6. Drainage


Almost all gravel road and forest road structures operate well in dry conditions, with only a few exceptions, such as dust issues. The reason for this is that these roads have been mainly built on dry terrain. Where gravel roads have had to be built on wet terrain, drainage structures have usually been designed to keep the road structures dry.
This chapter will focus on the issues that need to be considered when managing and maintaining drainage on gravel roads and forest roads. More detailed information about the water on roads and drainage systems can be found in ROADEX eLearning.

6.1 General, problems cause by poor drainage

A road drainage system should be designed, built and maintained to remove the water from the road surface, the road structures and also from the surroundings. The system can be broken down into two parts: dewatering and drainage. “Dewatering” means the removal of rainwater from the surface of the road. “Drainage” on the other hand covers all the different infrastructural elements to keep the road structure dry.

On gravel and forest roads, poor drainage creates many problems to both road users and road owners:

a) Traffic safety and drive comfort
A poorly performing gravel road or forest road drainage system can be the cause of a number of traffic safety and drive comfort problems.

Similarly wet and weak road shoulders can fail quickly under heavy loads resulting in truck or bus crashes. These weak areas can arise following road widening and Mode 2 rutting, and be compounded by poor drainage.

b) Erosion
If the surface dewatering system cannot function due to the presence of verges or other obstacles preventing water flowing from the road surface, this may lead to rapid erosion of the road structures during heavy rainfall.

c) Permanent deformation
Permanent deformation under heavy traffic can take place in road sections where road structures become saturated with water due to clogged ditches.

d) Frost heave and spring thaw weakening problems
The formation of segregation ice, i.e. ice lenses, requires unfrozen water to be available within the road structures. When these ice lenses start to thaw the road structures can become saturated with water and exposed to permanent deformations.

6.2. Drainage components

6.2.1. Dewatering, crossfall, road geometry

The dewatering components of gravel and forest roads are normally a) the road surface crossfall, b) the road shoulders and their shape and c) any impermeable road surface materials. In addition road geometry can also have a great effect on a gravel road drainage system.

The most important issue in road dewatering is that the gravel road surface should have a good crossfall gradient of 3-5%, so that rainwater will be able to flow away from the road surface to the ditches and keep the top road structures dry and stiff. Also there should not be any verges in the road shoulders that could block this water flow. All these components have a complex cause- effect relationship. For instance, flattening of the road surface over time is normally accompanied by road widening due to the road and subgrade material being pushed to the side. This can additionally lead to the formation of verges on the road shoulders that further accelerate the process.

Water impermeable road surfaces can be used on gravel roads with higher traffic volumes. On forest roads with less traffic this is not so important.

Finally on hilly areas with steep longitudinal gradients it is especially important to have good crossfall and no verges, otherwise the road surface can be exposed to major erosion.

6.2.2. Side ditches

Gravel or forest road should always have side ditches ensuring that road material stays as dry as possible. The only exceptions to this general rule are those locations where the subgrade soil consists of coarse graded and water permeable materials. If roads are designed and built without side ditches, deformation problems have to be accepted during rain or spring thaw weakening seasons.

The depth of the ditch bottom should about 0.2- 0.3 m deeper than the bottom of the road structure which means that minimum ditch depths should be about 0.4 – 0.6 m deep from the road surface. Side ditches should also have a proper longitudinal gradient to ensure that water does not start to pond on the bottom of the ditch.

6.2.3. Outlet ditches

Outlet ditches are drainage structures that lead the water from the side ditches away from the road area. The water from outlet ditches normally discharges into existing waterway systems, such as river channels and lakes. The outlet ditch is a critical part of a road drainage system but is often ignored. If the outlet is clogged, it can create significant problems to the road over a large area. Outlet ditches are normally located outside the road area with the result that the road owner may not always own the land that they pass through. This can create difficulties in gaining permissions from the landowner when the outlet ditch is clogged and requires re-opening.

6.2.4. Main road culverts

Main road culverts are pipes or boxes (diameter < 2 m) generally used as cross drains for ditch relief and to pass water under a road at natural drainage and stream crossings. If the construction is a large pipe with a clear opening of 2-4 m, the culvert is defined as a pipe bridge. The shape of a culvert is usually a round pipe, but culverts can also be pipe arch, structural arch or box. The shape depends on the site, the required area and the allowable height of soil cover.

In gravel and forest roads main road culverts were earlier made of concrete or steel, but currently most culverts are made of plastic. The benefits of the new culverts are their price and that they can be installed as one continuous “tube”. This eliminates the washout erosion problems, typical for old concrete culverts with joint problems. When installing main road culverts the foundation and transition wedges against differential frost heave should be constructed correctly. Where foundations are weak culverts often bend up from the site and do not act properly.

6.2.5. Access road culverts

The function of access road culverts is to ensure flawless water flow along the side ditches of the road as they pass under any access roads. Access road culverts and their condition is almost as important for gravel road or forest road performance as main road culverts, and many times poorly operating or missing access road culverts cause greater problems to the road than that of the main road culvert.

6.3. Drainage maintenance

6.3.1. General

A common feature of drainage maintenance in gravel and forest roads is that in most countries reactive measures are only taken after clear visual problems are seen on the road that have an obvious connection to poor drainage. At that time the reactive repair costs will always be much higher compared the cost if the maintenance operations had been preventative or proactive.

There is hardly any research made concerning the economic benefits of good drainage maintenance vs. poor maintenance on gravel or forest roads, but if the benefits are clear on paved roads there should be the same trend in gravel roads. Gravel road monitoring projects have shown that, for instance, potholes appear on the same gravel road section even within two weeks after grading or blading. And the same places have continuous problems year after year if the drainage has not been fixed. So fixing the drainage in the problem places must be much cheaper compared to sending a grader regularly to the road to fix only a few problem sections.

But this “knowledge based gravel and forest road maintenance management” requires some technologies to collect objective and repeatable data from the road and drainage performance.

6.3.2. Subjective drainage condition monitoring

Drainage condition has traditionally been monitored visually. The downside of this technique is that it has been made mainly from a moving car and with only detailed observations of those locations with poor drainage condition and any root causes.

Classic visual drainage evaluation has been made in three phases:

Phase 1. mapping the road sections suffering from inadequate drainage
Phase 2. making a basic diagnosis of the drainage problem sites
Phase 3. defining solutions for the problem sites

Visual inspection is still the most common monitoring tool for drainage condition, in spite of it being subjective. This consists of visual inspections of ditches and culverts supplemented by video material, interviews of road owners and maintenance crews.

For gravel roads, drainage condition can be classified into three categories: Class 1 for good and properly working drainage, Class 2 for fair drainage conditions and Class 3 for poor drainage condition or where there is no drainage structures on the road and there is a clear need for them.

The ROADEX description for drainage classification for gravel roads is described in the following.

Class 1: Good Drainage Condition

Description: Faultless drainage. The road cross section shape has preserved its form well and water flow from the pavement to the ditch has no obstacles. Unrestricted water flow in ditches.

Class 2: Adequate Drainage Condition

Description: Small changes in the road cross section shape can appear. The road shoulder has small verges or vegetation that prevents good water flow to the ditch. Vegetation in the ditches restrains water flow and causes dams. A small amount of soil flows from road slopes into the ditches and raises the bottom of the ditch, slows the water flow and raises the groundwater table.

Class 3: Poor Drainage Condition

Description: Deformations and damages are evident in the road cross section. The road shoulder can have a high verge and/or dense vegetation that causes ponding on the traffic lane or on the shoulder. Vegetation in the ditch restrains water flow and causes dams in the ditch. Unstable soil flows from the ditch slopes into the ditches and blocks the water flow. Blocked culvert or outlet ditch prevents the water flow in the ditch. The road does not have drainage system at all.

At the same time as visual drainage classification the condition of any culverts should be also checked.

Culverts can be classified into three classes

Class 1: Culvert operates well; no need for maintenance
• Water flows freely, no adverse remarks

Class 2: Fair operation; culvert needs to be cleaned / flushed out
• 25% or more of diameter of culvert is full of trash
• Culvert has stones or branches preventing the water flow through the culvert
• Vegetation at the ends of culvert prevents the flow of wate

Class 3: Poorly working culvert, need for further investigation and some maintenance operations
• Cracks in the culvert, culvert is broken
• Ends of the culvert have risen up
• Stability problems
• There is need for a culvert, but the culvert is missing

6.3.3. Objective drainage condition monitoring techniques

Over the last 10 years new objective road survey technologies have been developed and some of them, such as laser scanner (lidar) and ground penetrating radar (GPR) techniques as well as 360ᵒ cameras can be successfully used also in drainage monitoring and analysis. These techniques are described in detail in chapter 7 of this eLearning.

In drainage condition data analysis the key parameter is the ditch bottom level compared with the bottom level of the road structure. On roads built over frost or water susceptible soils the ditch bottom should be 0.2-0.3 m deeper that the road structure to ensure that water in the ditches does not infiltrate to the road structures and weaken them. This can be measured using the GPR and laser scanner technique. In gravel roads this depth is not always the case and drainage should be improved at least on those sections with known deformation or frost problems.


Example of the use of laser scanner and GPR data in a drainage analysis of a gravel road in Finland. The top field demonstrates the surface shape of the road by a “rainbow contour map” with links to photos of the road. The second field presents road structure and ditch bottom levels measured from the road surface. The third field presents dielectric value of the road surface, measured using GPR, indicating moisture content and moisture susceptibility of the road surface. The bottom field shows road surface elevation. NOTE: Point the green arrows in the picture to see more information

Another promising technique for the drainage analysis of gravel and forest roads is the use of laser scanner data in road width and cross slope analysis. This is based on the fact that road sections with poor drainage have a tendency to get wider and flatter due to mode 2 rutting. In addition laser scanner remission data can be used to detect sections where water is lying in the bottom of the ditch.


Drainage analysis using laser scanner data. The top field presents remission data from the road together with the position of road shoulders (red lines) and ditch bottoms (green lines). The black colour presents sections where water is lying in the ditch bottom. The middle field presents the surveyed road width compared to the design width. The lowest field presents the cross slope of right and left lane, where the green area presents the optimal cross slope in a straight road (3-5 %).NOTE: Point the green arrows in the picture to see more information

6.3.4. Drainage maintenance operations

Gravel road and forest road maintenance operations should be preventative operations, ie. drainage or dewatering measures should be carried out as early as possible when any issues are detected. This means, for instance, that road crossfalls should always be improved when road surface flattening is detected.

Similarly ditch cleaning should always be carried out when water can be seen lying in the ditch. Ditch filling can be a very slow process and difficult to detect visually. That is why in the future ditch bottoms should be monitored annually using techniques such as laser scanner. Where this is not possible a programme of ditch cleaning should be carried out every 5-8 years, and especially if deformation or spring thaw weakening problems are detected.

During winter sheet ice should be removed if it starts to fill the ditch. If this is not done water can infiltrate into the road structures leading to differential frost heave and spring thaw weakening problems. Additionally main road and access road culverts should be checked before snow starts to melt and any blocked culverts opened up if they are frozen to prevent erosion problems later in the spring.