Ch 5 - Streamflow Measurement (CEE335) PDF

HYDROLOY(CEE335) Chapter 5 Stream flow measurement 1 STREAMFLOW Streamflow is generated by snowmelt, rainfall and groundwater entering a channel During dry period streamflow may be sustained by groundwater discharges – however where groundwater is below...

HYDROLOY(CEE335) Chapter 5 Stream flow measurement 1 STREAMFLOW Streamflow is generated by snowmelt, rainfall and groundwater entering a channel During dry period streamflow may be sustained by groundwater discharges – however where groundwater is below the channel level streamflow ceases until next storm or melt event 2 How streamflow is generated 3 Streamflow continued Knowledge of the quantity and quality of streamflow is a requisite for: water supply flood control reservoir design hydroelectric generation navigation wildlife 4  Streamflow continued Published streamflows are based on field measurements using: flow-measuring devices such as weirs and flumes measurement of channel cross-sections along with flow velocities measurement of water levels Daily streamflow reported as a discharge (flow rate) volume of water that passes a particular reference point per unit of time commonly expressed in units of cubic metres per second (m3 s-1 or cms) 5 Stream System Network 6 Stream flow Streamflow depends on: area Slope Shape Flow length streams (location, density, nature) 7 Catchment Area The area of land draining into a stream or a water course at a given location is known as catchment area. Drainage area, drainage basin, watershed It is normal to assume the groundwater divide to coincide with the surface divide. 8  Watershed and watershed divide Watershed/ catchment Watershed/ catchment Wa ter sh ed d ivid e 9  Also need to consider Effect of Shape the storm duration and time of concentration. 10  Linear measures Catchment length -length along principle watercourse length to centroid (often G est. as point to 2 or more bisecting straight lines Order – 0 = overland flow – 1 = gets flow from 0 Lc L orders – 2 = gets flow when 2 1st order streams combine – etc. 11 Slope/catchment relief Relief - an elevation difference max relief = max elevation diff between highest & lowest points Relief ratio=max relief/longest straight length 12 Channel Shape and Roughness A. Narrow and Deep Less resistance Faster flow B. Wide and Shallow More resistance Slower flow C. Rough Streambed More resistance Slower flow 13  STREAM PATTERNS The combined effect of climate, soil structure and geology of the catchment area is noticed in the network of channel or stream pattern. The usual patterns observed are as follows 1) Dendrite type or tree like 2) Radial pattern 3) Trellis pattern 4) Pinnate type 5) Anastomosing pattern 6) Parallel 7) Rectangular These stream patterns have a significant effect on the draining period and the flow intensity of the surface14  Drainage 1. Patterns Dendritic Tree-like pattern. Efficient branch length minimized 2. capturing runoff from smaller streams and joining into larger rivers at right angles 15 3. Radial Drainage Streams flow from central peak or dome 4. Annular Drainage Occurs in dome structures with concentric patterns of rock strata 5. Parallel drainage Steep slopes - similar to dendritic, but steep slopes cause branches to appear almost parallel to one another 6. Deranged Drainage In areas with disrupted surface patterns 16 Stream Order The classification reflecting the degree of branching or bifurcation of stream channels within a basin is known as stream order. Consider a map of the catchment area showing the channel network as shown Ordering of streams 17 Stream Ordering 18 Flow Regime Classifications Based on degree of continuity of flow - 3 types of streams: perennial never ceases to flow intermittent flows only part of year ephemeral flows only after rain event reflects strong influence of climate factors 19 ANNUAL HYDROGRAPHS FOR 3 RIVERS IN CATCHMENTS IN 3 DIFFERENT CLIMATE ZONES. Note differences among streamflow in (a) Perennial stream (b) ephemeral Stream (c) Snowmelt stream 20 STREAM GAUGING STATIONS Streams Gauging stations are used to record flow depth (stage) as a function of time. The result is a stage hydrograph. 21 THE HYDROGRAPH 22 DISCHARGE-STAGE (RATING) CURVE A stage hydrograph can be converted to a discharge curve using the rating curve. 23 STREAM DISCHARGE HYDROGRAPH (BLUE LINE) AND HYETOGRAPH (BLACK BARS) 24 SO HOW DO WE MEASURE DISCHARGE? Direct method: – By measuring velocity – And cross sectional area – Product is discharge Indirect method – From high water marks or channel characteristics at a cross section – By construction of structures in the stream that have a fixed relationship between water stage and discharge rate (i.e. Rating curve) Weirs, flumes, culverts and bridges Once the stream cross section is rated, only stage needs to be measured. 25 SO HOW DO WE MEASURE DISCHARGE?...CONTD. Discharge is very easy to calculate: cross-sectional area of the channel multiplied by the velocity of the water 26  MEASURING THE CROSS- SECTIONAL AREA The cross-sectional area of a stream is the area of the stream perpendicular to flow. It should be estimated by first constructing an accurate cross ection of the stream, based on width and depth measurements. 27 MEASURING THE WIDTH The distance between stream’s banks in a perpendicular direction to flow. Using a tape measure. 28  MEASURING THE DEPTH The distance between stream’s surface and stream’s bottom. Because rivers depth may vary significantly from bank to bank, it is important to take many measurements to get an accurate cross- 29 sectional view of the stream.  MEASURING THE DEPTH Partition stream into equal length segments Measure the depth 30 CALCULATING THE CROSS- SECTIONAL AREA Consider the area as being composed of many rectangles/trapezoid. Each rectangle is defined by a depth measurement and the width of your sampling interval along the transect across the stream. 31 HOW DO WE MEASURE VELOCITY? Most Simplistic Float Method Current Meter Average at.6 of the total depth 32  Velocity 0.2 depth 0.6 depth 0.8 depth 33 FLOAT METHOD Two people are needed to run the float test. One should be positioned upstream and the other downstream. Distance between them should be measured. The upstream person releases the float and starts the clock and the downstream person catches the float and signals to stop the clock. Velocity is the distance traveled divided by the time it takes to travel that distance. If you have recorded the surface velocity, a good estimate of the average can be found by multiplying your surface value by 0.8. 34 FLOAT METHOD surface velocity = distance / time average velocity = (0.8*surface velocity)35  Using a Flowmeter Discharge = Velocity x Cross Sectional Area Average velocity at.6 * total dept Faces upstream 36 When using a flowmeter, a single measurement at approximately 60% of the depth of the stream will 37 give a reliable vertical average.  Pygmy Meter Rotations make clicking sound in headphones If current strong may need weight 38 Flowmeter in action  HOW ELSE MIGHT WE ESTIMATE STREAMFLOW? The Stage of a Stream (Indirect Method) The stage of a stream is the elevation of the water surface above a datum. The most commonly used datum is mean sea level. Gages are used to measure the stage of streams. Types of gages: - recording - non-recording 40 HOW CAN WE RELATE STAGE TO DISCHARGE? Rating Curve – relates stage to discharge Empirical relationship from observations Measure discharge at different flows 41  Rating curve We can do this in Excel Fit a mathematical equation 42 HOW DO WE MEASURE THE STAGE? Nonrecording gauges Staff Gauge Estimating Peak Flow Debris Line Crest Gauges - Cork 43 Continuous Measurement - Water Level Recorders 44  The Stage of a Stream Float moves up / down with water surface 45 DISCHARGE MEASUREMENT 46 FIXED GAUGING STATIONS - WEIRS Stable cross section with simple geometry rating curve – just measure stage 47  OPEN CHANNELS Different from pipe flow because water surface is at atmospheric pressure Velocity methods (Q = Vm Af) – Current meter (measure velocity at a number of points in the cross-section using a calibrated meter) – Float method (Vm = Kf Vs where Vs is surface velocity measured with a float, and Kf is a velocity correction factor ranging from 0.65 to 0.8) 48 Estimating Surface Velocity, Vs, of a Straight Stream with a Float and Stopwatch (seconds) (feet) Vm = Kf Vs Kf : 0.65 – 0.80 Distance, (feet) = Velocity, (feet/second) Kf = 0.65 (if d  1 ft) Time, (seconds) Kf = 0.80 (if d >4920 ft)  Estimating the Cross-Sectional Area of Flow, Af Dividing the Streambed into Triangles, Rectangles and Trapezoids Rectangle Area Ar = d w Trapezoid Area, Atz = (di + di+1)/2* w Triangle Area, Atr = ½ d w Water w = spacing between verticals Surface w 50 Measuring and Recording Streamflow For large stream systems discharge is normally estimated by measuring velocity and using cross-sectional area to translate measurement into discharge 51 RECTANGULAR WEIR AND V-NOTCH 52 WEIR SHAPES AND FORMULAS 53 54 Relating Point Velocity to Cross-Sectional Velocity - 1 mean velocity is closely approximated by: V  ( v0.2d  v0.8d ) 2 - for depths on the order of 0.5 ft use: v v0.5d - the problem is now how to determine Q for the river a given cross-section for a number of depth and velocities 55 - average velocity for each section 1 v i  ( v i 1  v i 1 ) 2 2 2 Area for section i is given by: Ai li (d i  1  d i  1 ) 2 2 - discharge for section i Qi  Ai vi n Qtotal   Qi i 1 56 Relating point velocity to cross-sectional flow velocity While point velocity measurements are important, what is desired is a method to translate them into the average cross- sectional flow velocity Average flow velocity when multiplied by the cross-sectional area yields the discharge of the stream A popular approach is the mean-section method 57 Relating river stage to discharge typically a water elevation (stage) is surveyed during the measurement of the stream flow with enough measurements one can construct a stage-discharge relationship for the site based on this relationship one need only to monitor river stage to obtain estimates of instantaneous discharge 58 WATER FLOW RATE MEASUREMENT IN A CHANNEL PARSHALL FLUME 59  W E IR E X P E R IM E N T  = 1 '-0 " 6 0.0 ° = 9 0.0 ° 1 0 3 /4 " 6" 6" 2 '-0 " 2 '-0 " 2 '-0 " o o R ectangular 90 Trian gular W e ir 60 Trian gular W e ir C ontracted W eir 60 RECTANGULAR WEIR AND V-NOTCH 61 SLOPE AREA METHOD TO DETERMINE DISCHARGE Some cases difficult to make velocity measurements e.g., floods Possible to take measurement of high-water lines, cross-sectional area, and channel slopes and then using Manning’s equation to obtain estimate of discharge  1. 49  2 1 Q    A R  S 2 3 where  n  Q = discharge (ft3 s-1) n = Manning’s roughness coefficient (empirical value) A = cross sectional area (ft2) R = hydraulic radius (ft)= area/ wetted perimeter S = the head loss per unit length of channel (slope) 62 OPEN CHANNEL FLOW Q=AV C 23 V R S n Cross-Section Area Wetted Perimeter 63 MANNING EQUATION C 23 12 V R S n V = velocity of flow in feet per second (meters per second) C = Constant = 1.49 for English units (1.00 for metric units) R = Hydraulic Radius in feet (meters) S = channel slope in ft/ft or m/m n = Manning roughness coefficient 64 MANNING ROUGHNESS COEFFICIENTS Smooth concrete n= 0.012 Corrugated pipe n= 0.025 Smooth soil n= 0.03 Cultivated soil n= 0.04 Sluggish weedy stream n 65 Example 66 Example 67 THANK YOU 68