SEESSEES 503503 SUSTAINABLESUSTAINABLE WATERWATER RESOURCESRESOURCES

STREAMFLOWSTREAMFLOW

Instructor

Assist. Prof. Dr. Bertuğ Akıntuğ

Civil Engineering Program Middle East Technical University Northern Cyprus Campus

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Introduction

„ are the best renewable sources of water for all kinds of demands.

„ Streamflow represents the integrated results of all meteorological and hydrological factors operating in the .

„ Engineering is generally concerned with estimating rates or volumes of flow or changes in these rates or volumes caused by the independent variables or by human activities.

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Introduction

„ Streamflow is the only element of the hydrologic cycle for which reasonably accurate measurements can be made.

„ Streamflow data are usually derived from continuous or frequent records of water level.

„ The data collected at most streamflow measurement stations are stage.

„ Stage values are then converted to values using the rating (stage-discharge) curve of that particular cross-section.

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Overview

„ Water Stage Measurement

„ Manual (non-recording) Gauges

„ Recording Gauges

„ Crest-Stage Gauges „ Selection of Station Site „ Discharge Computation

„ Discharge by Velocity Measurement „ Stage-Discharge Relationship

„ Effect of and Sedimentation

„ Extension of Rating Curves „ Interpretation of Streamflow Data

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Water Stage Measurement

„ River Stage: The elevation of the water surface above some arbitrary zero datum at a certain cross-section.

„ Datum: Slightly below the zero flow elevation in that particular cross-section of the

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Water Stage Measurement

Manual (non-recording) Gages

Manual flow gauges

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Water Stage Measurement

Manual (non-recording) Gages

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Water Stage Measurement

Manual (non-recording) Gages

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Water Stage Measurement

Manual (non-recording) Gages „ Two major disadvantages in the use of manual gages are:

„ The accuracy of the observations is dependent almost entirely upon the gauge observer.

„ The water level may change radically between gauge observations, especially for “flashy” streams, where the flow may change drastically in a matter of hours.

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Water Stage Measurement

Recording Gages

Recording stream gauge Stream Gaging Station, Carnation, Washington

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Water Stage Measurement

Recording Gages „ Although manual gages are simple and inexpensive, it is not possible to obtain the adequately when the flow is changing rapidly in a cross-section. „ Recording gages record the motion of a float on a paper wrapped around a clock driven drum. „ Such recording instruments, called float-type water-stage recorders are generally installed in a shelter house with a stilling well. „ The stilling basin well protects the float and the other parts from floating debris suppress fluctuations from surface waves in the stream.

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Water Stage Measurement

Crest-Stage Gages „ During high , staff gauges can not measure the stage, since they will be totally under water. „ For such times crest-stage gages provide information about the water levels. „ They are installed at electricity posts, trees or other suitable structures like bridges, at higher locations.

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Water Stage Measurement

Crest-Stage Gages

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Water Stage Measurement

Crest-Stage Gages

Stream gauge United States Geological Survey (USGS)

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Water Stage Measurement

Crest-Stage Gages

CSG at North Carolina Creek near Marshfield, Missouri. Photo courtesy of Paul Rydlund.

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Overview

„ Water Stage Measurement

„ Manual (non-recording) Gauges

„ Recording Gauges

„ Crest-Stage Gauges „ Selection of Station Site „ Discharge Computation

„ Discharge by Velocity Measurement „ Stage-Discharge Relationship

„ Effect of Erosion and Sedimentation

„ Extension of Rating Curves „ Interpretation of Streamflow Data

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Selection of Station Site

„ The most important point to be considered for the selection of station site is the accessibility of the location in all the seasons.

„ Unless it is a remotely sensed gaging station, the station is visited twice a day.

„ Another important point is the stability of the cross-section.

„ There should not be sedimentation or erosion occurring in the station site.

„ This may be obtained by choosing location of the station site away from the falls, or bends.

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Overview

„ Water Stage Measurement

„ Manual (non-recording) Gauges

„ Recording Gauges

„ Crest-Stage Gauges „ Selection of Station Site „ Discharge Computation

„ Discharge by Velocity Measurement „ Stage-Discharge Relationship

„ Effect of Erosion and Sedimentation

„ Extension of Rating Curves „ Interpretation of Streamflow Data

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Discharge Computation

„ The recorded stage values must be converted to discharge values by calibration.

Discharge by Velocity Measurements

„ Discharge at a certain cross-section is generally determined by making velocity measurements in the cross-section.

„ Instruments to measure the velocity:

„ Meter (most commonly used in Turkey)

„ Cup-type current meter

„ Propeller-type current meter

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Discharge Computation

Discharge by Velocity Measurements

„ Instruments to measure the velocity:

„ Cup-type current meter has a vertical revolution axis and a horizontal wheel with six conical cups. It can rotate in even very small velocities.

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Discharge Computation

Discharge by Velocity Measurements

„ Instruments to measure the velocity:

„ Propeller-type current meter has a propeller with a horizontal axis, and is preferred for turbulent and high velocity flows.

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Discharge Computation

Discharge by Velocity Measurements

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Discharge Computation

Discharge by Velocity Measurements

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Discharge Computation

Discharge by Velocity Measurements

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Discharge Computation

Discharge by Velocity Measurements

„ Velocity varies along the cross-section and vertical section.

„ Average velocity ≈ u0.6d (velocity at a depth of 0.6 x depth)

„ Average velocity ≈ (u0.2d + u0.8d)/2 „ Assumption: Flow is steady during the measurement.

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Discharge Computation

Discharge by Velocity Measurements

„ Velocity measurements are made at a number of axis.

„ Discharge = Σ {(u0.2d + u0.8d)/2 x StripAreai} „ Depending on the width of the cross-section, velocity measurements are made at a sufficient (20-30) of axes.

Qi<10% of QTOTAL

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Overview

„ Water Stage Measurement

„ Manual (non-recording) Gauges

„ Recording Gauges

„ Crest-Stage Gauges „ Selection of Station Site „ Discharge Computation

„ Discharge by Velocity Measurement „ Stage-Discharge Relationship

„ Effect of Erosion and Sedimentation

„ Extension of Rating Curves „ Interpretation of Streamflow Data

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Stage Discharge Relationship

„ Discharge computation takes considerable time and effort,

„ It is customary to make simultaneous stages and discharge measurements for various stages and prepare a calibration curve called rating curve or stage-discharge curve.

„ This curve is then used for converting the daily measurements of stage into discharge values.

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Stage Discharge Relationship

Effects of Erosion and Sedimentation

„ If in a cross-section there is a sedimentation and erosion problem different discharges flow with the same stage.

Therefore, the rating curve of a section must be checked from time to time and corrected if there is a change.

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Stage Discharge Relationship

Extension of Rating Curve „ To find the discharge for flood, the rating curve should be extented. „ One method of extension assumes that the shape of the rating curve is a parabola. q=k(s-a)b where q: discharge (m3/s) s: stage (m) k, a, and b: constants a: the distance between zero elevation of the gauge and the elevation of zero flow. Then q – (s-a) are plotted on a log-log paper Æ Straight line b: slope of the line k: the ordinate intersection of this line. k=log q where (s-a)=1.

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Overview

„ Water Stage Measurement

„ Manual (non-recording) Gauges

„ Recording Gauges

„ Crest-Stage Gauges „ Selection of Station Site „ Discharge Computation

„ Discharge by Velocity Measurement „ Stage-Discharge Relationship

„ Effect of Erosion and Sedimentation

„ Extension of Rating Curves „ Interpretation of Streamflow Data

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Interpretation of Streamflow Data

„ Units 3 „ The basic unit of flow in Turkey is m /s or lt/s.

„ The amount of flow coming from a unit area of the basin (yield of a basin) expressed in m3/s/km2 or lt/s/km2.

„ This unit is used for the comparison of the productivities of the basin.

„ To compare the flow with precipitation, the depth of flow over the drainage area is found:

total volume of water depth of flow = basin area

st „ For annual streamflow data: the water year (October 1 – September 30th) may be used.

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Interpretation of Streamflow Data

„ Hydrograph and Mean Daily Flows

„ Hydrograph: graph of stage or discharge versus time

Recorded chart should be used for detailed studies

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Interpretation of Streamflow Data

„ Hydrograph and Mean Daily Flows Streamflow variations are studied as: 1. Geographical variations of mean annual flow in depth terms on maps. 2. Variations of total runoff from year to year 3. Variations of daily rates of runoff throughout the year. 4. Seasonal variations in runoff

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