LANSLIDE INVENTORY MAPS AS THE BASIS OF SUSCEPTIBILITY AND HAZARD ASSESSMENT (TREMP, CATALAN PYRENEES) COPONS, R.(1,2); LINARES, R. (3); CIRÉS, J. (4); TALLADA, A. (1); ROQUÉ, C. (5) (1) Georisc Company; (2) Technical University of Catalonia; (3) Autonomous University of Barcelona, (4) Geological Institute of Catalonia, (5) University of Girona
1. INTRODUCTION AND OBJECTIVES
5. DISCUSSION
3. MATERIALS AND METHODS
Landslide inventories involves the location, classification, volume, activity and others characteristics of landslides in an area (Fell et al., 2008). Landslide inventories are the basis for susceptibility, hazard and risk assessment because provide information contrasted in the field. However, landslide inventories may be inaccurate because do not portray all the existing landslides, and landslides boundaries are usually approximate. Accuracy of inventories is influenced by several variables: abil ity of practitioners, work scale, used techniques, morphology preservation, forest density, anthropization and the available resources. A extensive landslide mapping project at 1:25.000 scale has been initiated in Catalonia. Landslide maps will be used for academic and applied purposes, and will be the basis for hazard mapping. Before undertaking exte nsive mapping over the years, it is crucial to establish procedures and rules to reduce mapping uncentainties. For governmental hazard practitioners and hazard managers is critical to know the accuracy and completeness of landslide inventories. The purpose of this work is to compare two landslide inventory maps performed by two different disci plinary working groups: a geological engineering group and a geomorphological academic group. This comparison will allow to asses the strengths and weaknesses of different mapping methodologies, to constrain the distinct technique s and criteria and, to propose a multi-disciplinary procedure to perform extensive mapping over a country with diverse climate, orographical and geological characteristics.
2. SETTING DESCRIPTION SELECTED AREA
5.1. Comparison of maps: 3.1. Geological engineering group
3.2. Geomorphological academic group
A landslide inventory was prepared by the Georisc geological engineering company. The purpose of this inventory was acquiring indispensable data for assessing susceptibility and hazard.
A landslide geomorphological map was performed by an academic group from the Autonomous University of Barcelona and the University of Girona.
Company Group
The inventory was prepared as follows: 1. Identify landslides and deduce their boundaries by using the oldest stereo aerial photographs available (1957 flight). Forest density, urban development and anthropization in 1957 were lesser than the present. These features allowed to identify easily morphologies related to landslides and prepare a preliminary inventory. 2. Complete the preliminary inventory using younger aerial photographs and remote sensing images available on the Cartographic Institute of Catalonia (http://www.icc.cat). This task yields a preliminary inventory with temporal aspect. 3. Verify the temporal preliminary inventory with field reconnaissance. During the field work data related to activity (cracks, damaged vegetation, etc.) was gathered. 4. Complete the inventory map on a GIS-based platform by comparing data gathered in the field and aerial photograph interpretation. 5. Prepare a database that contains alphanumeric data about landslide characteristics for any event: TIPOLOGY
MATERIAL AFFECTED
Photo 1
Slide Earth-flow
Hard rock Soft rock
Debris-flow
Alternated hard and soft rock
Lateral spread Fall Topple Complex
Photo 2
AGE
ACTIVITY ESTIMATION
BOUNDARY SLOPE GROUP RELIABILITY
based on Keaton and Degraff (1996) but without age
(based on Varnes 1978)
Photo 3
5.2. Analysis of results
Debris soil Earth soil
Active or suspended Dormant recent
Age according aerial Dormant mature photography identification Dormant old and historial Anthropized information
Accurate Approximate Estimate
Slope with superficial slides
The study area is located in the southern slope of the Pyrenees in the north-western Catalonia, near the village of Tremp. Elevation ranges from 400 to 1700 meters a.s.l.. The data recorded at several weather station s provide a mean annual temperature of 12-13ºC and average annual precipitation values ranging from 600 to 700 mm. Typically, a significant proportion of the rain falls as high intensity convective storms. The study area, showed in this poster, has a surface about 20km 2 (Figure 2). This area is one of the highest landslide density in the Pyrenees and has a high variability of landslide characteristics (typology, activity and extension).
Group according prone lithology to instability, structural geology, slope dip and orientation
Results
GEG
GEG
The inventory was prepared by means of interpretation of aerial photographs of different date flights and scale (1:33.000 from 1956-57, 1:18.000 from 1978; 1:22.000 from 1992 y 1:60.000 from 1993) and field work. Two reasons justifying the use of old aerial images: 1. Climatic and morphologic conditions in 1956-57 were different from at present. Fluvial, erosive and mass movements processes were more active than now (Fig. a) 2. During the second half of the 20th century, farming activity on slopes by means huge agricola machinery erases morphologies related to shallow landslides (Fig. b).
Figure (A) Landscape evolution near Tremp from the comparison of aerial photographs took in 1956 and 2009. 1: Conques river. Until the middle of the 20th century, Conques river was characterized by braided channels, unstable sedimentary structures and an almost complete lack of plant cover in the alluvial plain, due to the high sediment yield on hillslopes and the occurrence of frequent and intense flooding. This was probably related to strong demographic pressures, including the cultivation of steep slopes, frequent fires, deforestation and overgrazing. 2: Toes of shallow landslides are only identifiable by the photo from 1957. 3: Dip slopes are landslide scars. Figure (B) Example of shadow landslide in a farm Their identification is difficult due to identifiable only before plowing land. forest with high density of trees.
GEG: Geological Engineering Group. GAG: Geomorphological Academic Group
Academic Group
Portray the information more Represents the geological rellevant for further hazard history of instability assessment
GAG
Completion and accuracy
Depends on financial resources and death time
Extremely complete regardeless resources and time
Temporal aspect
Relative age of landslides according preservation
Material selected for dating. Instability evolution well kwon.
Activity
Taken special attention to silent witnesses.
Enquiries to eye-witnesses.
Time spended and financial resources
Suitable relationship cost/ benefit
Excessive time and resources
GIS-based
Each landslide has a polygon and an attribute table. Easy for further analysis by GIS.
Each landslide has a polygon Easy for further analysis by GIS.
Paleolandslides
Without morphology. Considered as colluvium.
Taking special attention. Looking for outcrops among forest and farms.
Scienfic research
Checking for methodologies
Research about environmental changes (hidrology, land uses, etc). Aerial-photografs used represents the highest instable scenario.
6. FINAL REMARKS
Strengths
Company Group
Academic Group
Suitable for extensive mapping.
Suitable for local mapping in order to supply additional information for extensive mapping
Gathered data suitable for further hazard assessment
Inventory map more complete. Suitable for academic and scientific purposes.
Good relationship cost/benefit Database obtained allows analysis by GIS
The geology of the area consist on a large synclinorium made up by two main lythologies: (i) the “Garum” Formation composed by plastic clays susceptible to instability, (ii) and calcareous rock forming rockcliffs faces from ten to hundred meters high where primary rockfalls occurs.
4. RESULTS: LANDSLIDES INVENTORY MAPS
CALCAREOUS ROCK
Hazard zoning optimistic. So requires further maps at more detailed scale for urban development.
CLAYS
Weakness
4.1. Geological engineering group
Several types of landslides are present: rockfalls, topples, slides, earth flows, large lateral spreads and slow earth flows also occurs in a large part of the slopes. The instability is active from Early Pleistocene to present. Photo 1
Photo 2
Photo 3
4.2. Geomorphological academic group
Poor relationship cost/benefit
Database management system is required to storage a great amount of data. Hardly for hazard assesment purposes.
Example of an attribute table for a landslide:
Ec: Scar surface. EC_a: Active or historic complex landslide. EC_d: Dormant complex landslide. ECr_i: Complex landslide with uncertain age involving hard rock. EF_a: Active or historic flow. EF_d: Dormant flow. El_a: Active or historic undifferentiated landslide. El_d: Dormant undifferentiated landslide. EL_a: Active or historic slide. EL_d: Dormant slide. EL_i: Slide with uncertain age. ELr_d: Dormant slide involving hard rock. ELr_i: Slide with uncertain age involving hard rock. Epl: Rock layers prone to instability.
For further information:
Acknowledgements
Ramon Copons Llorens. Georisc company is grateful to the Geological Institute of Catalonia for the economic support to perform a landslide inventory for hazard purposes.
Georisc S.L.P. C/Belgrad, 62. 43850 Cambrils ramon.copons@georisc.cat
The work of the academic group has been financed by the Geological Institute of Catalonia and by the Spanish Education and Science Ministry (project CGL2005-02404). Information exposed by webs of California Geological Survey (www.consrv.ca.gov) and American Geological Survey (www.usgs.gov) is truly appreciated by Georisc Company. For implementing geographical
data Georisc Company has been using Geomedia
Professional 6.1. with courtesy of Intergraph.
References
Technical University of Catalonia (UPC) Geothecnical Engineering and Geosciences Dpt. Campus Nord, 08034 Barcelona. ramon.copons@upc.edu.
Rogelio Linares Santiago Department of Geology (Area of Geomorphology) Autonomous University of Barcelona 08193 Bellaterra (Cerdanyola del Vallès)
Jordi Cirès i Fortuny. Geological Institute of Catalonia. C/ Balmes, 209-211. 08006 Barcelona.
Fell, R., Corominas, J., Bonnard, C., Cascini, L., Leroi, E., and Savage, W. (2008): Guidelines for landslide sus ceptibility, hazard and risk zoning for land use planning, Engineering Geology, 102, 85–98. Keaton, J.R., DeGraff, J.V. (1996): Surface obs ervation and geologic mapping. In: Turner, A.K., Schuster, R.L. (Eds.), Landslides, Investigation and Mitigation, Transportation Research Board. Special Report, vol. 247. National Academy Press, Washington D.C, p p. 178–230. Varnes D. J.(1978): Slope mov ement types and processes. In: Schuster R. L. & Krizek R. J. Ed., Landslides, analysis and control. Transportation Research Board. Special Report. vol 176, National Academic Sciences, pp. 11–33.
Carles Roqué Pau Department of Enrivonmental Sciences University of Girona Campus Montilivi. 17017 Girona