Determination of Infiltration Rate by INFILTROMETER

LABORATORY WORK

IN

HYDROLOGY

KANCHAN KASWAN

KANISHKA SAHNI

KAPIL DEV BANSAL

KARAN PREET SINGH

K. S. HARI PRASAD

DEPARTMENT OF CIVIL ENGINEERINGINDIAN INSTITUTE OF TECHNOLOGY

ROORKEE-247667FEBRUARY 2012Contents

Page No.

1. Determination of Average Rainfall over a Catchment

3

. Kapil Dev Bansal

2. Determination of In-situ Soil Hydrological Properties

10 . Kanishka Sahni

3. Determination of Infiltration Parameters with Double Ring Infiltrometer 15 . Kanchan Kaswan

4. Measurement of Discharge in an Open Channel with Current Meter

19 . Karanpreet Singh5. Determination of Hydraulic Conductivity with Constant Head Permeameter24 . K. S. Hari Prasad References

29Chapter 1Determination of Average Rainfall over a Catchment

Aim: To determine average rainfall over a catchment by (i) Arithmetic Average Method (ii) Thiessn polygon method and (iii) Isohyetal MethodApparatus Required: Thematic map of the catchment with location of raingauge stations, Planimeter, Tracing and Graph Sheets, Surfer softwareTheory: The average rainfall over a catchment is needed in the analysis and design of several hydrological process such stream flow estimation, flood analysis and identification of areas vulnerable during heavy rains. The rainfall recorded at a raingauge station represents only point a small part of the catchment. However, the rainfall distribution may not be uniform over the entire catchment and necessitates the need for arriving at an average value for the entire catchment. The rainfall distribution over a catchment depends primarily on the nature of the catchment (plain or hilly area) and one has to devise an accurate of way of deterimining the average rainfall for individual catchment. The commonly used methods for the determination of average rainfall in a catchment area are (i) Arithmetic Average Method (ii) Thiessn polygon method and (iii) Isohyetal Method

Arithmetic Average Method:When the rainfall measured at various raingauge stations in a catchment show little variation, the average precipitation over the catchment is determined as the arithmetic mean of the rainfall values recorded at each of the stations. If P1, P2,, PN are the rainfall values recorded at N raingauge station in the catchment, the average rainfall Pave is computed as the arithmetic average;

(1.1)

This method is very simple and can be used for very small catchments which are plain. Since, the methods does not take into account the terrain characteristics of the catchment, it is rarely used in case of large catchments. Thiessen Polygon Method:In Theissen polygon method (Subramanya, 2008), each raingauge station is given weight on the basis of the area closest to that station. Fig 1.1 shows a catchment with five rain gauge stations. Among these, one rain gauge station is outside the catchment.

Fig 1.1: Thiessen Polygon MethodThe raingauge stations are joined with one another to form a network of triangles. Perpendicular bisectors for each side of these triangles are drawn as shown in Fig. 1.1. These bisectors form polygons around each raingauge station and are called Thiessen Polygons. These polygons have the property that every point within the polygon will be closest to the rain gauge station representing that polygon. In case of rain gauges located out side the catchment, the intersection of the bisectors with the catchment boundary is considered as the polygon representing that station Station D in fig. 1.1). The areas of the polygons are determined either with a planimeter or using an overlay grid. If P1, P2,, PN are the rainfall recorded at N raingauge station in the catchment and A1, A2,, AN are the corresponding Thiessen polygons, the average rainfall Pave over the catchment is determined as

(1.2)

This method gives very good results for plain catchments. However, it is not very much suitable for mountainous catchments.

Isohyetal Method:Isohyetal method uses isohyets to determine the average rainfall over a catchment. An isohye is a line joining points of equal rainfall magnitude. In Isohyetal method, the catchment is drawn to a scale and the position of the raingauge stations both in the catchment and neighbouring stations are marked. Knowing the recorded values of rainfall at these stations, isohyets of various values are drawn through interpolation. The interpolation can be carried out either manually or using softwares such as SURFER. Fig. 1.2 shows the plotting of isohyets for a catchment having 13 rain gauge stations. Having drawn the contours, the area between the adjacent isohyets are determined with planimeter. If the isohyets go out of the catchment, the catchment boundary is used as the bounding line. . If P1, P2,, PN are the values of the isohyets and A1, A2,, AN-1 are the corresponding inter-isohyets, then the mean precipitation of the catchment is determined as

(1.3)Isohyetal method is the best among all the methods of determining the average precipitation of a catchment.

Fig.3.2: Plotting of Isohyets

Procedure and Computations:Arithmetic Average Method:

1. Knowing the rainfall at the raingauge stations, compute Pave using eqn. (1.1).

Thiessen Polygon Method.

1. Plot the location of rain gauges on the base map with a pencil.

2. Connect adjacent points with dashed lines using a straight edge and pencil.

3. Construct perpendicular bisectors across the dashed boundary lines.

4. Connect the bisector lines to outline polygons belonging to each station or region.

5. Count squares on the graph paper to determine the size of each area and fill in Table 1.1.6. Use eqn. (1.2) to compute Pave.Table:1.1: Thiessen Polygon Method

Station

iTheissen Area (Ai)

(L2)Rainfall (Pi)

(L)Ai x Pi(L3)

Isohyetal Method:Drawing Isohyets using Surfer 9.0

1. Open Surfer 9.0

2. Press Ctrl+W

3. In the first column give heading X-values and enter X-values in the subsequent rows. Similarly add Y-values in the second column and the precipitatipn values in the third column.

4. Save the file with dat extension

5. If you have the worksheet window open, click on the Window menu and choose Plot1, or click on the Plot1 tab. Alternatively, you can create a new plot window with the File | New | Plot command.

6. In the plot window, choose the Grid | Data command, or click the button in the grid toolbar. The Open Data dialog is displayed.

7. In the Open Data dialog, click the file TUTORWS.DAT (located in Surfer's SAMPLES folder). The name appears in the File name box below the list of data files.

8. Click the Open button and the Grid Data dialog is displayed. Alternatively, you can double-click the data file name to display the Grid Data dialog.

9. The Grid Data dialog allows you to control the gridding parameters. Take a moment to look over the various options in the dialog. Do not make changes at this time, as the default parameters create an acceptable grid file.

10. Click the OK button. In the status bar at the bottom of the window, a display indicates the progress of the gridding procedure. By accepting the defaults, the grid file uses the same path and file name as the data file, but the grid file has a [.GRD] extension.

11. By default, a Surfer dialog appears after gridding the data with the full path name of the grid file that was created. Click the OK button in the Surfer dialog. The TutorWS.GRD grid file is created.

12. If Grid Report was checked in the Grid Data dialog, a report is displayed. You can minimize or close this report.

13. Choose the Map | New | Contour Map command, or click the button in the map toolbar.

14. The Open Grid dialog is displayed. The grid file you created in Lesson 2 (TUTORWS.GRD) is automatically entered in the File name box. If the file does not appear in the File name box, select it from the file list.

15. Click the Open button to create a contour map.

16. The map is created using the default contour map properties.

17. If you want the contour map to fill the window, choose the View | Fit to Window command, or click the button. Alternatively, if you have a wheel mouse, roll the wheel forward to zoom in on the contour map. Click and hold the wheel button straight down while you move the mouse to pan around the screen.18. Having plotted the isohyets, determine the area between the adjacent isohyets and fill in Table 1.2.

19. Use eqn. (1.3) to compute Pave.Table:1.2: Isohyetal Method

Isohyets

IAverage value of P (Pi)

(L)Area (Ai)

(L2)Ai x Pi(L3)

Chapter 2

Determination of In-situ Soil Hydrological PropertiesAim:To determine and compare the in-situ soil hydrological properties using Core Cutter and Time Domain ReflectometryApparatus Required:

Cylindrical Core Cutter, Time Domain Reflectometer, Steel Rammer, Steel Dolly, Containers, Balance sensitive to 0.01 gms, Spade, Trowel, Trimming Knife and Oven

Theory:

Determination of soil hydrological properties is important in the analysis of many hydrological processes such as infiltration, percolation, surface runoff and in soil science and irrigation engineering,. The soil is a three phase system comprising solid particles, water and air. The water and air are present in the voids between solid particles. These pores may be filled up with only water, only air or with water and air. Correspondingly, the soil may be saturated, dry or partially saturated (unsaturated).Definitions: Let V be the volume of a soil sample, Vs be he volume of solid particles, Vv is the volume of voids (pores), Vw be the volume of water and Va be the volume of air in the soil sample respectively. The porosity () is defined as the volume of voids per unit soil volume;

(2.1)

The volumetric moisture content () is defined as the volume of water per unit soil volume;

(2.2)

Degree of saturation (S) is defined as the ratio of volume of water and the volume of voids;

(2.3)From eqn. (2.1) to (2.3) it can be proved that

(2.4)In laboratory, soil samples are collected and the soil moisture is determined gravimetrically by weighting the wet and dry soil samples and is reported as percentage of dry weight of the soil. Fig. 2.1 shows the core cutter assembly used for collecting the in-situ soil samples needed for the gravimetric analysis. The dry weight moisture fraction (W) is defined as (Walker and Skogerboe, 1987);

(2.5)Where Ww is the weight of water and Wd is the dry weight of the soil sample. As the hydrologists and irrigation engineers require water in volume units, the dry weight soil moisture fraction W has to be converted to corresponding volumetric moisture content. These two terms are related as;

(2.6)Where is the dry mass density of the soil sample and is the mass density of soil water.

Fig. 2.1: Core cutter assembly for gravimetric analysisThe technique of time domain reflectometry is being widely used now a days by hydrologists for the determination of in-situ volumetric moisture content. As shown in Fig. 2.2, the time domain reflectometer (TDR) consists of two metal probes and an assembly to generate electromagnetic pulses. The metal probes are either 12 cm or 15 cm in length and the TDR records the average volumetric moisture contneent over the length of these probes. It enables to measure moisture content of soil quickly and precisely in a non-destructive manner. The principle that a TDR follows is discussed hereafter. The probes are inserted at the desired depth at which moisture content needs to be measured. An electromagnetic pulse is injected into a waveguide inserted into or buried in the soil. The time required for the pulse to travel along the metal rods of the waveguide depends upon the bulk electrical permittivity (BEC) of the soil. The BEC is a function of the volumetric moisture content. Hence the time taken for the pulse to travel along the metal rods depends on the moisture content. The time taken by the pulse is converted to the corresponding volumetric moisture content through calibration which would be recorded digitally. Fig. 2.2: Time Domain ReflectometerProcedure:

1.Weigh the core cutter and measure its inner dimensions and calculate its volume.

2. Push the cylindrical cutter into the soil to its full depth by gently ramming it.

3. Lift the cutter up carefully with the help of a trowel.

4. Trim the top and bottom surface carefully.

5. Nearby the site where the core cutter was inserted, measure the volumetric moisture

Content using TDR6. Take the empty pans and weigh them.

7. Determine the weight of the soil inside the cutter by placing the soil in the pan.

8. Take the soil mass in the pan and place it in the oven for a period of about 24 hours.

9 After oven drying, again note down the weight of the pan in order to determine the dryweight of the soil mass.

Observations and Calculations:

Diameter of the core cutter (L) = Height of the core cutter (L) =

Volume of the core cutter (V) (L3) = Table 2.1: Comparison of volumetric moisture using gravimetric analysis and TDR

PanNo.Wt. ofEmpty

Pan (w1)

(M)Wt. of pan with wet soil(w2)

(M)Wt. of wet soil

(w2-w1)

(M)Wt. of pan with dry soil

(w3)

(M)Wt. of dry soil

(w3-w1)

(M)Wt. of water(w2-w3)

(M)Dry wt. moi. frac.(W) =

(M/L3)=

by TDR

Precautions:

-Steel dolly should be placed on the top of the cutter before ramming it down into the ground.

-Core cutter should not be used for gravels, boulders or any hard ground.

-Before removing the cutter, soil should be removed around the cutter to minimize the disturbances.

-While lifting the cutter, no soil should drop down

- The TDR readings should be taken carefully.Chapter 3Determination of Infiltration Parameters with Double Ring InfiltrometerAim: To conduct infiltration test using a flooding type double ring infiltrometer and determination infiltration parametersApparatus Required: Double ring infiltrometer, wooden hammer, Pointer Gauge with Vernier Scale, Stopwatch and Bucket

Theory: Infiltration is the process water entering from the ground surface into the soil. The infiltration rate depends on many factors such as soil type, texture, vegetative cover, soil properties such as hydraulic conductivity and porosity and the antecedent moisture condition. The infiltration rate f (L/T) is the rate at which water enters the soil at the surface and is usually expressed in cm/hr.

During infiltration, the infiltration rate f varies with time and many empirical relationships are proposed to describe the infiltration phenomenon. (Horton,1933, Green and Ampt, 1911, Philip 1969). Among these, Hortons infiltration equation is the simplest and is quite widely used for determining infiltration rates.

Horton (1933) observed that the infiltration rate begins at a high rate f0 and exponentially decreases with time until it reaches a constant rate fc and proposed the following empirical relation for infiltration rate f at any time t;

(3.1)

In eqn. (1.1), k is the decay constant and has the dimensions T-1. It is evident from eqn.(3.1) that f = f0 when t = 0 and as . It is clear from eqn. (3.1) that the determination of infiltration parameters f0, the initial infiltration capacity, fc, final steady state infiltration capacity and k, the decay constant, is necessary for the use of Hortons infiltration equation. The parameter f0 depends upon the soil type, vegetative cover and the antecedent moisture content prevailing at the ground surface whereas the parameters fc and k depend on the soil type. The parameter fc is essentially the hydraulic conductivity of the soil. The infiltration parameters are determined in the filed either with (i) Single ring infiltrometer or (ii) double ring infiltromter. The major drawback of single ring infiltrometer is that water infiltrates laterally at the bottom of the ring and as such does not accurately represent the vertical infiltration phenomenon and is rarely used in the field. Hence, the double-ring infiltrometer is most commonly used for measuring infiltration rate and determination of infiltration parameters.

Double ring infiltrometer: The double ring infiltrometer consists of two concentric hollow cylinders of length 25 cm made of cast iron as shown in Fig. 3.1. The diameters of the inner and outer cylinders are about 30 cm and 60 cm respectively.

Fig. 3.1: Double Ring InfiltrometerDuring an infiltration test, the field is first levelled and the infiltrometer is driven to a depth of 15 cm into the soil with a wooden hammer. Then Water is applied initially to the outer ring and water is applied to the inner rings after some time. The purpose of the outer ring to is to restrict the later flow of water from the inner cylinder which ensures vertical movement of water at the centre of the infiltrometer. The rate of downward movement of water at the centre is measured with a pointer gauge with Vernier scale.

Procedure:

1. Level the filed at which infiltration test is being carried out

2. Hammer the double ring infiltrometer at least 15 cm into the soil. Use the timber to protect the ring from damage during hammering. Ensure that the infiltrometer is driven vertically and approximately about 12 cm of the infiltrometer is above thr ground.3. Start the test by pouring water into the outer ring first. Add water into the inner ring up to the same level as in the outer ring.4. Start the stop watch and record the water level with a pointer gauge5. Keep recording the water levels at regular intervals6. The frequency of recording would 1 min for the first 15 minutes, 2 min fro 15 min to 45 min and 5 min from 45 min to 1.5 hrs.7. Continue the recording of water levels until the rate of fall of water level becomes constant.Observations and Computations:Lease Count of Pointer Gauge =

Table 3.1: Calculations

Time

(T)Pointer gauge

Reading

(L)Drop in

water level

(L)Infiltration rate f (L/T)f - fc(L/T)ln(f fc)

1. Plot f v/s t on a normal graph sheet and draw the best fitting curve for the data points. From the graph, Find the value of fc as the limiting (asymptotic) value of f at large time.

2. Use the value of f so obtained to compute ln(f fc) in Table 3.1

3. Plot ln(f fc) v/s t and draw a best fitting line through the data points and obtain the slope of the line and the y-intercept. The y intercept gives the value of ln(f0 fc)4. The slope gives the decay constant k5. Knowing ln(f0 fc) from step 3 and fc from step 1, f0 can be determined

Precautions:

1. Water is to be first filled in the outer ring, so that water in the inner ring moves vertically downwards

2. Presence of vegetation may effect observations.

3. The water depth in inner and outer ring must be same during the observation period.

Sources of error:1. Measurement of Vernier scale upto 0.1mm2. Stray points on the curve may be due to loose fixing of pointer gauge used.

Chapter 4Measurement of Discharge in an Open Channel with Current MeterAim: To measure discharge in an open channel using current meter and heck its accuracy.Apparatus:- Current Meter, Scale, Stop watch

Theory:

Measurement of discharge in rivers, wide open channels is essential to assess the surface water potential to meet domestic, agricultural and industrial needs. The discharge in a river or a canal can be measured by employing either direct or indirect methods (Subramanya, 2008). Among the direct methods, Area-Velocity method is the most commonly used technique. In this method, the measurement consists of measuring the area of cross section of the river/canal at a selected section called the gauging site and measuring the velocity of flow through the cross sectional area. The river/canal cross section is divided in to a number of vertical subsections. The average velocities in these subsections are measured using current meters. The accuracy of the measurement increases with number of subsections used. In rivers/channels having shallow depth up to 3 m, the velocity measured 0.6 times the depth of flow below the water surface represents the average velocity whereas in deep rivers/canals the average of velocities measured at 0.2 and 0.8 times the depth of flow from the water surface represents the average velocity, i.e.,

(4.1)

(4.2)Where d is the depth of flow,are the velocities at depths 0.2d, 0.6d and 0.8d from the water surface respectively. Current Meter:Current meters are the most commonly used instruments used to measure velocity at a point in rivers and open channels. It essentially consists of a rotating element which rotates due to the reaction of the stream current with an angular velocity proportional to the velocity. Current meters are commercially available in two types (a) verticalaxis meters and (b) horizontal-axis meters as shown in Fig. 4.1. Vertical-axis current meter consists of a series of conical cups mounted around a vertical axis. The horizontal-axis current meter consists of a propeller mounted at the end of horizontal shaft. To measure the velocity at a point in a river/ canal, the current meter is held against the current. Due to the stream velocity the propeller or conical cups rotate, the number of revolutions being proportional to the velocity at that point. A functional relationship is developed relating the number of revolutions to the stream velocity by calibration. A typical relationship is of the form

(4.3)

Where v is the stream velocity, Ns is the number of revolutions per second and a and b are calibration constants. Procedure:

1. Open the supply valve. Adjust the discharge and the tail gate to get a suitable depth of flow in the channel. 2. Wait until the flow becomes uniform.

3. Select a suitable section in the middle portion of the channel

4. Take the pointer gauge reading at the middle of the section to get the depth of flow

5. Divide the cross sectional area into three or four vertical subsections

6. Place the current meter at 0.6d from the water surface at the middle of each subsection and record the number of revolutions7. Measure also the pointer gauge reading of the sharp crested weir at the d/s of the channel to compare the discharge measured by current meter.8. Repeat the steps 1 to 7 for three or four discharges.9. Record the pointer gauge reading at the weir crest level.

Observations and Computations:

Current meter calibration constants: a =

b =

Table 4.1: Measurement of discharge using current meter

Run No1234

Pointer gauge reading at water surface, h1 (L)

Pointer gauge reading at bed h2 (L)

Depth of flow, y (h1 h2) (L) Width of subsection 1 (b1) (L)

Area of subsection 1, A1 (b1 x y)

Current meter revolutions per second Ns1Flow velocity in subsection 1 () (L/T)

Discharge in subsection 2Q1 ( v1 x A1) (L/T3)Width of subsection 2 (b2) (L)

Area of subsection 2, A2 (b2 x y)

Current meter revolutions per second Ns2Flow velocity in subsection 2 () (L/T)

Discharge in subsection 2 Q2 ( v2 x A2) (L/T3)

Width of subsection 3 (b3) (L)

Area of subsection 3, A3 (b1 x y)

Current meter revolutions per second Ns3Flow velocity in subsection 3 () (L/T)

Discharge in subsection 3 Q3 ( v3 x A3) (L/T3)

Total Discharge Q = Q1 + Q2 + Q3 (L/T3)

Width of broad crested weir, B (L) =

Pointer gauge reading at the crest of broad crested weir, yc (L)Table 4.2: Measurement of discharge using sharp crested weirRun No1234

Pointer gauge reading at water surface, yw (L)

Head over the wier, H = (yw yc) (L)

Coefficient of discharge

Discharge (L3/T)

Chapter 5

Determination of Hydraulic Conductivity with Constant Head PermeameterAim:

To determine the saturated hydraulic conductivity of a soil using constant head permeameter. Apparatus: Constant head permeameter, stop watch, scaleTheory:The fundamental law which is used to describe flow of water in soils is Darcys law (Raghunath, ). Darcy conducted several experiments on water flow through pipes filled with soil. Water was allowed to enter at one end and pass through a circular pipe of cross section A filled with sand and pair of manometers. The water was collected at the other end. The water was allowed until such time as all the pores were filled with water and the inflow rate Q is equal to the outflow rate. He conducted several experiments by changing the diameter of the pipe, the length of the pipe and the hydraulic head difference. He observed that the discharge Q was directly proportional to the area of the pipe A, and the hydraulic head difference and inversely proportional to the length of the pipe L, i.e.,

(5.1)Where V is the Darcy velocity. Introducing a constant of proportionality K, Eqn. (1.6) can be written as

(5.2)

Where K is called the hydraulic conductivity and is the hydraulic gradient. Eqn. (5.2) is popularly known as Darcys Law. Darcys law is valid when inertial forces are less dominant as compared to viscous forces. In general, the law is considered as valid when the Reynolds number (defined as where, d is the average diameter of the soil grains and is the kinematic viscosity of water) is less than unity. Beyond Darcys range (i.e., Reynolds number greater than unity) the mean velocity is given by

(5.3)Where is n an exponent greater than unity.

If velocity and hydraulic gradient are measured in Darcys range ( i.e., Reynolds number less than unity) and plotted on a log-log graph paper, the resulting plot will be a straight line inclined at an angle of 450 (i.e., slope equal to unity). Beyond Darcys range, the resulting straight line will have a steeper slope.

Constant Head Permeameter:Fig. 5.1 shows the constant head permemaeter commonly used in laboratoeis for determination of hydraulic conductivity. It consists of a square or circular column (approximate size 100m 100mm or 150mm dia.) made of Perspex sheet. The column is about 1.5 m long and is filled with sand grains of uniform size ( about 2 mm diameter). Pressure taps are provided on one side of the column. These pressure taps are connected to piezometer tubes. The supply pipe is connected to the lower end of the column. Outlet pipe is connected to the upper end of the column. Precautionary measures are taken to prevent the migration of sand grains along with the flowing water. Procedure:1. Check that there is no air bubble in the piezometer tubes.

2. Note the distances of various pressure taps from a reference point.

3. Open the supply valve and let the flow become steady.

4. Take down the piezometer readings.

5. Measure the discharge using collecting tank and stop watch.

6. Vary the discharge and repeat steps (4) and (5) to collect more sets of observations.

Fig. 5.1: Constant Head Permeameter ApparatusObservation and Computations:Average grain diameter, d (L) = Kinematic viscosity, (L2/T) =

Cross sectional area of permeameter, A (L2) =

Table 5.1: determination of Hydraulic Conductivity

Run

No.Vol. of water collected

S (L3)Time Taken forCollection

t (T)Velocity

(L/T)Piezometer Readings(L)Hydraulic gradientiHydraulic conductivityK

(L/T)

Distance of piezometers

from datum

1. Plot the piezometer readings (on y-axis) v/s distance of the corresponding pressure tap from the selected reference datum for each of the runs taken. Fit-in a straight line for each of these runs and find the slope of each of these lines. These are the mean values of hydraulic gradient for different runs. Enter these values of i in the corresponding column of the observation and computations table. Also calculate the velocity V for all these runs.

2. Plot V v/s i ( with i on x-axis) on a log-log graph paper. Fit-in a straight line having a unit slope to data points for relatively low velocity. For higher velocity data points, fit-in another straight line and measure its slope. 3. Determine the value of n in Eqn. (5.3) from V v/s i line for higher velocity data points.

PAGE 17

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