Vegetation - Hydrological effects (NBS)

Category: MODIFYING THE SURFACE WATER REGIME – Surface drainage

Description

Both soil erosion and shallow landslides may occur due to degradation or removal of land cover or vegetation. The use of vegetation to mitigate these phenomena has long been common practice. The role of vegetation in mitigating soil erosion is described in Nature-Based Solutions for surface protection and soil erosion and summarized in Figure 1. Another role of vegetation in contributing to slope stability is based on its hydrological functions.

Figure 1: Some influences of vegetation on soil stability (source: Coppin and Richards, 1990)

 

Effects of vegetation on induced soil suction 
It is well known that vegetation enhances evapotranspiration (ET) process governed by plants, which decreases volumetric water content (VWC) and increases soil suction (negative pore water pressure) in unsaturated soil with a consequent reduction in the permeability of soils and potential increment of soil shear strength through apparent cohesion (Greenwood et al., 2007; Ng & Menzies, 2007; Ng & Leung, 2012).

Many types of physical model have been realized in laboratory to quantify the induced soil suction given by different types of vegetation species (Ng et al., 2013; Leung et al., 2015; Capobianco et al., 2018), concluding that under the same atmospheric conditions, the induced soil suction in vegetated soils results higher than that in bare soil. This is due to the Evapotranspiration process (ET) which includes the activity of roots in taking up water from soil. Since the effect of vegetation on the induced soil suction is dependent on temperature, humidity and external stresses (i.e. wind: see Dainese et al., 2018), it typically shows a seasonal trend which has a high peak during summer, when temperature is higher and vegetation is more florid, and decreases in wet season, when evaporation is reduced by lower temperature and the transpiration is vanishing because vegetation is quiescent. Other factors influencing the vegetation induced soil suction are the roots and leaves characteristics. Ng et al. (2016) tried to correlate the suction induced by evapotranspiration process with plant traits and characteristics and they found a positive linear correlation between plant characteristics, mostly associated to roots (RAI) and leaves (LAI) and mean peak suction. This means that as the root network increases into the soil and the leaves cover higher canopy area, the higher is the suction induced by evapotranspiration (Figure 2).

Figure 2. To the left: relationships of mean peak of suction (Δs) and mean peak RAI. To the right: Relationships between mean LAI and peak of suction (Δs) during drying (Ng et al., 2016).

 

In addition, during plant growth, roots occupy soil pore spaces by potentially changing the whole soil structure (i.e. reducing the porosity) with a modification of both soil water retention curves and soil hydraulic conductivity (Buczko et al. 2007; Scanlan and Hinz 2010; Jotisankasa & Sirirattanachat, 2017).
 

Effects of vegetation on infiltration rate

During a rainstorm, several factors can affect the rainwater infiltration in vegetated soils. Firstly, vegetation can reduce raindrop impact on the soil by slowing the rainwater movement. As consequence, the net rainwater is reduced because of leaf interception, and the rainwater reaching the ground underneath the vegetation may have a better chance of infiltrating than on unvegetated soil (Morgan & Rickson, 1995). On the other hand, the water fluxing into the root permeated soil is changed because the soil water retention and soil hydraulic conductivity are modified (Inoue et al., 2000) due to the evapotranspiration activity of the plants.

Several past studies have been conducted to quantify the infiltration rate in natural soil vegetated with different vegetation species. Nevertheless, the results obtained seem to be confusing because of many factors affecting the hydraulic response of rooted permeated soil, but some general considerations can be done.  One factor affecting the hydrological response of root permeated soil is the state of roots: actively growing or decaying. It was found that actively growing plants could reduce infiltration rate and saturated permeability of the soil (Huat et al. 2006; Leung et al. 2015b, 2015c). On the other hand, for grass-covered soil, if the roots are actively growing the infiltration rate can be lower than that in bare soil (Gish and Jury, 1983; Huat et al., 2006; Ng et al., 2014); whereas, if roots are decaying the infiltration rate can be higher than those in bare soil (van Noordwijk et al., 1991; Mitchell et al., 1995). The presence of organic matter, earthworms, decaying roots helps to maintain a continuous pre-system and thus an higher hydraulic conductivity (Morgan & Rickson, 1995). Another factor affecting the hydraulic response of root permeated soil is the creation of possible preferential flow networks (Ghestem et al., 2011) or conversely the clogging of soil pores (Gabr et al., 1995; Scanlan and Hinz, 2010), but these aspects are still not well understood.  

In general, the influence of vegetation on the infiltration rate depends on vegetation species (e.g. plant species or grass species), state of roots (e.g. actively growing or decaying) as well as rainfall amount (e.g. heavy rainfall, low intense rainfall).

Based on the positive hydraulic effects vegetation is widely believed to improve the stability of slopes, especially on steep slopes and with respect to superficial or shallow movements. However, it can take a long time to become effective at depth and it can also have negative effects, as summarized in Table 1 (Greenway 1987; Wu, 1995).

Geotechnical Manual for Slopes (Geotechnical Control Office, 1984), reflects one of the most comprehensive research programs in the world on the engineering role of vegetation for slope stabilization (Barker, 1991).



Design methods

Selection of vegetation species.

There are several factors to consider when vegetation is selected to modify the surface water regime of slopes. Once defined the water regime conditions, the selection of the most suitable species to be implemented depends on many aspects of the slopes (e.g. morphology, type of soil, slope inclination, accessibility with equipment), as well as on climate conditions such as temperature, relative humidity, wind. Usually to ensure a good establishment in a relative short period, indigenous plants are preferred.

Another consideration concerns the role that the vegetation should have in providing "hydraulic" reinforcement to the soil. For example, if the vegetation is requested to act as protection from rainfall-induced landslides, its main role is the reduction of infiltration rate. In this case, the formation of organic matter should be avoided because it is the main cause of increase in permeability of soil. The formation of organic matter has been found to increase when there is a high tree-tree competition is available (i.e. low space between plants), so when the density of tree planting is high, and more decay roots are found (Ng et al., 2016). In addition, when there is enough space between plants a higher amount of suction can be preserved and thus the drop of suction due to rainfall infiltration is reduced. Some examples of plants that demonstrated to well preserve the soil suction into the soil and thus reduce the rainfall infiltration in tropical areas are Orange jasmine, Vetiver, Schefflera heptaphylla (Rahardjo et al., 2014; Garg et al., 2015; Ng et al., 2016).

When the role of plants is aimed to increase or preserve the induced suction and maintain a low ground water level, it was found that shrubs or trees are more suitable because their longer roots can affect a deeper soil region (Garg et al., 2015). On the other hand, there are some grass species more able than others in increasing suction and this is mostly related to the root density (Rahardjo et al., 2014). In some cases, there are some grass species able to reach great depths and this positively affect the induced suction at deepest depths (Capobianco et al., 2018).

Another factor to consider is the suction recovery after a rainfall event, which is found to be fastest along slopes vegetated with trees (Garg et al., 2015).

The induced soil suction in vegetated slopes is also influenced by the plants spacing: the lower tree spacing the higher induced soil suction. When mixed trees and grass vegetation is considered, the best solution is to have a wider spacing between trees to enhance the activity or grass roots on the induced suction (Ni et al., 2016).

The effect of vegetation on ground water level can be either observed indirectly by the measured volumetric water content and by the piezometric measurements at different depths. Is likely observed a more stable and lower ground water level when soil is reinforced with tree roots compared with grass or shrubs roots (Krzeminska et al., 2018).
 

Establishment period

After planting works, the Establishment period, when the vegetation is still weak and can require maintenance for its growth, starts. The Geotechnical Engineering Office in Hong Kong territories suggests an Establishment period to be 12 months, during which the contractor is required to provide horticultural care and maintenance of all plants to ensure their healthy establishment and growth (GEO, 2011). Often in this period the use of fertilizers is recommended to enhance the plant growth and thus increase the roots/leaves activity in the evapotranspiration process, which positively affect the induced soil suction (Ng et al., 2018).



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 Most suited to all types of slides and, to a lesser extent, flows, by attenuating the impact of intense precipitation and by inducing increase in soil suction.
Topple 0
Slide 9
Spread 9
Flow 7

Material type

Descriptor Rating Notes
Earth 9 Applicable to landslides involving earth and only to a lesser extent in debris. Applicability in rock limited by typical slope geometry and failure mode. Vegetation cannot establish easily along rocky slopes.
Debris 8
Rock 1

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 7 Typically applicable to landslides of any depth, but relative effectiveness decreases with increasing depth of movement.
Shallow (0.5 to 3 m) 8
Medium (3 to 8 m) 4
Deep (8 to 15 m) 2
Very deep (> 15 m) 1

Rate of movement

Descriptor Rating Notes
Moderate to fast 1 Seeding can be applied remotely, by helicopter if necessary. However, it needs time to become established (especially trees) and this may limit application in moderately to fast movements.
Slow 3
Very slow 3
Extremely slow 3

Ground water conditions

Descriptor Rating Notes
Artesian 4 Applicable irrespective of groundwater conditions. Effects on groundwater levels only indirect through reduced infiltration and suctions. Potential difficulties and/or extra maintenance required where groundwater is low or absent.
High 6
Low 5
Absent 4

Surface water

Descriptor Rating Notes
Rain 6 Water courses should be diverted. Even small localized flows may hinder establishment.
Snowmelt 6
Localized 5
Stream 2
Torrent 1
River 2

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 6 Effects on stability only indirect and strictly dependent on the type of plant used, root depth, leaf ratio.
Feasibility and Manageability 6 will be Apparently simple and long practiced technique, it requires careful selection of species. Ongoing discussion about real benefits and limits of applicability.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 8 Easily implemented with widely available equipment. However, it requires intense maintenance during early stages, say up to 3 years in certain cases.
Environmental suitability 10 Environmental friendly, it mostly involves the use of live plants
Economic suitability (cost) 10 Low costs, where applicable.

References

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  • Capobianco, V., Cascini, L., & Foresta, V. (2018). Long Root Grasses in Pyroclastic Soils: Vegetation Growth and Effects on Induced Soil Suction. In Proceedings of China-Europe Conference on Geotechnical Engineering (pp. 1260-1263). Springer, Cham.

  • Dainese, R., Belli, A., Fourcaud, T., & Tarantino, A. (2018). An infiltration column to investigate experimentally the response of the Soil-Plant-Atmosphere Continuum. HKUST.

  • Gabr MA, Akran M, Taylor HM. 1995. Effect of simulated roots on the permeability of silty soil. Geotechical Testing Journal, ASTM 18(1): 112–115.

  • Garg, A., Coo, J. L., & Ng, C. W. W. (2015). Field study on influence of root characteristics on soil suction distribution in slopes vegetated with Cynodon dactylon and Schefflera heptaphylla. Earth Surface Processes and Landforms, 40(12), 1631-1643.

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  • Greenwood J.R., Norris J.E., Wint J., Goodwin A.K., Mc Donald M. (2007). “Discussion: Assessing the contribution of vegetation to slope stability” Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, Vol. 160, 51-53.

  • Huat, B. B., Ali, F. H., & Low, T. H. (2006). Water infiltration characteristics of unsaturated soil slope and its effect on suction and stability. Geotechnical and Geological Engineering, 24(5), 1293-1306.

  • Jotisankasa, A., & Sirirattanachat, T. (2017). Effects of grass roots on soil-water retention curve and permeability function. Canadian Geotechnical Journal, 54(11), 1612-1622.

  • Krzeminska, D., Kerkhof, T., Skaalsveen, K., & Stolte, J. (2019). Effect of riparian vegetation on stream bank stability in small agricultural catchments. Catena172, 87-96.

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  • Ng, C. W. W., & Leung, A. K. (2012). Measurements of drying and wetting permeability functions using a new stress-controllable soil column. Journal of Geotechnical and Geoenvironmental Engineering, 138(1), 58-68.

  • Ng, C. W. W., Leung, A. K. & Woon, K. X. (2014). Effects of soil density on grass-induced suction distributions in compacted soil subjected to rainfall. Canadian Geotechnical Journal 51, No. 3, 311–321.

  • Ng, C. W. W. & Menzies, B. (2007). Advanced Unsaturated Soil Mechanics and Engineering. Taylor and Francis, USA.

  • Ng, C. W. W., Ni, J. J., Leung, A. K., Zhou, C., & Wang, Z. J. (2016). Effects of planting density on tree growth and induced soil suction. Géotechnique, 66(9), 711-724.

  • Ng, C. W. W., Tasnim, R., Capobianco, V., & Coo, J. L. (2018). Influence of soil nutrients on plant characteristics and soil hydrological responses. Géotechnique Letters8(1), 19-24.

  • Ng, C. W. W., Woon, K. X., Leung, A. K., & Chu, L. M. (2013). Experimental investigation of induced suction distribution in a grass-covered soil. Ecological engineering, 52, 219-223.

  • Ni, J. J., Leung, A. K., Ng, C. W. W., & So, P. S. (2016). Investigation of plant growth and transpiration-induced matric suction under mixed grass–tree conditions. Canadian Geotechnical Journal, 54(4), 561-573.

  • Scanlan CA, Hinz C. 2010. Insights into the processes and effects of root-induced changes to soil hydraulic properties. In Proceedings of the 19th World Congress of Soil Science, 2.: Brisbane, Australia; 41– 44.

  • Simon A., Collison A. (2002). “Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability”. Earth Surface Processes and Landforms, 27(5), 527-546.

  • Van Noordwijk, M., Heinen, M., & Hairiah, K. (1991). Old tree root channels in acid soils in the humid tropics: important for crop root penetration, water infiltration and nitrogen management. Plant and Soil, 134(1), 37-44.

  • Wu T.H. (1995). “Slope Stabilization”. In: Slope Stabilization and Erosion Control: a Bioengineering Approach, R.P.C. Morgan, R.J. Rickson (eds.), London, E & F. N. Spon London, Chapter 7, 221-263.

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