[1] It states that bed elevation increases (the bed aggrades) proportionally to the amount of sediment that drops out of transport, and conversely decreases (the bed degrades) proportionally to the amount of sediment that becomes entrained by the flow.
It was developed by the Austrian meteorologist and sedimentologist Felix Maria Exner, from whom it derives its name.
[2] It is typically applied to sediment in a fluvial system such as a river.
The Exner equation states that the change in bed elevation,
, times the negative divergence of sediment flux,
[3] A typical value for spherical grains is 0.64, as given by random close packing.
Often, for reasons of computational convenience and/or lack of data, the Exner equation is used in its one-dimensional form.
, as one is typically interested in the downstream distribution of erosion and deposition though a river reach where
is scalar sediment flux in the downstream direction.