PRELIMINARY DESIGN OF THE HYDRAULIC WORKS – DAM IN

THE VALE DOS FETOS RIVER

Irene Almeida Samora

Objective: The main purpose of this extended abstract is to present the design, on a

preliminary level, of the hydraulic appurtenant work of an rockfill dam. The chosen location is a

section of the Vale dos Fetos river, near the Luso village, and its main characteristics are

summarized. The studied hydraulic structures were the spillway, the temporary diversion, the

bottom outlet, the ecological discharge outlet and the intake tower.

Keywords: spillway, temporary diversion; bottom outlet; intake tower.

Topography and Hydrology

A hydrological study of the dam watershed was developed to obtain the flood hydrographs with

return periods of 20, 5000 and 10000 years. For that purpose, precipitation-runoff models were

used, such as the SCS’ HUD model and the rational formula, taking into account the intensity-

duration-frequency curve of the nearest meteorological station among those studied by Brandão

et al. (2001) and the meteorological data of the nearby climatic stations.

The dam’s axis was chosen based on the topographic characteristics of the construction site,

with an established NWL at elevation 198. The dam’s body is made of rockfill and both the

upstream and the downstream slopes are 1:1.3. Table 1 presents the main topographic and

hydrological characteristics of the dam.

Table 1 – Main topographic and hydrological characteristics.

Location Hydrology

Village Luso River basin área 1.44 km

2

Mean annual rainfall 1045.2 mm

River Vale dos

Fetos river

Flood runoff 32.0 m3/s

Return period 5000 years

Dam Reservoir

Type Rockfill Reservoir area 39 000 m2

Height above streambed 27 m Gross capacity 36 x 1000 m3

Volume of dam body 92 x 1000 m3 Effective storage 32100 m

3

Crest elevation 200.00 m Normal water level (NWL) 198.00 m

Crest length 244.00 m Maximum flood level (MFL) 198.80 m

Crest width 6.00 m Minimum operating level 180.80 m

Spillway

Due to the more favorable topographic conditions in this site, the spillway is located on the right

riverbank, and comprises an uncontrolled labyrinth weir and a chute spillway. The study of the

spillway included an analysis of two types of weir, the labyrinth and the WES, the design of the

channel in concrete and the study of two terminal dissipation structures.

Table 2 presents the main characteristics of the spillway.

Table 2 – Spillway. Main characteristics.

Sill Channel

Elevation 198.00 m Length 116.89 m

Effective length 17.38 m Width 2.00 m

Maximum discharge 17.6 m3/s Wall's height 2.00 m

Basin Convex curves 2

Elevation 162.50 m Slopes

0.12

Length 13.00 m 0.265

Wall's height 7.20 m Excavation volume 3000 m3

Width 3.00 m

The spillway was designed to guarantee the passage of 5000-year return period flood in safety,

and it was checked for the flood with 10000-year return period flood. For the later, the flood

should not overtop the dam’s crest. The labyrinth weir was chosen for because it is more

economical than the WES.

The final geometry of the labyrinth weir has one module, with a depht of 2.50 m and an angle of

8º. Inserted in a 5.00 m wide channel, it has an efective length of 17.38 m. To assure the right

approch conditions, the upstream leading walls have an hydrodynamic shape. The geometry of

the weir is represented in Figure 1.

Figure 1 – Spillway. Labyrinth weir. a) plant; b) longitudinal section.

The open concrete channel has a constriction downstream of the weir, two straight courses

connected by convex curves and a divergent transition to a silling basin, completing a total

length of 116.89 m. At the downstream edge of the constriction, the runoff is considered critical

and the width of the channel varies from 5.00 to 2.00 m.

At the bottom edge of the open channel, the energy dissipation is guaranteed by a BUREC type

III hydraulic jump stilling basin. This basin is located at elevation 162.50 m, and the water high

at its entrance, for the design flood, is 0.3 m, with a velocity of 19.8 m/s (Fr = 11.6), leading to

4.7 m of water high after the jump. The possibility of a flick bucket was also analyzed, but it was

concluded that the valley did not have the ideal conditions for that kind of structure.

The stilling basin is represented in Figure 2.

Figure 2 – Spillway’s stilling basin. a) longitudinal section; b) plant.

Figures 3 and 4 present the plant and longitudinal section of the spillway. The green line

represents the original land surface.

The discharge is delivered to the river through a short tailrace channel, downstream of the

stilling basin.

Figure 3 – Spillway. Plant.

Figure 4 – Spillway. Longitudinal section.

Temporary diversion

The temporary diversion, designed for 20 years flood, was planned to be a gallery of concrete,

with a circular cross section, below the dam embankment. This diversion includes the intake

and restitution structures as well as one stoplog.

The base of the intake structure was placed at elevation 177.75 m and the highest level

reached in the reservoir for the design flow is 181.30 m which means that the crest of the

upstream cofferdam must be at elevation 182.50, m considering a freeboard of 1.20 m. The

intake was designed to guarantee a free surface flow in the gallery for the maximum effluent

discharge of 7.8 m3/s, and to attain uniform flow at the upstream section with a flow height of

0.8D, where D is the diameter of the gallery.

In the present case, it was considered necessary to design an energy dissipation device in the

downstream edge of the gallery. The restitution structure was planned as a SAF stilling basin,

suitable for the energy dissipation downstream of conduits. This basin is located at elevation

168.60 m, and the flow depth at its entrance is 0.3 m, with a velocity of 10.3 m/s (Fr = 5.8),

leading to 2.5 m of water high after the jump.

Table 3 presents the main characteristics of the spillway. The SAF stilling basin is represented

in Figure 6 and Figure 5 presents the profile of the temporary diversion.

Table 3 – Temporary diversion. Main characteristics.

Gallery Basin

Top elevation 172.43 Elevation 168.60

Lenght 99.6 m Lenght 2.90 m

Diameter 1.1 m Wall's hight 3.30 m

Slope 0.039 Maximum width 3.35

Figure 5 – Temporary diversion. Longitudinal section.

Figure 6 – Temporary diversion. SAF stilling basin. a) longitudinal section; b) plant.

Bottom outlet and water intake

The gallery of the temporary deviation was adapted to operate as bottom outlet. The tower has

three intakes, one for the bottom outlet and two for water supply. The tower is accessible

through a 3 m wide and 57 m long bridge (Figure 7).

The diameters of the bottom outlet conduit and of the Howell-Bunger valve were calculated to

be able to empty the reservoir within 4 days. It is usual to consider a longer time to empty a

reservoir but, in the present case, the reservoir is very small and a smaller diameter could lead

to clogging problems.

The water supply intake is used for water supply and for the ecological discharge. The conduit

has a diameter of 0.50 m and, downstream, it splits in two conduits: one for water suply and

another for the ecological flow.

Air conduits were also designed to be placed downstream of the gates to avoid the formation of

vortexes and consequent vibration. Figure 8 shows some of the details of the intake tower.

Table 4 presents the main characteristics of the bottom outlet and water intake.

Table 4 – Bottom outlet and water intake. Main characteristics.

Bottom outlet Water intake

Location Inside diversion tunnel Maximum discharge 300 l/s

Section 1.10 m Section 0.50 m

Maximum discharge 2,3 m3/s Ecological discharge

Uptstream control Sluice gate Maximum discharge 5 l/s

Downstream control Hollow jet valve D400 Section D50

Figure 7 – Bottom outlet. Longitudinal section.

Figure 8 – Intake tower.

Final comments

The hydraulic works appurtenants to the dam in the Vale dos Fetos river are presented in

Figure 9. During construction, a temporary deviation, consisting of an underground gallery and

an earthfill cofferdam, ensures that the area of work is not flooded. A spillway in the right

riverbank was designed for the flood with 5000 years of return period. In case of emergency, the

evacuation of the stored water in the reservoir is regulated by a bottom outlet, controlled by a

planegate upstream and by a Hollell-Bunger valve downstream. An intake tower, with two intake

levels, assures the supply of 300 l/s and the release of 5 l/s for ecological discharge.

Figure 9 – Dam in Vale dos Fetos river. Layout.

Since the purpose of the dam was unknown, the incoming normal runoff was not studied and,

because of that, the MWL and the required water supply were postulated. The designed

temporary diversion is too complex considering the size of the dam and the magnitude of the

peak discharges, but it was interesting to design it for a free-surface flow. Some economical

aspects were considered, such as the reduction of the excavations.