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.
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| 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:
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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:
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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
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