Pilot channels are an option that was added for unsteady flow modeling. Occasionally, when modeling low flows (such as at the beginning or end of a storm event), the program will go unstable. This instability can occur for many reasons. The following is a list of some of the main causes for instabilities at low flows:

  1. At low flows the depths are very small. As the flood wave begins to come into the reach, the depths change dramatically percentage wise. Unsteady flow models use derivatives that are based on the change in depth with respect to time and distance. If the depth changes significantly during any time step, the derivatives can become very large, and oscillations will occur. These oscillations can grow to the point where the solution becomes unstable.
  2. Also during low flows, it is much more likely that your river may be flowing in a pool and riffle sequence. At the riffles, the flow may be passing through critical depth and going supercritical. By default, the unsteady flow solver in HEC-RAS cannot handle flows going down to or passing through critical depth (unless the mixed flow option is turned on). This again causes instabilities in the solution, and may eventually cause the solution to go unstable.

Pilot channels are one of the available options to help prevent the model from going unstable. A pilot channel cuts a rectangular notch into the bottom of the cross section. Generally this notch is not very wide (often 1 ft is used), but it provides depth to the cross section at low flows (typically make it 5 to 10 feet deep). Additionally, the use of a pilot channel can smooth-out irregularities in the channel bottom. This also helps the stability of the model solution. The pilot channel area and conveyance are barrowed from the lower portion of the main channel, such that the total area and conveyance properties of the cross section relate to the original cross section at higher flows. In other words, when the depth of flow gets higher, the area and conveyance of the pilot channel are ignored. To use the pilot channel option, select Pilot Channel from the Tools menu of the geometric data editor. When the pilot channel option is selected a window will appear as shown in Figure 5-89.

Figure 5 89. Pilot Channel Editor

As shown in Figure 5-89 the user selects a river, reach, and range of river stations to apply the pilot channel too. On the lower left hand side of the form are some utilities to enter the pilot channel information. The user enters the pilot channel width (typically the width should be narrow), and the Manning's n value (should be equal to or higher than the main channel n value). The user can either enter an elevation for the invert of the pilot channel and project it on a slope over the range of cross sections, or they can enter an upstream and a downstream invert elevation and have the program use linear interpolation for the cross sections in between. A list of the final pilot channel values for each of the cross sections is shown in the table on the lower right hand side of the editor. The user can modify the table directly and change any value on a cross section-by-cross section basis. The profile plot on the editor will display the invert elevation of the pilot so you can compare it to the actual channel invert. Once you have finished adding the pilot channel information, press the OK button, and then save the geometric data.