What is the influence of actual Evaporation (AE) while calculating the rainfall slope stability analysis.
The total Precipitation(P)= Infiltration (I)+Runoff(R)+AE+Potential E(PE)+Transpiration.
AE makes the soil dry which results in increase in effective stress due to suction pressure.What is the influence of AE on slope stability. Whether the magnitude of AE is small and hence negligible or should take into account for calculation factor of safety of soil slopes (landslide studies) during rainfall periods or monsoon seasons.
Actual Evapotranspiration (AE) can be important while calculating the slope stability. Soils (specially clayey soils) can generate significant amount of suction (even upto several mPa) when water is withdrawn from it. You can calculate the suction by knowing the volumetric water content in the soil using the water balance equation which is as below.
Rainfall - Evapotranspiration - Runoff - Interflow = change in water storage
Once you know the water content of the soil you can then relate it to the soil water characteristics curve (also know as water retention curve). It can be very difficult to do by hand calculation. There are programs that does it.
It can be argued that, the worst possible stress state of soil will be at full saturation. While it is true, the swelling and shrinkage caused by seasonal fluctuation of water content can lead to stiffness degradation where soil looses its strength over time. This can have detrimental effect on the stability of slopes. And also the swell and shrinkage can cause serviceability problems to structures.
Of the top of my head, swell and shrinkage related problems are responsible for the highest amount of insurance claims in UK.
Sir, Thank you for the reply. I have some more doubts:
Firstly during rainfall season or during rainfall duration magnitude of AE will be less as it is function of Daylight and Wind velocity (correct me if I am wrong, my specialization is not in Hydrology). Still does it have impact of Slope Stability Analysis (SSA).
Secondly if we perform Evapo-Transpiration (ET) calculation will it take care of AE, or separate calculation is needed.
Thridly will it be more precise to consider Runoff(R) and ET for SSA. Very few literature may be available on including ET, R and Infiltration(I) together for SSA.
Dear Chatra,
the actual evapotranspiration may be much less than the potention if the soil moisture storage capacity is limited.
Katia
One of major problem is predicting the amount of runoff
that will occur from a given rainfall.
The infiltration capacity varies with the soil type and moisture conditions. Katia
Dear Kátia Vanessa Bicalho ,
I agree that Actual Evaporation will be much less, but should it be considered while carrying out slope stability analysis or it can be neglected as it magnitude is small.
Secondly,
Runoff (R) depends on the soil type and moisture condition. If I am planning for different soil which having different hydraulic conductivity or permeability, will it be appropriate to consider R while carrying out slope stability analysis.
How about considering Eavpotranspiration for antecedent rainfall conditions along with R?
Thank you in Advance
Dear Adarsh,
There are some coupled numerical models that may calculate the predicted soil state profiles considering soil-atmosphere interactions. I have some publications on it and please let me know if you need the full text. There is a good book on it: Applied hydrogeology , C W Fetter. You can not neglect the actual evaporation that depend on soil properties and on the climatic conditions.
Best regards,
Katia
Dear Kátia Vanessa Bicalho,
It would be of great help if you could send me your research articles.
Thank you.
Hi Adarsh,
I tried to address the questions you raised. See below.
1. "Firstly during rainfall season or during rainfall duration magnitude of AE will be less as it is function of Daylight and Wind velocity (correct me if I am wrong, my specialization is not in Hydrology). Still does it have impact of Slope Stability Analysis (SSA)"
- During rainy seasons the slope will mostly remain saturated - which is usually the worst effective condition. If you look at the problem from this point of view, you wont need to calculate the PET or AET at all. Just do the slope stability condition with the water table at the surface or any possible flooding.
2. "Secondly if we perform Evapo-Transpiration (ET) calculation will it take care of AE, or separate calculation is needed."
- To calculate the actual ET, usually a potential value is calculated first for a standard crop. The actual ET can be equal to or less than the potential value depending on the water available in the soil.
3. "Thirdly will it be more precise to consider Runoff(R) and ET for SSA. Very few literature may be available on including ET, R and Infiltration(I) together for SSA."
-If you want to know the suction generated in the soil during the dryer months, and the amount of suction carried throughout the wet months of the year, a complete water balance is needed. Runoff is one of the important quantity in the water balance equation. There are many numerical programs that can automatically calculate the run-off. Simpler assumptions can also be made to estimate runoff by knowing the soil permeability and available storage capacity using a spread sheet calculations.
Everything depends on why you want to calculate the Evapo-transpiration and runoff. If you are trying to capture the strength degradation or swell-shrink behaviour or you want to know what amount of suction can be carried through the wet months of the year then yes you will have to calculate them. Otherwise I cant think of other reason why you will need to do it.
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