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An innovative automatic stream gaging method
doumai of Hydrology, 21 ( 1 9 7 4 ) 2 7 - 3 1 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD J.F. H A R P
School of Civil Engineering and Environmental Science, Uni~,ersity of Oklahoma, Norman, Okla. fU.S.A.) (Accepted for publication April 3, 1973)
ABSTRACT Harp, J.F., 1974. An innovative automatic stream gaging method. Jr. Hydrol., 2 1 : 2 7 - 3 1 . This brief paper presents an extension of the moving boat method whereby intermediate sized ,,treams may be gaged automatically, by means of a moving meter, by remote control with no operator at the point of the velocity measurement. The paper is clearly innovative in that no velocity magnitude measurement is required, only an angle measurement and a known input constant velocity of traversing meter. The technique pre~nted here is most unusual in that a new technique is combined with classical methodology to produce a reliable field measurement of streamflow rates in a remarkably short time.
INTRODUC~ON
The purpose of any technique of stream gaging is to measure the discharge of a stream at any given water surface elevation in a minimum time interval. Historically, streamgaging has been a slow, laborious procedure using a current meter, suspending cable and stability weight. The stream is divided into sections, the depth sounded, and the meter placed at 0.2 and 0.8 depths and the velocities measured there and averaged. The average velocity in each section multiplied by the area and the products summed. This procedure is accomplished at a given elevation and the stage-discharge rating-curve subsequently derived. A serious drawback to this procedure lies in the long period of time required to perform the measurements. If the river is on the rise, or fall, the water surface elevation may change significantly before the measurements can be completed. Also, errors in positioning the meter and -eading the velocities arise. Both precision and accuracy are difficult to achiew, under field conditions. The general idea of the dynamic concept is essentially extended in this paper whereby the extended technique is applicable to small streams which are normally gaged by classical current meter methods. In a sense, this method is an extension of the moving boat method, but there are important differences, which interested readers will immediately appreciate.
28
J.F. HARP
A NEW TECHNIQUE
Discharge measu~'ement at gaging stations is a colossal task in the United States and throughout the world. The U.S.G.S. is the appropriate agency which has the prime responsibility for establishing streamflow records for more than 5000 gaging stations at the present time° The research divisions of the Agriculture Research Service, Soil Conservation Service, the Bureau of Reclamation, and other governmental agencies are currently investigating small watershed streamflow measurements on an intensified basis. Most of these measurements are being accomplished by classical techniques describedby Corbett et al. in 1962. The U.S.G.S. has recently published a new technique manual by Buchanan and Somers (1969) which is an updated version of the original work by Corbett (1962). However, the techniques remain the same and except for newer equipment and minor revisions, the methodology is still the same. Existing techniques depend upon a point velocity measurement and classical methodology. With this in mind, the U.S.G.S. and others are currently researching the application of pressure transducers and pitot tube methods to measure point velocities. The techniques of streamflow measurement, however, remain the same. In 1968, the first innovation in streamflow measurements in half a century was presented by Smoot and Novak (1969). They used a moving boat and surface velocity measurements in a dynamic technique that was demonstrated to be a revolutionary new approach which provided great accuracy and precision. The greatest advantage of the method is the short time required to perform the measurement. On the Mississippi River at Vicksburg, Mississippi, a measurement by classical methods which required maay hours was performed in minutes by the moving boat technique. Since that time, Smoot and others from the U.S.G.S., have used a stationary and a moving boat on the Amazon River in South America with great success. The mov,~ng boat method was developed for use on large rivers and water-courses where a boat was feasible to operate. Presuming that the U.S.G.S. experience, and published characteristics for velocity distributions in streams, are correct and as stated below, certain presumptions can be made (see Fig.l): (I) The velocity distribution may be assumed parabolic. ~,2) The location of the maximum velocity is from 0.05d to 0.25d below the wa~:cr surface. (3) "lhe average velocity occurs at approximately 0 6d below the surface. (4) The average ve!ocily is approximately 85% of the surface velocity. (5) The average velocity is the average of the velocities at 0.2d and 0.8d below i:he surface. f h e technique presented here ties in the correctness of presumption (4),
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD
29
Vo.,o' \1 T-,,., 7_I o.~,0,oo.,,~ •
~]
d O.ed O.6a
II
I
v... A/°~=.~o,,=
I !
I I ,, / _L.._ ~
/
~ ~
/ll
verity
distribution
Vo..V i
Fig.1. Diagram of velocity distributions in streams. For explanation see text.
Fig,2, Diagram of the moving boat method, For explanation see text,
that is, that the average velocity is approximately 85% of the surface velocity. This being the case, assume that flow measurements are to be made at a river cross section where an existing cableway or bridge is available. A traversing mechanism is used to move the meter across the stream from A to B at a constant velocity so that the surface velocity of the water relative to the meter is measured continuously across the stream (see Fig.2). Now the basic relative velocity equation may be written as:
Vw =Vw/m+Vm where V--w is the velocity of the water; Vwlm is the vejocity of the water relative to the meter at an angle 0, shown in Fig.2; and Vm is the velocity of the meter as it transverses the distance s, from A to B in a time t. Expressing the equation in a simplified pictorial graphical form, we have:
Vw "k Vw/m+ Vm
(I)
30
J.F, HARP
whereby two scalar equations can be written as:
GO
Vm
(2)
Vwi m sinO
(3)
= Vwlm cos 0 -- V w = -
-
Two procedures are possible whereby the great pewer of this technique becomes apparent. The first procedure would be to measure the velocity of the water relative to the meter, Vw/m , and since Vm is known, the angle 0 can be computed from eq.2. From eq.3 the velocity of the water can be determined. Knowing the Vw term at all points across the stream, the flow rate can be determined by conventional procedures. The second procedure would be to introduce a directional sensing body only as the stream traversing instrument. The velocity of the water relative to the meter is not measured, but only the angle 0. From eq.2 the angle 0 being measured, and the velocity of the metes Vm being known, t h e Vw/m can be directly calculated. Now, from eq.3 the velocity of the stream Vw is computed. The resulting streamflow rate can now be computed by classical techniques. Now, Vw/m is continuously measured using a recorder as the meter traverses the stream. The angle 0 may or may not be measured depending on the procedure to be followed. Surface velocities will be measured with the meter, or not, as necessary. Fror,a prior measurements, the cross section of the stream has been presumed known Possibly a sonic depth-sounder could be mounted on the meter and this parameter measured also. The resultant velocities and area products will yield the stream discharge. Now, this method can be applied at a given location in a fraction of the time required by classical techniques and with greater accuracy. For example, a typical stream of about 2 0 - 3 0 yards wide would require a0out 1 h to "gage" by classical techniques. It is envisioned that this method will take only a few minutes or a small percentage of the usual time needed. The method will iequire a "track" or cableway whereby the moving apparatus can be suspended. A mechanism to move the meter across the stream at constant velocity is also required. Finally, a portable instrument console which can be moved from place to place and will record velocity, or angle, position and time. One of the new digital devices should be excellent. Usually a cableway or bridge is available anyway. This setup will require a cableway of less strength than those required to support personnel and apparatus, when the measurements can be made remotely, as this technique suggests. The savings in time will, in great likelihood, justify the additional apparatus that must be utilized. However, the same apparatus can be used at many stations. The procedure will have the great advantages of time reduction of at least
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD
31
80-90%, greater accuracy as a result of the shorter period of time to make the measurement and a reliably more constant water surface elevation during the measurement. CONCLUSIONS.
It is evidenced that this technique will be a powerful new tool. Development is the essential process. Probably a series of measurements will have to be made to determine the proper speed of traverse to minimize the errors incurred due to transverse meter velocity since the quantity being measured is the velocity of the water relative to the meter, or the angle should this prove feasible. The inertia of the meter cups will however have to be assessed, the inertia of the angular motions, of the apparatus, should be slight. Should the developmental measurements be made at a location where the stage-discharge. relationship has already been determined, direct comparison can be made and, accuracy assessed. Also, since a velocity input, Vm , is made, the angle 0, only, can be measured l and the streamflow thus determined. This aspect of the technique should be justification alone to develop and research the technique.
REFERENCES Corbett, D.M. et al., 1962. Stream GagingProcedure - U.S.G.S. Water Supply Paper 888. U.S. Government Printing Office, Washington, D.C., 244 pp. Buchanan, T.J. and Somers, W.P., 1969. Discharge measurements at gaging stations. In: Applications of Hydraulics (Book 3, Chapter AS). U.S.G.S., U.S. Government Printing Office, Washington, D.C., 66 pp. Smoot, G.F. and Novak, C.E., 1969. Measurement of discharge by the moving boat method. In: Applications of Hydraulics (Book 3, Chapter A1 I]. U.S.G.S., U.S. Government Printing Office, Washington, D.C., 22 pp.
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD J.F. H A R P
School of Civil Engineering and Environmental Science, Uni~,ersity of Oklahoma, Norman, Okla. fU.S.A.) (Accepted for publication April 3, 1973)
ABSTRACT Harp, J.F., 1974. An innovative automatic stream gaging method. Jr. Hydrol., 2 1 : 2 7 - 3 1 . This brief paper presents an extension of the moving boat method whereby intermediate sized ,,treams may be gaged automatically, by means of a moving meter, by remote control with no operator at the point of the velocity measurement. The paper is clearly innovative in that no velocity magnitude measurement is required, only an angle measurement and a known input constant velocity of traversing meter. The technique pre~nted here is most unusual in that a new technique is combined with classical methodology to produce a reliable field measurement of streamflow rates in a remarkably short time.
INTRODUC~ON
The purpose of any technique of stream gaging is to measure the discharge of a stream at any given water surface elevation in a minimum time interval. Historically, streamgaging has been a slow, laborious procedure using a current meter, suspending cable and stability weight. The stream is divided into sections, the depth sounded, and the meter placed at 0.2 and 0.8 depths and the velocities measured there and averaged. The average velocity in each section multiplied by the area and the products summed. This procedure is accomplished at a given elevation and the stage-discharge rating-curve subsequently derived. A serious drawback to this procedure lies in the long period of time required to perform the measurements. If the river is on the rise, or fall, the water surface elevation may change significantly before the measurements can be completed. Also, errors in positioning the meter and -eading the velocities arise. Both precision and accuracy are difficult to achiew, under field conditions. The general idea of the dynamic concept is essentially extended in this paper whereby the extended technique is applicable to small streams which are normally gaged by classical current meter methods. In a sense, this method is an extension of the moving boat method, but there are important differences, which interested readers will immediately appreciate.
28
J.F. HARP
A NEW TECHNIQUE
Discharge measu~'ement at gaging stations is a colossal task in the United States and throughout the world. The U.S.G.S. is the appropriate agency which has the prime responsibility for establishing streamflow records for more than 5000 gaging stations at the present time° The research divisions of the Agriculture Research Service, Soil Conservation Service, the Bureau of Reclamation, and other governmental agencies are currently investigating small watershed streamflow measurements on an intensified basis. Most of these measurements are being accomplished by classical techniques describedby Corbett et al. in 1962. The U.S.G.S. has recently published a new technique manual by Buchanan and Somers (1969) which is an updated version of the original work by Corbett (1962). However, the techniques remain the same and except for newer equipment and minor revisions, the methodology is still the same. Existing techniques depend upon a point velocity measurement and classical methodology. With this in mind, the U.S.G.S. and others are currently researching the application of pressure transducers and pitot tube methods to measure point velocities. The techniques of streamflow measurement, however, remain the same. In 1968, the first innovation in streamflow measurements in half a century was presented by Smoot and Novak (1969). They used a moving boat and surface velocity measurements in a dynamic technique that was demonstrated to be a revolutionary new approach which provided great accuracy and precision. The greatest advantage of the method is the short time required to perform the measurement. On the Mississippi River at Vicksburg, Mississippi, a measurement by classical methods which required maay hours was performed in minutes by the moving boat technique. Since that time, Smoot and others from the U.S.G.S., have used a stationary and a moving boat on the Amazon River in South America with great success. The mov,~ng boat method was developed for use on large rivers and water-courses where a boat was feasible to operate. Presuming that the U.S.G.S. experience, and published characteristics for velocity distributions in streams, are correct and as stated below, certain presumptions can be made (see Fig.l): (I) The velocity distribution may be assumed parabolic. ~,2) The location of the maximum velocity is from 0.05d to 0.25d below the wa~:cr surface. (3) "lhe average velocity occurs at approximately 0 6d below the surface. (4) The average ve!ocily is approximately 85% of the surface velocity. (5) The average velocity is the average of the velocities at 0.2d and 0.8d below i:he surface. f h e technique presented here ties in the correctness of presumption (4),
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD
29
Vo.,o' \1 T-,,., 7_I o.~,0,oo.,,~ •
~]
d O.ed O.6a
II
I
v... A/°~=.~o,,=
I !
I I ,, / _L.._ ~
/
~ ~
/ll
verity
distribution
Vo..V i
Fig.1. Diagram of velocity distributions in streams. For explanation see text.
Fig,2, Diagram of the moving boat method, For explanation see text,
that is, that the average velocity is approximately 85% of the surface velocity. This being the case, assume that flow measurements are to be made at a river cross section where an existing cableway or bridge is available. A traversing mechanism is used to move the meter across the stream from A to B at a constant velocity so that the surface velocity of the water relative to the meter is measured continuously across the stream (see Fig.2). Now the basic relative velocity equation may be written as:
Vw =Vw/m+Vm where V--w is the velocity of the water; Vwlm is the vejocity of the water relative to the meter at an angle 0, shown in Fig.2; and Vm is the velocity of the meter as it transverses the distance s, from A to B in a time t. Expressing the equation in a simplified pictorial graphical form, we have:
Vw "k Vw/m+ Vm
(I)
30
J.F, HARP
whereby two scalar equations can be written as:
GO
Vm
(2)
Vwi m sinO
(3)
= Vwlm cos 0 -- V w = -
-
Two procedures are possible whereby the great pewer of this technique becomes apparent. The first procedure would be to measure the velocity of the water relative to the meter, Vw/m , and since Vm is known, the angle 0 can be computed from eq.2. From eq.3 the velocity of the water can be determined. Knowing the Vw term at all points across the stream, the flow rate can be determined by conventional procedures. The second procedure would be to introduce a directional sensing body only as the stream traversing instrument. The velocity of the water relative to the meter is not measured, but only the angle 0. From eq.2 the angle 0 being measured, and the velocity of the metes Vm being known, t h e Vw/m can be directly calculated. Now, from eq.3 the velocity of the stream Vw is computed. The resulting streamflow rate can now be computed by classical techniques. Now, Vw/m is continuously measured using a recorder as the meter traverses the stream. The angle 0 may or may not be measured depending on the procedure to be followed. Surface velocities will be measured with the meter, or not, as necessary. Fror,a prior measurements, the cross section of the stream has been presumed known Possibly a sonic depth-sounder could be mounted on the meter and this parameter measured also. The resultant velocities and area products will yield the stream discharge. Now, this method can be applied at a given location in a fraction of the time required by classical techniques and with greater accuracy. For example, a typical stream of about 2 0 - 3 0 yards wide would require a0out 1 h to "gage" by classical techniques. It is envisioned that this method will take only a few minutes or a small percentage of the usual time needed. The method will iequire a "track" or cableway whereby the moving apparatus can be suspended. A mechanism to move the meter across the stream at constant velocity is also required. Finally, a portable instrument console which can be moved from place to place and will record velocity, or angle, position and time. One of the new digital devices should be excellent. Usually a cableway or bridge is available anyway. This setup will require a cableway of less strength than those required to support personnel and apparatus, when the measurements can be made remotely, as this technique suggests. The savings in time will, in great likelihood, justify the additional apparatus that must be utilized. However, the same apparatus can be used at many stations. The procedure will have the great advantages of time reduction of at least
AN INNOVATIVE AUTOMATIC STREAM GAGING METHOD
31
80-90%, greater accuracy as a result of the shorter period of time to make the measurement and a reliably more constant water surface elevation during the measurement. CONCLUSIONS.
It is evidenced that this technique will be a powerful new tool. Development is the essential process. Probably a series of measurements will have to be made to determine the proper speed of traverse to minimize the errors incurred due to transverse meter velocity since the quantity being measured is the velocity of the water relative to the meter, or the angle should this prove feasible. The inertia of the meter cups will however have to be assessed, the inertia of the angular motions, of the apparatus, should be slight. Should the developmental measurements be made at a location where the stage-discharge. relationship has already been determined, direct comparison can be made and, accuracy assessed. Also, since a velocity input, Vm , is made, the angle 0, only, can be measured l and the streamflow thus determined. This aspect of the technique should be justification alone to develop and research the technique.
REFERENCES Corbett, D.M. et al., 1962. Stream GagingProcedure - U.S.G.S. Water Supply Paper 888. U.S. Government Printing Office, Washington, D.C., 244 pp. Buchanan, T.J. and Somers, W.P., 1969. Discharge measurements at gaging stations. In: Applications of Hydraulics (Book 3, Chapter AS). U.S.G.S., U.S. Government Printing Office, Washington, D.C., 66 pp. Smoot, G.F. and Novak, C.E., 1969. Measurement of discharge by the moving boat method. In: Applications of Hydraulics (Book 3, Chapter A1 I]. U.S.G.S., U.S. Government Printing Office, Washington, D.C., 22 pp.