Floodplain Mapping is skipping about

If the floodplain mapping is skipping parts of the
river channel (see figure below), this can occur
due to a "false dam" being created when the digital
terrain is developed.

A false dam occurs during the floodplain delineation
tinning process when a TIN edge (one side of the TIN
triangle) straddles the natural channel, forming a dam
in the bottom of the channel.

A TIN is constructed by triangulating the vertices
that define the ground terrain. This vertice data is
extracted from whatever 3D data (contours, polylines,
points, existing TIN vertices, etc.) are defined as
layer information within the Configure Contour Layers
dialog box.

RMS' Floodplain Mapping Module connects the vertices
with a series of edges to form a network of triangles
to represent the terrain. The resulting triangulation
satisfies the Delauney criterion. The Delauney
criterion ensures that no vertices lie within the
interior of any circumcircles of the TIN triangles.

As the tinning triangulation process proceeds, adjacent
triangles are compared to see if they satisfy the
Delauney criterion. If necessary, the adjacent edge
of the two triangles is swapped (the diagonal of the
quadrilateral defined by the two triangles is changed
to the other two vertices) in order to satisfy the
Delauney criterion. If the Delauney criterion is
satisfied everywhere on the TIN, the miniumun interior
angle of all the triangles is maximized. The result
is that long thin triangles are avoided as much as
possible.

If there is not enough 3D data contained within the
channel insert, a false dam can be created by RMS
by using Delauney criteria. If this occurs, the
false dam can be easily eliminated by adding additional
3D break line data to represent the channel bottom in
the area of the skipping floodplain map.