The proposed route for High Speed 2 crosses miles of floodplains and could exacerbate flood risk in some areas, the Sustainability Statement for part of the scheme has revealed.

The Sustainability Statement, which applies to the route between the West Midlands and Manchester, describes the findings of the Appraisal of Sustainability (AoS) in developing the proposed route and the main potential impacts of the proposed scheme.

In the document it says that the proposed scheme could exacerbate flood risk where it crosses flood zones (as indicated by the Environment Agency) by obstructing flood flows and reducing flood storage capacity, so potentially affecting the severity and frequency of flooding upstream.

The least obstructive form of construction for floodplain crossings is viaduct, and at this stage, viaducts have been assumed for all sections where the route centreline lies within Flood Zone 2 (areas at risk of flooding on average more than once in every 1,000 years), except where tunnels exist, the document says. However, it adds that other solutions may be agreed in due course.

There is a risk of flooding at Manchester Piccadilly station, according to the document, where the scheme would cross a number of small watercourses.

Issues of flood risk would also arise at three other stations: East Midlands Hub, which may require flood defences; Sheffield Meadowhall Station, where ground-level vehicle access to the station may be impeded in an extreme flood event; and Leeds New Lane Station, which is largely in Flood Zone 2 and therefore should only be affected by flood events which exceed the 100-year return period.

HS2 Ltd said it will continue to work closely with the Environment Agency and other relevant parties to establish the most practical and effective solution for each crossing of the floodplain. The crossing of the underground or ‘lost’ Corn Brook at West Gorton would require flood defences to protect the railway and other measures to ensure risk of flooding is not increased, the document says.

In all cases, the detailed design would reflect the findings of hydraulic modelling and make allowances for flood compensation, according to the document.

A more detailed examination of flood risk is to be undertaken, HS2 Ltd said, and the AoS has “not addressed in detail” the risks to HS2 infrastructure from flooding, although the vertical alignment is generally designed to be at least 1m above the expected 1000-year fluvial water level.

The more detailed assessment will look at other flood risk issues including groundwater flooding, failure of reservoirs, flooding from sewers and surface water flooding.