In conventional drainage systems, rainwater that falls on impervious surfaces is drained away as quickly as possible. The swale-trench system does exactly the opposite: It retains precipitation where it falls, slows its runoff, releases it into the soil in a controlled manner, and thereby relieves the burden on sewer systems, water bodies, and the urban microclimate alike. Anyone who understands this principle understands one of the most effective building blocks of decentralized stormwater management in modern urban planning.

• What a swale-trench system is and how it differs from other drainage concepts
• What are the hydraulic and ecological principles underlying the system
• How basins, swales, and their connecting elements are structurally designed
• Which standards, regulations, and planning guidelines apply in Germany
• Where the system is best applied and what site requirements must be met
• What advantages it offers over conventional sewer systems and where its limitations lie
• How maintenance and operation are ensured in the long term
• What typical planning errors occur and how to avoid them

Definition and Classification: What Is a Trough-Trench System?

A swale-trench system is a decentralized stormwater management system consisting of two functionally distinct but hydraulically connected elements: the above-ground swale and the underground trench. The basin is a shallow, vegetated depression in the ground that collects stormwater from adjacent paved areas, temporarily stores it, and gradually allows it to infiltrate into the soil. The infiltration trench is an underground cavity filled with gravel or plastic media that receives the water transferred from the basin either through downward infiltration or via a lateral overflow. Together, these two elements form a two-stage system that buffers peak runoff, reduces pollutant loads through filtration, and returns the water to the natural water cycle.

The classification of the trough-swale system within the framework of stormwater management is clear: It belongs to the group of decentralized, nature-based drainage methods, which are collectively referred to in the English-speaking world as “Sustainable Urban Drainage Systems” (SUDS) or “Low Impact Development” (LID). In German technical discourse, the term “decentralized stormwater management” is commonly used; it encompasses all methods that treat, store, allow to infiltrate, or allow precipitation to evaporate as close as possible to its source, rather than discharging it into a central sewer system. Within this group, the basin-trench system is one of the most effective and widely used combined solutions because it integrates infiltration, storage, and filtration into a single structure.

Historically, the concept emerged in response to the overload of urban combined sewer systems, which reached their capacity limits in the second half of the 20th century due to increasing land sealing and heavy rainfall events. Research and standardization in Germany, driven primarily by the German Association for Water, Wastewater, and Waste (DWA), systematically developed planning guidelines for such systems beginning in the 1980s. Today, the trough-and-trench system is an integral part of stormwater management in zoning plans, development concepts, and open-space planning.

Hydraulic and Ecological Principles

The hydraulic operating principle of the trough-trench system is based on a combination of retention capacity and infiltration capacity. Retention capacity refers to a system’s ability to temporarily store water and thereby extend the duration of runoff. Infiltration capacity describes the ability of the soil or fill material to absorb water and transfer it to the aquifer. The key parameter for infiltration capacity is the saturated hydraulic conductivity value kf (in meters per second), which describes the permeability of the surrounding soil. DWA Information Sheet M 153 and DWA Worksheet A 138 define minimum requirements for this value: Soils with a kf value below approximately 1 × 10⁻⁶ meters per second are generally considered too impermeable for a pure infiltration system. However, the trough-trench system can also be useful even with lower soil permeability if the trench is operated as a retention structure with a restricted outflow.

The system’s ecological benefits extend far beyond its mere drainage function. Through infiltration, the natural water cycle is replicated: groundwater is replenished, water bodies are protected from hydraulic overload, and the evaporation capacity of the vegetated basin contributes to cooling the urban climate. The latter is of particular importance in the context of climate adaptation, as evaporative cooling produces a measurable cooling effect in urban heat islands. At the same time, the soil and the vegetation system of the trough filter pollutants out of stormwater: heavy metals, hydrocarbons, and fine particulate matter carried into the water from traffic areas are retained through adsorption onto soil particles and biological degradation. However, this purification capacity depends on the specific site and the level of contamination and must be carefully evaluated during the planning phase.

For urban biodiversity, the vegetated depressions of a depression-trench system provide additional habitats. Sites with fluctuating moisture levels—which are periodically flooded and then dry out again—are rare in the urban landscape and ecologically valuable. Adapted plant communities consisting of reed species, sedges, rushes, and meadow perennials can take root here and provide habitat for insects, amphibians, and soil organisms. This multifunctional ecological role makes the trough-swale system a tool that combines drainage technology, open-space design, and nature conservation.

Structural Components: Trough, Swale, and Connecting Elements

The trough is the visible, above-ground element of the system. It is constructed as a shallow depression in the ground, typically ten to thirty centimeters deep, with gently sloping sides to ensure safe access and even water distribution. The trough bottom is constructed using a root-permeable, permeable substrate that provides sufficient filtration capacity without becoming clogged too quickly. Clogging refers to the blockage of the soil surface by fine particles carried in by water, which permanently reduces the infiltration capacity. To slow down clogging, swales are often covered with vegetation consisting of hardy grasses or perennials whose roots keep the soil structure open. The swale is sized so that it can fully accommodate rainfall events up to a defined return period without overflowing.

The swale is located below or to the side of the basin and is separated from it by a filter layer or a geotextile. Geotextiles are water-permeable, synthetic nonwoven fabrics that retain fine particles and protect the interface between the basin substrate and the swale fill. Suitable fill materials include gravel (16/32 or 32/63 millimeters), crushed stone, or plastic drainage media. Plastic drainage elements, also known as drainage tunnel elements or drainage tunnels, offer a significantly higher void ratio than gravel (up to 95 percent compared to about 30 to 35 percent for gravel) and thus considerably reduce the required volume of excavated soil. However, they are more expensive to purchase and require more complex installation.

The connection between the basin and the infiltration trench is established via inlet structures, slits, or perforated pipes that direct water from the basin into the infiltration trench as soon as the basin’s infiltration capacity is exceeded. An emergency overflow, which discharges water in a controlled manner during extreme events, is an indispensable safety feature of every system. This overflow discharges either into a stormwater sewer system, a body of water, or a downstream infiltration system. The infiltration trench itself is equipped with an inspection pipe that allows for maintenance, flushing, and monitoring of the water level. Without this component, proper operation is not possible.

System Variants and Combination Options

In practice, the trough-drain system comes in various configurations. The simplest form is the single-unit system, in which a trough is positioned directly above a drain and collects water from a single area, such as a parking lot or a roof. More complex systems connect multiple basins via an underground swale network that runs in a linear pattern beneath sidewalks, green belts, or parking areas. These linear swales, also known as swale runs, can drain larger catchment areas and can be efficiently integrated, particularly along access roads with accompanying green belts. Another variation involves combining the system with green roofs: the roof handles the initial retention and filtration, while the trough-swale system at ground level handles the second stage. This cascade significantly increases overall performance and is particularly valuable in dense urban settings.

Standards, Regulations, and Planning Guidelines

The planning of swale-trench systems in Germany is based on a clearly defined set of regulations. The central document is DWA Worksheet A 138, “Planning, Construction, and Operation of Systems for Stormwater Infiltration,” which sets forth requirements for site suitability, hydraulic design, construction, and operation. It is supplemented by DWA Information Sheet M 153, “Recommendations for Rainwater Management,” which provides assessment criteria for pollutant loads in stormwater and the resulting treatment requirements. Both documents serve as the binding working basis for planning offices, municipalities, and permitting authorities.

The respective state water law governs water law approvals, as infiltration systems in Germany are considered water use within the meaning of the Water Resources Act (WHG). The requirements vary among the federal states, particularly with regard to the threshold values for small-scale systems exempt from permitting and the documentation requirements to demonstrate that the infiltration poses no harm. In many federal states, there are supplementary decrees or guidelines issued by the environmental ministries that specify how the WHG applies to infiltration systems. Planners must be familiar with these state-specific regulations and coordinate with the relevant water authorities at an early stage.

The system is designed based on design rainfall intensities derived from statistical analyses of precipitation data. The KOSTRA-DWD dataset (Coordinated Regionalization and Analysis of Heavy Rainfall by the German Weather Service) provides site-specific design rainfall values for different return periods and duration classes. The selection of the design recurrence interval—that is, how rarely a rainfall event that overloads the system is statistically permitted to occur—is a planning decision that depends on the use of the drained area and the consequences of an overload. For residential areas, a recurrence interval of two to five years is often used; for more sensitive areas, the interval is correspondingly higher.

Site Requirements, Applications, and Limitations

Not every site is suitable for a swale-trench system. The most important requirement is sufficient soil permeability, which is determined through infiltration tests in accordance with DIN 18130 or through pumping tests. Loamy or clayey soils with very low kf values are unsuitable for pure infiltration systems but can be used as retention systems with restricted outflow. The groundwater level is another exclusion criterion: A sufficient distance must be maintained between the bottom of the trench and the highest groundwater level to prevent backflow and contamination of the groundwater. DWA Worksheet A 138 specifies concrete minimum distances for this purpose.

The mulden-rigole system has a wide range of applications. In residential areas, roadside greenery and front yards are used as basins to collect roof and street runoff. In commercial areas, large-scale trough-and-ditch systems drain parking lots and operational areas, although the pollutant load in the runoff requires special attention. In public spaces—such as parks, school grounds, or along city streets—swale-trench systems are increasingly being integrated as a design element that combines drainage functionality with the quality of the public space. Particularly in new development areas, where infrastructure planning can be geared toward decentralized drainage from the outset, swale-trench systems can be integrated in a way that is both cost-effective and functionally optimal.

The system’s limitations arise where the pollutant load in stormwater is so high that uncontrolled infiltration would endanger the groundwater. DWA Information Sheet M 153 classifies areas according to their runoff load and defines the pollution class at which pretreatment of the water is required before it may be allowed to infiltrate. Heavily trafficked roads, gas stations, transshipment sites for water-polluting substances, and certain industrial sites either require an upstream treatment plant or are ineligible for direct infiltration. Special or complete restrictions on infiltration systems also apply in Zone I and II water protection areas.

Maintenance, Operation, and Common Design Errors

A trough-trench system is not a maintenance-free structure. Its long-term functionality depends crucially on regular and proper maintenance. The trough must be cleared of sediment that accumulates from fine particles carried in by stormwater and reduces infiltration capacity. A visual inspection is recommended after every major rainfall event, as well as a thorough cleaning at least once a year. The vegetation in the trough must be maintained in such a way that it keeps the soil structure open without damaging the geotextile layer through excessive leaf fall or root pressure. Avoid planting trees or shrubs in the immediate vicinity of the infiltration trench, as roots can damage the fill material and the piping.

The infiltration trench itself should be regularly inspected via the inspection pipes for blockages, settlement, and changes in fill level. Flushing the trench can restore functionality if infiltration performance declines, provided that clogging has not progressed too far. In practice, it has been shown that systems planned from the outset with a well-thought-out operational concept remain functional significantly longer than those in which the operational phase was neglected during planning.

One of the most common planning errors is underestimating the sediment load in the inflow. If runoff from roads or parking lots is discharged directly into the basin without pretreatment, the basin surface will become clogged within a few years. A simple sand trap or an activated soil filter as a pretreatment stage can significantly extend the system’s service life. Another mistake is failing to adequately account for the emergency overflow: If this is missing or undersized, extreme events can lead to uncontrolled flooding that endangers neighboring buildings or traffic areas. Finally, the importance of catchment area boundaries is often underestimated: If subsequent development or land-use changes cause more water to enter the system than was originally designed for, the system will fail hydraulically.

The Trough-Trench System as a Building Block of the Climate-Resilient City

The trough-and-trench system is more than just a technical drainage structure. It is a tool that restores urban water cycles, relieves pressure on sewer networks, replenishes groundwater, and simultaneously creates open spaces. At a time when heavy rainfall events are becoming more frequent and cities are seeking climate adaptation strategies, the principle of decentralized stormwater management—which this system embodies—is gaining in importance. Municipalities that consistently rely on trough-and-swale systems not only reduce the costs of sewer expansion and wastewater treatment plant capacity but also create open spaces that are ecologically sound and attractive places to spend time.

Integration into urban planning is most successful when the trough-and-swale system is conceived not as an afterthought but as a structuring element of the development. Street spaces planned from the outset with accompanying infiltration basins are not only functionally superior but also more cohesive in design than streets into which drainage elements are added later. Landscape architects, urban planners, and civil engineers must work closely together on this, because the system lies at the intersection of open space, traffic areas, and drainage technology.

The future of the swale-trench system lies in its further development into multifunctional blue-green infrastructure that combines water retention, evaporative cooling, biodiversity promotion, and quality of life within an integrated concept. Cities such as Berlin, Hamburg, and Munich have already incorporated guidelines into their urban development strategies and zoning plans that establish decentralized stormwater management as the standard. The trough-and-trench system is not the only tool available to planners, but it is one of the most proven and versatile. Those who apply it correctly help shape a cityscape that does not treat water as waste, but rather views it as a resource.