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Headloss - HEC-22 Energy (Third Edition) Loss Method

In recent years, the FHWA has defined an updated procedure for computing loss through a junction. Bentley provides the ability to switch between the previous the junction loss methods: HEC-22 Energy (Second Edition) and the newer HEC-22 Energy (Third Edition). The newer protocol is detailed the FHWA's Urban Drainage Design Manual - Hydraulic Engineering Circular No. 22, Third Edition, 2009). The Second Edition procedure computes correction factor used to adjust an overall loss coefficient term. In contrast, the Third Edition computes total junction loss through summation of multiple componentized losses. Computationally, and perhaps the most significant improvement with the Third Edition, is the explicit procedure in which plunging inflows are considered.

The Third Edition procedure has three calculation steps: solve for Entrance Loss, then the Additional Structure Losses, and finally the Exit Loss for each inflowing pipe to the junction. Each step solves for the relative energy loss in downstream to upstream order, relative to the energy grade line. Therefore, and to hold true to the FHWA protocol, this method can only be utilized when the calculation option of Structure Loss Mode set to Energy Grade. In summary, the method computes an access hole's energy level at the upstream boundary () before Exit Loss, as:

=Initial Access Hole Energy Level. See also: HEC-22 (Third Edition) Entrance Loss

) =Additional loss for benching. See also: HEC-22 (Third Edition) Loss for Benching

) =Additional loss for angled inflows. See also: HEC-22 (Third Edition) Loss for Angled Inflow

) = Additional loss for plunging flows. See also: HEC-22 (Third Edition) Loss for Plunging

After the Total Additional Structure Losses (See also: HEC-22 (Third Edition) Additional Structure Losses) are determined, and the structure’s upstream energy level is computed, the Exit Loss can then be calculated for each incoming piped flow. (See also: HEC-22 (Third Edition) Exit Loss.)