ABSTRACT: Multiple in-stream structures including cross-vanes, W-weirs, and bendway weirs have been developed and utilized for channel rehabilitation. In-stream rehabilitation structures can provide increased channel roughness, high velocity control, diverse flow conditions and scour pools. In-stream structures have been successfully used in gravel and cobble bed streams for rehabilitation; however, application of these structures in sand-bed channels poses many uncertainties. Structure effectiveness and integrity, filter layer necessities and depth of scour formations are all uncertainties associated with in-stream rehabilitation structures applied in sand-bed channels. Limited design guidelines are available for many in-stream rehabilitation structures and few have quantitative information on hydraulic effects, scour predictions and stability thresholds.

A channel rehabilitation project has been initiated by the Bureau of Reclamation on a 29-mile reach of the Middle Rio Grande located between Cochiti Dam and the Town of Bernalillo. Channelization and the addition of the Cochiti Dam have caused significant morphological changes within the reach. The Middle Rio Grande, which was historically straight and braided, has developed into an unstable actively meandering sinuous channel. In-stream structures composed of native rock material are being considered for inclusion in the rehabilitation project. However, research was needed to determine their applicability and effectiveness for this reach. Additionally, reliable design guidelines are necessary for including in-stream rehabilitation structures in the channel maintenance project.

Cross-vane, W-weir and bendway weir structures were modeled at the Hydraulics Laboratory of the Engineering Research Center at Colorado State University. The structures were modeled in sand-bed flumes to study their hydraulic effects, stability thresholds and scour formations. Multiple variations in bed slope, structure material, weir height and weir length were included in the cross-vane and W-weir models and data were collected on water surface elevations, bed surface elevations, velocity direction, velocity magnitudes, scour elevations and scour locations. Structure geometry was constant for the bendway weir models. Three of the four bendway weir structures were built with the same size rock material and one structure was built with a smaller size rock material. Data collected during bendway weir testing also included water surface elevations, bed surface elevations, velocity direction and magnitudes and scour elevations and locations.

Using the data obtained from the test program, multiple relationships were developed. An equation was developed to predict the upstream flow depth for cross-vane and W-weir structures. From the cross-vane and w-weir test data, effective Manning's n values were calculated using a step backwater model with the water surface profiles obtained from the structure tests and baseline channel geometry. Calculated effective Manning's n values were compared to the Manning's n values of the channel without a structure to obtain the effective increase in Manning's n value due to the structure. Empirical equations were developed to predict the effective increase in Manning's n and the corresponding reach lengths for cross-vane structures. Structural integrity was investigated for cross-vane, W-weir, and bendway weir structures. Instability modes and stability thresholds were identified for the structures. Relationships for maximum downstream scour depth prediction were obtained for cross-vane and W-weir structures. Bendway weir scour behavior was investigated and compared to multiple existing predictive equations. Additionally, relationships were developed to predict the maximum scour at the structure toe for the bendway weir structures. Structure testing resulted in multiple developed predictive equations, comparisons between existing relationships and valuable information concerning different modes of structural instabilities and stability thresholds.


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