Description
Turfing is the direct application of grass with developed roots onto the slope surface (Figure 1). Relatively matured grass can grow easier and extend the roots into the soil to strengthen the overall surface. This is one of the methods used for restoring man made slopes of Hong Kong territories (GEO, 2011). Turfing is also effective in protecting areas with concentrated flows, -*/ such as channels and drains, when the flow velocity does not exceed 1.5 m/s (Turf Australia). In the past it was used as restore vegetation over flat areas, gentle slopes, or stram banks, when the extraction of turf was possible in the vicinity (Evette et al., 2009). Recently the turf can be easily provided by greenhouses or private companies and thus it can be applied also in landscapes with scarce presence of the material.
The turf is preferred for runoff and rainsplash erosion because of the high capacity of grass in binding soil more effectively than other plants. Grass plants are ligth and up to 90% of their weight consists of roots. Moreover, healthy turfs can intercept and adsorb rainfall and consequently drastically reduce runoff.
If the grass is well established, it provides a long-term solution for surface erosion control with a high aesthetic value to the landscape. It is highly effective for slopes not steeper than 33%, since for higher inclination the grasses cannot establish for long-term.
Advantages
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Immediate protection from rainsplash erosion and reduction of runoff speed;
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Fast grass establishment;
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High aesthetic value to the landscape;
Disadvantages
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The slopes must be uniform and relatively smooth before installation to ensure complete contact with the soil. The associated labour cost may be high;
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Not suitable for steep slopes;
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Not adequate for slopes with high erosion rates: the turf can be flooded by deposited sediment.
Design methods
A well site preparation must be ensured for a successful installation. The soil should be smoothed and shaped if required. The soil conditions (PH and nutrient availability) should be suitable for hosting the turf. It may be necessary to clean the site from the presence of rocks, stones, or existing weeds, that can obstacle the perfect contact between underlying soil and the turfing mats. The turf is laid over a top soil that has been raked over, since an high compaction of the soil prior to installation could inhibit the establishment of turf roots. Then it is roll downwards slope without being stretched or overlapped. The contact between turf and underlying soil is fundamental to minimise any risk of water flooding underneath the layer. In some cases an heavy turf roller can be used during installation. A first irrigation should be provided to encourage the vegetation establishment.
Maintenance includes mowing the vegetation for an healthy and vigorous turf.
Period of installation: before the period with the peak risk of erosion;
Materials: evergreen turf (indigenous grass are preferred).
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Suitable for controlling surface erosion or slowing runoff flow along a slope/bank, it is also effective in protecting areas with concentrated flows such as channels and drains |
| Topple | 0 | |
| Slide | 7 | |
| Spread | 4 | |
| Flow | 5 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Mostly for earth and debris sediments, not efficient in protection of rock slopes. |
| Debris | 6 | |
| Rock | 2 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Vegetation is effective for controlling surface erosion. Furthermore the roots provide a reinforce for the soil, improving the slope stability at shallow depths. |
| Shallow (0.5 to 3 m) | 6 | |
| Medium (3 to 8 m) | 3 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 3 | Adequate for movements from extremely slow to slow, is not suitable for slopes with high surface erosion rate, since it can be flooded away from deposited sediments. Turfing can prevent triggering of landslides, but not stop their movement once started. |
| Slow | 2 | |
| Very slow | 3 | |
| Extremely slow | 4 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 4 | Applicable irrespective of groundwater conditions. Enough water availability is needed in the soil for turf to grow. Indirect effects on groundwater levels due to root-water uptake from plants during evapotranspiration. |
| High | 4 | |
| Low | 4 | |
| Absent | 3 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | Mostly used for contrasting rainsplash and runoff erosion. Despite they are used along stream banks or river banks for rainwater erosion protection, they are not adequate for shoreline erosion. |
| Snowmelt | 7 | |
| Localized | 5 | |
| Stream | 2 | |
| Torrent | 1 | |
| River | 1 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | Direct effect on surface erosion reduction when the vegetation is properly established. |
| Feasibility and Manageability | 10 | A well site preparation should be ensured before the installation. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | The time of implementation depends on the preparation of the site before the installation, but once installed the measure is immediately effective against rainsplash erosion and runoff. |
| Environmental suitability | 10 | The use of indigenous species is always preferred for consistency with surrouiding environment. |
| Economic suitability (cost) | 8 | Depending on the availability of turf in surrounding area, the cost can be relatively high. |
References
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City U- Slope protection http://personal.cityu.edu.hk/~bswmwong/pl/pdf/slope_protection.pdf
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Evette, A., Labonne, S., Rey, F., Liebault, F., Jancke, O., & Girel, J. (2009). History of bioengineering techniques for erosion control in rivers in Western Europe. Environmental Management, 43(6), 972.
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GEO (Geotechnical Engineering Office). (2011). Technical guidelines on landscape treatment for slopes.
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Turf Australia. Using turf for erosion and sediment control. https://www.turfaustralia.com.au/documents/item/136