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Streamflow Measurement and Analysis

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Streamflow Measurement and Analysis BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 CHAPTER 6 Streamflow Measurement and Analysis INTRODUCTION Streamflow is the primary mechanism by which water moves from upland watersheds to ocean basins. Streamflow is the principal source of water for municipal water supplies, ir- rigated agricultural production, industrial operations, and recreation. Sediments, nutrients, heavy metals, and other pollutants are also transported downstream in streamflow. Flooding occurs when streamflow discharge exceeds the capacity of the channel. Therefore, stream- flow measurements or methods for predicting streamflow characteristics are needed for various purposes. This chapter discusses the measurement of streamflow, methods and mod- els used for estimating streamflow characteristics, and methods of streamflow-frequency analysis. MEASUREMENT OF STREAMFLOW Streamflow discharge, the quantity of flow per unit of time, is perhaps the most important hydrologic information needed by a hydrologist and watershed manager. Peak-flow data are needed in planning for flood control and designing engineering structures including bridges and road culverts. Streamflow data during low-flow periods are required to estimate the dependability of water supplies and for assessing water quality conditions for aquatic organisms (see Chapter 11). Total streamflow and its variation must be known for design purposes such as downstream reservoir storage. The stage or height of water in a stream channel can be measured with a staff gauge or water-level recorder at some location on a stream reach. The problem then becomes converting a measurement of the stage of a stream to discharge of the stream. This problem Hydrology and the Management of Watersheds, Fourth Edition. Kenneth N. Brooks, Peter F. Ffolliott and Joseph A. Magner. C 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc. 141 BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 142 Part 1 Watersheds, Hydrologic Processes, and Pathways is solved by either stream gauging or installing precalibrated structures such as flumes or weirs in the stream channel. Measuring Discharge A simple way of estimating discharge is to observe the time it takes for a floating object that is tossed into the stream to travel a specified distance. A measurement of the cross section of the stream should be made simultaneously and the two values are then multiplied together: Q = VA (6.1) where Q is the discharge (m3/s); V is the velocity (m/s); and A is the cross section (m2). This method of estimating discharge is not accurate, however, because velocity of a stream varies from point to point with depth and width over the cross section of the stream. The velocity at the surface is greater than the mean velocity of the stream. Actual velocity is generally assumed to be about 80–85% of surface velocity. The velocity profile can be estimated by dividing the cross section of a stream into vertical sections and measuring the mean velocity of the stream at each section (Fig. 6.1). VS V Flow Flow Vertical velocity Horizontal velocity distribution distribution Flow Perspective view FIGURE 6.1. Measurements of stream channel cross sections and velocities needed to estimate the mean velocity of a stream BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 Chapter 6 Streamflow Measurement and Analysis 143 The area of each section can be determined and the average discharge of the entire stream computed as the sum of the product of area and velocity of each section as follows: n Q = Ai Vi (6.2) 1 where n is the number of sections. The greater the number of sections, the closer the approximation to the true value. For practical purposes, however, between 10 and 20 sections are commonly used depending on the channel width. The actual number of sections depends on the channel configuration and the rate of change in the stage with discharge. Depth and velocity should not vary greatly between points of measurement. Importantly, all of the measurements should be completed before the stage changes too much. The velocity and depth of vertical sections can be measured by wading into the stream or from cable car, boat, or bridge. Velocity of the stream is usually measured by a current meter with the following general rules: r Two measurements are made for each section at 20% and 80% of the total depth and then averaged for depths Ͼ0.5 m. r One measurement is made at 60% of the depth for depths Ͻ0.5 m. r A current meter smaller than a standard current meter, such as a pygmy current meter, is used to measure velocity of shallow streams less than about 0.5 m deep. The most critical aspect of stream gauging is the selection of a control section, that is, a section of the stream for which a rating curve (Fig. 6.2) is to be developed. This section should be stable, have a sufficient depth for obtaining velocity measurements at the lowest of streamflows, and be located in a straight reach without turbulent flow. Further information concerning measurement of streamflow and analysis of stream- flow data can be obtained from the US Geological Survey (accessed July 20, 2011; Gauge height (ft) Discharge (ft3 / s) FIGURE 6.2. Example of a rating curve of streamflow discharge in relation to depth measurements (from Brown, 1969). A logarithmic relationship is shown in the figure because of the range of measurements obtained BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 144 Part 1 Watersheds, Hydrologic Processes, and Pathways http://ga.water.usgs.gov/edu/measureflow.html) or Oregon State University (accessed July 20, 2011; http://streamflow.engr.oregonstate.edu/manipulation/index.htm). In the United States, the United States Geological Survey (USGS) measures streamflow discharge directly with an acoustic Doppler current profiler (ADCP) system (Mueller and Wagner, 2009; accessed July 20, 2011; http://pubs.water.usgs.gov/tm3a22). Acoustic energy is transmitted at a known frequency and the measured backscatter of acoustic energy is used to compute the velocity of water particles. Discharge can be determined by deploying the ADCP system by towing or remotely controlling a moving boat to determine the velocity in the water column from one streambank to the other. This system determines instantaneous streamflow discharge directly in the channel. Precalibrated Structures for Streamflow Measurement Precalibrated structures are often used on small watersheds, usually less than 800 ha in size because of their convenience and accuracy. The most common precalibrated structures are weirs and flumes. Because of their greater accuracy, weirs are often used for gauging small watersheds, particularly those with low flows. Flumes are preferred where sediment-laden streamflows are common. Weirs A weir is a small wall or dam across a stream that forces water to flow through a notch with a designed geometry. The notch can be V-shaped, rectangular, or trapezoidal (Fig. 6.3). Water is impounded upstream of the dam, forming a stilling basin that is connected to a water Water level recorder Water level in V-notch weir Float h Pipes from ponding Basin to stilling well FIGURE 6.3. A schematic illustration of a weir with a V-shaped notch on the weir blade BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 Chapter 6 Streamflow Measurement and Analysis 145 level recorder. A gaugehouse or other type of shelter is provided to protect the recorder. The cutoff wall (dam) used to divert water through the notch is seated into bedrock or other impermeable material where possible so that no water can flow under or around it. The stilling basin is sometimes constructed as a watertight box where leakage is apt to occur. The notch edge or surface over which the water flows is called the crest. Weirs can be either sharp crested or broad crested. A sharp-crested weir has a blade with a sharp upstream edge so that the passing water touches only a thin edge and clears the rest of the crest. This design provides accurate measurements at low flows. A broad-crested weir has a flat or broad surface over which the discharge flows and is used where sensitivity to low flows is not critical. The rectangular weir has vertical sides and horizontal crests. Whereas its major ad- vantage is its capacity to handle high flows, the rectangular weir does not provide precise measurement of low flows. The trapezoidal weir is similar to the rectangular weir, but it has a smaller capacity for the same crest length. The discharge is approximately the sum of discharges from the rectangular and triangular sections. Sharp-crested V-notch or triangular weirs are often used where accurate measurements of low flows are important. The V-notch weirs can have a high rectangular section to accommodate infrequent high flows. Flumes A flume is an artificial open channel built to contain flow within a designed cross section and length. There is no impoundment, but the height of water in the flume is measured with a stilling well. Several types of flumes have been used on small watersheds. Figure 6.4 shows a trapezoidal broad-crested flume that is used to measured open channel flow (Robinson and Chamberlain, 1960). HS-, H-, and HL-type flumes developed and rated by the U.S. Natural Resources Conservation Service (NRCS) have converging vertical sidewalls cut FIGURE 6.4. A broad-crested trapezoidal flume for measuring open channel streamflow BLBS113-c06 BLBS113-Brooks Trim: 244mm×172mm August 25, 2012 17:2 146 Part 1 Watersheds, Hydrologic Processes, and Pathways back on a slope at the outlet to give them a trapezoidal projection. These flumes have been used largely to measure intermittent runoff events.
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