16.—(1) The authorised development must not be commenced until a detailed drainage plan showing all new surface water drainage has been approved by the relevant planning authority in consultation with the lead local flood authority. The details must be in accordance with the details of paragraph 5.4.6 of the flood risk assessment and demonstrate that:
(a)all discharges to watercourses from railway drainage systems should be attenuated to a rate not exceeding the existing rate of runoff, that is 5 litres per second per hectare; and
(b)all discharges made from railway drainage systems should be discharged downstream of existing watercourse culverts wherever possible to reduce any impact on the capacity of each of the respective culverts.
(2) The authorised development must be constructed in accordance with the approved details.