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

 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

 Direct  Area velocity Method  Dilution techniques  Electromagnetic Method  Ultrasonic Method  In-direct  Hydraulic Structures  Slope area method

 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

 Float Gage Recorder To record flow depth as a function of time

 Often presented as Stage Hydrograph  Depth (stage) vs. time  Discharge hydrograph is not measured directly but inferred from the stage hydrograph

Relates stage to discharge Constructed by plotting measured discharge against stage Typically non-linear curves Rating Curves can be extrapolated

 Variation of surface velocity across a river section and at different levels

 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: lines joining the points having equal 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 = aN s + 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

 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)

 X-section area = depth at the i th segment * (1/2 width to the left + ½ width to the right)  Stream Cross-section

 Calculation of Discharge For 1 st and last segment

 Also known as chemical method  Depends on continuity principle applied to a tracer that’s allowed to mix completely with the flow  C o = Initial tracer concentration (background concentration)  C 1 = added concentration of tracer at section 1  C 2 = tracer concentration at section 2 downstream  Q 1 = tracer injection rate  Q= Stream discharge

 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:

 Basically area-velocity method  Average velocity is measured using ultrasonic signals  Transducers or sensors are used to send and receive ultrasonic signals

 Transducer A sends an ultrasonic signal received at B and B sends a signal that’s received at A after elapse time t 1 and t 2 respectively, then t 1 = L/(C + v p ) t 2 = L/(C – v p ) Where: L = Length of path from A to B C = Velocity of sound in water v p = component of the flow velocity in the sound path = vcos θ v = average velocity at a height ‘h’ above the bed

 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

 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)

 Weirs  90 degree V-notch weir  Sharp crested rectangular weir  Sharp crested trapezoidal (Cipolletti) weir  Flumes  Parshall Flume  Rectangular Flume  Trapezoidal Flume  U Flume

 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 = C w Lh 3/2 where: Q = flow h = head (depth of flow) above the weir Crest L = length of weir crest C w = weir coefficient

Other Shapes of Weir

 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

 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 reach 3. 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 = R 2/3 s 1/2 n Chezy: V = C R 1/2 s 1/2 alsoC = R 1/6 n Q = VA Where: V = mean flow velocity R = Hydraulic Radius (cross-sectional area divided by the wetted perimeter) s = slope of the channel n = Manning roughness coefficient of the channel C = Chezy roughness coefficient Q = volumetric flow A = cross-sectional area

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= In sub-section 1 n= 0.06 and in 2 n =