A weir is placed in a stream to determine the flow rate of a stream. A weir may be a simple metal plate with a V-notch cut into it. A weir backs the water up, causing a spill over the notch. The height of this spill way is related to the flow rate of the stream.
Permanent installation of a weir requires extensive permitting and may have negative effects on a stream’s ecology.
We developed this alternative that can be clamped onto a culvert, used to record flow rates and then removed.
We use a number of "V" notch weirs to measure stream flows in the Yukon.
- 12 gauge galvanized metal - Two pieces 18 inches by 3 feet for the 3 foot culvert and 2 feet by 4 feet for the 4 foot culvert.
- flat bar for brackets (6 pieces, 1/8 by 3/4 inch flat bar, six inches long)
- carriage bolts. washers and butterfly nuts
- self tapping metal screws
- ensolite (closed-cell foam)
- contact cement
Step 1: Forming the "V" Notch to Fit the Culvert
We cut a 45 degree angle with the 45 degree cut starting about 30 cm from the edge of the sheet metal and cut the metal to fit over the opening of 3' and 4' culverts.
Weld the two sections together to form a 90 degree "V"
Step 2: Culvert Brackets
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 that the bolts face the holes in the "V" weir.
Step 3: Reducing Water Leaks
Use contact cement to fix ensolite to the back of the weir, this will reduce leakage around the weir once in place.
Step 4: Attaching the Brackets to the Culvert
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 sheet metal weir against the metal flanges to mark the hole locations then drill 7/16 holes at these locations.
Step 5: Fastening the Weir to the Culvert
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 the water passing over the v-notch to accurately determine the stream flow rate.
Our weirs were marked with 1 cm graduations to measure the height of the flow. For more information on calculating the flow rate please see http://www.yukonenvirothon.com/hydrologic-calculations.html