Presentation on hydraulic conductivity and drainable porosity

Complete presentation on determination of hydraulic conductivity and also drainable porosity with diagrams and formulas.

1. Presentation on Determination of hydraulic conductivity 1 4/16/2018

2. Hydraulic conductivity is the ratio of velocity to hydraulic gradient indicating permeability of porous media. Symbolically represented as  K, that describes the ease with which a fluid (usually water) can move through pore spaces or fractures. It depends on the  permeability of the material, the degree of  saturation , and on the  density  and  viscosity  of the fluid. Saturated hydraulic conductivity K sat , describes water movement through saturated media. Hydraulic conductivity: 2 4/16/2018 Hydraulic conductivity in non saturated soil is called capillary conductivity . In which ‘Ks’ is hydraulic conductivity of saturating soil and ‘KI’ is soil intrinsic permeability or natural conductivity.

3. The definition of the hydraulic conductivity follows from the Darcy’s Law –Darcy's flow velocity for laminar flow is defined as the quantity of fluid flow along the hydraulic gradient per unit cross sectional area. There fore, V = KI 1. In the saturated flow conditions and according to the Darcy’s Law, the flow velocity v can be expressed as V= K (dh/ dx ) 2. where x is distance in the direction of groundwater flow, h is hydraulic head. Generalized table with the ranges of K-values for certain soil texture are :- Texture Hydraulic conductivity (m/day) Gravelly coarse sand 10 – 50 Medium sand 1 – 5 Sandy loam, fine sand 1 – 3 Loam, clay loam, clay (well structured) 0.5 – 2 Very fine sandy loam 0.2 – 0.5 Clay loam, clay (poorly structured) 0.002 – 0.2 Dense clay (no cracks, pores) < 0.002 3

4. Methods of Hydraulic Conductivity Determination Hydraulic conductivity determination methods Hydraulic methods Correlation methods Field methods Laboratory methods (soil samples) Large scale Small scale - Pumping test - Parallel drains method BELOW WATER TABLE Auger hole Piezometer ABOVE WATER TABLE Guelph Double ring Inverse auger hole -Constant head -Falling head -Pore size distribution -Grain size distribution -Soil texture -Soil mapping unit 4 4/16/2018

5. Hydraulic methods (in the field ) Small scale(Below water table) Auger hole method: Measurement of saturated hydraulic conductivity at a locality with available groundwater level in measured layer is best operated by using the auger hole method. This method is quick and easy and does not demand any expensive equipment. Moreover, natural water from the place being measured is used for the experiment.  A hole is made to a certain depth below the groundwater table. The water table in the hole is lowered with a bailer and then the rate of rise of the water table is measured. F rom the geometry of the auger hole, the value of hydraulic conductivity can be calculated . In the finer textured soils, the pressure required for the initial augering causes a thin, dense seal to form on the sides of the hole. This seal is hard to remove with a hole scratcher. But the removal of seal is essential to obtain reliable data from the test. 5 4/16/2018 Auger hole method Piezometer

6. Drilling of the hole The hole is bored into the soil to a certain depth below the groundwater (GW) level The depth where the GW level is reached for the first time is registered Observe the changes in soil characteristics (color, water saturation, etc.) Wait until the equilibrium with the surrounding GW is reached (until the GW level keeps constant), the stable GW level is measured   Ground water is removed manually by using a bailer or pumped out from the hole  Final GW level after removal is registered 6 4/16/2018 Measurement procedure 1) Drilling of the hole 2) Removal of the water from the hole 3) Measurement of the rate of the rise 4) Computation of the hydraulic conductivity from the measurement data

7. Use of float gauge with a measuring tape or electrical device. The observations are most often made at regular time intervals. About 10 readings are recommended The Durango and Orchard type augers are suitable for most soils, but the Dutch type auger is preferable for some of the high clay cohesive soils. Samples from Durango are less distributed than those from the other two types, thus permitting a more reliable evaluation of soil structure. Dutch or open type Orchard type Durango Types of hand soil augers 7 4/16/2018

8. Observed parameters in field H – stable GW level [m] y  – GW level difference from stable GW after its removal, at the beginning of the rise rate measurement [m] y n  – GW level difference from stable GW at the end of the rise rate measurement [m] y – GW level during the rise rate measurement [m] r – borehole radius [m] 8 4/16/2018 The equation for direct calculation of hydraulic conductivity K, where y 1 , y 2 are measured water levels in a hole at the corresponding times t 1 , t 2

9. Piezometer Methods The piezometer test measures the horizontal hydraulic conductivity of individual soil layers below a water table. This test is preferred over the auger hole test when the soil layers to be tested are less than 18 inches thick and individual layers below the water table are to be tested. This test also provides reliable hydraulic conductivity data for any soil layer below the water table. Non-perforated pipe is placed into the hole under the water level, leaving only a small cavity at the bottom. This means that water can only flows through this cavity. The water table in the hole is decreased with a bailer then the rate of rise of the water table is measured. From this water recharge and from the geometry of the cavity, can be calculated the value of hydraulic conductivity. 9 4/16/2018 Piezometer Soil surface Water table 2R Y2 Y1 d 2a z L ∆y in ∆t cavity

10. This field method can be used for measuring the hydraulic conductivity of layers at relatively great depth or of separate soil layers. Piezometer method is not used in practice very often. This method serves for estimation of impact of soils heterogeneity and also for differentiation of horizontal and vertical components. Disadvantages: The value of hydraulic conductivity represents only the direct surrounding of the small cavity. 10 4/16/2018

11. 4/16/2018 11 Guelph permeameter method The Guelph Permeameter is an easy to use instrument to quickly and accurately measure in-situ hydraulic conductivity. Accurate evaluation of soil hydraulic conductivity, soil sorptivity, and matrix flux potential can be made in all types of soils. The equipment can be transported, assembled, and operated easily by one person. Measurements can be made in 1/2 to 2 hours, depending on soil type, and require only about 2.5 liters of water.   The Guelph Permeameter comes as a complete Kit consisting of the permeameter, field tripod, borehole auger, borehole preparation and cleanup tools, collapsible water container, and vacuum test hand pump, all in a durable carrying case.

12. 4/16/2018 12 Measurements can be made in the range of 15 to 75 cm below the soil surface. The method involves measuring the steady-state rate of water recharge into unsaturated soil from a cylindrical well hole, in which a constant depth (head) of water is maintained. is the saturated hydraulic conductivity (cm/s) a is bore hole radius (cm) H1 is the first head of water established in borehole (cm) C1 is the shape factor Inserting Guelph permeameter into tripod stand

13. This procedure is very well known and is used very often, when the groundwater table is absent. The unsaturated soils two concentric infiltrometer rings are placed at the certain depth, where the infiltration properties (hydraulic conductivity) will be measured. The soil bellow and around the rings is saturated by infiltration. The rings are then filled the water and the rate of fall of the levels in both outer and inner ring is measured. Double ring infiltrometer method 13 4/16/2018

14. The procedure is repeated. The water level in the outer ring being constant and is kept approximately at the same level as is the water table in inner ring. The role of the outer ring is to minimize horizontal flow below the inner ring. The results are almost vertical flow path below the inner ring, where the data are measured. This process is known as cumulative infiltration by ponding. 14 4/16/2018

15. Disadvantages: Results depend on actual moisture content of the soil (sorptivity); only values for the top layer (measured layer) can be found. Boundary effects may cause errors. The rings are then refilled by water, the water level in outer ring is kept constant, and the rate of fall in the inner ring being recorded. It means, that in inner ring is recorded certain infiltrated height of water (e.g. 5,0 mm) and at the same time is recorded continued time. In inner ring is always refilled amount of water, which represented recorder infiltrated height (at this case 5.0 mm). 15 4/16/2018 Where ‘K’ is the saturated hydraulic conductivity and ‘A’ is the parameter

16. The inversed auger hole method, described in French literature as the Porchet method, consists of boring a hole to a given depth, filling it with water, and measuring the rate of fall of the water level. A hole is augered to a certain depth well above the groundwater table. Water is flowed into the dry hole, and then the rate of lowering of the water table is measured. The advantage of this method over the infiltrometer method lies in the difference between digging soil pits and making auger holes. Inversed Auger-Hole Method 16 4/16/2018

17. Moreover, by gradually deepening the auger hole and filling it with water over the corresponding depth, the hydraulic conductivity of successive layers can be measured in the same hole. From this rate of decreasing and from the geometry of the borehole, is calculated the value of hydraulic conductivity. Common procedure in field surveys for surface or subsurface drainage design, if groundwater table is not present. 17 4/16/2018 Hydraulic conductivity by inverted auger hole method In which, K = hydraulic conductivity (cm/sec) r = hole radius (cm) hn = Depth of water level inside the hole (cm) tn = End time (sec) to = Start time (sec)

18. 4/16/2018 18 Large-scale field methods are designed for determining hydraulic conductivity below the water table (i.e., K of the saturated zone). The methods available for large-scale K determination are of two types : the method that uses pumping from wells (known as ‘pumping test’) (b) the method that uses pumping or gravity flow from horizontal drains (‘parallel drains method’). The pumping test is the standard and most accurate method for determining ‘hydraulic conductivity’ and ‘storage coefficient’ of saturated zones (aquifers). Large scale field methods Pumping test Parallel drains

19. 4/16/2018 19 Using the parallel drains method, hydraulic conductivity (K) can be determined from the functioning of drains in experimental fields, pilot areas, or on existing drains and thus this method is very suitable for drainage. This method uses observations on drain discharges and corresponding elevations of the water table in the soil at some distance from the drains. From these observed data, the value of K can be calculated using a drainage formula (either steady­state or unsteady­ state formula) appropriate for the conditions under which the drains are functioning. The advantage of large­scale determination is that the flow paths of the groundwater and the natural irregularities of the K values along these paths are automatically taken into account in the overall K value found by the method.

20. The water head at one of the sides of the sample decreases with time. A high initial water head is desirable for low hydraulic conductivities. The calculation of the hydraulic conductivity from the velocity of total flux through the sample can be somewhat complicated, because the head difference is not constant. This laboratory method is suitable especially for layers with a low hydraulic conductivity, in horizontal or vertical direction. Disadvantages: Small sample area means the high possibility of a large random error. Falling-Head Method 20 4/16/2018

21. 4/16/2018 21 L is length through the soil y is the height of ponded water x is the height of water required to lower the gradient so that y can be maintained. Note: if the gradient is 1 then K s = q as per Darcy’s Law.

22. The constant-head  permeameter is a suitable method to determine the saturated hydraulic conductivity (K s ). However, for highly permeable soils, resistances to flow in tubing systems of conventional constant-head  permeameters may result in an underestimation of K s . A constant difference in head is created over an undisturbed soil sample in a Kopecky steel ring. At certain times, the volume of water that has flowed through the sample is measured. From this discharge, the size of the soil sample, and from the difference of head, can be calculated the value of hydraulic conductivity. Constant-Head Method 22 4/16/2018

23. Advantages Laboratory method is good tool to measure hydraulic conductivity of a certain layer in horizontal or in vertical direction. Disadvantages The measured value is valid for the relatively small soil sample area only, so there may be a large error. This laboratory method is not suitable for samples with extremely high or with very low hydraulic conductivity. 23 4/16/2018 a is the cross-sectional area of the burette A is the cross-sectional area of the soil column t 2 – t 1 is the time required for the head to drop from H 1 to H 2 .

24. Correlation Methods Correlation methods are based on predetermined relationships between an easily determined soil property Texture, Pore­size Distribution, Grain­size Distribution, Soil Mapping Unit A variety of empirical formulae are available which relate K with content of sand, silt and clay; K with grain diameter (mean or effective grain diameter); K with grain­diameter and porosity; K with grain­size distribution; and K with soil series and they can be used in the absence of field or laboratory values of hydraulic conductivity. For example, Smedema and Rycroft (1983) provided a generalized table with ranges of K­values for certain soil textures. However, such tables should be handled with care. Smedema and Rycroft (1983) warn that: “Soils with identical texture may have quite different K­values due to differences in structure and some heavy clay soils have well­developed structures and much higher K­values than those indicated in the table”. 24 4/16/2018

25. 4/16/2018 25 Sl. No. Soil Texture Range of K (m/day) 1 Gravelly coarse sand 10 – 50 2 Medium sand 1 – 5 3 Sandy loam, fine sand 1 – 3 4 Loam, clay loam, clay (well structured) 0.5 – 2 5 Very fine sandy loam 0.2 – 0.5 Values of K by soil texture ( Smedema and Rycroft, 1983)

26. 4/16/2018 26 Of the various empirical formulae, the Hazen formula is a simple relationship between the hydraulic conductivity and the effective grain diameter, and it is often used for the estimation of hydraulic conductivity from grain­size distribution data. It is expressed as (Freeze and Cherry, 1979):   K = A × d 10 2 Where , K = hydraulic conductivity, (cm/s); d 10 = effective grain diameter, (mm) which is determined from the grain­size distribution curve; A = constant, which is usually taken as 1.0 (Freeze and Cherry, 1979).   The advantage of the correlation methods is that an estimate of the K value is often simpler and faster than its direct determination. However, the major drawback of these methods is that the empirical relationship may not be accurate in all cases, and hence may be subject to random errors.

27. Determination of drainable porosity 27 4/16/2018

28. 4/16/2018 28 Determination of Drainable Porosity   Drainable porosity can be measured in the laboratory or in the field. In the laboratory, drainable porosity can be measured using Hanging Water­Column apparatus , which is suitable for a tension range of 0­150 cm. Hanging Water­Column apparatus consists of a glass funnel with a porous plate, a burette, and a flexible transparent tube connecting the glass funnel and the burette. Hanging­Water Column apparatus: (a) Initial saturated sand column; (b) Lowered burette.

29. 4/16/2018 29 Undisturbed soil sample is taken from the field and it is saturated in the laboratory with the help of Hanging Water­Column apparatus . The saturation may take 24 hours or more depending on the soil type. The water level in the burette is maintained at the same level as the top of the porous plate . Thereafter, the burette is lowered by a certain distance (usually in steps of 10 cm ), which imparts a suction to the saturated soil sample, and hence water starts draining slowly. The drained water raises the water level in the burette. At a given suction, this rise in water level is adjusted by readjusting the burette height such that the originally applied suction is closely maintained. When there is no further rise in water level in the burette, the elevation difference between the top of the porous plate and the water level in the burette are noted down, which gives the value of average suction applied to the soil sample. The difference between the initial and the final burette readings gives the volume of water drained from the soil sample due to the applied suction. This process is repeated by lowering the burette in steps of 10 cm initially and more lately until the desired suction (corresponding to the maximum possible depth of the subsurface drain or any other criteria) is obtained.

30. 4/16/2018 30 If the water retention characteristic of the soil is known and if the pressure­head profile is known for two different levels of water table, the drainable porosity (m) can be calculated from the following equation: Where, z 1 = water table depth (m) for Stage 1 (say at t = t 1 ), z 2 = water table depth (m) for Stage 2 (say at t = t 2 ), q 1 (z) = soil­water content as a function of soil depth for the water­table position at t 1 q 2 (z) = soil­water content as a function of soil depth for the water­table position at t 2 . Thus, ‘drainable porosity’ can also be defined as “the ratio of the change in soil­water content in the soil profile above the water table to the corresponding rise/fall of the water table in the absence of evaporation”.

31. THANK YOU Presented By Akhila Shiney (2017-18-015) 31 4/16/2018

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