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DISASTER REVIEW 2014 DWIDP
Use and Limitation of Landslide Hazard Map in Road
Alignment Planning: Case Study of Wamitaksar-Rudrabeni
Road Section, Gulmi District, West Nepal
Dinesh Pathak, PhD
Department of Geology, Tri-Chandra Campus
Tribhuvan University, Kathmandu, Nepal
(Email: dpathaktu@gmail.com)
1. INTRODUCTION
Landslide hazard mapping is the basic tool to assess, project
and mitigate the water induced disaster in a basin (Pathak,
2014, Pathak et al., 2005). Likewise, it is equally important
tool to assess and ensure the round the year functioning of
the mountainous road (Pathak, 2013). The damages due to
water induced disaster are quite significant in Nepal and it is
assumed to be more severe due to changing climate (Pathak
et al., 2010).
There exist various models for landslide susceptibility and
hazard mapping (Guzzetti et al., 1999; Chung and Fabbri,
2003; Remondo et al 2003; Van Westen et al., 2003; Dahal
et al., 2009). In this study, the information value method has
been used for the landslide hazard mapping. Van Westen
(1997) proposed the Information Value (InfoVal) method for
landslide hazard analysis, which considers the probability of
landslide occurrence within a certain area of each class of a
landslide causative factor.
Thus, assessment of landslide hazard and its use for
the planning and assessing the infrastructures like road
alignment is inevitable. In addition, the remote sensing
imageries plays vital role through reducing time, cost and
efforts in the process and also enhancing the quality of the
mapping and assessment (Subramani and Nanda Kumar,
2012; Pathak, 2013 and Pathak, 2014).
2. THE STUDy AREA
The study area lies in the Gulmi District, west Nepal (Fig.
1). The road section is important as it connects Butwal to
Burtibang, an old and important settlement in the Baglung
district. The road is under the process of upgrading from Ridi
bazar to Burtibang to ensure round the year connectivity.
The considered Wamitaksar-Rudrabeni road section runs
along the left bank of Badigad River.
Abstract
Better road network with enhanced connection to different parts of the country is envisaged as backbone of the development
goal of Nepal. Occurrence of geo-hazard condition along the road corridor is threat to the objective of timely, efficiently
and quality road construction. In addition, it causes the heavy demand of maintenance at the operational stage that greatly
restricts the better utilization of the road by the beneficiaries. An ultimate goal of any road project is to avoid unstable areas
prone to landslides and erosion processes. However, in case of Nepal with fragile mountain environments, road construction
projects inevitably face the road side geo-hazard condition. Road alignment should avoid areas near active landslides and
erosive gullies. In order to avoid such hazard prone areas, road should be aligned based on the landslide hazard map of
the catchment area. However, the landslide hazard map should be utilized with care during finalization of the alignment. A
landslide hazard map indicates the areas susceptible to landslide but does not tell anything about the impact at downslope/
upslope area if the event occurs. Therefore, it is necessary to careful evaluation of the hazard condition and its possible
impact to road before road alignment is finalized. These aspects have been presented here with example from Wamitaksar-
Rudrabeni road section at Gulmi District, west Nepal.
Keywords: Geo-hazard, Landslide hazard map, Road construction, Mountainous terrain
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DISASTER REVIEW 2014 DWIDP
Fig. 1: Location of the study area through which the Wamitaksar-
Rudrabeni road traverses.
The VDCs lying along the road corridor under the present
study are Wami Taksar, Kurgha, Ampchaur, Turang,
Rupakot, Juniya, Jahang, Hasara, Limgha, Thulolumpek and
Aslewa (Fig. 2). There are clustered settlements at various
locations along the road alignment indicating that this road
section serves a large number of people in the area. Gedhi
Khola, Hugdi Khola, Jumdi Khola, Lumdi Khola, Sisne Khola,
Tekwa Khola, and Dahare Khola are the major tributaries of
the Badigad River in the study area.
Fig. 2: VDCs and settlements lying along the Wamitaksar-
Rudrabeni road corridor.
3. MATERIALS AND METHODS
The Wamitaksar-Rudrabeni road section has been evaluated
with respect to the geo-hazard condition. The following data
have been utilized:
• Digital topographic map of Department of Survey
• Road alignment
• Geological map of the area published by
Department of Mines and Geology
• Satellite imageries
• Field data
The geological map of the area was digitized in GIS. Various
thematic layers like slope, aspect, drainage density, land
use, landslide distribution, and gully erosion were prepared
from the digital topographic data that was updated from the
satellite images. Bivariate statistical method was used for the
generation of landslide hazard map of the watershed.
The geo-hazards along the alignment have been assessed
by overlaying the road alignment on the satellite image,
geological map and hazard map.
4. GEOLOGy OF THE STUDy AREA
The geology of the area plays vital role in the occurrence of
geo-hazard condition. The present road alignment traverses
the Lesser Himalayan zone consisting mainly of the Nourpul
Formation, Dhading Dolomite, and Benighat Slate (Fig. 3).
Alluvial deposit is distributed mainly along the Badigad
River and its tributaries.
Fig. 3: Geological map of the study area.
Nourpul Formation consists of intercalated slate, phyllite and
quartzite; Dhading Dolomite Formation is predominantly
consisting of dolomites with stromatolites; while the Benighat
Slate Formation consists of slate with calcareous bed. The
Badigad River has developed terraces along the river.
The road alignment mainly traverses through the alluvium
deposits and occasionally through the rocks of different
formations. Slate of Benighat Formation is mostly distributed
rocks along the alignment.
Badigad fault traverses almost along the Badigad River and
other minor faults and joints are also present in the area.
These are mainly responsible for the unstable hill slope and
also disintegration of the rocks.
5. TERRAIN RELATED DATA
The terrain related data are obtained from the digital
topographic maps and satellite imageries. The road
alignment was overlain on the satellite image to observe the
terrain condition that the road alignment is passing through
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DISASTER REVIEW 2014 DWIDP
(Fig. 4). It is clear that the road is aligned at the valley part
with the minimum elevation that goes on increasing at the
upslope in northern part. The northern part of the catchment
is relatively steeper and hence the mass wasting process is
more active at the upper reaches of the tributaries of the
Badigad River.
Fig. 4: Road alignment overlain on satellite image and
assessment of geo-hazards.
The catchment area at the left bank of Badigad River,
influencing the road corridor has also been assessed in terms
of the land use condition (Fig. 5). It is observed that the area
is mostly consisting of forest area, followed by cultivation.
Thus, in terms of greenery of the catchment, it is good,
however, the geological condition is playing vital role in the
occurrence of landslide and other soil erosion condition. The
road alignment mostly passes through the cultivated area
lying at the valley floor with occasional crossing of the forest
area.
Fig. 5: Land use condition in the study area.
6. LANDSLIDE HAzARD ASSESSMENT ALONG THE
ROAD CORRIDOR
Preparation of landslide inventory map is pre-requisite for
the assessment of geo-hazard condition along the road
alignment. Landslide inventory map has been prepared
through extraction from digital topographic map, satellite
imageries and field data collection (Fig. 6). The satellite
imageries have been commonly used in geo-hazard
assessment (Pathak et al., 2005; Pathak et al., 2009;
Subramani, T. S. and Nanda Kumar, 2012.).
Fig. 6: Landslide inventory map of the study area.
The landslides are mainly distributed in the Turang, Rupakot,
Juniya, Jahang, Harsa, Limgha and Thulolumpek VDCs.
The severe condition is observed in Juniya VDC followed
by Thulolumpek, Limgha and Turan VDC. A landslide
hazard map of the study area has been prepared using
various thematic layers like slope, aspect, drainage density,
geology, land use, landslide (Fig. 7). The map was prepared
through statistical bivariate method and validated with the
independent landslide dataset that was not used to prepare
the hazard map.
The landslide hazard map shows that most of the road
stretch passes through high to very high hazard class. Even
if the road doesn’t exactly pass through the higher hazard
classes, the adjacent landslide hazard may affect the road.
Fig. 7: Landslide hazard map of the study area.
The road stretch around Ullikhola Village, Juniya VDC and
Kalwar gau, Aslewa VDC are considered to be the most
critical parts in the Wamitaksar-Rudrabeni road section. The
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DISASTER REVIEW 2014 DWIDP
alignment crosses several tributaries of the Badigad River.
These tributaries are transporting huge amount of sediments
and the bed load derived from the landslides at the upper
reaches of the tributaries. The site condition as observed on
satellite image, field photograph and the prepared hazard
map have been compared to evaluate the appropriateness
of the hazard map (Figs. 8, 9 and 10).
It is clearly observed that the massive landslide zone at the
upper reaches of the Dahare Khola (upstream of Ullikhola
village) that is identified on the satellite image is clearly
affecting the road stretch while passing through the landslide
mass deposited area at the lower reaches of the stream (Fig.
8). The sediments derived from the landslide appear to cause
continuous damage and affect the smooth functioning of the
road during the monsoon period. Likewise, the hazard map
also accurately delineated this area as very high landslide
hazard class (Fig. 9). In addition, the field study has
confirmed this site condition (Fig. 10) showing the massive
mass wasting at the upper reaches followed by thick pile
of sediments deposited at the downstream through which
the road passes. However, it is to be noted that the entire
risk area at the downstream part couldn’t be delineated by
the landslide hazard map though the source area has been
accurately delineated.
Fig. 8: Landslide observed on satellite image at the upper
reaches of Dahare Khola (upstream of Ullikhola village).
Fig. 9: Road alignment overlain on landslide hazard map
around Ullikhola village
Fig. 10: Field photograph of the landslides at upper reaches of
Dahare Khola (10a) and its impact to the road alignment at the
downstream, around Ullikhola village (10b).
Similar evaluation was carried out at further downstream
of Ullikhola village, around the Lumdi Khola crossing of the
road stretch at Aslewa VDC, near Kalwargau. The satellite
image shows significant mass wasting at the upper reaches
of the Lumdi Khola (Fig 11). The Kalwargau area has been
classified as high to very high landslide hazard classes (Fig.
12). This is quite interesting and important outcome as the
road stretch passing through this area is really at risk of the
debris derived from the upper reaches of Lumdi Khola.
Fig. 11: Mass wasting at the upper reaches of the Lumdi
Khola as observed on the satellite image.
10 a
10 b
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DISASTER REVIEW 2014 DWIDP
Fig. 12: Landslide hazard map shows high hazard class
around Kalwargau, Aslewa VDC.
7. DISCUSSION AND CONCLUSION
The Wamitaksar-Rudrabeni road alignment traverses through
the river valley along the Badigad River. The alluvial deposits
as well as Benighat Slates dominantly occur along the road
corridor. Various thematic layers have been prepared using
the digital topographic data, satellite imageries and data
collected in the field. Statistical bivariate method has been
used to prepare landslide hazard map of the study area and
the road corridor was evaluated with respect to the hazard
condition. The field observation greatly helped to accurately
assess the impact of landslide on the road alignment.
The Ullikhola village at the left bank of Badigad River is affected
by the transported debris from Dahare Khola, especially
during the monsoon season. The damages to the settlements,
infrastructure and agriculture land at the downstream site
is quite significant indicating that it has wider impact in the
society. The hazard map shows that the northern part of the
catchment is represented by the high hazard areas while the
low lying valley area is represented by the low to moderate
landslide hazard classes. The field condition at the upstream
of Dahare Khola is well represented by the landslide hazard
map. Similar was the case around Lumdi Khola catchment.
However, at the lower reaches of Dahare Khola, around
the Ullikhola village, the site is at high risk of impact due
to landslide at the upper reaches but not well depicted
by the hazard map. The landslide hazard map should be
understood as indicative of the presence of landslide in an
area, however, it may not provide direct information about
the possible risk at the downstream through which the road
alignment passes. This map should be further interpreted
based on the possible affected areas from the debris derived
from the upper reaches. Finally, rather than confining the
geo-disaster assessment only along the road alignment, it
is necessary to have thorough study and understanding of
the entire catchment area. If this approach is well addressed
during the road alignment planning and possible geo-
disaster areas are identified, a better alignment can be
selected and the applied mitigation measures at required
site would greatly enhance the performance of the road with
reduced risk.
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