DESCRIPTION

The module fills a lake to a target water level from a given start point. The user can think of it as r.grow with additional checks for elevation. The resulting raster map contains cells with values representing lake depth and NULL for all other cells beyond the lake. Lake depth is reported relative to specified water level (specified level = 0 depth).

This module uses a 3x3 moving window approach to find all cells that match three criteria and to define the lake:

cells are below the specified elevation (i.e., water level);
cells are connected with an initial cell (seed or E,N value);
cells are not NULL or masked.

The water level must be in DEM units.

NOTES

The seed (starting) point can be a raster map with at least one cell value greater than zero, or a seed point can be specified as an E, N coordinate pair. If the seed is specified as a coordinate pair, an additional check is done to make sure that the target water level is above the level of the DEM. When a raster map is used as a seed, however, no such checks are done. Specifying a target water level below surface represented by DEM will result in an empty map. Note: a raster lake map created in a previous run can also be used as a seed map for a new run to simulate rising water levels.

The module will create a new map (lake=foo) or can be set to replace the input (seed=bar) map if the -o flag is used. The user can use -o flag to create animations of rising water level without producing a separate map for each frame. An initial seed map must be created to start the sequence, and will be overwritten during subsequent runs with resulting water levels maps (i.e., a single file serves for both input and output).

Negative output (the -n flag) is useful for visualisations in NVIZ. It equals the mapcalc's expression "negative = 0 - positive".

MAPCALC EQUIVALENT - FOR GRASS HACKERS

This module was initially created as a script using r.mapcalc. This had some limitations - it was slow and no checks where done to find out required iteration count. The shell script code (using r.mapcalc) used in the original script is shown below:

${seedmap} = if( ${dem}, \
if( if( isnull(${seedmap}),0,${seedmap}>0), ${wlevel}-${dem}, \
 if( \
  if(isnull(${seedmap}[-1,0]),0, ${seedmap}[-1,0]>0 && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[-1,1]),0, ${seedmap}[-1,1]>0 && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[0,1]), 0, ${seedmap}[0,1]>0  && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[1,1]), 0, ${seedmap}[1,1]>0  && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[1,0]), 0, ${seedmap}[1,0]>0  && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[1,-1]),0, ${seedmap}[1,-1]>0 && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[0,-1]),0, ${seedmap}[0,-1]>0 && ${wlevel}>${dem}) ||\
  if(isnull(${seedmap}[-1,-1]),0, ${seedmap}[-1,-1]>0 && ${wlevel}>${dem}),\
 ${wlevel}-${dem}, null() )))

The ${seedmap} variable is replaced by seed map names, ${dem} with DEM map name, and ${wlevel} with target water level. To get single water level, this code block is called with same level numerous times (in a loop) as the lake grows by single cells during single run.

BUGS/KNOWN ISSUES

The entire map is loaded into RAM.
The module is NOT large file safe. (due to the previous point)
A completely negative seed map will not work! At least one cell must have a value > 0. Output from r.lake -n can NOT be used as input in the next run.

EXAMPLE

Example of small flooding along a street (North Carolina sample dataset):

g.region rast=elev_lid792_1m -p

# water accumulation next to street dam
r.lake elev_lid792_1m xy=638759.3,220264.1 wl=113.4 lake=flooding

# draw resulting lake map over shaded terrain map
r.shaded.relief elev_lid792_1m
d.rast elev_lid792_1m.shade
d.rast -o flooding
d.vect streets_wake 
d.vect bldg_cmcl_pol type=area

Small flooding along a street (r.lake, using Lidar 1m DEM)

AUTHOR

Maris Nartiss (maris.nartiss gmail.com)