What is the function of RRB

Session management

Informational printed matter no. 1609/2012:
Rainwater retention basin (function, design and renaturation)


Content of the printed matter:

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Informational printed matter
In the works committee for urban drainage
Number of plants

Rainwater retention basin (function, design and renaturation)

The sealing of surfaces for streets, residential and commercial buildings is associated with increased rainwater drainage and an increase in flood discharge peaks. Rainwater retention basins (RRB) are technical systems for intermediate storage and throttling of these runoffs.
Their task is to reduce the hydraulic load on the body of water and flood peaks through storage.
The throttling of precipitation runoff can be necessary, both from a higher-level water management point of view and as a measure for the management of precipitation water from new building areas. The retention of rainwater and its controlled transfer is a measure for preventive flood protection. The drainage of rainwater from built-up areas should be throttled before it is discharged into a body of water in such a way that residential developments, infrastructure and commercial operations in the urban area are protected as best as possible. The dampening of the runoff wave is accompanied by a reduction in the hydraulic stress of aquatic life and material pollution, which are considered to be the main cause of water pollution.
Rain retention basins are also formative parts of the landscape, are used for local recreation and are of great importance for flora and fauna.

Basically, rain retention basins are part of the sewer system and are used for the intermediate storage of rainwater from built-up areas.

Components of a rainwater retention basin are the inlet structure and the outlet structure with the throttle, which reduces the runoff to the calculated level, the emergency overflow and the maintenance routes.

Rainwater retention basins can be designed as basins in an open, technical or near-natural construction.

A distinction is also made between dry and wet basins:
Wet basin are called basins that are always filled with water. The storage volume is the difference between the permanent water level and the higher outlet.
As Drying basin is the name given to a hollow integrated into the landscape, which is only dammed in during rainy weather and which falls dry again after the stored rainwater has run off.
Nowadays, rain retention basins are mainly created as varied and natural dry basins, depending on the soil and groundwater conditions.
In accordance with the technical purpose and aesthetics of the time, wet basins have been designed functionally in the past. The classic rainwater retention basin had steep banks, a paved service road and massive inlet and outlet structures. The base of the embankment was often built with concrete, grass pavers or tropical wood to prevent embankments from breaking off. Water transition zones do not exist. The maintenance of the bank areas is intensive. For reasons of traffic safety, these basins have not been made accessible to the public.

Today, when planning a rainwater retention basin, close coordination with the relevant municipal authorities such as the planning office, property office, department for the environment and urban greenery, possibly the investor and the responsible authorities in the region (lower nature conservation authority, lower water authority) is required. The necessities of a water rights application, the type of construction (sealed, unsealed, dry or wet basin) or the requirements of the nature conservation authority depend on the location and have to be redefined each time. Rain retention basins, which are intended to meet the requirements of nature conservation and thus the regulation of intervention under nature conservation law, must be designed to be close to nature.

The near-natural design includes a varied shoreline, different depths in the basin for the development of different types of biotopes and site-appropriate planting.
By observing these principles, new habitats for flora and fauna are created. Near-natural RRB in urban areas represent important retreats and breeding grounds for bird species. They offer optimal spawning conditions for amphibian species and create the most varied of dragonfly species optimal space for development through the near-natural conditions.
The improvement of the ecological functionality and the technical purpose are therefore not mutually exclusive. A near-natural design can take on a part of the cleaning process for polluted rainwater and thus contribute to improving the quality of the water. A design that is close to nature saves costs when it comes to maintaining the pools.

Examples of the new construction of near-natural RRB are in the urban area:
RRB "Wettbergen West"
RRB "Lange Hop Str."
RRB "Bugstrasse"
RRB "In der Rehre"
RRB "Büntewiesen"

Redesign and ecological upgrading of existing rainwater retention basins
In the face of the lack of natural small bodies of water, retention basins designed to be close to nature can provide a substitute habitat for amphibians and dragonflies.
As far as the water management function allowed, therefore, within the framework of the still water program1 some rain retention basins remodeled in a natural way. The aim is to create small habitats, to increase the structural diversity, to improve self-cleaning power through reed plantings and to steer users through shielding plantings.


Rainwater retention basin "Im Othfelde"
The water area was expanded and shallow water zones were created in the southeast of the basin. Here, the soil was removed to a water depth of no more than 0.80 m and alternating slope shapes with an inclination of 1: 4 to 1:20 were created. This created larger shallow water areas, with parts of the shallow water zone being laid out in such a way that a water level of at least approx. 0.20-0.30 cm is permanently reached.
Part of the excavated soil was used along the north bank of the rainwater retention basin as an embankment to create a shallow water reed zone.
Reeds were planted both in the new shallow water zone on the south-east bank and on the north bank. Plants were introduced at the transition from the shallow water zone to the deeper water of the rainwater retention basin. Only native plants were used for all plantings.
The storage volume increased by approx. 550 m³, the water surface by approx. 1040 m². In addition, the landscape and townscape experienced an upgrade, as the water is in a much more natural state

Rainwater retention basin "Koenigsberger Ring"
A particularly intensive redesign was carried out in 2004 in the retention basin on Königsberger Ring. The concrete basin left no space for water-bound animal and plant species. Except for a permanent water-bearing area, it was converted into a drying basin.

To compensate for the reduced storage volume, additional storage space was created above the basin. The rough trench was relocated to the east and designed to be natural. Neighboring higher areas are flooded when the water runoff is higher.

The conversion of the rain retention basin ensures both an enlargement of the existing swamp biotopes, which are specially protected according to the Lower Saxony Nature Conservation Act, and a long-term protection of the structures of this very young biotope development. Embankments that were newly created during the construction phase were left in the open, ungrown state without woody plantings, so that plants can settle here again by themselves.

1(The standing water program supplements the flowing water program. The elements of the creation of small bodies of water, the creation of retention areas, bank renovation, the creation of shallow water zones, and natural remodeling of the rainwater retention basin are taken from the program of measures for the development of open spaces in the landscape areas of Hanover 1998-2002 Department of Environment and Urban Greenery))

Stadtentwässerung operates 65 rainwater retention basins with a total storage volume of approx. 280,000 m³. Approx. 50% of these are designed as wet basins and 50% as dry basins.

Consideration of gender aspects

Statements on gender differentiation according to the decision of the council of July 3rd, 2002 (see DS 1278/2003) are not relevant in the case of this printed matter and are therefore not detailed.

Cost table

No financial effects will occur.
Hanover / Jun 22, 2012