The following details pertain to the crossing of a canal and a drain. Bed level of Canal = + 211 m; Full Supply depth of Canal = 1.8 m; Bed level of Drain = + 209.4 m; depth of flow at High Flood Level = + 2.2 m. The suitable type of cross drainage work is:
Depending upon the constructional features, the aqueduct is of following types: (a) Pipe AqueductWhen the canal is very small, it passes over the drain through pipeline which is called pipe aqueduct. (b) Irrigation Culvert Where the width of drain is from 2.5m to 16m small culverts are provided for drain over which canal is passed. Canal passes unobstructed over the culvert. (c) Irrigation Box Culvert It is also called irrigation slab drain type aqueduct. When the width of drain is small i.e., up to 3m, the drain may be passed through rectangular tunnel or a culvert. (d) Syphon Aqueduct Syphon is the structure in which case the canal water passed through inverted syphon under a drainage. The canal water allowed to flow under pressure when tail water level below a syphon is higher than the under surface of the culvert and uplift pressure is exerted on the covering of the roof. As given, that Bed level of canal = + 211m; Full supply depth of canal = 1.8m; Bed level of drain= + 209.4m; Depth of flow at high flood level= + 2.2 m. Difference of bed level of canal and drain =211 – 209.4 = 1.6m Effective depth = Depth of flow at high flood level – Difference of bed level of canal = 2.2 – 1.6 = 0.6m Whereas, the full supply depth of canal = 1.8m; here the canal water allowed to flow under pressure when tail water level below a syphon is higher than the under surface of the culvert and uplift pressure is exerted on the covering of the roof. So, this is the condition of Syphon Aqueduct