bank stations

[Janet C]

I often encounter the situation where a channel has a low bank that is inundated on a relatively frequent basis and a high bank or terrace on the other side that is rarely or never flooded. This occurs where rivers migrate and come into contact with steep valley walls or where levees line only one side of a river.

On the low bank, the selection of the bank station is at the slope break beyond which water accesses the floodplain. On the high bank or terrace side, what makes sense? Locate both banks at the same elevation (where we typically start to see perennial woody vegetation)? Or at the slope break on the high bank or terrace where stage doesn’t reach?

I ask this question also for very incised urban streams that rarely flood.

From a computational perspective, HEC-RAS computes and tracks discharge for the channel, left overbank and right overbank. Is there a logical approach to this problem based on how HEC-RAS computes stage and discharge?

Any insight would be appreciated

Thank you!

[Anonymous]

This is a question I also grapple with. I have had discussion with experienced engineers and even with a full-time instructor for HEC-RAS classes, but never received a definitive answer. Some helpful points I have heard:

1. From an instructor: your channel stations should not be determined by changes in n-value. They should be determined by whether the wetted surface affects the flow in the main channel (ie. is somewhat vertical and exerts friction “sideways”.) If it is only affecting the flow directly above, then it is a floodplain. The same instructor also said you could have your bank stations at the very beginning and end of your cross sections if necessary.

2. Run approximately a 2-year flow and use that as a relative reference when selecting bank stations. For banks that continue to be steep, but never flood, just put that bank station close to the same elevation as the other side with a well-defined bank. Simple as that. Add an extra interpolated point if necessary to represent your bank. I would use this strategy with caution and be prepared to defend your decision, because it definitely does not always “look” right in graphical view.

My conclusion is that there is no absolute answer that will fit every situation. Also, I have decided that the following rules have exceptions:

1. Flowpaths must run outside channel banks. Case in point: a vertical bank.
2. Floodways shall never be inside channel banks. Case in point: any lake or pond.

In order to keep these types of rules, bank stations would necessarily have to be in illogical places.

With a 1-D modeling software like RAS, there will always always be a compromise somewhere. Hope that helps.

[huesmann]

I would agree with Randy’s point 1, above. On the high side, if the channel n-value is the same all the way up, using a high bank elevation makes sense.

You do have the option of horizontally varying n-values too…

[Author]

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