Inland waterways: categorisation of waters

Guidance for owners, operators and masters of vessels on inland waters, explaining the classifications and how to apply for categorisation.


Inland water categorisation policy and processes are dealt with by the Navigation Safety Branch (NSB) of Maritime and Coastguard Agency (MCA).

This guide is for commercial vessel operators, and anyone else who needs to know the correct categories of waters. Knowing the categories will affect:

  • the type of vessel you can operate
  • the safety equipment and crew qualifications you’ll need onboard

What inland waters are

Inland waters are any area of water, such as canals, tidal and non-tidal rivers, lakes, and some estuarial waters, not categorised as ‘sea’.

The UK has over 4,000 miles of inland waterways. They’re not regarded as ‘sea’ for the purposes of most merchant shipping legislation.

How inland waters are classified

Inland waters are classified as one of four categories:

  • Category A: narrow rivers and canals where the water is generally less than 1.5 metres deep
  • Category B: wider rivers and canals where the water is generally 1.5 metres or more deep, and where the significant wave height couldn’t be expected to be more than 0.6 metres at any time
  • Category C: tidal rivers, estuaries and large, deep lakes and lochs where the significant wave height couldn’t be expected to be more than 1.2 metres at any time
  • Category D: tidal rivers and estuaries where the significant wave height couldn’t be expected to be more than 2 metres at any time

Water categorisation matters are co-ordinated by the Navigation Safety Branch of MCA.

These categories are defined and listed in MSN 1837.

How to apply for categorisation or re-categorisation of waters

You (as an individual or organisation) can apply for categorisation, or to change an existing categorisation,of a defined area of inland water. You must have an interest in the area requested.

MCA will consider any request for the categorisation of new or for changes in the existing categorisation of areas. The NSB co-ordinates the categorisation of a defined area of water.

Contact your local marine office to get advice about categorisation of a particular stretch of water and making an application.

Your application to your local Marine Office needs to be fully supported with:

  1. Chart or chartlet of location;
  2. Recent baythemetric data with spot soundings and/or depth contours shown on charts;
  3. Levels of vessel movement;
  4. Types of vessel using the area;
  5. Physical description of the area;
  6. Details of significant wave heights;
  7. Predominant wind chart for the area;
  8. General rationale to support the application;
  9. Supporting letter from the local Harbour/Navigation Authority, as applicable.

More about significant wave heights

For your application, you must also give evidence showing how the significant wave height has been established.

In each category, a maximum significant wave height has been given. Your evidence needs to show that the wave height does not go above this limit at a representative point (or points) in the area you wish to have categorised or re-categorised.

This evidence could, for example, be based on information taken from a mathematical model of the geographic area. This should be validated by wave height observations from a wave rider buoy, or a seabed pressure sensor. Other evidence may be accepted if considered suitable by the MCA expert group.

Maximum significant wave height limits:

Category B: up to 0.6 metres

Category C: up to 1.2 metres

Category D: up to 2.0 metres

The time of year your application covers will also be a factor. For an application for a winter or summer seasonal area, measurements should represent the period of worst weather. For all-year-round applications, measurements should represent the worst winter conditions.

In addition to wave heights, other factors may have to be taken into consideration when determining categories of water. For example:

  • The strength of tidal stream;
  • Relevant local phenomena (for example, a bore).
  • The effect of underwater banks on the tidal streams;
  • The fetch of the wind, especially in long, straight estuaries with a low-lying hinterland;
  • The effect of shifting underwater sand banks which result in changing navigational channels;
  • Height of banks at the water’s edge and the degree of shelter that may be found in bad weather;
  • The effect of any surrounding high land which may give rise to sudden and unpredictable wind shifts and strengths;
  • The effect of the prevailing wind against the tidal stream which can shorten the sea, giving rise to steeper waves which are more liable to break; and
  • Any available data that can prove wave heights will be affected by climate change.

How your application is processed

  1. Send your application and supporting documents to the local Marine Office for the attention of the Surveyor-in-Charge (SIC) or Area Operations Manager (AOM).

  2. The SIC or AOM will consult the District Safety Committee or Small Passenger Ship Steering Group as appropriate.
    If they’re content with the application they’ll send their comments or recommendations to the NSB. If not, they’ll ask you for further or clearer information.

  3. The NSB will review the application and submit it to the next bi-annual meeting of the Limits Committee of Focal Point Group 2 (FP2).

  4. FP2 will make recommendations to MCA’s Technical Advisory Group for review, and whether or not to grant approval.

  5. The NSB will send the TAG’s decision to the local marine office. The marine office will then tell you the decision.

Further information

See the map and full list of UK navigation authorities on the Association of Inland Navigation Authorities (AINA). It represents the organisations which operate and manage navigable inland waterways in the UK.

Published 6 May 2014
Last updated 8 October 2019 + show all updates
  1. Updated section on how to apply for categorisation or re-categorisation of waters.

  2. First published.