In Germany, allowing rainwater to infiltrate is not a voluntary gesture of sustainability, but is legally required in large parts of the country, technically standardized, and deeply embedded in water law. Planning rainwater infiltration in accordance with regulations means mastering three levels simultaneously: federal and state water laws, the technical codes governing design and implementation, and the site-specific conditions of the soil and groundwater. Those who understand these levels and integrate them can plan rainwater management not only in compliance with regulations but also in a truly functional way.
- What rainwater infiltration means from a legal perspective and which laws and regulations apply
- Which technical standards—particularly DWA worksheets—guide the planning process
- What types of infiltration systems exist and how they differ in practice
- How soil suitability and hydrogeological conditions are tested and evaluated
- How to correctly size infiltration systems
- What role infiltration plays in the context of flood protection and the “sponge city” concept
- What mistakes frequently occur during planning, permitting, and construction
- How infiltration can be integrated into different types of open spaces
Rainwater Infiltration: Basic Principle and Significance for Water Management
Rainwater infiltration refers to the controlled transition of precipitation from ground level into the soil, where it passes through the unsaturated soil zone and ultimately replenishes the groundwater. This process is the natural water cycle in its simplest form: Rain falls, infiltrates, is purified, and replenishes the groundwater. On impervious surfaces, this cycle is interrupted; the water runs off as surface runoff, overburdens the sewer system, and deprives the soil of its ability to store and filter water.
The importance of infiltration for water management is significant. In densely built-up urban areas, where a large proportion of the surface is impervious, the groundwater recharge rate drops well below natural levels. At the same time, peak runoff increases during heavy rainfall because of a lack of retention capacity. Decentralized infiltration of rainwater at the point of origin—that is, as close as possible to the surface where the rain falls—is now considered one of the most effective strategies for addressing both problems simultaneously. It relieves the burden on the sewer system, reduces flood risks, and stabilizes the water balance of urban soils.
For landscape architects and open-space planners, rainwater infiltration is a central design element that goes far beyond its purely technical function. Troughs, swales, infiltration basins, and vegetated retention areas serve simultaneously as design elements of open spaces, habitats for plants and small animals, and climatically effective structures that regulate evaporative cooling and soil moisture. The question of how to plan infiltration in compliance with regulations is therefore always also a question of the quality of open spaces.
Rainwater Infiltration Regulations: Legal Framework from the Federal to the Local Level
Water law in Germany is structured in three tiers: The federal Water Resources Act (WHG) sets the framework, the state water laws specify its details, and municipal ordinances regulate local implementation. All three levels are relevant to rainwater infiltration, and their interaction is often complex in practice.
Section 55 of the WHG contains a fundamental obligation: Rainwater shall, to the extent possible without causing harm, be allowed to infiltrate, be dispersed, or be discharged into a body of water in the immediate vicinity. This wording is not merely a recommendation but a legal obligation, albeit subject to the condition that no harm be caused. What “without causing harm” means depends on soil conditions, the groundwater situation, and possible contaminants in the stormwater. The legislature has thus deliberately incorporated a duty to weigh the options, which requires a site-specific assessment.
Requirements vary considerably at the state level. Some states have enshrined explicit infiltration requirements for new construction and property renovations in their water laws or in specific ordinances. In other states, the requirement is implemented more extensively through municipal drainage regulations. Many municipalities have enacted stormwater ordinances that specify, for certain areas or lot sizes, what proportion of stormwater must be infiltrated or retained on the property before a restricted discharge into the sewer system is permitted. These ordinances are binding for planning purposes and must be obtained at an early stage.
Under permitting law, the infiltration of stormwater is generally considered a use of groundwater that does not require a permit, provided certain conditions are met. The relevant regulations, particularly the state ordinances on permit-free infiltration, attach conditions to this privilege: The stormwater must originate only from areas with low pollution levels, the system must be constructed in accordance with recognized technical standards, and the minimum distance from the groundwater table must be maintained. Anyone who does not meet these conditions requires a permit under water law, which necessitates a formal approval process.
Technical Guidelines: DWA Worksheets and Other Standards
In Germany, the primary technical basis for planning infiltration systems is DWA Worksheet A 138, “Planning, Construction, and Operation of Systems for the Infiltration of Stormwater.” This set of guidelines from the German Association for Water, Wastewater, and Waste (DWA) systematically describes the requirements for site suitability, sizing, design, and operation of infiltration systems. Although it is not a law, it is considered a recognized technical rule and thus has quasi-normative effect: Anyone who plans in accordance with DWA-A 138 generally meets the requirements of water law for proper implementation.
The worksheet distinguishes between different types of systems and assigns different requirements to them. It contains specifications regarding the minimum soil permeability (kf value), the minimum distance between the system and the groundwater table, and the pretreatment of stormwater from heavily polluted areas. For sizing purposes, it provides calculation methods based on statistical precipitation data and the applicable rainfall intensity. Planners who use the worksheet thus have a consistent tool at their disposal that combines legal requirements with technical practice.
In addition to DWA-A 138, other sets of regulations are relevant. The DWA Information Sheet M 153, “Recommendations for Rainwater Management,” provides guidance on assessing rainwater quality and classifying areas into pollution classes. The TRBD (Technical Rule for Soil and Drainage) and standards from the DIN series for drainage systems round out the picture. In practice, planners often work with a combination of these regulations, with DWA-A 138 forming the core.
System Types: From Infiltration Basins to Swales
The infiltration basin is the simplest type of system and the one most commonly used in open-space planning. It is a shallow, vegetated depression in the ground that collects rainwater, temporarily holds it back, and allows it to infiltrate through the vegetated soil surface. The basin acts both as a retention basin, which attenuates peak runoff, and as a filter zone, where suspended solids and bound pollutants are retained in the topsoil. Species tolerant of fluctuating moisture levels—which can survive both brief periods of flooding and dry spells—are suitable for planting.
The swale is an underground storage and infiltration feature consisting of a trench filled with gravel or plastic media. It is particularly suitable where there is no above-ground space for a basin or where the infiltration system should not be visible for aesthetic or functional reasons. Drainage trenches can be arranged as linear elements along paths or building edges and can be equipped with inlet shafts, overflow pipes, and inspection openings. Their disadvantage lies in the limited accessibility for maintenance and cleaning, as well as the lack of a filtering effect provided by the topsoil.
The trough-swale combination combines both principles: The trough handles pre-treatment and acts as a buffer, while the swale beneath it handles the actual infiltration. This combination is technically more complex but offers the best performance in terms of filtration, retention, and infiltration capacity. In practice, it is particularly suitable for larger catchment areas, such as industrial parks, schoolyards, or parking lots.
Infiltration basins are larger depressions, usually bare or with extensive vegetation, designed for larger catchment areas. They are frequently used at the end of drainage systems and serve both retention and infiltration purposes. Finally, surface infiltration refers to widespread infiltration through living topsoil, as occurs in extensively used green spaces, sports fields, or parking areas with water-permeable surfaces. Water-permeable surfaces such as grass pavers, gravel-covered lawns, or paving with open joints allow for infiltration on paved areas and are explicitly recognized as eligible infiltration areas in many municipal ordinances.
Check Soil Suitability and Hydrogeological Conditions
The basic prerequisite for any infiltration system is sufficiently permeable soil. The key parameter is the kf value (hydraulic permeability), which describes the soil’s ability to transmit water. The DWA-A 138 standard specifies minimum and maximum values: Soils with a kf value between approximately 1 × 10⁻⁶ and 1 × 10⁻³ meters per second are considered suitable for infiltration systems. Highly permeable soils (kf greater than 10 to the power of minus 3 m/s) can allow water to infiltrate too quickly without providing sufficient filtration; highly impermeable soils (kf less than 10 to the power of minus 6 m/s) do not allow for economical infiltration.
The kf value is determined by field tests, not by estimation. Common methods include the double-ring infiltration test, the pit test, and the pumping test. For smaller systems, a simplified infiltration test according to DWA-A 138 is often sufficient. The results must be interpreted on a site-specific basis, as kf values can vary greatly in the subsurface. A single measurement value always represents only a single point; for larger systems, multiple measurements at different locations are required.
The minimum distance between the lower edge of the infiltration system and the highest expected groundwater level is another key criterion. DWA-A 138 generally requires a distance of at least one meter; in some state regulations, the distance is even greater. This distance ensures the filtration path in the soil necessary for the purification of the infiltrating water. Where the groundwater level fluctuates significantly on a seasonal basis, the highest groundwater level must be used as the design basis, not the average. Hydrogeological reports or analyses of groundwater monitoring stations from the State Office of Geology are indispensable planning tools in this context.
Special caution is required in the case of contaminated soil or near contaminated sites. Infiltration can mobilize pollutants and introduce them into the groundwater. In such areas, a permit under water law is mandatory, and the permitting authority will require a detailed risk assessment. Stricter requirements also apply in water protection areas: In the inner protection zone (Zone II), infiltration is generally prohibited; in the outer protection zone (Zone III), it is permitted only under strict conditions.
Design of Infiltration Systems: Basic Principles and Design Parameters
The design of an infiltration system aims to collect the rainwater generated by a defined design event and allow it to infiltrate without causing damage. The design event is defined by the rainfall intensity and the frequency of the rainfall event. In Germany, statistical precipitation data from the German Weather Service (DWD) are used for this purpose; these are available as the KOSTRA dataset (Coordinated Regionalization and Analysis of Heavy Precipitation) and provide location-specific precipitation intensities for various return periods at each site.
The design method according to DWA-A 138 calculates the required storage volume of the system based on the difference between the inflowing rainwater volume and the volume that infiltrates during the event. Input parameters include the connected impervious area, the area’s runoff coefficient, the design rainfall intensity, and the soil’s kf value. The result is a minimum volume that the system must provide to handle the design event without overflow. For larger systems or more complex catchment areas, hydrodynamic simulation models are used that can account for time-varying inflows and infiltration rates.
One aspect that is often underestimated is the design for extreme events beyond the design rainfall. DWA-A 138 recommends providing for an emergency drainage system in the event of flooding, which safely diverts the water when the system reaches its capacity limit. This flood prevention measure is a core principle of modern stormwater management and links infiltration planning with the topic of heavy rainfall risk management.
Infiltration in the Context of the Sponge City and the Climate Adaptation Plan
The concept of the “sponge city” describes an urban structure that absorbs, stores, filters, and evaporates rainwater like a sponge, rather than quickly draining it away. Infiltration systems are a core element of this concept, but not the only one. Green roofs, tree plantings with large tree pits and substrate reservoirs, retention basins, water features, and urban streams round out the picture to form an integrated water management system that simultaneously provides flood protection, groundwater recharge, evaporative cooling, and an enhanced quality of life.
For landscape architects, the sponge city principle means an elevation of their role in the planning process. Stormwater management is no longer a secondary technical task addressed after the design phase, but rather a structuring element that shapes the design of open spaces from the very beginning. Basins determine terrain modeling and planting; infiltration trenches influence path routing and paving choices; retention areas create new typologies of open spaces. Those who view infiltration as a design resource from the outset create better open spaces than those who treat it as a leftover task.
Climate adaptation plans at the municipal level—which are now being developed in many German cities—regularly identify decentralized stormwater infiltration as one of the priority measures. The combination of increasingly heavy rainfall due to climate change and growing soil sealing caused by urbanization makes the question of how infiltration is planned and implemented in accordance with regulations one of the most pressing practical tasks in open space and urban planning.
Common Mistakes in Planning, Permitting, and Implementation
One of the most common mistakes is the lack of or insufficient soil testing. Many plans rely on soil type maps or general soil descriptions without determining site-specific kf values. This leads either to oversized—and thus expensive—systems or to undersized systems that fail during heavy rainfall events. Proper infiltration planning always begins with a field test.
Another mistake is underestimating maintenance requirements. Infiltration systems, particularly basins and swales, become silted up over time due to the accumulation of fine sediments. Without regular inspection and occasional cleaning, infiltration capacity decreases significantly. Many systems that cease to function after just a few years are not incorrectly sized, but simply have not been maintained. Operational plans with clear maintenance intervals and responsibilities are therefore an essential part of comprehensive planning.
Problems with permitting often arise from incomplete application documents or a lack of knowledge regarding the applicable municipal regulations. Since stormwater regulations vary considerably from municipality to municipality, early coordination with the responsible authority is essential. Anyone who only realizes after construction has begun that the planned system does not meet the requirements of the regulations will face costly corrective measures.
Finally, the quality of the stormwater generated is often given too little consideration. Not all stormwater is the same: runoff from heavily trafficked roads contains heavy metals, tire abrasion, and hydrocarbons; roof runoff from galvanized or copper-clad roofs contains elevated concentrations of metals. DWA Information Sheet M 153 provides a system for classifying areas based on their pollution levels and determining appropriate pretreatment measures. Anyone who skips this classification risks groundwater contamination and the loss of the right to discharge without a permit.
Infiltration as an Integral Part of Open Space Design
Planning rainwater infiltration in accordance with regulations is not a limitation on design, but rather a resource. Those who are familiar with the legal requirements, understand the technical regulations, and carefully assess site-specific conditions have a tool at their disposal that enriches open spaces functionally, ecologically, and aesthetically. Basins, swales, and infiltration areas are not compromises imposed on the design; they are elements that productively structure land modeling, planting, material selection, and land use options.
The requirements of water law, the guidelines of the DWA regulations, and municipal ordinances form a framework that provides planning certainty. Those who are familiar with this framework can act with creative freedom within its boundaries. Those who ignore it produce systems that are not approved, do not function, or fail after just a few years. The ability to plan infiltration systems in compliance with regulations while ensuring high quality is at the core of landscape architecture and open-space planning, and it will continue to grow in importance in light of climate change and the increasing demands for urban water resilience.












