Debris-flow risk assessment and land management at

Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome
Davide Murgese(1), Dario Fontan(1), Marina Pirulli(2), Claudio
Scavia(2), and Paolo Oria(3)
Debris-flow risk assessment and land management
at municipal scale.
(1) Sea Consulting Srl, Land Management, via Cernaia 27 10121 Turin Italy, +39 011
5162939
(2) Politecnico di Torino, Department of Structural and Geotechnical Engineering, Turin,
Italy
(3) Ingegneria e Territorio, Turin, Italy
Abstract Natural-risk levels assessment related to
debris-flow and flood occurrence is a crucial process for
the development of proper land management policies in
densely urbanized alluvial-fans of the Alpine region.
Mitigation measures are built to protect edified areas
lowering natural-risk level below defined thresholds and
to allow socio-economic development at local scale,
especially in mountainous areas, where urban expansion
is strongly conditioned by local morphology and natural
processes dynamics. In 2009, SEA Consulting was
instructed by Bruzolo Municipality (Susa Valley,
Piedmont Region, NW Italy) to assess the efficacy of a
levee along the Pissaglio riverbanks designed to protect a
densely populated area. The study was conducted
considering debris-flow hazard by following two different
approaches: monodimensional (“Colate detritiche”, that
implements Voellmy and Takahashi equations) and
bidimensional (RASH3D code, assuming a Voellmy
rheology) models. Flood hazard was assessed by means of
the program HEC-RAS. The results of the study are
presented in this paper.
Keywords debris-flow, natural risk, runout modelling.
Introduction
One of the most critical aspects in natural risk
management is related to the concept of residual risk.
According to UN definition, residual risk is the “risk
that remains in unmanaged form, even when effective
disaster risk reduction measures are in place, and for
which emergency response and recovery capacities must be
maintained” (UNISDR, 2009).
The PLANAT platform, set up by the Permanent
Committee of the Alpine Region, defines the residual risk
as “the risk that remains after all protective measures have
been implemented and is closely related to the question
which risk is accepted by the individual or by society”
(PLANAT, 2009)
The Po Basin authority recently introduced a
methodology for the assessment of residual risk with
regard to flood management (AdBPO, 2005). The
proposed approach relates the residual risk to the
residual flood hazard. This latter element includes the
occurrence probability of an event characterised by a
magnitude higher than the reference event and the
hazard related to mitigation measure failure (e.g. levee).
The Piedmont Region Land Management law
requires the assessment of natural risk levels at municipal
scale. This information is the reference element for the
definition of the urban management plan.
Natural Risk levels are classified according the
“Circular of the President of the Regional Council, 8th
May 1996, n.7/LAP – R.L. 5th December 1977, n.56 and
further modifications and integrations – Technical
guidelines for geological studies in urban planning”
(called 7/LAP thereafter): I low risk, II moderate risk; IIIaIIIb-IIIc high risk. Class IIIb is used to classify edified
sectors where, due to high natural hazard levels, it is
required the construction of mitigation measures. Based
on natural hazard level, IIIb sectors are distinguished in
sub-classes: IIIb2 (sectors where the implementation of
the mitigation measure and the assessment of their
efficacy allows urban expansion), IIIb3 (sectors where the
implementation of the mitigation measure only allows
the improvement of building construction standards and
functionality), IIIb4 (sectors where the implementation
of the mitigation measure allows maintenance of existing
building in their place and no relocation is required).
For IIIb classes, the efficacy of mitigation measures
and the consequent urban development scenarios are a
function of the residual risk.
In 2006 SEA Consulting srl was appointed by
Bruzolo Municipality (Susa Valley, Piedmont Region, NW
Italy) to prepare the natural risk map for the municipal
territory; in 2009 SEA Consulting srl was instructed by
municipal authorities to assess the efficacy of a levee
along the Pissaglio riverbanks built to protect a densely
populated area. The study aimed at assessing residual risk
levels for sectors that could be affected by debris-flow
and flood occurrence, characterised by risk class IIIb2.
Defined residual risk levels were then examined by local
and regional authorities in order to allow urban
expansion in the examined sectors. Residual risk levels
were evaluated following a multidisciplinary approach: in
D. Murgese, D. Fontan, M. Pirulli, C. Scavia, P. Oria – Debris-flow risk assessment and land management at municipal scale.
this paper the methods applied and the results obtained
are presented and discussed.
Geological and geomorphological setting
Geological setting
The study area is located in the Lower Susa Valley
(Piedmont Region, NW Italy). The Bruzolo municipal
territory falls within the catchment area of the Pissaglio
river. Bruzolo territory includes a portion of the right side
of the Susa Valley (80% of the total area) and a portion of
the Dora Riparia river flood plain.
The area is characterised by the presence of rocks
belonging to the following Alpine Structural Units:
- Piedmont Unit: calcschists, serpentinites,
prasinites and quartzites (Lower Jurassic –
Middle Cretaceous);
- Dora Maira Massif bedrock: metadolomites
(Upper Trias), gneiss (Permian)and micaschists
(Pre-Carboniferous).
Tectonic contacts are marked by the presence of
tectonic breccias.
Quaternary deposits are the following:
- alluvial deposits related to the Pissaglio river
activity: blocks, sandy gravels and cobbles.
- Colluvial deposits: cobbles, blocks in a sandy
matrix.
- Glacial
deposits:
(a)
moraine
deposits
represented by cobbles and blocks poorly
rounded in a sandy matrix; (b) fluvial deposits
represented by gravels, with cobbles in silt-sand
matrix often cemented.
Geomorphological setting
Pissaglio river catchment
Pissaglio river catchment characteristics are summarised
in Tab. 1.
Table 1 Main morphological features of Pissaglio river
catchment.
Feature
2
Area (km )
Max height (m asl)
Min height (alluvial fan apex) (m asl)
Catchment average slope (°)
Main stream length (m)
Main stream average slope (°)
Outcropping areas (catchment area %)
Landslide areas (catchment area %)
Value
5.31
2674
523
34
5064
23
12
8.5
Pissaglio river catchment is characterised by the
presence of a sector subject to Deep Seated Gravitational
Slope Deformations. Other types of slope instability
involve mainly Quaternary deposits and are represented
by rotational slides, earth flows and soil slips. Sectors
classified as soil slip-prone areas are mainly located along
the hydrographic network.
The main sediment sources identified for the
Pissaglio river catchment are the following: (1) small
2
alluvial fans originated by soil slips evolving as minor
debris-flow that reach the riverbed (Fig. 1A); (2) earth
slide from glacial deposits outcrops along the
hydrographic network, triggered by erosion processes at
the base of the riverbanks (Fig. 2B); (3) blocks and
cobbles along the Pissaglio riverbed.
Figure 1 (A) Alluvial fan located in the vicinity of Pissaglio
riverbed, originated from minor debris-flow occurring along the
riverbank; (B) Erosion processes at the base of the riverbanks.
Pissaglio river alluvial fan
Pissaglio river alluvial fan characteristics are summarised
in Tab. 2.
The Pissaglio river alluvial fan is the area where
about 90% of Bruzolo edified areas are located (Fig. 2).
Table 2 Main morphological features of Pissaglio river alluvial
fan.
Feature
Area (km2)
Apex height (m asl)
Min height (m asl)
Average height (m asl)
Average slope (°)
Main stream length (m)
Value
1.6
523
410
445
5
2089
The river flows on the right side of the alluvial fan,
west of the inhabited area (Fig. 2). The actual river bed is
the result of both natural processes and human activities.
Pissaglio river flooding area is characterised by the
presence of crops; distribution of woods is patchy. For
this sector abandoned channels were recognised (Fig. 2)
based on local morphology and by the presence of debrisflow deposits (e.g. blocks, lobes).
Upstream the apex of the alluvial-fan, mitigation
measures are represented by four over fall barriers. The
channel along the alluvial fan is limited on both sides by
levees. Riverbanks are protected from erosion by rip-rap
and retention walls along both riverbanks. To reduce flow
speed and to allow solid-load deposition, a retention
basin and a flow breaker are present.
The inhabited area is also protected by an ancient
wall, named “La mura” (Fig. 2). Recently, close to the
apex of the alluvial fan, a concrete levee has been built to
prevent debris-flow runout from affecting the portion of
Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome
settled areas non protected by “La mura” (Fig. 2 and Fig.
3).
Figure 2 Bruzolo urban area. The continuous red line represents
the limit of Pissaglio river alluvial fan. Red arrows indicate the
presence of abandoned channels. “La mura” is an ancient wall
built during the XVIII-XIX centuries. The recent levee to protect
the inhabited areas is located in the vicinity of the apex of the
alluvial fan.
Figure 3 Aerial photograph to show the potential debris-flow
runout direction and the location of the concrete levee and “La
mura” levee.
Risk assessment: methods
Field survey
Field survey was carried out in 2006 for the assessment of
natural risk levels for the definition of the Bruzolo
municipality urban plan. According to what was
prescribed by Piedmont Region directive 7/LAP the
following maps were produced: geological map,
hydrogeological map, geomorphological map, mitigation
measures map, natural risk map.
Based on the risk map, the alluvial fan area has been
classified into the following risk classes: IIIa, not edified
areas affected by debris-flow; IIa, edified areas non
affected by debris-flow processes; IIIb3, edified areas,
mostly close to “La mura” levee; IIIb2, edified areas
between sectors falling into IIIb3 and IIa classes (Fig. 4).
Data collected during the field survey were stored in
a GIS and were then used for risk analyses.
Following the production of the risk map, Bruzolo
Municipal Council required the assessment of the efficacy
of the levee located at the alluvial-fan apex, in order to
define urban development policies for all sectors falling
into risk class IIIb2.
Risk analysis was conducted following a
multidisciplinary approach. Aims of the analysis were the
modelling of debris-flow runout and the calculation of
debris-flow and flood flow height and velocity, to check
the adequacy of the levee indicated in Fig. 3. The analysis
considered a specific reference event characterised by a
return period of 200yr.
Hydraulic study (flood hazard)
Hydraulic study was organized into the following phases:
• Basin morphological analysis
• Reference event magnitude (rainfall level)
• Runoff calculation
• Flood modelling
Figure 4 Natural risk classes for the Pissaglio river alluvial-fan.
Sectors investigated for the risk analysis are indicated by the
green line. The alluvial-fan limit is indicated by the white line.
Flood event was simulated by means of the program
HEC-RAS.
Basin characteristics were determined in a GIS
environment and led to the calculation of runoff time
according to Giandotti’s equation (Caivano, 2003).
Calculated runoff time is 0.81 hr.
Rainfall levels for the event having return period
200yr were calculated considering the P.A.I (Po river
Catchment: Natural Risk Mitigation Plan) Grid (AdBPO,
2002). For each node of the grid, equation parameters for
calculating rainfall levels referred to defined Return
Period (i.e. 20yr, 100yr, 200 yr, 500 yr) are indicated.
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D. Murgese, D. Fontan, M. Pirulli, C. Scavia, P. Oria – Debris-flow risk assessment and land management at municipal scale.
Reference precipitation levels are obtained following
a geostatistical process that interpolates the precipitation
levels calculated for each cell of the grid.
The runoff was calculated with the Curve Number
(CN) method (USDA-SCS 1986), considering the
following information: soil permeability map (derived
from the geological map), land cover (derived from the
Piedmont Region Forestry Plan). In order to define the
model in the worst condition, the value of CN(AMCIII)
(referred to high-saturation of soil) was considered.
Contributions from the sub-catchments of the
Pissaglio river catchment were then calculated and
combined in order to obtain the water-discharge to be
used as the input of the hydraulic model (Tab. 3).
For the hydraulic study a detailed topographic
model of a section of the Pissaglio river was realized (Fig.
5). Flow levels calculated for section 13 were considered in
order to assess levee effectiveness (Figs. 5 and 6).
Table 3 Water discharge calculated for the Pissaglio river
catchment related to different return periods.
Return Period (yr)
20
100
200
500
Q (m3/s)
61.7
91.6
105.2
123.9
Figure 5 Topographic sections (yellow lines) considered for the
hydraulic model along the Pissaglio river. The white arrow
indicates section 13.
Flow level at section 13 for the event having return
period of 200 yr is 1.79 m. Contribution due to solid load
was considered by increasing the flow level by 1/3: waterflow level is then 2.4 m. The river bed altitude for section
13 is 520.77 m a.s.l. Considering that for section 13 the
levee height is 4.9m, the hydraulic model indicates that
the water-flow is contained within the channel, proving
the effectiveness of the mitigation measure (Fig. 6).
Figure 6 Hydraulic model results for section 13.
Debris-flow modelling runout (1)
Debris-flow runout, flow height and velocity were
calculated by means of the program “Colate detritiche”
(Bruschi, 2008). In order to define reliable scenarios,
main sediment sources were identified according to the
distribution of shallow-landslide prone areas, as indicated
in Bruzolo Municipality geomorphological map (Fig. 7A).
Source-areas instability-levels were assessed by
means of SHALSTAB (Montgomery and Dietrich, 1994).
Shallow-deposits geotechnical-parameters where selected
based on field observations (Tab. 4).
Figure 7 (A) Potential sediment-source areas considered for
debris-flow; (B) Debris-flow paths (T1, T2, T3 and T4)
considered for debris-flow modelling.
Table 4 Parameters value considered for this study.
Paramter
Cohesion (MPa)
Inner friction angle (°)
Unit weight (N/m3)
Deposit thickness (m)
Value
0
35
18
2.5
According to SHALSTAB results, 94% of the source
areas is unstable in case of rainfall event with a return
period of 200 yr (rainfall level: 166 mm; event duration:
24 hr; runoff coefficient: 0.5).
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Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome
Debris-flow runout was modelled considering the
four sediment sources (Fig. 7B); each selected path is
represented as a topographic section parallel to the
stream. For each node of the section, the program “Colate
detritiche” allows the calculation of debris-flow height
and velocity.
Debris-flow height is calculated according to
Takahashi (1991) and Arattano & Savage (1994) models.
Flow velocity is calculated following 5 different models:
Syanozhetsky et al. (1973), Tsubaki et al. (1982),
Takahashi (1991), Arattano and Savage (1994),
Rickenmann (1999). Runout distance is determined based
on Voellmy (1955), Takahashi and Yoshida (1979) and
Takahashi (1991).
Along the four topographic profiles (T1, T2, T3 and
T4), results were examined in order to define debris-flow
conditions in correspondence of the levee:
• Debris-flow height: based on model results debrisflow height in the vicinity of the levee ranges
between 2.5m (T2 and T3 paths) and about 1.5m (T1
and T4 paths).
• Debris-flow velocity: in the vicinity of the levee
considered models indicate a flow velocity always
ranging between 1.5 m/s and 3 m/s. Only
Syanozhetsky et al. (1973) model provides velocities
ranging between 4 m/s (T1 and T4 paths) and 5 m/s
(T2 and T3 paths).
• Debris-flow runout distance: Takahashi and Yoshida
(1979) model indicates runout distances for T1 and
T2 paths of 1161 m and 1728 m from the alluvial-fan
apex, respectively. Runout distances for T3 and T4
paths are smaller ranging between 355 m and 123 m,
respectively.
With regard to debris-flow height and velocity, the
obtained results are similar to those provided by the
hydraulic model.
Also in this case models prove the effectiveness of
the levee.
Debris-flow modelling runout (2)
A second approach was applied by using the program
RASH3d (Pirulli, 2005).
The program calculates the debris-flow path based
on 10 × 10 m grid size DTM. Debris-flow dynamic are
modelled referring to Voellmy (1955) equation. Source
areas are identified following Hungr (1984) method.
Pissaglio river catchment was divided into subcatchments according to average stream slope, river-bed
material, talus thickness. Based on these elements, for
3
each sub-basin a specific channel debris yield rate (m /m)
is given. According to the Pissaglio river morphology
(DTM) and geological setting (geological map of Bruzolo
municipal territory), three types of sub-basin, with a
specific debris-yield rate, were recognized (Fig. 8): type A
3
3
3
(0-5 m /m), type B (5-10 m /m) and type C (10-15 m /m).
Figure 8 Pissaglio river sub-catchments defined to determine
channel debris yield-rate.
Total debris volume related to each sub-catchment
is calculated as the product between average debris-yield
rate and total hydrographic network length. Debris
volumes range between 141,000 m3 (sub-catchment A,
3
with average talus thickness of 1.1 m) and 103,000 m
(sub-catchment C, with average talus thickness of 6.9 m).
In order to run the debris-flow simulation with
RASH3D, debris volumes were assumed to be localised in
specific sectors, rather than along the streams. Sediment
sources were identified following a similar approach to
that shown in the former paragraph.
Voellmy (1955) parameters were determined after a
back analysis that considered the debris-flows occurred
in 2000 along the Rocciamelone river (Bussoleno
Municipality, Susa Valley), whose catchment presents
morphological and geological condition similar to the
Pissaglio river catchment. The values of model
parameters ϕ (friction angle) and ξ (turbulence
coefficient) are shown in Tab. 5.
Table 5 Parameters value for RASH3D simulation.
Paramter
ξ (m/s2)
ϕ (°)
Value
450
11
Results indicate that the flow is contanied by the
levee as indicated by the previous simulation.
Nevertheless, the debris-flow path appears to be
strongly affected by DTM resolution. In correspondence
of the bridge, located downstream the section where the
levee was built (see Fig. 3), the model indicates that the
debris flow does not proceed within the channel. This can
be due to the resolution of the grid: cells having 10 × 10 m
size do not correctly describe the local morphology in
correspondence of the bridge. Indeed, for this sector the
maximum flow height calculated by the model is about 4
m, whereas the riverbanks height is 5 m (Fig. 9).
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D. Murgese, D. Fontan, M. Pirulli, C. Scavia, P. Oria – Debris-flow risk assessment and land management at municipal scale.
Figure 9 RASH3D results considering sediment source related to
sub-catchment B central sector. The blue arrow indicates the
bridge location.
Conclusions
This study focused on the assessment of residual risk
related to the occurrence of debris-flows for the
municipality of Bruzolo. The analysis involved the
assessment of the efficacy of a levee built along Rio
Pissaglio riverbanks to protect downstream inhabited
areas within the risk class IIIb2. The assessment was
conducted following two different approaches: flood
modelling (hydraulic model – HEC-RAS) and debris-flow
modelling (“Colate detritiche”, monodimensional model;
RASH3D bidimensional model). Results proved the
effectiveness of the levee in containing a debris-flow or a
flood, having a return period of 200 yr, and protecting
inhabited sectors of Bruzolo.
Accordingly, urban expansion policies for the IIIb2
sector could be implemented and new expansion areas
were defined by local authorities. The study allowed the
assessment of residual risk and provided a valid tool for a
cost benefit analysis for the investment made for building
the levee with regard to the possibility of guaranteeing
further socio-economic development for local population.
Also, the possibility of comparing results from
different models provided a more reliable residual risk
scenario to better support local authorities in the
decision-making process.
For models based on DTM, it is important to notice
how grid resolution has to be carefully considered as not
all the investigated features can be depicted by the digital
model of the landscape, leading to inaccurate results
compared to real conditions.
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