2014-01-29_HYDROLOGY - P2009025 (8)

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Appendix D — SOP for Stream Flow Measurement — Hansen Project 11 in flows as shallow as 0.1 feet. The meter is capable of measuring velocities ranging from 0.0 to 20.0 feet per second. 3.2.2 Theoretical Considerations The volumetric flowrate of water, which is commonly called discharge (Q), is the product of multiplying the average velocity (V) times the total cross - sectional area (A): Q =VA The current meter measures velocity at a point. The velocity-area method of making discharge measurements at a cross - section requires measurement of the mean velocity in multiple subsections of the total cross - section at each of the selected vertical intervals. A complete discussion of velocity observation methods is found in USGS Water - Supply Paper 2175, Measurement and Computation of Streamflow: Volume 1 - Measurement of Stage and Discharge, Chapter 5. By dividing the stream width into subsections, total discharge becomes the sum of discharges measured in each subsection. Individual point velocity (v) is measured at each subsection, and discharge becomes the sum of the products of each point velocity and cross - sectional area (a) of each subsection: Q = Eva The cross section is defined by depths at verticals 1,2,3,4.....n. At each vertical, the average velocity is measured by a current meter. In measuring discharges for use of the data in developing stream ratings for litigation and flood -plain insurance purposes, federal agencies typically base the number of subsections on the criterion that each subsection contains no more than five percent of the total discharge. While this method has reportedly resulted in measurement accuracy as high as 98 percent, factors such as the characteristics of the measurement section reduce this accuracy. Furthermore, measurements limiting flow to five percent of the total discharge are time - consuming. Therefore, in the interest of conserving time while maintaining acceptable measurement accuracy, current -meter measurements will be based on selecting subsections to contain approximately 10 percent, or slightly more, of the total discharge. As suggested in the United States Environmental Protection Agency (EPA) report, "A Compendium of Superfund Field Operations Methods," the procedure for flow measurement by the velocity-area method recommends that each subsection not contain substantially more than about 10 percent of the total flow for the cross - section; however, EPA recognizes, in the same document, that field conditions may, on occasion, produce cause to use fewer than the ideal number of subsections: "Ideally, the stream should be partitioned into sections small enough that less than 10 percent of the total stream flow passes through each section. In this manner, individual measurements that may be in error will have less impact on the overall average velocity determination. However, practical considerations, such as a rapidly changing stage or limited time available to conduct measurement, often may preclude the use of the ideal number of partial sections (10 -5p)." In general, depending on average depth and velocity distribution, a stream less than two feet wide will require no more than eight to 10 subsections. A stream up to four feet wide will require about 10 to 12 subsections. Further, subsections need not be of identical width. For example, because velocities near banks are generally lower than velocities near the center of streams, these subsections may be wider than subsections near the center. Subsections will also be more closely spaced if a stream has an unusually deep portion in the cross sections, or if velocities are higher than usual for the cross section. Table 3 is to be used to guide the selection of the approximate number of subsections to be used based on stream width. Velocity will be observed by current meter at each point for a period that ranges from 40 to 70 seconds. 4153A.140129 Whetstone Associates