2014-01-29_HYDROLOGY - P2009025 (8)

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Appendix D — SOP for Stream Flow Measurement — Hansen Project 1 1. INTRODUCTION The purpose of this document is to define the Standard Operating Procedures (SOPS) for measuring surface water discharge in streams and ditches. Discharge is defined as the volumetric rate of flow of water, including any substances suspended or dissolved in the water. Discharge (flow) will be expressed in cubic feet per second (cfs). This document outlines the methods for measuring flow which are (1) by the use of permanently installed flumes or weirs, (2) the velocity-area method using a Flow ProbeTM or similar electronic current meter, (3) the velocity-area method using a Price AA or Pygmy meter, (4) bucket and stopwatch, or (5) indirect methods at selected sites. Because of the dynamic nature of surface water behavior, flow measurement by the methods described in this document may, on occasion, be impossible at some sites. It is understood that if immeasurable flow conditions are encountered at any of the surface water data collection sites which are to be measured, the field team will attempt to measure flow at a point upstream or downstream of the site and will note this point relative to the marked data collection point. Whether or not a measurement is made, the team will note the conditions that inhibited accurate flow measurement. This situation will be brought to the attention of the Project Manager. Selection of the streamflow measurement methods in this SOP is based on the following conditions: • In the absence of a control structure, such as a flume or weir, a current meter will be used to measure discharge by the velocity-area method • Portable or permanent flumes or weirs may be used at some sites /stations • Staff gages may be installed at selected locations • A few locations may be selected as indirect discharge measurement locations The method of discharge measurement to be used at each site will be described in the field folder for the site. Each of these methods will be presented in the following format: • Method name • Theoretical considerations • Required measurement conditions • Equipment • Maintenance and calibration procedures • Field procedures • Discharge calculations 2. EQUIPMENT 2.1 Material Required for Flume Measurements For purposes of discharge measurements, existing flumes or weirs, if any, will have been inspected and dimensioned prior to use in the surface water program. Parshall Flume. The Parshall flume consists of a converging section with a level floor, a throat section with a downward sloping floor, and a diverging section with an upward sloping floor. The principal feature of the Parshall flume (developed by R. Parshall in 1922) is an approach reach having converging sidewalls and a level floor, the downstream end of which is a critical -depth cross section. Critical flow is established in the vicinity of that cross section by having a sharp downward break in the bed slope of the flume. The bed slope downstream from the level approach section is therefore supercritical. The primary stage measurement is made in the approach reach at some standard distance upstream from the critical -depth cross section. The flumes are designated by the width (w) of the throat. Flumes having throat widths from three inches (in) to eight feet (ft) have a rounded entrance whose floor slope is 25 percent. Smaller and larger flumes do not have that feature, but it is doubtful whether the performance of any of the flumes is significantly affected by the presence or absence of the entrance feature as long as approach conditions are satisfactory. 4153A.140129 Whetstone Associates