No. In fact this is the exact opposite of what you want to do! You want to set a flume so that it is level from front-to-back and from side-to-side. The hydraulic action of a flume is such that setting it on a slope would actually artificially accelerate the flow. This would in turn mean that the flume would under read the actual flow rate.
Corrections for longitudinal settling (the flume at a slope) have been developed for Parshall and Cutthroat flumes so that the indicated flow rate can be corrected. For other types of flume, however, no correction is available. So if those flumes have been set at a slope, the only way to correct those installations is to re-level the flume.