Module 4 Measurement and Processing of Streamflow Data

Transcription

1 Module 4 Measurement and Processing of Streamflow Data 4.1 Streamflow Data Measurement of River Flows Types of Station Control Main elements of a streamflow gauging station 4.2 Measurement and Recording of River Stage Non-recording river stage gauges Depth Sounding and Suspension Equipment Water Level Recorder 4.3 Direct Determination of Discharge 4.4 Velocity-Area Methods Velocity Measurement by Floats Measurement of Flow Velocity by Current Meters Stage-Discharge Gauging Stations in Natural Channels Discharge Measurements 4.5 Indirect Determination of Discharge Estimation of Discharge by Slope-Area Method 4.6 Moving Boat Method Determination of stream velocity 4.7 Discharge Measurement Using Artificial Structures Weirs Flumes Short-Throated Flumes 4.8 Advanced Discharge Measurement Techniques Dilution Methods for Measuring Discharge Electromagnetic Method 4.9 Measurement of Discharge under Difficult Conditions Special Problems in Streamflow Measurements in Arid and Semi-arid Regions Measurement of Streamflow under Ice Cover 4.10 Closure References 4.A International Standard ISO A.1 General requirements of a gauging station Keywords: Streamflow Data, Station Control, River Stage, Depth Sounding, Water Level Recorder, Velocity-Area, Float, Current Meter, Slope-Area, Moving Boat Method, Weirs, Flumes, Dilution, Electromagnetic Method. The objectives of this module are: To discuss the techniques of acquisition of river stage data, To discuss the techniques of acquisition of river discharge data, To discuss the advanced techniques of acquisition of river discharge data.

2 A stream is a flow channel into which the surface runoff from its catchment drains. Streamflow or discharge is measured in units of volume per unit time (m 3 /s). The measurement of discharge in a stream forms an important branch of "Hydrometry", the science and practice of water measurement. This module deals with methods of measuring discharge in a river and processing of streamflow data. 4.1 Streamflow Data Stream flow records are the continuous data of flow passing through a particular section on the stream. Since the measurements made at the gauging site may subject to various random, systemic and spurious errors, raw data is to be processed to transform them into their most usable forms through a variety of quality checks at appropriate stages to ensure data quality and reliability. Processing of stream flow data is not a single step process; several steps are required to produce reliable streamflow data. For effective and efficient water resource assessment and management and for proper river basin planning, development of flood forecasting system, etc. reliable, accurate, processed and easily accessible data base containing data of hydrologic variables is a pre-requisite. The most important hydrological data for management of surface water resources pertains to streamflow. Streamflow has served as the lifeline for mankind and continues to do so. Its importance is also relatively more, since this source is visible in contrast to ground water which is hidden. Streamflow records are primarily continuous records of flow passing through a particular location on the stream. Streamflow data are analysed to determine the magnitude and variability of surface waters. These records are input in planning, design, and operation of surface water projects and are also used in design of bridges and culverts, flood forecasting systems, and flood plain delineation Measurement of River Flows There are several methods for measuring river flows - the long-established and time-tested velocity-area method, structural (use of weir, notch) and dilution techniques and recent innovations such as the moving boat method, ultrasonic, and electromagnetic methods. Each of these methods has certain advantages and limitations which makes their choice dependant on site conditions and on the equipment and resources available with the gauging authority. When the channel cross-section is regular and of a known shape, an estimate of the discharge can be obtained by simply measuring the averaged flow either across the channel at some depth or over depth in the center of the channel. Fig. 4.1 shows the classification of various methods of discharge measurement.

3 The sections where river measurements are carried out are known as stream gauging stations. A network of these stations is established to collect data about river flows of a region. The location of gauging sites primarily depends upon the purpose of data collection. If the site is needed for a specific project, the general location will be in the vicinity of the project. However, if a network of gauging stations is to be established to study the general hydrology of a region and for planning and design of various projects than careful planning is required to identify locations so that optimum information is obtained for the resources deployed in the data collection. River gauging stations are of three types: basic data stations, operational stations, and special stations. The basic stations are established to collect data for a variety of uses, including planning and design of projects, and to understand the hydrological characteristics of the region. The operational stations collect data to run projects and issue forecasts. The objective of special stations is to meet specific data needs that may arise in cases, such as research, project investigation, special studies, legal cases, etc. Their operation is terminated when the specific need is fulfilled. Sometimes, auxiliary stations are set up to augment the database from the network or region. An auxiliary station may, for example, record only the peak discharge which occurs at that site during a certain (usually limited) period. Conventional Methods Advanced Techniques Direct Velocity area methods using current meter Velocity area methods by using floats By Dilution methods Indirect Slop area method By stage - discharge relations Measurement by hydraulic structures Moving boat method Ultrasonic (Acoustic) method Electromagnetic method Fig. 4.1 Classification of methods of discharge measurement. The number of gauging sites depends on the cost of installation and operation, the value of the data, watershed size, degree of development, objective of data collection, accuracy, hydrologic characteristics, etc. Some of these factors are interrelated. The streamflow data that are of immense use in water resources are river water level or gauge and discharge. A continuous observation of the river water level or stage may be made with comparative ease and economy.

4 At important stations, the stage is measured at short intervals and discharge is measured once or twice each day. At less important stations, only stage measurements are made regularly. A continuous measurement of discharge in a natural channel is comparatively difficult, time consuming, expensive and requires special skills. Therefore, the discharge at a site is measured less often and is estimated by indirect methods. Fortunately, there exists a relation between stage and discharge at a section. This relation is termed as stage-discharge relationship or rating curve. This relationship is used to transform the observed stages into discharges. Note that the reliability of such discharge records is dependent on the reliability of stage data and the correctness of the stage-discharge relation. At many sites, the discharge is not a unique function of stage; variables other than stage must also be simultaneously measured at such sites to correctly estimate discharge. For example, if variable backwater occurs at a site, the data on water surface slope is required. The slope can be measured by installing an auxiliary gauge downstream. The rate of change of stage is an important variable where the flow is unsteady and channel bottom slopes are flat Types of Station Control A permanent and stable stage-discharge relation is desirable from the point-of-view of hydrologic measurements and to establish such a relation, the river channel at the gauging station must be able to stabilize and regulate the flow at the station such that for a given stage, the discharge past the station is always the same. The stability, reliability, and shape of the stagedischarge relation are normally controlled by a section or a reach of the channel at or downstream from the station and are known as the station control. Station controls can be classified in four categories: section and channel controls natural and artificial controls complete, compound and partial controls permanent and shifting controls When any change in the physical characteristics of the channel downstream to the control has no effect on the flow at the gauging section, such control is termed as section control. Natural or artificial local narrowing of the cross-section (waterfalls, rock bar, gravel bar) creating a zone of acceleration (Fig. 4.2) are some examples of section controls. The section control necessarily has a critical flow section at a short distance downstream.

5 Fig. 4.2 A section control A cross section where no acceleration of flow occurs or where the acceleration is not sufficient to prevent passage of disturbances from the downstream to the upstream direction is called as a channel control. The length of the downstream reach of the river affecting the rating curve depends on the normal or equilibrium depth h e andd on the energy slope S [L h e /S, where h e follows from Manning s Q = K m Bh 5/3 e S 0.5 (wide rectangular channel) so h e = [(Q/K m S 0. 5 ) 3/5 ]. The length of channel effective as a control increases with discharge. An artificial control is specifically constructed too stabilize the relationship between stage and discharge. Thesee include weirs and flumes, discharging under free flow conditions. Natural section controls include a ledgee of rock across a channel, the brink of a waterfall, or a local constriction in width (including bridge openings). Natural controls can have a range of geometry and stability. Some natural controls may have a single featuree such as a rock ledge across the channel at the crest of a waterfall, thereby forming a complete control. Thus complete control governs the stage-discharge relation over the whole range of stage experienced. However, in many cases, station controls are a combination of section control at low stages and a channel control at high stages and are thus called compound or complex controls. Where the geometry of a section and the resulting stage-discharge relationship does not change with time, it is describedd as a stable or permanent control. Shifting controls change with time these may be section controls such as boulder, gravel or sand riffles which undergo periodic or near continuous scour and deposition, or they may bee channel controls with erodible bed and banks.

6 The amount of gauging effort and maintenance cost to obtain a record of adequate quality is much greater for shifting controls than for permanent controls. Since stage discharge observations require significant effort and money, it is always preferred to select a gauging site with a section or structure control. A complete control has many advantages: permanence, easy installation and running of the gauging site, and favourable conditions for current meter measurements. However, a complete control is not practicable in many cases and one may have to be content with either channel control or a compound control Main elements of a streamflow gauging station The main elements required at a streamflow gauging station are as follows: a) a stage measuring device (manual observation) or a stage sensing and recording device; b) a control section or reach; c) a section suitable for measuring discharge; d) set-up and devices necessary to measure discharge; e) office building and place to keep equipment, spare parts, and accommodation, etc. The International Standard Organisation (ISO) has brought out a large number of standards dealing with measurement of liquid flow in open channels. Technical committee TC113 of ISO deals with this topic. The publication ISO (1983) is a useful collection of standards dealing with various aspects of streamflow measurement. WMO has also brought out many publications related to streamflow measurement. Of course, the individual countries have their own standards. The topic is covered in detail in Herschey (1986 and 1995) and Boiten (2008). Site requirements for measurement of discharge using current meters or floats are given in ISO 748. Now, we will discuss measurement of river stage and discharge. 4.2 Measurement and Recording of River Stage The terms stage and gauge height are interchangeably used to express the elevation of the river water surface with respect to an established datum. As shown in Fig. 4.3, river stage is the vertical difference between the water surface of a river relative to an established gauge datum. The datum may refer to an arbitrary datum that is selected for convenience or the national reference (e.g., the mean sea level or the datum of the survey of India or an arbitrary point, slightly below the point of zero flow in the stream). To eliminate the possibility of negative values of the gauge height, the datum selected should be below the elevation at which the flow is zero. The gauge height is usually expressed in hundredths or thousandths of a meter. River stage is a basic variable representing the state of a river. River stage Gauge datum

7 Figure 4.3 Cross section of a river with gauge datum and river stage. Usually, it is difficult and expensive to measure the discharge of a river directly and continuously. Therefore, one frequently measures thee water stage which is easy and then converts the stage values to discharges. Records of stagee are used with a stage-discharge relation to obtain the records of stream discharge. Clearly, the reliability of the discharge record is dependent on the reliability of the stage record and the stage-discharge relation. The water level data is measured using a varietyy of equipment: staff gauges, autographic water level (chart) recorders, and digital type water level recorders. These can be classified in two broad categories: (1) Non-recording, manual gauges, (2) Recording gauges Non-recordin ng river stage gauges The simplest way to measure river stage is by the use of a staff gauge which is basically a scale installed such that a portion of it is always immersed inn the water. This gauge may be a vertical scale attached to a column, pillar, bridge pier, or other structure that extends into the low-water part of the channel. Such staff (manual) gauges are simple and inexpensive but must be read frequently. Where the range of water levels exceeds the capacity of a single vertical gauge other gauges may be installed in the line of a cross-section normal to the direction of flow (seee Fig. 4.2). The scales on such a series of stepped staff gauges should overlap by not less than 15 cm. A ramp gauge consistss of a scale marked on or securelyy attached to a suitable inclined surface, which closely follows the contour of the riverbank. Thee ramp gauge may lie on one continuous slope or on more slopes. It should lie on the line of a cross-section normal to the direction of flow. staff A or

8 ramp gauge is an inexpensive, simple, and reliable method of measuring water level. By using it, water level can be measured by relatively unskilled staff. Ramp gauges amplify surges and ripples but provides the opportunity to of a higher resolution. A staff gauge can only be used for spot measurements. It is difficult to obtain readings in the field with a true resolution higher than ±5 mm. Figure 4.4: Staff gauge pictorial view (left) and markings (right) Staff gauges can be either vertical or inclined. Vertical staff gauges are normally porcelain enameled iron sections with a scale (with graduations of 5 or 10 mm) marked on or securely attached to a suitable vertical surface graduated every 10 mm. The vertical staff gauge is used as an inside reference gauge (if installed in a well), or as an outside gauge if installed in the stream. Where the water level of the river varies over a large range, observations using a single staff gauge might be difficult. In such cases, the gauge consists of stepped sections installed at different locations in a line normal to the flow. Each of these stepped gages should refer to the common datum and they should overlap by not less than 15 cm to ensure continuity of readings and confirm their consistency with each other. Staff gauges are manually read, generally each day in the morning in lean season and at (multi) hourly intervals during high flows. An inclined staff gauge is usually a graduated surface attached securely to a permanent foundation. Rocky outcrops on river banks make good base for inclined gauges. When inclined gauges are built flush with the stream bank, they are less likely to be damaged by floods, floating debris, or drift than are projecting vertical staff gauges. Such gauges should be located as close as possible to the measuring section, without affecting the flow conditions. Manual gauges are to be read at fixed times whereas the recording gauge provides a continuous data of the variation of stage. In manual observation, commonly the stage is read daily in fair weather and (multi)hourly in monsoon season. In automatic recording stations, a

9 continuous record of stage is obtained by utilizing water level sensors interfaced with a analogue recorder (chart) or a digital recorder (logger or telemetry). The advantages of the non-recording gauge are low initial cost and ease of installation. The disadvantages are the need for an observer (and even then data will observed at limited times) and less accuracy. For a long-term operation, the advantages of a recording gauge outweigh those of a non-recording gauge. Sometimes, an automatic and a non-recording gauge are maintained together because the electro-mechanical recording gauge equipments are liable to breakdowns. The values of stage may be required as a single instantaneous measurement, as a short series of instantaneous measurements or as a continuous or practically continuous record of the fluctuations of stage. Even when a gauge recorder is used, the observer should visit the station from time to time to ensure satisfactory performance of the sensor and recorder. Observer should note the time and date of such checks along with the staff gauge and recorder values. It is essential that the staff gauge itself is maintained such that it is safely accessible and legible to the observer. Further, the observer should always visit the site following a major high flow event to ensure continued measurement and recording of data Depth Sounding and Suspension Equipment During field investigations, depth of water from surface to the river bed may have to be measured. Depending on the velocity and depth of flow, either a sounding rod or a sounding line is used for this purpose. A sounding rod is a graduated rigid rod with a base plate. It is used to measure depths up to 5 to 6 m in flows with velocities up to 2 m/s. For measurements by either sounding rod or wading rod, the rod must be held in a vertical position. For measurements by sounding line, appropriate weights must be attached to keep it as close as practicable to vertical. For smaller depths and velocities, a wading-rod is used; for greater depths, a sounding line is used. A sounding rod should be as lightweight as possible but sufficiently strong to withstand the force exerted by flowing water without undergoing significant deflection or vibration. It should remain straight and vertical during use; should not cause significant heading up of water and should not penetrate into the channel bed. To make observations in flowing water with the help of suspension equipment, the measuring equipment must be placed at the point of measurement in such a way that it does not cause appreciable disturbances, irrespective of the depth of water and velocity of flow Water Level Recorder As the name suggests, a water level recorder (WLR) is an instrument which senses and records water level. It basically consists of a time element and a gauge height element which together produce a time-series of water levels. The time element is controlled by a clock while the gauge

10 height element is activated by a float or a pressure actuated system. These recorders can be classified as either analogue type or digital type, depending on the way the data are recorded. The analogue type recorders produce a graphic record of fluctuations of the water level with respect to time. The water level recorders are generally of shaft-angular-input type, and the angular rotation of the shaft is recorded. The depth of water surface is sensed for automatic recording by a float in a stilling well which follows the rise and fall of the water level. A gas-purge system that transmits the pressure head of water in a stream to a manometer is known as a bubble gauge. A water level recorder gives a continuous record of the water level on a chart from which values are manually extracted at desired intervals. The data from a digital water level recorder are stored in an electronic memory and these are downloaded to a computer. The data can either be recorded at equal intervals of time, usually at (fraction of) an hour interval, or at only those instants when there is a change in water level by more than a pre-set amount. (a) Float-type water-stage gauges A float-type gauge consists of a float which is installed in a stilling well, a graduated tape or wire, a counterweight, a pulley and a pointer. The tape or wire runs over the pulley and is kept taut by the action of the counterweight. In this way the float that positions the tape with respect to the pointer senses the stage fluctuations. As shown in Fig. 4.5, this device records the motion of the float as it rises and falls with river water level. Motion of float and counterweight are translated to move a pen or stylus while the chart moves at right angles and thus a continuous record is produced. The stilling well protects the float and other accessories from floating debris as well as moderates the rapid fluctuations in the stream level. Such gauges are appropriate for streams with narrow, incised gravel-bed channels, so that the stilling well can be located close to the stream. For wide and sandy channels, the stilling well has to be located on the stream bank some distance away. Pipes connecting the stilling well to the stream are vulnerable to blockage by siltation and the water level in the stilling well may not faithfully rise and fall with river water level. A float gauge can provide a direct record of river stage and no external energy source or battery is required. It provides almost uniform resolution throughout the range and good accuracy. But being a mechanical device, it is subject to errors from hysteresis and friction. Construction and maintenance of stilling well is expensive, particularly in places where rivers carry large amount of sediments. A chart recorder produces a continuous hard copy record. Quality of the recording mechanism affects the accuracy of the chart record. Subsequently, the data is manually converted to digital format; this process is labour insensitive, time consuming, and may introduce errors.

11 Figure 4..5 Float type recorder (b) Bubble gauges Bubble gauges record the pressure required to maintain a small flow of gas from an orifice submerged in the stream and this is an indicator of the water level in the river. Fig. 4.6 shows the gauging arrangement by using bubble gauge. The advantage of such gauge is that is does not need a costly stilling arrangement like the one needed forr float operated gauges. Figure 4. 6: Bubbler gauge