3 Streamflow and Measurement The character, amount, and timing of discharge from a basin tells a lot about flow paths within the basin. Stream flow is one of the most important topics in engineering hydrology because it directly relate to water supply, flood control, reservoir design, navigation, irrigation, drainage, water quality, and others.
4 Need for Stream flow Measurements Floodplain management Flood forecasting & analysis Reservoir operations Low flows – water quality concerns Design structures – culverts, bridges, storm water systems Evaluate changes in land use on watersheds and/or changes in climatic regimes
6 Need for Streamflow Measurements Important to hydrogeologist to identify how to create stream hydrographs from discharge measurements
7 Measurement of discharge Method used depends on type of study, size of river and flow, data requirements, etc. Streamflow measurement techniques can be broadly classified into 2 categories: Direct determination – area-velocity method, dilution techniques, electromagnetic method, ultrasonic method Indirect determination – hydraulic structures, slope-area method
8 Streamflow Measurements Serves as the basis for many water resources engineering designs Three approaches Measurement of water stage (water level) Measurement of flow velocity Hydraulic Structure
11 Streamflow Measurements Measurement of Water Stage Water stage: the elevation above some arbitrary datum of water surface at a station Types of Gages Measuring River Stage: Staff gage – vertical or inclined Suspended – weight gage Recording gage (automatic data logger) Crest – stage gage ( used to indicate high water mark) Pressure sensor Float
23 Current Meter Method 3 types of current meter Propeller type : for high discharge Price type using anemometer Electromagnetic type : for low river flow Rating curve for current meter is given by: V = a + b N where V = flow velocity; a = starting velocity to overcome mechanical friction; b = equipment calibration constant; N = revolutions/sec.
24 For river velocity measurement, we need: Wading/Paddle Bridges Boat Cablecar Cableway
25 Velocity-Area Method Mostly/frequently used River cross-section determined Velocity measured using Float (for straight channel) Current meter Vertical velocity measured at 0.2d and 0.8d if depth,d >0.6m. If d<0.6m, velocity measured at 0.6dm.
26 Velocity-Area Method Q = [Velocity x Area] Need to know width of channel (w), Depth of channel (d), and Velocity of flow (V) (ft/s or m/s) Area = w x d Because depth & velocity vary across a channel: (1)Important to divide the channel into manageable segments (slices); Typically use 10-20 segments (2)For each segment measure depth, width and velocity
27 Measuring Streamflow Discharge Procedure: at each segment measure depth then velocity If Depth < 0.6m, take one reading @ 60% depth If Depth > 0.6m take 2 measurements and compute the average –One @ 20% depth –One @ 80% depth –Average the two readings
28 Measuring Streamflow Discharge Two method of measurement Mean section method Mid section method
35 Dilution gauging Using tracer/chemical at upstream For uneven stream base, good method for turbulent streams Q can be determined by tracer quantity and concentration at upstream and downstream (after dilution) using mass transfer equation. need to use tracer that is a) easily soluble, b) have no or very low natural concentrations in stream, c) be conservative, d) easily detectable at low concentrations, e) ecofriendly, f) affordable
38 Dilution gauging: Constant Rate Injection C 1,q C 2 (q+Q) Q
39 Example calculation 20 g/L of tracer injected at upstream of the river at rate 0.01 L/s. Concentration of tracers at downstream is 5 ppb. Estimate the discharge of the river at downstrean. Assume the initial concentration of tracer is very low. Solution: q =0.01 L/s = 10 -5 m 3 /s C 1 = 20 g/L = 20 000 g/m 3 C 2 = 5 ppb = 5 x 10 -6 g/L = 5 x 10 -3 g/m 3 Q = C 1 /C 2 x q = (20 000/5 x 10 -3 )x 10 -5 = 40 m 3 /s = 40 000 L/s
41 Dilution gauging: Sudden Injection Where: V = volume of tracers (m3) t1=time of tracer induced at upstream(point 1) t2=time of tracer detected at point 2 C 1,V 1 C 2, Q 2 Q
42 Example Calculation: 100 liter NaCL at concentration 10 g/L induced at river upstream. Average NaCl concentration after an hour at 800 m distance, at downstream are 0.02 mg/L. Estimate the river discharge at downstream. Solution:
43 - Measure speed of small particles in the flow - Can also track and subtract bottom speed Sonic methods
44 Some gages are designed to measure just high flows
45 Hydraulic Structures Used for small watersheds – such as experimental watersheds – where need accurate, continuous flow measurements. Two types: Weirs Flumes
46 Weirs Obstruct flow and force it through a notch Stage-Q relationship established mathematically for different types of notches
47 Weirs Generally used in small streams Various types V-notch for accurate low flow Rectangular Handles higher flows Less accurate at low flows Trapezoidal -- an intermediate weir Concerns Sediment & debris are trapped Leakage
62 Estimating Discharge (Q) from channel features: Manning’s Equation v = average velocity (m/s) R = hydraulic radius = [Area/wetted perimeter] S = Energy gradient, Approximated by water surface slope n = Manning’s roughness coefficient
63 Chezy Equation Based on Chezy formula, with A = flow cross-section area; C = Chezy Coefficient; R = hydraulic radius, A/P; and S = channel slope. For a given section, = constant whereas for a wide channel (W>10D) RD. Therefore,