The agriculture on terracements is closely linked to the problem of natural risk, in many a way: on one side the terraced structures offers a ’spontaneous’ form of defence against landslides, but on the other the ruined terracements can become a threat to the lower lands. So, this is a topic that can be confronted in many ways, and on many levels.
The interesting point coming from the presentations held in the last meeting in Goriska Brda (12th-13th Oct) seems to me that many partners have chosen a statistical approach to landslide hazard assessment. The Ligurian partners, coordinator of the discussion group, as well as the Slovenian friends working on Brda, and also the IREALP partners, they all collected data on terraced areas and on landslides and then statistically analysed them. The Slovenian guys already succeeded in defining different classes of risk and a ’risk map’, the others are on the way to do the work. Even considering the difference of the various methods, this general approach should definitely be carried on by ALPTER, and the final proposed method will use it. Some work could still be done in comparing the chosen indicators, the units used for analysis, and to define the working steps.
For a long time the forum of discussion on hazard assessment in terraced areas has developed through e-mail exchange and meeting between the partners of the project (sorry for this).
To upgrade the forum, here below are the last results achieved by the discussion:
As a way for assessing hydro-geologic hazard in large study areas (in an effective way) a method is being developed by Univ. of Genoa: the proposed approach makes use of multi-variate analysis applied to large sets of data - a full presentation of the proposal has been uploaded to the web-site (see the news or the documents);
Other methods are still under examination: in particular, Univ. of Milan in experimenting a modeling method for detailed analysis (at small scale), while the Universal Soil Loss Equation (USLE) has emerged as an important base point for this study subject.
I’d like to give thanks to the Milano working group for the contribution to the Risk forum.
The proposal follows what we pointed out in our meeting in Genova and we think is a good basis to develop the application of the model to our different local situations.
Kind regards to all of you,
Guido Paliaga, Gerardo Brancucci
Dear colleagues the following note is based of what contained in the two interesting papers on slope instability hazard suggested by our thematic coordinator:
1) â€œGIS modelling of slope stability in Phewa Tal watershed, Nepalâ€ and
2)â€œAssessing landslide potential using GIS, soil wetness modeling and topographic attributes, Payette River, Idahoâ€ .
The analysis of the papers shows some differences in the methodological approach, despite having similar data input.
To carry out the calculation of the final hazard map, â€œpaper 2)â€ assumes that cells with landslides and without landslides are compared, assuming the existence of a preliminary census of landslides in the investigated area.
As reported in Genoa these data are seldom available in all our areas, because of the lack of a real landslide census in terraced slopes and/or the lack of a statistically significant number of landslides.
On the other side â€œpaper 2)â€ contained a very interesting method to consider the influence of precipitation and the consequent development of pore-water pressures through a simple model.
The following rough proposal tries to combine methods of the two papers considering assumptions and limitations already pointed out in Genoa.
a) developing of a first landslides hazard model based on â€œpaper 1)â€ approach (in function of influencing parameters only) using:
a uniform grid, not based on geomorphic and geologically homogeneous terrain units;
model integration with the wetness index DYNWET proposed in â€œpaper 2)â€ ;
model integration with specific parameters or factors, characterizing terraces instability (e.s. terraces geometry, number of strips per unit heightâ€¦.);
no statistical analysis of the landslides distribution but local validation of the model through the comparison with existing landslides (few can be sufficient).
b) a successive phase where further specific data can be added:
a) state of decay of terraced elements and their relative location in the slopes;
b) drainage flow net
c) other suggestionsâ€¦â€¦â€¦â€¦.
Marco Masetti, Tiziana Apuani, Tecla Pagnoncelli
Dipartimento di Scienze della Terra UniversitÃ degli Studi di Milano
following our discussion during the last meeting in Genova and the approach that we have proposed as co-ordinator of the natural risk forum, I am pleased to point out some references that can be used to define the model.
David N. Rowbotham and Douglas Dudycha "GIS modelling of slope stability in Phewa Tal watershed, Nepal". Geomorphology, Volume 26, Issues 1-3, December 1998, Pages 151-170.
Mandy Lineback Gritzner, W. Andrew Marcus, Richard Aspinall and Stephan G. Custer "Assessing landslide potential using GIS, soil wetness modelling and topographic attributes, Payette River, Idaho". Geomorphology, Volume 37, Issues 1-2, March 2001, Pages 149-165
Just few words to sum up our proposal: the approach we have focused on starts from the modelling of instability and include in the assessment procedure the conditions of the terraces as they emerge from the field survey.
Then the first step is the delimitation of terraces from aerophotography and cartography; the second one is the field survey and the third one the synthesis between the DEM analysis and the field data.
Guido Paliaga and Gerardo Brancucci
I am trying to summarize here below what said in the work session in Genoa, to start a deeper discussion about the point that remained open.
A simplified method for definition of a map of risk in terraced areas has been defined. The method make use of GIS tecniques and follows this steps:
Census and mapping of terracements
Elaboration of DEM model (3D) for water flow simulation
Possibility of insertion of additional data
Use of a standard model for comparison of previous results and mapping of risk
This method, although simplified, has the advantage of being easily appliable, using GIS tools, to many cases and areas. It must be fully developed with details like omptimized scale, parameters, etc...
All the guys that took part to the discussion in the afternoon in Genoa are invited to give their response to this summary.
Dear colleagues, in view of the upcoming event of Genoa, two brief notes on risk thematic.
Considering time remaining to the end of the Alpter Project, the proposal of an original methodology for risk analysis seems not to be a possible solution; it would take a lot of time and then we wouldn’t have enough time to validate a completely new product.
A more practicable solution should be to use one of the existing techniques to analyse landslide risk in natural (non terraced) slopes, modifying them by adding some typical elements important for terraced slope stability. We could use worldwide known and efficient methodologies adding in these already tested techniques, new (and few) elements whose effectiveness should however be checked in pilot areas.
We think that these new elements should be contained in the census data sheet so that we can propose a "modified" technique of risk analysis for terraced areas coupled with a standardized method of collecting the required data.
In few words the need is to be in Genoa with some ideas and proposals to discuss on:
1) the existing methodology of landslide risk considered more appropriate for the use we need, or in alternative which is the preferred approach (statistical, empirical, deterministic.....)
2) the new elements to introduce in the existing methodology to make it suitable for risk analysis in terraced slopes
Marco Masetti, Tiziana Apuani
Dipartimento di Scienze della Terra
Universita’ degli Studi di Milano