Flood Focus of FGM Analysis

FGM analysis is often focused on higher frequency flood events, which is also the focus of much operational forecasting. Bankfull is a term used to describe a river channel just before flow spills onto the Floodplain. It is a term that comes before descriptions of river depth, river width, river area, and river discharge. Some of these terms, and their geomorphological characteristics, are given below. The use of these terms will be provided in later FGM Module Sections.

1. Bankfull Depth: Vertical distance from river bottom Datum to point of incipient flooding or Floodplain encroachment. It is often a depth relative to the river Thalweg, and is often noted on point bars. In cut-away banks, opposite point bars, localized scour may place the Bankfull Depth below Floodplain. In some entire Reaches of a river, the Bankfull Depth may be within the channel, and the Floodplain abandoned due to incision.
2. Bankfull Width: Shortest horizontal distance between left and right Bankfull Depth. The width is often taken, and distinguished, at riffles and pools.
3. Bankfull Area: Cross-sectional area of the Bankfull channel, either measured by survey, or derived as the quotient of Bankfull Discharge and average Bankfull velocity.
4. Bankfull Discharge: Volumetric flow that fills the Bankfull channel, beyond which the flow spills onto the Floodplain.

Bankfull concepts are important to FGM analysis because they have been shown to deliver the greatest channel forming energy, or the greatest product of flow frequency and sediment transport. This has been called effective channel forming flow, or the Product of Peak Discharge Frequency and Sediment Transport Rate, by Wolman and Miller (1960).

Bankfull is the discharge at which channel maintenance is the most effective at "moving sediment, forming or removing bars, forming or changing bends and Meanders, and generally doing work that results in the average morphologic characteristics of channels" (Dunne and Leopold 1978). It is important to note that despite the relative effective power of Bankfull flow, channels are formed by the integration of all flows up through Bankfull.

Bankfull does not have a fixed recurrence interval, however many researchers have discovered that when flows are assessed with the Water Resources Council 1972 Flood Frequency method, the frequency of Bankfull Discharge generally falls between 1 and 3 years, most often within 2 years. Even if Reaching Bankfull is classified as a 2-year event, it is possible to exceed Bankfull multiple times within a given year. This may occur in a single event, or it might occur in different isolated events.

Original Image from NCSU