# Clarification of overbank flows

Apologies for what may be a very basic question, but I am struggling to understand the definition of overbank flow.

Overbank flow is defined along the lines of this: Water flow over the top of the bankfull channel onto the floodplain.

Does this flow include the flow that is in the channel? or is it only the flow that is over the channel?

I am working through a document that seems to say one thing with the words, and a different thing with the mathematics.

If Qf = overbank flow; Qb = bankfull flow:

Is the total flow = Qf, where Qf >= Qb so if Qf = Qb all flow is in the channel

Or, is total flow = Qf + Qb, flow in channel plus flow out of channel?

Overbank flows means flows that are greater than the channel can handle. Thus the water flow goes over the (river/stream) banks and onto the flood plain.

Bankfull flow would be the maximum water flow that the channel could contain.

Note that flow in the channel isn't necessarily the bankfull flow. Of course if there is overbank flow then there must be bankfull flow.

total flow = Flow in channel + Overbank flow

• I am trying to understand whether that includes or excludes the flow in the channel. If you are given a value for overbank flow, do you need to add in the bankfull flow to get the total flow? Jun 19 '18 at 3:53
• @Esme_ Exactly that yes.
– Ash
Jun 19 '18 at 12:03
• In passing: If Qf were 20% of Qb, that doesn't mean that the cross section of Qf is 20% of the area of Qb. Overbank flow is often spread widely in a thin layer and so the stream velocity is much lower. Much of the time you treat it as a shallow lake in terms of analysis. Jun 23 '18 at 12:48