4.—(1) The Department may by order substitute a different volume of water for the volume for the time being specified in sections 1, 2(2)(b), 41(3) and (5), 45(3)(a) and 49(3)(b).
(2) The Department may by regulations make provision for the purposes of sections 1, 2, 41, 45 and 49 as to—
(a)how the volume of water capable of being held above the natural level of the surrounding land is to be calculated,
(b)how “natural level” and “surrounding land” are to be construed.
(3) Before making an order under subsection (1) or regulations under subsection (2), the Department must consult—
(a)the Institution of Civil Engineers,
(b)such other organisations representing engineering or concerned with the practice of water and environmental management as it considers appropriate.