Planning of surface drainage with channel systems

When planning a drainage system, both mechanical and hydraulic load criteria must be considered. The former determine the necessary design/class of the drainage channel, the latter the required size (nominal size, number of outlets, etc.). In addition, local conditions (terrain slope, connection options) and increasingly ecological aspects (infiltration, retention) also play a role. The procedure is explained step by step below.

Determining the traffic load: First, it must be determined which load class the channels will be subjected to. EN 1433 defines six load classes: A 15, B 125, C 250, D 400, E 600, and F 900. The numbers indicate the test force in kN (e.g., D 400 = 400 kN ~ 40 tons). Typical applications for these classes are summarized here: Class A 15, test load 15 kN: Areas used exclusively by pedestrians and cyclists, as well as green spaces. Class B 125, test load 125 kN: Sidewalks, pedestrian zones, terraces, private driveways, and parking areas. Class C 250, test load 250 kN: Applications: Public areas with light traffic, curb areas, unpaved shoulders, and car parks. Class D 400, test load 400 kN. Application: Roadways, parking areas for all types of road vehicles. Note: On expressways, drainage systems of class D400 may only be used in the verge area; they are not permitted for cross traffic. Class E 600, test load 600 kN. Application: Industrial and logistics areas with high wheel loads. Class F 900, test load 900 kN. Application: Areas with extremely high wheel loads, e.g., airport aprons, container terminals, airfields, racetracks. Principle: The channel must be able to withstand the traffic loads occurring at the installation site permanently without damage. If an area is driven on by heavy vehicles, at least class D or higher must be selected. If there is any uncertainty, it is better to choose a higher class (safety margin). This should be taken into account, especially if areas are also occasionally driven over by heavier vehicles (e.g., fire trucks, garbage trucks in residential streets). In Germany, DIN 19580 was previously the relevant standard, requiring proof of testing through the aforementioned load test. With the introduction of EN 1433, this has been harmonized: all CE-marked drainage channels carry a classification from A 15 to F 900. Public tenders often explicitly require: "Drainage channel class XY according to DIN EN 1433". In cases of doubt (mixed traffic), it is advisable to consult the REwS 2021: this standard describes drainage channel requirements in detail, depending on road class and traffic density, and recommends, for example, using E 600 instead of D 400 in parking and rest areas with high point loads. Summary Step 1: Determine the load case → define the load class → select the appropriate channel type within this class (choose a higher-rated class if the intended use is unclear or if a safety margin is desired).

Terrain and elevation: The local topography significantly influences the planning. First, it must be determined where the water should ultimately go (connection to a sewer? On-site infiltration?) and where the lowest points are located. Drainage systems must be arranged so that the slope of the terrain is utilized or created by the drainage channel's own gradient. On level terrain (e.g., a large parking lot without a natural slope), sufficient crossfall to the drainage channel must be ensured. Furthermore, sufficient outlets must be provided to prevent excessively long flow paths without drainage (see dimensioning). Fig. 19 (a) shows level terrain with a central drainage channel; here, the inlet boxes should be positioned centrally under the surface (a2), or the surface can be divided and drainage to both sides. On sloping terrain (e.g., roads on hillsides), the water naturally flows downhill. Here, the slope (combination of longitudinal and transverse gradients) must be given particular attention. Is the road, for example, a slope of 15° or 15°? For example, if the channel slopes 5% longitudinally and 3% transversely inwards, the resulting flow direction is oblique to the curb. In such cases, channels along the roadside should ideally have a continuous slope (using stepped slope elements if necessary), otherwise water could pool on the downhill side of the road. Inlet boxes can be installed either laterally at the lowest point (if the channel is located at the roadside) or, if the main channel is in the center of the road, central outlets (double boxes with connections on both sides) can be used. Constraints: There are often structural requirements, such as existing sewer connections at fixed heights. In such cases, a channel system must be designed so that the last channel or box has the required connection height. With long, continuous channel runs, there is a risk that the connecting pipe will not have sufficient depth if the slope is too shallow. If necessary, multiple channel runs with multiple outlets must be provided (see hydraulics). Constraints can also include junctions, property entrances, or structural obstacles, requiring the drainage system to be interrupted or adapted at these points (e.g., a lowered curb at a driveway). Lengths and segment numbers: Manufacturers supply channels with segmented markings, especially for gradient systems. The elements are often numbered (e.g., 1 to 20 for 20 m with a 0.5% gradient) to ensure they are installed in the correct sequence. For the planner, this means: create a drainage plan, determine the length of the run, and then specify the corresponding number of elements (or sets, if necessary). For very long drainage runs (e.g., a runway with a 300 m channel), multiple runs will be necessary, as the elevation difference would otherwise be too great or the gradient impossible to maintain.