Hi all,
We have been assessing several stream crossings that appear to have been affected by last fall's flooding events. This has caused deposits to buildup TALLER than bankfull heights, and has created bankfulls that appear to be "new" or not natural.
We are wondering if we should be taking bankfulls where the "new" banks are (Where deposits have settled on top of original banks). Or if they should be taken where the original banks were (if that can be determined).
Photo 1: If you look closely, you can see stream bed material dropped on top of eroding banks. More dense deposit photo right.
Photo 2: Original bank extends into photo right eventually to stream. As you follow bank downstream, you can see that it disappears underneath a deposit . Used vegetation growth to guess where original bank likely located.
Good question.
You will still want to take BFW measurements at the bank height indicated by the original banks, whenever possible. As far as I can determine from the photos, you are correctly identifying that deposits have built up (during high flood flows) on the top of banks, in other words, built up on the floodplain neighboring the channel that begins at the top of the bank and extends away from the channel.
These deposits will form during floods that leave the channel and enter / flow onto floods that are larger than the channel forming flows that generate the typical bankfull indicators.
I'm going to pick apart the first picture to demonstrate what I would try to do in such a situation:
Looking at the photo above, focusing on the yellow lines, there seems to be evidence of recent channel forming erosion marked by the yellow lines at points A and B. At A we see the edge of the top of the roots and that this lines up well with a sharper (as far as I can tell from the photo) slope in the bank at point B. These two yellow lines also match (not perfectly, it never will) the top of the deposit point out in location C, which seems to be more of a normal deposit not necessarily related to flood flows (though I could be wrong. The top of normal channel deposits (not those caused by flood flows moving stream bed substrate onto floodplains) are often considered the minimum starting height of your possible bankfull height.
So, yes, try and take the bankfull widths at the height that is indicated by the 'normal' bankful heights, ignoring deposits on neighboring floodplains (where it is obvious that's what they are) and whenever possible attempting to identify the normal bankful indicators through or next two the floodplain deposits. It is tricky though, especially in the first 1-2 years after such floods, since normal channel erosion takes at a minimum that amount of time to begin reshaping the channel again.
Please feel free to ask any follow-up questions.
Tom