As you’ll remember from last issue,  porosity is the total volume of space that can store water in a substance like soil or rock. Permeability is the rate at which water moves through the soil or rock.

Have you noticed how long is spent dumping and grading vast amounts of gravel in road or building construction? Now you know why. Gravel has low porosity and high permeability, so it drains water away from building walls faster than other materials. Outside, underground drains and weeping drains are required to be set into a bed of gravel to ensure proper drainage.

We know that when the water table rises even gravel can become saturated with water if it has no way to drain out. So you will frequently see drainage ports on the sides of roads to do just that.  If a stream crosses the road, it is redirected into a culvert under the road to separate the water from the road bed. 

If the water cannot escape and the ground becomes saturated, there is significant risk of underground collapse.  The gravel roadbed can suddenly wash away from under the paved surface and leave it hanging in mid-air. 

Our changing rainfall patterns, due in part to global climate change, challenge current  roadway construction methods.  Culverts that met codes years ago may no longer be sufficient to drain through the sudden large downpours of rain that we are seeing more frequently.  When the drain becomes overwhelmed with runoff and cannot separate the water from the roadbed the water will back up and saturate the ground on either side of the culvert.  Sometimes you will see a culvert where the surrounding gravel has been washed away; once this happens the culvert piping is in danger of collapse.

Once the water has filled all the space in the gravel road bed, you have perfect case for a slide of rock and gravel from underneath the paved road, washing out on the downstream side. Radio reports after a heavy rain support this when they talk about the number of slides and washouts on a highway route.  Road builders are trying to decrease this event.

Larger culverts are expensive and not the only way to handle this type of event.

We know from our other discussions that an open stream bed can expand the volume of water it carries much more than an enclosed pipe. We know that gravel or similar substrate with a high permeability will effectively drain away water. Therefore an open stream bed can drain away water much faster than a culvert when a ”gulley washer” incident occurs.  Next time you drive on Highway 14 take look at the sidings of the newest roadways.   On the side of the road look for open “stream beds” of large rocks and gravel being utilised to keep water from pooling up and saturating the road bed.  The high permeability of these gravel stream beds will keep the water away from the road.