Guidance for Developers
Internal Drainage Boards and Development Control
The Black Sluice Internal Drainage Board (the Board) is an independent authority constituted under the 1930 Land Drainage Act, with duties "to exercise a general supervision over all matters relating to the drainage of land within it's district".
The Board acts as a non-statutory consultee to Local Planning Authorities, but importantly the Board has it's own statutory powers with respect to drainage which also determines how and if a development may proceed. The Board's current powers derive from the Land Drainage Act 1991.
How the Board Appraises Developments
The following factors are considered by the Board when appraising proposed developments:
- Rainfall Run-off and Development Contributions
- Disposal of Foul or Dirty Water
- Discharge Outfalls
- Access to Watercourses
- Filling in or Culverting Watercourses
- Property Floor Levels
- Site Ground Level
- Environment and Biodiversity
Detailed guidance for each of these factors follows - emboldened sentences indicate the Board has statutory powers.
Application forms and other leaflets referred to may be obtained from the Board's offices (address at end) or the consent forms page.
Section 1 - Rainfall Run-off and Development Contributions
The Board's consent is required to increase the rate of run-off from a development.
Where possible sustainable methods of disposal should be used which do not adversely affect existing surface water management, nor adversely expose people or property to an increased risk of flooding. In most instances sustainable disposal will best be achieved by dealing with rainfall run-off at or as near as possible to source using Sustainable Drainage Systems (SuDS). For example
- Rainwater recycling
- Soakaways, Infiltration areas and Swales
- Filter drains and porous pavements
- Attenuation or balancing ponds
Soakaways and infiltration systems should be designed and proved with a percolation test in accordance with BRE Digest 365 or other approved code.
A Development Contribution shall be payable to the Board for any discharge from the site above the green field rate.
The development contributions (1/4/2007-31/3/2008) for un-regulated flows are:
- £3.21 +VAT per impervious square metre of development (£32,100/ha)
or - £500+VAT for a single property (no charge for soakaways)
If the flow is regulated or attenuated then the above charge is proportioned in accordance with the flowing flow rates:
- Green field rate: 1.4 litres/second/hectare (zero charge)
- Impervious area rate: 100 litres/second/hectare (full charge)
Where the discharge is via a third party system such as a private watercourse or Anglian Water surface water sewer then their permission is also required.
The one-off Development Charge covers the cost to the Board of improving the general drainage infrastructure to accommodate the increased flows. However the Board cannot guarantee to accept any water if it is unfeasible to increase the capacity of the existing system. Where localised off site works are required to a Board or a private watercourse then the improvement must be approved by the Board and paid for by the developer in addition to the development contribution. The Board may be prepared to carry out the work using it's powers under the Land Drainage Act.
Section 2 - Disposal of Foul or Dirty Water
The Board's consent is required to discharge any water into any surface water system.
Foul or dirty water, including water from vehicle wash downs, shall not be discharged directly to a surface water system. Where a separate foul water system, i.e. Anglian Water, is not provided then the water shall be treated before disposal.
If a developer wishes to make a discharge into a watercourse within the Board's district he will need to apply to both the Board and the Environment Agency (the EA) for consent:
- From the Board, to allow an increase in flow into the drainage system.*
- From the EA, who will agree the quality of the water to be discharged.
Septic tanks shall not discharge directly to a watercourse but to a soakaway system. If septic tanks are not approved by the EA then a package treatment unit will be required; the Board recommends that, unless there is a 600mm freeboard from the unit outlet invert to normal water level, the unit should have a pumped discharge.
Section 3 - Discharge Outfalls
The Board's consent is required before any structure is placed in a Board watercourse.
All outfalls shall have a suitable headwall to protect the banks from erosion. No part of the headwall unit shall protrude beyond the profile of the bank in order that flails and weed cutting machinery is not obstructed. Suitable scour protection shall be placed below and/or in front of the headwall if necessary. Details of a suitable headwall can be obtained from the Board alternatively the structure shown in "Sewers for Adoption 6th Edition Figure2.1" will be acceptable.
Outfall connections into pipe systems shall preferably be to a manhole, the use of a proprietary saddle connector may be permitted.
Section 4 - Access to Watercourses
No obstructions shall be placed within 9m of the edge of a Board Watercourse.
The Board's Byelaw states:
No person without the previous consent of the Board shall erect any building or structure, whether temporary or permanent, or plant any tree, shrub, willow or similar growth within 9 metres of the landward toe of the bank where there is an embankment or wall or within 9 metres of the top of the batter where there is no embankment or wall, or where the watercourse is enclosed within 9 metres of the enclosing structure
The Board has a separate policy leaflet regarding this byelaw.
Access to and maintenance of all other watercourses or piped systems (not vested with any authority) is the responsibility of the riparian owners i.e. the land owners on either bank.
Developers should take into account the future maintenance of riparian or private watercourses and piped systems when designing the site layout ; access may be required for weed cutting excavators or for jetting equipment for pipe systems.
Developers shall inform purchasers of the presence of a Board Watercourse and/or their responsibilities relating to a riparian watercourse.
Section 5 - Filling in or Culverting Watercourses
The Board's consent is required before any Board Watercourse or riparian/private watercourse is culverted, filled in or otherwise obstructed.
The Board considers that it is beneficial for watercourses to remain open wherever possible for both drainage and environmental purposes. Culverting or filling destroys wildlife habitats, damages a natural amenity and interrupts the continuity of the linear habitat of a watercourse. It can also remove functional flood plain storage and therefore increase the risk of flooding.
Access culverts will normally be consented, but in other instances consent will only be given if a benefit can be shown and/or mitigating environmental works and flood storage can be provided elsewhere.
The Board has a policy leaflet regarding the culverting of Board Watercourses.
Section 6 - Property Floor Levels
The Board may make recommendations to the Planning Authority in respect of good practice in relation to flood risk and land drainage.
Planning Policy Statement No 25 (PPS25) states that: "site layout and surface water drainage systems should cope with events that exceed the design capacity of the system so that water can be safely stored or conveyed from the site without adverse impact"
Sewers for Adoption specifies that site rainfall runoff systems should be designed not to flood any part of the site in a 1:30 year (3.3%) event.
The Board recommends that no property should flood in a 1:100 year (1%) site specific event, therefore flood storage above a 1:30 year event may need to be provided in areas such as roads, parking, open space etc. In addition the Board may recommend a minimum floor level based on recorded flooding or catchment modelling. Other authorities or insurance companies may require a higher standard of protection or floor level.
Section 7 - Site Ground Level
The Board recommends that the ground level of the site should not be raised above the level of neighbouring land unless it can be shown that it will not :
- Obstruct overland surface water flow from neighbouring land
- Cause surface water to flow overland off the site onto neighbouring land
- Raise the subsurface water table causing water logging of neighbouring land
In general if ground levels are raised above surrounding land then interceptor infiltration drains (French Drains) will need to be installed around the site boundary. The future responsibility (including maintenance) of such drains shall be private.
Section 8 - Environment and Biodiversity
It is the Board's statutory duty, when considering whether to issue consent, to take into account any likely adverse effect on the environment.
The Board is a signatory to the Lincolnshire Biodiversity Action Plan (BAP) and welcomes opportunities to work in partnership with developers to carry out environmental improvements on Board Watercourses.