Structures Specification

Specification and Standard Conditions for Structures in Watercourses 

Land Drainage Act 1991 – Section 23
Structures in Water courses – Standard Conditions

  1. Under the above Act no person shall in any drainage authority area install any structure, culvert or fill in any watercourse, (private or public) without the written consent of that drainage authority.
  2. In the Lower Severn Internal Drainage Board District the issuing of consents is charged for.  A charge of £50 (VAT exempt) is made in accordance with the Land Drainage Act.  A consent will be given by the Board under the provisions of the Land Drainage Act 1991 only.
  3. The consent of other relevant authorities may be required, e.g. District Council under Public Health Act 1936, adjacent owner, Highway Authority and Environment Agency for effluent discharges.  The Board may forward a copy of any application to the District Council and if deemed necessary to the Highways Authority also.
  4. The required pipe size and invert of any culverting will be stated in the consent.
  5. The application is not entitled by virtue of any consent to interfere with any other person’s property or impede the flow in the watercourse, or damage or interfere with any cable, wires or pipes which may be affected by the works.
  6. If any work is in a Board’s maintained watercourse then it must be carried out in accordance with the Boards specification, either by the Board itself or by a suitably experienced Contractor.  If the Board carries out the work pre-payment may be required; an estimate for the work will be given on request.
  7. Where a contractor carries out the work in a Board’s maintained watercourse the Board may make an additional charge to cover the cost of supervision and setting up of level pegs (this service can also be provided for private watercourses).  No overfilling of pipes shall take place until the work has been inspected and approved.
  8. The ownership of the structure will remain with the applicant or successors.  The owners will be fully responsible for the future maintenance of the structure and for its removal if so required by the Board.
  9. When a watercourse is piped and infilled to land level, the position of any removable fencing or boundary markers should be agreed with adjoining owners and the Board (if a Board’s maintained watercourse).

Specification for works in a Board’s Watercourse

A. Culverts

1. General
All pipes or units shall be laid in straight lines with manholes at each change of direction, level and size and at any junction.  Straight lengths of piping shall not exceed 30 metres between manholes unless agreed by the Board.  For long culverts layout plans must be submitted for approval.  Pipe inverts shall be 100mm below adjoining rhine hard bed level at the upsream and downstream headwalls.

2. Pipes
Pipes shall be concrete, plastic or corrugated steel. Concrete pipes shall be to BS 5911.  Plastic or corrugated steel pipes should be designed to meet Department of Transport specification for Highway Works.  Pipes or units of any material shall be adequate to carry the anticipated loading and shall be installed to the manufacturers instructions. The Board will not be responsible for specifying pipe or unit strengths.

3. Trenches
Trenches for pipes or units shall be excavated to a sufficient depth and width to enable the pipes or units to be adequately bedded and haunched with “as raised” material.

Pipes or units are to be laid level so that each one is in contact with the bedding material throughout its length and in a straight line between access points.

4. Bedding
Concrete pipes or units are to be connected with not more than a 12mm gap at the joint.  Other pipes are to be jointed using propriety connection pieces.  After jointing, backfill material of “as raised” granular fill should be compacted in not more than 150mm layers to 150mm above the top of the pipes or units.
Where the culvert passes under a public highway the conditions laid down by the Highways Authority shall be complied with.

5. Headwalls

Protection shall be given to the ends of the culverts or pipe outfalls by either:-

  1. a brick, blockwork or concrete vertical headwall minimum thickness 225mm, built on a proper foundation below the pipe or unit invert up to ground level. The sides of the headwall shall extend into each bank far enough to provided stability.


  2. for pipes under 225mm, precast concrete or GRC headwalls and troughs may be used. The trough units shall extend from the pipe to below lowest water level as specified by the Board.  No part of the structure shall protrude out of the bank in order not to impede cutting equipment.  The headwall and troughs shall be secured in accordance with the manufacturers instructions.

6. Manholes
Where manholes are to be incorporated in the culvert minimum internal dimensions shall be either 1200mm x 900mm, or 1200mm diameter from base to top, with walls of minimum thickness 225mm.  These structures shall be founded on suitable rafts of reinforced concrete not less than 150mm thick, the top of the raft being 150mm below the culvert invert. Manholes shall be covered with reinforced concrete slabs with a clear opening of at least 600mm square or 600mm diameter.  The opening shall be covered with a suitable removable cover (of at least class C250) giving the same clear opening.

B. Services in Boards Channels
Services such as water pipes, electricity cables, gas, oil pipes etc shall be buried beneath the true bed of the channel, a minimum of 1 metre deep and either be incorporated in a steel sleeve of appropriate diameter or provided with a protective concrete slab over.  The protection to the service shall extend 0.5 metres beyond the top of the bank of the channel and be able to be identified by the way of a concrete marker post on at least one side of the channel at the top of the bank.

C. Outfall structures
All outfalls discharging to a rhine shall have their inverts no lower the predicted water level for a 5 year flow, unless otherwise approved by the Engineer. Pipes shall be sized to limit discharges from the contributing area at a maximum rate of 3 litres/second/hectatre.