Adequate cross drainage and subsurface flow capacity

Roads in floodplains should also have adequate provision for cross drainage, both of surface and subsurface flow. Adequate cross drainage will help to maintain wetland functions on either side of the road and sustain several of the economic functions, for instance, keeping wells functioning on the downstream side of a road body. An understanding of wetland and floodplain hydrology will inform the placement of the appropriate number of cross-drainage facilities. This surface and subsurface cross-drainage capacity can be provided through culverts, sections with very coarse gravel (called French mattresses), or porous sections in the road embankment structure made of boulders and gravel that are graded from coarse to fine (from the bottom to top of the embankment).

Culverts

Culverts can provide cross drainage for both surface and shallow subsurface flows. If culverts are partly buried, they will convey both surface and subsurface flows. They will also mitigate (slow) flooding events. For partly submerged round culverts, the common embedment depth is 40 percent. Where culverts only carry surface flows, they will be placed above ground level.

The number of culverts is most critical. Given the slow flow of water in a floodplain, multiple culverts are required. The culverts balance the amounts of water on either side of the road. They rarely flow at capacity but are required for unusual events. The dimension of the culverts is a second consideration. For a seasonally fluctuating wetland, such as a floodplain, the amounts of water passing through for part of the year will be high and thus large culverts are required. Table 11.1 below indicates the preferred spacing and dimension of culverts.

Table 11.1. Road culvert spacing and dimensions for flood plains

Source: Partington et al. (2016).

Where the floodplain and wetland hydrology is not well understood, proactive spacing of culverts can be considered to maintain road connectivity and preserve the preferred wetland functions. This means that rather than spacing at 100 to 200 m, a distance of 50 to 100 m or even less may be maintained. In central parts of the wetland, the distance should be reduced, whereas at the dry edge of the wetland it should be increased. There is always scope to adjust, especially with unpaved roads, by adding culverts in sections where the water becomes ponded.

A special consideration for placing culverts in floodplains and wetland conditions is that the underlying soil may not have much bearing capacity, and reinforcement may be required to improve this capacity. There are several ways to do this, but the most common are:

• the placement of compacted gravel and geotextile (or a local substitute) beneath the culvert, often using a small notch; and
• the above measure, combined with small-diameter wooden logs in a “corduroy” pattern.

French mattresses

As an alternative measure to provide cross drainage to culverts, permeable sections may be provided. These sections typically consist of coarse clean rock enveloped in geotextile or a local alternative material. They are known as “French mattresses” or “rock sandwiches.” They have added value over culverts in a number of instances: 1

• Where water saturation risks destabilizing the road base (also between two culverts);
• Where a two-directional flow of water through the road base should be allowed;
• By making it possible to disperse flows, thus preventing gully erosion that may occur downstream of a culvert in areas with considerable slope; and
• Where the lowering of wetland water levels could occur as the result of having a large number of culverts: instead, the release of excess water through French mattresses is more gradual.

These French mattresses may be used in different ways depending on the local hydrology, either by installing a number of short sections at set intervals or, particularly in very wet conditions, by using a long section over a large area (up to 300 m).

Although the costs of transporting rocks may be considerable and result in high initial investment, French mattresses require virtually no maintenance and have a long service life. Unlike culverts, they are also difficult for rodents to block. Moreover, they help maintain natural vegetative communities and habitats by keeping different sections of floodplains connected.

French mattresses are constructed through the following steps:

1. Excavate a trench of the desired depth in the road body, allowing for a minimum 25 cms cover over the mattress.
2. Place geotextile fabric (preferably Class 2 woven) or a local alternative in the trench, leaving enough fabric on the sides to go around and overlap on the top of the finished mattress.
3. Place porous stone on top of the fabric and spread it out uniformly. The size of the stones should preferably be 6 to 10 cms.
4. Wrap the ends of the fabric over the top of the structure. Place a piece of fabric on the top if the existing fabric does not completely cover the mattress. Overlap all fabric joints by at least 25 cms.
5. Compact the fill material on top of the finished mattress.
6. Install French mattresses to match the slope of the land. In wetland situations, the slope may be minimal. In sloped areas, a 1 to 2 percent slope should be used to aid drainage.