According to Lacey, depth of scour in a river depends upon the straightness of the reach. If D is the depth of scour in regime flow in a right angled bend, it is A. 1.25 D B. 1.50 D C. 1.75 D D. 2.00 D

The correct answer is A. 1.25 D.

Lacey’s equation for scour depth in a river bend is:

$d = 1.25 D \sin \alpha$

where $d$ is the depth of scour, $D$ is the depth of flow, and $\alpha$ is the angle of the bend.

In a right-angled bend, $\alpha = 90^\circ$, so the equation becomes:

$d = 1.25 D \sin 90^\circ = 1.25 D$

Therefore, the depth of scour in a right-angled bend is 1.25 times the depth of flow.

The other options are incorrect because they do not take into account the angle of the bend.