Connection to rainwater retention/infiltration:

There is a growing demand to retain and/or infiltrate rainwater on-site, rather than diverting it into the sewer system. This is achieved by combining channel systems with swales, infiltration boxes, or pipe trenches, and, if necessary, incorporating additional components such as separators for purification or flow control devices. Early interdisciplinary coordination between civil engineering, landscape architecture, and water management is advisable. Given the increasing frequency of heavy rainfall events and overloaded sewer networks, decentralized rainwater management is gaining importance. The goal is to store and/or infiltrate rainwater locally to relieve the burden on sewers and watercourses. The "sponge city" concept describes this approach, which reduces flooding and simultaneously improves the urban climate. Drainage channels play a crucial role in this process, collecting water and channeling it to infiltration systems.

A simple decentralized infiltration system consists of three components: surface intake, storage space, and infiltration area. In the context of linear drainage, this means specifically: Inlet via channel: The drainage channel with its inlet boxes collects the runoff from the paved surface and conveys it in a concentrated manner. This already constitutes pretreatment, as dirt and coarse materials collect in the sediment trap of the inlet box. Infiltration line: A pipe leads from the inlet box into a trench. It is also conceivable to have the channel discharge directly into a vegetated swale (via an outlet in the curb). Important: If a separator is necessary (e.g., for oil in parking areas), it must be integrated into this inlet line, usually as a coalescing separator.

A distribution pipe is frequently used in underground infiltration trenches (e.g., gravel trenches or trench blocks). In swale-trench systems, the swale itself handles this distribution.

The core of the system is the storage chamber, where the water is temporarily stored until it infiltrates the ground. This can be a gravel-filled trench (approximately 30% void volume) or a trench block module (approximately 95% void volume). Trench tunnels (arched cavities) are also used; these are known, for example, from MEA's MEASTORM 1600 and offer very high storage volumes. Aboveground swales (basins) further increase the volume and ensure biological purification in the topsoil. The underground trench is lined with geotextile to prevent the ingress of fine soil particles. The water finally infiltrates into the groundwater via the contact surface with the surrounding soil.

Since extreme rainfall events can exceed storage capacity, an overflow is usually provided to direct excess water into the sewer system when the system is full. This can be, for example, an elevated outlet in the inlet shaft that is only activated once the infiltration trench is full. Important prerequisites for infiltration systems are: sufficiently permeable soil (kf value), sufficient distance to the groundwater (at least 1 meter), and water that is as clean as possible. Not all rainwater can be infiltrated untreated; polluted runoff (category II/III according to DWA-A 138-1) must be pre-treated. Therefore, direct infiltration from heavily polluted areas (e.g., areas with high traffic density, industrial sites with pollutant potential) is not permitted! Suitable alternatives include roof surfaces (provided they do not have metal roofing, which can introduce zinc/lead), and in some cases, even residential streets with low traffic and parking lots without significant leakage risks. In summary: Gutter system + infiltration trench = combined system for collection, storage, and infiltration. This relieves the pressure on the canal and water bodies, promotes groundwater regeneration and can even provide a cleaning effect in vegetated depressions (filtration by the soil).

The soil's coefficient of permeability and its water permeability (k_{_f}) are crucial. The k_{_f} value for complete infiltration typically ranges between 1 × 10^{^-3} m/s and 1 × 10^{^-6} m/s. If the k_{_f} value is less than 1 × 10^{^-6} m/s, a drainage system may be necessary. In such cases, partial infiltration is possible. If the k_{_f} value is greater than 1 × 10^{^-3} m/s, infiltration is possible, but additional measures for pollutant retention must be assessed on a case-by-case basis and coordinated with the relevant authorities.

According to DWA-A 138-1, when selecting the design frequency, the potential for damage and resulting impairment from possible flooding must be considered in accordance with DIN EN 752. For determining design frequencies, precipitation events ranging from once every two years to once every ten years are used. For determining flood frequencies, precipitation events ranging from once every ten years to once every fifty years are used. For drainage channels, this means either integrating filters into the inlet boxes (systems with filter cartridges are available) or routing the drainage channel pipe first to a treatment system (e.g., a sedimentation and filter chamber) and from there into the infiltration trench. With the new rainwater treatment guidelines (DWA-A 102), such combinations are becoming state of the art. Many manufacturers offer complete solutions, such as a drainage channel inlet box with a coalescing separator or sedimentation section. Finally, it should be noted that the design of decentralized systems can be complex and requires consideration of numerous factors (keyword: heavy rainfall risk management). For extensive calculations (such as simulations of various rainfall series), specialized software and expert consultation are recommended. In this summary, we focus on the essential parameters that a planner may need as guidelines.