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Non-modular, semi-modular outlet; module requirements

Short answer

Non-modular: discharge depends on both distributary and water course levels. Semi-modular: discharge depends only on distributary head (free orifice), unaffected by water course level. Good module: simple, cheap, robust, low head, proportionate silt, flexibility near 1, tamper-proof.

Explain the working principle of non-modular and semi-modular outlet. What are the requirements of a good module?

Non-modular outlet (working)

Its discharge depends on the water level in both the distributary and the water course: q=Cd a2g(Hd−Hw)q = C_d\,a\sqrt{2g(H_d - H_w)}. Raising the distributary level increases qq; raising the water course level (silting) reduces qq. Example: open sluice, drowned pipe.

Semi-modular (flexible) outlet

Its discharge depends on the distributary level only, provided a minimum working head exists. The flow leaves as a free jet, so the water course level has no effect. Example: Kennedy's gauge outlet, Crump's adjustable orifice semi-module (AOSM), free pipe outlet.

  distributary ~~FSL~~|--orifice--> free jet ~~ water course
  discharge = f(H_d) only; independent of H_w

Requirements of a good module (outlet)

  1. Simple and robust: no moving parts that can jam or be tampered with.
  2. Cheap and easy to construct and maintain.
  3. Minimum working head requirement, so that command is maintained.
  4. Correct share of silt: it should draw its proportionate share of the silt carried by the distributary (not set too low or too high).
  5. Proportional (flexibility near one) or at least sub-proportional, for equitable distribution.
  6. Tamper-proof: the discharge cannot be changed by the farmer.
  7. Adjustable to suit changes in the crop, if required (semi-modular).
  8. Little sensitivity to water level fluctuations.

✓ Answer checked by Claude Opus 5.5 (high effort)

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