Forecasting road conditions on gravel roads

Gravel roads constitute a significant portion of rural transportation networks. In Finland, 27 000 km of state roads and most of the 350 000 km of private roads are gravel roads. Gravel roads are subject to seasonal and weather-induced variations that greatly affect their drivability. The frost layer during winter increases the load-bearing capacity of the road, allowing extra heavy trucks to pass. However, the thawing of ice in spring causes muddy roads that are impassable by heavy vehicles. Information about gravel road conditions and timing of road freezing and thawing is vital for planning timber transportation and is useful for private road users and emergency services as well. Currently used methods for predicting gravel road conditions in Finnish Meteorological Institute (FMI) are coarse and do not account for local variations. However, FMI is in the process of developing a new model for predicting gravel road conditions. The model is based on the FMI’s road weather model RoadSurf that is used on asphalt roads. RoadSurf is one dimensional heat balance model that forecasts road surface temperature and road conditions. The model’s parameters will be adjusted, particularly those related to physical properties like thermal conductivity, to better represent gravel road conditions. The accumulation of snow, ice and water on the road will be also modified. Additionally, the model will be enhanced to account for water flow in the road. The weather during autumn and winter considerably affects the severity of the spring thaw weakening. Rainy autumn and mild winter cause higher water content and slower freezing, which causes more severe spring thaw weakening. On the other hand, fast freezing at the start of the winter usually leads to meager spring thaw weakening. To take into account the long-term variations, the length of the gravel road simulations needs to be considerably longer than when predicting asphalt road conditions. The model will be verified by using observations from stations that measure gravel road temperature and moisture through electric conductivity at different depths.