Difference between revisions of "Embankments:Embankment arcs in GSSHA"

Latest revision as of 20:56, 21 June 2008

Often embankment features are smaller (width-wise) than a grid cell and are poorly represented on a coarse or even fine 2D grid of elevations. In WMS, the embankments are represented as feature arcs and are given profile elevations that define the top of the embankment. These feature arcs are then mapped to the nearest cell edge. The embankment acts all along the edge, interrupting flow between the two adjacent cells.

In GSSHA, the embankments are represented as walls between cells. Once the water elevation in the cell reaches the lowest point of the embankment elevation profile that is in that cell, the embankment begins to act like a broad-crested weir. When the embankment acts as a wall, the water in the cells behind the embankment is stored and/or routed along the embankment to the nearest point of flow by the overland flow code.

Figure 25. The embankment arcs are represented in GSSHA as a series of walls between cells. The walls act as weirs when the water surface elevation overtops the embankment profile.

Low Points

Low points on the embankment arc should be set to be feature nodes and the low point attribute should be set for them. Once the water elevation has risen to this level then water will commence flowing over the embankment in a horizontal broad-crested weir fashion. A toggle box beneath the feature node type selector in the Feature Point/Node Type dialog sets the low point attribute. The elevation of the low point is set in the corresponding edit field. As an embankment acts as a horizontal broad-crested weir in the event of overtopping, the Cell Edge Length Fraction edit field is to designate a portion of the cell edge width as the weir width and ranges from 0.0 to 1.0. The feature node type may be set to any type desired and does not affect the status of the low point.

Figure 26.

The low points of embankment arcs are specified by making a feature vertex into a feature node and then checking the low point toggle box.

Related Topics

GSSHA Wiki Main Page
Primer Main Page

Embankments

Embankment arcs in WMS

Embankment arcs in GSSHA

Summary