Residual water

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In the case of a flowing water, the residual water quantity or residual runoff is the part of the runoff that remains in the water after water has been withdrawn by discharging water downstream or is continuously passed through a dam into the lower reaches of the water.

The minimum residual water volume or minimum water flow is the amount of residual water that must at least remain in the body of water in the area of ​​a diversion, dam or withdrawal. It is specified in the water law permit (water law permit or permit) and specified in l / s or m³ / s. The residual water is primarily intended to ensure the ecological functions of the water, in particular as a habitat for fish and other aquatic organisms, and to maintain the appearance of the landscape ( minimum runoff necessary for the landscape ).

Legal definition

The minimum amount of residual water is regulated in most countries by water protection or water law and is usually stipulated in the water law approval of the dam or the water abstraction. The methods for calculating and determining the amount are very different; While in some countries simple indicators or formulas are used, in others the individual situation with all influencing factors is examined and evaluated in each individual case.


According to § 33 WHG, the damming of an above-ground body of water or the withdrawal or drainage of water from an above-ground body of water is only permitted if the flow rate is maintained that is required for the body of water and other bodies of water connected to it in order to achieve the objectives of Article 6 (1) and to comply with §§ 27 to 31 (minimum water flow). These objectives are the general principles of water management named in Section 6 (1) of the WHG and specifically the management objectives for surface waters named in Sections 27 to 31 of the WHG.


According to Section 13 (4) WRG 1959 , the extent of water use in the permit is to be limited in such a way that part of the respective inflow is retained to maintain the ecological status of the water body and for other, higher-value purposes, in particular those of water supply. Exceptions to this can be permitted for a limited period, provided that there is no cause for concern that there would be any significant impairment of the public interest.


In Switzerland , the required amount of residual water is regulated by the Waters Protection Act (GSchG, Federal Act on the Protection of Waters , Title 2, Chapter 2).

This 2nd chapter, securing adequate residual water quantities, standardizes in Art. 29 when a permit is required, in Art. 30 the requirements for the permit , in Art. 31 the minimum residual water volume , in Art. 32 the exceptions , i.e. the conditions under which the cantons lower Can set minimum residual water quantities, in Art. 33 the increase in the minimum residual water quantity , i.e. the interests after which the authority can make this increase, in Art. 34 that for water abstractions from lakes and groundwater resources that "significantly influence the water flow of a river", the aforementioned Articles apply mutatis mutandis, in Art. 35 the decision of the authority, in particular on the amount of water to be spilled, and in Article 36 the control of the amount of water to be spilled .

The residual water quantity is determined according to the so-called Matthey formula (named after its developer, the hydrologist François Matthey from Service des Eaux, Sols et Assainissement (SESA) in the canton of Vaud ). This formula is based on the amount of Q 347 in the water; this is the amount of discharge that is reached or exceeded on 347 days a year, averaged over 10 years.

Ecological importance

Weir in Scotland : Due to the insufficient amount of residual water, the river bed is almost dry.

For ecological reasons, it is necessary that a certain minimum amount of water remains in the water. This amount of water is required so that the water can also fulfill its various ecological tasks in the residual watercourse below the extraction point.

This task consists on the one hand in the function of the water - including the bank - and alluvial areas - as a biotope for fauna (animals living in and on the water) and flora (plants and algae growing in and on the water) and thus to maintain biological biodiversity ( Biotope protection , species protection ).

In the case of animals, the fish populations are of particular importance, because the reproduction of many migratory fish species requires the minimum amount of water in connection with climbing aids at barrages.

The remaining amount of water helps to maintain the temperature buffer capacity against excessive warming in summer and freezing in winter and to prevent excessive eutrophication (algae and bacteria growth). In this way, the residual water has a considerable influence on the water quality ( water quality ), which is important on the one hand for the above-mentioned function as a biotope and on the other hand for the function of natural clarification (degradation of pollutants). A sufficient quantity and quality of the residual water are again important for the functioning of the groundwater supply .

The water body assumes a further function in landscape protection to preserve the diversity of the landscape as a landscape element. The residual water helps to maintain the morphology of the water and in particular to avoid silting up due to the deposition of alluvial sediments (especially sand and silt ) in the river bed .

The minimum amount of residual water required for ecological reasons depends very much on the type of water body , the runoff regime , the character of the water body and the individual situation at the extraction point and along the remaining water route. The most important parameter is the amount of water; the less water the river carries in the annual mean, the greater the relative minimum residual water volume tends to be. In addition, however, the gradient, the mean flow velocity, width and depth of the water and the geology of the river bed and many other influencing factors are of importance.


If, in a dam, the minimum amount of water that must flow over the weir due to its function or design ("excess water") is not sufficient to maintain the required minimum amount of water in the body of water, the amount of water must be increased by an additional, artificial addition of water ("doping" or " Endowment "called). The doping is done by opening an adjustable barrier.

If the utility water discharge is used to generate energy from hydropower, the required endowment reduces the yield, as less headrace water is directed to the hydropower plant. To reduce these losses, doping turbines are increasingly being provided. A doping turbine, sometimes also called residual water turbine , is a water turbine integrated into the dam system , through which the amount of doping water is diverted and used for energy. This allows the energy efficiency of the hydropower plant to be increased to up to 100%.

Computational estimation based on the MEFI model

For the computational assessment of an ecologically sound minimum discharge in rivers, investigations of the hydraulic, morphological, biological, physical and chemical parameters in the area of ​​diversion routes were carried out in the Bavarian Alpine foothills and the so-called "MEFI model" was developed from this.

The main parameters and sub-parameters for an ecological status description could be identified:

Measured parameter Included parameters
near-bottom flow velocity
u nb
River bed morphology
Soil substrate
Grain distribution (ground roughness)
Nutrient supply
Floor roughness
h A50
Intensity of the turbulence near the bottom
near-bottom flow velocity
Habitat offer
Measure of solar radiation
Riparian vegetation
Nutrient supply
Water temperature
Algae growth
Water chemistry (e.g. oxygen content, pH value)

To determine the minimum water runoff, the flow conditions at diversion sections were carefully examined. In particular, the height of the algae growth h A50 and the mean near-bottom flow velocity u nb were measured on characteristic cross-sections . The result is a relationship between the discharge Q and the Reynolds number Re nb near the bottom - in each case for the diversion section under consideration.

Re nb, in turn, is related to the biological parameter taxa rheo . This is representative of the biological living conditions in the flowing water, in particular concerning organisms that require good water quality, a gravelly bed and a certain minimum flow velocity. The rheotypic organisms (e.g. mayflies) thus represent an indicator of a good ecological status. By linking the two relationships, taxa rheo . determine, and consequently a minimum water discharge that meets the requirements of the ecological community in the water.

Another negative effect, caused by small water depths, low flow velocities and high levels of solar radiation, is the warming of the water. As a result, increased, harmful algae growth occurs. The Irradiation Factor (IF) was introduced to take this into account . With increasing solar radiation, this leads to an increase in the base discharge Q B and thus to the actual minimum discharge Q res .

Despite the consideration of flow conditions only near the bottom, the MEFI model represents a determination of the minimum water runoff on an ecologically sound basis, especially for rivers with gravelly bottoms. Likewise, the general, simple and inexpensive applicability should be pointed out.


  • Viviane Uhlmann, Bernhard Wehrli: Use of hydropower and residual water. Determination of the location for the implementation of the residual water regulations . Eawag , Kastanienbaum June 2006 ( Online [PDF; 1,3 MB ; accessed on May 13, 2017]).
  • Federal Office for the Environment, Forests and Landscape (ed.): Residual water quantities in rivers . Communications on water protection, No. 24. Bern 1997.
  • Michael Hütte: Ecology and Hydraulic Engineering: Ecological Basics of Water Management and Hydropower Use . Vieweg + Teubner, 2003, ISBN 3-528-02583-2 ( limited preview in the Google book search).

Web links

Individual evidence

  1. a b c Uhlmann, Wehrli: Use of hydropower and residual water. Determination of the location for the implementation of the residual water regulations . 2006.
  2. a b Uta Mürle: Morphology and habitat structure in the diversion section of an alpine dam (Spöl, Swiss National Park, Engadin) . Diploma thesis in geoecology. Institute for Geography and Geoecology University of Karlsruhe TH, Öschelbronn April 2000 ( Online [PDF; 5.2 MB ; accessed on May 13, 2017]).
  3. Aspects of determining residual water quantities in alpine rivers today and in the future. H&W Research Award 2009. Hintermann & Weber AG, accessed on October 6, 2011 .
  4. a b residual water. Federal Office for the Environment of Switzerland (FOEN), accessed on September 16, 2018 .
  5. surface waters. Use of hydropower and residual water. Eawag , accessed October 7, 2011 .
  6. ^ Alfred Wüest: Hydroelectric power plants in the CH. Overview and effects of WK use. (PDF; 1.3 MB) Eawag , 2010, accessed on October 6, 2011 .
  7. a b Causes of residual water and its spatial effectiveness. (PDF) University of Natural Resources and Life Sciences, Vienna , accessed on October 6, 2011 .
  8. a b Theodor Strobl, Franz Zunic: Handbuch Wasserbau . Springer, 2006, ISBN 3-540-22300-2 .
  9. Willi Maile, Thomas Heilmair, Theodor Strobl: Assessment of river biocenoses in the area of ​​diversion power plants (focus on macrozoobenthos). The MEFI model: a method for determining ecologically justified minimum runoffs in diversion sections of hydropower plants. In: Hydraulic engineering and water management: Reports from the Obernach experimental station and the chair for hydraulic engineering and water management at the Technical University of Munich . No. 80 . Technical University of Munich , 1997, ISSN  0947-7187 , p. 247-267 .
  10. R. Estoppey et al .: Adequate residual water volumes - How can they be determined? Guideline Enforcement Environment. Ed .: Federal Office for the Environment, Forests and Landscape . Bern 2000.