6. Subject to paragraph 8, if by reason of the construction of any specified work or of the failure of any such work the efficiency of any drainage work for flood defence purposes is impaired, or that drainage work is otherwise damaged, such impairment or damage must be made good by the promoter to the reasonable satisfaction of the Agency and if the promoter fails to do so, the Agency may make good the impairment or damage and recover from the promoter the expense reasonably incurred by it in doing so.