WRIS Time Series Data
Hydro Observation Data
WRIS time series data broadly covers three classes of databases – Hydro-observation (HO), Meteorological (Met) & Water Quality (WQ). HO database comprise mainly Discharge, Sediment, X-Section, River Level (RL) & Current Meter measured at variable frequency. Met database comprise of Rainfall, Temperature, Sunshine, Relative Humidity, Wind Speed, Wind Direction and Pan Evaporation. WQ database represents the concentrations of physical, chemical and biological constituents of a surface water body.
Discharge and Sediment data
Observations on water level, stage-discharge measurements and sediment concentration are the main raw hydrological data required to be entered. Water level is primarily observed in two ways:
- From staff gauges and
- From autographic or digital type water level recorders.
From the staff gauges the observations are generally made once in a day in lean season and at multiple times a day during flood times. For flashy rivers staff gauges may even be read at hourly intervals during flood season. However, standard timings are generally followed for the observations during the day by different agencies. An autographic type of water level recorder on the other hand gives a continuous record of water level in time. These autographic records have to be read manually and the data is normally reported at one hour time interval or lesser, if required. The digital data from a digital water level recorder can either be at equal intervals of time, usually at quarter or half hour interval, or can be reported for only those instants when there is a change in water level which is more than a pre-set amount. Stage-discharge observations are the primary data for establishing the relationship between the stage and discharge, called the rating curve, at any river gauging station.
The discharges are generally estimated indirectly by:
- Observing the flow velocities at a number of points across the area of flowing water and computing discharge by area-velocity methods,
- Observing the slope of the surface profile and computing discharge by slope-area method. Under normal circumstances the velocities are observed using flow meters (current meters) but in difficult circumstances and when the flow meters are not available the float method or slope-area method is also resorted to.The frequency of stage-discharge observations is primarily based on the type of control existing at the station. For perfectly stable controls, it is sufficient to establish the rating curve initially with 20-30 observations and then occasionally checking the rating by 3-4 observations every two to three years.On the other hand, for a constantly shifting control,like alluvial reaches in the plains,it may be required to take stage-discharge observations daily.Observations on suspended sediment concentration and associated characteristics are also taken along with the stage-discharge observations.Suspended sediment concentrations for various fractions form a part of sediment data.
Entry of Hourly Water Level Data
The hourly water level data is obtained either from the chart records of the auto graphic type recorders or by the digital data obtained from the digital water level recorders. The parameter for water level with respect to zero of gauge observed using an auto graphic water level recorder is HZA and for the records from the digital recorders is HZD.This hourly data is reported for each clock hour starting from the 0100 hrs and ending at 2400 (i.e. 0000 hrs). Each hourly data is the representative instantaneous water level recorded at these clock hours. The time interval code for these hourly data series is 4,1,0,0. The form functions for any parameter ID having first two characters as HH or HZ.
Entry of Flow Measurement Data
The velocity observations normally results in a huge amount of data for each discharge observation. For the purpose of reference many other details such as mode of crossing, type of equipment used, condition of water and weather etc. are also recorded. After each stage discharge observation the observer compiles the field notes in proper forms and then computes discharge and other characteristics of flow.Since the primary and summary stage-discharge data comprises of a huge number of types of data, its entry is accomplished using five sub forms:
- entry of gauge data and reference information,
- entry of current metering observations,
- entry of float observations,
- entry of slope observations,
- entry of summary stage-discharge data and,
- entry of summary suspended sediment data.
Secondly, fields for entry of reference information like:
- mode of crossing,
- method of velocity observation,
- location of discharge measuring section,
- equipment used for sounding and
- sounding weights used during the observations is displayed.
Slope-area method can be used to estimate the flow during high stages when it is very difficult to make velocity observations. For this, the average slope of the water surface profile is observed and velocity of flow is estimated using the Manning’s formulae.
Daily Suspended Sediment information
Under normal conditions samples are collected from a Boat/Motor launch with the help of Punjab Type Bottle sampler. Under high velocity conditions, when sampling is not possible with the help of Boat/Motor launch, the samples are collected from either a bridge or a cable way.The samples collected for each sampling vertical point are mixed to make groups having almost equal discharges. These groups divide the river hypothetically into almost equal compartments of flow. The suspended sediment samples are analysed by Gravimetric procedure. Entry of summary suspended sediment data Suspended sediment observations form the part of sediment data and is normally associated with the amount of flow at any section. The observations are normally taken along with stage-discharge observations. However, they may also be taken with only stage measurement and later computing the corresponding discharges by using rating curves.
River Level (RL) of Gauge Zero
At the stream gauging stations, water level is always measured with respect to the zero of the gauge. The zero of the gauge is established as per the requirement and flow conditions prevailing at individual stations. Thus, zero of the gauges for different stations are obviously at unequal elevation with respect to a common datum. For making any comparison of water level at two or more gauging stations it is necessary to bring all the water level observations at all the gauging stations to a common datum. Also, with the passage of time, gauges may be displaced or destroyed or they may be changed in elevation as the result of erosion of beds. In order that the records of stage may assuredly refer to the same datum throughout the period of record, the datum of each gauge must be referred to and occasionally checked with at least one and preferably two or more bench marks that are entirely detached from the gauge, its support or shelter, and that are not liable to destruction or change in elevation.The common datum is taken to be the mean sea level (M.S.L) and hence the reduced level (R.L) of the zero of the gauges with respect to M.S.L. for every station is a very important characteristic. Sometimes, due to floods or otherwise the gauges are required to be reinstalled or adjusted. As such this characteristic is semi-static in nature and every value of R L of gauge zero has a period of validity. To be able to compare the water level readings for different periods and for different stations it is very essential to properly store these values of R.L of gauge zero along with their respective validity periods.
R L of zero of gauge- This is the reduced level (R.L) of the zero of the gauge with respect to the mean sea level (M.S.L). The water level readings are always taken with respect to the zero of the gauge and so for obtaining the water levels with respect to the common datum, i.e. m.s.l, this R.L. of gauge zero has to be added to each water level reading.
Validity period- As stated earlier, the zero of the gauge has to be adjusted or re-installed, though not very frequently, due to variety of reasons. Such adjustments render the R.L of gauge zero to be non-unique. In these circumstances it is very essential to attach a validity period to every setting of the zero of the gauge. This information provides the complete reference about the changes in the R.L. of gauge zero for any moment in the past.
Datum of elevation- Water level readings are taken with respect to the local zero of the gauge. For the purpose of making use of these observations pertaining to different periods or comparing the records of two or more stations, it is essential to transform all the observations with respect to a common datum which does not change in time. Such a datum can be any arbitrarily defined datum and the mean sea level is taken as accepted for this purpose in the country.
Reference bench marks It is very necessary to have at least one bench mark very near to the station location (also called Musto Bench Mark) for making reference for the purpose of checking or adjusting the zero of the gauge at regular basis. R.L. of such reference benchmark and its distance from the zero of the gauge is recorded for the sake of information. Similarly the R.L. and distance of a secondary bench mark, from where the Musto bench mark has been carried, is also recorded.
Adjustment of gauge zero Since any changes in the setting of the zero of the gauge will be very crucial in interpretation of the water level observations, it is appropriate to record the following:
- reason of resurvey or adjustment,
- name and designation of the Surveyor involved in the re-survey or adjustment of the gauge and
- name and designation of the Inspecting officer.
Current Meter Ratings
Current meters or flow meters are one of the important equipment employed for measurement of flow velocities. The relation between the speed of rotation of the current meter to the velocity of the water which, causes the rotation, is defined by the meter rating. The current meter should be rated from time to time whenever it is repaired or modified in any way and in any event after a prescribed period of use. The entries for current meter ratings are maintained with the help of two forms. One form, Current Meter Information, is for maintaining the list of current meters and those properties which do not change with time. Other form, Current Meter Ratings is for entering the actual current meter ratings for each current meter for various validity periods. There are attributes required for describing the current meters. These are: the current meter number, its type, name of manufacturer and the date of manufacture of the current meter. For proper identification of the current meter it is necessary to enter the meter no. assigned by the manufacturer, the type of meter and the make or the name of manufacturer. The meter no. is the unique number allotted to every meter by the manufacturers themselves. The type of meter indicates the category to which the meter belongs.
For example, whether a meter is cup type, propeller type or pygmy type. Date of manufacture is also stored for purpose of reference. These four characteristics are static in nature and do not change with time for any current meter.
Validity period- Every calibration is valid for a certain period only and the meter has to be re-calibrated at regular intervals after making its use for the prescribed period. It is very important to associate each current meter rating with the respective start and end dates of the period of validity.
Calibration done- by It is very important to note the name of the agency which has carried out the calibration of the meter. Calibration of the meter is a very fundamental and important activity requiring special skills and equipment. Normally, all the agencies providing current meter calibration facilities must be certified to do so by appropriate authority.
Velocity limits- Every current meter calibration indicates two velocity limits between which the meter ratings are applicable and the meter is not supposed to be used beyond such boundaries. These limits are: (a) the threshold velocity and (b) the upper velocity limit. Limits are entered in units of m/sec and are used to exclude use of meter which indicate a velocity outside this range.
Details of meter ratings- The result of the meter calibration is presented in the form of one or more linear equations of the form:Y = A + B X. Here, A & B are the coefficients of the meter ratings, X is the number of revolutions the meter makes per second and Y indicates the velocity of flow of water in m/sec. Three characteristics are entered for the purpose of recording the details of any meter calibration and are: (a) number of rating equations, (b) coefficients A & B for each of these equations and (c) the limits of applicability of these individual rating equations in units of revolutions/sec.
Rated spin-It is important that a current meter, which is in constant use, be at all times in proper working condition, in other words, in as nearly as possible the same condition as it was calibrated. In order that the field engineer may be assured concerning the mechanical condition of the meter, a spin test is performed at the time of calibration. This is reported as the rated spin and is the number of revolutions which it makes after the meter is spinned by a quick movement of the hand until the meter comes to complete rest.
Figure-1: Example Plot of a current Meter Rating
Cross-section data comprise of the pairs of distance and elevation of several points on the cross-sectional profile of the river gauging section. The distances are taken with respect to an origin on the gauging section and elevation is reported with respect to the mean sea level as the datum. The date of survey is always associated with the cross-sectional data.
This is an added facility for making a certain addition or subtraction from all elevation values entered in the table. It is mainly useful if the manuscript lists the elevations of various points with respect to the local datum and it is required to add the reduced level of this datum with respect to the mean sea level to all the points.
Distance from CGL
For the upstream and downstream gauge lines it is required to indicate the distances from the central gauge line. These distances in meters can be given in the fields for distance from CGL available with upstream and downstream gauge lines.