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Provision of Low Level (Minor) River Crossings by the Ministry of Public Works and Transport

Splashes, bed-level causeway (drifts), culverts (concrete and Armco), vented causeways and submersible bridges are provided where the road crosses a natural water way. Even if the water way only carries water occasionally during the rainy season some kind of structure is necessary to prevent scouring and gully formation.
The key issues guiding the provision of such crossings are as follows:

  • The provision of reliable, cost-effective access to as many of the rural population as possible, rather than high access standards for a few;
  • The use of appropriate design standards to balance the transport requirements of core economic and social activities with the impact of geography (geology, topography and climate) on construction and maintenance costs;
  • The employment of cost-effective and innovative techniques such as labour-based approaches and low-cost structures, and;
  • Poverty alleviation through the implementation of projects using labour-based approaches with strong local community involvement.
  • The selection criteria for the type of structure to be used in the provision of access across a natural waterway vary from situation to situation depending on the terrain.

Drifts are best suited for a crossing where the river banks are relatively low and gently sloping and the stream is shallow. Concrete is the best suitable material for surfacing the crossing, however stone and gravel may be used if the water flow is not rapid.

Embanked drifts may be constructed in streams with deep-water flows in order to create shallow rapid flows across the top of the drift or to make shallower in narrow and deep-water causes.

Culverts


are best suited for streams with steep banks, since their construction requires some height difference between the level of the road surface and the stream bed.
Culvert construction consists of the following:
1. The actual culvert (one or more pipes) which carries the water under the road;
2. The embanked which carries the road across the water course;
3. Wing walls which protect the embankment from flood water and direct the flow into the culvert;
4. The apron at the discharge end, which prevents erosion of the stream bed.

Simple Bridges

are ideal where the river is narrow and the banks are solid and the bridge interferes as little as possible with the natural flow of the water.
Components of a bridge:
1. Abutments: structures provided to strengthen the stream banks and adequately support the shore end of the road bearing beams.
2. Intermediate supports (commonly made of timber trestles, or masonry piers or reinforced concrete columns) where the stream is too wide to be bridged in a single span;
3. Road-bearing beams that carry the weight of the roadway and traffic between abutments and any intermediate supports;
4. Decking or flooring which make up the road surface on the bridge;
5. Curbs and rails along the edges of the bridge for safety.

Low-level crossings often require routine maintenance after periods of flooding. This maintenance will include the following:

Structure repair

  • Gravel/ masonry replacement;
  • Replacement and or cleaning of warning signs, marker posts and depth indicators and;
  • Clearing silt and regrading the carriageway where necessary.

The low level crossings mentioned above are critical in providing basic access that connects the rural population, and therefore the majority of the poor, to their farms, local markets and social services, such as schools and health centres, potentially increasing their real income and improving their quality of life. According to the RTI World Bank (2001) report "basic access is one of necessary building blocks of poverty reduction

Chief Roads Engineer:Mr Trevor Tshabalala

Postal Address:
P. O. Box 58
Mbabane                               
Swaziland

Physical Address
Ministry of Works & Transport Building                       
Mhlambanyatsi Road
Mbabane

Telephone Number:(+268) 2409 9105
Fax Number:      (+268) 2404 5825
Email Address: This email address is being protected from spambots. You need JavaScript enabled to view it.

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