Tool risk reduction measures
«Keep the rain where it falls.»
Guiding principle by the European Commission
What do you find here?
Retention of surface water is a central element of heavy rain risk management. You find here information, links and downloads on reasonable storage design and management, addressed to property owners, municipalities and farmers.
Why is RETENTION important for heavy rain risk reduction
As result of human influence, natural water retention capacities of catchments have decreased and runoff has increased in the past. Small and big ﬂood events and in small to large catchments occurred and damages increased in Central Europe like in other regions worldwide. At present, the consequences are aggravated by climate change impacts.
Increasing the water retention capacity in catchment areas is a key element in the management of heavy rain risk and flood risk in general. The European Commission has been addressing the issue for a long time and promotes a simple message: ‘Keep the rain where it falls.’
What can be done to ‘keep the rain where it falls’?
Involve different stakeholders
A wide range of knowledge on stormwater storage options is essential for municipalities, decision-makers, stakeholders and practitioners. Especially in settlement areas it is the responsibility of the leaders of municipalities and local water management authorities to enforce water retention by the different land users and to make use from appropriate implementation tools and strategies for regulation and stimulation like spatial planning, planning in water management etc. and to approach the relevant target groups.
Consider the following influencing factors
1) Degree of sealing (permeability of the surface) – rainfall cannot penetrate and runs off where the soil surface is sealed. The parameter is related to the population density and can be influenced to some extent, e.g. by desealing and avoidance of sealing further areas.
2) Land form / terrain – Surface water flows always to the deepest point of a terrain and accumulates there. The steeper a slope is, the higher is the speed of the flowing water. In flat terrain, local depressions can fill up and form temporary ponds, a process also known as “ponding”. The parameter can’t be influenced by human activities so well, but it shapes the type and exact location of retention and drainage measures that can be implemented.
3) Vegetation cover – the more vegetation exists, the rougher is the surface in an area. A rough surface slows the runoff velocity and increases evaporation. The parameter is related to the type of land use and can be influenced, e.g. by change of land use in areas of a high risk.
4) Soil characteristics – clayey and loamy soils as well as compacted soils decreases the infiltration rate and increases runoff. The parameter is very important for areas with agricultural land use and can be influenced by appropriate farming practices.
5) Weather conditions – e.g. high moisture of soil or soil covered with snow and ice increase and speed up the runoff significantly. The parameter can’t be influenced by human activities so well.
Consider the parameters that are shaping runoff (see above) and stakeholders connected to different land uses when planning of measures aiming on increasing the water retention capacity of areas. Adapt approaches and solutions accordingly. Find further guidance in our tool RISK REDUCTION MEASURES.
Often a complex of small retention measures has the same or a better effect than ‘big’ solutions. ‘Green’ instead of ‘grey’ solutions as well as solutions for multiple purposes have a large number of benefits and are suitable almost everywhere.
Rainwater storage facilities have long term literature in conventional engineering. (valley dams, reservoirs, etc.). These facilities are built as a result of a large investment, with a certain level of safety.
In contrast, grey and green solutions are cheaper, more adaptive to climate change, better suited to an urban environment, and their long-term benefits are indisputable.
Follow the recommendations for the implication of natural water retention measures (NWRM) published on the European NWRM platform (http://nwrm.eu) and act according to the EU strategy on green infrastructure developed by the European Commission in 2019.
Sustainable soil and forest management, integrated water management, sustainable and flood-resilient urban development, informed by sustainable urban drainage and “sponge city” concepts are further concepts and strategies closely connected to RAINMAN recommendations for storm water retention that should be considered.
Furthermore, RAINMAN highlights the importance of climate change adaptation in cities and rural areas.
The study ‘retention concepts and optimization for storage management’ provides further insights into the topic. It contains the following chapters:
Runoff regulation and retention issues related to heavy rainfall
Problem description, in-depth information in influencing parameters, interdependencies and effects as well as an overview about existing concepts and solutions is provided and conclusions are drawn.
Technical practice guide for municipalities (storage types, designing, etc.)
Framing condictions and how-to guidance is provided for the realisation of retention measures in Hungarian and Polish planning context. The described method is primarily created for urban river catchments, in the classic sense of stormwater storage. It can also be used for smaller rural catchments as well.
Best practice examples, alternative measures & recommendations
A wide range of storage options through concrete examples of good practice is presented in this chapter. For each type, a factsheet was prepared summarizing the main parameters of the storage types. You can download best practice examples relevant for municipalities, private persons and farmers.
Full study “Retention concepts and optimization for storage management” | Available in English
Download [pdf; 4.4 MB]
Best practice examples relevant for municipalities | Available in English
Download [zip; 4.0 MB]
Best practice examples relevant for private persons | Available in English
Download [zip; 1.8 MB]
Download best practice examples relevant for farmers | Available in English
Download [zip; 1.7 MB]
Building Kakat-reservoir next to Kunhegyes town, Hungary
"A reservoir (a channel) was built in 2019 to primarily prevent the territory of Kunhegyes, Hungary, from being flooded. The reservoir decreases the peak of river flood and gathers the excess water faster from the catchment without inundations. It can store more than 12000 m³ water above the standard volume from the sub-catchment; its length is 550 m. The depth of the water in the storage area can reach 2 m. This amount of excess water can also be retained after rainy periods, to mitigate the effects of drought, and to refill ground water sources. Thus, the investment could also reduce other negative effects of climate change." (Gábor Harsányi, Middle Tisza District Water Directorate)
Improving water retention by adaptation of farming practices in Spitzkunnersdorf, Saxony
"After a heavy rainfall event in 2017 with big damages on farmland and in the village , the local agricultural enterprise decided to adapt its farming practices in order to increase infiltration capacity of the compacted soil and to prevent erosion. The avoidance of ploughing, intense mulching, adapted field division and strip till practices have already improved the water retention capacity of the ground." (Sabine Scharfe, Saxon State Office for Environment, Agriculture and Geology)