19.—(1) If during the construction of a specified work or a protective work, or during a period of 24 months after the completion of any such work any alterations or additions, either permanent or temporary, to a waterway are reasonably necessary in consequence of the construction of the specified work or the protective work in order to avoid detriment, and the Board gives to the promoter reasonable notice of its intention to carry out such alterations or additions (which shall be specified in the notice), the promoter shall pay to the Board the reasonable cost of those alterations or additions including, in respect of any such alterations and additions as are to be permanent, a capitalised sum representing the increase of the costs which may be expected to be reasonably incurred by the Board in maintaining, working and, when necessary, renewing any such alterations or additions.
(2) If the cost of maintaining, working or renewing the waterway is reduced in consequence of any such alterations or additions a capitalised sum representing such saving shall be set off against any sum payable by the promoter to the Board under this paragraph.