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Berlin: Soncino Gesellschaft, epub hydrometry ihe delft lecture note series Viereck, George Sylvester July 2, immigrants to Guy Raner Jr. Albert Einstein Creator and Rebel. Boiten, W. Hydrometry: A. Back to study programme. Hydraulic Works and Installations Year 5. Academic year Language of Instruction Portuguese. Position fixing is done by direct methods tape and range finder or by indirect methods linear and angular. At the same time the position fix is indicated on the echosounder recording. In estuaries or along open coasts, soundings are taken on lines per- pendicular to the bottom- contour lines in order to have the most accu- rate way of locating these lines.
The intervals between the tracks are more or less dependent on the purpose of the survey and the scale on which data should be charted.
Various instruments are used for sounding depth measurements The sounding rod and sounding line were formerly the conventional tools used for measuring the depth of water. The accuracy of both meth- ods is limited and the echo sounder has found wide application instead because it is not restricted by flow velocity and depth.
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It is, therefore, universally more acceptable than the outdated conventional tools. It also enables a fast measurement of the depth. Sounding rod The sounding rod is a wooden rod graduated in centimetres, or deci- metres. At the bottom end of the rod is a base plate which prevents the rod from penetrating the river bed and also serves as a weight to keep the rod in a vertical position on the bed.
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Sources of error are deviation of the rod from the vertical and the stagnation head related to the flow velocity, especially where there are strong currents. Therefore, the appli- cability of this tool in flowing water is limited. Sounding line The sounding line is used if the flow velocity or the water depth pre- vents the use of the rod.
It consists of a chain or wire to which a heavy weight is attached. Tags are tied to the line to indicate metres and decimetres. In other cases use of davit and winch the depth is read from a counter aboard ship. Corrections have to be made to the mea- surements to allow for deviations from the vertical above the water sur- face air line corrections and below the water surface wet line correc- tions. For this purpose the international standard ISO has been developed. The depths which can be measured by a sounding line are limited to some 12 to 15 m. Sources of error are the penetration of the weight into the river bed and the variations from the ideal conditions wire bending etc.
Furthermore, it may be difficult to ascertain whether the weight is actually in contact with the river bed. Echosounders The echo or ultrasonic sounder, the use of which is not limited by flow velocity or depth, is the most suitable tool for sounding. It enables accu- rate and rapid measurement of depth, based on the propagation speed of sound in water and the measurement of the running time of the sound wave. The propagation speed of a sound wave in water depends on tem- perature and salinity. Increase of each of them leads to increase of Cwater.
The principle is as follows: a short but strong electrical pulse is pro- duced by the signal generator, amplified and sent to the transducer which converts it into an acoustic signal which is sent to the river bed. The signal is then reflected from the bed, received by the transducer and converted into an electrical pulse, which is amplified.
The time-lapse. Relative 10 Propagation speed of a sound wave as a function of temperature and 0 relative density after: Nedeco, The depths are shown on a suitable indicator, recorded on a paper roll or given as a digital output. Because of the high transmission frequency, the individ- ual depths appear on the recording paper as an almost continuous bed profile, as is shown in Figure 3.
The transducer is located at a certain distance 0.
And as the distance between the bottom of the transducer and the reflected surface of the river bottom is measured, the draught of the transducer should be added to the recorded depth, so giv- ing the correct water depth See Figure 3. Most echosounders are operated on a 12 V battery. From time to time the echosounder must be calibrated.
Besides that, at the beginning and end of each sounding day, the zero-setting of the echosounder should be checked by means of a bar check. Zero setting depends on the depth of the transducer under the water surface, which may vary from day to day.
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In the case of an outboard transducer, a steel plate suspended to a marked line is lowered into the water and held at a certain depth beneath the transducer. The depth recording of the testbar should correspond with the actual depth of the testbar under the water surface. This is checked at various depths. If this is not the case, an adjustment has to be made in the speed of sound to cope with the. Correction of the sounded depth. Air bubbles under the transducer should be avoided by limiting the speed.
Usual speeds are 1. The echosounder is used for cross-sectional soundings of the river, lon- gitudinal soundings talweg and local complete soundings of a part of the river, a bay or a lake. The paper speed can be adapted to the different types of soundings: — river cross-sections: the highest paper speed — longitudinal soundings; a lower paper speed.
If soft mud layers are expected or the survey is intended as a pre- dredging survey or a post-dredging survey, a low frequency echosounder should be used in order to detect layers and sedimentation or degrada- tion, as the sound pulse of a low frequency echosounder penetrates more deeper in the bottom. A high frequency echosounder, however, gives only a recording of the top of the bottom regardless the bottom compo- sition. By using both frequencies simultaneously the multiple layers in the riverbed are detected.
Determination of water level For conversion of the measured depth into a level with respect to the datum plane, gauge readings have to be made during the soundings. Usually, the water level in the sounding area is determined by linear interpolation between two readings at existing gauging stations. If such interpolation introduces an error exceeding a few centimetres, a tempo- rary gauge should be installed close to the sounding area.
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As the error due to linear interpolation over time should not be more than a few centimetres, the frequency of the readings depends on the rate of change of the water level. Therefore, during periods of rapidly changing water levels, frequent readings are required. Each of the components of a sounding procedure, i.
To arrive at a suitable presentation of the results of the soundings, these data have to be processed. Check P 4 O. A, B, C, D etc. In other cases sighting might be obstructed by bushes or trees and if nessesary sighting paths should be cut. Example of When the sounding takes place in cross sections, the data for the deter- a triangulation network to mination of the location of a sounded point consist of the number of the get the river alignment after: Nedeco, When the free sounding system is applied, the data consist of, for instance, the co- ordinates of the base points used and two angles measured simulta- neously.
The location can then be determined either mathematically or graphically. The water level at the sounding point is determined using records from one or two gauges. From these data, as well as the gauge-zeros and their locations, the required water level is obtained by linear interpolation over distance and, if necessary, over time. The time link between soundings and water level is essential.
The water level and the sounded depth, obtained from an echogram or a list of depths measured, are then used to determine the elevation of the sounded points of the river bed with respect to a horizontal datum plane MSL. All components of the processing are rather simple and can easily be done manually. However, if the extent of the work justifies it, the entire processing, or parts of it, can be programmed for computer. Other data such as gauge readings and time are limited in number and can easily be brought into digital form.
Example of Figure 3. For soundings taken in a river according to cross sections, the water depths related to the reference plane for each point in the cross section can be plotted on mm-paper and a cross sectional profile can be drawn see Figure 3. If so required, a complete contour chart of the river can be made derived from the cross sectional profiles.
On the sounding chart, contour lines indicating equal depths can be drawn. This work requires experience and understanding of the draughtsman, and it can be improved by occasional detailed measure- ments between the sections which are normally sounded. The spacing of sounding lines sounding tracks or transit lines for making a sounding chart depends on: — the water depth and river alignment, — the shape of the river bottom, — the purpose of the survey.
In most cases a spacing of metres is acceptable for river charts, depending on the river width and the river alignment. Classification of flows Flows can be classified by two parameters: time and distance 1. The majority of flows in open channels will fall into one of the clas- sifications listed below. Steady or permanent uniform flow. The discharge is constant with time, and the cross-section through which the flow passes is of constant area. A typical example is that of constant flow through a long irrigation canal with uniform cross-section, free from back water effects. Steady non-uniform flow.