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Flood-proof paving

As a lifelong resident of a water-scarce region, I never complain about the rain. However, living in a city, one is often conscious of storm-water when it causes flooding, or even pooling in the carpark exactly in an arc around your car! For several years, I have been part of a study group on permeable paving, a Nature-based Solution to urban storm-water, headed up by Prof Neil Armitage of UCT.

Permeable paving solution
Creative commons licensed: https://laturfandpaver.com/shop/turf-block/

Most Nature-based Solutions use vegetation: many of us are familiar with constructed wetlands, green roofs, swales and rain gardens, where the planting does much of the work in absorbing the rainwater, trapping it in its roots and directing any water that is not needed down into the sub-surface "groundwater". This contrasts with conventional storm-water engineering, where the rainwater is encouraged to flow away as quickly as possible, using impermeable sloping surfaces (roads, paving, parking areas) to concrete channels and pipes that discharge into natural water courses. In this conventional approach, no water percolates into the groundwater, disrupting the natural cycle, and the discharged water is usually concentrated into a point-source, causing stress on the receiving natural water course.

Permeable paving aims to address these two problems: groundwater recharge and the reduction of point discharge. The idea is that water flows over the paved surface but is allowed to percolate through the paving, which is often laid on a bed of stone to create a temporary water storage layer. If the natural soil is permeable (sandy or porous rock), the stored water will gradually percolate down to the water table. If it is impermeable, either clay soil or impervious rock, the rock sub-base may be criss-crossed with a network of perforated pipes that channel the water to a conventional storm-water outlet or a natural water body. The time delay for the water to move through the paving, down through the stone layer and into the pipes, means that the water only reaches its destination long after the storm has passed and any floodwater has subsided.

There are three main approaches to permeable paving, depending on the function of the paved space. The one that allows the greatest permeability is vegetated - specially designed concrete or clay paver blocks that have spaces in them large enough to take planting, usually a hardy ground-cover or low-growing succulents.

Vegetated permeable paving
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This system is only really feasible for very low traffic, so for domestic use or for office parking where one occupant will use the space for the whole day, to minimise driving over the planting. In this system, the vegetated paving is often reserved for the actual parking bays, with the driveways and roadways using one of the other systems.

The most common approach is  to use interlocking concrete pavers that have small gaps designed in, creating small pockets that are filled with graded stone - the permeable component. These are highly engineered, so are strong enough to be used for roadways with low-volume, slow-speed traffic. They are also great for parking areas for malls and other uses where there is a high changeover of parking-bay usage.

Permeable Interlocking Concrete Pavers (PICP)
Creative commons licensed: https://lawnking.co.za/6-reasons-to-repave-your-business-parking-lot/

Another approach which is being implemented on higher-grade roads, is to make the asphalt or concrete of a conventional road porous. This is a technique used for many years for other storm-water applications called "no fines". Normal asphalt or concrete has sand (fine aggregate) and stone (coarse aggregate) to provide strength and consistency. With "no fines" the small aggregate is left out, creating a porous layer that allows water through.

A demonstration of permeable asphalt
Creative commons licensed: https://upload.wikimedia.org/wikipedia/commons/b/b7/Demonstration_experiment_of_the_permeable_paving_%282012.10.07%29.jpg

Often, permeable paving will look like any other conventional approach, whether interlocking blocks or porous asphalt or concrete. We only really notice when it is pouring with rain - the permeable paving has no standing water or pooling, allowing us to cross the paved area with relatively dry shoes and with the reassurance that we are doing all we can for the environment.  

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