OBJECTIVES:
To successfully complete field test infiltration determination using a double ring Inflitrometer.
To appreciate the importance of Infiltration determination in Irrigation engineering.
To analyze the infiltration data to obtain essential information.
APPARATUS REQUIRED:
THEORY:
Infiltration is the process by which water on the ground surface enters the soil. The infiltration capacity is defined as the maximum rate of Infiltration. Accumulated infiltration is defined as the total quantity of water that enters the soil in a given time.
Double ring infiltrometer are the most commonly used infiltrometer consisting of two concentric rings driven into the soil uniformly without disturbing the soil. The purpose of outer tube is to eliminate to some extent the edge effect of the surrounding drier soils and to prevent the water within the inner space from spreading over a larger area after penetrating below the bottom of the ring.
PROCEDURES:
A representative area of the field to be tested was selected by avoiding disturbed surfaces, animal burrows, stony soil paths, roads and the soil condition on the area selected.
The inner cylinder was placed onto the ground and the outer cylinder placed over the inner cylinder with both sharp surfaces facing the ground. A driving plate was placed on top of both cylinders. Using a mallet,the driving plate was knocked firmly. The impact was absorbed and distributed so that the ring penetrated the ground at right angles up to a depth of 10cms.
The outer cylinder was filled with water to saturate the soil. The purpose of the water in the outer cylinder was to moist the soil beneath the Inflitrometer and to form a buffer zone. The water infiltrated from these zones prevented lateral seepage of the water from the inner cylinder.
The inner cylinder was filled with water. The water levels of both cylinders were observed to ensure that they were the same.
Water is poured into the rings to maintain the original constant depth at regular time intervals (after the commencement of the experiment ) of 5, 10, 15, 20,..etc minutes and the drop in water level with time was recorded.
The above process was repeated until the rate of infiltration reached a constant reading.
Once the infiltration measurements were complete, the cylinders were extracted from the soil.
OBSERVATIONS:
Area of inner cylinder=`(pi d^2)/4=pi/4*30^2=225pi (cm)^2`
S.no. | Time (Min) | Volume of infiltration (L) | Depth of infiltration (cm) | Depth of infiltration (cm) | Infiltration Rate (cm/hr) |
1 | 0 | - | - | - | - |
2 | 5 | 3.180 | 4.4988 | 4.4988 | 53.9856 |
3 | 10 | 1.450 | 2.0513 | 6.5501 | 24.6156 |
4 | 15 | 0.930 | 1.3157 | 7.8658 | 15.7884 |
5 | 20 | 1.000 | 1.4147 | 9.2805 | 16.9764 |
6 | 25 | 0.975 | 1.3792 | 10.6598 | 16.5516 |
7 | 30 | 0.975 | 1.3793 | 12.0391 | 16.5516 |
8 | 35 | 0.975 | 1.3793 | 13.4184 | 16.5516 |
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The plot of infiltration rate versus time is shown in the natural graph. Best-fitting curve for plotted points are shown in the graph.
`f_0=191.15`cm/hr
`f_c=16.55`cm/hr
`F_c=`Shaded area
Or, `F_c=11*5*10/60+½*2.5*10/60+½*5*1.5/60+½*10*0.5/60 = 9.4792 cm`
Now Horton's Constant is,
`K=(f_0-f_c)/F_c`
`=(191.15-16.55)/9.4792`
`=18.42 (hr)^(-1)`
We know, the Horton's equation is,
`f=f_c+(f_0-f_c)*e^(-kt) `
`=16.55+174.60*e^(-18.42 t) `
This is the required equation for the infiltration capacity curve, where f is in cm/hr and t is in hr.
Let us check this equation:
For `t=5`min, `f=16.55+174.60*e^(-18.42*5/60) =54.1686` cm/hr, which is very near compared to the observed value of 53.99 cm/hr.
Similarly, for `t=10` min, `f=16.55+174.60*e^(-18.42*10/60) =24.6551`cm/hr, which is very near compared to the observed value of 24.62 cm/hr.
Hence, our equation is correct as it represents the general equation for infiltration rate.
DISCUSSION:
The rate of infiltration decreases with time. The rate of decrease is rapid initially and the infiltration rate tends to approach constant value known as the basic infiltration rate.
When soil becomes saturated, rate of infiltration will decrease because soil only will take in water which can be transmitted down.
After ponding infiltration rate decreases approximately exponentially, initially driven by both capillary gradients and gravitational gradients. However, after certain time, the infiltration through profile capillary gradients is minimized (approaches zero) and is driven by gravity gradients up to an asymptotic value `K_(sat)`.
The basic infiltration rate can be used to provide an estimate of run-off given rainfall data and evaporation losses. Various cases are highlighted below:
For rainfall rates less than saturated conductivity of soils all rainfall will infiltrate, no runoff will occur.
For rainfall rates > Ksat but less than the soils maximum infiltration capacity, initially all water will infiltrate. Since rate > Ksat all water cannot be transmitted down, water storage in soil will increase until soil is saturated.
For rainfall rates greater than maximum infiltration capacity get immediate ponding and Exponential decay from maximum infiltration capacity toward minimum infiltration capacity.
There are consequences for both overestimation and underestimation of infiltration rate. If infiltration rate is overestimated during the infiltration test, system performance suffers once the storm water control measure is constructed. Volume reduction, mitigation of peak flow rate and groundwater recharge will all be overestimated during the design phase resulting in a lack of hydrologic function and pollutant mitigation. Consequences in underestimation of infiltration rate are typically observed in increased construction costs due to the addition of under drains, deeper bio-retention media depths and reduced ability to utilise internal water storage zones as part of the design.
SOURCES OF ERRORS:
CONCLUSION:
Infiltration rate decreases with an increase in elapsed time up to a time when the infiltration rate becomes constant due to soil saturation.
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