FEATURE OF TECHNICAL SERVICE Watersheds tributary to the ... · RESERVOIR «DIGA DI RIDRACOLI»...
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Watersheds tributary to the Ridracoli reservoirFEATURE OF TECHNICAL SERVICE
Subject
Management project of the reservoir «Diga di Ridracoli» according to the
instructions of Ministerial Decree 30 june 2004, under Article 40 Comma
2 of D.L. 11 may 1999 N.152
Carried out by SGAI Srl di E. Forlani & C.
Client Romagna Acque-Società delle Fonti S.p.A
Service length 2005
View of the reservoir «Diga Ridracoli»
Geologic map of the Ridracoli reservoir
RESERVOIR «DIGA DI RIDRACOLI» – Investigation stage, characterisation of the
reservoir and sedimentation process
The reservoir of the Ridracoli dam is located in the province of Forlì-Cesena, within the Foreste Casentinesi
National Park, Campigna Park and Monte Falterona, along of the Bidente river at the confluence with the Rio
Celluzze, at about 10 km from the town of Santa Sofia. The dam is of the arch-gravity type with a height of
103.5m and with a crowning development of 432m. The basin covers an area of 1,035 km2, allowing the
storage of 35.5 million m3 of water. The primary use of the basin is for water supply purposes with annual
regulation of the flow rates of the Bidente River, while the secondary one concerns the hydroelectric production
in the nearby Isola plant. According to the disposal of the Ministerial Decree 30 june 2004, an accurate study of
the basin, dam and hydraulic equipments was carried out, thanks to define the forecast framework of the
management and maintenance activities to carry out for ensure the maintenance and gradual restoration
of the useful capacity of the reservoir, ensuring the operation of the discharge and catchment systems in all
operation conditions. The design approach has seen the realization of accurate geological-geotechnical,
hydrological, hydraulic and environmental studies, thanks to which it is possible to carry out the qualitative
characterization of sediments and suspended solid material; calculate the annual solid transport volume
and the volume of sedimented material; as well as investigating the water quality of the basin. Analyzing
the geological and geomorphological framework of the reservoirs of the Ridracoli reservoir, the presence of
medium tenacity and little erodible lithologies was found, consisting mainly of the marly-arenaceous
formation of Romagna in its various components. The vegetal coverings, made up of thick coppice woods, help
to limit the surface erosion of the soil, reducing even more the supply of debris. Therefore, the volume of
sediment produced annually in each sub-basin was estimated, obtaining an accumulation of debris in the
reservoir for about 41,000 m3/year. Conducting geotechnical, physical-chemical and mineralogical analyzes
on the sediments, the presence mainly of angular-shaped gravel emerged with strong heterogeneity in the
various parameters, indicating a very short moving of the material before deposition. The values of heavy
metals, hydrocarbons, pesticides, Phosphorus, Nitrogen and BOD5 are very low and all parameters verify the
standard value limits. The suspended solid material is present with an amount less than 1% of the solid
transport intercepted by the tidal barrage. In relation to the parameters described and the trends recorded, it is
noted that the temperature, dissolved oxygen and pH are characteristic of reservoirs with good water quality.
The investigation survey showed that the sedimented material for the entire duration of the concession is less
than 5% of the reservoir capacity and, if it was filled from the bottom, it would not reach the elevation of the
bottom discharge tunnel without affecting the useful basin capacity.
Parametri Acque vs. Solidi Sospesi
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 50 100 150 200 250 300 350
Solidi Sospesi (mg/l)
BO
D5
(m
g/l)
0
5
10
15
20
25
30
P e
d N
(m
g/l)
Fosforo totale (mg/l P)
Azoto totale (mg/l N)
Azoto ammoniacale (mg/l N)
Azoto nitrico (mg/l N)
BOD5 (mg/l O2)
Water quality analysis: suspended solids, BOD5, Nitrogen and Phosphorus
Spillway crest and stilling basin Bottom discharge tunnel Outlet of the bottom discharge tunnel
Outlet of the middle discharge tunnel
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Checks of the turbidity levels during the draw-off and maintenance operation
OUTLET OF WATER
SUPPLY TUNNEL
OUTLET OF THE MIDDLE
DISCHARGE TUNNEL
ARCH-GRAVITY DAM
WITH SPILLWAY CREST
Aerial view of the reservoir «Diga di Ridracoli»
STILLING BASIN AND
BOTTOM DISCHARGE
TUNNEL
Torbidità Totale e Torbidità Caratteristica Scarico di Fondo
0
50
100
150
200
250
300
350
400
-5 0 5 10 15 20 25 30 35 40 45 50 55 60
Tempo (min.)
To
rbid
ità (
mg
/l) Torbidità Corretta Allo Scarico di
FondoTorbidità Misurata nei punti A,B,C
Volume Rilasciato Totale e dallo Scarico di Fondo
0
20000
40000
60000
80000
100000
120000
140000
-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
tempo (min.)
Vo
lum
e (
mc)
Volume Totale in Alveo
Volume Totale Rilasciato dallo
Scarico di Fondo
Curva di Torbidità/Volumi Scarico di Fondo
apertura dopo 1 anni
0
50
100
150
200
250
300
350
400
0 5000 10000 15000 20000 25000 30000 35000
Volumi Rilasciati dallo Scarico di Fondo (mc/s)
To
rbid
ità
(m
g/l
)
torbidità
Rilasci Totali a Valle
0
5000
10000
15000
20000
25000
30000
-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
tempo (min).
Vo
lum
i R
ilascia
ti (
mc)
Scarico di Fondo - mc rilasciati
Totale Volumi in Alveo
Torbidità - Attesa e Limiti
0
2000
4000
6000
8000
10000
12000
14000
16000
-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Tempo (min.)
To
rbid
ità (
mg
/l)
Torbidità Scarico di Fondo in Assenza Diluizione
Torbidità Attesa nel Corpo Recettore
valore massimo
Soglia di Attenzione
Operazione Azione Effetti
-L’invaso è colmo, al termine delle operazioni se di manutenzione si può essere in condizioni di sfioro
-I rilasci, sempre se in condizione di sfioro, seguono già portate nel corpo recettore
INIZIO FASE A -Le fasi iniziali hanno una miscelazione con acque di ottima qualità
-Le condizioni stagionali di portata e torbidità sono quelle naturali
Apertura dello Scarico di
Mezzo Fondo . (se necessario)
Sarà in modo da consentire di diluire opportunamente la successiva onda di maggiore torbidità
all’apertura dello scarico di fondo
INIZIO FASE B -La necessità o meno di aprire lo scarico di mezzo fondo dipende dall’eventuale flusso dallo sfioro
Apertura Parzializzata dello
Scarico di Fondo .
INIZIO FASE C
Modulazione Portata Scarico di
Fondo
INIZIO FASE D
Chiusura Scarico di Mezzo
Fondo
-Dopo il raggiungimento di valori di torbidità bassi dallo scarico di fondo sarà possibile chiudere lo
scarico di mezzo fondo, pur mantenendo accettabili valori di torbidità
INIZIO FASE E -La chiusura di questo avverrà in tempi differenti in caso di sfioro in atto o meno
Fine Operazione – Chiusura
Scarico di Fondo
INIZIO FASE F
4
5 -Dopo le operazioni di manutenzione/spurgo dello scarico
Periodo: da Ottobre ad Aprile.
Preferibile condizione di Sfioro.0
1
2
-I primi volumi d’acqua rilasciati dallo scarico avranno la maggiore torbidità, la parzializzazione ha la
finalità di consentire il massimo rapporto di diluizione con le eventuali acque di sfioro e/o di scarico di
mezzo fondo
3 -Dopo la fase iniziale lo scarico di fondo verrà portato alla portata maggiore definita
Spurgo Scarico di Fondo
0
5
10
15
20
25
30
35
40
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
tempo (min.)
Q (
mc
/s)
Sfioro+Mezzo Fondo
Scarico di Fondo
Totale
1 2 3 4 5Operazione n.
Characteristic turbidity curves of the bottom discharge tunnel
MANAGEMENT PROJECT - Opening procedures of the hydraulic discharge system
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FEATURE OF TECHNICAL SERVICE
Subject
Management project of the reservoir «Diga di Ridracoli» according to the
instructions of Ministerial Decree 30 june 2004, under Article 40 Comma
2 of D.L. 11 may 1999 N.152
Carried out by SGAI Srl di E. Forlani & C.
Client Romagna Acque-Società delle Fonti S.p.A
Service length 2005
RESERVOIR «DIGA DI RIDRACOLI» – Management project of the basin, design issues and their resolution
Management project of a reservoir, according to the instructions of the Ministerial Decree 30 june 2004, consists of the preventive definition of the
management and maintenance procedures to carry out for ensure the maintenance and gradual restoration of the useful capacity of the basin,
while limiting the effects, related to the outflow of muddy waters, in the river bed downstream of the dam. In order to define the procedures for opening
the discharge system, for each of these, it is necessary to establish the permissible turbidity curves by estimating the movement of sedimented
material at the site of the floodgates, based on the typical speeds of runoff water. In detail, the Ridracoli dam is equipped with the following hydraulic
equipment: spillway crest, middle discharge tunnel, bottom discharge tunnel and emptying drainage. The spillway crest consists of an overflow
threshold composed by 8 spans of 14m each. To protect the foundation of the structure there is a stilling basin with a tidal barrage of 14m height. The
middle discharge drain, consisting of a 4m diameter tunnel under the left abutment, allows to drain flow rates up to 130 m3/s. The bottom discharge
drain consists of a tunnel on the right abutment, of a length of about 400 m, with a disposal capacity of up to 170 m3/s. To best calibrate the opening
procedures of the bottom discharge tunnel, an opening test was performed, in order to obtain the "characteristic" curve of turbidity according to the
volume released. During the test, not being aware of the sediment intake downstream, it was expected that a good mixing of the water from the bottom
discharge drain with high quality reservoir water would be ensured. It is important to define the frequency between one maneuver and another, since
the turbidity values could increase a lot. In fact, by opening the bottom discharge tunnel after 3 years, a turbidity of 2050 NTU was obtained, equal
to about 6,000mg/l. The turbidity curve obtained made it possible to define the correct opening procedures for the discharge system. Since there are no
sedimentation problems in the reservoir and on the quality of the water, the study focused on the activity of the bottom discharge, so that the turbidity
levels remain acceptable during the flaring and maintenance operations. For this purpose, it was considered to perform turbidity sampling at the height
of the valley bridge every 5 minutes from the beginning of the operations with verification of the limit values and reconstruction of the real curve. In
detail, an operating sequence was considered with activation of the middle discharge drain, in preferable overflow condition, and following the opening
of the bottom discharge drain at a flow rate of 10m3/s for 3 minutes. In this phase the maximum turbidity will be diluted in a 1.5:1 ratio. Once the
peak of turbidity has passed and entered areas compatible with the thresholds, the background drain will be divided into 10m3/s and the background
drain will be raised to 25m3/s. The management plan, in order to avoid local phenomena of abnormal sedimentation nearest the discharge organs,
provides for opening operations of the hydraulic organs regularly, ensuring the opening of the bottom and middle discharge drain at least once a year.