Solid obstacles of the size of the falling block cause a strong loss of energy, as the impact reaction with such an obstacle takes place at an blunt angle. In practice, block scree heaps or torrent channels, for example, form zones of increased roughness.
The roughness acts directly on the movement component parallel to the slope. This means that the roughness has a greater influence on a uniformly inclined slope, where the movement of the block takes place mainly in flat jumps parallel to the slope, than on a profile with large inclination changes, where higher jumps and consequently larger impact angles result.
The roughness is calculated in the model with an empirical approach. Depending on the size ratio between the falling block, the size and number of roughness components (stones, blocks, deadwood, terrain structures such as gutters, ditches), a defined percentage of the translational energy is subtracted for each impact on the ground. The roughness number is specifically modified within the program so that the roughness is weighted less for large crash blocks than for small blocks/stones.
ROFMOD calculates with empirical roughness numbers between 1 and 19. These are defined by the size and the density of the roughness components:
The value 1 means a low roughness, which is achieved with small stones lying far apart and the maximum roughness number of 19 is chosen for large blocks lying close to each other.
It should be noted that the roughness components lie on the terrain or on the profile. They should not be confused with the modelled fall blocks. The roughness number can be read from the following table. In ROFMOD 5.0 a roughness calculator is implemented, which determines the roughness number from diameter and distance of the components (obstacles).
|
Distance between obstacles [m] |
Diameter of obstacles [m] |
||||||||||
|
|
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
1.0 |
1.2 |
1.6 |
2.0 |
|
|
10.0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
|
8.0 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
|
|
6.0 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
|
|
4.0 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
|
|
3.0 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|
|
2.5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
|
|
2.0 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
|
|
1.5 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
|
|
1.0 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
|
|
0.5 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
|
Important note:
The roughness should be understood as the "micro topography" of the terrain surface, which is not represented in the digital terrain profile. This is the case if the grid width of the profile (distance between the profile points) is greater than the dimensions of the roughness structures.
The point density of a profile and the roughness values used must therefore be matched to each other:
•In the case that a detailed profile (e.g. export from a high-resolution DTM) is modelled for a slope profile, rather low roughness values should be defined because the roughness is already included in the profile definition.
•For profiles, which are defined with few support points, rather high roughness numbers should be used; the topography between two 'distant' profile points is usually more or less 'structured', which should be recorded with the roughness value.
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