Specific energy in open channel flow

The specific energy concept was introduced by Boris Bakhmeteff (1912). It is commonly used in open channel flows, especially for smooth and short transitions.
By definition, the specific energy E is equal to the total head using the bed elevation as the datum. The specific energy is similar to the energy per unit mass, measured with the channel bottom as the datum. The specific energy changes along a channel because of changes of bottom elevation and energy losses (e.g. friction loss).
The specific energy is commonly used during the application of the Bernoulli principle to smooth and short transitions in open channels. These are discussed is the relevant Youtube video movies in the same Playlist at:

Fundamentals of open channel hydraulics [Playlist]
Advanced hydraulics of open channel flow [Playlist]

Minimum Specific Energy and Critical Flow Conditions in Open Channels
Bernoulli equation in open channel flows
Bernoulli principle and its application to smooth and short transition in open channels
Critical flow conditions in open channels
Critical flow calculations in open channels of irregular cross-sections
Relationship specific energy versus water depth in open channels
Relationship discharge versus water depth in open channels

References
BAKHMETEFF, B.A. (1912). "O Neravnomernom Dwijenii Jidkosti v Otkrytom Rusle." ('Varied Flow in Open Channel.') St Petersburg, Russia (in Russian).
BAKHMETEFF, B.A. (1932). "Hydraulics of Open Channels." McGraw-Hill, New York, USA, 1st ed., 329 pages.
CHANSON, H. (2004). "The Hydraulics of Open Channel Flow: An Introduction." Butterworth-Heinemann, 2nd edition, Oxford, UK, 630 pages (ISBN 978 0 7506 5978 9).
CHANSON, H. (2004). "Environmental Hydraulics of Open Channel Flows." Elsevier-Butterworth-Heinemann, Oxford, UK, 483 pages (ISBN 978 0 7506 6165 2).
CHANSON, H. (2006). "Minimum Specific Energy and Critical Flow Conditions in Open Channels." Journal of Irrigation and Drainage Engineering., ASCE, Vol. 132, No. 5, pp. 498-502 (doi:10.1061/(ASCE)0733-9437(2006)132:5(498)).
CHANSON, H. (2008). "Minimum Specific Energy and Critical Flow Conditions in Open Channels - Closure." Journal of Irrigation and Drainage Engineering, ASCE, Vol. 134, No. 6, pp. 883-887 (DOI: 10.1061/(ASCE)0733-9437(2008)134:6(883)).