Drainage is a critical part of every construction project, large or small. Yet all too often the specification of drainage systems, especially at the design stage, is either overlooked altogether or carried out as an afterthought, says Peter Jennings, Technical Director for ACO Building Drainage.

With any construction project, time spent on planning, specifying and the co-ordination of tasks is key to successful implementation. In fact, the built environment is becoming more complex with sophisticated designs and increasing requirements from legislation and standards, providing further challenges for designers and specifiers. Although sometimes out of sight, drainage systems are also an important part of this jigsaw; doing the homework to find a system that is easy to install which functions reliably and effectively will provide long-term cost and time benefits.

Calculating the risks

Any area with impermeable surfaces needs careful drainage design to ensure efficient conveyance of surface water. To achieve this, the capacity of the drainage system must be designed to carry the expected resultant flow at any point in the system to prevent unwanted surcharge or flooding. The design requirements need to consider geographical location, rainfall intensity, storm duration and return period, effective catchment area, risk category of building which in turn will determine the hydraulic capacity of the drainage system. It is normal practice to strike a pragmatic balance between the cost of the drainage system, the risks and the subsequent consequences of flooding.

The UK's varied and complex rainfall patterns must be considered. For example, although the western and northern parts of the UK experience higher average rainfall, more significant in surface water drainage terms is the fact that lowland areas experience more intense shorter duration rainfall. As a guideline for normal situations in England and Wales, the design rainfall intensities of 0.014 litres per second per square metre may be assumed and by applying a complex spatially varied, non-uniform flow theory using ACO CAD software which has been laboratory verified, the parameters for the drainage system are readily determined.

Material specification

As well as flow capacity specification, the designer needs to consider the most appropriate drainage materials that will give maximum durability and performance in use. Depending on the application, building drainage elements can be manufactured from a variety of materials including galvanised and stainless steels as modern fabrication techniques prove to be very cost effective in providing not only the required hydraulic capacity for the installation, but also to satisfy the aesthetic needs of the application. Using state-of-the-art CNC manufacturing processes, it is becoming increasingly common to provide fully curvilinear channel and grating forms to match building contours to satisfy the requirements of architects.

Making the right choice

Choosing a modular design drainage system rather than an equivalent bespoke system can offer many significant advantages during the installation and offers the designer versatility to meet most situations likely to be encountered. As these items can also be sourced readily and quickly, this can reduce costs and avoids delays on site. Various standard off-the-shelf lengths, widths and depths, together with sloping invert options optimise flexibility that can be critical during installation and in meeting project performance commitments.

An aspect the designer must address is load bearing capacity. The prescribed location of the drainage system, for example in precinct areas, may have different load bearing requirements than, say, off-highway applications such as car parks, loading facilities or industrial depots. Here, load classifications in accordance with industry recognised standards provide guidance on the selection of gratings and covers that are suitable for specific applications. The designer should also bear in mind any requirements for emergency vehicle access, particularly in areas where vehicles are not normally present.

Often overlooked by designers are the higher performance drainage products that offer enhanced water flow capabilities through advanced grating geometry, compared with conventional units, as well as security fixings that prevent vandalism and theft. These products can make the difference in long-term performance and maintenance costs.

Design aesthetics can also be an important factor for some applications, with the drainage system working in harmony with the overall design of the construction project, ensuring it remains consistent in style and appearance. This is achieved through the use of a range of high quality material finishes to complement the project style, or bespoke design such as three-dimensional gradient drainage solutions.

The challenges facing drainage

Foremost, drainage systems must be leak-free, durable and above all functional to meet the purpose of the particular application in mind.

Climate change has challenged that need with increased cases of flooding in some areas, while conversely the need for water conservation has never been greater. The introduction of sustainable drainage systems (SUDS) offers a new approach for drainage design that can help minimise flooding risk.

Therefore, it is essential that designers and specifiers of construction projects consider the importance of effective drainage systems that are simple to install, discreet and safe which will have a considerable bearing on saving costs in the short and long term.