(a) Subject to this section, where for the purposes of any qualifying trade carried on by a person the person incurs capital expenditure on dredging, and either the trade consists of the maintenance or improvement of the navigation of a harbour, estuary or waterway or the dredging is for the benefit of vessels coming to, leaving or using any dock or other premises occupied by the person for the purposes of the trade, then -
(i) an initial allowance equal to 10 per cent of the expenditure shall be made for the first relevant chargeable period to the person incurring the expenditure, and
(ii) writing-down allowances shall be made in respect of that expenditure to the person for the time being carrying on the trade during a writing-down period of 50 years beginning with the first relevant chargeable period; but, where a writing-down allowance is to be made for a year of assessment to such a person and such person is within the charge to income tax in respect of the trade for part only
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