Novak P. Developments in hydraulic engineering - Vol. 5
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two generating units. The width of a pier accommodating generating machinery is, of
course, greater than that of a pier of a normal gated weir; nevertheless, the total length of
the plant is reduced. The designers also argued that the layout provided for a
FIG. 14. Station with conventional
Kaplan aggregate in siphon setting for
utilizing very low head; Randersacker,
Main, German Federal Republic, 100
m
3
/s, 2·7–3·3 m, 2 MW.
31
less disturbed inflow to the turbines when compared with the curved flow pattern in front
of an in-bay located block powerhouse.
39–42
Despite the suggested beneficial properties, this design has only been used on two
small rivers in Europe (Drau and Inn). Lively discussion had been, for a long time, taking
place about the advantages and drawbacks of this solution.
28,28a,43,44
It is plausible that,
with growing unit capacities, both advantages, i.e. saving in construction width and
higher hydraulic inflow efficiency, are diminishing or even disappearing. Also, the
designers of equipment (mechanical and electrical) and, even more, the engineers
responsible for the operation, are for various reasons averse to a scattered powerhouse.
Yet this solution can still be justified in exceptional conditions: no possibility for any
widening of the river bed, small or medium-size plant with fairly small units, and where
piers are not seriously endangered by drifting ice.
4.1.4 Submersible Power Station
The powerhouse incorporated in the body of the overflow weir is the characteristic
feature of this type of plant. Passage of floods can be alleviated by additional bottom
outlets (Fig. 22).
45
In order to increase the spilling capacity of the weir and to prevent an
excessive rise in backwater
Water power development: low-head Hydropower utilization 29
FIG. 15. Block power station with
conventional Kaplan aggregates and
elevated spillway conduits;
Ottmarsheim, Upper Rhine-Grand
Canal d’Alsace, France, 1952, 1400
m
3
/s, 15·5 m (max.), 144 MW.
34
levels, the weir crest is usually well below the designed headwater elevation. Hence
regulation by crest control gates is indispensable. If bottom outlets are also necessary,
they alternate with the generating units. In the first stations of this type rim-generator
tubular turbines were installed, as shown in Fig. 22, while later bulb-type sets were given
priority (Fig. 23). If the attainable head makes the construction of a sufficiently high dam
Developments in hydraulic engineering–5 30
FIG. 16. Block power station with
conventional Kaplan units and deep-
intake release (spillway) flumes;
Volgograd, Volga, USSR, 19 m, 2530
MW.
35
possible, vertical-axis (conventional) aggregates can also be chosen (Fig. 24).
46
The decision about the use of a submersible layout depends primarily on the discharge
and sediment and ice regimes.
47,48
Sealing problems, considered earlier as possible
drawbacks of this arrangement, no longer exist. During periods when Q>Q
p
the head-
increasing effect of the overflowing jet can be taken into account. The formula derived by
the author gives, with fair approximation, the tailwater depression:
(24a)
where v is the mean velocity in the tailwater (after mixing), v
e
is the
Water power development: low-head Hydropower utilization 31
FIG. 17. Block power station equipped
with bulb-type tubular Kaplan units;
Iffezheim, Upper Rhine, France/FRG,
1977, 1100 m
3
/s, 10·8 m (max. 12·5
m), 100 MW.
36
entering velocity of the excess flow into the tailwater (slightly smaller than the spouting
velocity), Q
e
is the excess discharge, and Q=Q
p
+Q
e
= total flow.
4.1.5 Underground Layout
The development of underground hydropower plants shows that, despite a few cases of
clearly defence objectives, the main intention was to make use of technological and
economic advantages. Though the beneficial effects of an underground location are fairly
obvious in the majority of high-head plants, it has been found that, under specific
topographical and geological conditions, this layout is favourable also for low-head
stations (Fig. 25).
The terrain along the river banks and sharp bends which can be cut by short tunnels is
in some instances favourable for an underground location of the plant. When comparing
an open-air with an underground variant it may be found that for economic reasons, i.e. to
reduce investment and operating costs, the latter has to be given priority. Moreover, even
when the economic balance is not in favour of the underground alternative, for
Developments in hydraulic engineering–5 32
FIG. 18. Block power station with rim-
generator Kaplan (STRAFLO)
aggregates; Weinzödl, Mur, Austria,
1982, 180 m
3
/s, 9·8 m, 16 MW.
37
Water power development: low-head Hydropower utilization 33
FIG. 19. Low-capacity S-type
(propeller or Kaplan) set in low-head
plant (TUBE turbine of Allis-Chalmers
Corporation).
FIG. 20. Layout of pier-head plant;
Lawamünd, Drau, Austria, 1944, 380
m
3
/s, 9·0 m, 23 MW.
39,40
Developments in hydraulic engineering–5 34
FIG. 21. Lawamünd pier-head station
(layout on Fig. 20); horizontal section
through the pier, cross-sections of pier
and gated weir.
39,40
Water power development: low-head Hydropower utilization 35
FIG. 22. Early development of
submersible plant equipped with rim-
generator propeller turbines: (a)
section through turbine; (b) bottom
outlet; (c) longitudinal section; Rott-
Freilassing, Saalach, FRG, 1951, 60
m
3
/s, 8·5 m, 4100 kW.
45
FIG. 23. Submersible station equipped
with twenty bulb-type Kaplan sets;
Kiev, Dnepr, USSR, 9·4 m, 320 MW.
(After E.Gruner.)
Developments in hydraulic engineering–5 36
FIG. 24. Submersible station with
conventional Kaplan units located
within the weir body; Pavlovsk,
USSR.
46
the sake of environmental and landscape protection this solution may still be adopted.
Figure 26 shows a very-low-head project where bulb-type tubular aggregates are
installed.
4.2 Diversion Type Projects
4.2.1 Power Canal
Physical (hydrological, topographical and geological) conditions, environmental and
economic requirements, further anthropogenic developments, even political aspects, may
favour this solution. The flow is partly (seldom completely) blocked and, if necessary,
impounded in the watercourse by a diversion weir and is diverted into a power canal
which, at a shorter or longer distance, again joins the river. If the diversion is short it is a
loop development or power branch of the project.
Water power development: low-head Hydropower utilization 37
FIG. 25. Underground arrangement of
low-head plant equipped with
conventional units. (After
K.I.P.Wittrock and K.G.G.Pira.)
The power station can be located next to the intake, or at the outlet, or at any section of
the canal (Fig. 27). The siting of the station depends on the complex effects of the above
listed factors, where control of the groundwater table may have a predominating role.
In preventing, or rather reducing, intrusion of bedload into the canal,
FIG. 26. Low-head underground
station housing a single tubular bulb-
type aggregate; Shingo 2, Agano,
Japan, 200 m
3
/s, ~23 m, ~41 MW.
49
Developments in hydraulic engineering–5 38