90. Subject to paragraph 89(5)(b), if by reason of the construction of any specified work or of the failure of any such work the efficiency of any ordinary watercourse for flood defence or land drainage purposes is impaired, or that watercourse is otherwise damaged, so as to require remedial action, such impairment or damage must be made good by the undertaker at its own expense to the reasonable satisfaction of the drainage authority and if the undertaker fails to do so, the drainage authority may make good the same and recover the expense reasonably incurred by it in so doing from the undertaker.