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Système de contrôle des crues et des barrages des réservoirs d'écrêtement des crues au sud ouest de l'Allemagne
Water management measures are closely linked to the various
requirements of human society. They have to facilitate the water utilization in
order to satisfy many different needs, but must also provide a protection against
the destructive effect of water. However, they cannot be realized without farreaching
consequences for the natural, social and ecological environment of
human beings.
In many catchment areas, meaningful management of the water available
in a region has to be based on storage basins. Storage reservoirs make it
possible to meet a lot of multipurpose objectives, among them the essential flood
protection for larger areas. Damaging flood events became more frequent during
the past two decades.
Responsibility for water resources management is shared between the
Federal Government of Germany and the State Governments. With the exception
of the federal waterways, all main river systems including their flood control
facilities are under the water management boards of the 16 German States. The
water management administrations usually have a three-level-structure, the
lowest level instances in the districts and major towns being in charge of the
practical execution of flood control measures. An important role in regional water
management tasks is played by water associations, formed by communities
bordering the same waters for the purpose of handling specific water resources
management including water supply, sewage treatment, maintenance of water
courses and flood protection.
Germany occupies a central location in Europe. It covers an area of
357 000 km2 and has a population of 80 million. The average population density
is 225 inhabitants per km2. Although the country is heavily industrialized and
densely populated, the area covered by building developments and traffic
infrastructure is only some 12 % of the total area, 58 % of the total area is utilized
for agriculture and 30 % of forest.
One of the 16 German states is Baden-Württemberg located in the
Southwest with the most important two river basins of Rhine and Neckar. In
December 1993 and April 1994, extreme flood events in Germany and Baden-
Württemberg caused damage to water courses, built up areas, industrial facilities
and communal transportation and communication infrastructure. Just in Baden-
Württemberg alone, the damage was of an order of magnitude of 500 million DM.
Historically, these floods were by no means unusual. Recurrence periods are, for
example, at Cologne 65 years, and on the rivers in the Stuttgart region 20 to 100
years.
The main flood waves were caused by extreme precipitation falling on
water saturated ground. The soil pores were largely filled by previous intensive
rainfalls. Prior to the rainstorms, the storage capacity was exhausted. Thus the
soils were naturally made impervious, with the consequence that almost the
entire water falling during the rainstorms ran off rapidly. This meant that building
over of land due to urbanization did not play any decisive part in the generation
and the intensity of the floods.
Extreme floods occur along the major German rivers predominantly in the
winter and spring months, whereby in many cases heavy and long-lasting
precipitation falls on ground which, in part, is still frozen and on melting snow,
particularly in the uplands, which reinforces flood run-off.
For flood protection, the number of alternative countermeasures available is
considerably greater than generally assumed. Conventional hydraulic
engineering measures like retention and water course development are an
economic proposition and justifiable only when other approaches have been
exploited to the full. First and foremost, provisions have to be made to reduce
damage in the inundation areas of rivers like measures to match land utilization
to the natural risk situation, measures to protect property (backwater gates,
erecting dikes around properties, flood- and uplift-proof structural design,
emergency escape routes); last but not least flood defences (action plan,
safeguarding measures, placing flood fighting material at readiness, warning
service).
Only after these have been implemented are expensive structural flood flow
regulation installations appropriate, by the construction of water retention basins,
flood channels, watercourse development, dams and protective walls.
The River Neckar, the second largest river in Baden-Württemberg,
occupies a particular position. Its source as well as its outlet in the River Rhine as
well as virtually its entire catchment area lie within the State boundaries. The
catchment area is surrounded by the Black Forest and Swabian Alb and forms
with its south-eastern boundary the European watershed between Rhine and
Danube. It has a size of 14 000 km2, thus covering 39 % of the total area of
Baden-Württemberg of 35 751 km2 - an area, where about half the population
lives out of altogether 10.2 million. Apart from the water scarcity the most
prominent characteristics are the relatively high mean annual temperatures,
which are next to the Rhine Valley at the top of the scale in Germany, and the
favourable soil and growth conditions. This explains why Baden-Württemberg
has a high population density, currently amounting to 285 inhabitants per square
km.
Tremendous flood flows occur in the river Neckar, which in the past have
greatly influenced population and industrial development, and continue to do so.
Additionally, the development of settlement in the flood plains has resulted in a
reduction of natural impoundment zones and flooding areas.
In earlier times, protection against flood dangers was largely implemented
by local measures, such as development of water courses and building dikes, but
less by flood water retention measures. It was thus possible to achieve flood
protection of settlements and transportation routes, and at the same time new
industrial and residential areas could be erected in river valleys. The
consequences of this, though, were that flows were accelerated, and in many
places the danger of flooding downstream became more acute. The
topographical and geological situation in the catchment area of the Neckar, the
widespread creation of built-up areas in the valley flood plain, the exploitation of
the waters for the economy, industry, transportation facilities, drinking water
protection zones, as well as sewage disposal and treatment plants hinder the
implementation of major damming schemes.
For this reason, protection against flooding has been realized largely by the
creation of smaller retention basins and basin systems in the tributary valleys.
The required impounded volume in order to protect the valley of the Lein, a
tributary of the river Kocher as a first example, against flooding amounts to some
15 million m3. Up to now, some 13 million m3 has been created, costing 30 million
DM.
In parallel to this, planning of surface water courses for flood protection has
concentrated more on existing retention areas. At the same time, their
preservation and protection serve the interests of nature conservation and contribute to stabilisation of the ecological balance. Water retention areas which
have been lost by building development, can and should be replaced on principle
by retention basins.
Due to the increase of built-on areas and infrastructure systems in the
valley floors, implementation of major reservoir schemes is not possible any more
in the densely populated regions of Baden-Württemberg. Instead, retention basin
systems have been developed for flood protection purposes in tributary valleys.
Over 300 retention basins are in operation in Baden-Württemberg alone, most of
them under the responsibility of water associations, set up by riparian
communities under statuary laws. The flood protection associations receive
funding from the State for construction, maintenance and operation as well as
technical support services and support by the water resources administration.
Within the Stuttgart Administrative district there are 18 flood protection
associations which from 1955 till now have constructed and operated a total of
over 90 flood retention basins with capacities greater than 100.000 m3. The State
government promotes this investment by a subsidy of 90 % and maintenance at a
proportion of 15 % less than the capital investment subsidy.
In principle communities have to contribute at least 30 % as compensation
for the benefits, they derive. This is regulated in each individual case in a contract
between the State government and the community concerned.
In the past decades, flowing waters in Baden-Württemberg have been
greatly changed from their natural course. Due to water course development,
large parts of the natural flooding and retention areas have disappeared and the
water courses have been greatly shortened. During this process, many ecological
systems of the landscape along the rivers were lost. In part, the danger of
flooding downstream was increased. Heavy urbanisation, and in particular the
concentration of commercial and industrial locations as well as traffic routes
along the flood plains, and also intensification of agriculture, constrained the
water courses into narrow corridors. At today's state of knowledge full flood-proof
development of a river is, as a rule, not a desirable solution.
Flood protection measures have to be implemented with the highest
possible degree of environmental compatibility if they are to fulfil today's general
aims of water management along river areas. For this reason, a sensitive
approach has to be taken. Prior to their implementation, not only the locally
achievable flood protection, but also their impacts as a whole on the discharge
behaviour of the water course system, the ecology and the landscape
appearance have to be taken into account. From this it is possible to evaluate the
advantages and draw-backs of the planned measures, as well as of various
combinations and options. This makes it possible to minimize impacts on the
natural surroundings, and to optimize flood protection measures with respect to
all important aspects.
Flood retention with storage basins has the advantages of great
effectiveness, if applied just upstream of properties to be protected. Then only
small areas will be flooded, no adverse impacts along the river basins, last but
not least the utilization of the impounded area as public amenity. Draw-backs are
the lack of topographical possibilities for reservoirs near the communities to be
protected, the high maintenance costs, the adverse ecological impact on the
inundated area as well as interruption to the continuity of the water course
systems, furthermore the intervention in the sedimentation behaviour along the
river bed and the intrusion in the landscape.
The various flood protection measures are to be evaluated and selected,
depending on the hydrological, topographical, geological and ecological
constraints, on the basis of the criteria degree of attainable flood protection,
economic efficiency and reasonable, i. e. optimal water management.
In earlier times, flood retention basins were usually designed on the basis
of actual flood events. However, the hydrological findings and processes that
have been developed on a large scale since approximately the mid-seventies
have opened up new perspectives and it has become possible to consider the
safety and operational abilities of flood retention basins with greater accuracy.
Systematic safety examinations of impoundments have revealed
shortcomings, considerable in some cases, usually as a result of the
underestimation of the design flow for the emergency spillways. The aging of
materials, the subsequent placing of structures in the outflow cross sections of
spillways, such as the installation of accident prevention grates, changing in use
in the catchment areas, the storage areas and the areas downstream of the flood
retention basins - all these have led to safety - related problems which have been
tackled using the improved rules of the art.
Fig. 1 shows the main features of a common flood retention basin. The
layout demonstrates distinctly that for the technical solution reservoir, dam, intake
structure, bottom outlet, spillway, chute and stilling basin are the significant
constructions just as well as for big impounding reservoirs.
Progress in the art is reflected in an ever increasing number of standards,
guidelines and codes of practice. According to them for normal operation of the
flood retention basin the dimensioning follows the aspects of function and
operation, in order to guarantee the protection of downstream populations from
floods, for extra-normal operation the dimensioning follows the aspects of safety,
in order to prevent the destruction of the installation if extreme events occur.
Fig. 1 Flood retention basin/ Réservoirs d'écrêtement des crues
1. dam / barrage
2. inlet structure / prise d'eau
3. bottom outlet / vidange de fond
4. valve chamber / chambre de vanne
5. spillway / évacuateur des crues
6. chute / coursier
7. stilling basin / bassin d'amortissement
8. developed river / section aménagée de la rivière
9. maximum reservoir head / niveau d'eau maximal du réservoir
In the codes of practice guide values are given for the specification and
dimensioning of storage level (permanent storage level), usual flood retention
capacity, bottom outlet and operating outlet, emergency spillway, further intake
structures, stilling basin and freeboard. The differences between installations with
and without movable gates are the basis for the clear distinction between
operation in flood-free periods and during floods. The operating instructions
(together with the associated responsibilities) include the measurement-related
requirements for operation, specifications of the basis discharge, control of the
gates, the maintenance schedule and commissioning including test
impoundment. The operation principles are the same for flood retention basins as
temporary impoundment areas, or as basins with a permanent water level.
The required storage capacity of flood retention basins depends on the
intended form of flood regulation (uncontrolled or controlled) and the flood
frequency, with the extent of any additional channel development being
determined by landscape appearance, urban character and cost. When planning
flood retention basins for which, over and above the flood protection capacity,
sufficient area remains in the valley for water storage, often the specific task of
flood protection is superimposed on multiple water management tasks, such as
the creation of permanent lakes and public amenities areas, fish breeding,
groundwater replenishment, augmentation of low river flows, agricultural
irrigation, and upgrading of water quality. In the case of dry basins, the
opportunity presents itself of using basin areas which are only flooded over short
times for agriculture and forestry.
It is self-evident that such multiple tasks, and particularly the combination of
reservoir management with flood retentions, require detailed preliminary
investigations in compliance with the precepts of framework for water
management planning.
In most cases, it is not possible to capture completely the flood loading
established on the basis of hydrological investigations in individual retention
basins, and then to release these after a time delay into the discharge waters,
since the available capacity is not sufficient. Only by means of operational
regimes matched to local conditions is it possible to achieve retention of critical
discharges, and to limit as a rule the downstream discharge to the permissible
level.
The simplest operating regime of retention basins is the lake retention,
which is to be understood as the smoothing effect on the discharge due to lakes
or expansions of a water course bed in the nature of a lake.
In contrast to this is controlled retention for which the outflow is held as
uniform as possible depending on the developed discharge in the downstream
water course. This depends on the ratio of the normal flood retention capacity to
the flow volume which has to be retained. Also the degree of smoothing of the
peak flow depends on this ratio.
If the normal flood retention capacity is the same as the flood volume to be
accommodated, the basin just fills up, without triggering operation of the spillway,
although this presupposes the correctness of the flood prediction with regard to
shape and content of the flood wave, and the basin capacity dimensioned to
match this.
As it could be seen flood protection reflects to a high degree of the basic
human need of protection against the force of nature. The expansion of urban
development observed in many regions has, particularly in conurbations, resulted
in an appreciable reduction in the natural areas available for water retention,
which has often been promoted by measures to prevent flooding along lengthy
river sections. So, for example, along the middle reaches of the River Neckar
between Stuttgart and Heilbronn, in the past four decades around four-fifths of
the natural flooding area has been lost. The danger of flooding area has also
been made more acute by water management and agricultural measures such as
draining and canalisation of water courses, transformation of pastures into arable
land, draining and land consolidation.
Fig. 2 Catchment area of Kocher-Lein (A) and upper Jagst (B) (Table 1 and 2) with the
locations of 13 flood control reservoirs along the rivers Kocher (A) and Lein
(tributary), furthermore with 21 flood control reservoirs along the river Jagst (B)/
Bassins versants de Kocher-Lein (A) et haute Jagst (B) (table 1 et 2) avec les
sites de 13 réservoirs d'écrêtement des crues le long de la rivière Kocher (A) et
Lein (tributaire) ainsi que 21 réservoirs d'écrêtement des crues le long de la
rivière Jagst (B)
Table 1
Flood retention basins along the rivers Lein (tributary) and Kocher (Fig. 2)/
Réservoirs d'écrêtement des crues le long des rivières Lein (tributaire) et Kocher
(Fig. 2)
| No. |
Name |
catchment area [km²] |
capacity level [m a.s.l.] |
maximum flood [m a.s.l.] |
max. discharge for the spillway [m³/s] |
max. capacity of the reservoir [10³ m³] |
max. capacity of the bottom outlet [m³/s] |
height of the dam [m] |
| A1 |
Aichstrut |
5,8 |
502,07 |
502,34 |
8,4 |
550 |
5,0 |
8,00 |
| A2 |
Eisenbach |
7,4 |
460,20 |
460,50 |
14,7 |
540 |
7,0 |
13,50 |
| A3 |
Leineck |
31,9 |
452,47 |
453,63 |
38,0 |
2.180 |
15,0 |
14,00 |
| A4 |
Hagerwald |
14,0 |
450,90 |
451,68 |
24,2 |
800 |
12,0 |
11,00 |
| A5 |
Hüttenbühl |
15,5 |
448,87 |
450,19 |
27,5 |
530 |
7,0 |
11,00 |
| A6 |
Reichenbach |
8,8 |
440,57 |
441,59 |
14,3 |
900 |
9,0 |
16,00 |
| A7 |
Täferrot |
108,7 |
417,88 |
419,28 |
99,0 |
2.200 |
27,0 |
14,00 |
| A8 |
Rehnenmühle |
45,1 |
427,30 |
428,30 |
48,1 |
2.900 |
32,0 |
16,00 |
| A9 |
Götzenbach |
17,4 |
431,93 |
432,91 |
30,2 |
1.750 |
16,0 |
23,00 |
| A10 |
Federbach |
9,6 |
417,84 |
418,92 |
19,7 |
800 |
16,0 |
25,00 |
| A11 |
Laubach |
2,9 |
392,00 |
392,70 |
3,5 |
190 |
9,0 |
13,00 |
Table 2
Flood retention basins of upper Jagst (B1-B15) and Brettach (tributary, B16-B21)
(Fig. 2)/
Réservoirs d'écrêtement des crues des rivières haute Jagst (B1-B15) et Brettach
(tributaire, B16-B21) (Fig. 2)
| No. |
Name |
catchment area [km²] |
capacity level [m a.s.l.] |
maximum flood [m a.s.l.] |
max. discharge for the spillway [m³/s] |
max. capacity of the reservoir [10³ m³] |
max. capacity of the bottom outlet [m³/s] |
height of the dam [m] |
| B1 |
Stockmühle |
19 |
501,80 |
502,80 |
39,90 |
961 |
4,10 |
8,00 |
| B2 |
Buch |
81 |
449,70 |
450,12 |
86,60 |
1070 |
8,60 |
9,80 |
| B3 |
Sonnenbach |
9 |
488,50 |
489,50 |
23,90 |
599 |
3,30 |
9,40 |
| B4 |
Haselbach |
8 |
482,30 |
483,30 |
6,20 |
1350 |
5,40 |
10,20 |
| B5 |
Häsle |
10 |
479,50 |
480,60 |
18,50 |
750 |
6,40 |
7,40 |
| B6 |
Rötlen |
22 |
470,40 |
471,08 |
17,00 |
610 |
2,80 |
5,00 |
| B7 |
Schlierbach |
8 |
482,50 |
483,02 |
8,70 |
635 |
5,20 |
8,40 |
| B8 |
Schwabsberg |
175 |
447,08 |
447,50 |
80,00 |
2720 |
40,0 |
12,20 |
| B9 |
Glasweiher |
9 |
448,50 |
449,52 |
7,80 |
505 |
6,70 |
7,50 |
| B10 |
Kreßbach |
8 |
438,90 |
439,60 |
15,80 |
641 |
7,00 |
10,50 |
| B11 |
Fischbach |
17 |
431,90 |
432,56 |
25,50 |
641 |
4,00 |
11,30 |
| B12 |
Holzmühle |
12 |
? |
? |
? |
190 |
? |
7,80 |
| B13 |
Orrot |
18 |
437,50 |
440,00 |
32,80 |
1190 |
11,50 |
13,50 |
| B14 |
Reiglersbach |
19 |
431,20 |
432,50 |
59,70 |
964 |
4,20 |
9,00 |
| B15 |
Degenbach |
8 |
420,60 |
421,80 |
19,90 |
650 |
1,70 |
9,20 |
| B16 |
Wiesenbach |
19,1 |
464,77 |
465,75 |
60,0 |
720 |
10,5 |
8,00 |
| B17 |
Breitloh |
60,4 |
412,70 |
414,00 |
58,0 |
242 |
25,0 |
8,00 |
| B18 |
Wallhausen |
31,3 |
423,90 |
424,90 |
50,7 |
36 |
9,4 |
4,50 |
| B19 |
Seebach |
45,1 |
406,50 |
409,10 |
66,0 |
39 |
1,0 |
6,50 |
| B20 |
Bemberg |
37,1 |
417,60 |
419,00 |
90,0 |
1.800 |
43,0 |
28,00 |
| B21 |
Beimbach |
157,2 |
473,5 |
475,50 |
240,0 |
2.660 |
0,0 |
28,60 |
For this reason, protection against flooding has been realized largely by the
creation of retention basins and basin systems in the tributary valleys. For the
first example the required impounded volume in order to protect the valley of the
river Lein, a tributary of the river Kocher (Fig. 2) flowing into the river Neckar,
against flooding amounts to some 15 Mio. m3. Up to now, some 13 Mio. m3 has
been created, costing 30 Mio. DM. Characteristic datas are presented in Table 1.
One typical example of an executed flood retention basin within the
catchment area of the tributary Lein is the reservoir Hagerwald (Fig. 2, number
A4). Following the specific datas given in Table 1 the maximum height of the
earthfill dam reaches to 11 m (Fig. 3). The spillway, the intake structure of the
bottom outlet and the joint stilling basin are pointed out in Fig. 4.
Fig. 3
Cross section of the earthfill dam Hagerwald (Fig. 2, number A4)/
Section transversale du barrage en remblai Hagerwald (Fig. 2, numéro A4)
1. maximum head (451,7 m a.s.l.) / niveau d'eau maximal de la retenue
2. permanent water level (443,4 m a.s.l.) / niveau d'eau permanent
3. cutoff wall and clay sealing embedded in rockfill
/ parafouille et matériaux argileux
encastré en remblai d'emrochement
4. hydraton sealing / masque d'étanchéité en "hydraton"
5. dam body / corps de barrage
6. rock fill / enrochement
7. drainage / drainage
In addition of them Fig. 5 contains the cross section of the 23 m high
earthfill dam for the impounding reservoir Götzenbach, acting as the most
important flood retention basin within the catchment area of the reservoir Lein
resp. river Kocher (Fig. 2 and Table 1, number A9).
For the second example the river Jagst is a right hand tributary of the
Neckar with the total catchment area of 1 862 km2. Its catchment area (816 km2)
of the upper Jagst lies in the eastern part of Baden-Württemberg and contains a
system of fiteen storage basins with a capacity of around 12 Mio. m3, and whose
discharges can be regulated (Fig. 2 and Table 2).
Originally in this region, despite the low long-term average annual
precipitation of 865 mm, over many centuries disastrous floods have occurred. In
the fifties, much thought was given to regulating the flood waves, which led in
subsequent years to setting up the "Upper Jagst Water Association" and to the
construction of storage reservoirs and water retention basins. Apart from the
flood control function, nearly all basins serve for low-flow augmentation of the
river Jagst, which carries little water during the summer months. In the past
several years, at some of the larger reservoirs and retention basins, facilities
have been provided for local recreation amenities, such as swimming, sailing and
fishing. Some lakes and parts of lakes not greatly used for leisure activities have
been converted into, and designated as, biotopes.
Fig 4
Hagerwald: cross sections of intake structure, spillway, bottom outlet and joint
stilling basin/
Hagerwald: sections transversales de l'évacuateur des crues, prise d'eau de fond
et bassin d'amortissement adjoint
1. weir crest of spillway inlet (450,9 m a.s.l.) / cote du seuil déversant d'entrée
de l'évacuateur
2. bottom outlet with gate / vidange de fond avec vanne
3. valve chamber with bulkhead gate ("Mönch") / chambre de vanne ("Mönch")
4. fixed weir for permanent water level / seuil déversant au niveau d'eau
de la retenue permanent
5. bifurcation of bottom outlet / bifourcation de la prise d'eau de
fond
6. ground sill / seuil de fond
7. end sill / seuil de sortie
Fig. 5
Cross section of the earthfill dam Götzenbach (Fig. 2, number A9)/
Section transversale du barrage en terre Götzenbach (Fig. 2, numéro A9)
1. sloped material of dam excavation / talus en matériau de fouille du
barrage
2. rock fill / remblai en enrochement
3. dam body / corps de barrage
4. asphaltic sealing / masque d'étanchéité asphaltique
5. cutoff wall and clay sealing embedded in rockfill / parafouille et matériaux argileux
encastré en remblai d'enrochement
6. permanent water level / niveau de retenue permanent
In the following, the focus will be on the Buch and Schwabsberg basins.
The water storage and retention basin Buch (Fig. 6) was constructed
between 1975 and 1991. Its catchment area amounts to 84 km2, the mean water
outflow to 1.1 m3/s, and the hundred-year-flood inflow to 54 m3/s. The basin has a
total reservoir capacity of 1 150 000 m3, occupying an area of 29,6 ha. During the
summer months, for local amenity purposes, a permanent water volume of
680 000 m3 is retained, covering an area of 25.8 ha; which means that in summer
the capacity available for flood control is 470 000 m3. In the winter time, with a
lowered water level in the reservoir, flood control capacity is increased to
1 070 000 m3.
Fig. 6
Schwabsberg (1) and Buch (2) reservoirs/
retenues de Schwabsberg (1) et Buch (2)
1. Schwabsberg reservoir / retenue de Schwabsberg
2. Buch reservoir / retenue de Buch
3. dam with outlet / barrage avec prise d'eau
4. sedimentation reservoir with bird islands / basin de sedimentation avec île d'oiseaux
5. playing and swimming area / aire de jeu et natation
6. swimming area / aire de natation
7. nature area / aire naturelle
8. road/railway / route/chemin de fer
The impounding structure consists of an earthfill dam with a clay core. The
slope on the reservoir side is covered with concrete paving. The dam of 240 m
length and up to 9.8 m in height is founded on impermeable marl, and its crest
width is 7.0 m. The square (2.0 x 2.0 m) bottom outlet is located in the northern
part of the dam, and leads to a gate shaft with upflow baffle. Installed in the gate
shaft is a bottom outlet sluice gate (1.5 x 1.5 m). This roller sluice gate can be
operated by an electrically driven spindle winch as well as manually. The
discharge capacity of the gate amounts to 12 m3/s. For fine regulation of the
water level or low-flow augmentation, a further regulating gate of 0.5 m diameter
with mechanical drive is installed.
The spillway installation consists of two independently controllable gates of
the fish belly type 0.8 m x 5.4 m and 1.5 m x 14 m in size, which drain by way of
a stilling basin into the adjacent Schwabsberg basin. At the maximum water level,
the total outflow capacity of these gates amounts to 77 m3/s.
The Schwabsberg retention basin was constructed between 1972 and 1974
(Fig. 7). Its catchment area is 175 km2. The mean water outflow amounts to
1.9 m3/s, and the hundred-year-flood outflow to 100 m3/s. The Schwabsberg
basin is located immediately downstream of the Buch basin described before.
This basin has a gross reservoir capacity of 2 740 000 m3 and occupies an area
of 83 ha. The low permanent storage capacity of 20 000 m3 and water covered
area of 2.0 ha is negligible, which means that it can be designated as a "dry
basin". Characteristic datas are presented in Table 2 with respect to number B8
in Fig. 2.
Fig. 7
Cross section and layout of the flood retention basin Schwabsberg (Jagst) with
bottom outlet and stilling basin (Fig. 2, number B8)/
Section transversale et vue en plan du réservoir d'écrêtement des crues
Schwabsberg (Jagst) avec prise d'eau de fond et basin d'amortissement
(Fig. 2, numéro B8)
1. maximum water level (446,9 m a.s.l.) / niveau d'eau maximal
2. inlet structure with trash rack / prise d'eau avec grille d'entrée
3. dam body / corps du barrage
4. radial gate / vanne segment
5. stilling basin / bassin d'amortissement
6. tail water level / niveau d'eau aval
7. drainage / drainage
The impounding structure comprises a rock/earthfill dam, waterproofed on
the reservoir side with clay. Its length is 450 m and its height 8.5 m. The crest
width is 20.4 m. The operating outlet with a cross section of 5 m x 2.5 m can be
closed by a radial gate driven hydraulically and electrically. In the event of a
power failure, the gate may be operated manually. The maximum discharge
capacity of the operating outlet amounts to 95 m3/s. The spillway consists of a
lowered dam section, 60 m long, serving as a fixed overflow sill. The discharge
capacity of the overflow sill is, at maximum water level, 115 m3/s.
For the summer operation of the Buch storage and retention basin, as a
rule, the basin is filled up to its permanent top water level. The water flows over
the gate into the "Mönch" outlet structure. The inspection gate remains open. All
other outlets are closed. In the event of impending floods, the water level is
regulated up to its maximum continuous point by way of the Mönch. If the water
level continues to rise, control of the basin is transferred to the automatic bottom
outlet gate, which is opened in increments. If, despite opening this gate fully, the
water level continues to rise, then in sequence fish belly gate 1 (small) is
operated, and after an interval fish belly gate 2 (large). Control of the gates is
automatic, depending on predetermined water level monitored by means of a
float gauge. The levels can be adjusted according to the requirements of water
management. In winter, the basin is normally emptied down to its minimum level.
Basin outflow is by way of the regulating gate. The inspection gate is opened. All
other water outlet devices are closed. In the event of impending floods the bottom
outlet gate is opened to a clearance of 80 cm. This position is retained even if the
basin level, regulation is transferred, like in summer operation, to the fish belly
gates.
During normal operation, the water level of the river Jagst is regulated to its
desired value by the fish belly gate in the weir downstream of the dam
Schwabsberg. This gate is moved automatically in accordance with the arriving
water flow, controlled by way of a level gauge switch. The radial gate in the dam
outlet structure is completely open, and the radial gate drive is disconnected
during normal operation of the basin. In the event of impending floods, the radial
gate is regulated manually so that the basin outflow will not exceed 40 m3/s.
Upon attaining the maximum impoundment level, the radial gate is opened to
95 m3/s, the maximum discharge, in order to maintain the basin water level. In the
event of a still increasing flood wave, the non-controllable spillway (overflow sill)
becomes effective. If the flow increases in an extraordinary way, the radial gate is
opened fully. The downstream fish belly gate is operated to evacuate the arriving
flow.
One of the most significant benefits resulting from construction of a dam is
the long-term and efficient protection that detention reservoirs provide against
floods. They modify the natural flow of a river respectively its hydrograph
suppressing or even avoiding the occurence of flooding downstream areas.
Flood-control reservoirs, created by concrete or earthfill dams, not only store the
flood water but also reduce the peak flow and considerably slow down the
discharge in the river reaches downstream of the dam. This management of flood
flows depends upon the outletworks and gated or nongated spillways.
For this kind of river basin management nearly 350 flood retention basins of
different size have been constructed in the state of Baden-Württemberg during
the last decades. In all cases they form a unit with the river course. Therefore
flood protection measures have to be implemented with the highest possible
degree of environmental compatibility. A sensitive approach has to be taken.
Prior to their implementation, not only the locally achievable flood protection but
also their impacts as a whole on the discharge behaviour of the water course
system, the ecology and the landscape appearance have to be taken into
account.
For the latter criteria, together with the economic efficiency, earthfill dams
are mostly preferred. The heights of them reach from a few meters to 30 m
refering in particular to the mentioned flood control reservoirs. There are no
differences to the conventional impounding reservoirs with respect to aspects of
planning, construction, operation, maintenance and security, except the fact
being temporarily used for flood retention by normally empty, i. e. agriculturally
used, or partly impounded basins, advantageous for leisure centres and water
sports.
The paper describes in detail the general significance of flood control
basins, their constructional design including the concerning standards and
regulations, furthermore the operational aspects. These principles will be
illustrated by the outstanding example of a system combining 14 detention
reservoirs along the river Lein with a catchment area of 247 km2. This river basin
is located in the northwestern part of Baden-Württemberg, Germany, a hilly
region with a high rate of precipitation and dangerous flood events due to a small
storage capacity of the underground. Thanks to the total retention volume of
more than 15 Mio. m3 the peak discharge at the mouth into the main river Kocher
(tributary of the river Neckar) will be reduced to 60 %.
Un des plus grands et importants résultat de la construction d'un barrage
est la protection efficace et à long terme contre les événements de crues et
d'inondations. Ces réservoirs d'écrêtement des crues modifient le cours naturel
des rivières, autrement dit des hydrogrammes, en évitant la formation de crues
dans les surfaces en aval.
Les barrages de contrôle de crues construits en béton ou remblai,
n'emmagasinent pas seulement l'eau des crues mais réduisent aussi le débit de
pointe considérablement et ralentissent l'écoulement en aval du barrage. Le
management des crues dépend des travaux de vidanges des déversoirs avec ou
sans vannes.
Si les avantages de tels systèmes de contrôle des crues pour les régions
en aval qui sont proches du barrage est évident, il y a encore beaucoup de
progrès à faire en ce qui concerne l'optimisation des lâchers d'eau lors de la
superposition des ondes de crues provenant de différentes rivières.
Pour ce genre de gestion des bassins de rivière, on a construit dans l'état
fédéral du Baden-Württemberg plus de 350 bassins de rétention des crues ces
dernières décennies. Dans tous les cas, ces constructions forment une unité
avec le cours de l'eau. C'est la raison pour laquelle l'implémentation de mesures
de protection contre les crues requit une compatibilité environnementale élevée.
Une approche sensible du problème est indispensable.
Avant q'une telle implémentation soit faite, ce n'est pas seulement la
protection locale contre les crues mais aussi l'impact du comportement global de
décharge des eaux dans un système de rivières, l'écologie, la nature
environnante ainsi que l'aspect visuel qui doivent être considérés
Pour le dernier critère, en plus de l'efficacité économique, les barrages en
remblai sont préférés. Leur hauteurs peut aller de quelques mètres jusqu'à 30 m,
se référant en particulier aux réservoirs de contrôle des crues cités. Il n' y a pas
de différence entre les dits bassins et les bassins conventionnels
d'emmagasinement des eaux en ce qui concerne la planification, la construction,
les opérations, l'entretien et la sécurité, excepté le fait que les bassins de
rétention sont utilisés temporairement. Pendant les périodes d'écoulement
normales, ils sont habituellement vidés et utilisés dans l'agriculture ou
partiellement remplis, ce qui est avantageux pour des centres de loisirs et des
sports aquatiques.
Cet article décrit en détail la signification générale des réservoirs
d'écrêtement des crues, leur conception en construction en incluant les normes
et régulations appropriées ainsi que l'aspect opérationnel. Ces principes seront
illustrés par un remarquable exemple du système combinant 14 réservoirs de
rétention au long de la rivière Lein avec une superficie du bassin de 247 km2.
Celui-ci est localisé au nord-est de l'état du Baden-Württemberg, Allemagne, une
région de collines avec une pluviométrie abondante et des événements
d'inondations dangereux dus à la capacité de rétention restreinte du sous sol.
Grâce au volume de rétention total de plus de 15 millions m3, le débit de pointe à
l'embouchure de la rivière Lein dans la rivière Kocher (tributaire de la rivière
Neckar) sera réduit à 60 %.
COMMISSION INTERNATIONALE
DES GRANDS BARRAGES
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VINGTIEME CONGRES
DES GRANDS BARRAGES
Beijing, 2000
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