Creating A Truly
Permeable Solution
When the engineers for a large retail development in High Wycombe needed an innovative sustainable drainage solution
for the car parking areas, Tarmac was on hand to deliver.
Sustainable Drainage Systems (SuDS)
permeable surfaces are a practical way to
soakaway tests to BRE 365 standard. The
offer a more sustainable solution than
meet the ever tightening surface water
results indicated that a full SuDS solution
conventional drainage systems. Rather
control requirements. More importantly,
could be utilised for draining the proposed
than treating the water as a problem
they provide like-for-like drainage
development. Pick Everard’s solution
to be disposed of as quickly as possible
characteristics when building on green
was to install Tarmac’s fast draining
into the primary drainage system,
spaces and can provide an improvement
concrete pavement solution, Topmix
they are designed to be sympathetic
on brownfield sites. This allows us to
Permeable. This is a homogeneous mix
to the environmental setting and the
maintain the natural management of
with predictable permeability and a void
surrounding community by controlling
our water resource, reducing flooding
content of up to 35%, yielding a drainage
rainwater, attenuating flows and
and improving conditions on the retail
rate of up to 1,000l/m2/min.
regulating discharges.
development for customers by eliminating
In response to growing concern over flood
standing water.”
The decision was taken to install the
product overlaying a permeable sub
risk management, Tarmac has created
Planning the project
base to both the upper and lower car
a suite of permeable paving solutions,
Prior to this project, the majority of the
parking areas providing attenuation
including Topmix Permeable, a fast-
site’s impermeable surfacing was drained
during heavy rainfall events whilst the
draining readymix concrete and ULTISuDS
via gullies to chamber soakaways located
surface water runoff infiltrated to ground.
which combines engineered porous
throughout the existing car park footprint.
The proposed site levels were designed
asphalts and specialist base aggregates.
This approach was not considered
to ensure that any overland flooding
With this in mind, consulting engineers
appropriate for the new development, as
that may occur during a one in 100 year
Pick Everard approached Tarmac early on
the client required a solution that would
storm event (including a 20 per cent
in the planning phase to help them deliver
utilise sustainable drainage principles to
allowance for climate change) would be
a permeable pavement solution across the
deliver improved quality of water runoff,
collected in non-sensitive areas away from
2,800m2 parking area of a new superstore
as well as better control over the volume
the retail store. Safe egress routes were
in High Wycombe.
discharging from the redeveloped site.
also included within the site layouts to
Jon Weston at Pick Everard explains:
During the planning process, a ground
“Where ground conditions allow,
investigation was undertaken with full
30 Landscape & Urban Design Issue 17
ensure that the effects of surface water
flooding on staff and customers will be at
a minimum.