Permeable Paving

Content Table

Permeable Paving

General Overview

The permeable paving allows storm water to percolate through the surface and infiltrate down to the soil below. It can reduce the risk of flooding and overflow in storm water systems when heavy rain falls in an urban environment. Problems with erosion in watercourses and siltation from surface runoff to watercourses can also decrease.

There are several types of material, some of them are:

- Pervious concrete (widely available, can bear frequent traffic)

- Porous asphalt (used on highways)

- Single-sized aggregate (gravel, have no binder, least expensive, used for low-traffic)

- Porous turf (grass, used for occasional parking)

- Open jointed blocks (have openings between the units, can bear heavy traffic)

- Clay pavers

- Grid system (grid of high strength plastic or concrete, filled with soil/gras/gravel)

The technique behind permeable paving consist of different layers of material with membranes between. Three different systems can be used. The first is solely infiltration to the ground and to the ground water. The second system has a drainage pipe at an appropriate level, which lets water who reaches that level to be transported away. This is good for areas which has a poor infiltration capacity or are exposed to frequent, heavy precipitation. The third system has ha impermeable membrane at a certain level in the ground and above lays a pipe that transport the water to another place. This is good for areas with much pollutant on the ground surface. (Paving Expert a, b)

The ground water level, structure of the soil, requirements for pollution control are some factors that can restrict use of permeable paving.

This measure deals with cities that are flooded directly from heavy precipitation. When there is heavy rainfall the water has to be able to be transported away from the streets and city areas and this measure provide a way to transport the water. The measure has the effect of infiltration, storage and transportation.

It can probably take some time to prepare areas and construct the different soil levels and any drainage pipes necessary. When this is done, the measure can be used at once. But then there can be a limit on how much water the system can take. The system without drainage pipe has a limit. Flooding or heavy, intense rainfall over such area can probably fill up the soil and rise the ground water level. The other two systems with drainage pipes can transport water, beneath the surface, to other places when the soil under the pavement fills up.

Pro and Con to use this Measure

Pros
Without permeable paving the water from precipitation fills up the storm water systems. These might be overloaded and lead to water accumulating on the pavements and water pouring out, untreated, to watercourses. With permeable paving the water infiltrates the ground, and makes use of the storage potential that are beneath the surface. (Paver Search)

Pollutants can be stored in the soil or in other material under the pavement and decrease the risk of having pollutants washed of the surface and into sensitive environment. (Davis Langdon)

Trees in urban environment will get more rooting space also air and water can more easily reach the roots.

Depending on which system is used heavy polluted areas, like fuelling stations, can be more or less suitable for permeable paving. The third system, with an impermeable membrane is good in these kinds of areas. Is there a sudden, heavy discharge of pollution, it can be accumulated in the area and be shield of. If a discharge happens when using the two other systems, the pollution can infiltrate down to the ground water.

Foto:Ramsey-Washington Metro Watershed District

Cons
The paving require frequent maintenance since the pores can be blocked by grit and gravel. (CRD) The maintenance is usually done with industrial vacuum or jet-washing. The drainage capacity will reduce as time goes by. Research show that the capacity can be reduced with 10-20% after 3-6 years.For permeable paving with concrete blocks the grit between the blocks will not be a problem, it will only enhance the filtering properties.

Some areas in an urban environment are more likely to have pollutants, like fuelling stations, recycling facilities and so on. These areas are not good places for permeable paving (with permeable membranes deeper in the soil), because of the risk that pollutants reach the ground water.

Porous pavement might not be appropriate to have in areas with high traffic or loading docks. Some companies have however developed concrete paving specifically for areas with heavy traffic, like for industrial activities.

Permeable paving should not be down slope or where there is piped drainage at the foot of buildings. Water from other areas must be dealt with in another way.

Permeable paving in a cold climate can have some drawbacks. Chlorides from road salt can infiltrate down to the ground water. During snow plowing blades can catch block edges and damage the pavement. Sand can plug pores in pervious asphalt and water beneath the surface can freeze and cause frost heave. Through modifications in design, the risk might be reduced.

A crystalline deposit of salt migrates from inside the concrete/masonry to the surface and form insoluble calcium carbonate that harden on the surface. This calcium carbonate can make the surface slippery and miscoloured. The slippery surface can be a big problem for both pedestrians and cars. Moisture makes the formation of calcium carbonate quicker, but there is a chemical that can remove the insoluble coating.

The technique can not be used at one small area only since storm water runoff from close by areas without permeable paving can overflow these areas. The area will not manage to handle storm water from areas close by because the infiltration effect will be overloaded.