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Modular, semi-modular, non-modular outlets

Short answer

Non-modular outlet discharge depends on both canal and watercourse levels (e.g., submerged pipe). Semi-modular outlet discharge depends only on distributary level if free flow is ensured (e.g., AOSM). Modular outlet provides constant discharge independent of both levels, using energy dissipating mechanisms (e.g., Gibb’s module).

Explain the working of modular, semi-modular and non-modular outlets (with neat sketches).

Non-modular outlet

Its discharge depends on the water levels in both the distributary and the water course: q=C A2g (Hd−Hw)q = C\,A\sqrt{2g\,(H_d - H_w)} when submerged. Any change of either level changes the discharge. Examples: open sluice, drowned pipe outlet.

  distributary FSL ~~~~~|--pipe--|~~~~ water course
                       (submerged)

Semi-modular outlet

The discharge depends only on the water level of the distributary if the minimum working head is available, the water course level not influencing the flow (the jet discharges freely or the flow is made independent by a flume). Examples: pipe outlet with free discharge, Kennedy's gauge outlet, Crump's open flume, adjustable orifice semi-module.

  Crump's adjustable orifice semi-module (AOSM)
 distributary                          water course
 ~~~~~ FSL ~~~~|_roof block_|~~ flume ~~
 ______ bed ___|  orifice   |__expanding__
                free jet, hydraulic jump in flume

The roof block can be raised or lowered to set the opening; the diverging flume gives a free flow.

Modular outlet

Discharge is practically constant and independent of the water levels in both distributary and water course, within a working range. Rigid modules have no moving parts: extra head is destroyed by a standing wave against baffles in a spiral eddy chamber (Gibb's module) or by a sloping roof with air vents (Khanna's module). Examples: Gibb's module, Khanna's module.

  Gibb's rigid module
  distributary ~~~~ | eddy chamber with |~~~~ water course
                    | baffles (no moving parts) |

Modular outlets are accurate but costly and need careful maintenance, so semi-modular outlets are preferred in practice.

✓ Answer checked by Claude Opus 5.5 (high effort)

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