Water Quality And Erosion Control Through Terracing

Terracing is a soil conservation practice applied to prevent rainfall runoff on sloping land from accumulating and causing serious erosion. Terraces consist of ridges and channels constructed across the slope.

Fremont, CA: To some people, the word `terrace' brings to mind bench terraces used in various mountainous regions.

What are the advantages of a terrace system?

The major advantage, of course, is soil and water conservation. Terraces reduce the amount and velocity of water moving across the soil surface, greatly reducing soil erosion. Terracing hence permits more intensive cropping than would otherwise be possible.

PTO terraces offer these extra benefits:

1. The total area can be farmed as grassed waterways are not required.

2. Eliminating grassed waterways also eradicates the difficulty they cause when tilling or applying herbicides.

3. Peak discharges are decreased because runoff is temporarily stored.

4. Sediment and other contaminants settle outside the terrace ridge before dirtying water in a receiving stream.

To what field situations are terraces best adapted?

Fields with long, fairly consistent slopes that are not too steep (generally less than 8%) are best adapted to broad-based terraces. However, if the slopes are irregular or the soil is shallow (less than 6 inches), alternative BMPs should probably be used.

The overall slope of a terraced field can be improved by taking fill material from the `right' locations in that field. Topsoil should be eliminated from the cut and the fill areas and stockpiled, particularly with shallow soil. It can later be expanded over the terrace and borrowed areas.

Terraces are sometimes placed across minor watercourses on land where a total terrace system is not practical. This type of terrace employs subsurface outlets same as PTO terraces. It controls sediment ejection but does not check soil erosion effectively as steep slopes are likely to exist above the terrace.

How effective are terraces in controlling erosion?

Usually, very effective! They prevent rainfall runoff as it starts down a slope, thus avoiding a large accumulation of flow on the surface. This decreases the potential for sheet and rill erosion.

Terraces break up one long slope into several short ones. As already referred, PTO terraces also give soil particles that do wear a chance to clarify in the basin backside the terrace ridge while the extra water is slowly released through a subsurface drain.

To what span do terraces interfere with field operations?

Any forever-installed practice will somewhat obstruct field operations and/or take some land out of production. However, these obstructions are greatly minimized under the present methods of design. For instance, a PTO terrace system eliminates the grassed waterways more than offsets the interference generated by cross-slope ridges and channels.

Does terracing require changes in tillage practices?

As terraces are constructed beyond the slope, tillage will also be made in that direction. Cross-slope tillage may be the only extra change needed. Still, conservation tillage practices also might permit greater distances between terraces.

What type of maintenance is required?

The main thing is yearly preventive maintenance, like removing debris from outlets and smoothing out silt bars in the channels. Still, yearly tillage activity will eventually wear down the terrace ridges, demanding they be rebuilt occasionally. This is done by plowing against the ridge top from both sides with a moldboard plow. Conservation tillage methods are a good means of assuring long life for a terrace system.