# Notch Weir

Open channel fluid flow rates can be computed from the pool height (head) developed behind an obstruction, such as the right angle triangular notch weir.

We constructed such a weir from plywood and installed it in the UpperWikiCreek section of our creek.

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We're making measurements manually now, with the intention of automating this once we have some experience at various flow rates.

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We've found it difficult to read the gauge in dark or rainy weather. On December 25th, a sunny holiday, we inspected the weir and found that high water was marked by mud deposits on the rule at 8 cm (diagonal). This corresponds to a peak flow rate of 90 gallons per minute. We suspect flow was higher during the WikiCreekFlood but had no measures back then.

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We also found that after three weeks of operation water had found a way around both sides of the plywood v-notch. We had only dug a few inches into the creek bank before placing the appropriately trimmed plywood board. We encountered many roots while digging and choose to cut a minimum of them hoping that the clay soil would be water tight when replace. This seems not to be the case.

 date condition flow rate dec 5 dry for days 8 gpm dec 25 previous high 90 gpm jan 17 rain for days 22 gpm

We develop the following conversion table from information available at these sites:

The formulas we used are as follows:

• flow rate (cubic ft / sec) = 2.50 * pool height (ft) raised to 2.5
• flow rate (us gal / min) = 448.9 * flow rate (cubic ft / sec)

We tabulate this calculation for diagonal measurements at one half centimeter intervals:

 diagonal pool height flow rate flow rate cm ft cubic ft / sec us gal / min 0 0.00 0.000 0.00 0.5 0.02 0.000 0.09 1.0 0.05 0.001 0.52 1.5 0.07 0.003 1.43 2.0 0.09 0.007 2.94 2.5 0.12 0.011 5.14 3.0 0.14 0.018 8.11 3.5 0.16 0.027 11.93 4.0 0.19 0.037 16.65 4.5 0.21 0.050 22.35 5.0 0.23 0.065 29.09 5.5 0.26 0.082 36.92 6.0 0.28 0.102 45.89 6.5 0.30 0.125 56.06 7.0 0.32 0.150 67.47 7.5 0.35 0.179 80.17 8.0 0.37 0.210 94.20 8.5 0.39 0.244 109.62 9.0 0.42 0.282 126.46 9.5 0.44 0.322 144.76 10.0 0.46 0.367 164.57 10.5 0.49 0.414 185.91 11.0 0.51 0.465 208.84 11.5 0.53 0.520 233.39 12.0 0.56 0.578 259.59 12.5 0.58 0.640 287.48 13.0 0.60 0.706 317.10 13.5 0.63 0.776 348.48 14.0 0.65 0.850 381.65 14.5 0.67 0.928 416.64 15.0 0.70 1.010 453.49 15.5 0.72 1.097 492.23 16.0 0.74 1.187 532.89 16.5 0.77 1.282 575.51 17.0 0.79 1.381 620.10 17.5 0.81 1.485 666.71 18.0 0.84 1.594 715.35 18.5 0.86 1.707 766.07 19.0 0.88 1.824 818.89 19.5 0.90 1.947 873.83 20.0 0.93 2.074 930.92 20.5 0.95 2.206 990.20 21.0 0.97 2.343 1,051.69 21.5 1.00 2.485 1,115.41

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Last edited January 18, 2010
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