Commenting on the hydraulic parameters of the sewage drainage pipes

In this post, I will analyse the reasons why the hydraulic parameters are demanded on the design codes for the underground drainage, either for the foul and storm sewage. In other words, I will look for the fundamental purpose of those hydraulic parameters. I will focus on Scottish Water design parameters, comparing its requirements with different recommendations, codes of practice and guides. However, I am not entering in the mathematical analysis; there is enough bibliography available.

The hydraulic design of sewers entails finding out the pipe section and gradient. I would remark two variables that determine the hydraulic design of the sewage: the design flow and the relation section-slope-velocity. The design flow is the primary input of the net. On the other hand, the slope, the section and the flow velocity are parameters that the engineer can play with to transport all the sewage to the discharge point.

Until the 80’ in Scotland, foul sewage generated from domestic and industrial uses were mixed with the surface water (rainfall water) into a single or combined pipe. However, environmental and flood management considerations demanded to separate out in two different systems the foul sewage from the surface water. Nowadays, it is not usual finding a combined sewer system, so here we will be focus only on separate sewage systems.

Source: Scottish Water (https://www.scottishwater.co.uk/)

Design flow estimation

Foul line

The total foul flows would be the total discharges produced by domestic and industrial uses. Hypothetically, the wastage flow consumption is roughly the same as the water consumptions. However, for industries, it could not be accurate if water is added to the final product. The peak hours of consumption will be similar as well as the discharges to the sewage. Also, when estimating the foul discharges to the drainage system, the potential growth of the city or industries in the area must be considerate. So, the quantity of flow depends on the rate of water supply (consumptions) and population growth.

In the codes of practices (BS EN 752, BS EN 12056-2 and Sewers for Scotland v4.0) provide chads with average values of discharge according to the source of water consumption.

Storm line.

When rainfall takes place a part of it infiltrates into the ground surface, and the remaining part flows over the ground surface. The part of rainwater flowing over the ground surface (or runoff) needs to be drained through the sewers; otherwise, the entire area would be flooded.

The quantity of stormwater (or rainwater) that will reach sewers depends on intensity and duration of rainfall, characteristics of the catchment or drainage area such as its shape, imperviousness, topography, and the time required for the flow to reach the sewer.

The intensity and the duration of rainfall are determined according to the rainfall history of the area. In Scotland, the standard considered is 1 in 30 years’ period event of rainfall (return period or recurrence interval). That means the drainage has to be capable to drain the maximum event (storm) that could happen every 30 years’ time (average time or an estimated average time between events such as high-intensity storms).

The 1 in 30-year rainfall level of service could be increased if the flood risk assessment of the area demands it. According to the SEPA, in Scotland checks shall be made for the 1-in-200 year return period considering allowances for climate change and potentially new urban developments to ensure that properties on and off site are protected against flooding for all these possible scenarios. The design of the site layout or the drainage system shall be modified where the required flood protection is not achieved.

The characteristics of the catchment area due to its capacity to prevent flooding problems have an important repercussion in urban design. As said before, the rainfall than infiltrates into the ground surface do not produce runoff, so, the total runoff would be higher in impermeable surfaces such as roads, house roofs or concrete sidewalks than in a forest or green areas. As a consequence, the new urban designs trend is to avoid extended impermeable areas locating more green spots and infiltration systems.

For estimating the stormwater flow for the design of sewers, the following two methods are commonly adopted:

• Rational method: this method is useful for large catchments areas.
• Empirical formulae: The use of the rational formula for estimating the stormwater flow for the design of sewers is usually limited to small catchment areas or drainage areas, say up to about 400 hectares.

Hydraulic requirements: relation gradient-velocity

The sewers design (foul and storm) usually tends to optimize the excavation volume adapting the gradient of the pipelines to the ground level. However, the pipes transport the water by gravity, so the pipes should be laid at a continuous gradient in the downward direction to the discharge point under hydraulic requirements. Therefore, the pipes’ gradient is limited; subsequently, the pipes cannot always adapt to the ground level. Sometimes it is impossible to avoid deep excavations, pump stations to lift the water or backdrops when the pipes cannot face a steep slope.

The sewage pipe hydraulic requirements are usually related to accommodating a finite volume of sewage under certain velocity limits, which are commented as follows.

The sewers contain considerable particles in suspension. The heavier particles could settle down at the bottom of the pipe if the flow velocity is low. The deposition of sediments could result in the blockage of the pipe. In order to avoid silting of sewers, the laid sewers need to have a gradient that ensures a flow velocity capable of cleaning the sediment at different possible discharges. Therefore, the hydraulic design looks to provide a flow velocity to generate an automatic self-cleansing effect.

On the other hand, the maximum velocity is limited to avoid erosion or scouring problems due to the friction between the pipe and the suspended solids in the flow. Also, high flow velocities damage junctions and potentially generate leaking problems. The non-scouring velocity depends on the material of the pipe.

As follows, I will comment on the limitations imposed on the storm and foul sewers:

Foul sewers

The design requirements for foul gravity according to Scottish Water are as follows:

Section 2C - 2.23. Hydraulic design – Foul sewers. Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, 2018.

Surprisingly, Scottish Water do not consider any maximum velocity for foul sewers. It only mentions that energy dissipation and safety measures may be required without further guidance than the requirement of bedding arrangement for gradients steeper than 1 in 10. As an example of other guidances, the maximum velocity has a value between 3 and 6 m/s depending on the conditions and local authority in Spain.

Also, it is remarkable that no matters the material of the pipe, the roughness parameter of the pipe is set in 1,5 mm for the calculations. This criterion is based on extensive research into the behaviour of drains and sewers in service (BS EN 752:2017).

Lastly, in some norms ask the designers that sewers (storm and foul) should be designed to run at 75% of pipe full conditions (Water UK, Canal Isabel II), while I couldn’t find any mention on it in the Scottish Guide.

Storm sewers

The hydraulic design parameters for storm sewers according to Scottish Water are as follows:

Section 2B - 2.7.2. Sewers. Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, 2018.

The guidance set the minimum velocity of 1 m/s at pipe-full flow for self-clearing reasons, with the possibility to be reduced to 0.3 m/s if the discharges received by the pipeline are free of sediments. Other ways to ensure the pipe self-clearing in storm sewers could be checking the minimum velocity is archived for a reduced return period. In Spain, it is usually checked that the minimum velocity is higher than 0.6 m/s for a 2-year event, so that, the drainage system is expected to be cleaned at least one time every two years.

As well as for the foul sewage, Scottish water set the hydraulic roughness value for all pipe materials (0.6 mm). As said before, that value is established based on extensive research into the behaviour of drains and sewers in service (BS EN 752:2017).

Also, like the foul sewage, it is required by several norms that the drainage system should be designed to run at 75% of pipe at full flow conditions (Water UK, Canal Isabel II). In this case, the 1 in 30-year rainfall should not overcome the 75% of the pipe capacity.

References and useful related information:

• Scottish Water, Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, Version 4.0 – October 2018.
• Water UK, Sewers for Adoption – A Design and Construction Guide for Developers, Eighth edition – August 2018.
• Sustainable Urban Drainage Scottish Working Party (SUDSWP), Water Assessment and Drainage Assessment Guide, 2016
• CIRIA, the SuDS Manual C753, London 2015
• Canal de Isabel II, Normas para redes de saneaminto – Version 2, 2016
• BS EN 752:2017, Drain and sewer systems outside buildings. Sewer system management, March 2018

Setting-out pipes

When laying pipes, it is critical to ensure the right position since the beginning because a small error in the gradient, level and direction could be unacceptable at the end of the line pipe.

Setting out is time-consuming, but it is common knowledge that site engineers are often rushed when setting out to give all the information to the squad to start laying pipes and run to the next job. Quality and safety should not be compromised for the program, but it is widely known that in construction, the productivity is in everyone's minds, time is money. Here I will try to compare several methods of setting out pipes analysing the pros and cons to meet the three goals mentioned previously: time, quality and safety. For this kind of task, the cost difference between methodologies is predominantly determined by the time needed to perform it. Therefore, the pure cost analysis has been overlooked.

Traditionally, the pipes have been set out placing pegs (position), and profiles (highs) along the pipeline with an offset from the centreline of the line (around 3m). The profiles and the pegs describe a reference line needed by the squad to start digging the track and laying pipes.

Layout, profile and section of the traditional setting out method (BM Sadgrove, 2007)

The availability of beam or line laser levels on sites gave more control, precision and productivity to these jobs. When laying pipes, this device creates a laser beam that gives a straight line at whatever slope needed that the squad uses as a reference. So once the laser is set, the construction process is monitored continuously increasing the accuracy and productivity of this task. The line laser levels, as expected, substituted the traditional straight line and also generated new methodologies. Then, the issue with this new technology is the method to put in place the laser at the start of the line with the right gradient, level and direction.

Pipe laser positioning (BM Sadgrove, 2007)

The laser could be set using the profiles and pegs of the traditional method. However, the information given by the profiles has to be transferred to the bottom of the excavation (usually 2-4 m depth and an offset of 3m), losing precision. It entails the acceptance of wide margins of tolerance. The increasing demand for high-quality standards implied that a site engineer (with a total station) join the squad to set the laser on the bottom of the track. Notice that the sitting out process, in this case, is a critical task in terms of time because the squad cannot start laying pipes until the laser is set. Also, it demands the presence of a site engineer when required.

From my professional experience, I have been applying two methods which I describe and compare next.

1. Setting out the first concrete pipe

The first method consists of setting out the first pipe. Being the first pipe on position and buried to ensure it doesn't move, the laser can be placed inside of the pipe, giving the required baseline. So, setting out the pipe is key, which is tedious, especially with large and heavy concrete pipes. Also with this method, the direction of the pipe could be compromised because the two control points are at the beginning and the end of the pipe which usually is only 2.5-3 meters long for a full pipe or 0,6-1m long when setting out the stub and rocket.

The first step for this method is to level the gravel bed where the pipe will lay. It is a good practice to level the bed a bit high (around 10mm) because when placing the pipe, this margin probably is lost (especially with heavy concrete pipes). Next, the pipe has to be positioned in the right direction, checking that the two extremes of the pipe are inside the reference line. The following step is to check the levels in both ends to make sure that the pipe has the right gradient on the correct elevation. If the pipe is low, the gravel bed has to be risen, going back to the first step. If the pipe is high, the pipe could be "shacked” or pushed down until meeting the desired position. It is an interactive process; it could be the possibility after levelling the pipe that the direction has been compromised, which means repeating some of the previous steps.

After the first pipe is on place, the laser is placed inside the pipe with the desired slope programmed. The end of the pipe could be taken as a reference point to point the laser in the right direction. Still, it is highly recommendable that the site engineer goes as far as possible along the reference line to give another point to ensure the direction of the laser beam. Despite being accurate laying the first pipe, it is recommended to double-check the line after 3 or 4 pipes.

In terms of security, sometimes when checking the position and highs of the control points the site engineer hast to stand on the pipe with the measurement tools (prism and controller), which can be risky when laying big diameter pipes.

2. Setting out the laser in a concrete block

In this method, the laser is placed on a concrete block at the beginning of the line, usually in the centre of the manhole. As well as the previous method, the site engineer with a total station will join the squad to set the laser.

First, the concert block must be positioned. The top of the concrete block (or log) has to be on the invert level and it roughly needs to be in the right direction. The direction accuracy is archived when positioning the laser on the block. The second step is to set the laser on the block with the gradient desired. Next, the site engineer will give a reference point on the block and another reference along the line to set the direction. In order to minimise errors, it is recommended to provide the second reference point as far as possible. It is also recommendable doing periodic checks along the line. Once the first pipe is on place, the laser could be paced inside the pipe, and the concrete block retired.

Site engineer setting out the laser on a concrete block

Conclusion

After putting into practice these two methods, setting out a concrete block has been proved to be less tedious and less time-consuming than setting out the first pipe. Additionally, as a consequence of this time optimisation, the site engineer has more time to do other tasks.

In terms of quality, the concrete block method is more simple and straightforward than the rest of the methods described. It offers more accuracy, mainly because the process needs fewer adjustments. It is essential to mention that the site engineer with this method has more control over the process. He knows exactly the level of the block where the laser is placed, so the gradient could be adjusted to meet the endpoint at the right level.

Despite setting the laser on a concrete block seems to be the proper method to lay pipes, setting out the first pipe could be the only solution in narrow places without space to place the concrete block.

The main drawback of the two methods exposed is that require a site engineer with a total station when the squad needs to start laying a pipeline. In contrast, the traditional approach the setting out could be independent of the task of laying pipes.

References:
- BM Sadgrove (2007). Setting-out procedures for the modern built environment.
 (C709) CIRIA, London.