I have a cross-section with a channel and a floodplain and I get different answers when I put the Top of Bank Markers at the extreme ends of the cross-section compared to when I put them at the top of the channel. Shouldn't the water level in the channel be the same in each case?
The position of the Top of Bank Markers is very important when you have a well defined channel with wide "floodplain" areas on one or both sides. It is important in these situations because when the flow depth rises to just higher than the main channel, the wetted perimeter can suddenly increase dramatically for only a small additional increase in cross-sectional area, and the overall roughness or impedance to flow can also increase.
In these situations, if the Top of Bank Markers are located at the ends of the cross-section, the program calculates a flow at the top of the main channel (say X cumecs at depth Y), and, when it calculates the flow at the next incremental height (Y + dY), because of the increased wetted perimeter or roughness, the flow calculated can be less than X cumecs. However, if the channel could carry X cumecs at a depth of Y, clearly it would normally be capable of carrying more than that at a greater depth. By ensuring that the Top of Bank Markers are located at the point where the channel meets the floodplain, the calculated flow will increase from X at the depth of Y + dY. The Help within PC-Convey contains further information on this, including an example which uses two files that come with PC-Convey to show the difference.
Having said that, it is not always clear where the channel meets the floodplain, or where the “main channel” is for a road cross-section! As a guide, in general the Top of Bank Markers need to be placed at the location where the grade changes "substantially". It can help to view the cross-section using the 1:1 scale option to help determine where this occurs, and where the Top of Bank Markers should be located.