Design considerations: free and submerged outlets

Free (semi-modular) outlets

Water leaves as a free jet, so the discharge depends on the distributary level only: $q = C_d\,a\sqrt{2g\,H}$, with $H$ the head over the centre of the opening.

Design considerations:

1. Fix the required discharge $q = \dfrac{A\,\Delta}{T}$ from the command area, the depth of water (duty) and the period.
2. Choose the setting $H/D$ (sill level) to give the required flexibility, $F = \dfrac{m}{n}\dfrac{D}{H}$: a deeper (lower) setting gives a lower flexibility but draws more silt; a higher setting gives a higher flexibility. For proportionality, $H/D = m/n$.
3. Provide a minimum working head so that the jet is not drowned by the water course level (modular limit).
4. Size the opening from the discharge equation; a pipe also needs a check on its length and friction.
5. Set the sill at a suitable height above the distributary bed so that it draws its share of silt.
6. Provide a free fall or stilling space d/s, and protect the water course with stone pitching.

Submerged (non-modular) outlets

The water level in the water course is higher than the outlet crest, so the discharge is $q = C_d\,a\sqrt{2g\,(H_d - H_w)}$ and depends on both levels.

Design considerations:

1. Required discharge as above.
2. Determine the available head difference $(H_d - H_w)$ from the levels in the distributary and the water course at FSL.
3. Size the opening: $a = \dfrac{q}{C_d\sqrt{2g(H_d - H_w)}}$.
4. Check that the outlet remains submerged over the range of flows and does not become hyper-proportional.
5. Prevent tampering: the opening should be fixed, preferably a pipe set in masonry.
6. Provide a settling basin in the water course if silt is a problem.

Free outlets are preferred, since they are not affected by the water course.