If D_{c} is the critical depth, D is the depth of water down stream and H_{L} is the total head loss at the cistern of a vertical fall, the depth x of the cistern below the bed level down stream suggested by Blench, is

Flexibility: It provides a criteria for judging the behaviour of modules and semi-modules. It is generally denoted by letter 'F'. It is the ratio of the rate of change of outlet discharge to the rate of change of discharge of the parent channel.