Design of non-vented drifts

Road drift on a small river near Kitui town, Kenya

The design of a non-vented road drift consists of several elements: the body of the drift, the approach road, the upstream protection of the stream, and the downstream apron. Annex 2 provides a detailed description of the design of a non-vented drift.

However, the main points with respect to the design of the non-vented drifts are presented below:

  • Expand the drifts by 5 m to 10 m on either riverbank, depending on the width of the river.
  • Extend the approach above the experienced flood level to prevent damage when floods are high.
  • In sandy riverbeds, anchor the structure at 1.5 m below the existing riverbed level; in rocky riverbeds, the foundations should be laid on the bedrock.
  • Foundations should have a minimum width of 500 mm and a construction depth of 250 mm.
  • The top slab, on which traffic passes, should have a constructed thickness of at least 150 mm to 200 mm, depending on traffic volume and load.
  • The drift should be filled with hardcore material and compacted to a maximum depth of 1 m on sandy riverbeds and 0.6 m on rock riverbeds.
  • The foundations, walls, and slab should be rigidly tied together to give the drift great resistance to being washed away by floodwaters.
  • The width of the roadway slab should vary between 3 m and 5 m, depending on the anticipated type and volume of traffic.
  • The height of the drift above the existing riverbed should be a maximum of 1 m to ensure sufficient depth for the accumulation of sand and water upstream.
  • At the foundation of the drift on the downstream side, gabions should be installed to prevent the foundation from being undermined by the overflow of floodwater.
  • The drift should have a curvature toward the center of the river to ensure that the water concentrates in the middle of the river, minimizing erosion along the riverbank
  • The elevation of the drift and the walls above the riverbed determine the additional material deposited and the amount of water retained. Coarse material is deposited in the riverbed, while finer material is washed off the drift and walls to areas downstream of the river. The deposition takes place over a number of years. The height of the drift can be increased in stages so that mainly coarse material is deposited each year.