The discharge over a broad crested weir is maximum when the depth of flow is (where H is the available head) A. $$\frac{{\text{H}}}{3}$$ B. $$\frac{{2{\text{H}}}}{3}$$ C. $$\frac{{\text{H}}}{2}$$ D. $$\frac{{2{\text{H}}}}{5}$$

The correct answer is $\frac{{\text{H}}}{2}$.

The discharge over a broad crested weir is maximum when the depth of flow is equal to half the head. This is because the maximum discharge occurs when the water surface is parallel to the crest of the weir. When the depth of flow is less than half the head, the water surface is curved and the discharge is less. When the depth of flow is greater than half the head, the water surface is curved in the opposite direction and the discharge is also less.

Option A is incorrect because the depth of flow is equal to one-third of the head when the discharge is minimum.

Option B is incorrect because the depth of flow is equal to two-thirds of the head when the discharge is less than the maximum.

Option C is incorrect because the depth of flow is equal to the head when the discharge is zero.

Option D is incorrect because the depth of flow is equal to two-fifths of the head when the discharge is less than the maximum.