The landslide map of Bogota updating
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| License | CC Attribution 3.0 Unported: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
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Digital photography
Transcript: English(auto-generated)
00:07
I don't have time for any more questions. Please come to Bogota. There are a lot of students here. In Germany, there are photographs here, informatics here, and... ...here in Germany.
00:20
And I'll leave the floor. Good morning. I'll leave my floor in English, because my German is not good enough,
00:40
and all of this is technical. I will talk about the Landslide Hazard Map of Bogota and updating. This project was developed with the Risk Management Institute and Climate Change of Bogota, Colombia. So, first of all, the agenda. We have the contents of Landslides in Bogota.
01:02
There are two methodologies. The first one is the semi-quantitative evaluation stability. Then I will talk about the natural slopes, contrasting of maps, software and use cases, impact and results, and then conclusions. First, the contents of Landslides in Bogota.
01:23
Well, we'll look where Colombia is located. Colombia is in the north part of Latin America. And then here is Bogota. It's in the middle of the country.
01:40
So, some basic information of Bogota. The height of Bogota is about 2,600 meters. This is located in Andes. The study area is about 16,000 hectares. And this is situated in an inter-tropical convergence zone. The population exceeds 8 millions of inhabitants.
02:04
And here you see this area. This magenta area is the area that the study covers, because this is the area where the mountains are. But why the Landslides occur in Bogota?
02:21
So, we have a cocktail of many factors that make the Landslides happen. So, the first is that the mountains have natural susceptibility due to the geomorphology. And then also there are frequent precipitation. And then we have the global climate change
02:42
and El Nino and La Nina. And also Bogota is located in an intermediate earthquake zone. So, Bogota mountains are prone to Landslides. But why Landslides occur? What is this problem? What happened?
03:02
Here, there are three different situations. And you see that Landslides, the more important things are rain, angle and material. And the first one, you see a block. And they are the forces that act in the block. So, the gravitational force, the normal force,
03:20
shear force and shear strength. So, when this shear force is smaller than the shear strength, the block will not move. But that will happen if the angle is steeper, as in this situation. Then the shear force is equal to shear strength.
03:44
Then the block will move or will not move. But when there is rain, for example, strong rain, then the block will move. And then in the third situation, the angle is steeper than in this situation.
04:01
And then, of course, the shear force is much bigger than the shear strength and they will move. And then we have a Landslide. But there is not also these problems that I already explained. There is also a social problematic that we have in Bogota.
04:23
And then, you know that there are many refugees, not only from all the country, but also from Venezuela now. And they live in such risky zones, like you see here. So, there is a social problematic. We have accelerated and disordered urban growth.
04:41
And people live in these regions, like you see, like I already showed. So, there is human activity and queries and a slope of rivers and streams and steep slope zones and dams. And here, an important number, approximately 3.5 million people live in these such risky zones.
05:05
And here, you see, for example, the situation is dangerous, like you see. This point is located in the western part, in the western south part of Bogota.
05:21
And this is this phenomenon. And this is considered the largest Landslide phenomenon in urban areas of Latin America, with these 73 hectares approximately. And then here, you see a crack of about 1 meter and the house is parted in half.
05:46
Then, we talk about many, many different factors that make the necessity of a land management plan updating of Bogota. And that was include the Landslide and flooded maps updating.
06:01
And that was necessary to visualize and you process basic and complex information. So, the municipality of Bogota hired 32 professionals to perform this project and was used open source software, QGIS, GRAS and SAGA.
06:20
This part, it was not easy to convince these people to use this software. Because at the beginning, they wanted to buy RGS software, many new licenses and modules. Finally, they accepted to do it like I suggested.
06:41
And then, there was the training of open source software. They save about 90,000 euros. The hardware capacity was exploited because all people had installed these programs. And then, we have a good result. So, they updated maps. So, first, the methodology,
07:02
semi-quantitative stability evaluation and then consist of categorizing and weighting the factors that cause instability according to the influence of this in the Landslides. So, there is a stability rate that this is the sum of all these factors that you see here.
07:23
So, material relief, drainage, land use, erosion, climate, earthquake and of course, human factor is a very important aspect. And I will show you a series of maps where the best rating is represented in green and the worst is represented in red.
07:43
The higher the rate is, the better the stability is. So, first, we have drainage and I will not explain in a very short detail each factor because there is not enough time.
08:00
But the drainage was evaluated like you see here the classification and average slope. And then, these two variables were combined and then you see that the average slope of drains, the higher here and drainage density, of course, if you had so much water, that is worse.
08:20
And then here you see that the rating is six. So, the areas in six are represented in red, like I already explained. And do you remember this phenomenon that I talked about, the largest Landslide phenomenon in Latin America is here. This is about 13 rate for this phenomenon. And then 10 rate conditions.
08:44
And for this variable, it was not easy to have a result because the methodology says that you have to consider all these subsums and all these slopes and afterwards, you have to combine this first result with this profile,
09:00
with this convex, linear or concave. So, it was not so easy to perform this variable. And then I used ZAGA. And for this project, this algorithm, this is the rain surface classification algorithm and you combine the convexity, the slope and the texture.
09:22
And if you compare this for the previous study and this with the updated study, you see that here there is a much more higher level of detail. And then the third variable was the material and land use.
09:45
And there are another group of variables, the triggering factors. And then you see the human factor, erosion, climate and earthquake. And for the human factor here, again,
10:00
the largest Landslide in Latin America is represented with a bad rate. So, erosion and climate. And climate is not bad for this region, for the largest Landslide in Latin America and then earthquake.
10:21
Afterwards, you combine all these variables, so these eight variables are combined and they accept normal distribution and then there are five categories defined between very high and very low hazard level. And here this means that this part
10:45
has a very high hazard level. The second methodology is a little bit more complex because it's a probabilistic methodology and then the basic principle says that
11:02
in this methodology, a family of measurements of heights in meters and the corresponding lengths in meters or vertical gradient lines in a natural high slope of homogeneous composition and origin are linked by this equation. So the reason is only that the heights
11:21
and lengths in a high slope are related with this equation. But important to see here is that these regions should be homogeneous and the composition should be homogeneous and the origin should be homogeneous also. Then for applying this equation,
11:42
it was used the hydrological modeling. And for doing this, grass was used because it was necessary to define small watersheds. So if you had here a comparison between the results of commercial sulfur
12:01
and here for example you see that the watershed doesn't follow the natural landforms of the terrain because it's completely horizontal. But if you see these watersheds in red, they are much better. Then I use this grass sulfur
12:24
to define these watersheds and then afterwards it was necessary to find the height and length and this is like this, that you define these families like I already showed here. So families of slopes like here.
12:40
So many different groups of slopes which have different size. So here this is the family and this is an envelope and here the methodology says
13:00
that you have to find the distance between this point and this point, this point and this point, this point and this point and so on for the first contour line and afterwards you have to find the distance between this and the next contour line and so on until the whole family is covered. So this is not easy to find to do it manually.
13:23
It would be horrible, cumbersome and the probability of making mistakes would be high. So it was necessary then developed in Visual Basic in Excel and then here you see a table with the coordinates in X and Y
13:42
and afterwards the table, sorry that I left this in Spanish and then you have the average, the maximum and the minimum for each contour line like I already explained. Afterwards was necessary the geotechnical model in the line. We're not explained here
14:01
and the results gelled the safety factor and these categories were defined like in the previous methodology using a normal distribution. The contrasting of maps then you have the two partial results for the first methodologies and then it was used this matrix
14:23
to say what is the result when the natural is loss methodology and semi-quantitative methodology and then with many, many combination of these maps these two partial results but like you know
14:44
a model without calibration is not good enough. So there was an inventory of the processes and these processes says how a landslide, the landslide that had already occurred in Bogota and then you have this table
15:01
where if the process is active the probability is very high and so on. So for calibrating this map was these processes were used. Instruments are another input for this map
15:22
and in this study the landslide of each block was studied and was included afterwards in the map. So at the end you have the components of the maps so the first result is the combination of the two methodologies and then the processes
15:41
the instruments and then you have the landslide hazard map of Bogota and the processes are the most important because in these processes like I already explained you already know the landslide the hazard and then for example if the methodology here says that the hazard is very low
16:02
but the processes says the hazard is very high the result would be very high. So far unused cases so there were many many issues because the amount of information
16:21
was big, was not so easy to deal with this information so for example to merge small areas to adjacent areas was used with clean vector factor with remove small areas or to erase polygons was used was solved with overlapping vector map not operator
16:41
obtained a digital elevation model from contour lines was solved with interpolating surfaces a combination of the polygons it was solved with overlapping map of course most of these processes you can solve with a commercial software but for example with this one
17:00
when you use RGs for doing this you have to do it three times and then the time that you spend for doing these processes is bigger than this with grass and for example for this one erase polygons is a simple process but you need the most expensive license of RGs for doing it
17:22
for smoothing was also used grass and for topology fixing was used of course grass the impact and results then there is the map
17:42
of landslide hazard in Bogota and here you see that 64% of the area is in medium hazard it doesn't mean that Bogota is so dangerous you can visit Bogota so from this map
18:03
new suitable areas for housing projects can be defined so this is a very important input in the planification of the city the success of risk management depends on continuous monitoring with this map it doesn't mean that there is something fixed
18:21
because there is very dynamic the situation in Bogota like I already explained and updated such a map playing an important role in the risk management this is the first input for prevention and do you remember this? the largest
18:41
landslide in urban areas in Latin America they made a wall of about 200 meters and when necessary then the relocation of 3000 families the conclusion is that in processing of large volumes
19:02
of geographical information different alternatives should be explored when you find large volumes of geographic information grass is very efficient a model is a simplification of the reality
19:22
that means that there will be variables that they will not take into account and that's why it's necessary that more specific studies are executed validation of the map was used with the processes like I already explained because in these processes the hazard was already known
19:44
so that was my presentation
20:01
thank you very much for your presentation I have one question you said that you need land cover data, land use data for your model
20:22
you need land use data for your model where did you take the data from from which sources do you have this data? they were a combination of different things but mainly was from Google Earth ok
20:42
so it was from remote sensing and there was also an input from an entity but the resolution was not so good so it was not so detailed because we need a scale of 5,000 so mainly Google Earth yeah I'm asking because I think it's a very interesting
21:00
development now we had yesterday this session about code and Copernicus data and it's also an open source approach and I think in the future we can use open source GIS and open source remote sensing data to get data like for your problems about land cover, about DEM and so on
21:23
thanks you told us about how you validated your results from your model how the validation
21:42
how were the results I didn't understand say it again you validated the model using real life data correct there is a model yes yeah you built the model and then you validated the model using the real life data I validated the model with the processes
22:00
how were the results of the validation how was the results the results of if the model was correct validated you mean yes there were many many different approaches because they used the results of the previous study so the previous study was 20
22:21
years ago the map that we have was not so update was not so current and then we use many approaches until we find the best one and the approach is very reliable model
22:41
this one
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