Location and height of road embankments and controlled overflow sections

There are important fundamental choices to be made with regard to the design of roads in flood-prone areas, in particular with regard to the location of the road and the height of the road embankments.

First, the location of the raised road embankment will divide the floodplain, with one side of the embankment free from inundation and the other side more exposed. People living on the downstream side of the road will not face flood hazards. However, during normal and low-flow periods, these same people may not have access to the beneficial uses of floods: fishery, grazing area, and farming using residual moisture once a flood recedes On the other hand, the upstream side of the road will face greater risk of floods during the rainy season, since the spreading of the flow is restricted to only part of the original floodplain. Therefore, the siting of the road in a floodplain must be done carefully.

A closely related choice is whether to opt for high or low road embankments. In some cases, high embankment roads are prone to breaches during peak flood periods as the floodplain area becomes more restricted. During extreme floods, the road can be overtopped, and damage can occur in an uncontrolled manner. In the wake of this, the movement of traffic may be disrupted for an extensive period. Damage to road embankments can be substantial, not only because of breaches. Debris and silt may accumulate on and along road embankments, and scouring may necessitate repairs to road shoulders. In addition, the stability of stream channels in low-lying floodplains is uncertain because the debris and silt that floods deposit may cause channel shifts and, in general, changes to the floodplain’s morphology.

The alternative to high embankment roads is low embankment roads. These low embankment roads can be designed to allow overflowing, or to route floodwater through designated sections called floodways (see Annex 5). These designated overflow areas will make it possible to lower the height of the road embankment along its entire length, resulting in considerable cost savings.

Overflow embankment sections, or floodways (Annex 5), allow high water to pass over part of the embankment in a controlled manner, when necessary. They prevent overtopping of the embankment in an uncontrolled manner. Low embankment roads will conserve floodplain functions and prevent unpredictable damage.

Floodway (Sado Plain, Portugal)
Floodway (Sado Plain, Portugal)

When overtopped, a floodway typically operates as a broad-crested weir with a large potential overflow capacity. The following aspects should be considered in its design:

  • The depth of flow over the embankment should be inversely related to the width of the embankment’s overflow section. Deep flow over the road can interfere with transport. Therefore, the overflow depth should be kept to a minimum.
  • The upstream and downstream faces of the embankment should be blanketed with impermeable material such as stone masonry or concrete lining.
  • While designing the top of the road surface, consider the scouring effect of the overtopping flow and select a material that protects the road base from saturation: rigid pavement (ford or vented ford) is a good option.
  • The downstream side of the embankment and its toe need protection from scouring by the overflowing water. A toe apron, stilling basin, downstream pool, or stone riprap are good alternatives for this purpose.
  • The downstream side should be well aerated to avoid sub-atmospheric pressure. Flow splitters should be positioned at the top edge of the downstream face of the embankment.
  • Trees on either side of the floodway will provide further protection against scouring by overflowing water.
  • The overflow should lead to an area where it does not do harm but serves useful purposes: for instance, the recharge of groundwater, the improvement of grazing land, or the preservation of a wetland.

A disadvantage of the floodway is that its overtopping during floods causes inconvenience and is a possible hazard to road users, because the road section is not passable during the floods and traffic will be disrupted. Therefore, careful consideration and calculation are required to assess the periods during which the floodway will be inundated and the implications in terms of traffic disruption. However, the use of poles along the floodway will make it possible to pass sections inundated with water. Annex 2 discusses floodway design.