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A unique relation between water-surface elevation and water discharge usually does not exist for sand-channel streams. The relation is affected by changes in bed roughness and changes in bed elevation because of scour and fill. An artificial control on a sand-channel stream must control both the resistance to flow and the bed elevation in order to stabilize the relation between water-surface elevation and water discharge. The weir (control structure) in the Rio Grande conveyance channel near Bernardo, N. Mex., was designed on the basis of a model study and field data (Harris and Richardson, 1964). About 72 percent of the measurements used to define the base relation between water-surface elevation and water discharge falls within plus or minus 5 percent of the mean relation for the prototype. The stage-discharge relation is not affected by backwater for values of submergence less than 90 percent. There is no consistent relation between the ratio of measured discharge to rated discharge and. submergence for values of submergence greater than 90 percent. The control does not restrict the channel capacity to less than the strted design capacity of 2,000 cubic feet per second. When the control is drowred out, or ineffective, the relation of water-surface elevation to water discharge is virtually the same as that prior to construction of the control for discharges greater than 1,500 cubic feet per second. When the control is not drowned out that is, free-fall conditions exist the water-surface elevation for a discharge of 2,000 cubic feet per second is greater than the minimum elevation, but is less than the maximum elevation that occurred at that discharge prior to construction. The model study was only partially successful in predicting the operating characteristics of the prototype. Some of the differences between prototype operation and model predictions may exist because the prototype was not built exactly as recommended on the basis of the model study. In general, the prototype has operated somewhat better than the model predicted. INTRODUCTION Sand-channel streams often present problems in the determination of the total volume of water passing a gaging station because a unique relation between water discharge and water-surf ace elevation generally does not exist. The non-cohesive sand beds of the channels, Al A2 STUDIES OF FLOW IN ALLUVIAL CHANNELS which control the stage-discharge relation, are subject to changes in roughness and to scour and fill. As a result, the stage-discharge relation often is not well defined, and corrections, based on waterdischarge measurements, must be applied. Simons and Richardson (1962) have described the effects of changing bed roughness on the stage-discharge relation for several hydraulic conditions. Changes in bed roughness in some sand-channel streams may be gradual because the bed form consistent with a given discharge does not completely develop under unsteady How conditions as a flood wave passes. This condition results in a stage-discharge relation in the form of a loop. Changes in bed roughness, particularly from dune to plane bed, in sand-channel streams sometimes are rather abrupt, resulting in stage-discharge relations that are discontinuous (Colfoy, 1960; Dawdy, 1961). For sand-channel streams the change from dune bed to plane bed may occur over a wide range of discharges. The cause for the change is not easily defined. The bed elevation of a long reach of sand-channel stream generally changes very little owing to scour and fill during the passage of a flood wave. Howrever, the change in bed elevation at sections in reaches that are narrower than average can be large (Colby, 1964). Gaging stations often are located at narrow sections, and the stage-discharge relations are subject to the effects of scour and fill. Natural controls seldom exist on sand-channel streams; however, properly designed artificial controls (weirs) can be used to stabilize the stage-discharge relation. The elevation of the water surface at a given discharge is dependent on depth of flow associated with the prevailing bed form and the prevailing elevation of the bed. The structure must, in effect, control two variables, bed elevation anc1 roughness, to yield a stable rating. The Rio Grande conveyance channel near Bernardo, N. Mex. (fig. 1), is an example of a sand-channel stream that can present a problem in determining the discharge and the volume of flow at a gaging station. The conveyance channel has steep banks that are fairly well stabilized by native vegetation. In the vicinity of the control, the channel is straight and has a width of approximately 80 feet. Flows greater than about 100 cfs (cubic feet per second) occupy the full channel width. The banks are of alluvial material and the streambed is composed of fine sand (median diameter, 0.17-0.24 mm). The eH.nnel, constructed in 1954, was designed to convey 2,000 cfs, which is diverted from the river through the headworks 5 miles upstream from the control. The headworks structure consists of seven gated corrugated metal culverts, 7 feet in diameter. Discharges greater than 2,000 cfs are carried by the floodway. Median flow in the conveyance channel is 315 cfs, and flows exceed 2,000 cfs less than 0.3 percent of the time (fig-2). STAGE, DISCHARGE, WEIR IN SAND-CHANNEL STREAM A3 Control structure ,and total sediment sampler FIGURE 1. Rio Grande conveyance channel. P E R C E N T A G E O F T I M E ^ N D I C A T E D D I S C H A R G E W A S E Q U A L E D O R E X C E E D E D i r o o o o o o ^ h o m o o c . 3 1 3 X H V H O I V I A f l T I V K I J A . O 1 & STAGE, DISCHARGE, WEIR IN SAND-CHANNEL STREAM A5 The bed form can be either dune or plane bed over an approximate range in discharge of 300 to 1,200 cfs. The depth, on the average, is about 35 percent less for plane bed than for dune bed at a given discharge. The channel is also subject to scour and fill, and a change in bed elevation of as much as 4 feet could occur at the original gaging station on the channel (Harris and Richardson, 1964, fig. 36). Because of these changes in bed form and in bed elevation, the stage-discharge relation was very unstable and streamflow could not be accurately determined without frequent measurements. In 1961, the U.S. Bureau of Reclamation began a water-salvage study in the reach of the Rio Grande between Bernardo and San Acacia. An agreement was made between the Bureau of Reclamation and the Geological Survey to design and build a control structure at Bernardo that would provide a stable stage-discharge relation. The structure was designed on the basis of a model study and field data (Harris and Richardson, 1964). The prototype structure was completed in September 1963. Construction was in cooperation with the Bureau of Reclamation under the general supervision of W. L. Heckler of the Geological Survey. The purposes of this paper are (1) to describe the field installr.tion and construction, (2) to evaluate the performance of the control, including a comparison of the prototype performance with that predicted by the model study, and (3) to evaluate the effect of the controlled streambed elevation on channel capacity. Basic data used in the analysis are given in table 1. Basic datT. include measured water discharge, gage height for gages B, C, and D, depth above the crest for gages B, C, and D, and percent submergence computed on the basis of depths above the crest for gages B and D. PROPOSED CONTROL STRUCTURE The Geological Survey obtained data on water-surface and streambed elevations at the proposed location for the control and also conducted model studies at the Colorado State University Hydraulics Laboratory. The water-surface and streambed elevations at the proposed site were obtained to determine the proper elevation of the control crest. The crest elevation was to be .such that the channel capacity would be as much as 2,000 cfs and the crest would be above mean bed elevation and therefore free of sand most of the time. The model studies were conducted to determine (1) a control configuration which would eliminate, or at least minimize, the effects of changes ir bed configuration and changes in bed elevation on the stage-discharge relation, (2) the maximum elevation of the control crest which would not create sufficient backwater to interfere with the maximum design capacity of the channel (2,000 cfs) and the backwater characteristics of the control under various degrees of submergence, (3) the design 335-122 O-H69 2 A6 STUDIES OF FLOW IN ALLUVIAL CHANNELS TABLE 1. Basic data for the conveyance channel

Stage-discharge characteristics of a Weir in a sand-channel stream