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11.1.1 Open Top Culverts
11.1.2 Sub-base Culverts

11.1.1 Open Top Culverts

An effective way to control road surface erosion is to put in open top culverts or simply earth cross-drains, which will lead the surface water from the road. Open top culverts can be made of steel, concrete, timber, round wood, or only earth debris. Open top culverts need to be placed into the road with a certain cross gradient in order that they will be self-clearing, and normally a cross-gradient of 6 - 7% is sufficient.

The effectiveness of open top culverts depends very much on the correct spacing and maintenance work (clearing of soil particles, leaves and twigs, etc.) in order to keep them functioning effectively all the time.19 The spacing of open top culverts depends mainly on the gradient of the road, amount of precipitation, steepness of terrain and soil conditions. In a watershed forest in steep terrain with high rainfall, Sessions (1974) proposed an open top culvert spacing in metres derived by 800 divided by the gradient in percent. However, in areas with heavy rainfall and large catchment areas, a shorter spacing (20 - 40 m), especially on roads with 9% or a higher gradient, may be required. The correct spacing of open top culverts may be worked out through experience in the respective areas; the table below should serve as a guideline.


Open Top Culvert Spacing (in metres)

Road gradient in percent

800 per gradient in percent

Suggested spacing in steep terrain with heavy rainfall
























19 It is advisable to employ a road maintenance man who cleans ditches, culverts and other water courses and detects major future problems on the road.


Wooden Open Top Culverts

Roundwood used with ø 8-15 cm. Wooden nails keep roundwood in correct position; placement of culvert with 6-7% cross gradient. Gravel Layer

Three roundwood logs with wood nailed on underneath. Spacing of roundwood 10 cm apart. Gravel Layer

Split logs fixed with crossboards

Roundwood fixed together with clamps at 10 cm distance

Clamps fix the side boards of the culvert. The bottom board is fixed with nails and side boards. Dimensions on side boards 23x8 cm; bottom board 10 x 8 cm.

Spacing of side boards at the top 6 cm; at bottom 12 cm. Dimensions of side boards 23x8 cm; bottom board 12x8 cm.

11.1.2 Sub-base Culverts

For seasonal stream flows and smaller water flows, sub-base culverts may be constructed to allow the natural passage of water. Very often on forest roads of minor importance only wooden culverts are used, but for access or permanent roads, it is best to install precast concrete pipes.

Because there is a tendency for small culverts and pipes to block with debris, it is preferable to provide either an open ditch or pipes with a minimum diameter of 300 mm but in areas of low rainfall and high permeability or exceptionally where the topography requires the provision of culverts at short intervals 225 mm culverts may be installed. In no circumstances should pipes of less than 225 mm diameter to be used.

A very simple type of wooden culvert can be constructed with three logs with mid-diameters of 50 -100 cm each. Two logs are used for the basic layer and the third log is put on top of them. Instead of putting a third log on top, one could use boards or split logs which are then nailed onto the two bottom logs. Sometimes wooden box sub-base culverts are installed. As these need more labour to install, they are more expensive.

Generally, one may say that a culvert made of durable untreated hardwood species may serve very well for 5 - 6 years; whereas with chemical preservation it could last for 10 years. The costs of wooden sub-base culverts, depending on the type of construction, are normally only 10-40 percent of the cost of precast concrete pipes. To put a culvert in a road, 3-8 man-days are required, depending on whether the sub-base culvert is built into a newly constructed road or in an already existing road.

The diameter of culvert required at a particular site is governed chiefly by the catchment area and the characteristics of the catchment: ie shape, slope, topography, climate, soil type and vegetal cover, together with the frequency of failure and flooding which can be tolerated.

In mountainous areas where streams carry large quantities of gravel and other debris in time of flood special considerations apply. The aim must be not only to discharge the flood flow but to do so without ponding upstream of the culvert. If this is not achieved deposition at the culvert inlet may cause blockage, and subsequent wash out. Larger culverts are therefore required.

Twin or multiple culverts are more expensive and more easily blocked than single culverts of the same capacity and should not be used except in exceptional cases where headroom is unavoidably restricted, where the greater portion of the waterway area is to be kept near the invert elevation so as to prevent flooding or ponding on the upstream side of the culvert.

Open cross ditches can be used where it is difficult to find the depth necessary to provide a culvert. Care should be taken to provide a profile which allows reasonable traversing by traffic. Support to the sides can be given by sawn or round timber, but concrete channels are available.

In cases where there is difficulty in obtaining the required discharge from the culvert size available, consideration should be given to improving the hydraulic characteristics by designing a flumed entry to the culvert using wing walls. Care should be taken to ensure that the increased velocity of discharge is catered for by providing paving to invert downstream of the culvert where necessary.


In flat ground it is normally cheaper to collect drainage by ditches to a single culvert rather than provide separate culverts at a number of adjacent low points on the road. Conversely on a steep gradient it is usually necessary to site culverts at closer intervals than the natural drainage channels down the hillside; any attempts to economise by installing a small number of large diameter culverts is likely to cause erosion of the roadside drain. Where there is a tendency for the roadside drain to erode, the best and cheapest answer is usually the provision of more culverts rather than such expedients as check-weirs.

A culvert taking the flow of a stream which crosses the road at an angle should follow the line of the stream if the latter is subject to high velocity and there is the possibility of erosion. But if these conditions do not apply the culvert should be placed at right angles to the road with its inlet in line with the stream thus reducing the length of culvert required.

No culvert on a stream crossing (or other important culvert taking the flow of a forest drain) should be placed on fill. Where the culvert departs from the line of the stream it should be laid in a trench and where it follows the line of a stream it should be laid at bed level; the bed being regulated as necessary by excavation or by filling of local hollows with hard materials well tamped into place.

All culverts should be laid in the natural ground if possible rather than in fill. Laying culverts in recently placed fill leads to differential settlement and even to a vertical displacement between adjacent pipes in some cases and requires the provision of some means of preventing scour at the outlet as described in para 32. Where access is reasonably good culverts can be laid in the solid before the earthworks are carried out and this must be done where drains are to be cut in advance. Where culverts are installed after completion of earthworks on moderate cross slopes the excavation for them should be taken down through the fill to the solid as the additional stability of the culvert and prevention of scour at the outfall more than compensate for the additional length and extra excavation.

Where the laying of culverts in fill cannot be avoided it is an advantage if installation can be delayed for a time since the rate of settlement decreases very rapidly after the fill is placed. The culverts at stream crossings normally have in any case to be laid before the earthworks are carried out; on some sites it is possible to delay construction of intermediate culverts by carrying the water on in the roadside drain to the stream and draining low areas temporarily by grips across the sub-grade. In considering the advisability of adopting this expedient the possibility should be borne in mind of extensive damage which may result from storms occurring before the construction of the culverts particularly in friable soil; furthermore the grips must be kept open if they are to be effective and this is sometimes impracticable.


Corrugated steel culverting may provide an economic alternative to concrete pipes, especially where access is particularly difficult. It is not possible to say that these are cheaper above any particular size but the economics are worth examining for culverts upwards of 600 mm diameter and certainly in locations requiring diameters of 1 150 mm and upwards.

The thickness of the metal to be used in these culverts should be taken from the manufacturers design manuals.

The life of steel culverting is controlled by the protection afforded by the galvanised coating. This is attacked by acid water and abrasion by the bed load. The site condition as well as comparative cost of concrete pipes will influence the decision as to the use of these.

Where the conditions are less severe the life of corrugated steel pipes can be extended by additional protection. Where the water is acid such as in peaty or moorland areas bitumen-coated material should be used, if the water course carries a heavy bed-load rivetted pipes can be obtained with an asphalt paved invert; the latter adds 10% to 20% to the total cost of the culvert and the bitumen coating alone about half this figure but the protection aforded may double the life of the structure. For the larger steel structures built in situ the invert can be protected with an in situ concrete lining; this should usually cover 20% of the pipes periphery.


Headwalls are not normally required at culvert outlets except on very steep gradients where there is a risk of the end pipe becoming displaced. Apart from this function and to prevent scour upstream it is generally more economical to lengthen the culvert than to construct headwalls. The desire to reduce the length of culverts must not lead to attempts being made to place fill at unstable angles, nor should dry stone or timber retaining walls be built in an attempt to support such slopes, although drystone revetment may be used to prevent scour where masonry head walls and wingwalls are not justified. Where the culvert unavoidably discharges on to fill some protection is necessary. To allow for settlement it is advisable to provide a temporary installation such as a simple timber flume in the first instance which can be replaced later by a masonry or precast concrete flume.

Where culverts are required beneath roads constructed over soft ground it may be necessary to excavate to a sufficient depth to enable a 1 m depth of suitable granular material to be placed and compacted below the pipe invert level. The width of this granular base should be twice the diameter of the pipe with a minimum of 1.200 m. The length of the culvert should be such that the ends of it are clear of the road fill.

Installation of corrugated steel structures should be in accordance with the makers installation manual.


In order that the installed culverts can function as designed it is necessary to ensure that the entrance and exits to the culvert are clear of debris, and that the passage through the culvert has not silted up.

PHOTO NO. 26 Placing of prefabricated concrete sub-base culvert, by excavator

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