Detention basin

A flood retention basin ( HRB ) is a storage system whose main purpose is to regulate the runoff of a river in flood. It dampens the flowing flood wave by emitting caches excessive water loads and controlled by an event subsided again there. The basin is normally empty (so-called dry basin or green pools) or partially filled (permanent dams). Large flood retention basins can be constructed similarly as dams, but their main purpose is flood control. However, dams also serve other purposes as well, such as the drinking water supply or electricity generation. As a shut-off dams or dams are used.

• 3.1 The shut-off
• 3.2 The operational and bottom outlet
• 3.3 Spillway

Location of the pool for water

Main circuit

Flood retention basins in the " main circuit" are flowed directly from the waters. The shut-off ( 1) is transverse to the river, where the river runs freely through the bottom outlet at low tide. Increases the flow of the water over the control output of the bottom outlet, this increased runoff is retained and the pelvis (2) is dammed. Only when the flow falls below the control runoff, the basin slowly emptied again. The bottom outlet is set so that only as much water can flow through, as the underflow can withstand without damage. The lowest portion of the basin, which is regularly affected with minor flood is typically designed as a pure natural area (3). An example of such a basin is the flood retention basins Jonen Bach.

Shunt

Flood retention basins in the " shunt " (also called polders ) are traversed and not directly by water, but the pool (2) is arranged laterally next to the river. The pelvis is oriented in the flow direction by a longitudinal dam ( 1) separated from the river. During floods, a portion of the water from the river through a reconciliation ( 5) can be passed into the pelvis. It later flows through the conduit or other outlet (6) back into the river. The river remains in the normal state in its natural course. The ecosystem of the river with shoreline and wetlands is therefore retained throughout the flood retention basins in the creek. An example of such a basin is the flood control basin Langeler bow.

Classification

A subdivision of flood retention basins in accordance with DIN 19700-12 first of all due to the size according to the following table:

In addition, deviations from this schedule may be set by the operator in consultation with the licensing authority in up or down because of the importance and the hazard potential. This is especially true if the two values ​​are the total storage space and height of the dam are in different classes.

The primary purpose of flood retention basin is to protect against flooding. Are there other uses, flood retention basins can be associated with duration of storage dams in the sense of DIN 19700. The decision shall be plant-specific. Criteria for the decision not so much the size - finished in 2006, flood retention basins Lauenstein in Saxony for example, has a height above valley floor of about 40 meters - but rather the ratio of the size of permanent storage space for flood storage space or other tasks such as low water levels, water supply or electricity.

Construction of a flood basin

A flood retention basins normally consists of a shut-off with the purpose dam up the water as necessary on the reservoir. The operating and bottom outlet allows the pelvic emptying and pelvic management. A spillway serves to protect against unexpected high water levels.

The shut-off

The shut-off can be performed either as a dam or a dam. Dams are poured and can be built both on rock and on sufficiently load-bearing soil ( loose rock ). If waterproof layers pending only in greater depth, specific measures to Underground seal shall be provided in the rule. Underground seal and dam seal must form a complete seal system. Dams are to be constructed solely on firm rock. The exact design depends largely on the rock properties and the valley shape.

The decision on the type of the dam is determined not only by the decisive geological subsoil textures also of economic and design considerations. Dams should be preferred, especially in earthquake-prone areas. On the other hand, must be in the local vicinity of the proposed dam is available for a dam construction material in sufficient quantity and quality.

Depending on the size of the plant are different security credentials to be provided. These relate to the structural safety, serviceability and durability of the structure. The fatigue strength shall be demonstrated with a service life of 80 to 100 years by regular visual and metrological controls. According to DIN 19700-11 must be such as earthquakes considered in the design permanent actions such as dead load, traffic and ballast water and pressure at permanent traffic jam, but also rare effects such as an extreme flood storage and exceptional load cases.

The operating and bottom outlet

Operating and bottom outlet are summarized in many flood retention basins in a structure and form the "heart " of a dam. The common operational and bottom outlet runs mostly at the level of the river bed by the foot of the dam and consists of an intake structure with flotsam, a choke or lock area, a passage ( Transportgerinne ) and an outlet structure with stilling basin.

The most important task of the core operating indulgence is the outflow boundary. The release of water is adjusted by a reduced cross-sectional area in the throttle so that only a harmless amount is excreted in the lower reaches. The throttle range can be located on the water side, center, or on the air side bottom outlet. The reduction to the design discharge ( " control output " ) is carried out either by a fixed size of a flow-through opening in the bottom outlet or by a movable throttle in the form of contactor, valve or damper. The uncontrolled bottom outlet has the advantages of low susceptibility to interference, and lower installation and maintenance costs. On the other hand, are to be mentioned as disadvantages of a poorer utilization of the pelvic contents, and the lack of ability to adapt to the flood wave (pointed or flat peak outflow).

In an uncontrolled basin the size of the orifice must be adjusted to the maximum allowable outflow rate Qmax at full angestautem pool. At a lower level in the retention basin, however, the water pressure and thus the flow rate is lower than would allow the performance of the lower reaches of the stream. This results in the beginning of the flood wave in a stronger impounding basin than is actually necessary ( striped area in the upper diagram ). Inflow falls below the allowable discharge rate Qmax, then the basin drained relatively slowly ( delayed release ).

The adaptive, event control system adapted other hand, allows a uniform, coordinated at the critical flow in the lower reaches of release of water. Especially at the beginning of a flood wave, the discharge rate increased by a wide opening of the impounded reduced ( striped area in the lower diagram ) as the retention space be better utilized and. Since at the beginning of a flood of mostly Schwemmholzanteil is increased, a larger opening in the bottom outlet at this time has the further advantage that the risk of blockage is reduced by flotsam ( Verklausungsgefahr ). After the flood wave decay of the normal level is reached faster and the pool is faster for more high water. The benefits described are stronger than smaller or medium-sized plants with larger systems. The emitted high amounts of water of different flood retention basins can be coordinated in order to reduce a superposition of flood waves on river mouths by a slightly delayed delivery. On the other hand, requires an adaptive control of a complex planning, as well as reliable information on total runoff situation of the affected area.

To increase operational safety DIN 19700-12 writes a bypass in the closure area before ( greater than 6 m, a volume greater than 100,000 cubic meters height ) for medium and large pool. This involves either a bypass for the throttle range or a second separate opening with possibility. Normally, the bypass is closed. In a Verklausung the bottom outlet, the basin can be drained via the bypass. Emergency exit opens directly into the stilling basin ( separate pipe culvert ) or is managed separately only in the first part and then flows into the bottom outlet (common outlet ).

Spillway

Flows at high water so much water that the basin is completely filled, the more incoming water must run over the spillway. This must therefore have a fixed overflow, a flume and stilling basin. Thanks to the retention Even a overflowing pool still has a high water -absorbing function.

Other examples

• Flood retention basins Salzderhelden, the largest in Lower Saxony
• Retention basins Auhof, flood retention basin of the river Wien in Vienna