r.avalanche User manual and model outline

r.avalanche User manual and model outline
r.avalanche
Version 2012-02-08
A model for simulating the motion of granular flows (debris
flows and snow avalanches)
based on GRASS
GIS
User manual and model outline
by Martin Mergili
Institute of Applied Geology, BOKU University of Natural Resources and Life Sciences Vienna, Austria
[email protected]
www.mergili.at
February 2012
2 r.avalanche
r.avalanche is a GIS-supported model framework for
simulating the motion of granular flows like debris
flows or snow avalanches over inclined surfaces.
r.avalanche is designed as raster module for the software GRASS GIS. The scientific concepts behind
r.avalanche are summarized in Chapter 1 (Model outline).
In contrast to most of the other GRASS raster modules, management of the data and the parameters (input and output) is not done by adding parameters to
the r.avalanche command, but by an additional shell
script (r.avalanche.sh) with various functionalities,
including input of data and parameters as well as
post-processing and display of the results. Instructions how to operate the model framework are given
in Chapter 2 (User’s manual).
The model shows a large potential for refinements.
Chapter 3 (Prospected improvements) will give a
short overviev of ongoing and prospected further development of r.avalanche, regarding the scientific
concepts behind it as well as its ease of use.
Every user is encouraged to report encountered bugs
or errors to
[email protected]
Furthermore, the developer would be grateful for receiving comments regarding
•
experiences with the program, shortcomings,
recommendations for improvements (scientific
concepts, ease of use);
•
parameters chosen for certain study areas;
•
interest in cooperation with application and further development.
The model, as applicable with GRASS, is running under the GNU General Public License (www.gnu.org).
r.avalanche has been created with the purpose to be
useful for modelling of debris flows. It has been developed with care, and much emphasis has been put
on ensuring its scientific value. Nevertheless, every
user has to be aware that it is only a computer program created by a human being, which may contain
technical and topical errors and shortcomings. No
responsibility can be taken by the developer for any
types of deficiencies in the program or in this document, or for the consequences of such deficiencies.
___________________________________________________________________________
1 Model outline 3
1.1 General aspects 3
1.2 Model layout 3
1.3 Implementation into GRASS GIS 4
1.3.1 Dimensionalization of the variables 4
1.3.2 Adaptation of the coordinate system 4
1.4 Acknowledgements 5
1.5 References 5
2 User’s manual 6
2.1 System requirements 6
2.2 Test dataset 6
2.3 File management 6
2.4 Installation 6
2.5 Data management 7
2.5.1 Parameter and data input 7
2.5.2 Execution of simulations 7
2.5.3 Post-processing of model output 7
2.5.4 Display of results 8
2.5.5 Removal of result files 8
2.5.6 Cleaning of file system 8
2.5.7 Exit 8
3 Prospected improvements 9
3.1 Scientific concepts 9
3.2 Ease of use 9
Model outline 3
1 Model outline
1.1 General aspects
r.avalanche constitutes a fully deterministic model for
the motion of granular flows (suitable for debris
flows, flow avalanches, and other types of movements). The model builds on a solution of the Savage-Hutter (SH) model for simple concave topographies with an only vertically curved flow line, running
out into a horizontal plane (Wang et al. 2004; Figure 1). This means that r.avalanche is only suitable
for relatively simple terrain with straight channels. Future development, however, shall be directed towards
overcoming this problem (compare Chapter 3).
A curvilinear coordinate system is used instead of a
simple rectangular system (compare Figure 1).
where ζ is the downslope inclination angle of the reference surface, κ is the local curvature of the reference surface, b is the elevation above the reference
surface, and ε and λ are factors (compare below). β x
and β y are defined as
β x = ε cos ςK x
Eq. 6,
β y = ε cos ςK y
Eq. 7.
K x and K y are the earth pressure coefficients in
downslope and cross-slope directions.
Eq. 8 top
K y , act / pass =
1.2 Model layout
r.avalanche requires far less input than r.debrisflow as
it only simulates one part of the process. In contrast,
the
mathematical-technical
concept
behind
r.avalanche is much more complex. A full description
of the way how the SH model was solved for this
specific topographic situation would go beyond the
scope of this study – a detailed description is given by
Wang et al. (2004). The most fundamental aspects are
summarized below.
Eq. 1 to 3 are the basis of the SH model:
∂h ∂
∂
+ (hu ) + (hv ) = 0
∂t ∂x
∂y
)
(
K x , act / pass = 2 1  1 − cos2 ϕ cos2 δ sec2 ϕ − 1 ,
Eq. 9 bottom
1
 Kx +1
2
(K x − 1)2 + 4 tan 2 δ  .

φ is the angle of internal friction, and δ is the bed friction angle. Active stress rates (subscript act) are connected to dilatation of the material, passive stress
rates are connected to compression – it depends on
acceleration or deceleration of the flow whether active or passive stress rates are valid (compare Wang
et al. 2004). Eq. 8 and 9 are only valid as long as the
flow moves predominantly in downslope direction.
z
Eq. 1,
y
z0
2

∂
(hu ) + ∂ hu 2 + ∂ (huv ) = hs x − ∂  β x h
∂t
∂x
∂y
∂x  2
( )

 ,

ζ
y0
x0
Eq. 2 top
Eq. 3 bottom
x
 β h2 
∂
(hv ) + ∂ (huv ) + ∂ hv 2 = hs y − ∂  y  .
∂t
∂x
∂y
∂y  2 
( )
h is the avalanche thickness, and u and v are the
depth-averaged downslope and cross-slope velocities.
s x and s y are the net driving accelerations:
sx = sin ς −
u
u +v
2
Eq. 4 top
sy = −
u +v
tan δ (cos ς + λκu 2 ) − ε cos ς
∂b ,
∂x
Eq. 5 bottom
u
2
2
2
tan δ (cos ς + λκu 2 ) − ε cos ς
∂b ,
∂y
talweg
Figure 1: The topography for which the solution of the
SH model used in r.avalanche was elaborated
(adapted from Wang et al. 2004)
The system of equations described above is only valid
for cohesionless and incompressible granular materials which can be considered as fluid continuum. It
has to be emphasized that all variables are dimensionless, meaning that the model is scale-invariant and
small-scale laboratory tests can be used as reference
for large-scale problems in nature.
The differential equations Eq. 1 to 3 were solved using a NOC (Non-Oscillatory Central Differencing)
scheme, a numerical scheme useful to avoid unphysical numerical oscillations. Cell averages are computed
4 r.avalanche
CFLx = max
∆xi
alli , j
CFLy =
(β x )i , j hi , j
ui , j +
[
max vi , j +
alli , j
Eq. 10,
(β )
h
y i, j i, j
]
∆y
Eq. 11,
The implementation of the SH model and its solution
by Wang et al. (2004) into GIS bears two problems:
(1) the solution provides dimensionless values – in
the GIS, however, it is necessary to use dimensional values;
(2) the solution is valid for a curvilinear reference
system only, in contrast to a GIS which usually
uses a simple rectangular system.
1.3.1 Dimensionalization of the variables
The first problem was solved using equations from
Pudasaini (2003). They are based on the variables L,
H, and R (compare Table 1). L is the typical avalanche length, H is the typical avalanche depth, and R
is the typical radius of curvature (all in meters). Dimensional variables are derived using
Eq. 16,
Eq. 18,
terrain
surface
reference
surface
z
until z 0,test =z 0,terrain :
z 0,test =x 0 *tan(90-ζ )
90-ζ
b

y = Ly

u = u gL

t =t L g
2
45
increasing or decreasing x 0
x0
3
b =(x 0 ²+z 0,test ²)
1.3 Implementation into GRASS GIS
Eq. 12,
Eq. 14,
z0
1
where ∆x and ∆y are the cell sizes in x and y direction
and i and j are the coordinates of the cells in x and y
direction, respectively. The length of one time step ∆t
has to be smaller than 0.5 times the minimum from
CFL x and CFL y . ∆t is determined dynamically while
running r.avalanche, based on the CFL condition
from the previous time step. Instead of 0.5, a factor
of 0.2 was chosen in order to add some tolerance.

x = Lx

h = Lh

v = v gL

κ = Rκ
where the variables denoted with a cap are the dimensional counterparts of the variables discussed
above and g is gravitational acceleration (9.81 m s-2 on
the earth surface). r.avalanche proved to work best
when setting x, y, and h to the dimensional values (in
m) while setting L, H, and R to 1. The factors
ε = L/H and λ = L/R (Pudasaini 2003) were therefore 1. This contradicts the statement by
Wang et al. (2004) that the factors should be far lower
than 1, but appears appropriate when using dimensional values from the beginning with h being much
smaller than the length of the flow.
z0,test
using a staggered grid, meaning that the system is
moved half of the cell size with every time step (the
values at the corners of the cells and in the middle of
the cells are computed alternatively). The numerical
scheme derived by Wang et al. (2004) was used for
r.avalanche.
The degree of diffusion of the flow material is governed by using slope limiters, restricting the gradients
of flow depth to a certain range. The so-called
minmod limiter has already been used by Wang et al.
(2004) and also for the present study since it is known
as the most diffusive one, reducing numerical oscillations.
The simulation is run for a number of time steps until
a certain break condition is fulfilled. The time steps
have to be kept short enough to fulfil the CFL
(Courant-Friedrichs-Levy) condition required for obtaining smooth solutions:
Eq. 13,
Eq. 15,
Eq. 17,
x
step of iteration
Figure 2: Iterative determination of b based on the coordinate system defined by the reference plane. Designed by M. Mergili.
1.3.2 Adaptation of the coordinate system
The solution of Wang et al. (2004) builds on a curvilinear coordinate system based on a horizontally
straight “talweg” which shall be the predominant flow
direction. Three steps are performed for converting
the original rectangular coordinate system in which
the input raster maps are provided into the coordinate system for the simulation:
(1) the coordinate system is rotated around the z
axis so that the expected predominant flow direction derived from the starting material and
the elevation model is aligned with the new x direction. This rotation is based on two userdefined pairs of coordinates in the flow channel
(compare Table 1);
(2) a reference surface is created, based on the defined “talweg” and an inclination of zero in y
(cross-slope) direction;
(3) based on this reference surface, the cell size ∆x
for each x parallel to the reference surface is
computed. The elevation b (m) – defined as the
distance between terrain surface and reference
surface perpendicular to the reference surface –
is derived. This has to be done iteratively, varying the horizontal shift until the tested value of b
converges with terrain height (Figure 2).
After completing the calculation, the entire system is
reconverted into the rectangular coordinate system
Model outline 5
used in the current GRASS GIS mapset in order to
enable a proper display of the results.
1.4 Acknowledgements
Funding was provided by the Tyrolean Science
Funds, the Vice Rector for Research of the University
of Innsbruck (“Doktoratsstipendium aus der Nachwuchsförderung der LFU”), and the Austrian
Academy of Sciences.
Furthermore, the valuable remarks of Wolfgang Fellin (Institute of Infrastructure, University of Innsbruck), Katharina Schratz, and Alexander Ostermann
(Institute of Mathematics, University of Innsbruck)
are acknowledged.
1.5 References
Pudasaini, S.P. 2003: Dynamics of Flow Avalanches Over Curved and Twisted Channels. Theory, Numerics and Experimental
Validation. Dissertation at the Technical
University of Darmstadt, Germany. 200 pp.
Wang, Y.; Hutter, K.; Pudasaini, S.P. (2004): The
Savage-Hutter theory: A system of partial
differential equations for avalanche flows of
snow, debris, and mud. ZAMM - Journal of
Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik 84:507-527
Table 1: Input information required for running r.avalanche.
map/parameter
catchment
elev
delta
riskobj
dinit
phi
L
H
R
deltatout
tmax
vmin
xchan1, ychan1
xchan2, ychan2
symbol (unit)
– (boolean)
z (m)
δ (degree)
– (boolean)
h 0 (m)
φ (degree)
L (m)
H (m)
R (m)
∆t out (s)
t max (s)
-1
v min (m s )
– (m)
– (m)
input format
raster map
raster map
raster map
raster map
raster map
value
value
value
value
value
value
value
values
values
description
catchment of interest map
elevation map
basal angle of friction map
objects at risk (1 = present, 0 = absent)
starting material map
angle of internal friction of the debris flow material
typical avalanche length (recommended: 1; compare text)
typical avalanche depth (recommended: 1; compare text)
typical radius of curvature (recommended: 1; compare text)
interval between writing output
time of simulation until it stops
velocity at maximum flow depth at which simulation stops
coordinates of first point in flow channel (for rotating coordinate system)
coordinates of second pt. in flow channel (for rotating coordinate system)
6 User’s manual
2 User’s manual
2.1 System requirements
r.avalanche was developed and tested under Fedora
Core 6 with GRASS 6.2.1. It probably works on most
other UNIX systems as well as with other versions of
GRASS. The module itself should also be usable under cygwin, but the related shell script r.avalanche.sh
(compare below) would probably not work.
Please make sure to have a proper installation of
GRASS before installing r.avalanche. In case of
doubt, please consult www.grass.itc.it.
•
main.c: the source code for r.avalanche;
•
r.avalanche.sh: a shell script facilitating data input, management, and output;
•
and install.sh: a shell script helping to compile
the source code (main.c).
data
It contains the parameter file (test_param.txt as part
of the test dataset). The subfolder /colors contains
colour tables for the display of the results.
temp
2.2 Test dataset
The temp directory contains temporary files created
during the execution of r.avalanche.sh. Its content
shall not be manipulated manually, but only using the
functionalities of r.avalanche.sh.
A test dataset is provided together with the program,
consisting of some text files and a GRASS location
with the name test_ravalanche. All data is packed in
the file test_ravalanche.zip.
Table 2 shows the names of the raster and vector
maps. The test dataset is suitable to run r.avalanche at
spatial resolutions of 5 m or 10 m.
It contains some simulation results (summary file,
profile file). However, the main results are stored as
rasters in the active GRASS mapset.
Table 2: Names of the maps of the test dataset
docs
name of the map
description
The docs directory contains this manual.
test_mask
test_elev
mask for study area (raster)
elevation map at 5 m resolution
(raster)
elevation map at 10 m resolution
bed friction angle (raster)
road potentially at risk (raster)
depth of flow initiation (raster)
observed patterns of flow initiation
(line vector)
observed patterns of deposit from
flow (line vector)
test_elev10
test_delta
test_road
test_dinitdef
test_init_observed
test_depd_observed
2.3 File management
The file system r.avalanche consists of two parts:
•
a GRASS mapset with all the spatially distributed
input information as raster or vector maps, and
•
a folder named r.avalanche which may be stored
at any location of your home directory. The internal structure of the folder r.avalanche may not be
manipulated, otherwise the functionalities are not
more given.
The directory r.avalanche contains the following subdirectories:
results
2.4 Installation
r.avalanche has to be added to the GRASS raster library as a new module, based on the source code of
the file main.c. For performing this task, call the
script install.sh in the folder r.avalanche/tools:
cd dir/r.avalanche/tools
sh install.sh
dir may be any location in your
following prompt is displayed:
home
directory. The
Full path to GRASS source (slashes at beginning and end):
Here, enter the path to the GRASS source, for example
/usr/local/src/grass64_release/
The Makefile is created, and compilation and installation are run automatically, so that r.avalanche is ready
to use.
Please note that you have to change to the tools directory as described above – if just entering
sh dir/r.avalanche/tools/install.sh
tools
The tools directory hosts the scripts required for installing and running r.avalanche:
an error message will display and r.avalanche will not
be installed.
User’s manual 7
2.5 Data management
r.avalanche uses text files and rasters with predefined
names as input. The shell script r.avalanche.sh serves
for creating these datasets.
r.avalanche.sh offers the following modules:
1
2
3
4
5
6
7
-->
-->
-->
-->
-->
-->
-->
Parameter and data input
Execution of simulations
Post-processing of model output
Display of results
Removal of result files
Cleaning of file system
Exit
When entering a number, the corresponding module
is executed. r.avalanche.sh has to be called from
within the GRASS mapset with all the required raster
maps.
2.5.1 Parameter and data input
Module 1 consists of a sequence of prompts for input
of data and parameters (Table 1). If no input is given
for a prompt (by just pressing ENTER), the dataset
specified earlier is kept.
•
--> Catchment map (boolean):
Boolean raster map defining the catchment of interest (identified by cell values of 1, areas out of
the catchment are defined by 0 or no data).
•
--> Elevation map (m):
Raster map of elevation (meters).
•
results have shown that the chosen cell size may have
a considerable influence on the simulation results.
After specifying the cell size, the GRASS raster module r.avalanche is called by pressing ENTER. Additionally to an array of raster maps, a summary file
(summary.txt) and a profile file (profile.txt) are written
and stored in the
dir/r.avalanche/results/
directory. The summary file contains some variables
(time step length, maximum depth and velocity of the
flow, and flow volume) for each time step. The profile file contains a profile of flow depth along the
predefined flow channel for each time step.
Please note that if you wish to run r.avalanche manually, not from within r.avalanche.sh, you have to copy
the parameter file to the dir/r.avalanche/temp/ directory as param.txt. This file shall not contain labels,
but only the values.
2.5.3 Post-processing of model output
All the resulting raster maps are cleaned (cells outside
of the defined catchment are set to no data) and a
color table is assigned to the flow depth map for each
time step. The colour tables are stored in
dir/r.avalanche/data.
Furthermore, three prompts do appear when calling
the module. Each task is accepted by typing 1, or denied by typing 0.
•
--> Bed friction angle map:
The flow depth which should serve as maximum
for colouring the maps to be displayed (compare
below) has to be specified in full centimetres.
Raster map of bed friction angle (decimal degrees).
•
--> Objects at risk map (boolean):
•
Boolean raster map denoting objects at risk (1 for
presence of potential objects at risk like roads or
buildings, else 0 or no data).
•
•
--> Parameter file:
File with list of parameters for the simulation.
The parameters have to be specified in a defined
order (compare Table 1 and test_param.txt).
2.5.2 Execution of simulations
A prompt for cell size displays:
--> Cell size (m):
The cell size has to be chosen in accordance with the
input datasets and the required level of detail. For test
simulations it is recommended to choose larger cell
sizes in order to reduce computing time. However,
--> Interval of contour lines for display
(m):
Contour lines of flow depth (not of ground elevation!) are shown on the maps during display
(compare below). The interval between these
contours has to be specified here.
--> Estimated depth of mobilization of
soil map (m):
Raster map showing the patterns of estimated initiation of the debris flow or snow avalanche
(depth in meters).
--> Maximum flow depth for display (cm):
•
--> Export maps to ascii (boolean) ?
If 1 is entered, the resulting raster maps are exported as ascii rasters in order to be usable with
other GIS software products. The resulting files
are stored in dir/r.avalanche /results/asc.
Please enter 0 if you do not wish to export the
maps.
In the output rasters, the displays during program execution, and in the summary and documentation
files, every variable is addressed by a shortcut (Table 3). The names of the resulting raster maps have
the prefix r_. The number at the end of the raster
map names indicates the time step. For example, the
raster r_dflow0020 shows the flow depth at the end of
time step 20. Attention: These time steps are different
from those used for the computation itself (compare
Chapter 1).
8 Appendix 3: Manuals
2.5.4 Display of results
A sequence of flow depth maps over all time steps
can be displayed using this module. The following
parameters have to be specified:
•
--> Azimuth of the sun for shaded relief
map:
All maps are displayed with a shaded relief as
background, the azimuth of which has to be
specified (in decimal degrees; recommended for
most areas: 315).
•
--> Interval for export of maps to jpg:
The displayed maps can be automatically stored
as jpg graphics in dir/r.avalanche/results
/jpg. If you wish to do so, please specify an integer number larger than 0, else 0. The number you
enter defines the interval of output – for example, if you enter 4, the map from every 4th time
step is exported.
•
--> Height of monitor in pixels:
Please specify the height of the monitor for display – a value between 500 and 800 is recommended, depending on the size of your monitor.
•
--> Observed patterns of debris flow initiation (line vector):
A line vector map with areas of debris flow initiation observed in the field may be specified, if
available, for facilitating the evaluation of the
model results.
--> Observed patterns of debris flow deposition (line
vector):
A line vector map with areas of debris flow deposits observed in the field may be specified, if
available, for facilitating evaluation of the model
results.
A monitor opens, and a prompt with instructions appears in the terminal. The maps are displayed in a defined sequence (over all time steps forward) – please
enter the number of steps to move forward or backward, or exit to quit. If you have moved a defined
number of steps fore- or backwards and would like to
apply the same action again, you can just press ENTER to do so.
If you have chosen to export the maps as jpg, no
prompts appear, but all maps are displayed and exported automatically.
Please note that the size of the monitor and the
placement of some of the elements of the maps (legend, bar scale) are not suitable for all map formats. It
may happen that some of the placement options have
to be changed in the shell script r.avalanche itself in
order to design satisfactory layouts.
The text on the display is partly copied from the summary.txt file in the dir/r.avalanche/results/ directory – please do not rename or remove this file
before the display of the maps is completed.
2.5.5 Removal of result files
All results (rasters and text files produced by
r.avalanche and Module 3) are deleted. All temporary
files created in Module 1 are kept, so that new simulations may be performed immediately.
2.5.6 Cleaning of file system
All temporary rasters and text files created in Module
1 are deleted. Module 1 must be re-run in order to
perform new simulations.
2.5.7 Exit
r.avalanche.sh is exited and the default cell size is re-
stored
Table 3: Output from r.avalanche (after running Module 4). r=raster map, s=summary file, p=profile file
shortcut
dflow
vx
vy
dflow_max
hmax
vmax
vol
vol_riskobj
deltat
tsum
cfl
description
flow depth
flow velocity in x direction
flow velocity in y direction
maximum flow depth over all time steps
maximum flow depth over all cells
maximum flow velocity over all cells
flow volume
flow volume on object at risk (road, etc.)
length of time step
time elapsed since start of simulation
Courant-Friedrichs-Levy condition (comp. Chapter 1)
unit
m
-1
ms
-1
ms
m
m
-1
ms
m³
m³
s
s
–
versions
r
s
x
x
x
x
x
x
x
x
x
x
x
time steps
p
x
all
all
all
–
all
all
all
all
all
all
all
Prospected improvements
9
3 Prospected improvements
3.1 Scientific concepts
This document describes Version 1.1 of r.avalanche.
It constitutes a first try to implement a fully deterministic model for the motion of granular flows into
Open Source GIS. Therefore it has been kept relatively simple and shows some limitations. It is
planned to improve the model by the following steps:
(1) to select and adapt a sound method for modelling rapid granular flows over arbitrary topography, in order to overcome the severe limitation
that the model can only be applied for relatively
simple topographies. The method shall be extended by incorporating particle entrainment,
which can have important implications for the
runout behaviour of granular flows, and and the
role of pore fluid. The differential formulation
of the model shall be derived analytically, using
and extending the existing theories;
(2) to devise an appropriate numerical scheme (including shock capturing) for solving the equations derived in (1). Numerical solutions of the
analytical model for arbitrary topography shall
be elaborated and implemented into GRASS
GIS, analogous to the present version of
r.avalanche;
(3) and to evaluate the quality of the developed approach by comparing it to existing methods and
models (DAN, SAMOS) and to validate the results with data from past snow avalanches and
debris flows.
An additional improvement would be to find a more
efficient way of data management during the computation in order to decrease computing time and to facilitate computations at higher spatial resolutions.
3.2 Ease of use
One of the next steps shall be to improve the quality
of display of the results (Module 4 of r.avalanche.sh).
In its current version, the operation of r.avalanche
runs very much on text files and the command line.
For the future it would be desirable to have a graphical user interface facilitating data management and at
least partly replacing r.avalanche.sh.
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