Measurement of Small Discharges in Open Channels by Slit Weir

A rectangular slit weir is designed for measurement of small (<0.005 m3/s) discharges. The discharge coefficient is determined experimentally using the measured discharges and the corresponding heads over the weir. The relationship between the discharge coefficient and all relevant dimensionless parameters is investigated. It is concluded that the discharge coefficient can be represented solely as a function of Reynolds number.     

Laboratory Measurements of Flow through Culverts

Laboratory apparatus to simulate flow through culverts has been used to collect discharge and water level measurements. Two different shapes of culvert barrels, namely square and circular, were tested. The measurements presented in this note are intended to provide useful information regarding the variety of flow regimes (including overtopping) through culverts, and the transitions from one flow regime to another. It is known that modeling the culvert flow regimes and capturing the transitions among these regimes numerically is a challenging task. To that effect, the laboratory measurements presented herein can provide a testing and validation data set for numerical modeling of hydraulic structures such as culverts.     

Performance of Slit Weir

Slit weirs designed to measure small discharges in open channel flows are investigated. Experimental data available in the literature are added to study the effect of the slit width. An explicit relation for the discharge coefficient is obtained by a conventional Reynolds number definition. The discharge coefficient is modified to increase the working range of slit weirs for small values of head to slit-width ratios.     

Discharge Characteristics of Weirs of Finite Crest Length

This technical note presents a critical analysis flow over weirs of finite crest length, with square-edged or rounded entrance, for free-flow conditions. Using the flow equation for the broad-crested weir with parallel flow in the critical state as the basis, we have defined the discharge coefficient Cd, with the head on the weir as the length scale. Based on an extensive analysis of the experimental observations in the literature, we have confirmed the classification of finite crest length weirs into four classes of long-crested, broad-crested, short-crested, and sharp-crested weirs. For the square-edged entrance, we have developed robust correlations for Cd when the Weber number is greater than 1. For weirs with a rounded entrance, for which the data set is not that extensive compared to the square-edged case, we have developed good correlations for Cd.     

Discharge Formulas of Crump-De Gruyter Gate-Weir for Computer Simulation

One of the problems of interest to professionals in the field of irrigation and drainage is the computer simulation of discharge or level control structures. Particularly troublesome are structures that display a marked change of behavior when the downstream water level exceeds a certain limit. The Crump-de Gruyter gate displays several such changes of behavior. Not only does it exhibit a transition from free to drowned flow when the downstream water level rises, it can also go from weir to gate flow. A series of experiments in a laboratory flume provided the basic data to test a simple mathematical model of this structure. The model assumes the structure is located between two reaches with sub-critical flow in the upstream and downstream reach.     

Discharge Characteristics of Chimney Weir under Free-Flow Conditions

The assumption of a constant coefficient of discharge in the linear head-discharge relationship of a chimney weir is reinvestigated. Based on dimensional analysis and subsequent experiments conducted with three different chimney weirs at various crest heights and channel widths, it is found that the coefficient of discharge in the linear relationship is not a constant, but is found to vary with the ratio of head to altitude, h/d; half-vertex angle in the form of w/d, w being the half crest width; h/(h+P), P being the crest height; and the channel width contraction ratio, w/C, C being the channel width. A linear regression equation correlating the coefficient of discharge with the above variables is proposed that, along with the linear head-discharge relationship, provides an accurate prediction of free-flow discharge.     

Effects of draining and desilting a small weir on downstream fish and macroinvertebrates

Draining a small off-take weir in February 1991 before desilting introduced large amounts of sediment (at least 100m³) into the downstream creek. Increased suspended solids concentrations up to 4610 mg/l were recorded downstream during the release. Large volumes of sand were deposited close to the weir, with finer fractions (silt) deposited up to 2 km further downstream. Very fine silt was washed through the system to a major river 4.5 km below the weir. Over the first day of the release, at a site 2.1 km below the weir, there was a reduction of 19.4% in the total number of benthic macroinvertebrate taxa, and average reductions of 63.9% in the abundance of and 39.7% in the number of taxa per sample. Over the following 45 days the fauna recovered towards pre-release diversity and density. This recovery was not sustained and further reductions in diversity and abundance were recorded over the following five months, probably due to fresh disturbance from desilting works and further mobilization of sediment during high flows. A major storm and flood in September 1991 apparently flushed all additional silt from the creek and the macroinvertebrate fauna subsequently recovered to higher diversity and density levels than before the release. Compared with densities recorded in 1985 and 1988, the population of river blackfish (Gadopsis marmoratus) was reduced by 93% after the release at a site 250m downstream of the weir, with reductions of 81 and 59% at distances from the weir of 2.1 and 2.7 km, respectively. An additional reduction in this fish population occurred at the lower site in late 1991, probably due to further flushing of sediments during the September flood. Although reductions in fish numbers occurred across the whole population, greater reductions were recorded for the younger age classes. Small subsequent increases in fish numbers at upper sites during 1992-1993 indicated that a recovery of the blackfish population may be occurring through recruitment, but such a recovery will only be confirmed over the long term.     

Conditional Assessment of Flow Measurement Accuracy

A study conducted by the Utah Water Research Laboratory assessed the accuracies of a wide variety of flow measurement devices currently in service. During the study, a wide variety of flow measurement devices, including flumes, weirs, and rated sections in open channel systems, were evaluated; magnetic and ultrasonic meters in closed-conduit systems were also tested. The specified design accuracies for each device are presented. Actual flow measurements were determined at 70 sites and were compared with the theoretical discharges of each device. Comparison of actual and theoretical flow indicates that only 33% of the measurement devices tested currently measure flow within manufacturer-designed specifications. Field data is presented, and possible reasons for the flow measurement errors and their corrections are discussed.     

New Approach for Predicting Flow Bifurcation at Right-Angled Open-Channel Junction

An unsteady mathematical model for predicting flow divisions at a right-angled open-channel junction is presented. Existing dividing models depend on a prior knowledge of a constant flow regime. In addition, their strong nonlinearity does not guarantee compatibility with the St. Venant solutions in the context of an internal boundary condition treatment. Assuming zero crest height at the junction region, a side weir model explicitly introduced within the one-dimensional St. Venant equations is used to cope with the two-dimensional pattern of the flow. An upwind implicit numerical solver is employed to compute the new governing equations. The performance of the proposed technique in predicting super-, trans-, and subcritical flow bifurcations is illustrated by comparing with experimental data and/or theoretical predictions. In all the tests, lateral-to-upstream discharge ratios (Rq) are successfully reproduced by the present technique with a maximum error magnitude of less than 9%.     

Hydraulic Evaluation of W-Weir for River Restoration

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects.