ES classes have made a number of "V" notch weirs to measure stream flows. We have made these from 12 gauge galvanized sheet metal and cut the metal to fit over the opening of 3' and 4' culverts. We then fasten brackets on the culvert, and screw the weir onto the brackets for short periods of time. A accurate method for calculating stream flow is to use a "V" notch weir. A weir is an overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure (not unlike a dam), but allows water to flow over the top. Weirs give hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams, or in industrial discharge locations. Since the geometry of the top of the weir is known, and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. Triangular or V-notched weirs measure low discharges more accurately than horizontal weirs. We have made a number of "V" notch weirs to measure stream flows. We have made these from 12 gauge galvanized sheet metal. Two pieces of 12 gauge galvanized metal 18 inches by 3 feet for the 3 foot culvert and 2 feet by 4 feet for the 4 foot culvert. We cut a 45 degree angle with the 45 degree cut starting about 30 cm from the edge of the sheet metaland cut the metal to fit over the opening of 3' and 4' culverts. Weld the two sections together to form a 90 degree "V" Make 6 brackets 6 inches long, from 3/4 by 1/8 inch flat bar. Bend the flat bar at two inches from one end. Drill the 2 inch section with a 5/16 drill at the mid point. Then measure the 4 inch part against the culvert so that holes in this section fall on the ridges. Drill 5/16 holes in the brackets at these locations Weld a 5/16 by 3" carriage bolt on to the short part of the bracket so rhat the bolts face the "V" weir. Screw the brackets into the outside of the culvert using self drilling self tapping metal screws so the bracket is set back 1/2 inch from the lip of the culvert. Hold the 12 gauge weir against the metal flanges to mark the hole locations then drill 7/16 holes at these locations Use contact cement to fix ensolite to the back of the weir, this will reduce leakage around the weir once in place Place the weir so that the bolts from the hinges stick out the holes in the weir. use washers and butterfly nuts to tighten the weir against the culvert lip. ONCE IN PLACE, MEASURE THE HEIGHT OF WATER PASSING OVER THE "V" NOTCH. THIS WILL GIVE YOU AN ACCURATE FLOW RATE The V-notch is most commonly a 90° opening with the sides of the notch inclined 45° with the vertical. Since the V-notch weir has no crest length, much smaller flows are represented by a given head than for a rectangular weir. For example, at a head of 5 cm, the discharge through a 30 cm rectangular weir with end contraction would be about six times that through a 90° V-notch. Stream flow may also be calculated by measuring the height of the water flowing over a "V" notch weir. A "V" notch weir may be cut to fit a culvert outlet and bolted to brackets attached to the culvert. The weir can be removed when the flow calculations have been completed. V-notch weirs should be used when low flows occur frequently and need to be included in the overall range to be measured. The following website provides a tool to calculate the flow the flow rate given the height of water above the base of the notch. http://irrigation.wsu.edu/Content/Calculators/Water-Measurements/90-Degree-Triangular-Notch-Weirs.php |