E3S Web of Conferences 415, 03027 (2023)
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https://doi.org/10.1051/e3sconf/202341503027

Pulse-Doppler radar measurements of debris flows: Highresolution monitoring of surge dynamics from two events in theGadria Creek (Italy)
Tobias Schöffl1*, Georg Nagl1, Johannes Hübl1, and Roland Kaitna1
1University of Natural Resources and Life Sciences, Institute of Mountain Risk Engineering, Peter Jordan Strasse 82, Vienna 1190,

Austria.

Abstract. As a consequence of their spontaneous occurrence, and frequent formation of multiple surges
with high sediment loads, debris flows are considered one of the most hazardous gravity-driven mass
movements in montane regions. Field measurements of surface velocities are an essential link in the chainof understanding fundamental process dynamics and applied protection against debris flows. In order tomeasure the velocities of multiple consecutive surges within a single debris-flow event, a PD radar (pulseDoppler high-frequency radar) sensor for high-resolution real-time debris-flow monitoring has beendeveloped. In this contribution we present PD radar measurements of two debris flows, that occurred at theGadria creek in Italy on July 26, 2019, and August 10, 2020, over a torrent length of 250 meters. We record
over 55 high-amplitude surges that overlap and superimpose at the front of the debris flow, but also
subsequently throughout the debris-flow body. Our results demonstrate applicability of a PD radar for
debris-flow monitoring and serve as a data source for modelling surge dynamics in debris flows.

1 Introduction
In recent decades, various non-contact debris-flow
monitoring methods for velocity measurement havebeen developed [1]. A sensor system that is particularly
promising is the pulse-Doppler high-frequency (PD)radar [2]. In this contribution, we present firstmeasurements of this system at the study site Gadria
creek. We first provide a description of the method, witha special focus on the PD radar system. Subsequently,
we use derived surface velocities and independently
measured flow depths to characterize the surge
dynamics of two debris flows. Finally, we discuss thechallenges and limitations we have experienced andprovide an outlook.

2 Test site and methods
The Gadria creek is located in the autonomous provinceof Bolzano in the Eastern Italian Alps. The steep slopes,
the highly weathered metamorphic rocks and the glacial
deposits, which can easily be eroded by heavy
convective rainfall in the summer months, form thebasis of this remarkably high debris-flow active
catchment area [3, 4] In 2016, a monitoring barrier wasinstalled that provides flow height measurements inaddition to a wide range of flow parameter [5]. In 2017,a PD radar unit was installed on the dam of the retention
basin. The radar's main antenna beam is directed* Corresponding author:

upstream towards the monitoring structure, which islocated 135 meters from the dam.

Fig. 1. (a) Normalized Doppler spectrum (frame) of a debris
flow front (#frdf). The class width in this example corresponds
to 0.056 m/s. Sketch (b) of the basic PD radar principles.

PD radar features the capability to detect moving objects
at velocities > 0.2 m/s with a size of 1 m2 up to a distanceof 2,500 m by emitting a pulsed electromagnetic beamand processing the echo signal in a discrete-timemanner. Within this range, a distance resolution, socalled range gates (Fig. 1), with a width from 15 to 150
m can be selected [2]. Velocities are determined using
the Doppler effect. A fast Fourier transform (FFT) isused to convert the discrete-time echo signal into
Doppler spectra (frames). Note that a single frame
consists of an absolute frequency distribution where thevelocities correspond to the classes and the frequency
itself is defined as the echo intensity [6]. To simplify thetobias.schoeffl@boku.ac.at© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 03027 (2023)
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information from the Doppler spectra for further
calculations, we extract a maximum, median and mode
velocity value per frame (Fig. 2).

3 Results and analysis
During the summer months of 2019 and 2020, a total offour debris flows occurred in the Gadria creek. One
event took place on July 26, 2019, and the others wereinitiated at short intervals on August 02, 10, and 12,2020. In this study we focus on the July 26, 2019 (t0 =12:11:46 UTC) and August 10, 2020 (t0 = 17:50:33
UTC) debris flows. Both events were characterized by adistinct granular front, a solids-rich body with highamplitude surges, and a watery tail similar to [7]. We
were able to fully record both debris flows with PD radarand flow height sensors.

Fig. 2. Debris flow 2020 – (a) stage hydrograph of threedebris-flow surges. Highlighted is the peak flow height h,depositional or inter-surge height h+ and the resulting
difference is given by A. In (b), the same section, but measured
with the PD radar, is visualized in a velocity-time-intensity
plot.

Fig. 3. Boxplots of peak values derived from the debris-flow
surges 2019 & 2020 – the center line shows the median of thedata series, the edges of the box represent the 25th and 75th
percentiles and the whiskers represent 5th and 95thpercentiles, whereas the crosses indicate the outliers. In (a)peak flow height h, intermediate height h+ and their difference
A, in (b) maximum peak surface velocity vmed of the PD radarand in (c) the Froude number calculated with vmed are shown.

For characterizing and comparing surge dynamics, we
calculate the amplitude A of each surge as the difference
between the maximum flow height h with thedepositional height h+ in the intermediate region with A= h – h+ and the surge period T as the time from themaximum flow height of a surge to the subsequent one[8]. The Froude number is based on the medianvelocities vmed at the peak of the surges andcorresponding flow height h (Fig. 3). Finally, we alsocalculated the peak discharge Q with Q (t) = f (h (t)) ×vmed (t). Here, f (h (t)) is composed of the flow height hand the corresponding cross-sectional area of the
trapezoidal channel at respective times. For a suitableaveraged surface velocity value, we opted for the
median of the Doppler spectra vmed. Following thiscalculation, the estimated total volume is about 2000 m3
for DF19 and about 10000 m3 for DF20.

The 2020 event has significantly higher flow height atthe front, but thereafter the median value of flow heighth and depositional height h+ is found to be notably lower
than in the 2019 event (Fig. 3). Examining the flow
velocity vmed, DF20 also shows significantly higher peakvelocities. Interestingly, the average surge period T andthe mean surge amplitude A are rather similar. The
averaged values are given as A equals 0.57 m and Tequals 20 s for DF19 and A equals 0.54 m and T equals
27 s for DF20.

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Fig. 4. Debris flow 2020 - 4-dimensional range-time-intensityvelocity (RTIV) plots of the debris-flow front being
superimposed by the following surge and then merged.

Fig. 5. Debris flow 2019 & 2020 – Scatter plot of key peakvalues over all surges. The Froude numbers are plotted against
the deposition height h+.

Fig. 4 shows the binned and normalized echo intensity
for each range gate plotted against time as well as therespective distance of 25 m between the range gates. For
the fourth dimension, we plotted the medium velocity
vmed in the same way as echo intensity, but assigned it acolor scale. The individual surges have been highlighted
to provide a better sense of the dynamics. Around t840,
at a range of ~ 250 m the front approaches with vmed ofover 6 m/s, but between t850 and t870 a small surge
subsides from a distance of 160 m. In nearly the sametime slot from ~ t860 a following surge comes into thepicture, where vmed even increase to ~ 9 m/s whenentering range gate 8 at a distance of ~ 205 m. Shortlythereafter, at range gate 5 the front which is additionallyslowed by the impact on the monitoring barrier, iscaught up by the following surge and the two merge.

What is the reason for the different flow regimes in thetwo events? According to independent measurements ofbasal normal stress and flow height [9], the bulk densityof DF19 was between 2000 and 2300 kg/m³ and themean bulk density of DF20 varied around 2000 kg/m³.
What these measurements do not explicitly reveal is thewater content and the overall grain size distribution,
especially the fines content in the form of clay and silt,
which controls the viscosity of the fluid. Based on visual
video analysis, we hypothesize that DF20 had a higherfines content than the DF19, causing higher flow
velocities and lower deposition heights. For DF19, we
speculate that flow dynamics were more likely
controlled by inter-particle collisions.

4 Discussion

In this study we apply a novel non-contact and highresolution PD radar to monitor surface velocities innatural debris flows. We analyze the results together
with independent flow height and basal normal stressmeasurements. The key conclusions are:
 The PD radar system is applicable forcontinuously monitoring surface velocities ofdebris-flow surges.
 Data analysis in the form of range-timeintensity-velocity (RTIV) diagrams shows thatsurges can superimpose and merge with thedebris-flow front, but also with one-another.
 Surges of two debris flow events at Gadria
creek show surprisingly consistent surgedynamics.
 Depositional height h+ in surge-intermediate
stationary regions can be linked to different
flow regimes.
 High-amplitude surges do not necessarily flowat higher velocities than low-amplitude surges.
 High Froude-numbers surges seem to be
associated with lower inter-surge flow depth.

5 Conclusions

The major difference between DF19 and DF20 resides
in the flow regime. In DF19, the Froude numbers occur
almost exclusively in the subcritical flow regime, while
for DF20, they are found in the supercritical regime
greater than 1. We tested the Froude numbers from both
events against h+. DF20 exhibits moderate correlation(R2 = 0,421) with h+ and DF19 shows no correlation atall (Fig. 5). Note that for DF20, the higher the Froudenumbers (in the supercritical regime) at the crest of thesurge, the lower the deposition h+. However, thisobservation does not hold for DF19, where a clustering
occurs when the Froud numbers are less than 1. We notethat in the event of DF20 h+ also represents a criticaldeposition height. The reason is that at an average heighth+ of ~ 0.52 (m), the flow became stationary. At DF19,
6 of the total 16 surges came to a complete halt in theintermediate sections with an average h+ of ~ 1.20 m.
We could not find any relation between vmed andrespective amplitudes A and do not find high-amplitudesurges traveling faster than smaller ones.

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