a report on hydrology

Engineering Hydrology Hydrology means the science of water. It deals with the occurrence, circulation, and distribution of water of the earth and earth’s atmosphere. In a general sense, hydrology is a very broad subject of an interdisciplinary nature drawing support from applied sciences such as meteorology, geology, fluid mechanics, physics and statistics. Hydrology is classified as – 1. Scientific Hydrology which is concerned with academic aspects. 2. Engineering Hydrology which is concerned with its engineering applications. Engineering Hydrology deals with - Estimation of water resources - Study of processes such as precipitation, runoff, evapo-transpiration,etc - Study of problems such as floods, droughts and strategies to combat them. Sources of data The main components of hydrological cycle are rainfall, evaporation, Transpiration, Infiltration, Runoff and Ground water. Depending upon the problems in hand, a hydrologist would require data relating to the various relevant data such as: A. Weather records – Temp, Humidity, wind velocity B. Precipitation data C. Stream flow records D. Evaporation and transpiration data E. Infiltration characteristics of area F. Ground water characteristics G. Physical and geological characteristics of area General Knowledge of the measurement and reading of these data and the instruments used for their measurement is of great importance in Engineering Hydrology. CALCULATION OF DISCHARGE AT LAB Time (sec) Revolutions Revolution per second 20 15 0.75 25 19 0.76 30 25 0.83 60 48 0.80 The mean rev/sec is 0.786 Given V=0.7002*Ns +0.0004 So, the required velocity is 0.55076 m/s Area=0.1791*0.300= 0.0537 m2 The required discharge is A*V =0.0296 m3/s FIELD VISIT OF METEOROLOGICAL STATION AT KHUMALTAR The study of hydrology necessitates the collection of data on humidity, temperature, precipitation, radiation, evapotranspiration and wind velocity. For the collection of these meteodata, number of meterostations are fixed around the country. Around 400 meterostations are presently operating in our country. The station we visited at Khumaltar, Lalitpur was an Agro-meteorological station. It is one of the 14 major stations in Nepal. This metreostation operates under Department of Hydrology and Meteorolgy, Nepal. The various equipments and instruments we studied at this station were: 1. Rain gauge - Symons’s Gauge ( Non-recording Type) - Tipping Bucket ( Recording Type) 2. Anemometer 3. Campbell Sunshine Recorder 4. Evaporimeter 5. Steven Box ( Psychrometer)- Thermometers for measuring Min & Max temp. and Relative Humidity. The study tour was conducted on 14th January 2011. The summary of Study conducted is as described in following. 1. Rain Gauge Rainfall is expressed in terms of the depth to which rainfall water would stand on an area if the rainfall was collected in it. The rainfall is collected and measured in a rain gauge. A rain gauge essentially consists of a cylindrical vessel assembly kept in the open to collect rain. Rain Gauge is broadly classified as: i. Non – recording Rain gauge ii. Recording Rain gauge ( I ) Symon’s Gauge ( Non-recording Gauge) Symon’s Gauge consists of a circular collecting area of about 12 cm diameter connected to a funnel. The funnel discharges the rainfall catch into a receiving vessel. The funnel and receiving vessel are housed in a container. Water contained in the receiving vessel is measured by suitably graduated measuring glass with accuracy upto 0.1 mm. Fig: Inside cylinder of Symons’s gauge Fig: Symons’s Gauge ( II ) Tipping Bucket rain Gauge( Recording Type ) It can give permanent automatic rainfall record without any bottle reading. There is mechanical and electrical arrangement by which the total amount of rain fallen gets recorded automatically on a graph paper. The gauge thus produces a record of cumulative rain Vs. time in the form of a graph which is known as mass curve rainfall. Rain water is first caught in a collector and is funneled into a two compartmental bucket of 0.25mm each. These buckets are so balanced that when 0.25mm of rainfall collects in one bucket, it cannot withstand more and tips round to bring the adjacent one in collecting position. The water from tipping bucket can be collected. Further while tipping it actuates an electrically driven pen to trace a record on clock work driven chart. This instrument can also digitize the output signal as seen to the automatic data logger. Fig: Tipping bucket with Automatic Fig: Tipping Bucket (Inside) Data logger 2. Campbell Sunshine Recorder: Campbell Sunshine recorder is used to measure the duration of bright sunshine hours which mainly depends on the latitude of the place and environment. The main principle of Campbell Sunshine recorder is that when bright sun rays are converged using a lens into a paper it gets burnt. The special paper used is known as Campbell Sunshine card. Campbell Sunshine recorder consists of a special arrangement of transparent spherical lens provided with hourly graduated paper (Campbell Sunshine card). The sphere is faced directly towards the sun by fixing the latitude of place. The card is concentrically mounted with the sphere. Continuous burnt line on paper represents continuous sunshine and as paper is graduated for time in hours, the no. of hours can be determined. The glass sphere is usually 10cm in diameter. The card is held below the sphere in any one of the three grooves depending upon the seasons of the year. There are different cards for different hemispheres of the earth as well as different card for various seasons: In northen Hemisphere, 3 kinds of cards are generally used: 1. Winter card ( For 15th October to 29th February) 2. Summer card ( For 12th April to 2nd September) 3. Equinox card (For 1st March to 11th April & For 3rd September to 14th October) Fig: Campbell Sunshine Recorder Fig: Campbell Sunshine card 3. Anemometer Anemometer is used for measuring th speed and direction of wind. The speed of wind varies with temperatures, pressure and altitude. There are two horizontal arms over which the instrument for measuring direction and magnitude are mounted. Direction of wind is usually expressed in terms of 16 compass points (N, NNE, NE, NEE, E, SEE,…) The anemometer used was a three cup anemometer with a vertical axis of rotation. Anemometer converts the rotational speed of the cups into linear speed. The speed of wind is measured in km/day in the particular anemometer. It was placed 2m above the ground surface. (Determinination of Direction) (Measurement of Magnitude) Fig: Anemometer 4. Steven Box: Steven Box consists of special arrangement of 4 thermometers with a view to determine the maximum & minimum temperatures and Relative Humidity. Two small clips are placed inside the thermometers to measure the maximum and minimum temperatures. When the temperature gets to the minimum, the mercury remains at the same position even if temperatures rises (In case of minimum thermometers) whereas in case of maximum thermometers its exactly the opposite. When the mercury rises to the maximum temp, the mercury remains there and hence maximum and minimum temperatures are recorded. For the measurement of relative Humidity, two thermometers were placed vertically. One of the thermometer’s bulb was wrapped around a wet cloth and the bulb of the other was kept dry. These two thermometers’ combination is also called psychrometer. Wet bulb loses water from the cloth constantly due to evaporation due to which the temperature of this thermometer is lower than dry bulb temperature. The rate of capillarity in the cloth depends upon Relative Humidity. Wet bulb Deficit or Wet Bulb depression is the difference in temperatures of these two thermometers. Fig: Steven Box 5. Soil temperature measuring thermometers: As Khumaltar is an agro-metero station, the instruments for agricultural as well as meteorological data collection are present. The thermometers installed for soil temperature measurement are important especially for agriculture. But the amount of energy received or lost by the soil is determined using these thermometers. The 4 thermometers can give the temperatures of the soil at 10cm, 20cm, 30cm & 40cm. The variation of temperature with depth is different during different parts of the day. During summer: In the morning, the temperature at all the depths is nearly equal. As sun rises in the afternoon, the temperature at the surface is highest while the temperature inside the soil gets cooler. But in the Evening, outer atmosphere cools hence cooling the surface and temperature at the surface are lower than that at the bottom. Fig: Thermometers for soil temperature measurement 6. Evaporimeter An Evaporimeter is used to measure the evaporation rate and hence estimate the approximate rate of evaporation from a natural water body. Evaporimeters are large water containing pans which are exposed to the atmosphere. If rate of evaporation is to be determined using an evaporimeter, other meteodata such as humidity, wind movement, air and water temperatures and precipitation should be noted on a regular basis as these parameters affect evaporation significantly. Evaporation rate = ( Evaporated volume) / Time taken to reduce the depth Evaporimeters are generally placed at a height of 15cm above the ground so as to allow free air circulation and on wooden blocks so that heat transfer by conduction can be checked effectively. Evaporation pans are not the exact models of reservoirs and posses some drawbacks. So there is a pan Coefficient. (Cp) Lake evaporation = Cp x Pan Evaporation Fig: Evaporimeter 7. Radiometer: Radiometer is an instrument which is used to measure the net radiation. It gives the net radiation by subtracting short wave and long wave. It is also known as pyranometer. Fig: Radiometer 8. Automatic Data log It is an automatic device to record wind speed, rainfall, direction of wind, air temperature and solar radiation. In tipping bucket, every tip is recorded in this Data logger, the value of solar radiation, the velocity of wind, etc are also recorded. (For Figure Ref. Tipping bucket) Conclusions and Discussions With this field visit, we had a chance to be familiar with the different hydrological parameters measured in meterostations and the instruments & equipments used to measure these parameters. The different hydrological parameters measured were – wind velocity, R.H. of air, Rainfall intensity, Sunshine duration, Soil Temperature, Air temperature, Evaporation etc. Discharge Measurement The measurement of discharge in stream forms an important branch of Hydrometrics, the science of water measurement. In the hydrological cycle, streamflow is the only part which can be measured accurately. A stream is a flow channel into which surface runoff from a specified basin drains. After visiting the Khumaltar Meteostation we went off to Nakkhu Khola for its discharge measurement. Discharge measurement was done by velocity area method. Floatation method was also done for velocity determination. Velocity-Area method: This method of discharge measurement consists essentially of measuring the area of cross-section of the river at a selected section called gauging site and measuring the velocity of flow through the cross sectional area. This method is the most practical method of measuring stream discharge. In this method, the width of the stream is divided into a number of increments. The following are some of the guidelines to select the number of segments: 1. The segment width should not be greater than 1/15 to 1/20 of the width of the river. 2. The discharge in each segment should be less than 10% of the total discharge. 3. The difference in velocities in adjacent segments should not be more than 20% But most of the time all of these conditions cannot be fulfilled so we must take steps according to the site condition. For velocity measurement, for shallow sections, it is measured at 0.6 of the distance from the water surface to streambed whereas for deep sections, the average velocity is taken by measuring velocities at 0.2 and 0.8 of the distance from the water surface to streambed. The product of velocity, depth and width of the section gives sectional discharge. The total sum of the sectional discharge is the total discharge of the stream. Q= A x V Where, Q = discharge (m3/sec) A = sectional area ( m2 ) V = velocity (m/s) To measure depth, for shallow streams, height-calibrated rod can be used whereas for deep streams and for accurate depth measurements electro acoustic instrument called echo-depth recorder is used. Velocity is measured using current-meter so this method is also sometimes called standard current meter method. Current Meter: The most commonly used instrument used in hydrometrics to measure velocity is a current meter. It consists essentially of a rotating element which rotates due to the reaction of the stream current with an angular velocity proportional to stream velocity. There are two types of current meter: 1. VERTICAL AXIS CURRENT METER The vertical axis meter consists of a series of conical cups mounted around a vertical axis. The cups rotate in a horizontal plane and a cam attached to the vertical axial spindle records generated signals proportional to the revolutions of the cup assembly.The disadvantage of such meters is that these are useless in situations where there are appreciable vertical components of velocities. The accuracy is higher in higher velocities than in smaller ones. 2.HORIZONTAL AXIS CURRENT METER The horizontal axis meters consist of a propeller mounted at the end of horizontal shaft and these rotate in vertical plane w.r.t. horizontal axis.These meters are fairly rugged and are not affected by oblique flows of as much as 15°.The accuracy of the instrument is about 1% at the threshold value and is about 0.25% at a velocity of 0.3m/s and above. The current meter is so designed that its rotational speed varies linearly with the stream velocity at the location of the instrument. A typical relationship is given by, V = aNs + b Where, V = velocity of flow in m/s Ns = no. of revolution per sec of the current meter a and b are meter constants given by the manufacturer, or generally determined by experiments. We used vertical axis current meter for velocity measurements. Fig: Measuring rod, Current meter 2. Float Method: A floating object on the surface of the stream when timed can yield the surface velocity by the relation, Vs = S/t Where, S =distance traveled in time t. This obtained velocity Vs when multiplied by the cross sectional area A of the stream along the mid point of the S distance gives the discharge Q using the relation: Q = Vs*A This method of finding velocity and hence discharge finds applications in special circumstances although is primitive method: ( I ) a small stream in flood ( II ) a small stream with rapidly changing water surface and (III) preliminary surveys and also for checking purposes. There may be surface floats (simple float moving on the stream) or there can also be the Rod Float, in which a cylindrical rod is weighed so that it can float vertically. Field Procedure: - After fixing the gauging site, a distance of 10 m. both upstream and downstream was measured along the centre line of the river. Two people stood at the two extreme points of the 20 m. length. - Then a surface float was allowed to flow through the above marked 20 m. and the time taken to do so was measured using a stop watch. - The process was repeated as per required. Observations and calculations: The time taken by the float to flow 20m along flow direction, S.no Time(sec) Distance(m) Velocity(m/s) 1 104.84 20 0.191 2 118.73 20 0.168 3 100 20 0.2 Mean velocity of flow at mid line of flow in stream (v) = 0.186 m/s. S.no Distance(m) Depth(cm) 1. 0 0 2. 0.5 10.2 3. 0.5 10.1 4. 0.5 10.3 5. 0.5 10.9 6. 0.5 10.7 7. 0.5 9 8. 0.3 0 The cross sectional area at the midpoint (A) = 0.297m2 Thus, discharge of the stream (Q) = 0.8*V*A = 0.8*0.186*0.297 = 0.0442m3/s Result: The discharge of the Nakkhu khola obtained from float method is given below. Q=0.0442 m3/s Comments: The results obtained from different groups are different which is due to the reason that practical was not done accurately. If more accurate results are required, small increments in depths are recorded. Also if a floating rod or a specified object was used in floatation, it could give more accurate results. In the field, we floated small papers, plastics, etc which may also be the reason that the results given by different groups didn’t match. We believe that such educational tour will be a lot of help for us in understanding the actual situation while undertaking any professional tasks in the future and we hope that IOE will continue such field tours.