Flood retention basin

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The Mordgrundbach flood retention basin on the Bahra in the Eastern Ore Mountains

A flood retention basin (HRB) is a dam , the main purpose of which is to regulate the flow rate of a river during floods . It dampens the outflowing flood wave by temporarily storing excessive water loads and releasing it in a controlled manner after an event has subsided. The basin is normally empty (so-called dry basin or green basin) or partially filled (permanent storage basin). Large flood retention basins can be constructed similarly to dams , but their main purpose remains flood protection. Dams also serve other purposes, such as B. the drinking water supply or electricity generation. When shut-off are dams or dams used.

Position of the basin to the water

Flood retention basin (HRB) in the main section (left) and bypass (right), consisting of a dam (1), retention basin (2), natural area (3), stilling basin (4), overpass (5) and outlet (6)

Main conclusion

Flood retention basins in the "main circuit" are flowed through directly by the water. The barrier structure (1) lies across the course of the river, with the flowing water draining unhindered through the bottom outlet when the water level is low . If the inflow of the water rises above the standard discharge of the bottom outlet, this increased discharge is held back and the basin (2) is dammed. Only when the inflow falls below the regular outflow does the basin slowly empty again. The bottom outlet is set so that only as much water can flow through as the underflow can handle without damage. The lowest area of ​​the basin, which is more regularly affected by smaller floods, is usually designed as a purely natural area (3). An example of such a basin is the Jonenbach flood retention basin .

Shunt

Flood retention basins in the "shunt" (also called polder ) are not directly traversed by the water, but the basin (2) is arranged to the side of the river. The basin is separated from the river by a longitudinal dam (1) oriented in the direction of the river. In the event of flooding, some of the water from the river can be channeled into the basin through a transfer line (5). It later flows back into the river through the overpass or another outlet (6). In the normal state, the river remains in its natural course. The river's ecosystem with bank strips and floodplains is therefore retained throughout the flood retention basin in the tributary. An example of such a basin is the Langeler Bogen flood protection basin .

Classification

A subdivision of flood retention basins is done according to DIN 19700-12 first of all based on the size according to the following table:

Classification Total storage space Height of the barrier structure
tiny less than 0.050,000 m 3 less than 04 m
small less than 0.100,000 m 3 less than 06 m
medium less than 1,000,000 m 3 less than 15 m
big greater than 1,000,000 m 3 larger than 15 m

In addition, deviations from this classification upwards or downwards due to the importance and the risk potential can be determined by the operator in coordination with the approval authority. This applies in particular if the two values ​​total storage space and height of the barrier structure are in different classes.

The primary purpose of flood retention basins is to protect against flooding. If there are other uses, flood retention basins with permanent damming can be assigned to reservoirs in the sense of DIN 19700. The corresponding decision is made on a system-specific basis. The criteria for the decision are less the size - the Lauenstein flood retention basin in Saxony , for example, has a height above the valley floor of over 40 m - than the ratio of the size of the permanent storage space to the flood retention space or other tasks such as raising the low water level, supplying drinking water or generating electricity.

Construction of a flood basin

A flood retention basin usually consists of a barrier structure with the purpose of damming the water in the reservoir if necessary. The operating and bottom drain enables the basin to be emptied and managed. A flood relief serves to protect against unexpectedly high water levels.

The barrier structure

The barrier structure can either be designed as a dam or a dam . Dams have been poured in and can be built on rock as well as on sufficiently stable soil (loose rock). If watertight layers are only to be applied at a greater depth, special measures must usually be taken to waterproof the substrate. Subsurface sealing and dam sealing must form a seamless sealing system. Dams are to be erected exclusively on stable rock. The exact design depends largely on the rock properties and the shape of the valley.

The decision on the type of barrier structure is determined not only by the decisive geological subsurface conditions but also by economic and design aspects. Dams should be preferred especially in seismic areas. On the other hand, building material must be available in sufficient quantity and quality in the local vicinity of the planned dam to build a dam.

Depending on the size of the system, various proofs of safety must be provided. These concern the structural safety, serviceability and durability of the structure. The fatigue strength must be proven with a service life of 80 to 100 years by regular visual and metrological controls. According to DIN 19700-11, constant effects such as dead load, traffic and surcharge as well as water pressure in the event of permanent congestion, but also rare effects such as extreme flood accumulation and unusual load cases such as earthquakes must be taken into account in the design.

The operating and bottom outlet

In many flood retention basins, the operating and bottom drains are combined in one structure and form the "heart" of a dam. The common operating and bottom outlet mostly runs at the level of the river bed through the foot of the dam and consists of an inlet structure with flotsam rake , a throttle or closure area, a passage (transport channel) and an outlet structure with stilling basin .

Cross-section of the dam at the Jonenbach flood retention basin (1): culvert, (2): flood relief with driftwood rake , (3): bottom outlet (inlet) with rake and inlet restriction, (4): outlet structure with fish ladder, (5): stilling basin (fast flowing water is slowed down), (6): footpath, (7): dam crest
Inflow (red) and discharge hydrographs (blue) of the design flood for a) uncontrolled basin and b) basin controlled for constant discharge
Inlet structure for the bottom outlet with rake and inlet throttling, as well as opening for bypass (right)

The most important core task of the operational drain is to limit the discharge. The water release is adjusted by a reduced cross-section in the throttle area so that only a harmless amount is released into the underflow. The throttle area can be arranged on the water side, in the middle or on the air side at the bottom outlet. The reduction to the rated discharge ("regular discharge") is done either by a fixed size of a flow opening in the bottom outlet or by a movable throttle in the form of a contactor, slide or flap. The uncontrolled bottom outlet has the advantages of low susceptibility to failure as well as lower construction and maintenance costs. On the other hand, the disadvantages are poor utilization of the pool's contents and the lack of the ability to adapt to the flood wave (pointed or flat top discharge).

In the case of an uncontrolled basin, the size of the flow opening must be matched to the maximum permissible flow rate Q max for a completely dammed basin. At a lower level in the retention basin, on the other hand, the water pressure and thus the flow rate is lower than the capacity of the lower course of the stream would allow. At the beginning of the flood wave, this results in a stronger build-up in the basin than is actually necessary (striped area in the upper diagram). If the inflow falls below the permitted outflow quantity Q max , the basin empties again relatively slowly (delayed discharge).

The adaptive, event-adapted control, on the other hand, allows a uniform delivery of water that is matched to the critical flow in the underflow. Especially at the beginning of a flood wave, the flow rate can be increased through a wide opening, the damming can be reduced (striped area in the diagram below) and the retention area can be better used. Since the proportion of driftwood is usually increased at the beginning of a flood event, a larger opening in the bottom outlet at this point in time has the further advantage that the risk of blockage by floating debris (risk of blockage) is reduced. After the flood wave has subsided, the normal level is reached more quickly and the basin is available for further floods more quickly. The advantages described are greater with larger systems than with smaller or medium-sized systems. The flood quantities released by different flood retention basins can also be coordinated with one another in order to reduce the overlapping of flood waves at river mouths by means of a slightly delayed release. On the other hand, adaptive control requires complex planning and reliable information about the overall runoff situation in the affected area.

To increase operational safety, DIN 19700-12 prescribes a bypass in the closure area for medium and large pools (height greater than 6 m, volume greater than 100,000 m³) . This is either a bypass line for the throttle area or a second separate opening with a closure option. Usually the bypass is closed. If the bottom outlet becomes blocked, the basin can be emptied via the bypass. The emergency outlet either opens directly into the stilling basin (separate pipe opening) or is only routed separately in the first part and then opens into the bottom outlet (common outlet).

Flood relief

Retention basin Glashütte , dam rebuilt in 2007

If so much water flows in during flooding that the basin is completely filled, the additional water flowing in must drain off via the flood relief . This must therefore have a fixed overflow, a channel and a stilling basin . Even an overflowing pool has a flood-absorbing function thanks to the retention .

Further examples

See also

Web links

Individual evidence

  1. ↑ a b c d e f Clemens Höfer: Design and dimensioning of bottom outlets of flood retention basins in small catchment areas, master's thesis to obtain the academic degree of graduate engineer, Institute for Water Management, Hydrology and Structural Hydraulic Engineering, University of Natural Resources and Life Sciences, Vienna, 2010 ( online as PDF downloadable )
  2. ↑ Andrew Faeh, Susanne Eigenheer Wyler and Heinz Hochstrasser: Flood retention basin: progressive and proven. (PDF; 318 kB) (No longer available online.) Basler & Hofmann, archived from the original on April 30, 2015 ; accessed on June 8, 2013 (review article on the functioning of flood retention basins from UMWELTPRAXIS No. 55 / December 2008 pp. 17–20).
  3. ↑ a b c State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg: Working aid for DIN19700 for flood retention basins , PDF (1.8 MB), 2007, accessed on June 16, 2013.
  4. ↑ Ronald Haselsteiner: Normative innovations of DIN 19700-12 / 2004 "Flood retention basin" ( Memento from January 7, 2014 in the Internet Archive ) , PDF, accessed on June 20, 2013.
  5. ↑ a b Konrad Bergmeister , Frank Fingerloos, Johann-Dietrich Wörner: Concrete Calendar 2011: Focus: Power plants, fiber-reinforced concrete , Verlag Ernst & Sohn, 2010, ISBN 978-3-433-02954-1 (online preview on Google Books )
  6. ↑ AWEL: Office for Waste, Water, Energy and Air: Notice board: detention basin on Jonen Bach to protect Affoltern am Albis