Basin Flooding/Sea Level Rise
Accessed with "Calculate, Drainage, Flood basin" menu choice.
- Flood basin from single point:
pick a point and a reservoir top level;
all points below that level will be flooded and shown in blue.
- Use file with starting locations: specify point shapefile or dBase table (Create
locations from which flooding will start. This allows you
to deal with bridges, which act as dams in 2.5 D LIDAR DEMs, and the
fact that there might be unconnected patches of water of
different sides of the land.
After you pick the point to start flooding from (or a shape file with
multiple points), you will get the form below
which lets you select options and parameters.
If you want to export this to a KML file, you must use a lat/long DEM. You
can Reinterpolate a
UTM DEM to lat/long:
These require that depths have negative z values. You can quickly
edit the DEM to multiply the depths by -1.
- Reservoir top: flood up to this level (only level used for the
- Low level: for the movie option, the first level to depict.
- Interval: amount to increase each frame of the movie.
- Label maps: put the water level label on each map.
- HTML: copy the results of the Complex algorithm in the
data grid into an HTML file.
- Flood: flood a single water level. The value will be in
the "Reservoir top" edit box.
- Movie: create animation with a variety of reservoir
levels. Frames are saved with the format selected on
tab of options form,
- Start points: if you have a high resolution LIDAR image, you
can start flooding from a number of points because bridges will act
like dams. Select a point database file, which you can
create with the Create
shapefile option (point shape file) on Annotate Map icon of map
This choice only applies to a DEM map. If you use an image, the
Flooding algorithm will be used.
- Elevations: color codes elevations above
the flood level green, and those below in blue. Points (like
Death Valley) will "flood" even if they have no path to the water.
- Flooding: uses a fast graphical flood fill
operation to rapidly find all points with a path to the
water. This uses the current screen resolution. For
maximum accuracy, you should be at 1:1
map view with the
DEM, having a screen pixel size that is equal to or less than then
- create a white bitmap the same size as the map
- set all points about the flood elevation to green
- from all the starting points (one per point in the water
that is separated by a bridge or peninsula from the others),
flood fill to a green boundary, with blue fill
- turn all the green to white, as those points are above the
- in blue the bitmap now has all the flooded points, which are
below the top of the flooded level, and which have a clear path
to the water
- Complex: methodically computes all points
with a path to the water. It will be slow, but uses all
points, and produces the table seen below.
Table from complex algorithm.
Save using HTML button
||Movie showing flooding of downtown Annapolis with
For this option, set:
- Low level (m), for the water at the start of the animation.
- Reservoir top (m), for the final water level.
- Interval, the value in meters between frames in the animation.
There are some anomalies present due to the 2.5D nature of DEMs; bridges
and some other features will block flooding, so you should use multiple
Larger example on web, and a KML model.
- Does not consider friction, and slopes to the water
surface, which would apply in rivers.
- Does not consider breaks in seawalls not captured by
the DEM, or water backing up storm drains.
- Assumes there is sufficient water available to maintain the
water level as it floods onto the land.
- Carefully consider the Vertical datums.
LIDAR DEMs are most likely to use NAVD88, and tide gauges and
storm surge are likely to use MLLW or more recently MSL.
Last revision 9/11/2018