Hydrology and Water Resources RG744 Institute of Space Technology December 11, 2013.

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)