Infiltration of rainwater in soil is only possible if the soil allows this. The soil must allow sufficient water to pass through it to allow rainwater to be absorbed by the ground. Soil permeability is expressed as the so-called K value. How is this value determined? When is the K value used? And what can you infer from this value?
What is the K value?
In hydrology, the permeability of the soil is expressed using the K value, also known as permeability or hydraulic conductivity. The K value is expressed in metres per day. The more efficiently water is absorbed, the higher the K value of a material. For instance, sandy soils generally allow water to pass through relatively easily, in contrast to clay or loam. Sandy soils therefore have a considerably higher K value than clay.
What does the K value tell you?
A K value of 1 implies that a raindrop moves 1 metre per day. Fine sand has a K value of between 1 and 10. This means that fine sand allows 1 to 10 metres of water to pass per 24 hour period. The K value not only tells you something about the permeability of the material, but also about the resistance factor. This resistance factor can be vital for the correct functioning of a water management system. To prevent overloading, it may be desirable that water is not passed too rapidly from a buffer to the soil or, during extreme downpours, it can be discharged to surface water or the sewer with a delay.
How is this value determined?
The K value depends on several factors such as the proportion of fine particles and the average particle size of a material. Permeability therefore varies according to location. There are several methods of measuring the K value, e.g. the double ring test in which two concentric rings are stuck into the soil. The speed at which the water in the rings is absorbed is then monitored. For one type of auger hole test, a hole is bored in the ground. By flooding this hole and recording how much water has to be added to keep the water level in the hole constant, it is also possible to calculate the K value.
When is the K value used?
Like the groundwater level, the permeability of the soil type plays a major role in the effectiveness of an infiltration system. Municipalities, water authorities and project developers therefore often carry out permeability tests. The permeability measured can then be used during infiltration system design. A K value of 1 to 2 metres per 24 hours is generally considered the minimal K value for infiltration.
What is Rockflow’s K value?
The Rockflow water infiltration system is part of the ROCKWOOL Group. The Rockflow infiltration system consists of stone wool elements that absorb water and return it to the soil at a measured rate. The K value of these stone wool elements is 50 - 200 metres per 24 hours, depending on the type of stone wool used. The K value of a Rockflow water management system is therefore comparable to that of coarse sand or fine gravel.
What makes Rockflow unique?
One cubic metre of stone wool can absorb no less than 950 litres of water within 10 minutes. Coarse sand and fine gravel have a similar K value, but a far lower storage capacity. The capillary action of Rockflow stone wool elements can be invaluable, for instance during extreme rainfall when excessive rainwater has to be discharged to surface water or into the sewer. By draining the water with a delay, overloading can be avoided.