Once river levels recede, and when re-flooding is not thought to be imminent, we clear riverside footpaths of silt, mud and debris. Along the rivers in York this is known as 'warping'.
Warping policy
Our warping policy details our approach to warping (or 'warping-off') the riverside pedestrian paths and riverside cycle paths in our area.
In the context of our Warping Policy, the definition is:
warping: the removal of silt from riverside paths, following flood events.
Find out about:
- reasons for warping
- timing of warping operations
- warping operations and order of treatment
- warping on tow paths
Reasons for warping
City of York Council is the owner of:
- the majority of the banks of the River Ouse, between Clifton Bridge and the Millennium Bridge
- the towpath alongside the River Foss, between Blue Bridge and Castle Mills Lock
Located on the banks of both rivers are pedestrian and cycle paths which form an important part of York's transport network. Many of the riverside paths are affected by flooding, because they're in unprotected areas. The rivers carry silt in their flow, and as the water level falls, following a flood, the silt is either deposited on paths, or flows back into the river.
The amount of silt deposited and the extent of its effect on York's riverside paths depends on:
- the level rivers reach; the higher the flood, the more paths are affected
- the duration of the flood; the longer the duration of the flood, the greater the depth of silt deposits
Timing of warping operations
The 4 factors we consider before warping can commence, are:
- visual inspection: affected areas are inspected to ascertain where action is required
- magnitude of silt deposits: we'll check how much silt has been deposited to determine what action is needed
- behaviour of the river: during a prolonged flood event the river level can rise and fall several times; at low levels paths may appear usable, however, we use information from the Environment Agency to decide when warping should commence - if there is a chance of riverside paths being flooded again within a few days, warping operations will be delayed
- weather conditions and surface temperature of riverside paths: we use weather forecasts provided to our highways winter maintenance service to decide when warping should commence - if paths are freezing, or likely to freeze, and hazardous conditions may be created or aggravated, warping operations will be delayed
Warping operations
Silt is removed from the affected riverside paths when the river level has fallen and the risk of re-flooding has receded. The network of paths affected by flooding is known, and the depth and location of silt deposited on each path is generally predictable, based on the height of the flood and its duration.
Our warping operations consider:
- warping is best carried out when the silt is wet, since it becomes solid and dense and difficult to remove when it dries out
- sweeping is not effective, as wet silt spreads and dry silt is too compacted to remove
Wherever possible warping uses portable pumps using river water (rather than clean/mains water). The silt is returned to the river using:
- high pressure jetting either straight to the river or through the gullies and pipes which have been provided to drain low points on the paths (gullies and pipes are 'jetted' as part of the operation to maintain their effectiveness)
- other heavy 'plant' equipment, as required, to push the silt back into the river, following more extreme events
The order which paths are cleared is based on their level of use; paths in York City Centre are cleared first. However, the order of warping operations can change, depending on river levels and operational requirements. The usual order for warping operations is:
- King’s Staith
- South Esplanade
- Tower Gardens
- St George’s Field
- New Walk
- Queen’s Staith
- Terry Avenue
- Museum Gardens - Lendal to Scarborough Bridge
- Riverside Walk - Scarborough Bridge to Clifton Bridge
- West Esplanade
- Cinder Track - Scarborough Bridge to Clifton Bridge
- Skeldergate
- Foss Basin - Blue Bridge to Castle Mills Lock
While we endeavour to clear all paths in these locations in a timely manner following a flood, our operations take into account prevailing conditions, the weather forecast and partner organisations operational flood activity.
Warping on tow paths
Lower-level tow paths we own are normally cleared once, at the end of the winter flooding season. However, we'll monitor conditions and take action if necessary, in the interests of public safety, at:
- New Walk
- St Georges Field
- Between Scarborough Bridge and Lendal Bridge (both sides)
- North Street Gardens
- Almery Terrace
- Terry Avenue from junction with Clementhorpe to the City wildlife site near the caravan park