A question we often get asked is does the presence of panel markers in a river reach section matter? Does it affect results? The answer to both is yes, particularly in the case of extended river sections to represent out-of-bank flow.
With a river reach section, a conveyance profile is generated based on the cross-section area and wetted perimeter of each vertex within the river section as shown by the equation below:-
Where K is the conveyance, A the area, n the Mannings’ n roughness value and P the wetted perimeter. This conveyance term is then used in the St Venant equations which determine the flows and depths within the river sections.
With a cross-section similar to the one Figure 2, once flow just overcomes the top of the river section bank, there is a large increase in wetted perimeter but a relatively small increase in cross-section area. Based on the equation in Figure 1, this can lead to the situation where conveyance decreases with increasing water levels…which isn’t realistic.
For example take the following river section, which shows a main channel and is extended to provide a representation of the floodplain for the out-of-bank flow. The extended river section contains no panel markers.
Within the ICM river reach, since version 6.5, it is possible to see the conveyance data for each river section. Without panel markers you can see the following behaviour where the conveyance reduces with increasing water levels, as shown by the kickback in the trace in Figure 4.
The conveyance should always increase monotonically. Panel markers should be added where there is a major break in slope (ie, at the transition between the main channel and the floodplain), and effectively split the river section into a series of panels, on which the conveyance term is calculated as per Figure 5.
In the original example they should be put at the top of the river banks in our extended section.
Conveyance is calculated for each panel and therefore ensures that conveyance increases with increasing stage. Figure 7 shows the conveyance table for the same section but this time with panel markers. There is no longer the kickback in the curve.
As conveyance is increasing with increasing stage, the results will be more accurate and the model more stable. Panel Markers should also be added at major changes in Mannings’ n roughness values, and are automatically added at the boundary of any existing roughness zones when the river sections are built.
When simulating river reaches in InfoWorks ICM, a single results value for flow and velocity is provided in the results for each river section. Where panel markers are present, and the sections have been split into panels, the Velocity Profile Report in the grid report can be used to provide flow, velocity and panel area for each of the panels within the river section.
So to summarise, panel markers are an important part of modelling river reaches, particularly where river sections are used to represent some aspect of out-of-bank flow, or where there are two-stage channels. The addition of panel markers should be used to ensure that the conveyance increases monotonically with increasing water level.