Experimental study of sedimentation effects on discharge coefficient in V-notch weirs

Accuracy in water flow measurement is always considered as one of the most important issues in water resources management. Bed and wall sediment of alluvial rivers transfer in watershed and after entry into channels, the sediment deposit behind flow measurement structure and therefore, influence their performance. Weirs are widely used to measure discharges. In excellent installations, they provide accurate measurements of discharge over a wide range of flows with a constant discharge coefficient. However, many weirs around the world are filled with sediment and require manual rating curve adjustments or dredging. Including flow measurement structures is V-notch crested weir that is more accurate because of higher sensitivity in the flow depth measurement. The relationships that have already been presented according to experimental results for discharge coefficient has been extracted and developed in without-sediment conditions.
In the present study, effect of sedimentation level and weir angle on discharge coefficient in v-notch weir was investigated, experimentally. Parameters included: flow discharge, weir angle, weir height, channel slope, and degree of sedimentation were varied. All tests have been performed by taking six weir angles (75, 90, 105, 120, 130 and 140 degree) and three weir heights which were appropriate with every angle. Four sedimentation levels consisted of no-sediment bed and the sediment levels set to one-third, two-thirds and equal to the height of the weirs. Also, four bed slops were considered as: 0, 0.8, 1.6 and 2.4%. The experiments were performed in the University of Tarbiat Modares water structures laboratory. The flume used with dimensions of 10 m in length, 0.3 m in wide, 0.4 m in height, and adjustable slope was varied from -1.5 to 2.5 percent. Framed walls were constructed with glass. The flume was equipped with a measurement traverse. Discharge rang was from 0.3 to 16.3 L s-1. In all tests weirs installed in middle of flume long (5 m after flume entrance). Sedimentation was simulated by placing coarse gravel with 2 cm diameter in an area within 1 m of the weir and at an elevation equal to the invert of the weir. In all tests, the material did not move during the experiment. For the given geometrical condition (weir angle, weir height, bed slope and degree of sedimentation), the flume was first filled with water up to the elevation exactly equal to the invert of the weir. Afterward the flow was decreased to each target value using Electro Magnetic flow meter with ±5 percent accuracy, then water level measured with point gauge that its accuracy was 0.1 mm. Approximately in each experience 12 data collected of water levels and flow discharge. Measuring of water surface elevation was repeated for statistical inference of the effect of turbulent unsteadiness on discharge measurements.
Test results indicated that increase in the weir angle, increased the discharge coefficient for all weir heights and slopes, but increase in the weir height, caused to decrease the discharge coefficient. Comparing result between the sedimentation degree and discharge coefficient indicated that increase in the sediment level, leaded to increase the discharge coefficient. Also, a direct relationship between the discharge coefficient and bed slope was observed so that the maximum discrepancy of the discharge coefficient in without and full sediment condition was related to 140 degrees weir angle and in 2.4 percent flow bed slope, which was equal to 14.8 percent. Also, the results showed that an increase in the ratio of upstream water depth to the weir height (H/P), leaded to an increase in the discharge coefficient for all experiments. These results were favorably acceptable comparing with other studies.
Ultimately, according to the experimental data, and by using SPSS software, also formation of non-linear regression, the non-dimension equation has been suggested in order to estimate discharge coefficient in v-notch weir. The equation resulted in a high correlation coefficient (R2=0.956) for discharge coefficient estimated angle, weir height, flow bed slope and sediment level. By comparing between calculated values of the discharge coefficient of suggested equation and laboratory measurement values for calibration data and validation data considering ±5 percent of the levels error observed that suggested equation has high accuracy for calculating the flow discharge. In the following, the present study suggests investigating the effect of sedimentation level on discharge coefficient in other weirs for further researches.