Allowing rainwater to seep into the ground instead of directing it into the sewer system closes a cycle that has been disrupted for decades in densely populated cities. Infiltration is the decentralized return of stormwater to the natural water cycle: it replenishes groundwater, cools urban surfaces, relieves pressure on the sewer system, and lays the foundation for thriving vegetation. Those who understand the physics, planning principles, and technical possibilities of infiltration have a tool at their disposal that, in the context of climate change and soil sealing, ranks among the most important instruments of urban development.

• What infiltration means in physical terms and how it affects the water cycle
• What types of infiltration systems exist and how they differ in planning and construction
• Which soil parameters, standards, and regulations are relevant for design
• How infiltration is incorporated into urban land-use planning and water law
• What role infiltration plays in the context of the “sponge city” and blue-green infrastructure
• When infiltration is not possible or not practical, and what alternatives exist
• What mistakes frequently occur in planning, construction, and operation, and how they can be avoided
• How infiltration systems can be integrated into open spaces without compromising their function

What infiltration is: definition, operating principle, and significance for water management

Infiltration refers to the process by which water penetrates from the ground surface or from near-surface layers into the soil, where it is stored or sinks further into deeper layers until it reaches the groundwater. In technical terminology, a distinction is made between natural infiltration, which occurs on unpaved surfaces as part of the hydrological cycle, and technically controlled infiltration of stormwater, which is planned and constructed in the context of decentralized stormwater management. The latter is the subject of this article, even though both forms follow the same physical laws.

The water balance of a natural landscape distributes precipitation across three pathways: evaporation, surface runoff, and infiltration. In a near-natural watershed, depending on soil type, vegetation, and topography, a significant portion of precipitation infiltrates directly at the point of impact. In densely built-up urban areas with a high degree of soil sealing, this proportion is drastically reduced. Asphalt, concrete, and compacted soils channel water as surface runoff into the sewer system, which quickly becomes overloaded during heavy rainfall events. Groundwater is no longer replenished, evaporation decreases, and the temperature of urban surfaces rises. Thus, infiltration is not merely a technical measure for drainage but also contributes to restoring a disrupted natural balance.

From a water management perspective, the infiltration of stormwater closes the local water cycle. Groundwater recharge—that is, the replenishment of the groundwater table through infiltrating water—is threatened in many regions by increasing impervious surfaces and changing precipitation patterns. Decentralized infiltration systems make a measurable contribution to groundwater recharge, even if this contribution may seem small in individual cases. When the effects of many such measures are combined across an urban area, the contribution is significant. At the same time, infiltration reduces runoff in combined and separate sewer systems, which lowers the frequency of sewer overflows and associated flooding.

Fundamentals of Soil Physics: Permeability, Infiltration Resistance, and Soil Type

The key parameter for any infiltration planning is the permeability coefficient kf, also known as hydraulic conductivity. It describes the volume of water flowing through a defined cross-sectional area of soil per unit of time and is expressed in meters per second (m/s). The kf value varies by many orders of magnitude depending on soil type: coarse gravel reaches values of 10 to the power of minus 2 m/s and higher, medium-grained sands are around 10 to the power of minus 4 m/s, silty soils are around 10 to the power of minus 6 m/s, and cohesive clays can have values below 10 to the power of minus 8 m/s. For the engineered infiltration of stormwater, soils with kf values between approximately 10 to the power of minus 3 and 10 to the power of minus 6 m/s are considered suitable. Soils with higher values infiltrate too quickly, which poses a groundwater protection problem when dealing with contaminated water; soils with lower values infiltrate too slowly for a cost-effective system.

The kf value is determined as part of the site investigation, either through laboratory tests on collected soil samples or through field tests such as the double-ring infiltrometer test or the pumping test. For planning purposes, guideline values from soil maps and site investigation reports are often used, but these must be verified by on-site measurements. Particularly in heterogeneous urban soils characterized by fill, contaminated sites, and past land uses, the actual kf value deviates significantly from guideline values. A thorough site investigation is therefore not an optional supplement but a prerequisite for any serious infiltration planning.

In addition to the kf value, the groundwater table depth is of central importance. Groundwater table depth refers to the vertical distance between the ground surface and the mean or highest groundwater level. For infiltration, a sufficient safety margin must be maintained between the bottom of the infiltration system and the highest expected groundwater level; relevant regulations typically specify this as at least one meter. This distance ensures that the unsaturated soil zone can act as a natural filter and that the groundwater is not directly affected by the system. In areas with high groundwater levels, infiltration is therefore often not possible or only possible to a limited extent.

Infiltration Systems: Types, Design, and Sizing

The practice of decentralized stormwater infiltration involves a range of system types that differ in their geometry, storage capacity, and suitability for various sites and catchment areas. The authoritative German regulatory framework for the planning, construction, and operation of infiltration systems is DWA Worksheet A 138 of the German Association for Water, Wastewater, and Waste (DWA). It defines system types, specifies design procedures, and sets requirements for the substrate, distance to the groundwater, and catchment area characteristics.

Surface Infiltration

Surface infiltration is the simplest and most natural form. Stormwater is distributed over a permeable, vegetated area and infiltrates evenly there. Typical applications include vegetated depressions, lawns, meadows, or water-permeable surfaces. Surface infiltration requires no complex structures, is low-maintenance, and simultaneously offers additional ecological benefits through the vegetation. However, it is suitable only for areas with sufficiently permeable subsoil and is limited to absorbing relatively small amounts of water.

Basin Infiltration

Infiltration basins are shallow, vegetated depressions in the ground that temporarily store stormwater and release it into the soil through the basin floor and slopes. They are among the most commonly used forms of infiltration in open spaces and can be easily integrated into green spaces, parks, and residential neighborhoods. The basin also serves as a retention space that temporarily stores peak runoff and releases it with a time delay. DWA Worksheet A 138 provides detailed specifications for calculating the basin depth, base width, and slope gradient. It is crucial that the trough can drain completely within 24 to 48 hours after a rainfall event so that it is ready to receive water again for the next event.

Swales and Pipe-Swale Systems

A swale is an underground storage space filled with coarse-grained gravel or plastic media that collects stormwater and allows it to infiltrate through the surrounding soil layer. Drainage trenches are used where there is a lack of above-ground space or where an underground solution is preferred for aesthetic or functional reasons. Pipe-trench systems combine a drainage pipe with a surrounding gravel bed and allow for even distribution of water along the entire length of the system. These systems are widely used in practice because they can be installed beneath paths, parking lots, and green spaces without interfering with surface use. Their disadvantage lies in the limited ability to monitor and clean them: blockages caused by fine sediment or root ingrowth are difficult to detect and costly to remedy.

Infiltration Wells

Infiltration shafts are vertical structures that direct stormwater directly into deeper, permeable soil layers. They are particularly suitable where the upper soil layers are poorly permeable, but layers with good infiltration capacity are present at depth. Due to the lack of a filtration path through the unsaturated soil zone, infiltration wells are critical from a groundwater protection perspective and, according to DWA A 138, may only be used for water with low pollution levels. They are generally not suitable for street drains or areas with heavy traffic.

Design According to DWA A 138

The design of all infiltration systems follows the principle that the system must be able to accommodate the design rainfall event and allow it to infiltrate completely within a defined time. Precipitation frequency statistics serve as the basis for design; in Germany, these are typically the KOSTRA-DWD data from the German Weather Service, which provide rainfall amounts for various duration levels and return periods. The system is designed for a specific return period, often five or ten years. To calculate the required storage volume, the soil’s kf value, the connected impervious area, the runoff coefficient of the catchment area, and the system’s geometry are taken into account. DWA A 138 provides a simplified design method as well as a more detailed simulation method for this purpose.

Legal Framework: Water Law, Urban Land-Use Planning, and Permitting

The infiltration of stormwater is regulated by water law in Germany. The federal Water Resources Act (WHG) provides the overarching framework; state water laws specify the requirements at the state level, leading to significant regional differences. In many federal states, the infiltration of uncontaminated stormwater from roof surfaces and similar areas is classified as a use of groundwater that does not require a permit, provided certain conditions are met. For larger systems or areas with potential pollutant contamination, a permit under water law is required. The competent authority is generally the lower water authority of the respective county or independent city.

In urban land-use planning, the municipality can establish requirements for stormwater infiltration through the zoning plan. Based on the Building Code (BauGB), municipalities may require that stormwater be infiltrated on the property, thereby excluding or limiting discharge into the sewer system. Such requirements are common in new development areas and are often accompanied by the condition that proof of the soil’s infiltration capacity must be provided. For planners, this means that the site’s infiltration capacity must be assessed early in the planning phases, as it significantly influences the development concept and land use.

Quality requirements for the water to be infiltrated are another legal consideration. Stormwater runoff from roof surfaces made of uncoated metals such as copper or zinc may contain elevated concentrations of heavy metals and is not readily infiltratable. Water from heavily trafficked areas contains hydrocarbons, heavy metals, and other pollutants that must be retained before infiltration. For such areas, regulations require pretreatment measures, such as sedimentation tanks, light-liquid separators, or activated soil filters. The issue of water quality is therefore an integral part of any infiltration planning and must not be deferred to the construction planning phase.

Infiltration in the Context of the Sponge City and Blue-Green Infrastructure

The concept of the Sponge City describes an urban structure that absorbs, stores, purifies, and releases stormwater gradually—like a sponge—rather than draining it away quickly. Infiltration is one of the central strategies of this concept, alongside retention, evaporation, and utilization. The Sponge City responds to the dual challenge of climate change: more frequent and intense heavy rainfall events on the one hand, and longer dry spells and heat waves on the other. Both extremes require a different relationship between the city and water than that established by the traditional drainage technology of the 19th and 20th centuries.

Blue-green infrastructure combines water infrastructure (blue) with green infrastructure (green) to form multifunctional systems. Infiltration basins, which are designed to double as green spaces, play areas, or recreational spaces, are a classic example. Tree trenches along streets, in which stormwater from the roadway is channeled via inlet structures into the root zones of urban trees, combine infiltration with tree irrigation and evaporative cooling. Such systems have been tested and documented in Scandinavian cities, in Berlin, Hamburg, and Munich, as well as in numerous other European metropolises. They demonstrate that infiltration is not merely a technical issue, but rather a design challenge that brings together landscape architecture, civil engineering, and urban planning.

The effectiveness of blue-green infrastructure on the urban climate is well documented. Permeable surfaces that store water and release it again through evaporation measurably cool their surroundings. This effect is particularly valuable in dense urban neighborhoods, where impervious surfaces and a lack of vegetation contribute to the formation of urban heat islands. Infiltration is thus not only a tool for water management but also a contribution to climate adaptation, which is increasingly recognized as such in open-space and urban planning and is being incorporated into planning documents.

Limitations of Infiltration: When It Is Not Possible or Not Practical

Infiltration is not a universal panacea. There are sites and situations where it is technically impossible, legally prohibited, or ecologically unsound. The most important exclusion criteria include a groundwater table that is too close to the surface, a soil kf value that is too low, excessive contamination of stormwater or the soil, and locations within water protection areas where special protection requirements apply.

In areas suspected of being contaminated or where soil contamination has been confirmed, infiltration must always be carefully evaluated. The infiltrating water can mobilize contaminants and discharge them into the groundwater. In such cases, either extensive remediation measures or alternative drainage concepts are required. Infiltration is also problematic in areas with very shallow soils over bedrock or with highly swellable clays, as the water cannot be absorbed and may lead to waterlogging, landslides, or damage to buildings.

If infiltration is not possible at the point of precipitation, other components of decentralized stormwater management are available: retention with controlled discharge, rainwater harvesting for irrigation or toilet flushing, green roofs as retention and evaporation areas, or discharge into a water body. In practice, these measures are often combined, with infiltration used where possible and other strategies handling the remaining water volumes.

Common Errors in the Planning, Construction, and Operation of Infiltration Systems

In practice, infiltration systems rarely fail due to the basic concept itself, but often due to errors in execution or operation. A classic planning error is sizing the system based on soil map values without site-specific measurement of the kf value. Soil maps provide guideline values for soil types, not for the specific site with its history of fill, compaction, and land-use changes. Anyone who skips a site investigation risks ending up with a system that fails to drain during rainfall events and remains permanently waterlogged.

A common construction error is the compaction of the infiltration base by heavy construction equipment. If the base of an infiltration basin or trench is driven over during construction, the kf value at that point drops dramatically. The system then does not infiltrate through the base but only through the slopes, which significantly reduces the available infiltration volume. Proper construction requires that the base be created only after all earthwork has been completed and that it not be driven on thereafter.

During operation, clogging is the most common problem. Clogging refers to the blockage of the infiltration surface by fine sediment, biofilms, or organic material carried in by stormwater. Clogging reduces the system’s effective kf value and leads to failure in the long term. Regular inspection, removal of sediment deposits, and, if necessary, renewal of the upper substrate layer are essential operational measures that must be factored into the planning and included in the cost estimate. Infiltration systems that are left to their own devices after completion lose their functionality within a few years.

Finally, the pretreatment of contaminated stormwater is often underestimated or omitted in practice. Water from parking lots, streets, and commercial areas contains pollutants that, without pretreatment, enter the soil and groundwater directly. Sedimentation chambers, filter substrates, or activated soil filters are not optional extras, but rather requirements under water law and ecological necessities. Their sizing and maintenance are central to proper infiltration planning.

Infiltration as a Planning Task: Design, Integration, and Future Prospects

Infiltration is one of the few technical infrastructure tasks that can be fully integrated into landscaped open spaces. An infiltration basin serves simultaneously as a green space, a habitat for insects and amphibians, a retention area, and a design element. Tree trenches combine roadside greenery and drainage infrastructure in one. Permeable surfaces made of pavers, gravel, or grass pavers serve access functions while also allowing for infiltration. This multifunctionality is a significant advantage over conventional sewer systems, which serve solely a drainage function and do not create synergies with other open-space functions.

For landscape architects and open-space planners, this means that infiltration should be understood as a design consideration from the outset, not as an afterthought or technical requirement. The topography of an open space, the choice of surface materials, the placement of trees and woody structures, and the shaping of the terrain—all of these factors influence how stormwater is distributed across a site and where it can infiltrate. Those who take these interrelationships into account during the design phase create open spaces that are both effective in terms of water management and aesthetically compelling.

The demands on infiltration planning are more likely to increase than decrease in the coming decades. Climate change, urban population growth, increasing soil sealing, and political pressure to reduce combined sewer overflows are making decentralized stormwater management a core issue in urban development. Infiltration is not the only solution, but it is an indispensable one. Those who understand its physical principles, master its technical capabilities, and harness its design potential contribute to making cities more resilient, cooler, and more livable. This is not an abstract vision of the future, but a concrete planning task that begins today on every construction site.