Hydraulic Drainage Design - Pipes
There are two main categories of drainage:
Surface or Storm water systems which generally discharge untreated into receiving bodies such as rivers and water courses. Surface water includes agricultural, roof or paved areas and highway drainage.
2. Foul water systems that feed into sewage treatment plants. Foul water can be from either domestic or industrial sources.
‘Sewerage’ is the entire system of pipes, manholes, gullies and channels.
‘Sewage’ is the foul water effluent that flows within a sewerage system.
A ‘Sewer’ is the pipeline, either for foul or for surface water.
Storm sewers and foul sewers have been the traditional drainage conveyance method since the 1860s where concrete sewer design has generally been the preferred choice. More recently, an alternative approach to the use of storm sewers has led to the introduction of sustainable drainage systems (SuDS). SuDS principles aim to reduce flood risk, improve surface water quality and provide amenity to an area by mimicking the behaviour of surface water on a developed site as if it had remained undeveloped.
Sewer Design considerations
Similar criteria must be considered with the design of a storm sewer or a foul sewer. These considerations include average and peak flows and their duration gradient, the ranking of the sewer and its environs, the depth of the sewer, any topographical or structural feature, surface characteristics and access to the sewer for maintenance.
Storm Sewer Design
The volume of water can be estimated by applying one of the traditional methods such as the Lloyd- Davies or ‘Rational’ method which was modified by TRRL and widely used in the UK for many years. More recently the Wallingford Procedure was introduced by the Hydraulics Research Station, now HR Wallingford.This incorporates sophisticated computer programs that take into account the catchment geography, predicted rainfall intensity, return period and duration of storms, nature of the soil, percentage of impermeable area (i.e. roads, flags and roofs) and the ranking of the area. The procedure includes a simplified method that can be applied without need to refer to the suite of computer programs.
Foul sewer Design
Traditionally the volume of flow has been calculated using the general rule of thumb equations of 4 x dry weather flow for a new sewer with joints inherently sound or 6 x dry weather flow in the case of a sewer where infiltration might be expected. More recently, domestic flow according to Sewers for Adoption has been based on 4000 litres/unit dwelling/day. Foul sewage from industrial sources should be assessed taking account of the type of use of the property; this should be discussed with the local authority’s planning department to ascertain projected usage and capacity.
There has been extensive research on the comparative roughness - Ks factor - of pipes of different materials. The findings of HR Wallingford suggest that regardless of material a Ks value of 1.5mm should be used for all foul sewers and 0.6mm for storm sewers. These recommendations have been incorporated within Sewers for Adoption and EN 16933-2 as a practical and conservative approach for hydraulic design. Check our factsheet on roughness coefficients to find out more.
Concrete Drainage Design
Concrete pipes to BS EN 1916 and BS 5911-1 readily satisfy these Ks requirements, whether for storm sewers or foul sewers. For self-cleansing properties, the foul sewer must flow at a minimum of 0.75 m/sec at one third of the design flow, the main governing factors being the pipe diameter, the gradient and the volume of effluent. When considering concrete drainage design with larger pipes and flatter gradients, higher flow rates will generally be required to achieve self-cleansing velocity.
Sewer Design methods
The various sewer design methods used in the UK have been Crimp and Bruges, Manning, Hazen-Williams, Colebrook-White, Kutter, Chezy, Bazin and Darcy. In recent years the Colebook-White sewer design method for transitional flow, has been adopted by HRL as the basis for their design tables and has gradually become accepted nationally.
Full details relating to the hydraulic design of pipelines are explained in the The Complete Technical Design Guide.
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