SuDS in London - a guide

4. Attenuation in tree trenches 5. Discharge to watercourse
6. Surface water sewer outfall
4. Attenuate rainwater by storing in tanks,
sealed water features, permeable paving
and tree trenches for gradual release:
Although these systems can be configured
to suit a variety of below-ground conditions,
carefully consider utilities to avoid damaging
long-term tree-rooting potential. Surface
water can potentially be re-used in tree
trench planting.
5. Discharge rainwater directly to a
watercourse: this can offer a low-cost
option for surface water dispersal,
provided the surface water is pollutantfree.
Liaise closely with water body
authorities as they may put limits on how
much water can be discharged into the
conveyancing system.
6. Discharge rainwater to a surface water
sewer/drain: this has traditionally been
London’s default approach to rainwater
management. However, London’s sewer
network is so intertwined with the foul
network that there is a need to segregate
conveyance systems to minimise
contamination and effluent
treatment costs.
7. Discharge rainwater to the combined
sewer: the economic justification needs to
be set exceptionally high to mitigate the
commercial demand for such a choice. In
retrofit scenarios, the combined sewer may
be the only option. If so, control of discharge
rate and water quality will be critical.
7. Combined sewer outfail
130 6 Implementation