E3S Web of Conferences 415, 05012 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341505012

Proposal of Hazard Connectivity Index for debris flow disastermanagement
Masato Kobiyama1*, and Alessandro Gustavo Franck1
1Graduate Course of Water Resources and Environmental Sanitation, Hydraulic Research Institute, Federal University of Rio Grande

do Sul, Av. Bento Gonçalves, 9500, Porto Alegre/RS, Brazil.

Abstract. The concept of connectivity and its index have been widely discussed and applied toenvironmental studies such as ecology, hydrology, geomorphology and hydrosedimentology. Supposingthat this concept is very useful also in disaster risk management, we define the hazard connectivity
associated to debris flows as the connectivity degree between the debris flow occurrence (trajectory) andthe point of interest (location of house, person, and so on). Based on this definition, the Hazard Connectivity
Index (HCI) associated to debris flow is proposed as: HCI = L/(x+L), where x is the shortest distance
between the debris flow trajectory and the point of interest; and L is the runout distance of the debris flowor just mass movement. The HCI value varies from 0 to 1, being that its higher value indicates larger hazard
condition. The HCI application to one case study showed that it can be useful in the analysis of the debris
flow related hazard.

1 Introduction
Social vulnerability has been enhanced by land-use
changes and population increase in mountainous regionsin Brazil. Furthermore, climate change has intensified
torrential rainfalls, and debris flows have frequently
occurred in mountainous environment. Therefore,
mountain management including a debris flow disastermanagement should be urgently established in Brazil[1]. One of the important measures to reduce the impact
of such disasters is hazard mapping. For this purpose,
various approaches have been proposed over the world.
After its use in ecology, the connectivity concept has
been discussed in various sciences, for example,hydrology[2],geomorphology[3]andhydrosedimentology [4]. To express quantitatively theconnectivity degree, i.e., disconnected, weaklyconnected, moderately connected, strongly connected,
etc., a formula or index is required, for example,
connectivity index of sediment [4].
This connectivity concept can be used in the context
of the natural disaster management. If a certain natural
phenomenon does not reach any human-activity areasuch as a house, a factory and a bridge, or if a humanactivity area is not connected to this phenomenon, thedisaster never takes place. Thus, the probability ofdisaster’s occurrence can be considered similar to thepossibility for human-activity area to be connected to
the natural hazard.
Various indices have been proposed so far, but none
has been directly aimed at disaster management [5]. For
disaster management purposes, a simpler index would
be better in terms of practicality.
*Corresponding author: masato.kobiyama@ufrgs.br

In this circumstance, we aimed to propose theHazard Connectivity Index related to debris flow
disasters. To promote the use of this index, we applied
it to a case study in Brazil and showed one of its areasof application in hazard mapping.

2 Hazard Connectivity Index
In case of the debris flow occurrence, the importantconcern is if the initial mass movement reach theelements at risk, i.e., if the initial point of mass
movement and the point of interest such as houses areconnected by mass movement runout or not. In otherwords, the understanding of sediment connectivity canhelp to improve the assessment methodologies of hazard
which causes disasters.
Therefore, in the debris flow disaster management,
we suggest to use the term “hazard connectivity”. Here,the hazard connectivity in this management is defined as
the connectivity degree between the debris flow
occurrence (trajectory) and the point of interest (localityof house, person and so on). In this sense, the distancebecomes an essential parameter in case of debris flow
assessment.
Hence, using two distances, a kind of safetyparameter (SP) is firstly defined as follows:
𝑆𝑃 =𝑥𝑥+𝐿(1)

where x is the shortest distance between the debris flow
trajectory and the point of interest such as house
location; and L is the runout distance of the debris flowwhich influences on the point of interest. Theoretically,
SP values vary from 0 to 1, where the smaller SP thelarger hazard connectivity becomes.

© 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, 05012 (2023)
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https://doi.org/10.1051/e3sconf/202341505012

Then, we propose the Hazard Connectivity Index(HCI) which is dimensionless:
𝐻𝐶𝐼 = 1 − 𝑆𝑃 = 1 −𝑥𝑥+𝐿=𝐿

60 m

(2)

𝑥+𝐿50 m20 m

To explain HCI determination, Figure 1
demonstrates how to measure the values of x and L. We
suppose here that an initiation of landslide at a zeroorder basin [6] triggers one debris flow and thesediments are deposited downstream. In Figure 1, apainted area (pink colour) presents a debris flow
inundation area, and a point of interest is a house
location. Then, a distance x is firstly measured. After
that, a distance (L) between the end point of x on thedebris flow area and the debris flow initial point can bemeasured. Thus, the L value can be sometimes a part of
the total runout of the debris flow; meanwhile x is theshortest distance to the inundation area. Note that L isnormally a curved distance.

HCI = 0.75

HCI = 0.5460 m

80 m

40 m

HCI = 0.60
L60 m

L0m

HCI = 1.0Fig. 2. Some examples to calculate HCI values.

Residents usually look at common hazard maps andtend to be interested only in whether their houses are
inside or outside a hazard area. However, the HCI value
gives a relative understanding of how safe their houses
are, even if they are outside the area.

xx

3 Application of Hazard ConnectivityIndex

Lx

With a combination using SHALSTAB [7] andKANAKO-2D [8], a hazard map of debris flows waselaborated for a certain area of Igrejinha municipality,
southern Brazil [9]. Then, we added the HCI zoning onto
this map. Here we considered a landslide initiation point
as the starting point of the runout distance. And in thiscase study, this point was determined with theSHALSTAB simulation [9].
Figure 3 demonstrates the different zones based on
the HCI values from 1.0 to 0.6. The debris flow transportand deposition areas generated by the Kanako-2D
simulation are presented as the zone with HCI=1.
Supposing that this simulation performance issatisfactory [9], this zone can be considered very highly
hazardous.
As one of HCI applications, Table 1 shows thenumber of possibly-affected houses according to HCI
zoning of Figure 3. Many houses are located in zones
with relatively-higher hazard. These residents should beinformed this situation.

Fig. 1. Determination of the shortest distance between thedebris flow trajectory and the interesting point such as house
location (x); and the runout distance of the debris flow (L).

The determination of x and L permits to calculate
HCI values. Some examples of its calculation are shownin Figure 2 where the HCI values indicate the hazard
degree at the house locality, i.e., the locality’sconnectivity degree to debris flow occurrence. When thedebris flow reaches the house, the house will bedestroyed substantially, independent of their sizes.
Therefore, the situation with HCI=1 indicates thedisaster occurrence. In debris flow disastermanagement, the situation with HCI value over 1 should
be considered equal to that with HCI=1. In other words,
the whole area of the debris flow occurrence (pinklypainted area) has HCI=1. Hence, we set up that HCI
values vary between 0 and 1.

4 Final remarks
Introducing the connectivity concept, we proposed theHazard Connectivity Index for debris flow disastermanagement. Just after establishing the debris flow
occurrence maps, the determination of this index can be2

E3S Web of Conferences 415, 05012 (2023)
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https://doi.org/10.1051/e3sconf/202341505012

References

performed only with geometric analysis on them, which
does not require much knowledge or technique.
Since there are different mechanisms of debris flow
incitation, the starting point of debris flow needs to be
determined correctly, which can be the challenge fordisaster managers. Furthermore, the definition of thewhole trajectory (runoff distance) is also an important
procedure to have better performance of HCI. Anyway,
HCL is useful in cases where L and the entire debris flow
runout have uncertainties, because – at the end – this
proposal simply adds buffer lines around the inundationarea.
To verify the HCI applicability more, several casestudies should be necessarily executed in different
geographic settings.

1.
2.
3.
4.
5.

6.
7.

8.

9.

Fig. 3. Hazard Connectivity Index map in a part of Igrejinhamunicipality, southern Brazil.
Table 1. Number of possibly-affected houses corresponding
to different values of HCI in Figure 3.
HCI zoning

Possibility (%)

1.0

100%

No. possiblyaffected houses
28

0.9-1.090-10027

0.8-0.980-90

59

0.7-0.870-80

98

0.6-0.7

60-70

201

The present work was partially supported by the CAPES-ANA
project (grant numbers 16/2017, Finance Code 001). We thankthe CNPq for research scholarship.

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