39Size and stability of crossings

39 Size and stability of crossings

39 Size and stability of crossings

(1) Any crossing over a river must be designed, constructed and maintained—

(a) so as to enable conveyance of—

(i) the peak flow from a 1-in-5 year storm event (that is, a rainfall event predicted to occur on average once in 5 years), or

(ii) the flood plain level of flow (being the level of flow that needs to exist for water to break out of the defined banks of the river and enter a flood plain),

whichever is the lesser, and

(b) so that the flow velocity through the crossing exceeds by no more than 10% the flow velocity through the natural channel of the river at the site of the crossing, that is, before the crossing existed (the aim being to prevent impediment to fish passage).

Note—

If a culvert is chosen as a crossing, arch and box culverts are to be preferred to pipe culverts.

(2) Any causeway or culvert base used to cross a river must be set at the level of the stream bed so as to provide continuity in flow and so that a bed of natural sediment can form at the bottom.

(3) Any crossing over a drainage feature (other than a river) must be designed to convey the peak flow from a 1-in-5 year storm event.

(3A) Any causeway or culvert base used to cross a drainage feature (other than a river) must be set at the level of the stream bed unless the average longitudinal gradient of the drainage feature within 50 metres upstream or downstream of the crossing is more than 6°.

(3B) The outlet of any causeway or culvert referred to in subclause (3A) must be stable and must facilitate the passage of fish.

(3C) Subclauses (3A) and (3B) apply only in respect of a causeway or culvert base constructed or replaced after the commencement of those subclauses.

(4) The surface of any crossing, and the approaches on either side of it, must be made of stable material that is unlikely to be displaced during normal use of the crossing or approach, or by any flood up to and including the peak flow of a 1-in-10 year storm event (that is, a rainfall event predicted to occur on average once in 10 years).

(5) Any causeway must consist of gravel, rock, bitumen, concrete, logs, or other stable material that is unlikely to produce water turbidity.