Behavior of Submerged Ogee Crest Weir Discharge Coefficients

Weir head-discharge relationships are typically described using the discharge coefficient-dependent standard weir equation. The submerged weir (tailwater exceeds the weir crest elevation) head-discharge relationship can vary from the free-flow head-discharge relationship, particularly at high submergence levels. The accuracy associated with predicting the upstream head or discharge, corresponding to submerged weir flow conditions, is dependent upon the accuracy with which a representative submerged discharge coefficient can be determined. A submerged ogee crest weir discharge coefficient predictive method developed by the U.S. Bureau of Reclamation (USBR) is reviewed and its predictive accuracy compared to laboratory-scale submerged ogee crest weir experimental data associated with a wide range of submerged flow conditions for nine different ogee crest weir geometries. Discussion is presented in an effort to partially explain the relatively poor correlation between the USBR method and the experimental data set. Alternative submerged discharge coefficient relationships are also presented.     

Channel changes associated with two adjacent weirs on a regulated lowland alluvial river

Survey data from 1906 and 1988 for the lower River Murray, Australia, reveal changes in channel morphology associated with the construction of Locks 2 and 3 (1925–8), which form contiguous pools in the Valley and Gorge sections, respectively. Regulated annual flows are less than half the volume of natural flows, and the frequency of maximum flows has decreased. Upstream weirs have reduced the sediment supply to the study reach by 1.05 × 10⁶ tonnes per annum. The trap efficiencies of Pools 2 and 3 from 1906 to 1988 were 8 and 13% respectively, with retention channel of 80723 and 267470 tonnes. Responses over the past 60–70 years have differed between the pools-some channel characteristics have attained a new dynamic equilibrium and others have not. In Pool 3 the average bed slope has been reduced by 000004 (44%) and has attained equilibrium, but in Pool 2 the supply and redistribution of sediment have not been sufficient to reduce the bed slope. There are continuous areas of degradation and aggradation in Pool 3 but discontinuous areas in Pool 2, reflecting different stages in slope adjustment. Cross-sections have become wider and shallower in Pool 3 but narrower and deeper in Pool 2. These adjustments are influenced by local boundary conditions, floodplain morphology and the position of the study reach in the sequence of weirs. In general, the situation in the lower Murray does not conform to conventional models of the impact of regulation, which are based on data from upland dams. The Murray's gross morphology remains as a relict of the natural, semi-arid regime. Low contemporary stream energies and cohesive bank materials restrict the initiation of channel metamorphosis and prolong the time required for the completion of channel adjustment.     

Critical Flow over Circular Crested Weirs

The critical flow principle is a useful approach for the hydraulic analysis of round-crested weirs due to their single head-discharge relationships. The hydraulics of circular-crested weirs is examined using simplified models incorporating streamline curvature effects, comparing their predictions with experimental data. A generalized one-dimensional model based on the critical flow in curvilinear motion has been developed. The discharge coefficient increases with the specific energy normalized with the radius of curvature, E/R, when streamline curvature effects are included. The relative flow depth at the crest decreases as E/R increases. The flow at the weir crest is only critical for a normalized specific energy value of E/R ≈ 0.5–0.6. For larger heads, the flow at the weir crest has been found to be supercritical.     

Flow over Gabion Weirs

A conventional weir typically consists of an impermeable body constructed of concrete, since its primary functions are to heading up water and efficiently regulate flow. However, an impermeable body prevents the longitudinal movement of aquatic life and transportation of physical and chemical substances in water, eventually having a negative impact on the water environment. One of the advantages of gabions as a building material is that the motion of individual stones comprising the gabion is not of much concern. The wire mesh of the gabion basket serves to restrain any significant movement. Also, gabion weirs offer an alternative design that could be adopted for flash flood mitigation. In this study, a series of laboratory experiments was performed in order to investigate the flow over gabion weirs. For this purpose, two different gabion weir models were tested in two horizontal laboratory flumes of 10-m and 17-m length, 0.3-m width, and 0.3- and 0.5-m depth, respectively, for a wide range of discharge, upstream water depth, downstream water depth, weir height, weir length, and gabion filling gravel material size. The results of the gabion weir were compared with those of experiments carried out on solid weirs having the same dimension and it was found that there is a large deviation when the solid weirs equation is applied to gabion weirs (permeable weirs). So, using one of the existing solid weir flow formulas would lead to an erroneous calculated value. Multiple regression equations based on the dimensional analysis theory were developed for computing the discharge over gabion weirs for both free and submerged flow regimes. Also, equations were introduced for computing the discharge coefficient to be applied with the traditional solid weir equation.     

Effects of Vanes and W-Weir on Sediment Transport in Meandering Channels

Sediment transport patterns in a meandering channel with instream restoration structures (vane and W-weir) have been studied. Laboratory experiments were conducted in a large-scale mobile-bed channel with graded materials under bank-full and overbank flow conditions. Bed-load samples were collected with a calibrated minisampler. Vanes, constructed against the outer bank in a meander bend, relocated the scour hole toward midchannel, thereby protecting the bank from erosion. The sediment sizes (d50,d90) in the bend became slightly more coarse and more uniform in the center of the channel. The W-weir installed immediately below a riffle section created two scour holes without affecting the upstream bed or the natural sediment transport of the channel. Predictions of bed-load transport by selected deterministic and stochastic methods showed large deviation from measurements using Helley–Smith sampler in sections downstream of the bend apex. In addition to creating local scour holes, the structures also relocated the locus of sediment transport at downstream sections. This issue should be considered when installing vanes and weirs in meandering rivers with significant bed-load transport.     

Impacts on fish transported in tube fishways

Experimental data and numerical predictions of steady and unsteady flow in a 4 m high, 86 mm internal diameter tube fishway were compared quantitatively, and reflected expected uncertainties characteristic of the experiments and flow hydraulics. We then measured the response of a neutrally-buoyant fluid sensor and the behaviour of live fish transported vertically within the tube fishway. Ten repeat tests using the sensor and tests with seventy individual live fish demonstrated transport with 100% reliability. No ill effects were observed over a post-test monitoring period for two species of Australian native fish (Australian bass (Percalates novemaculeata) and Silver perch (Bidyanus bidyanus)) or as a function of size of the Silver perch that can be related to their passage through the fishway. There may have been temporary bruising of a few of the largest Silver perch tested. The largest Silver perch averaged 137 mm in length. The spatial distributions of the inert sensor and fish relative to the moving front during the transport process were quantified. Consequently, the volumes of water required during each operational cycle to ensure reliable delivery of fish over vertical distances less than 4 m were determined. The sensor measurements indicated negligible interactions with straight pipe walls but exposure to significant accelerations at sharp bends. Further experiments with live fish are required to quantify the possible adverse effects of alternative pipe transition designs on animals transported through them. Safe transport of fish up to a fish length/tube fishway delivery diameter ratio of 1.6 is demonstrated.     

Use of Brink Depth in Discharge Measurement

The behavior of free surface flow at a rectangular free overfall is studied experimentally to obtain a relation between the brink depth and the flow rate. A series of experiments were conducted in a tilting flume with wide range of flow rates covering subcritical, critical, supercritical regimes, and two different roughnesses in order to develop a relationship between the discharge and the brink depth. An equation is proposed to determine the flow rate using the brink depth for a channel of known roughness and bed slope.     

Combined Flow over Weir and under Gate

Problems related to sedimentation and deposition can be minimized by using a system where weirs and gates are combined. Given its applications, the hydraulics of simultaneous flow over weir and under gate, in particular, the determination of the stage–discharge relationship, is of interest. Although previous approaches have been based on regression or dimensional analysis, the current work describes a physically based approach. Models of sharp-edged weirs and gates with no lateral contraction are combined. To calibrate and validate the proposed model, experiments have been carried out in a laboratory flume applying different submergence conditions. It was found that the model is able to predict the stage–discharge relationship with reasonable accuracy.     

Experimental study for measuring discharge through compound broad crested weir

This paper deals with a new compound broad crested weir for measurement of discharge. Calibration of broad crested weir is mainly dependent upon the length of the weir crest (L), weir height and upstream water head over weir crest (h). Hence measurement of discharge varies as per change in the flow characteristics due to change in the geometry of weir for a given flume. Based on experimental analysis, past researchers have shown that discharge coefficient (C_d) changes proportionally with h/L ratio, maintaining an average range of C_d for various h/L values. In the present study a novel approach towards design of compound broad crested weir is proposed which can effectively measure wide range of discharges thereby maintaining a constant discharge coefficient irrespective of the head over weir. Experimentation has been carried out in a laboratory horizontal tilting flume 2.5 m long, 20 cm wide and 30 cm deep. The investigation is carried out for discharge ranging from 10 lps to 2 lps. In the earlier experimentation conducted C_d was varying in the range from 0.518 to 0.648, after which the broad crested weir model is modified for suiting best results thereby resulting in the reformed discharge coefficient values of 0.546–0.599 which is in close proximity to the design input value of 0.6.     

Hydraulic Relations for Clinging Flow of Sharp-Crested Weir

Available discharge coefficient formulas for sharp-crested weirs are only applicable to the free-flow regime. To extend the range of discharge measurement by a rectangular sharp-crested weir, critical heads of the transition flow regime, the head-discharge relation for clinging and free flow, and the discharge coefficient for clinging flow were investigated experimentally based on more than 300 experimental points with head ranging from 0.0048 to 0.0455 m. The results indicate that the transitions from clinging to free flow and vice versa do not occur at the same head. Upper and lower critical heads, Hu,crit and Hl,crit, can be identified at which these transitions occur. For the condition studied, the head relation between clinging and free flow is found to be linearly correlated at the same discharge. Expressions for the discharge coefficient for clinging flow are developed.