In a syphon aqueduct

• drainage passes over the canal and F.S.L. of the canal is below the bottom of the drainage trough
• drainage passes over the canal and F.S.L. of the canal is above the bottom of the drainage trough
• canal passes over the drainage and H.F.L. of the drainage is above the bottom of the canal trough
• canal passes over the drainage and H.F.L. of the drainage is below the bottom of the canal trough.