Modified drop-box weir for monitoring watershed flows under extreme approach channel conditions

摘要

Flows under extreme weir approach conditions are difficult to measure accurately They occur when there are rapid flows, steep channels, skewed flows in sinuous channels, and flows laden with sediment. Building on previous studies, the drop-box weir was modified and evaluated for use in steep and skewed stream channels found in small watersheds. Modifications to the modified drop-box weir (MDBW) included preventing water from overtopping the back wall of the box forcing all water to enter the sides of the box), extending the vertical side walls at the V section of the weir to preclude water flow out of the weir-measuring section at high flows, and use of a channel baffle for energy dissipation. Three energy-dissipation configurations upstream of the box and 20 upstream approach conditions were tested. The approach-channel angles ranged from 0° to 45°, and approach-channel slopes ranged from 5 to 75. The investigations showed that an upstream baffle yielded a stable rating curve compared with two other energy-dissipation measures. Statistical analyses of the rating curve parameters of the MDBW for each of the 20 approach conditions were similar, leading to the conclusion that the rating was independent of approach channel angle and slope at all flows when an upstream baffle was used to dissipate flow energy. The rating curve was composed of five linear segments on a log-log grid with relative gauge-height (R = gauge height/depth of V notch of weir) boundaries at 0.058, 0.11, 0.27, and 0.49. Data suggest that departures in the low-flow rating (R ≤ 0.27) between weirs may be due to differences in weir fabrication, and field rating using the results and procedures of this study are recommended. Changes in the slopes of these linear rating segments were associated with observed flow conditions in the weir A maximum R of 0.63 was recommended for sizing a MDBW but R = 0.78 could be used if monitoring equipment had sufficient water-depth resolution. The rating curve developed in this study was nearly identical to those developed in another independent study, in which flows were directed at a 90° approach angle by gutters, and with part of the rating curve of the original drop-box weir. This suggests that the rating in this study is applicable to extreme approach angles beyond the 45° angle tested in this study. Guidelines for weir sizing and developing rating curves for the MDBW are given.