Introduction to Open-Channel Flow

An open channel is a watercourse that allows part of the flow to be exposed to the atmosphere. This includes streams, rivers, culverts, stormwater systems, roadside ditches and swales, and roadway gutters.
Proper design requires that open channels have sufficient hydraulic capacity to convey the flow of the design storm. All open-channel flow assessments require a hydrologic analysis with procedures and methodologies presented in Chapter 2. In the case of earth-lined channels or river channels, bank protection may also be required if the shear stress is high enough to cause erosion or scouring.
This chapter provides guidance for determining design velocity (Section 4-2) and critical depth (Section 4-5) for designing roadside ditches (Section 4-4), stormwater systems, swales, and roadway gutters. All other transportation hydraulic features require the use of a 2D hydraulic model; FHWA has developed a reference document for 2D hydraulic models, titled Two-Dimensional Hydraulic Modeling for Highways in the River Environment (FHWA 2019).
River stabilization (Section 4-7) may be necessary for highly erosive, high-energy stream and river channels, to help stabilize the banks and/or channel bottom. The success of stabilization measures is dependent on the ability of the methods and materials used to withstand the hydraulic forces. For example, it is important to properly size the rock materials used for armoring; the methodology for sizing rock materials used in river stabilization is described in HEC-23 Volume 1 and Volume 2.

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