Compound weirs are combinations of weirs of different geometries and crest elevations. In addition to measuring discharge over wide range of flows, compound weirs are useful in controlling the discharge and water level in irrigation canals, rivers, and lakes. Compound weirs are also incorporated in the design of outlets from stormwater and sedimentation ponds and underground reservoirs. Transitions between the components of compound weirs are producing unsteadiness and instabilities. The analysis of experimental data is revealing noticeable fluctuations in values of the discharge coefficient associated with transitional instabilities. These undesirable effects can be eliminated by simply redesigning compound weirs as continuous polynomial weirs. Polynomial weirs have smooth geometries defined by polynomial equations of any order. Furthermore, the geometric versality of polynomial weirs can help address effects of climate change by allowing wider ranges of head-discharge curves to be incorporated in the design of new and retrofitted hydraulic structures. The hydraulic properties of polynomial weirs are reviewed, and methods of redesigning compound weirs as polynomial weirs are discussed.
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