Recommended practices and preferred options

PHOTO 7.2

A too-narrow bridge causing the floodplain to silt up and change, Portugal

Source: © MetaMeta (www.roadsforwater.com). Used with the permission of MetaMeta (www.roadsforwater.com). Further permission required for reuse.

Road development in low-lying floodplains will depend on several decisions:

• The preferred land use and wetland functions in the floodplain area • The objective of the road, and whether it should be passable under all circumstances • The financial resources available for the road’s development

In general, either of two key strategies can be followed when developing infrastructure in floodplains: the resistance strategy and the resilience strategy (Beevers et al. 2012). The resistance strategy, in principle, aims at preventing and regulating floods, whereas the resilience strategy aims at minimizing the consequences of floods while maintaining natural floodplain dynamics as much as possible. Typically, the resistance strategy will overcome the risk of floods to the road and traffic by providing ample freeboard, which protects against all flooding. The road will be designed to withstand adverse situations without necessarily considering the road’s potential adverse effects on the surrounding area. The resilience strategy, on the other hand, considers the best possible road alignment while carefully siting water crossings (at the bottom of sag curves) to minimize flood damage and ponding on the road surface. The resilience strategy calls for provision of flow-through and flow-over relief structures.

RECOMMENDED PRACTICES AND PREFERRED OPTIONS

once the overall strategy for road development in floodplains is clear, several specific decisions must be made:

• Selection of the location and height of the road embankment • Consideration of the use of controlled overflow sections • Provision of adequate cross drainage and subsurface flow capacity • Control of the upstream water level with cross-drainage structures • ensuring fish passage

Location and height of road embankments and controlled overflow sections

The location of the road and height of the road embankments are critically important decisions when developing roads in flood-prone areas.

The location of the raised road embankment will divide the floodplain, with one side of the embankment free from inundation and the other side exposed. Although people living downstream of the road will not face flood hazards, they will also not have access to beneficial uses of floodwater for fisheries, grazing areas, and farming using residual moisture after a flood recedes. Those living upstream of the road will face greater risk of floods during the rainy season because the flow is restricted to only part of the original floodplain. Considerable care is necessary to manage these impacts when siting roads in floodplains.

A second crucial decision is whether to opt for high or low road embankments. High-embankment roads that restrict floodplains are sometimes prone to breaches during peak flood periods. during extreme floods, the road can be overtopped, and damage can occur in an uncontrolled manner. The resulting damage is often severe and may disrupt traffic movement for an extended period. damage to road embankments can be substantial, not only because of breaches. debris and silt may accumulate on and along road embankments, and repairs to road shoulders may be necessary. In addition, the stability of stream channels in low-lying floodplains is uncertain; the debris and silt deposited by floods may cause channels to shift and, in general, change the floodplain’s morphology.

Low-embankment roads are an alternative to high-embankment roads. Lowembankment roads can be designed to allow overflowing, or to route floodwater through designated sections called floodways (photo 7.3). These designated overflow areas 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, prevent overtopping of the embankment in an uncontrolled manner by allowing high water to pass over part of the embankment in a controlled manner, when necessary. Low-embankment roads will conserve floodplain functions and prevent uncontrolled overtopping, but they will suffer predictable flood damage when overtopped.

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

• 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. • during the design of the top of the road surface, the scouring effect of the overtopping flow should be considered, and a material that protects the road

PHOTO 7.3

Floodway, Sado Plain, Portugal

Source: © MetaMeta (www.roadsforwater.com). Used with the permission of MetaMeta (www.roadsforwater.com). Further permission required for reuse.

base from saturation should be selected: 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 subatmospheric pressure. Flow splitters should be positioned at the top edge of the downstream face of the embankment. • Trees on either side of the floodway can provide further protection against scouring by overflowing water. • The overflow should be guided to areas where it serves beneficial purposes: groundwater recharge areas, grazing land, or a wetland.

one disadvantage of floodways is that overtopping renders a section of the road unusable during flood events. This lack of access causes inconvenience, can be harmful to the welfare of adjacent communities, and is a possible hazard to road users. Careful consideration and calculation are required to determine the direction of inundation to floodways and the implications of the resulting traffic disruption. Installation of poles with height markings along floodways can help road users estimate water depth and determine when crossing an inundated floodway would be unsafe and may also reduce the disruption associated with inundation.

Adequate cross drainage and subsurface flow capacity

Roads in floodplains should also have adequate cross drainage for surface and subsurface flows. Adequate cross drainage will help maintain wetland functions