Hydrology and SuDS: The Mathematical Reality of Permeable Resin Systems
For modern urban developments, the management of surface water runoff is a critical engineering challenge. As “hard landscaping” continues to replace natural soil, the risk of “flash flooding” increases.
Resin-bound surfacing is often marketed as SuDS (Sustainable Urban Drainage Systems) compliant. However, compliance is not a property of the resin itself, but a function of the entire hydrological system from the surface to the sub-grade.
1. The Legal Framework: Why SuDS Matters
Since 2015, UK planning legislation has mandated that all new developments must incorporate SuDS to mitigate flood risk. Under the Flood and Water Management Act 2010, failing to prove that a driveway or car park can manage its own water runoff can lead to the refusal of planning permission or costly retrospective enforcement.
2. Calculating the Infiltration Rate
A common mistake among students is confusing “porosity” with “permeability.”
Porosity: The measure of “empty space” (voids) within the material.
Permeability: The rate at which water can actually move through those voids.
The Hydraulic Conductivity of Resin
A high-specification resin-bound system typically offers a drainage capacity of over 50,000 litres/m²/hour. To put this in perspective, even during a “1-in-100-year” storm event in the UK, the rainfall intensity rarely exceeds150mm/hour 150 litres/m².
This means the resin surface itself is almost never the “bottleneck.” The bottleneck occurs in the sub-base.
3. The “Void Ratio” and MOT Type 3
To maintain SuDS compliance, the sub-base must act as a temporary reservoir (attenuation layer).
Why MOT Type 1 Fails
Standard MOT Type 1 contains a high percentage of “fines” (dust and small particles). When water moves through Type 1, these fines migrate and clog the system, essentially turning the sub-base into an impermeable block of concrete over time.
The MOT Type 3 Requirement
For a professional install, MOT Type 3 (open-graded crushed rock) is mandatory.
The Physics: Type 3 aggregate has a reduced fines content, creating a high void ratio (typically 30%).
The Math: For every 100mm of Type 3 sub-base, you have approximately 30mm of “storage space” for water before it even begins to soak into the ground.
4. Attenuation vs. Infiltration
Depending on the local soil’s CBR (California Bearing Ratio) and percolation rate, your system will function in one of two ways:
Full Infiltration: Water passes through the resin and the sub-base directly into the natural water table. (Ideal for sandy soils).
Attenuation (Tanking): In clay-heavy regions like Lancaster and Morecambe, the soil may not absorb water fast enough. In these cases, the sub-base is lined with a geomembrane to “hold” the water, releasing it slowly into the drainage network via a controlled outflow.
5. Mitigating the Urban Heat Island (UHI) Effect
Beyond hydrology, students of environmental science should note the thermal benefits of permeable resin.
Because the system is open-matrix, it allows the ground to “breathe.” As water evaporates from the sub-base through the resin pores, it creates a latent heat of evaporation effect, cooling the local air temperature compared to traditional “black-top” tarmac, which absorbs and radiates heat.
Conclusion: Specifying for the Future
When specifying a resin-bound system, the hydrology must be calculated based on the total catchment area. Simply “laying stones” is not enough; the professional must ensure the hydraulic conductivity of every layer—from the aliphatic binder to the geo-textile membrane—is synchronized.
