Control Section Lab
Explore how discharge, channel geometry and downstream water level decide which part of a river controls the water surface.
About this model
Backwater calculations are boundary-condition problems, but the boundary that controls a reach is not always at the end of the model. In subcritical flow, a downstream water level can influence the river far upstream. A contraction, a bed high point or a sufficiently steep transition can instead force the flow through critical depth and create an internal control section.
Control sections matter in natural rivers, canals, bridge openings and hydraulic structures because they determine which boundary conditions can influence each part of the water-surface profile. The narrowest or steepest section need not control at every discharge. Several possible controls can compete, the active control can move as flow changes, and high downstream water level can suppress a control that was decisive at a lower level.
Scientific basis
The model solves steady one-dimensional flow in a rectangular, non-prismatic channel with the standard-step energy equation and Manning friction. Subcritical and supercritical branches are calculated from the applicable boundary conditions, and active control sections emerge from the selected complete water-surface profile.
Where solution branches meet, momentum balance determines whether the transition is a hydraulic jump or a continuous critical transition. The model assumes hydrostatic pressure and a section-averaged velocity, and does not resolve three-dimensional or unsteady rapidly varied flow.
- Chow, V. T. (1959). Open-Channel Hydraulics. McGraw-Hill.
- U.S. Army Corps of Engineers, HEC-RAS River Analysis System, Hydraulic Reference Manual: Mixed Flow Regime Calculations.
- Sturm, T. W. (2001). Open Channel Hydraulics. McGraw-Hill.