Nepal is taken a good example of bio-engineering for erosion control and slope stability. Species like Alnus nepalensis (tree), Cymbopogon citratos (herb), Eulaliopsis binata (herb), Thysanolaena maxiama (perennial grass) are few widely used plants. Are there any research on performance of commonly used species including aforementioned ones taking consideration of their efficiency in decreasing pore water pressure and also increasing soil strength of the instable slopes?
Interesting question Susmita . Infact , this si the only basis of using such palnt species for successful bio-engineering , not only in Nepal , nut elsewhere as well. Here is interesting work..
Assessing the joint development of vegetation cover and soil properties is crucial to evaluate the efficiency of soil bioengineering techniques, especially during the most critical initial phase of vegetation colonization. We set up a laboratory experiment to quantify and disentangle the effect of Alnus incana roots on soil permeability and aggregate stability. Plants were grown in pots in a climate chamber for four different growing periods (1, 2, 4 and 8 months). Pots were filled with a soil coming from a moraine of a landslide area in Central Switzerland. After each growing period, surface permeability, soil volume permeability and soil aggregate stability were measured together with the development of the root systems. Our results show that alder roots significantly improve both surface and whole soil volume permeability already after 2 months of growth. Nevertheless, an increase in root length density does not necessarily correspond to an increase in permeability. We could set as a threshold a root length density of 0.1 cm/cm3 until which an increase in root development corresponds to an increase in soil permeability, whereas after this threshold we observed a decrease in soil permeability. A significant increase in soil aggregate stability could be observed only with a root length density of 2 cm/cm3 . No obvious correlation between soil permeability and aggregate stability could be found. Future work should validate these laboratory results with field data. Source : Vergani and Graf, ) DOI: 10.1002/eco.1686
PDF enclosed for further reading ..
I am not aware of any specific research in Nepal to address the issue of vegetation effects on soil pore water pressure or strength! Indeed it is an interesting and important area for further work.
Interesting discussion...
Vegetation helps stabilize forested slopes by providing root strength and by modifying the saturated soil water regime. Plant roots can anchor through the soil mass into fractures in bedrock, can cross zones of weaknessto more stable soil, and can provide interlocking long fibrous binders within a weak soil mass. In Mediterranean-type climates, having warm, dry summers, forest evapotranspiration can develop a substantial soil moisture deficit which can reduce both piezometric head and slope mass. Pore water pressures change seasonally in response to precipitation and are often the driving mechanism which ultimately leads to slope failure. When trees are cut, the root system begins to decay, and the soil-root fabric progressively weakens. The loss of root strength or increased soil moisture content or both after-tree removal can lower the slope safety factor sufficiently that a moderate storm and associated rise in pore water pressure can result in slope failure. After trees are removed, the frequency of landslides can increase. Source :Robert R. Ziemer,
Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Dept. of Agric., 1700 Bayview Dr., Arcata, CA, US A
Dear All,
Many thanks for the information. It is possible to find different ages of Alnus nepalensis in the field. If planned one can do such research then.
Susmita
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