Sediment transport in storm sewers
Literature survey made in this research indicated that
the roughness has a significant effect on the design of a
self-cleansing sewer. A conceptual model was therefore
developed taking into account the effect of roughness in the
sediment transport in sewers. This model was later tested
against experimental data obtained from a test rig. The rig
had a pipe which was specially fabricated in two halves so
that it could be divided to coat the interior with uniform
sand grains to provide roughness.
The experimental data shows a good correlation with the
model developed. Two relationships i. e. for smooth and rough
pipes, were derived from the results. The effects of
volumetric sediment concentration, pipe diameter, sediment
size and effective roughness on these relationships were in turn examined. Head loss formulae acquired by past
researchers were slightly modified to suit the range covered
by the experimental data.
The relationships developed in this research were
subsequently applied to sewer design. These were later
compared to the criterion of 0.76 m/s proposed by the
British Standards. In the case of smooth pipes, it shows conclusively that the criterion of 0.76 m/s produces
excessive slopes for pipe diameter up to 1.0 metre. However,
for rough pipes the criterion gives insufficient slopes to
maintain a self-cleansing sewer. This is valid up to a pipe
diameter of 0.3 metre. Beyond this diameter the criterion
gives high slopes.
Comparisons were also made with studies in the wider
field of sediment transport on fixed bed. These have set in
context the proposed formulae for smooth and rough pipes in
the design of storm sewers.