Rain is not a waste product that should be removed from the city as quickly as possible. Rainwater infiltration is one of the most effective strategies for restoring the natural water cycle in built-up areas, reducing flooding, replenishing groundwater, and improving the urban climate. Anyone who understands how infiltration systems work, their planning principles, and their technical requirements will also understand why decentralized rainwater management is now one of the core tasks of sustainable open-space planning.
- What rainwater infiltration means from a physical and hydrological perspective and how it functions within the water cycle
- What types of infiltration systems exist and how they differ in function, design, and application
- What site requirements and soil investigations are absolutely necessary prior to planning
- Which standards, regulations, and water law requirements apply in Germany
- How infiltration systems are integrated into open-space planning from both a design and ecological perspective
- What common mistakes occur during planning, construction, and operation—and how to avoid them
- How infiltration systems interact with other elements of the “sponge city” strategy
- What is required for the maintenance and long-term functionality of infiltration systems
Rainwater Infiltration: Definition, Function, and Hydrological Basis
Rainwater infiltration refers to the controlled, engineered, or near-natural process by which precipitation water from paved or built-up areas is directed into the ground and fed into the groundwater there. In the natural water cycle, a significant portion of precipitation infiltrates directly where it falls, is absorbed and filtered by the soil, and slowly replenishes the groundwater. Sealed surfaces disrupt this cycle: asphalt, concrete, and paving stones cause water to run off the surface, increase peak runoff, and place a strain on the sewer system. Infiltration systems are the technical and planning tools used to at least partially compensate for this loss.
From a hydrological perspective, infiltration systems fulfill several functions simultaneously. They reduce peak runoff during heavy rainfall events by allowing water to seep into the subsurface with a time delay. They replenish groundwater, which has dropped significantly in many urban areas due to decades of complete soil sealing. They improve the urban climate because infiltrated and stored water helps cool the environment through evapotranspiration. And it relieves the combined sewer system, which is often overloaded during heavy rain events and discharges untreated wastewater into water bodies. Rainwater infiltration is therefore not an isolated measure but rather an element of integrated stormwater management.
The key physical concept is the infiltration rate—that is, the amount of water that soil can absorb per unit of time. It is measured in millimeters per hour or meters per second and depends on the soil type, texture, degree of compaction, and current moisture content. Sandy soils have high infiltration rates, while clay soils have very low ones. Saturated hydraulic conductivity (kf value) is the key parameter in the design of infiltration systems: It describes how quickly water flows through water-saturated soil and is a decisive factor in determining which type of infiltration system is even feasible at a given site.
Types of Infiltration Systems: Basins, Trench Systems, Shafts, and Surface Systems
Technical planning distinguishes several basic types of infiltration systems, which differ in their geometry, storage volume, treatment capacity, and space requirements. The DWA-A 138 worksheet published by the German Association for Water, Wastewater, and Waste is the authoritative set of standards for the planning, construction, and operation of infiltration systems in Germany. It classifies the systems and specifies design criteria.
The infiltration basin is the most natural form and the one most commonly used in open-space planning. It is a shallow, vegetated depression in the ground that collects stormwater at the surface and allows it to seep into the ground through the vegetated bottom. The basin combines retention, purification, and infiltration: suspended solids and bound pollutants are retained by the vegetation and the topsoil before the water penetrates deeper. Basins can be easily integrated into the landscape design but require sufficient space and soil with adequate infiltration capacity. Their base should be at least one meter above the highest groundwater level to ensure a sufficient filtration path.
Trench infiltration uses underground trenches or pipes filled with gravel or plastic media to temporarily store water and slowly release it into the surrounding soil. Drainage trenches are space-efficient and can be installed beneath paths, parking lots, or green spaces. They are particularly suitable where there is no above-ground space for basins, but they require careful pretreatment of the inflowing water, as they are difficult to clean once silted up. The combination of a basin and a drainage trench—known as a basin-trench system—combines the treatment capacity of the basin with the storage volume of the drainage trench and is considered a particularly effective solution for urban locations.
Infiltration shafts direct water directly and locally into deeper soil layers. They are technically simple but offer little purification capacity and are therefore suitable only for lightly polluted water from roof surfaces. Finally, surface infiltration systems use permeable paving materials such as grass pavers, pavers with wide joints, or water-bound surfaces to direct precipitation into the ground directly at the point of impact. They are the most decentralized form of infiltration and require a stable, permeable subsoil.
Site Analysis and Soil Testing as Prerequisites for Planning
No infiltration system can function without a thorough site analysis. The most important preliminary test is determining the kf value through an on-site infiltration test. DWA-A 138 describes various methods, including the simple double-ring infiltrometer test for shallow basins and more complex pumping tests for deeper systems. Laboratory analyses of soil samples can provide an approximate determination of the kf value but do not fully replace the field test because soil structure, macropores, and layering cannot be replicated in the laboratory.
The kf value determines whether a site is fundamentally suitable for rainwater infiltration. DWA-A 138 specifies a kf value of approximately 1 × 10⁻⁶ meters per second as the lower limit for most infiltration systems. Soils with lower values—that is, heavy clays and silts—infiltrate so slowly that economically feasible systems cannot provide sufficient capacity. In such cases, basins with extended retention periods, combined retention-infiltration systems, or discharge into water bodies may be considered as alternatives.
In addition to the kf value, the groundwater level and its seasonal fluctuations are critical. According to DWA-A 138, the minimum distance between the base of an infiltration system and the highest expected groundwater level is generally one meter to ensure a sufficient filtration path for contaminants. This distance is non-negotiable because it ensures the protection of groundwater as a drinking water resource. In areas with high groundwater levels, such as river floodplains or marshes, infiltration is therefore often not a viable solution.
Other site-specific parameters include the geology and potential contaminated sites in the subsurface. Infiltration systems located near areas suspected of contamination can mobilize pollutants and release them into the groundwater. A search of the contaminated sites registry and, if necessary, a preliminary soil investigation are therefore mandatory before any planning begins. The distance to building foundations must also be taken into account: Infiltrated water can cause moisture to penetrate foundations, lead to settlement, or flood basements if the system is located too close to the structure.
Water Law, Standards, and Permitting Practices in Germany
Rainwater infiltration is regulated by water law in Germany. The Water Resources Act (WHG) defines the principles according to which stormwater is to be infiltrated on-site, sprayed, or discharged into a body of water, provided this can be done without impairing the public interest. This principle of on-site management is enshrined in Section 55 of the WHG and forms the legal basis for the planning requirement that rainwater infiltrate on the property.
The specific permit requirement depends on state law and on the size and type of the system. Many federal states have established thresholds below which small infiltration systems for roof runoff may be installed without a formal permitting process. Larger systems, systems for stormwater runoff from roads, or systems located in water protection areas are generally subject to a permit under water law, for which an application must be submitted along with a hydraulic analysis, soil investigation, and site plan. The responsible water authority reviews, in particular, groundwater protection and the system’s design.
DWA-A 138 is the central technical standard for design calculations. It describes the design procedure based on design rainfall derived from KOSTRA-DWD data (Coordinated Regionalization and Analysis of Heavy Precipitation by the German Weather Service). The system must be sized so that it can safely collect and infiltrate the design rainfall with a defined return period—typically two years for simple systems—without causing damage. For systems in sensitive areas or with higher protection requirements, longer return periods are applied.
In addition to DWA-A 138, other regulatory frameworks may apply depending on the type of system. The DWA-M 153 technical guideline addresses the assessment of rainwater with regard to its infiltration capacity and provides guideline values for the pollutant load of various surface types. Street runoff, parking lot drainage, and runoff from heavily trafficked areas generally require pretreatment before the water may be discharged into the ground. Runoff from copper, zinc, or lead roofs may also contain elevated levels of heavy metals and requires a separate assessment.
Design, Ecological Integration, and the “Sponge City” Strategy
Infiltration systems are not merely technical infrastructure; they are elements of open space and can be designed and implemented to high aesthetic and ecological standards. Infiltration basins can be integrated as part of green corridors, roadside greenery, schoolyards, residential courtyards, or parking areas. Their vegetated bases and slopes provide habitat for insects, amphibians, and birds when planted with site-appropriate, drought-tolerant plants that can also withstand occasional flooding. Suitable species include, among others, water buttercup, purple loosestrife, sedges, and meadow fescue, which tolerate short-term waterlogging and can survive with little water during dry periods.
The “sponge city,” known as “Sponge City” in English-speaking countries, is the overarching planning concept that incorporates rainwater infiltration. The vision of the Sponge City describes a city that absorbs, stores, purifies, and slowly releases rainwater like a sponge, rather than quickly draining it away. Infiltration systems are one element among others, such as green roofs, green facades, retention basins, stormwater retention ponds, urban trees with tree trenches, and permeable paving. Only the interaction of these elements results in robust rainwater management capable of buffering even heavy rainfall events.
Tree trenches are a particularly interesting example of the integration of infiltration and urban vegetation. They consist of underground, substrate-filled cavities beneath tree sites that collect rainwater from adjacent streets and sidewalks, temporarily store it, and slowly release it to the tree’s root system. The urban tree benefits from the water supply, infiltration takes place in the root zone, and the burden on the sewer system is reduced. Projects such as Berlin’s street trees with tree-lined infiltration trenches or comparable approaches in Copenhagen and Stockholm demonstrate that this combination works and is aesthetically appealing.
For open-space planning, the integration of infiltration systems represents an expansion of the design repertoire. Basins and infiltration trenches must be incorporated into the design from the outset, not as a technical add-on at a later stage. This requires close collaboration between landscape architects, civil engineers, hydrologists, and developers as early as the preliminary planning phase. Those who do not consider infiltration systems until the construction planning phase lose design flexibility and risk conflicts with other underground utilities and structures.
Common Mistakes in Planning, Construction, and Operation
The most common planning error is the absence of, or insufficient, site investigation. Infiltration systems are sized based on soil maps or empirical values without conducting a field test to determine the kf value. The result is either oversized systems that take up an unnecessary amount of space, or undersized systems that overflow during rainfall events and flood the connected areas. Both errors can be avoided if soil investigation is treated as an integral part of the preliminary planning.
Another common mistake is the lack of pretreatment of the inflowing water. Infiltration systems that receive runoff directly from streets or parking lots—without sediment separators, filter shafts, or vegetated pretreatment basins upstream—become clogged with silt within a few years. The bottom of the basin or the pores of the infiltration trench become clogged with fine particles, the infiltration rate drops to a fraction of its original value, and the system ceases to function. Regular inspection and cleaning are therefore not optional operational measures, but rather prerequisites for long-term functionality.
Construction errors often involve the compaction of the soil in the area of the infiltration basin’s base. When heavy construction equipment compacts the base of a basin or the area around a swale, the kf value decreases significantly. DWA-A 138 therefore recommends that infiltration areas be constructed only after the structural work is complete and that the area be protected by construction fences during the construction phase. Placing topsoil with too high a clay content on the basin floor is also a typical construction error that permanently impairs infiltration performance.
During operation, infiltration systems are often neglected because they function invisibly under normal conditions and are only noticed when they fail. A sensible maintenance routine includes an annual visual inspection for silt buildup, vegetation growth, and damage to intake structures; cleaning sediment traps after major rainfall events; and verifying that emergency overflows are unobstructed. Publicly owned systems should be included in municipal maintenance contracts, as responsibility for their operation otherwise remains unclear.
Rainwater Infiltration as a Building Block of Resilient Urban Development
Rainwater infiltration is not a niche technical topic, but rather a cornerstone of resilient urban development. Climate change is intensifying both heavy rainfall events and periods of drought, and both extremes hit cities with high levels of soil sealing particularly hard. Flooding of basements, underpasses, and underground parking garages during heavy rains, on the one hand, and groundwater drawdowns causing drought stress for urban trees, on the other, are two sides of the same problem: too much water is drained away too quickly instead of being retained on-site and returned to the water cycle.
Municipalities that consistently incorporate infiltration systems into zoning plans, drainage regulations, and open-space planning are building, in the long term, an infrastructure that provides both flood protection and groundwater recharge. Provisions in zoning plans under Section 9 of the Building Code make it possible to mandate infiltration for new development areas. Municipal wastewater regulations can create incentives or requirements for disconnecting from the sewer system. These planning tools are available; their consistent application is a matter of political will and the technical expertise of the planning administration.
For landscape architects and open-space planners, the topic of rainwater infiltration represents an expansion of their professional responsibility. The planning of outdoor spaces does not end at the surface but extends into the subsurface, the water cycle, and the climate resilience of the urban neighborhood. Those who view infiltration systems as a design and ecological opportunity—rather than a burdensome technical requirement—create open spaces that are at once beautiful, functional, and sustainable. This is not a utopian ideal, but a planning task that can be implemented today using existing knowledge, regulations, plants, and materials.
