fans develop
through complex series of processes, including backfilling,
avulsion, channelization, which occur through successive
debris flows as shown by laboratory experiments (De Haas
et al. 2016). Those natural processes are also influenced byhuman impacts and the reworking of fan surfaces, such as
controlled channelization for instance (Wasklewicz andScheinert 2016).
In such environments, the security of the public using
those tracks rely on a good understanding of the geomorphology at a given time and how this latter influences
the flows that occur at its surface. The present contributionconsequently focuses on the fan of Yellow Creek, in theFox Glacier valley of New Zealand (Fig. 3), because it is
one of the largest interfaces transferring sediments to thevalley and also because the tourist track crosses it forGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

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Fig. 3 Presentation of the main tributaries and fans connected to the main Fox Glacier valley, based on the data built for this study. The aerial
photograph and the topographic data were first use here in order to introduce the fieldalmost a kilometer, eventually putting tourists at risk during floods or debris flows. Presently, the valley is managed
by the Department of Conservation in collaboration with
the Fox Glacier guide and the valley is closed to tourists
when potential hazards are forecasted or evidenced.
In such environment, it is therefore necessary tocollect high-resolution topographic dataset at highfrequency to keep up with the geomorphologic evolution
and the changing hazards.

Methods/Experimental
Study location: the Fox Valley

The study area is located on the West Coast of New
Zealand, in the 2 km downstream the glacier terminus(as per its location in ~ 2015). If the adjacent valley still
has prominent moraines that are visible across thecoastal plain (as in Fig. 3), the Fox Valley is at a stage,
where the moraines that could play a role in trapping
sediments have already been buried (if they exist, andthis is a long-standing discussion). We are thus alreadyoperating in a sedimentary environment that aggradedwhen the present glacier recession started to occur.
If the long-term issue is for the settlements on the
coastal plain of the West Coast, who see the release of
temporarily trapped sediments from the Alpine catchments, issues also arise in the upper part of the catchment
especially because infrastructures located in the Alpine
Valleys themselves are potentially at risk of flood andother sediment-related hazards (Purdie et al. 2015). A
location where this problem is particularly acute is theFox Glacier valley, located upstream of the Fox valley
township, which has developed over the alluvial fan of theriver, and the aprons of the Southern Alps. Before emerging from the Southern Alps, the valley count 6 majorfans that have formed in the valley following historical glacial recession (Purdie et al. 2014), of which 4 are locatedbetween the car park and the “point of view” where thetourists go and watch the glacier (Fig. 3)—with an annual
influx of about 700,000 tourists (www.westcoastnz.com).
Purdie et al. (2014) demonstrated that at Fox Glacier, this

Gomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

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process of glacier recession was very rapid, with the glacierbeing more than 15 km long in the late nineteenth century
and between 12 and 13 km in length in 2014. Hence, thetributaries along a ~ 2.5-km length have been freed fromtheir “ice-cork” accumulating important amount of sediments at the junction with the main valleys. Althoughthose valleys have already created very large fans, thelongitudinal profiles indicate that the sediment reservoir is
far from empty (Fig. 3), with also nonlinear transfer fueledby the landslides like the one that can be seen in the
Yellow Creek catchment (Fig. 3).

therefore, the spatial data from the Samsung camera wereignored during calculation, and only the ground controls
were used to calibrate the models. The spatial position
and accuracy of the 3D point cloud generated by photogrammetry was corrected using geodetic points. For thispurpose, we used a Trimble R8 base-station and a Trimble
R8 Rover, using the RTK-GNSS (real-time kinematic—differential global navigation satellite system) method and arecording time of 4 s at each point. To ensure a perfectconnection between the base-station and the rover, a relay
radio was set in the central part of the valley on a topographic high. The RTK-GNSS dataset was collected on
the 19th September 2015 between 9.30 AM and 4.00 PM.
The RTK-GNSS data includes 3 files (441 points, 331
points, and 23 points) used to constrain the model(Fig. 4).
The imagery and the RTK-GNSS data were brought
together in the software PhotoScan Professional Edition(Agisoft LLC), in order to derive a 3D point cloud dataset
by using the Structure from Motion and Multiple View
Stereophotogrammetry method used in geomorphology at
various scales (Gomez 2014; Gomez et al. 2015; Mosbrucker et al. 2017). The dataset was calibrated using a setof 12 points marked on the ground using a visible target.
Once the sparse 3D point cloud is created, further vertical correction was performed by forcing RTK-GNSS data
to the point cloud, reshaping it. The generated point cloudpresented a deviation (positive or negative) ranging from< 5 cm to locally ~ 40 cm. As the altitude in the survey
area varies between 241.3 and 292.58 m, the maximum
recorded error only represents 0.78% of the vertical variation and can be neglected. The produced data were thenbrought in the Geographic Information Sciences software
ArcGIS ArcMap commercialized by ESRI, in order toproceed to the geomorphological analysis and also prepare
the data for running the flow simulation, where a 50-cm
horizontal resolution grid was created.

Point cloud acquisition and registration

The present contribution is articulated around field-datafrom Fox Valley, New Zealand, and numerical modelingof flood water on fans.
The field data is comprised of aerial photographs takenfrom helicopter (250 to 300 m from the ground) and froma Quadcopter UAV-phantom 2-Vision+ (9 September2015), in order to perform SfM-MVS photogrammetry andgenerate a 3D point cloud of the studied surface (Fig. 4).
The camera used in the helicopter is a Samsung
EK-GC200 Camera with a 86.1-mm 4.1 focal length, withphotographs recorded with a resolution of 4608 × 3456
pixels. The UAV-based aerial imagery is a set of 320 photographs taken between 60 and 80 m above the ground usinga camera angle tilting of 0° (perpendicular to a horizontal
imaginary plane) and 20°. Both the Samsung camera usedin the helicopter and the camera on-board the UAV use an
integrated GNSS, and the target on the position of the features was then corrected using the Trimble R8 RTK-GNSS.
However, if the Phantom 2-Vision+ corrects the distance
between each camera position using its internal accelerometer, the GNSS receiver integrated to the Samsung camerain the helicopter is not equipped with the same feature,and the GNSS positioning was of too low spatial resolution, presenting delay in the position acquisition;

Fig. 4 Point cloud of the portion of the study area, where the simulation has been run, as generated by Structure from Motion Multiple View
Stereophotogrammetry, as well as the position of the RTK-GNSS points and their altitude. The imagery also points out another issue in the valley:
the lack of aerial and satellite imagery to monitor change and evolution in the valley because of the important cloud cover. a Perspective view ofthe point cloud with the tourist track indicated by the red dotted line, managed pathways on the valley talweg and on the fans, allowing a walkto a vantage point on the glacier. b Plan view of the area under cloud coverGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

Simulation using NAYS 2D following different scenarios

The Nays 2-D model is a computational fluid dynamics(CFD) flood model for unsteady 2D depth average flow,which uses curvilinear fitted coordinates (i.e., a Cartesian
grid projected over the topography). The model has beendeveloped by Prof. Y. Shimizu from Hokkaido University,and it has been widely used for academic and consulting
purposes in North Japan (iRic Nays2D Flood n.d.).
The hydrodynamic model resolves the equation ofcontinuity:∂h ∂ðhuÞ ∂ðhvÞ
þ
þ
¼qþr
∂t
∂x
∂y
and the equations of motion:∂ðuhÞ ∂ðhu2 Þ ∂ðhuvÞ
∂H τ xþþ¼ −hg:− þ Dx
∂t
∂x
∂y
∂x ρ

∂ðvhÞ ∂ðhv2 Þ ∂ðhuvÞ
∂H τ yþþ¼ −hg:− þ Dy
∂t
∂y
∂x
∂y ρ
for which h is the water depth, t is the time, u is theflow velocity in both the x and v in the y directions, g is
the gravitational acceleration, H is the elevation of thewater surface, τn are the shear stress in both x and y
directions (directions denoted by n here), and ρ is thedensity of water, q is the inflow from extra culvert and r
is the rainfall. The shear-stress is then described in termof velocity and roughness coefficient (Manning formula).
Those basic equations in Cartesian coordinates are thenmapped on the topography into curvilinear equivalent.
As the simulation is meant to be a rapid answer topotential hazards, it uses a simplified approach that does
not take into account the water table nor the water loss
in the pores of the sediments by infiltration. The
Manning coefficient was chosen empirically as n = 0.05
corresponding to the normal value for mountain streams
with cobbles and large boulders (Manning’s Empirical
Table n.d.). The simulation hydrographs are for a periodof 5 min each, with instantaneous maximum peak flows
at 50, 100, 200, and 300 m3 s−1 with the peak flow
reached at 60 s. The flow then decelerates to 0 (Fig. 6).
The choice of such short interval was motivated by theneeds to represent a flash flood, as they have beenreported during heavy rain events. The choice of a
Newtonian flood simulation over a debris flow model
has been motivated by the quasi-absence of fines (finesand, silt and clay) that would support the larger clasts
in a typical debris flow. Their transport is mostly therole of gravity and steep slope. Walking on the fans, one
would notice the dominance of large clasts and the lack

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of fine materials. For this reason, the model describes
best the flood series on the fan. Finally, the simulation ismeant as a hazard analysis system for rapid response,
not for sediment transport analysis.

Results
Geomorphological mapping from the 3D point clouddatasetThe geomorphology of the valley consists in a valley bed
and steep slopes with different geomorphologic featuresconnecting them. The valley walls are steep and command
topographic variations of more than 1000 m on both sidesof the valley (Fig. 3). The slopes are characterized by bedrock eroded by the Fox Glacier as well as a mantle of debris
also deposited on the side of the glacier. As the glacier recedes in length, it also loses volume and height, explaining
the topographic break that can be observed along the valley(a and b on Fig. 5). These topographic breaks decrease inaltitude towards the center of the valley.
These step-like features are then reworked by two
processes. If the location of the slope breaks, or the “noseof the step” is related to the ice position, it is being
reworked by fluviatile activity that cuts rills through it,
perpendicularly to the main valley (Fig. 5) and the horizontal surface of those features adopts a gentle sloping
profile as the debris flow and fluviatile processes createfans covering partly the glacial deposition features.
As the lower parts of the slopes have only been uncovered by ice in recent years (see “1 s” on Fig. 5c), they are
active locations that tend to produce material depositedin 6 (Fig. 5c) while also being progressively packed while
the interstitial ice melts and part of the material is being
eroded away in the Fox River.
The geomorphology of the bottom of the valley isshared between the straight channel of the Fox Rivergrowing into braids and the debris flow fans that make thecontact between the tributaries and the valley. The fanthat developed downstream Straight Creek and Fork
Creek (Fig. 5c “3”) is uneven between its true left and rightside. The true left side of the fan is much more developed
spatially and it is thicker, while the right side is less developed and also thinner. On this thinner section, we canalso observe two different levels with a slight nose feature.
This characteristic has been observed to be related to theice retreat as late as 2013. The fan of Yellow Creek is thesecond fan in size in the valley and it is located slightly
downstream the Straight Creek fan (1 in Fig. 3), and thisone is comparatively more homogeneous on both its true
left and right sides. Longitudinally, however, it is nothomogeneous and the topographic “noses” that also exist
on the slopes are to be found at the same altitude on the
fan as well. This indicates that a single process must haveshaped both the valley walls and remodeled the fans: theGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

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Fig. 5 Geomorphological mapping of the valley below Fox Glacier. a Shaded relief map generated from the UAV and helicopter-based
photogrammetry. b Geomorphological map with the principal landforms and their break locations. c The principal landform units in the valley.
Unit (1) unvegetated material plastered by the glacier, unit (2) rockfall and debris cone, units (3–5) the main fans of the valley, unit (6) a mix ofdebris flow fan and apron material, unit (7) undertermined wavy feature being recovered by the recent fan extension, and unit (8) braided-river
fluviatile features

presence of ice and its retreat, which leaves the step-likefeatures.
Results of different scenarios on Yellow fan

For all the fans, the grain size of the material range fromsandy and coarse sand matrix to boulder size material
typically less than 1 m in diameter. The material angle issemi-rounded showing the influence of water in the transport but not sufficient to generate typical river gravels.
Among this material, one also can find outsized blocks
that can be more than a 5-m diameter and that typicallydisplay subangular characteristics, relating them to fall
events from the upper slopes. The geomorphology of thefans therefore indicates high-energy events, even if one
keeps apart the occasional rockfall events. As the tourist
tracks run over Yellow fan and White Creek fan (see thetrack on Fig. 4), it is necessary to better understand someof the related hazards, and we present here flow simulation results on Yellow fan, which is the largest portion of
the present track to the view point to the glacier (Fig. 6).
As potential scenarios are numerous, the authors generated different wave-phenomena that can be linked to apeak rainfall or a dam break from the upstream landslides. For the scenario “series 1” (Fig. 6), the short-peakrelease (effectively a second at peak only) shows that thewater diverges from the main channel on the right sideand flows into another channel about the center of thefan. Another scenario with a slightly larger amount ofwater (series 2) shows that no spill happens on the true
left side of the channel on the fan, but on the true rightside, the flow is not constrained in a channel anymore
and runs as a sheet-flow over a large portion of the fan.
It is therefore possible to determine areas that are potentially safer than others when crossing the fan, and thiswould be on the upstream side of the fan.

Discussion
Geomorphological interpretation of the results

The present contribution is organized around two steps:(1) a geomorphologic description and analysis of the valleyand (2) example of flood simulations on Yellow Creek fan
using the geomorphologic data as boundary conditions.
The geomorphology shows that the steep slopes areconnected to the valley by debris flow fans at the exit ofGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

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Fig. 6 CFD (computational fluid dynamics) results of catastrophic floodwater release, with two different peaks, showing that in the present fan
configuration, the flood water would tend to flow over the true right side of the main channel on the fan

tributaries and by sediment aprons at the bottom of theslopes. The slopes present several topographic steps ornoses on both the aprons and the debris flow fans. By
comparison with the glacial recession (Purdie et al.
2014), they can be related to past locations of the limitsof the glacier. On Straight Creek fan, the ridges areperpendicular to the valley and divide the fan between
an overgrown half downstream the Fox Valley and an
upstream section thinner and smaller. This pattern was
also created by the presence of the glacier that stopped
the development of the fan on its true right (upstream
half ) while developing the true left hand side of the fan.
On the other side of the valley, the debris flow fan of
Yellow Creek presents a series of deep trough that control water flows. Flood simulations show that these controls can be overcome, and instead of being channeledin other sub-channels, the flow tends to spread on theright (downstream) end of the fan as a sheet flow. In the2014 state of the fan, the safest location was thus on thetrue left half of the fan towards the glacier. Such information is essential as tourists occupy the valley and walktowards the glacier the whole year (Purdie et al. 2015).
Technologies for rapid 3D topographic data construction

This short study presents a set of compelling evidences
explaining the necessity to develop further 3D point
cloud technology in order to monitor our environment
with higher frequencies than the traditional methods.

Active Alpine Valleys combine sets of difficulties to whoever wants to capture the geomorphology of those environments. Firstly, the valleys are often narrow offering only avery limited sky-opening. Topographic, and often vegetation (Zimbelman and Keefe 2018), often affect the qualityof the topographic measurement, through a process known
as positional dilution of precision (PDoP), which is anindex representing the impact of satellite visibility on themeasured topography (Johnson and Barton 2004). PDoP
increases the error margin on measurement in mountainareas. In Northern America, research on PDoP has shownthat a low-visibility below 20° azimuth towards the Southwas an important source of error (Karsky 2004). Such error
further increases under vegetation cover.
This lack of sky-access has a direct effect on the potential use of satellite positioning, but it also has indirect
effects how much of the terrain is effectively captured
through satellite or any other airborne technology, because
the position above the valley only offers NADIR or near
NADIR angles, while the slopes develop profiles that canbe subvertical, offering little grip to a NADIR image. It istherefore essential to downscale the data sampling methodin such a way that the tool can be brought inside the valley
itself, to capture the subvertical slopes. This method is
however not a silver bullet because the limited sky-accessputs a constrain on the potential use of satellite-based
geodetic system and navigation system of the UAV. At
Fox, although there are enough satellites to performGomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

RTK-GNSS surveys, there were not a sufficient number ofsatellite visible near the valley walls, and for this reason,
the UAV-automated flight testing was not successful.
Providing that instruments are brought in the valley andthat data are collected, the next difficulty to be dealt within such dynamic environments is the fact that “everything
moves,” even the slopes that seem rather stable creep andthere is little if not virtually no potential control point thatcan be kept from one season to another. This renders thecomparison of DEMs, point clouds, and any form ofgeodetic measurement difficulties. It is therefore important
to know that reasonable amount of error might be introduced and that one should not pay too much attention to
centimeters to decimeter variations in X, Y, and Z. One ofthe possibilities to overcome this difficulty would be to“bring” reference points from outside the valley, through aseries of theodolite or total station measurement, in order
to measure the variation of a reference point, in such a
way that any movement of such point could be quantified
and then all relative change can be also measured. Similar
results could be obtained from having a fix GNSS station
recording data continuously in the valley.
The advantage of 3D point cloud technology for rapid
hazard risk assessment

Because of the rapid glacial retreat phase that is happening in the Fox Valley (Purdie et al. 2014), the sediments
trapped in the tributaries make their ways very rapidly
to the main stem of the Fox Glacier valley. Therefore,
the junctions between those valleys evolve very rapidly
as well at the week to month scales. In turn, this rapid
evolution poses a real challenge to the valley manager
because the hazards are ever changing and especially
rockfalls and debris flows can have very different
pathways (Purdie et al. 2015).
Consequently, collecting a high-resolution dataset
using traditional theodolite surveys or solely
RTK-GNSS to construct a terrain model is tootime-consuming and it is likely that the fan shape will
have changed between the start and the end of the survey. Furthermore, the resolution is not sufficient either
to run flow models at the adequate resolution. The useof a “dense” point cloud is therefore essential, in orderto capture the complexity of the terrain, which will play
an essential role in controlling potential debris flows on
the fan. This has been emphasized by the recent survey
on debris flow channels in Switzerland (Schürch et al.
2011). It is also important to note that the method usedin the present study is well suited for hazard estimation, but for diachronic analysis for instance, there is aneed to increase the number of control points andunderstand how the error propagates through the point
cloud and then the subsequently constructed DEM(Brasington et al. 2003).

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Fan flood hazard simulation

The flood simulation shows that rainstorm or a flood dueto the release of an upstream impoundment (the headwaters have several active landslides that will block the main
channel when they move) will produce an increase of flowheight in the upstream reach, due to the narrow and deepchannel, which then spill over the fan. Although the flow
seems to be unlikely to run over the man-made levee on
the true left of the main channel, the overland flow islikely to flow across the true right of the fan.
One must however remain cautious and only use it for
hazard likelihood assessment, because the model works
over a fix bed, which does not allow erosion or sedimenttransport during the flow. Moreover, the driving equation is the one of a Newtonian flow, that works fine fora typical flood, but is less accurate for debris flows.

Conclusion
Glacier recession is the present trend, but the Fox Glacier
has shown through a general recession trend series of shortadvances, which, if they were to occur again, could block
the tributary valleys once more and trigger temporary
reduction in sediment feeding before eventually freeing further sediments again. Further developing the monitoring
methods that we discussed in the present discussion istherefore essential. Ideally, data could be crowd-sourced orcollected semi-automatically, in order to match valley
rhythm and survey frequency, and by then reassess various
hazards such as flood hazards on fans, using photographs
taken by people including local managers and tourists,
which are used to generate multitemporal topographic
models, as it has been already proposed for the conservation of cultural heritage (Inzerillo and Santagati 2016) andarcheological sites (Griffiths et al. 2015) using donatedphotographs.
In relatively data-poor areas like the Fox Glacier valley,
UAV-based photogrammetry and the point cloud generated afterwards can be a very strong combination for
managers and scientists to collect (even crowd-source)
the necessary data to ensure the security of the public,
and with the decreasing prices of the technology, it
might become available to the greater number, even in“developing” nations where data is often the weak link.
Abbreviations
2D: Two dimension; CFD: Computational fluid dynamic; DEM: Digital
elevation model; DSM: Digital surface model; RTK-GNSS: Real-time
kinematic—global navigation satellite system; UAV: Unmanned aerial vehicle

Acknowledgements
This research has been funded by the Mason Trust Funding and supportedby the Geography Department at the University of Canterbury in New
Zealand. The authors are in debt to the technical team of the Geography
Department (Justin Harrison, Paul Bealing, and Nicholas Key) for their help,
advice, and equipment maintenance.

Gomez and Purdie Progress in Earth and Planetary Science (2018) 5:50

Funding
The research was funded by the Brian Mason Trust Fund to study the Fox
Glacier and awarded to the two authors.
Availability of data and materials
The RTK-GNSS points are available upon request to the author, providing
proper citation of the present work.
Authors’ contributions
Both authors (CG and HP) contributed to the field work and the dataprocessing, while the write-up was led by CG. Both authors read andapproved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Graduate School of Maritime Sciences, Laboratory of Volcanic Risks at Sea,
Kobe University, Higashinada-ku, Fukaeminami-machi 5-1-1, Kobe 658-0022,
Japan. 2Department of Geography, College of Sciences, University ofCanterbury, Christchurch, New Zealand.
Received: 5 July 2017 Accepted: 19 August 2018

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