Water Engineering Glossary
Essential terminology in stormwater management, urban water systems, and hydraulic engineering – explained clearly with technical context and practical examples.
It represents the top soil layer in technical or nature-based systems where microorganisms break down pollutants and purify water through biological processes.
Hygienic and chemical safety measure towards surface waters used for bathing. In the EU, the bathing water quality and its management is based on the Directive 2006/7/EC.
Underground tank for the storage and use of rainwater. It can also serve as a retention volume, functioning like a rainwater retention basin.
A drainage system in which both wastewater and stormwater are conveyed together through a single sewer to a treatment plant. During heavy rainfall events, however, the system can overload. In such cases, excess water is discharged into surface waters through so-called combined sewer overflows. This can result in untreated wastewater and stormwater entering rivers or lakes, potentially impacting water quality. Therefore, retention volumes and treatment facilities are essential components to reduce such discharges.
A system using wetland vegetation and soil to filter and treat rainwater or wastewater through biological processes. Plants and microorganisms break down pollutants, improving water quality.
The restoration of natural infiltration, evaporation, and hydrological balance by dismantling sealed areas. This measure helps adapt to heavy rainfall events and enhances groundwater recharge.
A regulatory permit for discharging wastewater or rainwater into a watercourse or the public sewer system. In rainwater management, direct discharge releases water, after any required treatment, directly into a surface water body, while indirect discharge routes water first into the public sewer system for subsequent transport and treatment if necessary.
Technical system used to remove excess water from soils or structures. It can also be used to convey water into infiltration systems, such as infiltration trenches or soakaways.
The removal of deposited sediment from water basins or water bodies to preserve their functionality and water quality. It also eliminates nutrients, such as phosphorus, that can lead to eutrophication
Legally designated area in drinking water catchment zones where stricter requirements for rainwater infiltration apply to protect drinking water from contamination.
A parameter defining the temporal length of rainfall events, typically derived from the “KOSTRA” dataset. Duration steps, ranging from 5 minutes to 72 hours, are crucial for the design of drainage systems and retention storage, since intense events require different considerations than long-lasting, lower-intensity rainfall.
Additional pathway for stormwater that diverts water when the regular sewer or retention systems are overloaded, helping to prevent flooding.
The excessive accumulation of nutrients, particularly phosphorus and nitrogen, in water bodies. It can cause algal blooms, oxygen depletion, and degradation of aquatic ecosystems. Treatment measures, such as vegetated soil filters, can help mitigate these effects.
Evaporation of water from, for example, soil, vegetation, or open water surfaces is a key process in the natural water cycle. As evaporation withdraws heat from the surrounding environment, measures that enhance evaporation are an important component of climate adaptation in urban areas.
The process by which water evaporates from plant leaves, returning rainwater to the atmosphere and influencing the natural water cycle.
The provision of water for fire protection, for example from hydrants, (artificial) water bodies, retention basins, or cisterns.
Measures to prevent or reduce damage from flooding, such as retention areas, emergency drainage routes, and floodable surfaces.
A flood risk assessment evaluates the potential for flooding caused by heavy rainfall and determines whether a site, development, or drainage system can safely manage excess stormwater. It considers rainfall intensity and duration, runoff volumes, drainage capacity, local topography, and potential flow paths to identify flood hazards and assess the risk of damage to buildings, infrastructure, and surrounding areas. Depending on the project and applicable regulations, assessments may involve hydrological and hydraulic calculations, surface runoff modelling, and the evaluation of temporary storage capacity. Potential flood mitigation measures include retention basins, infiltration swales, designated flood storage areas, and controlled overland flow routes. The assessment methods and design requirements vary between countries and jurisdictions. In Germany, DIN 1986-100 provides a regulatory and technical framework for demonstrating adequate protection against flooding on building plots during heavy rainfall events. Comparable assessments in other countries are carried out in accordance with relevant national standards, local regulations, and established engineering practices.
Green roof substrate layer designed to retain rainwater, enhance the microclimate, and decrease runoff. It can be part of a retention roof system.
Lightly polluted wastewater from showers, bathtubs, or washbasins. After technical treatment, it can be reused as service water.
Process in which water infiltrates into deeper soil layers, contributing to groundwater recharge. Technical measures, such as swales or infiltration trenches, can enhance this process.
A heavy rainfall event is characterized by high rainfall intensity, with large amounts of precipitation occurring within a relatively short period. Such events can exceed the capacity of drainage systems and lead to surface runoff, sewer surcharge, and localized flooding. Rainfall events are assessed using parameters such as precipitation depth, storm duration, and statistical return period. Meteorological agencies, hydrological institutions, and national or regional authorities provide rainfall statistics, intensity-duration-frequency (IDF) data, and warning criteria used in engineering practice. Local topography, overland flow paths, land use, and available storage capacity are particularly important when assessing flood risks and developing effective stormwater management strategies.
Infiltration is the process by which water penetrates the soil through unsealed surfaces. In stormwater management, infiltration is a key factor in the proper functioning of systems such as swales, infiltration trenches, or soakaway pits. The infiltration capacity depends on the soil type and is quantified, the kf-value. For example, sandy soils are particularly well suited for infiltration. It is essential to ensure that the soil can absorb the incoming rainfall within specified timeframes to maintain effective drainage.
Technical facility that directs rainwater into the ground in a controlled manner to promote groundwater recharge and reduce surface runoff. To achieve this swales, infiltration trenches and soakaway shafts can be used.
Underground, permeable storage structures where rainwater is collected, stored, and infiltrated. In gravel trenches, the storage medium consists of gravel or crushed stone, while modular infiltration trenches are typically made of plastic modules that provide a high storage capacity.
The Kf-value, also known as the permeability coefficient, indicates the water permeability of the soil. It is a key parameter for designing infiltration systems and assessing a site’s suitability for infiltration.
The collection and derivation of stormwater from paved surfaces such as roads, pathways, or plazas using channel drains.
Spaces that can be used as open public areas and also serve for stormwater management, for example for temporary retention or infiltration of rainwater.
An open water basin that purifies, temporarily stores, and infiltrates rainwater. It can be integrated into landscape planning and serve as a recreational area and habitat.
Pollutant load refers to the mass of pollutants transported by water over a specified period. In stormwater management, pollutant loads originating from roads, roofs, and other impervious surfaces are particularly relevant when assessing runoff quality and designing treatment systems. An important parameter used in Germany is AFS63, which describes the concentration of suspended solids with particle sizes of up to 63 µm. Assessing pollutant loads and runoff contamination provides a basis for selecting and designing appropriate stormwater treatment measures, including sedimentation and filtration systems.
Planting area that temporarily stores rainwater and allows it to infiltrate slowly. Its function is similar to a swale but is more space-efficient, making it suitable, for example, as a roadside installation.
It represents the rainfall events statistics over different return periods (e.g., 2 years, 5 years, etc.) used to design drainage systems. The data are, for example, provided as “KOSTRA-Daten” by the German Weather Service (DWD).
Controlled circulation of water in artificial water bodies using pumps and treatment systems to remove nutrients, improve water quality, and mix the entire water body.
Temporary storage of rainwater for delayed discharge or reuse. The goal is to reduce peak flows during heavy rainfall and retain water as close to its source as possible. To achieve this, retention roofs, cisterns, infiltration trenches, or multifunctional areas can be used.
A roof system designed for the temporary storage and controlled, delayed discharge of rainwater. When combined with green roofing, it helps relieve drainage systems and improve the urban climate.
Statistical average time after which a specific rainfall event of a defined intensity is expected to occur once. Events with return periods of 30 or 100 years are often considered, based on data from the KOSTRA system of the German Weather Service (DWD)
It indicates the rainfall that does not infiltrate or evaporate, but instead runs off. A distinction is made between peak and average runoff coefficients, depending on the given period or scenario.
A system for removing particles from rainwater through sedimentation. It serves as a preliminary treatment before the water is further treated, infiltrated, or discharged.
Drainage system in which wastewater and stormwater are collected and conveyed separately, reducing the load on treatment plants and preventing flooding.
Water intended for technical, commercial, or domestic use that does not need to meet drinking water quality standards, such as for toilet flushing and irrigation.
A vertical structure to infiltrate rainwater. In Type A, both the walls and the base are permeable, allowing water to infiltrate directly into the surrounding soil. In Type B, the shaft is watertight except for the permeable base, so water infiltrates solely through the bottom.
Replacement of contaminated or unsuitable soils with appropriate material, depending on the intended purpose (e.g., to enhance water infiltration).
The process of sealing land surfaces through various constructions, which limits infiltration, evaporation, and other essential soil functions.
Underground channels that temporarily store rainwater and release it in a controlled manner to prevent overloading the sewer system during heavy rainfall events.
A stormwater drainage fee is a charge imposed by municipalities or water utilities for managing and conveying stormwater runoff from roofs, roads, and other impervious surfaces into public drainage systems. Such fees are used in various countries to help finance the operation, maintenance, and development of stormwater infrastructure. Depending on local regulations, fees may be calculated based on the area of impervious surfaces connected to the drainage network, the estimated runoff volume, or other assessment criteria. Sustainable stormwater management measures, such as permeable surfaces, infiltration systems, green roofs, rainwater harvesting, and disconnecting impervious areas from public drainage systems, may qualify for fee reductions or financial incentives. The applicable conditions and potential savings vary between municipalities and countries.
Measures and systems for the controlled use, storage, infiltration, and discharge of rainwater. Their aim is to prevent flooding, reduce the load on sewer systems, promote groundwater recharge, and minimize the input of pollutants and nutrients into water bodies.
Infiltration swales are shallow, vegetated depressions used for decentralized stormwater management. They collect surface runoff and allow it to infiltrate through the biologically active soil layer. By temporarily storing water at depths of typically up to 30 cm, they also provide stormwater retention capacity. Filtration through the soil helps remove pollutants and improve water quality.
Controlled water discharge limitation from retention structures, typically implemented using flow control devices. In Berlin, it is permitted on a case-by-case basis, with a maximum of 10 l/s per hectare.
A sewage treatment system that filters water through vegetated soil layers, with either vertical or horizontal flow. It is commonly used in small-scale wastewater treatment plants, as well as for rainwater or surface water management.
The entirety of the water cycle in a given area, including precipitation, evaporation, infiltration, and runoff.
The zero-discharge principle aims to manage stormwater entirely on site under defined design conditions, avoiding discharge into the public drainage network. This approach combines measures such as infiltration, evapotranspiration, rainwater harvesting, and temporary storage. Its feasibility depends on factors including soil permeability, groundwater conditions, available space, and applicable regulatory requirements. Safe emergency overflow routes and appropriate flood protection measures must still be considered for rainfall events exceeding the design conditions.