Introduction to Hydrology, Stream Gauging
- 1. HYDROLOGY & WATER RESOURCE ENGINEERING SPPU, Pune T.E. Civil UNIT-I • Introduction to Hydrology • Precipitation: Types & Forms • Evaporation and Infiltration • Stream Gauging Prepared by : Prof. A. A. Inamdar Associate Professor, KJCOEMR
- 2. INTRODUCTION TO HYDROLOGY • Definition:- It is a science which deals with the occurrence, circulation and distribution of water over and beneath the earth surface. 2
- 4. SOME IMPORTANT DEFINITIONS 1. Precipitation:- It is the fall of water in various forms on the earth from the clouds. 2. Evaporation:- It is the process by which water is changed to vapours (gaseous state) at the free surface below the boiling point of water. 4
- 5. 3. Transpiration :- It is the process by which water vapours escape from the leaves of the plant & enter to the atmosphere. 4. Evapo-transpiration :- It is the total water lost from the forest land due to evaporation from the soil & transpiration by the plants. 5. Infiltration :-It is the process by which water enters into the soil from the ground surface. 6. Runoff :-The water that runs over the ground & enters into streams and rivers. 5
- 6. Need for the Hydrologic Studies Due to… –Increased population, –Increased standards of living of people, –Unpredictable Rainfall. Hydrological studies are very important for planning and development of water resources to meet these demands. 6
- 7. Importance of Hydrology in Civil Engineering • Design and Operation of water resources engineering projects like – Irrigation – Flood control – Water supply schemes – Hydropower projects and – Navigation 7
- 8. PRECIPITATION • Types of Precipitation… 1) Convective Precipitation, 2) Orographic Precipitation, 3) Cyclonic Precipitation, a) Frontal Precipitation, b) Non-frontal Precipitation. 8
- 9. PRECIPITATION • Forms of Precipitation – Drizzle (dia. 0.1 to 0.5mm) – Rain (dia. < 0.5mm) – Glaze (Freezing Rain) – Sleet (dia. 1 to 4mm snow pallets) – Snow (Solid ice crystals) – Hail (dia. <0.5mm lumps of ice) – Dew (It forms due to condensation on ground) 9
- 11. What if you need to know the rainfall in a catchment? - Type of rain gauges? - Where to put gauges? - How many gauges? 11
- 12. RAINGAUGES • Following are the type of Rain Gauges… - Non-recording type a) Symon’s Rain gauge b) IMD Standard Rain gauge - Recording type a) Tipping Bucket Rain gauge b) Weighing Bucket Rain gauge c) Float type Rain gauge - Measurement by RADAR 12
- 13. Non-recording type a) Symon’s Rain gauge b) Indian Meteorological Department (I.M.D.) Standard Rain Gauge 13
- 14. Non-recording type… • Working:- These type of rain gauges consists of a cylindrical vessel of dia. 12.7cm with funnel. The vessel is fixed in a masonry foundation block of size (60 x60 x 60)cm. The height of vessel is so fixed that the top rim is 30.5cm above the ground. The glass bottle (receiver) is of 7.5 to 10cm in dia. The rain water collected in receiver is measured by measuring glass jar in mm. 14
- 15. RECORDING TYPE 1. TIPPING BUCKET RAIN GAUGE Working :- 1. It consist of two small buckets placed below the funnel of 30 cm dia. receiver. 2. The buckets are balanced in unstable equilibrium. 3. So that, at one time only one bucket remains below the funnel at higher level. 4. When the higher bucket receives 0.25mm rainfall it becomes unstable and tips to empty itself. 15
- 16. TIPPING BUCKET RAIN GAUGE… Working… 5. At the same time other bucket comes below the funnel for collection of rainfall. 6. The tipping of bucket actuate an electric circuit which causes a pen to make a mark on a drum chart. 7. Each mark indicates the rainfall of 0.25mm. 8. By counting the no. of marks & noting the intensity & amount of rainfall can be determined. 9. The amount of rainfall determined can be verified by measuring the total water collected in the measuring tube. 16
- 17. • Working:- 1. The rain is collected in a weighing bucket which is connected to the spring balance. 2. As the rainwater increases in bucket, the weighing platform(Pan) moves downwards. 3. The movement of that pan is transmitted to pen through a link & lever system. 4. The pen makes the trace of the accumulated amount of rainfall on a chart attached to a drum revolved by a clock driven mechanism. 5. The record is continuous & the slope of line gives intensity of rainfall. 2. WEIGHING BUCKET RAIN GAUGE 17
- 18. 3. FLOAT-TYPE RAIN GAUGE • Working:- 1. The rainfall collected in a float chamber filtered & passes through the funnel. 2. The filter prevents the dust debris etc. from entering the float chamber. 3. The float chamber contains a float with a vertical stem fixed over it. 4. As the rain is collected in the float chamber, the float rises & the pen which is attached on the top of the vertical stem moves on the drum chart. 18
- 19. FLOAT-TYPE RAIN GAUGE • Working….. 5. From the chart the intensity & duration of rainfall is determined. 6. When the float chamber is fully filled, the pen reaches the top of the chart & the syphoning action occurs automatically. 7. At the time of syphoning action of 15 seconds, the pen draws a vertical line on the drum chart which resets the pen to zero. 19
- 20. Rainfall measurement by radar 1. Electromagnetic waves are produced by a transmitter & are radiated by a narrow beam antenna. 2. These waves are reflected by the clouds and are intercepted by the same antenna, which now acts as a receiver. 3. The receiver detects these reflected signals and amplifies them. 4. The returned signals are transformed into a visual display on the radarscope. 20
- 21. Estimation of missing rainfall data 1. Comparison Method, 2. Isohytal map Method, 3. Normal Ratio Method Case I: When the mean annual rainfall at each of index station A, B, & C is within 10% of the mean annual rainfall of station X, a simple avg. of the values of the index station is taken… Px = 1/3 (PA + PB + PC) Case II: When the mean annual rainfall at each of index stations differs from the station X by more than 10% the Normal Ration Method is used… Px = Nx/M (PA/NA + PB/NB +……….+ PM/ NM ) Where M = No. of index stations, N = Mean annual Rainfall(Avg. Annual Precipitation) P = Precipitation 21
- 22. Mass curve & hyetograph Mass curve hyetograph Definition - The mass curve is a plot between the accumulated rainfall at a station as ordinate (Y-axis) & time as abscissa (X-axis). Definition – The hyetograph is a bar diagram plotted between the avg. intensity of rainfall as ordinate & time as abscissa. 22
- 23. MASS CURVE 1. The recording type rain gauges gives directly the mass curve. 2. It is used for determination of the intensity, amount & duration of the rainfall. 3. The amount & duration of the rainfall are obtained directly from the curve. 4. The rainfall intensity is obtained from tangent to the mass curve. HYETOGRAPH 1. The hyetograph is obtained from the mass curve. 2. It is used for determination of Average Rainfall Intensity with a constant time interval. 3. The smaller is the time interval the greater is the accuracy of avg. rainfall intensity. 4. The area under the hyetograph gives the total rainfall occurred in that period. • Rainfall intensity is expressed in mm/hr or cm/hr. 23
- 24. problems - Mass Rainfall curve & Hyetograph 24 Time (O’clock) 8:30 9:00 9:30 10:00 10:30 11:00 11:30 Rainfall (mm) 0 2 8 15 25 30 32 ?? Draw mass-curve & hyetograph from the following data.
- 25. Depth-area-duration curve (Dad curve) Depth-Area Relationship:- - A storm over a particular catchment never produce uniform rainfall depth. - Each storm has its centre called as EYE where the precipitation is maximum. - As the distance of a point from the eye increases, the precipitation decreases thus the depth of precipitation also decreases. - The depth-Area relation for different storm durations are different. (DAD curve-see fig.) The Depth-Area-Duration Curve 25
- 26. Problem DAD Curve ?? The computation of an Isohyet map of 1250 km2 basin, a 6-hr storm gave following data. Determine the avg. precipitation for the basin and compute & draw depth-area curve. 26 Isohyet (mm) 200 175 150 125 100 75 Area (km2) 100 275 400 520 600 650 650 km2 100 km2
- 27. Evaporation - Definition: It is the process by which the precipitation that falls on earth surface is returned to the atmosphere as vapour. - Factors affecting evaporation… 1. Temperature 2. Wind 3. Atmospheric Pressure 4. Impurities in water 5. Shape & size of Water surface 27
- 28. Evaporation - Methods of Measurement of evaporation… a) Evaporation pan (Evaporimeter) Method, b) Empirical Method, c) Analytical Method. 28 Evaporation pan (Evaporimeter)
- 29. infiltration • Factors affecting infiltration Rate, 1. Soil Moisture 2. Type of Soil Medium 3. Permeability 4. Vegetal Cover 5. Compaction of Soil 6. Temperature of water 29
- 30. infiltration • Measurement of Infiltration capacity, - Horton’s Method - Field Methods… a) By Flooding type Infiltrometer, b) Rainfall Simulator type Infiltrometer. 30
- 31. 1. Horton’s Method • It is an empirical formula that says - Infiltration starts at a constant rate f0 and is decreasing exponentially with time, t. After some time when the soil saturation level reaches a certain value, the rate of infiltration (ft) will level off to the rate, fc. ft = fc + ( fo - fc) e-kt Where 31 ft = Infiltration rate at time t; fo = Initial infiltration rate or maximum infiltration rate; fc = Constant or equilibrium infiltration rate after the soil has been saturated or minimum infiltration rate; k = The decay constant specific to the soil.
- 32. 2.a. Flooding type infiltrometer 1. It consists of a metal cylinder open at both ends. 2. It has 22.5 cm diameter & 60 cm height. 3. The cylinder is driven into the ground by hammer such that 10cm length remains above the ground. 4. Water is filled in the cylinder. 5. A pointer is set to mark water level. 6. As the infiltration takes place, the water level goes down. 7. Water level is maintained constant through the burette. 8. Reading on the burette are taken at a regular interval to determine the rate & amount of infiltration. 9. The experiment is generally continued till a constant infiltration rate is obtained. 32
- 33. 2 b. Double-ring infiltrometer 1. It consists of two cylinder called rings. 2. The inner ring is of 22.5cm in dia. & outer ring is of 35cm dia. 3. The inner ring is used to determine the infiltration rate. 4. The water level in between the two rings is maintained at same level to reduce the other effects. 5. The outer ring provides a sort of water jacket to the inner ring to infiltrate the water vertically downward without spreading. 6. It is more accurate than simple flooding type infiltrometer. 33
- 34. Infiltration indices - The infiltration capacity of the soil decreases with an increase of duration of rainfall. - For computing the runoff & flood discharge, the use of infiltration capacity curve is not convenient. - It is more convenient to use an average constant value of infiltration rate called Infiltration index. - There are two types of indices, ф-index & W- Index. Infiltration Capacity Curve Ф- Line 34
- 35. Infiltration indices ф-index • It is the average rate of rainfall such that the volume of rainfall in excess of that rate is equal to the volume of surface runoff. • For determination of ф-index, a horizontal line is drawn on a hyetograph such that the shaded area above that line is equal to the volume of surface runoff. • The unshaded area below the line represents all losses including interception, depression storage & infiltration. W- Index 35
- 36. PROBLEMS Surface Runoff ф-index & W-index 36 ?? A 6-hours storm produced rainfall intensities of 5, 15, 25, 13, 9 & 2 mm/hr in successive 1 hr intervals over a catchment of 900 km. sq. The resulting runoff is observed to be 2750 ha-m. Determine the ɸ- index.
- 37. Stream gauging • Definition:- The process of measuring discharge of a stream is called Stream Gauging. • Stream gauging is done for determination of Runoff from catchment by measuring discharge of river at outlet of catchment. CURRENT METER PROPELLERE TYPE CURRENT METER 37
- 38. Selection of stream gauging site The following factors are considered while selecting a stream gauging… 1. The site should be easily accessible. 2. The river reach at the site should be straight & uniform. 3. The c/s of the river at the site should be well-defined. 4. The banks & bed should be firm & stable. 5. There should be a good permanent control section located at the downstream of the site. 6. The best site is where the river c/s is regular & V-shaped with sufficient depth for immersing the current meter. 38
- 39. Measurement of discharge of stream Methods used for measurement of discharge of river… A) Direct Measurement Methods: 1. Area-Velocity Method, 2. Moving boat Method, 3. Salt concentration/Tracer method, B) Indirect Measurement Methods: 1. Slope-Area Method, 2. By using Notches, Weirs, Venturi-flumes & Spillways. 39
- 40. 1. AREA-VELOCITY METHOD 1. In this method, the discharge is determined from the area of c/s & the mean velocity. 2. The area of c/s of the river is determined from the profile of the river bed obtained by sounding. 3. The river c/s is divided into a suitable number of vertical segments (generally 15-25 segments). 4. The discharge in each segment is equal to the area of the segment multiplied by mean velocity of flow. 5. The total discharge in the river is computed as the sum of the discharges in various segments. A. DIRECT MEASUREMENT METHOD… 40
- 41. 1. AREA-VELOCITY METHOD a) Mid-Section Method b) Mean-Section Method There are two methods… 41
- 42. 1. AREA-VELOCITY METHOD • Mid-Section Method :- 1. In this method, the area of c/s is divided into several segments & the mean velocity is measured on a vertical line in the middle of the segment. 2. The mean depth of the segment is taken at the middle of the segment. 3. Now by multiplying mean depth to area of segment determine the discharge in that segment. 4. The total discharge is the sum of all segment discharge. • The discharge in the end tow triangular strips is usually neglected. 42
- 43. 1. AREA-VELOCITY METHOD • Mean-Section Method :- 1. In this method, the segment is taken between two vertical lines on which the velocity & depth are measured. 2. The velocity in the segment is taken as the average of the mean velocities of the two adjacent vertical lines. 3. Similarly, depth is also taken as the average of two depths of vertical lines. 4. The discharge of segment is determined by multiplying avg. velocity to avg. depth of segment. 5. The total discharge is calculated by sum of all discharges of segments. 43
- 44. 2. Moving boat METHOD Fig.a Fig.b 1. Discharge measurement of large rivers by velocity-area method is quite time consuming & tedious. 2. This method is very useful when river is in spate (full & flowing fast). 3. In this method, the boat is towed rapidly to cross the river normal to the direction of flow. 4. A current meter is fixed to the boat by vertical axis at a constant depth of 0.5m from the surface. 5. The velocity recorded by the current meter is the resultant velocity of the boat & the stream. (as shown in fig.a) 6. An angle indicator is attached to measure the angle. 7. An echo sounder is used to record the depth of flow at different sampling points. 44
- 45. 3. Salt concentration or tracer METHOD 1. This method is used for small & turbulent streams in mountainous regions. 2. The main advantage of this method is that it gives discharge directly. 3. In this method, a reach is selected & a solution of a common salt or sodium dichromate is injected of a known quantity at a constant rate at the injection point of reach. 4. The solution get mixed & diluted with river water. 5. The concentration of salt at the measurement point of reach is determined after taking a sample of water. 6. The total discharge is determined from the continuity equation assuming that the total amount of salt remains constant. 45
- 46. B) Indirect Measurement Method • It is commonly used to determine the maximum discharge in the river during flood. • In this method, discharge is computed using Manning’s formula (or Chezy’s Formula) from the area of c/s & the slope of water surface. • Manning’s Formula, V = 1/n R2/3 S1/2 • Discharge is Q= A x V = A/n R2/3 S1/2 Where, A = c/s Area determined upto High Flood Level(HFL), n = Manning’s Roughness coefficient, R = Hydraulic Radius = A/P, P= Perimeter of River Bed, S = Water surface slope= h/L, h= difference in water levels in the Length L of the reach V = Velocity of Water 1. Slope-Area Method 46
- 47. • The c/s area (A) is obtained by taking soundings at intervals of 3 to 6m. The c/s is area plotted & determined upto the HFL. • The water surface slope (S) is determined by noting water levels at upstream 1 km & also downstream 1km of site ( h) with measured length (L) of that reach (e.g. 2km). • The Manning’s Coefficient (n) depends on the nature of the bank, bed surface & condition of river. • Sometimes, Chezy’s Formula is used to determine Discharge of river… Q = A C R1/2 S1/2 1. Slope-Area Method 47
- 48. MODERN STREAM GAUGING METHODS 1. Conventional measurements of river flows are costly, time-consuming, and frequently dangerous. 2. The purpose of the modern methods are to directly measure the parameters necessary to compute flow, surface velocity (converted to mean velocity) and cross- sectional area, 3. The aim of these methods is to avoid the uncertainty, complexity, and cost of maintaining rating curves. 4. They are also called as Non-contact methods. 5. The results are accurate & methods are safe. 48
- 49. Advance techniques/ equipment used in gauge discharge measurement 1. Radar System, 2. ADCP (Acoustic Doppler Current Profiler), 49
- 50. 1. Types of Radar systems used 1. Continuous Wave Radar, 2. UHF Doppler Radar, 3. Pulsed Doppler Radar, 4. Ground Penetrating Radar 50
- 51. • River surface water velocity is obtained by - Pulse Doppler Radar, - Continuous Microwave Radar, - UHF Doppler radars • River channel cross sections is measured by - Ground Penetrating Radar (GPR) Microwave Radar GPR Reflections Display 51
- 52. Current meters • It is generally used to measure velocity of water in the river. • ADCP is the hydro acoustic current meter used for measure velocity profile of water in river. • BoogieDopp (BD) is a fairly new acoustic instrument is designed for small and shallow rivers. PROPELLERE TYPE CURRENT METER CUP TYPE CURRENT METER ADCP 52
- 53. 2. Acoustic Doppler current Profiler (ADCP or ADP) • It is a Type of current meter, attempting to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. • It is used to determines water velocity profiles by transmitting sound pulses at a fixed frequency and measuring the frequency (or phase) shift in acoustic echoes reflected back from scatterers in the water. Head of an ADCP with the four transducers53
- 54. Velocity Contours at c/s of river by using ADCP & BD Complete velocity distribution in a cross-section of San Joaquin River at Vernalis, California.54
- 55. System used for river Staging 1. Shaft Encoders, 2. Bubblers System. 55
- 56. Bubbler system used in river staging 1. In a bubbler system, an orifice is attached securely below the water surface and connected to the instrument by a length of tubing. 2. Pressurized gas (usually nitrogen or air) is forced through the tubing and out the orifice. 3. Because the pressure in the tubing is depends on the depth of water over the orifice, a change in the stage of the river produces a corresponding change in pressure in the tubing. 4. Changes in the pressure in the tubing are recorded and are converted to a record of the river stage. 56
- 57. Shaft encoder used for river staging • Shaft encoders convert shaft rotation to pulses for accurate and precise measurements of river stage. • Mechanical float and pulley assembly rotates with fluctuating water level. • As a float rises or falls, a pulley rotates the shaft of the encoder, generating signals for both rotation direction and amount. • Encoders read the signals and send a calculated water level to the datalogger. 57
- 58. 58