2014-01-29_HYDROLOGY - P2009025 (8)

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Appendix D — SOP for Stream Flow Measurement — Hansen Project 14 Vertical axis current meters do not register velocities correctly when placed close to a vertical wall. A Price meter held close to a right -bank vertical wall will under - register because the slower water velocity near the wall strikes the effective (concave) face of the cups. The converse is true at a left -bank vertical wall. (The terms "left bank" and "right bank" designate direction from the center of a stream for an observer facing downstream.) The Price meter also under - registers when positioned close to the water surface or close to the streambed. 3.2.4 Field Procedures Overview. Based on approximate depths, either the Price AA meter or the Pygmy meter will be selected to perform a velocity -area measurement. Neither the type AA meter nor the Pygmy meter should be used for measuring velocities slower than 0.1 fps unless absolutely necessary. If depths or velocities under natural conditions are too low for a dependable current meter measurement, the cross section will be modified, if practical, to provide acceptable conditions. A shovel will be used to remove aquatic vegetation, ice, or rocks which may interfere with meter operation or discharge measurements. Before velocity -area measurements are taken, use the staff gage to measure and record the gage height to the nearest 0.02 feet. At each measurement point (or station) across the stream cross - section, depth is measured prior to measurement of velocity. Therefore, it is recommended that the wading rod be set with the current meter suspended out of the water and above the tagline, which is used to measure width and to identify stations across the cross - section. Placement of the rod about 0.5 feet downstream from the tagline prevents contact between the tagline and the current meter when the meter is lowered into measuring position. The wading rod will be placed in the stream so the base plate rests on the streambed, and the depth of water will then be read from the graduated main rod. The main rod is graduated into 0.1 foot increments: these increments are indicated by a single score in the metal. Half -foot increments are marked by two scores in the metal, and each foot is marked by three scores in the metal. A vernier scale on the upper handle of the rod corresponds to 0.1 foot increments, and has one through nine in raised numbers next to raised marks. A sliding, adjustable rod, known as the setting rod, to which the meter is attached, has single scored marks which are aligned with values on the vernier scale. The hydrographer reads water depth directly from the main rod. In high velocity areas, it is recommended that depth be read as the value between the depth on the upstream side of the rod and the depth on the downstream side of the rod. Depth is measured to the nearest 0.02 foot. This depth is used to set the vertical location on the current meter. The setting rod is next adjusted downward so that the scored mark of the setting rod which corresponds to the range of depth in feet (e.g., if depth = 0.46, range in feet = 0: or if depth = 1.72, range in feet = 1) is aligned with the stream depth value transposed to the vernier scale. This automatically positions the meter for use in the six - tenths method as the meter is then 0.6 of the total depth from the surface of the water. If depths are less than 0.30 foot, the five- tenths method may be used. The observation depth recorded will then be 0.5 of the total depth. The hydrographer must stand in a position that least affects the velocity of the water passing the current meter. That position is obtained by facing upstream while holding the wading rod vertically and close to the tagline or measuring tape. The hydrographer stands at about a 45- degree angle downstream from the wading rod and at least 1.5 feet from the wading rod. This angle is an imaginary angle between the extended arm holding the wading rod and the tagline or measuring tape. The hydrographer should avoid standing in the water if his or her feet and legs occupy a significantly large percentage of a narrow cross section. For narrow streams, it is often possible to stand astride the stream. The wading rod should be held in a vertical position with the meter parallel to the direction of flow while the velocity is being observed. When measuring streams that have shifting beds, the soundings or velocities can be 4153A.140129 Whetstone Associates