This research project is focused on the mechanisms of road drainage during intense precipitation. By means of rainfall experiments and specific measurements, information is yielded to verify and validate simulations performed with a two-dimensional model. The results are used to evaluate rules of road drainage.
The objective of the project is to provide a verification and validation of two-dimensional model simulations according to depth of water on roads during intense rainfall. Furter the project has following objectives:
- collection of information on hydraulic roughness of different pavements
- collection of information on the susceptibility of aquaplaning on different pavements
- evaluation of rules for testing the capacity of inlets
The methodology presumes the following elements:
- Analysis authoritative test parameters such as longitudinal and transverse gradients, curve radii, cross-downslope alternately, crests, troughs, roughness of the surface layer and geometry of the grids of the mud collectors and the rain intensities (intensities in accordance with standard and hydroplaning-related precipitation events)
- Development of an experimental concept for efficient test execution
- Search of favorable trial sites regarding feasibility, the decisive test parameters and an efficient implementation
- Comparison of measured and calculated using a computerized 2D model water gauges
- Analysis of the watercourse at the edge termination and the inflow of waste water into the street sludge collectors
The Swiss norms for the drainage of roads have been developed based on the report "Discharge estimation from sealed und unsealed areas and catchments" (VSS, 2000). The norms consider the processes on the surface of the road in great detail. This calls for an evaluation of the generally used formulas to estimate flows on the surface of roads and a better understanding of the efficiency of the water intakes along the road.
To the purpose, sprinkling experiments have been made on three highway sections. On each location, two types of experiments were performed. The flow on the street surface has been observed and flow depths, flow velocity, time to peak and drainage times measured.
The data was used to check commonly used formulas and estimate water depth as function of rainfall intensity, slope, and surface roughness. This allowed the assessment of the retention volume and the risk of aquaplaning.
The second set of experiments considered the water flow along the edge of the road, especially during extreme precipitation. Of special interest was the capacity of the water intakes and how much water was flowing around these intakes. Additional experiments were made with dry leaves in the water to simulate the clogging of the intakes.
Although the flow processes on the surface were complex, the formula of Strickler (eng. Manning; Dracos, 1987) with calibrated roughness coefficients produced acceptable results. The resulting water depths were smaller than assumed in the original report (VSS, 2000). The risk of aquaplaning is usually small, however, it cannot be neglected when the slopes approach zero. The Strickler formula produced also acceptable results for the flow along the edges of the road. However, the estimated roughness coefficients were much higher than the one used for the surface of the road (ks= 92 instead of 40). The intake capacities were not limiting, even for large discharges. More important were width and velocity distribution of the water flowing along the edge.
At the location Birmensdorf more than 20 l/s could enter the intake, due to the large longitudinal and cross slopes. In Mt Russelin, a site with small longitudinal slopes, less than half that amount could be taken.
The results of this study allow a meaningful update of the existing norms. However, a decision on the priorities has to be taken, whether the number of intakes along the road should be minimised, the width of the water flow constrained or the water retention on