Lecture 4: Streamflow and Stream Gauging

Transcription

1 Lecture 4: Streamflow and Stream Gauging Key Questions 1. What is stream discharge and what controls its magnitude? 2. What is a hydrograph? 3. Where is the velocity highest in a stream? 4. Where is the average velocity measured? 5. What is baseflow? 6. What is the USGS Midsection Method? 7. How is a Real-Time Hydrograph produced? 8. What relates stage height to discharge? Stilling well on the North Fork of the Nooksack river

2 Streamflow is the heartbeat of a watershed Austin Creek Watershed (8.3 square miles) 300 Austin Creek Hydrograph 250 Discharge (cfs) Oct 1 Apr 1 Sep 30

3 Click on this web site and read about streamflow measurement

4 watershed Q = stream discharge Q = volume of water passing a plane in a unit of time Q

5 cross sectional area average velocity Q = average velocity x cross sectional area

6 Q is usually measured in cubic feet per second (cfs) velocity is in feet/second (ft/s) area is in square feet (ft 2 ) Q = average velocity x cross sectional area = ft 3 /s

7 A hydrograph is a plot of stream discharge as a function of time stream discharge (cfs) time

8 Base Flow is sustained streamflow in between rain events Hydrograph Q Q (cfs) base flow Q = stream discharge Time

9 In lowland streams, baseflow is primarily sustained by groundwater Austin Creek

10 Deming Glacier and the Middle fork of the Nooksack River In mountain streams, baseflow is primarily sustained by snowmelt, glacier melt, and groundwater Photo by John Scurlock

11 rain Hydrograph Q Q (cfs) rising flow Q = stream discharge Time

12 rain stops Hydrograph Q receding flow Q (cfs) Q = stream discharge Time

13 Base Flow is sustained streamflow in between rain events Hydrograph Q Q (cfs) base flow Q = stream discharge Time

14 How is stream discharge measured? Q

15 How is stream discharge measured? Measure cross sectional area determine average velocity Q = average velocity x cross sectional area

16 Stream velocity varies through the stream profile higher stream velocity (near the top) highest stream velocity (in the middle) velocity contour lowest stream velocity energy lost due to friction along the stream channel

17 Click on the web site below and finish the tutorial and TURN IN the certificate on WED January 19.

18 Measure stream discharge by the USGS Midsection method:

19 Measure stream discharge by the USGS Midsection method: break up the stream into rectangles

20 Measure stream discharge by the USGS Midsection method: break up the stream into rectangles measure the average velocity (vi) and area (Ai) in each rectangle

21 Measure stream discharge by the USGS Midsection method: break up the stream into rectangles measure the average velocity (vi) and area (Ai) in each rectangle estimate the discharge in each rectangle (qi = Ai x vi)

22 Measure stream discharge by the USGS Midsection method: break up the stream into rectangles measure the average velocity (vi) and area (Ai) in each rectangle estimate the discharge in each rectangle (qi = Ai x vi) sum up the discharges in all the rectangles (Q = qi)

23 String a measuring tape across the width of the stream

24 Measure the depth with a top-setting wading rod marked in tenths of a foot

25 Measure the stream velocity in a rectangle with a current meter Pygmy meter

26 Measure the stream velocity with a electronic-flow meter Marsh-McBirney Flowmate

27 If the water depth is greater than 2.5 feet, then measure the velocity at two locations: 0.2 and 0.8 of the depth below the water surface. The two velocity values are averaged. 4.3 ft

28 If the water depth is less than 2.5 feet, then measure the velocity at one location: 0.6 of the depth below the water surface. 1.7 ft

29 Stream Name: Date Time Weather: Researchers Names: Staff Gauge Water Level (to closest 0.01 ft.) Distance from Bank (10ths of a ft) Estimate to the closest 0.01 ft. Stream Depth (10ths ft) Estimate to the closest 0.01 ft. V (ft 3 /s, or cfs) At 0.6 of the depth 20 or 40 second average Comments Right bank, left bank, stone in stream affecting flow, etc.

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31 Calculating the discharge in a rectangle b i d i b i = distance from bank d i = stream depth q i = v i b i+1 b i -1 2 d i A i

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33 Software that applies the Midsection Method

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35 Real Time Hydrographs are used to monitoring stream flow for flood forecasting North Fork of the Nooksack River

36 How are real-time hydrographs generated? North Fork of the Nooksack River

37 Real-time hydrographs are generated using a rating curve that relates the stream stage (or height) to a discharge. Discharge (cfs) Stage Height (feet)

38 Streamflow stage height measurement Austin Creek Stream Gauge

39 Stilling Well: float and chart recorder

40 Streamflow Stage Height Measurement Austin Creek Stream Gauge Time Stage (feet) 10: : : feet pressure transducer

41 Austin Creek Stage Height: 2010 Water Year Stage Height (feet) Oct 1 Apr 1 Sep 30 Note: A water year in hydrology goes from Oct 1 to Sep 30

42 Time Stage (feet) 10: Measure the discharge at a specific stage height

43 Austin Creek Rating Curve Discharge (cfs) Stage Height (feet) Each red dot. on the plot represents a measured discharge at a respective stage height.

44 4.5 Stage Stage Height (feet) Rating Curve Equation Q = s s Discharge Discharge (cfs) Oct 1 Apr 1 Sep 30

45 Austin Creek Hydrograph: 2010 Water Year Discharge (cfs) Oct 1 Apr1 Sep 30 Note: A water year in hydrology goes from Oct 1 to Sep 30

46 Real Time Hydrographs are used to monitoring stream flow for flood forecasting North Fork of the Nooksack River

47 22.76 feet