Drainage Design from Planning to Construction
Common Sense Designs
Chartered Drainage Engineer Experts in Sustainable Drainage System Design (SuDS) and Highway Drainage Design
We work closely with our clients throughout the entire process: from design outline to construction. Our designs include drainage strategies, tender drainage drawings, detailed drainage drawings, sewer permit applications such as sewer requisition, adoptions and connections, and detailed designs of pumping stations.
Our designs aim to maximize the use of sustainable drainage solutions where use of material and energy is minimized.
We have extensive experience in dealing with water companies and the lead local flood authority standards for the adoption of sewerage systems.
We use the latest WINDES micro-drainage software and 2D modelling of surface water run-off to evaluate the foul and surface water designs.
Detailed Drainage Design Services
Our detailed drainage design services are fully compliant with building regulations, Sewers for Adoption: A Design and Construction Guide for Developers, lead local flood authorities guidance and the CIRIA SuDS Manual.
Our Methodology for Drainage Design
We have worked with developers, contractors, architects and planning consultants to design drainage at different stages of the project. During the planning stages we work with planning consultants and developers to get planning approval. In order to create our designs we work with architects, developers and contractors to produce the best designs for the development.
Our ethos of working in partnerships allow us to incorporate benefits into every stage of the design process.
Obtaining your development’s drawings and levels in mAOD.
We consult the local water company, the lead local drainage authority and the local council and obtain their records to give the development a better chance of being approved.
We scope the site using CCTV surveys, Topographical and service information, and complete an initial model to check the location of the main infrastructure and how it fits within the development.
We work together with multidisciplinary teams to get the most sustainable and cost effective drainage design for your development.
We strive to maintain ongoing partnerships with architects and developers.
Sequential and exemption reports in compliance with the local councils.
Frequently Asked Questions
What is the minimum slope for drainage?
The minimum slope for surface water drainage in a house is: 1:100 (1%) for a 100mm pipe. Foul Sewer drainage is 1:40 with a 100mm pipe for a single connection that carries a flow less than <1l/s (this is usually a combination of a WC, sink and shower).If there are more fittings discharging to the pipe then the slope is 1:80.
For more than one house and industrial areas, the minimum slopes for a pipe should be able to carry the design flows without surcharge and maintain a self-cleansing velocity when carrying the design flow. A self-cleansing velocity is one that is able to carry away any solids within the water and prevent sediment or scaling and eventual blockage.
Foul flows are relatively low and can on most small sites be conveyed in 100mm diameter pipework. It is however considered good practice to provide a minimum size of 150mm for main drainage runs where a number of WC’s may be connected as this helps prevent blockages. The individual branch runs or laterals installed can remain at 100mm diameter. Anything over the flow from 10 houses should connect to a DN150 with a minimum slope of 1:150. If there are less than 10 houses connected the pipe size should be 100mm and have a slope of 1:100. This needs to be agreed in advance with the water companies at the design stage.
How deep should drains be?
The minimum depths for drains without protection should be as follows:
- 0.35m for domestic gardens or pathways without car loading.
- 0.50m for domestic driveways and areas where the gross vehicle weight is less than 7.5 tonnes.
- 0.9m for domestic driveways, parking areas and narrow roads with limited access to gross weight higher than 7.5 Tonnes.
- 0.9m for agricultural land and public spaces.
- 1.2m for other highways and parking areas with unrestricted vehicle access.
What is a drainage structure?
A drainage structure is a structure that aids the movement and management of water. These structures can be manholes, pipes or any sustainable drainage system and outfall.
What surveys and investigations are required for the drainage design?
It is important to get surveys and investigations commissioned at an early stage to provide information to develop the drainage design.
- Topographical Survey – If not already available, this needs to be organized and the surveyors should be asked to record both cover levels and invert levels to any drainage chambers on the site. They should also be asked to plot the routes of drainage runs where possible. Quite often the survey information relating to drainage is limited and it will be necessary for the drainage engineer to carry out their own additional survey.
- Services Information – Details of existing statutory services in and around the site will be needed. This information may be provided as part of the topographical survey or can be provided by the mechanical or electrical engineer. If this is not the case then searches will need to be arranged with the various service providers. It is important to know the positions and depths of existing services to avoid clashes with any new drainage.
- Soil Investigation – A soil investigation is usually carried out for structural engineering purposes but it gives the opportunity to include tests for percolation rates. If soakaways are thought to be an option for surface water disposal then a test to BRE 365 should be requested and if the site requires treated effluent to be discharged to ground, then a test to BS 6297 should be requested. Likewise, if a new road is to be built CBR tests are appropriate.
- Contamination – Some sites may contain contamination and this may affect where drainage is installed or what materials are used. Generally soakaways cannot be installed in contaminated land. The parameters of any investigations for contamination will come under the scope of the environmental engineer. It is critical that both engineers work together closely, as one element leads the other and vice versa.
- CCTV Surveys – Closed circuit television surveys may be needed to ascertain the position or condition of existing drainage. Where information on existing systems is limited it may be necessary to commission a CCTV survey to establish exact routes. If it is the intent to reuse existing drainage as part of a new scheme, the CCTV survey is useful to establish the condition of the drainage and if it is able to provide suitable long term use or if any remedial works should be carried out.
What are the consultations with the water companies?
The initial consultations with the water authorities are:
- Sewer Plans: these must be acquired from the local water authority. An annotated street plan must be provided to indicate the specific area for which the information is required. The results are usually received within a week.
- Capacity Checks: If it is possible that a proposed development will increase flows to a sewer, then it is a requirement to get a capacity check carried out by the local water authority. The process does vary across the different authorities and it usually requires a form to be filled out and returned. These capacity checks can incur a fee from the local water authority.
- It is also important to obtain the baseline discharge flows, as well as to ascertain the potential for the adoption of sections of the sewerage system.
What are the consultations with the Environment Agency?
The Environment Agency are responsible for the many aspects of a development. The common issues relating to drainage design are:
- Consent to discharge: This is an application that is required if sewage treatment and disposal is to take place on the site and necessitates a great deal of information for completion. This will require details such as borehole logs, percolation tests and local environmental information etc. The approval process for a consent to discharge has a statutory limit of 4 months but can often exceed this.
- Light Liquid Separators: Petrol interceptors are needed wherever there is a risk of oil or petrol pollution. They are usually provided as a matter of course in large car parks and where oils or fuels are stored or delivered. Consultation may be needed with the EA in some specific installations. Most small car parks are best treated using source control sustainable drainage methods. A small car park is generally defined as having under 20 spaces.
- Planning Consents: On the majority of projects the EA will be consulted by the local planning authority to gain their input on environmental matters that may be relevant to the development. The Environment Agency’s feedback is usually incorporated into the planning conditions which are not always available to the design team until after the project has begun. It is therefore important to speak to the EA at an early stage to avoid any surprises when the planning conditions are released.
- Flood Risk Activities Permit: If the design involves connecting to a water course or carry out works within 8 metres of a water course then a flood risk activities permit may be needed and the Environment Agency should be consulted to determine if your proposals require a formal application.
What are the design standards used for the design of underground drainage?
- Building Regulations: Part H
- British Standard EN 752 2008: Drain and Sewer Systems Outside Buildings. This will provide information on the design of both small and large scale drainage systems.
- BS 12056 2000: Gravity Drainage Systems Inside Buildings. This document essentially covers above ground drainage systems and contains detailed information on the sizing of soil and waste pipes together with rainwater pipe and gutter sizing.
- Sewers for Adoption: A Design and Construction Guide for Developers: This is a design and construction guide for drainage and pumping stations to be adopted by water authorities. If drainage on your site is to be considered for adoption then it will need to meet the requirements of this document. The local water authority may also have its own additional requirements and this should be checked with them to see if they have a supplementary document.
- The SUDS Manual: This is a very comprehensive document that explores in detail the best practice methods for the design of sustainable drainage systems.