§ 7.5.2. Types Of Flow For Bridge Design

Three (3) types of flow caused by bridge construction on a flood plain are shown in Figure 7-5 in Appendix D of this manual. The three flow types are described below:

A.

Type I Flow.

Referring to Figure 7-5A in Appendix D of this manual, it can be observed that normal water surface is above critical depth at all points. This has been labeled Type I, or subcritical flow, the type usually encountered in practice. The backwater expression for Type I flow is obtained by applying the conservation of energy principle between cross-sections 1 and 4.

B.

Type IIA Flow.

There are at least two (2) variations of Type II flow which will be described here as Types IIA and IIB. For Type IIA flow, Figure 7-5B in Appendix D of this manual, normal water surface in the unconstricted channel again remains above critical depth in the constriction. Once critical depth is penetrated, the water surface upstream from the constriction, and thus the backwater, becomes independent of conditions downstream (even though the water surface returns to normal stage at cross-section 4).

C.

Type IIB Flow.

The water surface for Type IIB flow, Figure 7-5C in Appendix D of this manual, starts out above both normal water surface and critical depth upstream, passes through critical depth in the constriction and then returns to normal. The return to normal depth can be rather abrupt as in Figure 7-5C in Appendix D of this manual, taking place in the form of a poor hydraulic jump, since normal water surface in the stream is above critical depth.

D.

Type III Flow.

In Type III Flow, Figure 7-5D in Appendix D of this manual, the normal water surface is below critical depth at all points and the flow throughout is supercritical. This is an unusual case requiring a steep gradient but such conditions do exist, particularly in mountainous regions. Theoretically, backwater should not occur for this type, since the flow throughout is supercritical. It is more than likely that an undulation of the water surface will occur in the vicinity of the constriction, as indicated on Figure 7-5D in Appendix D of this manual.

A more thorough and complete discussion of these parameters and preliminary design procedures are presented in Chapters 1 and 11 of Hydraulics of Bridge Waterways by U.S. Department of Transportation Federal Highway Administration, Second Edition, September, 1973.