Non-modular outlet working, merits, demerits

Working of a non-modular outlet (3 marks)

A non-modular outlet is one whose discharge depends on the water levels in both the distributary and the water course. The flow through the outlet is orifice or sluice flow: $q = C_d\,a\,\sqrt{2g\,(H_d - H_w)}$ when submerged, where $H_d$ and $H_w$ are the water levels in the distributary and in the water course. If the level in the distributary rises, the discharge rises; if the water course silts up or its level rises, the discharge falls. Examples: open sluice, submerged pipe outlet.

  Distributary                         Water course
  ~~~~~ H_d ~~~~~|======pipe=====|~~~~~ H_w ~~~~
  ______ bed ____|               |______ bed ____

Advantages (about 1.5 marks)

1. Simple, cheap and easy to construct.
2. No moving parts, so little maintenance is needed.
3. Needs only a small working head, so it can command a large area.
4. The discharge can be adjusted by changing the opening size or setting.

Disadvantages (about 1.5 marks)

1. The discharge varies with the water levels in both channels, so distribution is not equitable.
2. The farmer can alter the discharge by changing the water course level (tampering).
3. Silting or weeds in the water course reduce the discharge.
4. It cannot give a fixed discharge.