Downstream Interactions
Explore how downstream apron position and tailwater combine to change the flow regime and reduce the discharge coefficient of an ogee spillway.
Linked hydraulic state
Regime and coefficient chart
Current state
About this model
Downstream conditions can change both the flow pattern and capacity of an ogee spillway. A raised downstream floor changes the available drop, while high tailwater can submerge the jet and alter the pressure field around the crest.
The two effects are not independent. As tailwater rises, the flow can pass from a free or supercritical jet through a hydraulic jump and into drowned or surface-jet regimes. The same downstream change can therefore affect both the visible flow regime and the spillway capacity.
Scientific basis
The regime boundaries and discharge-coefficient contours reconstruct Figure 9-26 in Design of Small Dams. A describes the downstream-apron effect alone, S the submergence effect alone, and q controls how strongly the two reductions interact in the reconstructed coefficient surface.
The linked longitudinal sketch is a pedagogical interpretation of the chart regimes rather than a solved free-surface profile. The marginal plots show the two single-effect relations together with conditional C/C₀ slices through the selected state.
Primary source: U.S. Bureau of Reclamation, Design of Small Dams, 3rd ed. (1987), Figure 9-26.