Reduce erosion with contour maintenance

September 2025

WELL-DESIGNED CONTOURS THAT ARE PROPERLY MAINTAINED ARE THE ONLY PROVEN SYSTEM TO REDUCE EROSION ON CULTIVATED LAND IN SOUTH AFRICA. THOUSANDS OF FIELDS ARE PROTECTED BY THIS SYSTEM.

Contours are measured with a gentle slope to prevent erosion in the channel and to ensure the contour embankment remains undamaged. The length and spacing of the contours are determined by the slope, landform, rainfall zone and soil erodibility.

It is important that an expert in the field measures and designs the contours. The maintenance of contours is also extremely important – it must be part of the farm’s work planning. The dimensions of the contour must always be in such a way that it can safely carry away the stormwater for which it was designed.

WHY MAINTENANCE IS IMPORTANT
There is hardly anything that is built that does not require some form of maintenance. The same applies to a contour ridge system.

Maintenance is important for the following reasons:

• Contours do not prevent erosion from occurring, but they limit it to acceptable levels. The soil that does wash off ends up in the contour and reduces its flow area. See Figure 1.
• When the contour ridge is not properly compacted during construction, it settles and causes the flow space to decrease.
• Since many contours are measured and built on already eroded land, subsidence can occur where old erosion gullies were.
• When a contour breaks, it causes excessive erosion further down the field, as the water is concentrated at one point. This can also cause the lower contours to break.
• Where work is done across the contours (which is not desirable), the ridge is lowered and the flow space also becomes smaller. Such contours must be rebuilt every year.
• The contour inlets tend to silt up. This is where most of the water flows over the ridge, causing erosion on the side of the field.
• Livestock damage the ridges of the contours where the fields are used for grazing in the winter.

REGULAR INSPECTION

• It is important to do regular inspections of the contours on the farm. Take note of the following:
• Is there any contour that has broken?
• Are there any subsidence areas in the contour?
• Is the ridge still high enough and undamaged?
• Are the contour inlets still open enough?
• Does the contour still meet the prescribed requirements?

REPAIR
The same implements and processes used to build the contour can be used for maintenance.

• Firstly, repair the ridge where it has broken.
• Fill up any subsidence areas in the contour.
• The contour inlets must be properly opened.
• Ensure that the contour meets the requirements as shown in Figure 1.

HOW OFTEN SHOULD MAINTENANCE BE DONE?
When work is done across the contour, maintenance must be carried out every year. Where no work is done across the contour, it is recommended that the contours should be rebuilt after the first year. After this inspections will determine when it is needed again.

Once a contour is properly stabilised, maintenance may only be necessary every five to six years.

Always measure the contour according to Figure 1. If there is a significant deviation from the standards, adjust the contour accordingly. Otherwise, it can wait a little longer.

DIVERSION CHANNELS
Remember that maintaining diversion channels is just as important. It’s no use if the contours are open but the diversion channels are silted up or eroded. If erosion occurs in the channel, it must be filled in and re-established as soon as possible.

Ensure that the ground cover of the diversion channels is still in place. If there are bare patches, re-establish them as soon as possible. The more grass that grows, the better. Just make sure that the grass does not restrict the water flow, so mow it regularly.

Publication: September 2025

Section: Pula/Imvula

Author: MARTIN STRUTHERS, RETIRED CONTROL ENGINEER TECHNICIAN. FIRST PUBLISHED IN SA GRAAN/GRAIN, OCTOBER 2024.