Measures to prevent failure

The constructed drift is an investment that provides livelihoods for communities. Therefore, it is important to prevent its failure and washing away by floodwater. Figure 8.2 illustrates the possible failures of a drift. The risks should be considered during design and construction of a drift. There are a number of designated measures:


Figure 8.2. Overview of non-vented drift with preventable failure features.

Overview of non-vented drift with preventable failure features. (Source: Neal 2014)
Overview of non-vented drift with preventable failure features. (Source: Neal 2014)

Body of the drift. There should be adequate support to the top slab of the drift to avoid collapse under traffic load and to create mass resistance so as not to be washed away by floodwater. Fill material of adequate strength should be placed in the structure to give it firm resistance to floodwater. The fill material also ensures adequate resistance to traffic load and thus prevents crashing of the drift’s top slab. Rock fill is typically recommended, with a minimum depth of 1 m underneath the top slab in sand riverbeds and 0.6 m in drifts constructed on rock riverbeds. (See Figure 8.2.)

Robustness. Care should be taken to ensure that the foundations, walls, and top slab are rigidly tied together by high-tensile reinforced steel to ensure the structure’s firmness, so that floodwater does not penetrate the structure and carry it away. The materials for making concrete should meet all requirements in accordance with the recommended standard specifications for road and bridge construction. The structure should be extended and anchored adequately beyond the riverbanks to prevent floodwater from going over the edges and undermining the drift ends. If the water undermines the drift ends, this could cause approach failure and cut the road off from the drift. If the drift links directly to an unpaved road, this can trigger erosion. As a rule, the drifts should be extended by 5 m beyond the banks of the river with spans less than 50 m, and by 10 m for rivers with spans greater than 50 m. Historical flood levels should also be assessed together with estimations of total river discharge. The drifts should be extended at least above the (expected) peak flood levels.

Lowered middle section of the drift. A middle curvature should be introduced in the drift body to ensure that floodwater concentrates in the middle of the river. This will prevent floodwater from washing away the approaches of the structure. The depression at the center should be up to 50 cm in a drift that spans 50 m in order to ensure adequate floodwater spreading and prevent bankside erosion.

Anchorage to the bedrock. The drift should be firmly anchored to the bedrock of the river across its full span in rocky riverbeds. Anchoring ensures that no water flows under the foundation, which would undermine the drift. Excess stormwater passes over the drift. For drifts constructed in a sandy riverbed, seepage underneath is allowed through the sand to infiltrate water to the downstream side of the river.

Downstream gabion protection. The structure’s foundation should be protected from being undermined by the river’s flood flows. Undermining of the structure washes away the rock fill, which results in the structure collapsing under the traffic load and the drift being washed away. Gabion boxes measuring 2 m x 1 m x 1 m should be placed at the foot of the foundation on the downstream side of the structure.