WRE-IUNIT-III :
WRE-IUNIT-III STREAM GAUGING Prof S S JAHAGIRDAR
BE (CIVIL), ME(ENV.),PhD(persuing)
NK Orchid college of Engg. and Tech. , Solapur CONTENTS :
CONTENTS Selection of site, Various methods of discharge measurements, Area velocity method, Area slope method, SWF method and other modern methods BASIC DEFINITIONS :
BASIC DEFINITIONS Discharge :- is the volume per unit time that passes any point in a stream. Direct measurement of discharge is not possible, but must be calculated from velocity and cross-sectional area of the stream, i.e. from the Discharge Equation
Velocity : -is the rate of water movement, but doesn't specify how much (volume of) water is moving. The volume rate is needed to determine flooding, etc. Slide 4:
Cross-sectional Area: - is the area on a vertical plane cutting the stream
Stage: - is the elevation of the river above its bed, i.e. water depth Stream gauging :
Stream gauging It involves
Obtaining a continuous record of stage(Depth),
making periodic discharge measurements,
Establishing and maintaining a relation between the stage and discharge, and
Applying the stage-discharge relation to the stage record to obtain a continuous record of discharge. SIGNIFICANCE OF Stream Gauging :
SIGNIFICANCE OF Stream Gauging Stream gauging provides water quantity information for:
The design of water supplies, dams and other engineering works;
Monitoring the sustainability of water allocation and water management;
Implementing water restrictions;
Flood mitigation and drainage;
Monitoring climate change and drought; and
Assessment of changes in water yields resulting from altered land use. Ways to measure Gauge :
Ways to measure Gauge Staff Gauge
The simplest stream gage. A "giant ruler" mounted on a pier, bridge support, post or column. Stage height is read manually. Wire Weight Gauge :
Wire Weight Gauge Permanently mounted on the side of a bridge a wire weight gauge has a weight, a reel of wire, and a manual crank. The weight is lowered until it touches the water. A calibrated spool accurately determines how much wire was required for the weight to reach the water. This number is used to calculate the stage of the stream. Gauge House :
Gauge House A permanent house that holds stream gauging equipment - typically a gauge of some type, a computer, and a satellite uplink. Frequently a stilling well or a vertical pipe is beneath the gauge house. Measuring Cross-Sectional Area :
Measuring Cross-Sectional Area The simplest approach to measuring cross-sectional area is to locate a number of points on the stream bottom by measuring down from the tagline (or yardstick) at regular intervals . Then draw these locations (and the water surface) to scale on graph paper, and count the squares to determine the area.
A second method is to approximate the area by a series of rectangles. If you measure depth at regular intervals , then the width of each rectangle is constant. Ways to measure discharge :
Ways to measure discharge 1) Wiers
- Work in smaller streams, culverts
- Put a dam with a notched spillway (weir) in the stream
- Measure the length of the weir crest(L)
- Measure the head (water height) above the weir crest (H)
- Plug these numbers into a formula to calculate discharge
Q= CLH3/2
C=constant 2) Venturiflumes :
2) Venturiflumes 3) Slope area method :
3) Slope area method Water surface slope is calculated by means of gauges placed at the ends of the reach, say 1km upstream and 1 km downstream of the gauging station(in a straight reach), then
S=∆h/L
Formulae used are
V=C√RS ----------Chezy
V=1/n (R2/3 S1/2) --------------Manning 4) Area velocity method :
4) Area velocity method Discharge is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic meter per sec. In general, river discharge is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section:
discharge = area × velocity.
numerous methods and types of equipment are used to measure velocity and cross-sectional area, including the following
1.current meter and
2. Acoustic Doppler Current Profiler. Slide 23:
Introduction 1: Measuring stage 2: Discharge measurement 3: Stage-discharge relation 4: Converting stage to streamflow Slide 24:
Stream discharge is sum of discharges in
all sub-sections Total Discharge = ((Area1 x Velocity 1) + (Area2 + Velocity2) + ….. (Arean x Velocityn)) Slide 25:
CUP TYPE CURRENT METER Slide 30:
CURRENT METER Measuring Velocity :
Measuring Velocity Velocity can be measured directly, using a flow-meter (Current meter)(essentially a speedometer for water or inferred by timing the movement of a float in the water . Velocity varies across a stream and with depth, depending primarily on the proximity of the streambed . When using a flow-meter, a single measurement at approximately 60% of the depth of the stream will give a reliable vertical average. Slide 38:
Velocity is sometimes measured at 0.2 and 0.8 times the water depth, and these two measurements are averaged.
Note: velocity varies from bank to bank
Many velocity measurements must be made RATING OF CURRENT METER :
RATING OF CURRENT METER Relation ship between the revolutions/sec (N rps) of meter and the velocity of flow past the meter (V m/s) has to be first established and need to be verified.
The process of calibration of the meter is called rating of the current meter.
V=aN+b
A and b are constants Slide 44:
Velocity measurement by floats FAQ :
FAQ Question: how often do we measure discharge?
- Answer: Not very often: it is time consuming and difficult: monthly, or several times per year
Question: How do water agencies report hourly discharge measurements?
- Answer: Use rating curves STAGE-DISCHARGE RELATION :
STAGE-DISCHARGE RELATION Converting discharge to stage- The solution: construct a rating curve:
Discharge measurements are made at the same locality (called a gauging station) for many different water levels (stages)
Both discharge and stage are noted
Construct a graph of stage vs. discharge Slide 49:
Now: for future events, just need to go out and measure the stage (water level), then use the graph to estimate the discharge
This is an empirical process
Stage is also measured continuously (electronically), and broadcast back from remote gauging stations. This give a continuous record of stream discharge
Potential problems: scour, channel geometry changes, fill Discharge measurements are used to develop rating curves :
Rating Curve Discharge Measurements Discharge measurements are used to develop rating curves Selection of stream gauging site :
Selection of stream gauging site 1. The section should be straight and uniform for a length of about 10 to 20 times the width of the stream.
2. Bed and banks of the stream should be firm and stable so as to ensure consistency of area-discharge relationship i.e. the cross section should not be subjected to change by silting or scouring, during different stages of flow. Smooth rock , clay bed is favourable. Slide 54:
3. The bed and banks should be free from vegetable growth, boulders or other obstacles.
4. V-shaped section is preferred for immersing current meter.
5. The sites above the confluence of rivers are best avoided.
6. Site should be easily accessible.