Rainwater does not infiltrate on its own. In densely built-up cities with high levels of soil sealing and overburdened sewer systems, well-designed systems are needed to collect precipitation, store it temporarily, and release it into the ground in a controlled manner. Trough-trench systems combine two principles of decentralized stormwater management into a high-performance unit: surface infiltration in the trough and underground temporary storage and infiltration in the trench pipe. Those who plan trough-and-trench systems are not merely designing drainage technology; they are actively shaping the water balance of urban spaces.
- What trough-drainage systems are, how they work, and how they differ from simple troughs or drainage pipes
- Which hydraulic and soil science principles determine the sizing
- Which standards, regulations, and guidelines provide the framework for planning
- How trough-swales are integrated into various types of open spaces
- What vegetation and substrate requirements apply to systems that function long-term
- How operation and maintenance must be organized to ensure that infiltration capacity is maintained
- What errors occur particularly frequently during planning and construction
- How swale-trench systems are embedded in the overall blue-green infrastructure strategy
Definition and System Logic: What Defines Mulden-Rigolen
A swale-trough system is a combination of a planted or vegetated surface depression and an underlying storage and infiltration structure called a trough. The rigole typically consists of a perforated pipe or a gravel-filled conduit embedded in a water-permeable substrate that collects rainwater, temporarily stores it, and releases it gradually into the ground. Both elements work together hydraulically: The depression handles the initial collection and pre-treatment of the water as well as surface infiltration, while the rigole serves as a buffer and extends the infiltration time when the depression alone is insufficient.
The term “trough-ditch” or “trough-ditch system” is used in German technical literature and in the relevant regulations, particularly in Worksheet DWA-A 138 of the German Association for Water Management, Wastewater and Waste, as a combined system of area infiltration and linear infiltration. The basin alone would be an area infiltration system; the trench alone, a linear infiltration system. It is only this combination that creates a system capable of allowing surface infiltration during light rainfall events, while also absorbing excess water underground during heavier events and releasing it with a time delay. This hydraulic redundancy is the key advantage over simpler, standalone solutions.
Trough-ditch systems must be distinguished from retention basins, which are primarily designed for retention and delayed discharge, as well as from ditches without a trough attachment, which operate exclusively underground. The distinction from infiltration basins is also relevant: basins are larger in size, often unplanted, and designed for higher inflow volumes. Trough-and-trench systems, on the other hand, are small-scale, can be integrated into roadside green spaces, parking lanes, schoolyards, and residential neighborhoods, and are therefore the preferred tool for decentralized stormwater management in urban contexts.
Hydraulic Principles and Sizing
The performance of a trough-trench system depends on three main factors: the design rainfall, the infiltration rate of the subsoil, and the available storage volume. The design rainfall is specified as the precipitation amount for a defined return period; in Germany, this is often defined as rainfall with a two-year return period (r(t,2)) for basic design or with a five- to ten-year return period for increased protection requirements. The corresponding rainfall amounts are taken from the KOSTRA data of the German Weather Service, which provides regionally differentiated precipitation statistics.
The infiltration rate of the subsoil is described by the saturated hydraulic conductivity value kf, measured in meters per second. For swale-trench systems, kf values between 1 × 10⁻⁶ m/s and 1 × 10⁻³ m/s are considered suitable according to DWA-A 138. Soils with very high permeability (kf greater than 1 × 10⁻³ m/s) require special evaluation with regard to groundwater protection, Soils with very low permeability (kf less than 1 × 10⁻⁶ m/s) are generally not suitable for infiltration systems and require alternative drainage concepts. The kf value is determined through field tests, in particular using the double-ring infiltrometer test or pumping tests, not merely by referring to soil type.
The storage volume of the infiltration trench is calculated as the product of the trench’s cross-sectional area, length, and the void ratio of the fill material. With gravel fill, the usable void ratio (pore volume) typically ranges between 30 and 40 percent of the total volume. For plastic infiltration trench bodies consisting of infiltration elements or infiltration trench cassettes, the void ratio can rise to over 90 percent, resulting in a significantly larger storage volume for the same construction volume. The choice of fill material affects not only the hydraulic performance but also the cost, installation depth, and long-term stability of the system.
A key design parameter is the minimum distance from the highest groundwater level as well as from buildings and traffic areas. DWA-A 138 generally requires a minimum distance of one meter between the bottom of the infiltration system and the mean highest groundwater level (MHGW). For buildings with basements, minimum distances of several meters must be maintained; these must be assessed on a case-by-case basis depending on soil conditions and the building’s structural integrity. These distances are not formal requirements but rather physically justified protective measures against foundation saturation and uncontrolled groundwater rise.
Planning Integration: Trough-Type Drainage Channels in Open Spaces
Trough-type swales are most effective when they are integrated from the outset as a design and functional element in open-space planning, rather than as a retrofitted drainage solution at the end of the planning process. In residential neighborhoods, roadside green belts serve as ideal natural locations: The trough is at the level of the adjacent sidewalk or slightly below it, collects road runoff via side inlets or curb cuts, and discharges the water into the ground via the swale. This approach has been tested in numerous German cities and can be found, for example, in Berlin’s “Sponge City” pilot projects as well as in new development areas in Hamburg.
In schoolyards, parks, and public squares, trough-swales can serve multiple functions: they collect rainwater, cool the microclimate through evaporation, provide habitat for insects and small animals, and add spatial structure to the open space. The trough is not merely a technical depression but can be experienced as a design element featuring fluctuating water levels, moisture-loving vegetation, and visible water flow. This experience of water in urban spaces has a unique quality that goes beyond the mere function of drainage and contributes to environmental education.
When planning trough swales in traffic areas, coordination with the road cross-section and traffic safety is crucial. Troughs must not pose a fall hazard to pedestrians, must be passable by vehicles if located near fire department access routes or emergency escape routes, and must remain stable in freezing conditions. The slope gradients of the troughs are generally limited to a maximum of 1:3 to ensure they can be mowed and are accessible on foot. Deeper troughs with steeper slopes require special safety measures or appropriate fencing.
In urban land-use planning, swale-trench systems can be ensured through provisions in the zoning plan, such as by designating areas for infiltration systems in accordance with the Building Code or through provisions for decentralized stormwater management. Water management permits under water law are required in most federal states when infiltration systems of a certain size or larger are constructed, or when they are built in water protection areas. Jurisdictions and threshold values vary considerably between the states, which is why early coordination with the relevant water authorities is essential.
Substrate, Vegetation, and Soil Structure
The vegetation layer in the basin is not merely a decorative addition but a functional component of the system. Plant roots aerate the soil, increase its macroporosity, and thereby permanently improve the infiltration rate. At the same time, plants absorb water and release it into the atmosphere through transpiration, which replenishes the system’s storage capacity between rainfall events. A dense, well-rooted turf composed of species that are both drought-tolerant and able to withstand waterlogging is the basic prerequisite for a long-term functioning depression.
For planting infiltration basins, meadow grasses and herbs that tolerate fluctuating moisture levels have proven effective: species such as Agrostis stolonifera (white bentgrass), Festuca rubra (red fescue), Deschampsia cespitosa (tufted hairgrass), and Poa trivialis (common bluegrass) form robust mixtures that can withstand both short-term flooding and periods of drought. For more aesthetically sophisticated depressions, tall perennials and marsh plants such as Iris pseudacorus (yellow iris), Lythrum salicaria (purple loosestrife), or Carex species (sedges) are suitable options, as they enhance the ecological value and improve the aesthetic appeal.
The substrate structure of the depression follows a defined layering principle. Beneath the vegetation layer lies a filter geotextile or a strip of filter gravel that retains fine particles and slows the clogging of the swale. The swale itself is constructed in a gravel bed or with plastic drainage elements and is lined on the sides and bottom with a water-permeable geotextile that keeps fine material from the surrounding soil out without impeding water flow. An overflow, which directs water in a controlled manner into the sewer system or a downstream infiltration basin when the storage volume is exceeded, is a mandatory component of every system designed in accordance with regulations.
The quality of the substrate on the surface of the basin significantly influences the infiltration rate. Loamy or clayey soils with low permeability require the installation of an infiltration substrate consisting of a defined mixture of sand, gravel, and organic matter that ensures sufficient kf performance. Excessively high humus content can reduce permeability over the long term because organic material releases fine particles as it decomposes. The recommendations of the FLL (Research Association for Landscape Development and Landscape Construction) regarding substrates for infiltration systems provide guidance here, even though specific standard values for trough substrates are less standardized compared to those for green roof substrates.
Operation, Maintenance, and Common Mistakes
Trough-trench systems require more maintenance than conventional channel inlets, but are significantly easier to maintain than is often assumed, provided they have been properly planned and installed. The most important maintenance task is the regular inspection and removal of sediment and fallen leaves that accumulate in the trough and reduce the infiltration rate. Troughs should be checked for silting and clogging of the inlets, especially after heavy rainfall events and in the fall. A maintenance schedule of two to four times a year is realistic and sufficient for most locations.
The vegetation in the trough must be mowed to maintain a dense turf and prevent the growth of woody plants, whose roots could damage the infiltration trench. Woody plant growth in the trough is a common problem that must be addressed consistently and early on. Trees and shrubs have no place in the trough itself; their roots can displace swale pipes, penetrate geotextiles, and permanently damage the filter media. However, trees and shrubs can be planted at an appropriate distance from the trough and benefit from the increased soil moisture in the vicinity of the system.
A common design error is underestimating the storage volume while simultaneously overestimating the infiltration rate. If the kf value of the subsoil is estimated during the planning stage based solely on soil type rather than being measured, the actual infiltration rates may deviate significantly from the assumptions. The result is permanently wet basins that neither infiltrate nor drain, damage to vegetation due to persistent waterlogging, and, in the worst case, waterlogging of adjacent areas. Field measurements of the kf value are therefore not an optional refinement but a fundamental prerequisite for reliable design.
Another common error concerns the inlet design. If road runoff is discharged into the basin unfiltered and at high flow velocity, the vegetation layer at the inlet erodes, and fine particles are carried deep into the filter bed. Impact surfaces made of gravel or natural stone, gently sloped inlet channels, and a sufficient inlet width distribute the inflow and reduce the erosive effect. These details are often neglected in the construction planning, even though they are crucial for the long-term functionality of the system.
The quality of the inflowing water must also be taken into account. Road runoff contains heavy metals, hydrocarbons, and fine particulate matter, which are retained in the filter bed and accumulate over time. The basin acts as a biologically active purification filter: microorganisms in the soil break down organic pollutants, plants absorb nutrients, and the filter media retains particles. For heavily polluted catchment areas, such as major thoroughfares or industrial sites, regulations recommend upstream settling basins or sedimentation chambers that retain coarse particles and suspended solids before the water reaches the swale.
Basin-Trench Systems as Part of Blue-Green Infrastructure
Basin-swale systems are not an end in themselves, but rather building blocks of an overarching strategy for transforming urban water cycles. The “sponge city” model, which is enshrined as a planning goal in numerous German and European municipalities, aims to keep stormwater as close as possible to where it originates—allowing it to infiltrate, evaporate, or be reused—rather than discharging it into the sewer system as quickly as possible. Trough-and-trench systems are a central tool in this concept because they are decentralized, can be implemented over a wide area, and can be integrated into existing open-space structures.
Combining them with other elements of blue-green infrastructure enhances the system’s performance. Green roofs reduce runoff from buildings and slow the inflow to the trough, thereby reducing the design load. Tree swales—that is, infiltration structures located beneath trees—can be connected to basin-swale systems and benefit from the water supply, which makes urban trees significantly more resilient to drought stress. Rainwater harvesting systems that use roof runoff for irrigation or toilet flushing reduce inflow volumes and effectively supplement the infiltration system.
From a climate-adaptive perspective, trough-and-trench systems make a measurable contribution to cooling the urban climate. Infiltrating and evaporating water removes heat from the environment; planted troughs with transpiring vegetation cool their immediate surroundings through latent heat release. During heat waves—which are becoming more frequent and intense due to climate change—this cooling effect should not be underestimated. Studies on urban heat islands consistently show that green and irrigated areas significantly lower surface temperatures in their surroundings, even though the exact effects depend heavily on location, vegetation, and water availability.
Biodiversity also benefits. Infiltration basins with extensive meadow vegetation provide nectar sources for insects, habitat for small animals, and structurally rich transition zones between dry and wet habitats. In a city that is increasingly dependent on ecological connectivity and biodiversity, these areas are not mere leftover spaces but active building blocks of the urban biotope network. Anyone who views swale-trenches merely as a drainage technique underestimates their potential as multifunctional open-space elements that simultaneously manage water, provide cooling, promote vegetation, and create habitat.
Conclusion: Trough-drainage systems as a systematic planning task
Trough-swale systems are technically precise, ecologically effective, and can be seamlessly integrated into the landscape. Their strength lies in the combination of surface and subsurface infiltration, which combines hydraulic safety with the quality of open space. Those who plan them must integrate soil science, hydraulics, vegetation engineering, and open-space design. This is not a weakness of the system, but rather its true strength: it demands interdisciplinary planning and rewards it with long-term functional reliability and high added value for the urban environment.
The fundamentals are well documented in the relevant standards, particularly DWA-A 138. Nevertheless, practical experience shows that errors in site suitability assessments, sizing, and design details occur frequently and jeopardize long-term functionality. Professional planning begins with measuring the kf value, continues with the careful detailing of inlets, substrates, and overflows, and does not end with completion but with a realistic maintenance plan that is integrated into the maintenance planning of the respective municipality or developer.
Trough swales are not a panacea for all urban drainage problems, but they are an indispensable tool in the arsenal of decentralized stormwater management. At a time when heavy rainfall events are on the rise, sewer systems are reaching their capacity limits, and cities are simultaneously suffering from heat and drought, the consistent implementation of such systems is not an option but a planning necessity. Landscape architects, urban planners, and engineers who are proficient in trough-and-trench systems contribute to the city’s resilience, one building block at a time.












