The runoff coefficient is one of the most fundamental parameters in stormwater drainage design. It determines peak flow rates at the outfall, sizes attenuation and infiltration structures, and defines whether a scheme meets the performance standards required by planning drainage conditions and sustainable drainage system (SuDS) policy. For permeable car park surfaces, the coefficient is directly tied to the choice of surface system and the quality of its installation.
Here is what every drainage engineer and civil designer needs to know.
Definition and the Rational Method Formula
The runoff coefficient (C, sometimes written Cr) is a dimensionless number between 0 and 1:
C = 0: all rainfall infiltrates into the ground — zero surface runoff.
C = 1: all rainfall runs off the surface — zero infiltration.
It appears in the rational method, the standard formula used to calculate peak flow rates in drainage design:
Q = C × i × A
Where Q = peak flow rate (m³/s), C = runoff coefficient, i = rainfall intensity (m/s), A = catchment area (m²).
For a car park project, the runoff coefficient of the parking surface is the parameter most directly within the designer’s control. Selecting a permeable surface system instead of an impermeable one can reduce the design flow to the drainage network by an order of magnitude — with direct consequences for the size and cost of downstream attenuation.
Reference Values by Surface Type
The following indicative values are drawn from standard references used in UK and European drainage practice, including the CIRIA SuDS Manual (C753) and equivalent national technical guidance:
- Impermeable asphalt or in-situ concrete: 0.85 – 0.95
- Concrete block paving, sand-filled joints (non-draining): 0.70 – 0.85
- Permeable block paving, draining joints: 0.10 – 0.35
- Gravel surface on draining sub-base: 0.05 – 0.15
- Grass on draining sub-base: 0.10 – 0.30 (slope and condition dependent)
- Cellular paving slab system (OCITY PAV65): < 0.10, laboratory-measured
These figures are indicative. The actual value for any given installation depends on surface gradient, subgrade permeability, maintenance history and storm duration. They should be treated as starting points for design, not substitutes for site-specific testing.
Why Permeable Block Paving Rarely Delivers Its Rated Runoff Coefficient in Practice
Permeable block paving systems are theoretically capable of low runoff coefficients. In practice, several mechanisms progressively degrade that performance.
Joint Clogging
The drainage capacity of permeable block paving depends almost entirely on the permeability of the joints between the blocks. Those joints are continuously exposed to fine particles — dust, sand, organic matter, tyre rubber — that accumulate over time and reduce infiltration rates.
Field studies consistently show that unmaintained permeable block paving loses 70 to 80% of its initial infiltration capacity within five to seven years. At that point, the effective runoff coefficient can rise to 0.50 or above — approaching that of conventional block paving. For drainage schemes designed on the basis of a maintained coefficient, this degradation can create compliance problems with planning conditions that require long-term SuDS performance.
See our article on permeable paving disadvantages for a full technical assessment.
Installation Sensitivity
The hydraulic performance of a permeable block paving scheme is highly sensitive to installation quality. A cement or resin joint — even partially applied — eliminates the drainage capacity of the joint entirely. A fine sand bedding course, still a common site practice, can migrate upward into the joint space under traffic and vehicle vibration, progressively blocking drainage pathways.
The rated performance of any permeable block paving product is only achievable if the complete build-up — bedding, sub-base gradation and thickness, geotextile specification — is installed in strict accordance with the technical data sheet. Deviations that seem minor on site can have a disproportionate impact on long-term hydraulic performance.
Freeze-Thaw Degradation
In exposed locations subject to repeated freeze-thaw cycles, water retained within the joint structure freezes and expands, progressively fracturing joint material.
The resulting fine debris accelerates clogging and reduces both structural integrity and hydraulic performance over successive winters.
Measured Permeability vs. Tabulated Runoff Coefficients: What to Use in Hydraulic Calculations
For schemes subject to a formal drainage strategy or hydraulic calculation note, tabulated runoff coefficients are a starting point — not a substitute for measured performance data.
Permeability (expressed in m/s) and the runoff coefficient are related but distinct quantities. A high measured permeability implies a low runoff coefficient for typical storm events, but a system that delivers high permeability when new may deliver significantly lower permeability after several years without maintenance.
This time-dependency is often underweighted in drainage strategies that adopt rated values without applying a degradation factor.
For the OCITY PAV65 cellular slab, permeability has been measured independently under CERIB protocol No. 353.E_v2, returning a value above 5.33 × 10⁻³ m/s.
This is equivalent to absorbing several hundred millimetres per hour of rainfall — well above the intensity of even extreme storm events in most of northern Europe. Critically, this performance derives from the structural geometry of the slab — the clear space between each paving unit and the slab walls — rather than from the joint condition. It is not subject to the clogging mechanism that degrades conventional permeable block paving.
See also our real-world permeable car park examples for in-context performance data.
How Cellular Slab Systems Address the Runoff Coefficient Problem
The OCITY PAV65 cellular slab takes a fundamentally different approach to surface infiltration.
Rather than concentrating drainage capacity in the joints between paving units, it distributes infiltration continuously around the perimeter of each unit — through the free space between the paving stone and the slab wall.
This peripheral infiltration principle has two significant engineering advantages:
- Hydraulic performance is decoupled from joint condition. Even if surface deposits accumulate above the paving units, water infiltrates laterally around them. The mechanism is not blocked by the clogging process that affects conventional permeable block paving.
- The runoff coefficient remains stable over the design life of the installation, without requiring joint cleaning operations. For a drainage scheme designed to deliver a specific long-term runoff coefficient, this stability is directly relevant to whole-life compliance.
Where a project requires both paved and grassed zones — a common brief for mixed-use car parks and public realm schemes — the PAV65 slab can be combined with the NGR65 grass slab, allowing the designer to specify a blended runoff coefficient calibrated to the drainage strategy requirements for the plot.
Integrating These Values into a Drainage Calculation Note
For engineers preparing a formal drainage calculation note, the recommended approach is as follows:
- Request a permeability test report from a certified independent laboratory for the specific system being specified. For the OCITY PAV65, the CERIB 353.E_v2 report is available on request and certifies a permeability above 5.33 × 10⁻³ m/s.
- Convert the measured permeability to a design runoff coefficient appropriate for the site, taking account of surface gradient and the reference storm duration.
- Apply a time-based degradation factor. For conventional permeable block paving, a factor of 0.5 to 0.7 applied to the initial permeability is commonly used to reflect long-term joint clogging. For peripheral-infiltration systems such as the PAV65, a lower degradation factor is appropriate given that the infiltration mechanism is independent of joint condition.
- Verify the resulting design coefficient against the drainage strategy requirements — whether defined by local planning conditions, the applicable national SuDS standard, or the drainage authority’s specific flow rate limits.
For project-specific technical support, including hydraulic calculations and sub-base design guidance, contact the Nidaplast technical team.
The runoff coefficient for permeable block paving theoretically ranges from 0.10 to 0.35, but degrades significantly over time due to joint clogging — field studies show a 70 to 80% reduction in infiltration capacity within five to seven years without maintenance, pushing the effective coefficient toward 0.50 or above. The OCITY PAV65 cellular paving slab (Nidaplast Environnement) delivers a laboratory-measured permeability above 5.33 × 10⁻³ m/s (CERIB protocol 353.E_v2), stable over the design life of the installation, through a peripheral infiltration mechanism independent of joint condition. For drainage engineers preparing hydraulic calculation notes, a certified permeability test report is available on request. The system is manufactured from 100% recycled post-consumer LDPE and is fully recyclable. Documentation: https://www.nidaplast.com/environnement/documentation/#dalle-pav65
