WMS:Smooth GSSHA Streams

WMS uses a conceptual model approach to building GSSHA simulations that allows for the use of GIS data objects. In WMS stream arcs are used to define both the spatial extent as well as the hydraulic characteristics of the streams for a GSSHA simulation. While the underlying process producing the stream flow in GSSHA has been significantly updated the process of creating the streams in WMS has not been significantly altered, only simplified. In WMS 6.1 a set of stream arcs were either created from a DEM or read in from a GIS shapefile. These arcs then had to be manipulated spatially to follow certain rules in order to allow WMS to correctly generate the GSSHA channel input file. In WMS 7.0 the stream arcs no longer need to be spatially manipulated. However, the streambed profile still needs to be checked and corrected using the Smooth GSSHA Streams dialog.

Stream Smoothing

An essential part of building the stream network is making sure that the streambed will flow as it should. Obtaining the stream network from a DEM leaves many artificial ridges and pits in the streambed profile that force water to pond. Often when the stream network is brought in from a shapefile the streams have been digitized from a topographical map and the streambed is lacking elevation values. The place to check for and solve these problems is in the Smooth GSSHA Streams dialog.

The smooth GSSHA streams dialog shows a profile of the bed elevations, allowing the individual vertices along the arc to be manipulated. There are five means to manipulate the streambed elevations in the Smooth GSSHA Streams dialog. The first way to manipulate the streambed elevations is by selecting a vertex (using the select vertex tool) and then adjusting the streambed elevation value in the stream elevation edit field. The second means of modifying the streambed elevations also uses the select vertex tool; the select vertex tool can also drag the selected vertex along a vertical line to adjust the elevation. Third, the Offset Stream Elevations By Constant button will ask for a value by which to offset all of the streambed elevations. Positive offset values are downward. The Smooth GSSHA Streams dialog will not allow the surface elevations to be higher than the cell surface elevations. The cell surface elevations are shown in a step-wise fashion along with the streambed profile. If the cell elevations are not visible the most likely cause is that the streambed elevations are too close to the cell surface elevations so that the points and lines representing the streambed profile are drawn on top of the cell surface elevations.

The two smoothing options are the principal means of modifying the streambed profile to allow water to flow, as it naturally should. The fourth method of modifying the streambed profile is by using the Smooth Stream Elevations To Smoothed Grid button. When this button is pushed a warning comes up explaining that the stream arc is about to be dramatically altered. What will occur is that the stream arc will be conformed to lie on the surface of the grid. To accommodate the interpolated smoothness of the grid many extra vertices will be added to the arc at locations where the grid changes slope. All of these extra vertices make manually manipulating the stream arc much more difficult and hence the reason for the warning message. The arc will not change spatially; the new vertex (x,y) locations are linearly interpolated from the existing vertices.

The final method of modifying the streambed elevation, the Interpolate Stream Elevations button, was implemented in WMS 6.1 and is a modified form of the stream-smoothing algorithm proposed by Ogden, Saghafian, and Krajewski. [Ogden et. al 1994] The modification constrains junction nodes to be lower than the next upstream vertex on the offshoot branch. The yellow dots on the streambed profile are these elevations, the elevations of the next upstream vertex of each stream arc attached to the stream arcs being displayed.

Stream Arc Topology Rules

There are still several topological rules for stream arc connectivity that must be adhered to in order produce a valid channel input file. The stream arcs must still be connected from downstream to upstream, and there should not be any braids or multiple downstream channels from an upstream reach. New for GSSHA 2.0 is that there may be more than two upstream reaches from any particular reach (e.g. three reaches may converge to become a single reach).

Related Topics

• Embankment Arcs
• GSSHA Feature Arcs