hydrology and water resources rg744 institute of space technology december 11, 2013
TRANSCRIPT
STREAM FLOW MEASUREMENT/MONITORING
Hydrology and Water Resources
RG744
Institute of Space Technology
December 11, 2013
STREAM FLOW MEASUREMENT Hydrometry is the science of water measurement
It is measurement of flowing water per second (flow rate discharge)
Measurement is required to develop hydrograph, mass curve, for flood warning, distribution of water among users, and determining seasonal variation in runoff
Discharge = area x velocity
Q = AV
TWO CATEGORIES OF MEASUREMENT Direct
Area velocity Method Dilution techniques Electromagnetic Method Ultrasonic Method
In-direct Hydraulic Structures Slope area method
MEASUREMENT OF RIVER STAGE Stage is defined as water surface elevation measured
above a datum
Continuous measurement of discharge is difficult whereas observation of stage is easy, inexpensive and continuous
Simplest device for this purpose is a staff gage – scale graduated in feet or meters
STREAM GAGING STATION
Float Gage Recorder
To record flow depth as a function of time
STAGE DATA
Often presented as Stage Hydrograph Depth (stage) vs. time Discharge hydrograph is not measured directly but inferred from
the stage hydrograph
RATING CURVE
Relates stage to dischargeConstructed by plotting measured discharge
against stageTypically non-linear curves
Rating Curves can be extrapolated
STREAM FLOW VELOCITY
Variation of surface velocity across a river section and at different levels
VERTICAL VELOCITY DISTRIBUTION
In a deep stream subsection, the average velocity is estimated by the average of velocities measured 20% depth (0.2D) and 80% depth (0.8D)
Average velocity for flow in a shallow subsection of a river is observed to be equivalent to the actual velocity measured at 0.6h depth from surface of water
ISOVELS
Isovels: lines joining the points having equal velocity
VELOCITY PROFILES
MEASUREMENT OF VELOCITY Current meters (mechanical device)
To measure the velocity at a point in the flow cross-section Rotates by the stream current with an angular velocity
proportional to the stream velocity
v = aNs + b
Floats Floating object on the surface of a stream Measure distance ‘S’ it travels in time ‘t’ Surface velocity ‘V’ can be calculated using the relation:
V = S/t Mean velocity can be determined by multiplying the surface
velocity with a reduction coefficient
AREA- VELOCITY METHOD Involves measuring
area of cross-section of a river at various sites called gaging sites velocity of flow through the cross-sectional area (by current
meters or floats)
CALCULATING X-SECTION AREA X-section area = depth at the ith segment * (1/2 width to
the left + ½ width to the right)
Stream Cross-section
AREA- VELOCITY METHOD Calculation of Discharge
For 1st and last segment
EXAMPLE 4.1: ENGINEERING HYDROLOGY BY K. SUBRAMANYA
SOLUTION:
DILUTION TECHNIQUE
Also known as chemical method
Depends on continuity principle applied to a tracer that’s allowed to mix completely with the flow
Co = Initial tracer concentration (background concentration)
C1 = added concentration of tracer at section 1
C2= tracer concentration at section 2 downstream
Q1= tracer injection rate
Q= Stream discharge
EXAMPLE:
ELECTROMAGNETIC METHOD Based on Faraday’s principle Large coil buried at the bottom of the channel carrying current I that produces a magnetic field
Small voltage produced due to the flow of water is measured by electrodes
Signal output E (millivolts) is found to be related to discharge Q as:
ULTRASONIC METHOD Basically area-velocity method Average velocity is measured using ultrasonic signals
Transducers or sensors are used to send and receive ultrasonic signals
ULTRASONIC METHOD Transducer A sends an ultrasonic signal received at B and B sends a signal that’s received at A after elapse time t1 and t2 respectively, then
t1 = L/(C + vp)
t2 = L/(C – vp)
Where:
L = Length of path from A to B
C = Velocity of sound in water
vp = component of the flow velocity in the sound path = vcosθ
v = average velocity at a height ‘h’ above the bed
INDIRECT METHODS OF STREAM FLOW MEASUREMENT Use the relationship between the flow discharge and depths
at specified locations
Depths are measured in the field
Two broad classifications: Hydraulic Structures (weirs and flumes) Slope area method
HYDRAULIC STRUCTURES FOR STREAM FLOW MEASUREMENT These structures produce a unique control section in the
flow
At these structure discharge Q is a function of water surface elevation h at measured at a specified upstream location
Q = f (h) (equation A)
TYPES OF HYDRAULIC STRUCTURES Weirs
90 degree V-notch weir Sharp crested rectangular weir Sharp crested trapezoidal (Cipolletti) weir
Flumes Parshall Flume Rectangular Flume Trapezoidal Flume U Flume
WEIR
Weirs are structures which are inserted in the channel to measure flow
Depth or "head" of the water is measured as water flows over a weir
For weirs equation A becomes
Q = K (h)n
H = Head over the weir
K and n = system constants depending on the geometry of the weir
90 degree V-Notch Weir
Q = 2.49 (h) 2.48
Where:
Q = flow in cubic feet per second
h = head (depth of flow) above the notch invert (lowest point) in feet
Sharp-Crested Rectangular Weir
Q = CwLh3/2
where:
Q = flow
h = head (depth of flow) above the weir
Crest
L = length of weir crest
Cw = weir coefficient
Other Shapes of Weir
FLUME Used for small stream flow measurements Device formed by constriction in the channel (narrowing in a channel or/and hump)
Head is measured in the flume upstream of the throat
When manufactured and installed according to the specification rating can be taken directly from published tables
SLOPE-AREA METHOD
Indirect determination of flood discharge Consists of estimating 3 basic factors
1. Area of average x-section in a longitudinal reach of channel of known length
2. Slope of the water surface in the same reach3. Roughness of the streambed
If the channel cross-section, slope, and roughness are known, flow can be estimated by: Manning Equation or Chezy Equation
Manning: V = R2/3 s1/2 n
Chezy: V = C R1/2 s1/2
also C = R1/6
n
Q = VAWhere: V = mean flow velocityR = Hydraulic Radius (cross-sectional area dividedby the wetted perimeter)s = slope of the channeln = Manning roughness coefficient of the channel C = Chezy roughness coefficientQ = volumetric flowA = cross-sectional area
ESTIMATING CROSS-SECTIONAL AREA AND WETTED PERIMETER
MANNING'S ROUGHNESS COEFFICIENT FOR CHANNEL
EXAMPLE 6.7:NANCY D. GORDON Calculate the discharge through a section where
the stream has overflowed onto the floodplain and
the dimensions of the water area as shown. For
both sub-sections, S=0.005. In sub-section 1 n=
0.06 and in 2 n = 0.035.
THANKS!
WEIR
FLUME (HUMP)