E3S Web of Conferences 415, 06001 (2023)
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https://doi.org/10.1051/e3sconf/202341506001

Design of two retention basins along the torrent Liera on theGares Valley (Dolomites, North East Italy) after the storm Vaia
Matteo Barbini1, Martino Bernard1*, Mauro Boregggio1, Fabio Da Re2 and Carlo Gregoretti1
1University of Padova, Land Environment Agriculture and Forestry Department, 35020 Legnaro (PD), Italy
2Forest Department of Veneto Region, 32100 Belluno, Italy

Abstract. A storm called “Vaia” affected the North East Italy and South West of Austria at the end ofOctober 2018. On the Gares Valley (Canale d’Agordo, North East Italian Alps), the abundant runoff
triggered several in-channel debris flows that transported hundreds of thousands of cubic meters ofsediments on the valley bottom. Some control works for retaining the sediment volume have then been built
in the most threatened sites. Herein we show the design of two retention basins that should protect both theLiera torrent and the main road parallel to it. We computed the debris-flow volume to be retained both using
an empirical law and simulated solid-liquid hydrographs corresponding to a return period of 300 years. The
estimate of the debris-flow volume in the empirical law depends on the basin area upstream of the deposition
zone, whereas, in the solid-liquid hydrographs, it depends on the area of the basin closed at the initiationarea, as well as on the design rainfall.

1 Introduction

average slope of 42 and 37% respectively, while theslope of reach of the Liera torrent between them is 4%.

The “Vaia” storm strongly hit the Gares Valley on theDolomites (North East Italian Alps) at the end ofOctober 2018 [1]. The Gares Valley is a straight andnarrow valley in the direction south to north, on which
the torrent Liera runs (Figure 1). The slopes flanking thevalley are very steep and incised by about 40 channels.
During “Vaia”, 33 of them were affected by in-channel
debris flows with a total mobilized sediment volume ofabout 700,000 m3. The rain gauge of Gares (Figure 1)
recorded two distinct events: a rainfall lasted one day
and a half with an average intensity of about 5 mm/h(184 mm), followed by a rainfall long half a day with anaverage intensity of 11 mm/h (167 mm). The second
rainfall event occurred on a saturated terrain andprovided the abundant runoff that triggered the 33 debris
flows. Most of the channels routed by debris flows ends
in uninhabited areas far from the main road of the valley,except for two (channels 33 and 34 of Figure 1) For thisreason, two retention basins are designed at the end of
the two channels, to protect the main road from debrisflow hazard.

2 Material and methods
2.1 The site
Channels 33 and 34, located on the left flank of theGares Valley, are straight (Figures 1 and 2) with anFig. 1. The Gares Valley with channels 2-34 routed by debris
flows including the affected areas (red) and the Liera torrent(blue). The yellow circle is the Gares weather station.

* Corresponding author: martino.bernard@unipd.it© 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/).

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https://doi.org/10.1051/e3sconf/2023415060012.2 The design volume of the two
retention basins: methods

The drainage areas of the triggering zones ofchannels 33 and 34 are 0.25 and 0.2 km 2 respectively.
Figure 3 shows the “Vaia”-related erosion-deposition
pattern on the two channels, whereas in Table 1 thesediment volumes mobilized along them during thestorm are resumed.

The design volume for the two retention basins is thedebris-flow volume, VDF. The value of VDF is searchedusing two approaches. The former is the relationship of[2] between VDF and AB, the drainage area of the basin
at the apex of the fan:

Table 1. Solid-liquid peak discharges (Qp) with sediment(VSED) and debris-flow volumes (VDF) for different events andmethods on channels 33 and 34.
Event/Method

3QP (m /s)

VDF =  AB

3VSED, VDF (m )where  and  are two numerical constants that dependon the percentile. [2] examined 809 values of sedimentvolumes of in-channel debris flows that occurred in theperiod 1950-2015 in Northeast Italy, and provided aprobability distribution. The values of  and  dependon the chosen percentile of that probability distribution.
In the present case, as suggested by the Northeast Italian
Alps River Authority [3], it is assumed the 98th
percentile for which  = 520004000 and  = 0.940.04(4000 and 0.04 are the factors corresponding to theuncertainty estimated using the errors on the measured
value of debris-flow volumes and basin areas). The
latter approach computes the debris-flow volume
according to [4]: the solid-liquid volume, i.e., the debrisflow volume, is the sum of the runoff volume
contributing to debris flow, VR, with the sedimentvolume, VSED, that includes also the water saturating thesediments:
VDF = VR + VSED
(2)

Channel 33
Vaia (VSED)-17360Equation (1)-25300-31100

11.4

42600

4.33

47100

Equation (4)simulation withmaximum peakdischarge
Equation (4)simulation withmaximum volume

Channel 34
Vaia (VSED)

15740

Equation (1)34300-41100

Equation (4)simulation withmaximum peakdischarge
Equation (4)simulation withmaximum volume

7.2

23600

3.95

40050

(1)

The runoff volume contributing to debris flow, V R,
is the runoff volume corresponding to the hydrographwithout the parts with a discharge lower than the criticalvalue for debris-flow occurrence, QCRIT. The runoffhydrograph is computed by hydrological modellingwhere the debris-flow surge forms, i.e., in the triggeringarea. The sediments volume, VSED, is equal to the solid
volume, VSOLID, divided by the solid volumetricconcentration of the dry bed, c*:
VSED = VSOLID /c*(3)

Substituting the second member of equation (3) into
equation (2) and posing VSOLID = cVDF (with c, solid
concentration of the debris flow), yields:
VDF = VR /(1-c/c*)(4)

Two simulated runoff hydrographs are considered:(a) the first one with the maximum value of the peakliquid discharge because it provides the greater peak forthe solid-liquid discharge; (b) the second one with themaximum value of the runoff volume contributing todebris flow, because it provides the maximum solidliquid volume. The former is obtained by searching therainfall duration that provides the maximum value ofpeak runoff discharge using the alternate block method
for building the hyetograph [5]. The latter is obtained
after computing the simulated runoff volume that
contributes to debris flow for increasing rainfall
duration and using a constant intensity hyetograph. TheFig. 2. The two channels 33 and 34 with the drainage areas
corresponding to the triggering areas (yellow line) and to thebeginning of the deposition zones (red lines). Superimposed
the two retention basins.

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duration of the latter rainfall is much longer than that of
the former one.

of the hydrograph corresponding to the maximum peakdischarge is assumed equal to 0.5, the largest
experimental value [8,10]. In the case of the simulation
with the maximum volume, data about c are missing.
Therefore, it is assumed equal to the values estimatedfor the storm “Vaia” that had a long duration: 0.30 and
0.42 for the channels 33 and 34 respectively. Thesevalues are estimated by dividing the solid volume, i.e.
the product of the sediment volume and c*, by the sumof the sediment volume and the runoff contributing
volume. The value c* = 0.65 is the average value of four
samples of debris deposits. Results are shown in Table
1.
In the case of channel 33, equation (4) provides thelargest value of debris-flow volume for the case of therainfall providing the maximum value of VR. In the caseof channel 34, equation (1), considering the associateduncertainty, provides the largest value of debris-flow
volume. This value is just a bit larger than that provided
by equation (4) for the case of the rainfall providing themaximum value of VR.
The two approaches for the estimation of the debrisflow volume provide results that seem in contrast. Thisis explained by considering the quantities involved. The
former depends on the basin area upstream of thedeposition zone, whereas the latter on the area of thetriggering zone. In the present case, the triggering area
corresponding to channel 33 is larger than that ofchannel 34. Vice versa, the basin area of channel 33 issmaller than that of channel 34. This explains the larger
value of the debris-flow volume for channel 34 in thefirst approach and the larger value of the debris-flow
volume for channel 33 following the second approach.
As a confirmation of the role of the triggering area,
Table 1 shows that the entrained sediment volume
during the event Vaia on channel 33 is larger than thaton channel 34.

Fig. 3. The deposition-erosion pattern of debris flows
occurred on channels 33 and 34 during Vaia storm withsuperimposed the two retention basins.

3 Evaluation of thevolumes

designThe values of AB corresponding to the channels 33 and34 are 0.52 and 0.71 km2 respectively. The basins areclosed just upstream of the planned works, where theslope of the channels sharply changes from over 25° toless than 15°. In this case the apex of the fan is notrecognizable because the stepness of the two channels.
In Table 1, it is shown the range of the values of thedebris-flow volume computed by means of equation (1)considering the uncertainties. The second method
requires the computation of the design rainfall and of therunoff hydrograph, before estimating the solid-liquidvolume. The design rainfall for the simulation of therunoff hydrograph is obtained using the depth-duration
frequency curve corresponding to a 300-year return
period. The curve has been obtained by applying thePeak Over Threshold technique (POT) to the rainfallevent depths recorded by the rain gauge station of Garesin the period 1985-2019 [6]. The rainfall events weredetermined using the criterion suggested by [7]. The
simulations of the runoff hydrograph were carried out
by using the model of [8], with the values of theparameters suggested by [9] that allow the best matching
between the observed and simulated runoffhydrographs. The runoff volume contributing to debris
flow is computed using the value 0.3 m3/s as the criticaldischarge for debris-flow occurrence, QCRIT, according
to [4]. Using this value in equation (4), it provides thevalue of debris-flow volume. The parameter c in the case

4 The two retention basins
The two basins intercept the channels upstream of theirconfluence with the Liera torrent at the valley bottom.
Both are left-oriented where the terrain has a gentler
slope and is occupied by a forest stand (Figure 4). The
areas of the two basins are 11500 and 9000 m 2
respectively, with an elevation of the retaining wall of 5m higher respect to that of the basin bottom. Theelevations of the level of full filling of the basins are 4.1
and 4.4 m higher respect to that of the bottom
respectively. This allows a freeboard of 0.9 and 0.6 m
respectively. With the aim of considering the residual
risk [11], i.e., the occurrence of a debris flow with thebasin already filled by a previous event, a broad-crested
weir is built on the left side of each basin, 4 m higher
than the bottom level (Figure 5). Both the broad-crested
weirs are 6 m long with a maximum hydraulic load of 1m. The opening and depth of weirs allow the flow of thesolid-liquid peak discharges of Table 1. In both cases,
the material flowing over the broad-crested weir iscollected by a canal running between the retaining walland the Liera torrent. The two weirs are located on theleft side of the basins because the flow in the basins is3

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5 Conclusions

left-oriented: the bottom of the basins has a slope in thesame sense of the torrent of about 2.5%. However, thespreading of a debris flow on the retention basin could
be not uniform because of preferential paths due to thestopping of big boulders. The preferential paths could
address the flow far from the weir and the retaining wall
could be overflown. This possibility leads to theupstream elongation of the collecting channels between
the walls and the torrent. Moreover, just upstream of theretaining wall, some grids will be arranged to permit thedrainage of the debris-flow liquid phase to the collectingchannels.

The design of two retention basins protecting the mainroad of the Gares Valley in North East Italy is based on
the estimation of the debris-flow volume. This value iscomputed following two approaches. The former is anempirical law based on a probability distribution
obtained thorough 809 observations of deposited
sediment volume. The latter is the direct determinationof the debris-flow solid-liquid volume corresponding to
a 300 years return period rainfall, through the runoff
volume contributing to debris flow obtained byhydrological simulations and a reference sedimentconcentration. Two design rainfalls are considered:those providing the maximum value of the solid-liquidpeak discharge and of the runoff volume respectively. In
the case of channel 33, the second design rainfall
provides the largest value of the debris-flow volume. In
the case of channel 34, the second design rainfall
provides a value of the debris-flow volume just a bit
smaller than that computed through the former
approach. After considering the residual risk of a debrisflow event just after the one filling the basin, a broadcrested weir is positioned normal to the main flow
direction in the basin. Below the weir, it is located acollector channel joining the main creek downstream ofthe basin.
The authors wish to thank the Forest Department and theProtection Environmental Agency of Veneto Region for theirsupport in the work.

References
1.

L. Giovannini, S. Davolio, M. Zaramella, D. Zardi,
M. Borga, Atmospheric Research, 253 (2021)
2. L. Marchi, M.T. Brunetti, M. Cavalli, S. Crema,
Earth Surf. Process. Landf., 44 (2019)
3. Northeast Italian Alps River Authority – Technical
Report of PGRA (2021)4. C. Gregoretti, L.M. Stancanelli, M. Bernard, M.
Boreggio, M. Degetto, S. Lanzoni Journal ofHydrology, 568 (2019)
5. M. Bernard, M. Boreggio, M., Degetto, C.
Gregoretti, Science Total Environ. 688, (2019)
6. J. Pickands. The Annals of Statistics, 3(1) (1975)
7. J.A. Coe, D.A. Kinner, J.W. Godt,Geomorphology, 96 (2008)
8. C. Gregoretti, M. Degetto, M. Bernard, G. Crucil,
A. Pimazzoni, S. De Vido, M. Berti, A. Simoni, S.
Lanzoni Water Resour. Res., 52(10) (2016)
9. M. Bernard, C. Gregoretti, Water Resour. Res.,
57(3) (2021)
10. T. Takahashi, Debris Flow. Taylor and Francis(2007)
11. S. Hartmann, L. Pedoth, C. Dalla Torre, S.
Schneiderbauer, Int. J. Disaster Risk Sci., (2021)

Fig. 4. Location of broad-crested weirs and of the collecting
channels for the designed retention basins.

Fig. 5. Typical cross-section and frontal view of a broadcrested weir.

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