A floodway consists not only of the roadway embankment but also waterway openings which may be required for one or more of the following functions:
- to reduce the afflux or rise in water level upstream due to the obstruction (embankment)
- to raise the tailwater level so that less batter protection is required on the downstream side e.g. grass instead of concrete
- to act as anti-ponding structures for low flow stream conditions.
Flow over roadways may be free flow or submerged flow. In the initial stages of overtopping a low tailwater usually exists and free flow occurs. Under these circumstances flow passes through critical depth over the road and the discharge is determined by flood levels upstream.
Free flow may be either:
- plunging flow which flows over the shoulder and down the downstream face of the embankment. The flow then penetrates the tailwater surface producing a submerged hydraulic jump on the downstream slope. Velocities are likely to be high and erosive
- surface flow which separates from the surface of the road embankment and rides over the surface of the tailwater. This flow will have less erosion potential downstream.
Submerged flow occurs when the discharge is controlled by the tailwater level as well as the headwater levels. This occurs when the depth of flow over the road is everywhere greater than the critical depth.
Where a floodway is constructed above ground level, it may or may not act as a control on flow, depending upon the headwater and tailwater conditions. In the initial stages of overtopping of the road a low tailwater condition usually exists and a free outfall condition occurs.
Typical velocities of flow over a floodway are shown in Figure 4.2. This figure illustrates the conditions that result from rising tailwater (e.g. submerged hydraulic jump).
Source: DTMR (2010).