Complex sentence: They may be designed to set spillway crest heights in dams, to adjust flow rates in sluices and canals, or they may be designed to stop water flow entirely as part of a levee or storm surge system.
Split into simpler sentences: They may be designed to set spillway crest heights in dams, or to adjust flow rates in sluices and canals. Or they may be designed to stop water flow entirely as part of a levee or storm surge system.