K = flume discharge constant (varies by flume size / units) With the above known, the flow rate in the flume can now be calculated.įor free flow, the flow equation is: Q = KH a n Submergence TransitionĪs Parshall Flumes increase in size, their resistance to the effects of downstream submergence increases. This ratio (H b/H a) is used to indicate the degree to which downstream conditions limit free flow out of the flume. The submergence ratio of a flume is defined as the ratio of the downstream level (H b) to the upstream level (H a). Note that the zero reference level for both H a and H b is the floor of the inlet / converging section (even through the inlet for the H b location is below this level). These locations can be found on the Parshall Flume Master Dimensions drawing. Short throated flumes like the Parshall have defined locations for the primary (H a) and secondary points of measurement (H b). So long as the flume conforms to these standards, the flume size can be checked by the throat width alone. As Parshall flumes have rigid, defined dimensions (per ASTM D 1941 and ISO 9826). The waist - or narrowest portion of the flume - is defined as the throat. However, corrections for Parshall Flume settlement have been developed. Note that this assumes that the flume is experiencing good approach conditions and that the flume has not settled or shifted out of position after installation.Ĭorrecting for poor approaching flow conditions cannot be done numerically - the flow itself must be corrected. With this information and the appropriate free or submerged flow equation, the flow rate can be quickly calculated. Whether the flume is experiencing free or submerged flow (as determined by the submergence ratio and flume size and submergence transition).Submergence ratio (secondary point of measurement head H b / primary point of measurement head H a).Head at the primary (H a) and secondary (H b) points of measurement.Flume size (Parshall Flumes are sizes by throat width).Slowly apply the gage, pulling up only as much of the cover over the adhesive backing as is necessary.To calculate the discharge flow rate of a Parshall Flume several pieces of information are required:.Starting from the bottom, peel back the paper over the adhesive backing.Paint thinner and mineral spirits work well with aluminum and galvanized steel construction.Make sure the area where the gage will be installed is clean.If you are unsure of your Parshall flume's depth, take a look at the Parshall dimensions drawing.We recommend trimming it so that is 1/2-inch shorter than the flume's sidewall is tall.If you are unsure of where this is, take a look at the Parshall dimensions drawing.Mark the location of the gage on the sidewall of the Parshall flume.Level gages are also know as head or staff gauges (the difference in spelling is due to the first head of the USGS Irrigation Survey - Fredrick Newell)! While intended for use with Parshall flumes, the level gage can be used to measure water level on most smooth vertical surfaces (including H-type, Cutthroat, and Montana flumes) Suitable for new and retrofit installation into Parshall flumes from 1-inch to 96-inches in size. The white background and blue / red graphics, the gage is designed for readability. The gage is a peal-and-stick style with a tough polycarbonate top coat and an industrial grade adhesive backing. The right of the gauge is in feet, 1/10-foot, and 1/100-foot increments. Red markings are ever foot to help you determine the level in your Parshall flume.
On the left of the gauge are inch and 1/4-inch increments. Level gages in Parshall flumes give you a quick way to see what the level of the water (and thus flow rate) is in the flume.