advanced drainage system - Polypipe

General Properties 3.4
Minimum Gradients
Sedimentation reduces a pipe systems hydraulic capacity, increases pollution concentration and in extreme cases
may lead to partial or complete blockages. Minimum pipe gradients are therefore specified to ensure the pipe flow
regularly achieves a ‘self-cleansing’ velocity, limiting long-term sedimentation.
A velocity flow of 0.75m/s is typically used as the minimum self-cleansing velocity in system design. However, certain
applications are required to comply with particular codes of practice.
• Non-adoptable drainage, within a properties curtilage, is defined by The Building Regulations. Prescribing specific
minimum pipe gradients for small diameter sewers.
Sewer systems proposed for adoption should be designed in accordance with Sewers For Adoption (6th Edition). It
specifies that foul sewers should achieve a minimum flow velocity of 0.75m/s at one-third design flow and surface
water sewers 1.0m/s at pipe full flow.
Design Example
Pipe Size Determination
A pipeline is to carry a discharge of 0.015 cubic metres per second (15 litres per second) when laid at a gradient
of 1 in 180.
Data: Design discharge (Q) = 0.015m 3 /s
Pipe gradient (S) = 1 in 180
Roughness height (ks)
= 0.6mm
Using Figure 3.3.3 for ks = 0.6mm, find the intersection point of a discharge of 0.015m 3 s -1 on the left hand scale and
a gradient of 1 in 180 on the bottom scale. The intersection point yields the following:
Pipe size selected
Discharge capacity
= 225mm
= 0.038m 3 /s at full bore flow
Using a rearrangement of the continuity equation,
Velocity =
Q
A
Velocity
= 0.038m 3 /s
0.040m 2
= 0.95m 3 /s at full bore flow (above the self-cleansing velocity of 0.75m/s
As the discharge capacity of the pipe exceeds the design discharge, the flow velocity should be checked.
Step 1: Calculate the proportional discharge.
This is the ratio of the design discharge to the full bore flow discharge capacity.
Proportional discharge
=
0.015m 3 /s
0.038m 3 /s
= 0.39
Step 2: Using Figure 3.4.1, the proportional discharge curve (read on the bottom scale) yields a proportional depth
of 0.44 (read on the left scale). A proportional depth of 0.44 yields a proportional velocity of 0.95.
Step 3: Calculate the velocity of the design discharge by multiplying the full bore flow velocity by the
proportional velocity.
0.95 x 0.95m/s = 0.90m/s
64
Therefore, the velocity of flow at the design discharge exceeds the self-cleansing velocity.