For those of us living around Banderas Bay through the summer, this time of year we are reminded of the powerful force that is water. Our roads, when poorly designed, can imitate ephemeral streambeds that leave gullies in their wake and maintenance without end. Today we will address several principles that can make all the difference when you are creating or re-shaping a road in these parts.
The type of road these principles improve are low-volume, unpaved roads like dirt jungle access roads on hills. However, “proper” paved roads can also benefit from these considerations and are sometimes modifiable.
When rain strikes, plants absorb a large amount of the water volume. But water in excess of this absorption, and in excess of what the soil can hold, runs down slopes in sheets. Water, combined with gravity, wants nothing more than to wash the contents of your road into the ocean. That is how you interpret the sight of a gully: a little message that says ‘this way to the ocean.’
When roads are pitched incorrectly, they actually channel sheet erosion into gully-forming erosion, by concentrating the force of the water. A poorly designed road will accelerate the force of the water as if humans were designing a faster delivery route to the beach. It stands to reason, therefore, that slowing the water down has the opposite effect. This is what we want.
In general, roads in low, flat areas have very bad quality results. You never want a road sited where the full volume of water wants to be: the lowest point. Flash floods and boggy conditions will continually eat away at your road. Even with impermeable surfaces like bitumen, erosion can attack the subsurface, creating air pockets that eventually result in sink holes.
Ideally a road is placed on the contour of a gentle slope, higher than the lowest point. It should be perpendicular to the slope (like the lines on a topography map), which acts like a speed bump to sheet erosion. Think of roads as terraces! A comfortable grade in many circumstances is 10 degrees. But the other important grade to consider is the pitch of the road itself.
Because water follows the path of least resistance, if the grade of the road’s length is steeper than the grade of the road’s width, water will then follow that course, swiftly creating a ravine. If the grade of the width is steeper than the length (causing you to drive at a slight angle), the road will effectively shed the water without concentrating its force. This tactic de-accelerates the destructive potential. A good rule of thumb is that the pitch degree of the width should be double the pitch degree of the length, and at least 5 percent.
Another strategy is to “undulate” the road every 5-10 meters (designing it to rise and fall, usually following natural grade changes). The water that does manage to follow the road is reduced in speed and channeled off the road intentionally at the low points of the undulation. This tactic is also about interrupting momentum.
Channeling water off a road requires attention. Usually an effective method is to install a French drain—a ditch with a perforated pipe and backfilled with gravel. Any perforated pipes should have a sleeve that protected them against getting clogged from sediment build-up. Ultimately water should be dispelled from these pipes onto thickly vegetated zones, boulders, gabion, geotextiled slopes, constructed dams, or any discharge method that will prevent new gullies from forming. Road bases should always be well compacted and, where gravel is chosen, the gravel should be angular and not rounded for greater effectiveness.
Incredible amounts of maintenance can be avoided when proper design and material selection is prioritized. There are many more best practices that you can delve into online. If there is one takeaway notion from all of the above, it’s ‘design roads to dissipate water’s force, not enhance it!’ And enjoy those extra hammock naps where you no longer are overseeing the bulldozer grading your road for the umpteenth time.