| A training manual in conducting a workshop in the design, construction, operation, maintenance and repair of hydrams |
Using a weir
Session 3, Handout 3A
A weir may be defined as an overflow structure built across an open channel, usually to measure the rate of flow of water. Weirs are acceptable measuring devices because, for a weir of a specific size and shape (installed under proper conditions) only one depth of water can exist in the upstream pool for a given discharge. The discharge rates are determined by measuring the vertical distance from the crest of the overflow portion of the weir to the water surface in the pool upstream from the crest, and referring to tables which apply to the size and shape of the weir. For standard tables to apply, the weir must have a regular shape, definite dimensions, and be set in a bulkhead and pool of adequate size so the system performs in a standard manner.
Whenever the flow from a creek is too great to be measured in a bucket ant yet is small enough to be dammed by a board, the weir method of measurement should be used.
Determine the dimensions to be used for the weir notch. The width of this notch is related to the measurement of the flow rate by the height of the water in the pool formed behind the weir. This height is measured in inches and by using a weir table, the inches can be converted to gallons per minute. A number of notches of different widths and height can accommodate a stream's flow. A rule of thumb is to make the width of the notch 3 times the height.
From your estimate of the flow of the stream, look at the weir table and guesstimate what size notch will accommodate your flow. Keep in mind that the whole stream must pass over the notch end that the pool formed behind the weir should become deep enough for you to easily get a decent height measurement, i.e., 2½" vis a vis 1/16". Example: you estimate the stream is flowing at 150 gal/mint If you made a notch 12" wide and 4" high, at full flow this weir would read approximately 290 gal/mint (4"-- 23.936 gal/mint x 12" = 286.89 gal/min). This weir would fit your stream if an actual weir reading of 28½" water height were obtained, it would indicate a flow rate of 11.818 gal/min/inch of notch or 141.8 gal/min (11.818 x 12") for the stream.
Once you have determined the dimension of the notch, cut the notch in the board and place the weir board in the stream making certain that it is kept level and seal off the stream completely. Support it with stakes and large rocks.
Measure 2 feet upstream from the weir board and drive a stake. Using a level, put a mark on the stake even with the top of the weir board. Next, measure down from this mark to the water level, subtract this measurement from the depth of your notch and that will give you the height of the water level above the bottom of the weir notch.
Using the weir table attached, locate the integer on the Left hand column and the fraction on the top column. Where these two rows intersect is the amount of gallons per minute flowing past the weir for every inch of width. Next multiply this figure by the width and this gives you the total flow of the creek.
Water is flowing through a creek three feet wide and about 3 inches deep. It looks like about 30 gallons per minute. After looking at the weir table we decide that a notch 6" wide and 2" deep would probably work. After cutting the notch in a 4 foot 1x6 piece of lumber, the weir board was placed in the stream. Two feet upstream a stake is driven in the water in front of the notch. A level is used to place a mark on the stake level with the top of the weir board. The water level is then measured to be 1" down from this mark.
We now know by subtracting this measurement from the depth of the notch that the water level is 1½" above the bottom of the notch. Now looking at the weir table we find 1 on the left hand column and ½ on the top row. These two rows meet at 5.46. We multiply this by the width of the notch (6") to find that the flow rate was 32.26 gallons per minute.